Recent development in lead-free perovskite piezoelectric bulk materials

Progress in Materials Science 98 (2018) 552–624

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Recent development in lead-free perovskite piezoelectric bulk materials

T

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Ting Zheng, Jiagang Wu , Dingquan Xiao, Jianguo Zhu Department of Materials Science, Sichuan University, 610064 Chengdu, PR China

A R T IC LE I N F O

ABS TRA CT

Keywords: Lead-free materials Recent advances Piezoelectric effect Phase structure Domain configuration Physical mechanism Application

The elimination of lead in piezoelectric applications remains challenging. Since the advances in the piezoelectricity were found in the perovskite family in 2000, studies into lead-free piezoelectric materials have grown exponentially in the fields of condensed matter physics and materials science. In this review, we highlighted the compelling physical properties of lead-free piezoelectric perovskite materials and summarized their state-of-the-art progress, with an emphasis on recent advances in the piezoelectric effect. We mainly introduced the unique advances in lead-free perovskites piezoelectric bulk materials, along with the descriptions of phase boundaries, domain configurations, and piezoelectric effects, and then the main physical mechanisms of high piezoelectricity were summarized. In particular, the applications of lead-free materials were also introduced and evaluated. Finally, challenge and perspective are featured on the basis of their current developments. This review provides an overview of the development of lead-free piezoelectric perovskite materials in the past fifteen years along with future prospects, which may inspire material design toward practical applications based on their unique properties.

1. Overview Piezoelectric materials generate electrical signals from the application of a mechanical force or displacement in response to an external electric field. This electrical–mechanical energy conversion is defined as the piezoelectric effect, which was first discovered by the Curie brothers as early as 1880 [1]. The interesting discovery of the piezoelectric effect has promoted the rapid development of information technology, optoelectronic technology, and so on. In particular, piezoelectric materials have very important and wideranging applications in electronic devices, such as actuators, transducers, and sensors. After years of developments, the exciting progress in electrical properties, physical mechanisms, and corresponding applications of lead-free materials have been realized. Fig. 1 presents the historic development of lead-free piezoelectric materials. It can be observed that the Curie brothers first discovered the piezoelectric effect in quartz crystal as early as 1880 [1]. The first discovery in piezoelectricity was made after the discovery of barium titanate (BaTiO3) ceramics in 1946, resulting in rapid progress in piezoelectric materials [2]. In 1954, Jaffe B et al. discovered PZT ceramics, which have better piezoelectricity than BaTiO3 [3]. In 1960, Smolenskii et al. discovered the high ferroelectricity of lead-free Bi1/2Na1/2TiO3 (BNT) ceramics [4]. Before the 21st century, the advances in electrical properties can be few found in lead-free piezoelectric materials. However, since entering the 21st century, the families of lead-free perovskite materials {K0.5Na0.5NbO3 (KNN), BaTiO3 (BT), Bi1/2Na1/2TiO3 (BNT), and BiFeO3 (BFO)} have received much attention in the last ten years because some significant progresses in their physical properties were realized [5]. For

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Corresponding author. E-mail addresses: [email protected], [email protected] (J. Wu).

https://doi.org/10.1016/j.pmatsci.2018.06.002 Received 23 September 2017; Received in revised form 18 January 2018; Accepted 27 June 2018 Available online 30 June 2018 0079-6425/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Historic development of lead-free piezoelectric materials.

example, in 2004, high piezoelectricity (d33 ≈ 416 pC/N) was achieved in textured KNN-based ceramics with orthorhombic-tetragonal (O-T) phase boundary fabricated by the reactive template grain growth (RTGG) method [5], and ten years later (2014), a series of KNN-based ceramics with high piezoelectric constants (d33 = 490–570 pC/N) were obtained by designing new phase boundaries of rhombohedral (R) and tetragonal (T) phases [6–9]. After that, ultrahigh piezoelectric coefficient d33 of 700 pC/N and large kp of 0.76 were reported by developing the textured KNN-based ceramics [10]. As the earliest-discovered polycrystalline ceramics, the BT-based ceramics exhibited much higher piezoelectricity than other lead-free piezoceramics. In 2009, Ren et al. observed a large piezoelectric effect (d33 ≈ 620 pC/N) in lead-free Ba(Ti0.8Zr0.2)O3–(Ba0.7Ca0.3)TiO3 ceramics with a R-T phase boundary, and after that, many researchers have focused on this effect and illuminated the underlying physical mechanisms [11]. In 2017, giant d33 of 755 pC/N and d33* of 2027 pm/V were simultaneously achieved in highly textured BCTZ ceramics [12]. The giant strain response of BNT-based materials makes them the preferred material for actuator applications. In 2007, Zhang et al. reported a large strain value (S = 0.45%) for Bi1/2Na1/2TiO3-BaTiO3-K0.5Na0.5NbO3 ternary ceramics due to an antiferroelectric-to-ferroelectric phase transition [13]. In 2016, a superior strain value (S = 0.7%) was obtained in Sr and Nb co-modified BNT-based ceramics by Tan et al. [14]. Numerous studies on the giant remnant polarization (Pr > 55 µC/cm2) of BFO-based thin films have been reported since 2003 [15], but unfortunately, it is difficult to fabricate high-quality BFO-based ceramics due to the high leakage current induced by both impurity phases and the valence change of Fe. However, in 2015, a large piezoelectric constant was achieved in BFO-based ceramics by modifying the preparation process, that is, both a large d33 of 402 pC/N and a high TC of 450 °C were simultaneously obtained for 0.67Bi1.05(Fe0.97Ga0.03)O3-0.33BaTiO3 ceramics with a R-T phase boundary [16]. As a result, the enhancement in the piezoelectric effect is mainly related to the phase boundaries in lead-free perovskite families. To better understand this situation, we collected the statistics on the percentage of publications in 2000–2017 related to the piezoelectric effect [Fig. 2(a)]. It can be clearly found that KNN and BNT piezoelectric materials received more attention due to the excellent piezoelectric and strain properties. In addition, we also collected publications on lead-free piezo/ferroelectric materials in the form of ceramics, thin films, single crystals, and nanostructures, as shown in Fig. 2(b). Bulk materials comprise approximately 87% of the total share due to their simple preparation and good comprehensive electrical properties. Due to the great advances in electrical properties, the development of lead-free piezoelectric materials has been recently quickened. Up to now, toxic lead-based piezoelectric materials, represented by Pb(Zr,Ti)O3 (PZT), have dominated the electronic device market worldwide [1]. Although some weak toxicity also exists in some lead-free piezoelectric materials [17], it is necessary to 553

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Fig. 2. (a) Publications on lead-free piezo/ferroelectric KNN, BNT, BT, and BFO-based materials; (b) publications on lead-free piezo/ferroelectric materials with the form of ceramic, thin film, single crystal, and nanostructure in refereed journals in the time period of 2000–2017.

develop the lead-free candidates for sustainable development, because related laws and regulations have been legislated to prohibit the use of lead in electronic devices, such as “The Restriction of the use of certain Hazardous substances in Electrical and Electronic Equipment” (RoHS) and “Waste Electrical and Electronic Equipment” (WEEE). Therefore, it is essential and urgent to fabricate leadfree piezoelectric materials with high performance for practical applications [18]. As claimed above, the enhanced piezoelectricity of perovskite lead-free materials was attributed to the coexistence of multiple phases [3,5–9,11,13,14,16]. The concept of “morphotropic phase boundary” was first proposed by Jaffe et al. [3], who observed the peak piezoelectric constant of PZT ceramics in a composition with or near an MPB [3]. Therefore, phase boundaries should play an important role in the electrical enhancement of lead-free perovskite piezoelectric materials, especially for the improvement of piezoelectricity. In the past 15 years, advances in lead-free piezoelectric perovskite materials have been reviewed from different viewpoints, greatly promoting research progress [19–26]. For example, in 2012, Jo et al. briefly presented the status and perspective of electric field-induced strains in BNT-based ceramics for actuator applications [19]. The processing technologies, phase structure, electrical properties, and potential applications of lead-free piezoelectric ceramics were reviewed by Rodel et al. in 2009 and 2015 [20,21]. In 2014, Damjanovic et al. summarized the processing, electrical, and electromechanical properties of bismuth ferrite ceramics. [22] In 2014, Wu et al. first focused on the relationship between the electrical properties and the phase boundaries in KNNbased materials [23]. In 2017, fundamentals, current status, and perspectives of BaTiO3-based piezoelectrics have been reviewed by Acosta et al. [27]. According to the previous reviews, it was found that few systematic reviews concerned recent advances in bulk lead-free piezoelectric perovskite materials in terms of the piezoelectric effects, domain configuration, and phase boundary. More importantly, the relationships among the phase boundaries, domain configurations and electrical properties in lead-free perovskite families are rarely systematically discussed. In particular, some important advances have recently been realized in bulk lead-free piezoelectric perovskites, but few reviews have included them. As a result, we addressed the research progresses into bulk lead-free piezoelectric perovskite materials in terms of piezoelectric effect since 2000, the relationship among phase boundaries, domain configurations and electrical properties was especially emphasized, and then the potentical applications were also introduced and evaluated. Fig. 3 shows the outline of this review. In this review, we summarized the state-of-the-art progress into bulk lead-free piezoelectric perovskites, with a particular emphasis on the relationships among phase boundary, domain configuration and electrical properties, and recent advances in the piezoelectric effect were well discussed. In addition, the physical mechanisms of high piezoelectricity were also proposed, and some promising applications were prospected. At last, challenges and outlooks were provided and analyzed on the basis of their current development. We expect that this thorough review can enlighten researchers and inspire ideas for further improving the electrical properties of lead-free piezoelectric perovskite materials. 2. General consideration 2.1. Piezoelectricity and related parameters Piezoelectricity is a unique property for some crystalline materials with non-center-symmetries. Piezoelectric effects, including the direct piezoelectric effect (generating electric polarization after applying mechanical stress) and converse piezoelectric effect (generating mechanical strain after applying an electric field), are fundamental for various electronic devices, such as transducers, actuators, and sensors. To address this wide audience, it is necessary to introduce the fundamentals of piezoelectricity and related parameters as a starting point. 2.1.1. Piezoelectric coefficients As mentioned above, piezoelectricity can reflect the relationships between mechanical variables (stress, X and strain, S) and electrical variables (electric field, E and electric displacement, D). Based on the selection of independent variables, there are four sets of piezoelectric coefficients, including the piezoelectric charge or strain coefficient (d), piezoelectric stress coefficient (e), 554

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Fig. 3. Outline of this review.

piezoelectric voltage coefficient (g), and piezoelectric stiffness coefficient (h). 2.1.1.1. Piezoelectric charge or strain coefficient (d). The piezoelectric charge or strain coefficient (d) can be defined when the independent variables are E and X and are formulated as below:

∂D ⎞ d=⎛ E ⎝ ∂X ⎠

(1)

∂S d = ⎛ ⎞X ⎝ ∂E ⎠

(2)

Eq. (1) refers to the direct piezoelectric effect. The piezoelectric charge coefficient or simply called the piezoelectric coefficient (dij) is one of the most important physical properties for sensor applications; this value can reflect the intrinsic piezoelectric effect and can be directly measured by a quasi-static d33 meter. Because the applied external mechanical stress is small, the piezoelectric charge coefficient is also called the small signal piezoelectric coefficient (dij = [pC/N]). Representative parameters include the longitudinal piezoelectric coefficient (d33), shear piezoelectric coefficient (d15), and transverse piezoelectric coefficients (d31) and (d24). Eq. (2) refers to the converse piezoelectric effect. That is, an electric field-induced strain, which is an important parameter for actuator applications. The piezoelectric strain coefficient is also called the large signal piezoelectric coefficient (dij* = [pm/V]) because of the large input electric field. The value of dij* can be simply calculated through unipolar S-E curves (dij* = Smax/Emax), where Smax and Emax represent the maximum strain and maximum electric field. Different from the small signal piezoelectric coefficient (dij), the large signal piezoelectric coefficient (dij*) can reflect the extrinsic domain wall motion. 2.1.1.2. Piezoelectric stress coefficient (e). The piezoelectric stress coefficient (e) can be defined when the independent variables are E and S, as shown below:

∂D ⎞ ∂X e=⎛ E = −⎛ ⎞S ⎝ ∂S ⎠ ⎝ ∂E ⎠

(3)

2.1.1.3. Piezoelectric voltage coefficient (g). The piezoelectric voltage coefficient (g) is also a crucial parameter for piezoelectric sensors, and can be obtained when D and X are selected as independent variables, as shown in Eq. (4).

∂S ⎞ ∂E g=⎛ X = −⎛ ⎞D ⎝ ∂D ⎠ ⎝ ∂X ⎠

(4) 555

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2.1.1.4. Piezoelectric stiffness coefficient (h). At constant D and S, the piezoelectric stiffness coefficient (h) is formulated as below:

∂E ∂X ⎞ h = −⎛ ⎞D = −⎛ S ⎝ ∂S ⎠ ⎝ ∂D ⎠

(5)

2.1.2. Dielectric permittivity The dielectric permittivity (εij ) is related to the electric displacement (D) and applied electric field (E) and can be defined as below:

εij =

Di Ej

(6)

Meanwhile, the commonly reported dielectric constant in the literature is the relative dielectric permittivity (εr (ij) ), which can be calculated according to Eq. (7).

εr (ij) =

εij ε0

(7)

where ε0 refers to the permittivity of free space. For piezo/ferroelectric materials, the dielectric constant can be influenced by polarization rotation and domain wall motion. Another critical parameter of material dielectric properties is dielectric loss (tan δ). Under an external electric field, dielectric loss can be considered as the process by which a portion of the electric energy is changed into thermal energy, and it can be calculated by Eq. (8).

tan δ =

εr ′ ′ εr ′

(8) ′

where εr ′ and εr ′ are the real and imaginary components of the dielectric permittivity, respectively. Dielectric loss can be affected by some factors, such as polarization hysteresis, dielectric leakage, the external environment, and experimental conditions. 2.1.3. Elastic constants Piezoelectric materials should obey Hooke’s law based on the mechanical effects. That is, stress (X) and strain (S) should obey the elastic relationship, as shown below:

c=

X S

(9)

s=

S X

(10)

where c and s are the elastic stiffness and elastic compliance, respectively. The elastic constant of the piezoelectric material determines the inherent frequency and dynamic characteristics of the piezoelectric device. 2.1.4. Other critical parameters 2.1.4.1. Electromechanical coupling factors. Electromechanical coupling factors (k) can reflect the conversion ability between electrical and mechanical energy, which can be directly calculated from resonance (fr) and antiresonance frequencies (fa), as shown below:

k2 =

π fr π f cot ⎜⎛ r ⎞⎟ 2 fa ⎝ 2 fa ⎠

(11) 2

In general, the value of k is less than 1 because electrical and mechanical energy cannot be completely converted. In addition, the electromechanical coupling factors only indicate the effectiveness of energy conversion and cannot represent the actual working efficiency in practical applications. The mechanical energy of piezoelectric elements is closely related to element dimensions and vibration modes. Thus, there are several corresponding electromechanical coupling factors, such as the planar coupling factor (kp), lateral coupling factor (k31), longitudinal coupling factor (k33), thickness shear coupling factor (k15), and thickness extension coupling factor (kt). 2.1.4.2. Mechanical quality factor. The mechanical quality factor (Qm) is an important parameter for resonant applications and can reflect the degree of energy dissipation. The mechanical loss (tan δm) is the reverse of Qm. Therefore, it can be seen that the larger the Qm, the smaller the power loss. The reason for mechanical loss is the existence of internal friction. Similar to the dielectric behavior, the polarization rotation, domain wall motion, environment, and measurement conditions can also affect the mechanical loss. 2.2. Background of phase boundaries 2.2.1. General consideration A phase boundary is a powerful tool to enhance the electrical properties of lead-based and lead-free piezoelectric perovskite 556

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Fig. 4. Cross-section of the Gibbs free energy for 90/10 and 60/40 Pb(Zr1−xTix)O3 along [MB] path [28]. Reproduced from [28], with the permission from Wiley.

materials, and in particular, the type of phase boundary can strongly affect the corresponding piezoelectric activity [23]. For example, the largest piezoelectric constant can be achieved in PZT ceramics with a Zr/Ti ratio of 52:48 due to the formation of an R-T morphotropic phase boundary (MPB), which is considered to be the most classical example [3]. In lead-free piezoelectric perovskite materials, compositions with an R-T phase boundary exhibited a higher piezoelectric response than those with other phase boundaries [23]. As early as the 20th century, the Landau phenomenological theory and first-principles calculations were used to elaborate the phase transition in piezoelectric materials from a theoretical viewpoint [28–30]. The main concept in the Landau theory is the Gibbs free energy, which is dependent on the temperature, composition, electric field, and other external parameters [28]. It is commonly considered that the flattening of the Gibbs free energy profile (instability of the Gibbs free energy) is largest near the MPB (Fig. 4), thus leading to facilitated polarization rotation and piezoelectricity enhancement [28]. Therefore, considering the close relationship among phase boundaries, Gibbs free energy, and piezoelectricity, it is essential to first introduce concepts of phase boundaries in lead-based and lead-free piezoelectric materials. 2.2.2. Phase boundaries in lead-based piezoelectric materials The proposed MPB concept of PZT was a major advancement in the development of piezoelectric materials [1]. In 1954, Jaffe et al. observed the peak d33 value for PZT when the compositions are located at the MPB [3]. Since then, lead-based piezoelectric materials have been widely studied and applied in practical applications due to their excellent properties. Subsequently, we briefly introduce the development of lead-based piezoelectric materials from the view of phase boundaries. 2.2.2.1. Pb(Zr, Ti)O3. PbZrO3 and PbTiO3 [(1 − x)PbZrO3 − xPbTiO3] solid solutions were first investigated by Shirane and coworkers in 1952 [31]. After that, Jaffe et al. constructed the phase diagram of (1 − x)PbZrO3 − xPbTiO3 and observed superior

Fig. 5. (a) Phase diagram of (1 − x)PbZrO3-xPbTiO3 reported by Jaffe et al. [1] and (b) composition-dependent piezoelectric properties of (1 − x) PbZrO3-xPbTiO3 ceramics [3]. Reproduced from [1], with the permission from Elsevier. Reproduced from [3], with the permission from AIP Publishing. 557

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Table 1 d33 and TC of some typical PZT ceramics [24]. Reproduced from [24], with the permission from Elsevier. Hard PZT

d33 (pC/N) TC (oC)

Soft PZT

PZT-2

PZT-4

PZT-8

PZT-5A

PZT-5H

152 370

290 328

225 300

375 365

590 190

electrical properties in the nearly temperature-independent MPB regions, as shown in Fig. 5(a) and (b) [1,3]. Over the years, the electrical properties of PZT were well modified by doping small amounts of cations, thus leading to the appearance of “hard” and “soft” PZT materials [32,33]. Table 1 shows the d33 and TC values of some typical PZT ceramics [24]. For example, doping with lowervalance cations (e.g., K+ for Pb2+ and Fe3+ for Ti4+ or Zr4+) can generate oxygen vacancies, hindering the domain wall motion and thus degrading the piezoelectric and dielectric properties. Both PZT-4 and PZT-8 are the classical representatives of “hard” PZT materials. However, the addition of higher-valance cations (La3+ for Pb2+ and Ta5+ or Nb5+ for Ti4+ or Zr4+) can deliver the opposite effect (soft PZT), as represented by PZT-5A and PZT-5H. Isovalent substitutions (e.g., Sr2+ or Ba2+ for Pb2+ and Sn4+ for Ti4+ or Zr4+) can reduce the Curie temperature and ferroelectricity, but the piezoelectric and dielectric properties can be greatly enhanced. Up to now, some disputes still concerned the phase boundaries in PZT [34–40], which is mainly responsible for the enhanced piezoelectric properties. For example, the generally accepted view is that the MPB in PZT consists of R and T phases, and in 1993, Cross et al. proposed a theoretical model for such MPB [35]. However, Noheda et al. found a monoclinic ferroelectric phase between the previously established R and T regions using high-resolution synchrotron XRD in 1999 [39], and later, the experimental results were confirmed by first-principles calculations [41]. It was believed that the monoclinic phase acted as a bridge between these two phases, and the origin of high piezoelectricity should be ascribed to the facilitated polarization rotation due to the existence of a monoclinic phase in the R-T phase boundary [40,42]. However, in 2009, Damjanovic et al. suggested the origins of the enhanced piezoelectric properties in ferroelectrics in terms of theoretical and experimental evidence and especially emphasized the role of polarization rotation and monoclinic phases [43]. In this review, the authors claimed that the highest piezoelectricity was often observed in the phase transition regions [43], where polarization either changes directions or appears from the nonpolar state and has no relation to monoclinic phases. As a result, the disputes about the intermediate phase still exist. 2.2.2.2. PZN-PT or PMN-PT. Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals are the most promising relaxor ferroelectrics due to their ultrahigh piezoelectricity (d33 = 1500–2500 pC/N) and electromechanical coupling factors (k33 > 0.9) [44]. As early as 1969, Nomura et al. studied the ferroelectric properties in the PZN-PT system and found that the MPB of R and T phases existed in the composition range of 9–9.5 mol% PT [45]. Shrout et al. also proposed that a R-T phase boundary exists in the (1 − x)PMN-xPT single crystal over the composition range of 0.3 ≤ x ≤ 0.4 [46]. After that, Noheda et al. found a ferroelectric monoclinic phase of space groups Cm and Pm around the MPB between the R and T phases in PMN-PT and PZN-PT single crystals [47,48], which is similar to the monoclinic phase in PZT. In addition, a new orthorhombic phase was also discovered in PZNPT single crystals after applying an electric field [49]. Regardless of whether the intermediate phase existed, most studies indicated that the piezoelectric properties of relaxor-PbTiO3 single crystals can be promoted in the vicinity of phase boundaries due to the largest flattening of the free energy. 2.2.2.3. Temperature stability of piezoelectricity. Importantly, the MPB in PZT is dependent only on the compositions and is not sensitive to the temperature, thus resulting in piezoelectricity with good temperature stability [50–55]. More interestingly, many theoretical or experimental results have indicated that PZT-based ceramics exhibit piezoelectricity with good temperature stability, as shown in Fig. 6(a) [5]. Unfortunately, the relaxor-PbTiO3 single crystals show strong temperature-dependent piezoelectricity due to the low FE-FE phase transition temperature and TC [56]. For example, piezoelectricity with insufficient temperature stability is obtained in PIN-PMN-PT single crystals because the R-T phase transition temperature was shifted below room temperature, as shown in Fig. 6(a) [57]. Therefore, piezoelectric properties with excellent temperature stability can be obtained in those PZT-based ceramics, while relaxor-PbTiO3 single crystals still show piezoelectricity with poor temperature stability [50-59]. Fortunately, piezoelectricity with enhanced temperature stability can still be achieved in some relaxor-PbTiO3-based ceramics due to the formation of diffused phase transition induced by composition modification [60]. 2.2.3. Phase boundaries in lead-free piezoelectric materials 2.2.3.1. Design principle of phase boundaries. Different from PZT, the phase boundary construction of lead-free piezoelectric materials often needs chemical composition modification, such as ion substitution and/or the addition of second or third members. In a word, the additives should have an ability to shift the phase transition temperature. Therefore, some fundamental characteristics of the additives should be considered, such as ionic radius and valence state. Table 2 summarizes the fundamental characteristics of some typical substitutes and their influence on the phase transition of lead-free K0.5Na0.5NbO3, BaTiO3, and Bi1/2Na1/2TiO3-based materials. According to the principle of crystal chemistry, the additives with similar ionic radii and valence states to the matrix elements are usually selected as substitutes. For K0.5Na0.5NbO3, the monovalent elements (Li, Ag) are commonly used to substitute 558

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Fig. 6. (a) temperature dependence of d33 or d33* for PZT-4 ceramic and 〈0 1 1〉 PZN-PT single crystals [5,57]; (b) temperature stability of d33 or d33* in lead-free 0.7(Bi0.98Nd0.02)FeO3-0.3BaTiO3 + 0.1 wt%MnO2 [61]; 0.955(Bi1/2Na1/2)TiO3-0.045Ba(Al0.5Ta0.5)O3 [62]; 0.965K0.48Na0.52Nb0.95Sb0.05O30.035Bi0.5(Na0.82K0.18)0.5HfO3 [9]; Ba(Ti0.8Zr0.2)O3-(Ba0.7Ca0.3)TiO3 [11]. Table 2 Fundamental characteristics of some typical substitutions and their influence on phase transition of KNN, BT, and BNT-based materials (↓: decrease, ↑: increase, →: basically unchanged). Materials

Substitutes +

Sites

Ionic radii (nm)

K0.5Na0.5NbO3 K+: 0.138 Na+: 0.102 Nb5+: 0.064

Li Ag+ Bi3+ Zr4+ Hf4+ Sn4+ Sb5+ Ta5+

A site

BaTiO3 Ba2+:0.135 Ti4+: 0.06

Ca2+ Sr2+ Zr4+ Sn4+ Hf4+

A site

Bi1/2Na1/2TiO3 Bi3+:0.103 Na+:0.102 Ti4+:0.06

Li+ Ag+ Zr4+ Hf4+ Sn4+ Ta5+ Nb5+

A site

B site

B site

B site

TR-O

TO-T

TC

Td

0.076 0.115 0.103 0.072 0.071 0.069 0.060 0.064

↑ ↑ ↑ ↑ ↑ ↑

↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

↑ ↓ ↓ ↓ ↓ ↓ ↓ ↓

[63] [64] [6] [6] [9] [65] [6] [66]

0.100 0.118 0.072 0.069 0.071

↓ ↓ ↑ ↑ ↑

↓ ↓ ↑ ↑ ↑

→ ↓ ↓ ↓ ↓

[67] [68] [69] [69] [69] → → ↓ ↓ ↓ ↓ ↓

0.076 0.115 0.072 0.071 0.069 0.064 0.064

Ref.

[70] [71] [72] [73] [74] [75] [76]

the A site (K or Na), while the tetravalency elements (Zr, Hf, Sn) and pentavalent elements (Sb, Ta) are often employed to substitute the Nb site. Through many experimental verification, Li substitution at the A site can decrease TO-T and increase TC [63], while the addition of Ag at the A site can decrease TO-T and TC [64]. Eitel et al. proposed that a smaller tolerance factor (t) can lead to a higher Curie temperature [77]. Based on the concept of the tolerance factor (t) [see Eq. (12)], the increased TC in Li-doped KNN ceramics is due to the decreased tolerance factor caused by the smaller Li ionic radius.

t=

rA + rO 2 (rB + rO )

(12)

In addition, substitutes at the B site can increase TR-O and decrease TO-T at the same time, which is very important for the construction of phase boundaries at room temperature. Furthermore, according to these rules of ion substitution, some compounds can be used to modify the KNN matrix, such as Bi0.5A0.5BO3 (A = K, Na, Li, Ag; B = Zr, Hf, Sn, Ti) [65,78], ABO3 (A = Ba, Ca, Sr; B = Zr, Hf, Sn) [8,79,80], BiMO3 (M = Fe, Sc) [7,81], and so on. For BT, some different phenomena can be found. For example, the Ca- and Sr-substitutions at the A site can simultaneously decrease TR-O and TO-T [67,68], while the addition of Zr, Sn or Hf at the B site can simultaneously increase TR-O and TO-T [69], as shown in Table 2. However, for BNT, the ion substitutions at the A site have nearly no influence on the depolarization temperature (Td), while the high valence ions substitutions at the B site can greatly reduce the Td values, leading to the formation of MPB(II) at room temperature and then achieving a large strain response. As a result, selecting the substitutions with ionic radii and valency states similar to those of the matrix elements is the foundation, and the effects of the additives on the phase transition temperature can be considered as the theoretical direction. However, there has been little investigation regarding about how the fundamental characteristics (e.g., ionic radii, valence states, covalency, etc.) of the various 559

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additives influence the compositional shifts of the phase boundaries. Further exploration should be carried out to better understand the design principle of the phase boundary. 2.2.3.2. Phase boundary vs. piezoelectricity. It is no exaggeration to say that the discovery of PZT with a MPB greatly stimulated the development of lead-free piezoelectric materials [1]. In the past, it was commonly thought that the phase boundaries in most leadfree piezoelectric materials can be defined as a polymorphic phase boundary (PPB), which is dependent on not only the composition but also the temperature. For example, the phase boundaries in KNN, BNT, and BT-based materials are similar to those in relaxorPbTiO3 ferroelectrics, and dielectric anomaly peaks corresponding to the phase transition can result in the fluctuation of piezoelectric/strain properties as well as temperature stability. However, there is a similar phase boundary in BFO and PZT materials. As a result, it can be thought that both a large piezoelectric constant and excellent temperature stability can be attained in phase-boundary materials without dielectric anomaly peaks. Although enhanced piezoelectric properties are often observed in lead-free piezoelectric materials with phase boundaries, the types of phase boundaries strongly affect the piezoelectricity [23]. For example, three types of phase transition temperatures (TR-O, TO-T, and TC) can be found in KNN-based materials. When doped with additives, different phase boundaries can be driven by shifting TR-O and/or TO-T, thus leading to the formation of R-O, O-T or R-T phase boundaries [23]. Generally, the order of piezoelectric coefficients in KNN-based materials is R-T > O-T > R-O. Therefore, KNN-based materials with R-T phase boundaries have become one of the most popular candidates in recent years because of the enhancement in piezoelectric properties. Similar to KNN, BT-based materials also exhibit different types of phase boundaries, and the optimal piezoelectric response is also obtained with the R-T phase boundary [11]. However, two kinds of phase boundaries were found in BNT-based materials, MPB(I) and MPB(II) [19]. The ferroelectric rhombohedral-tetragonal (R-T) phase boundary is denoted MPB(I), at which a maximum d33 can be attained; the ferroelectric and relaxor pseudocubic phase boundary is denoted MPB(II), at which a giant strain (d33*) can be observed. Finally, it is difficult to construct phase boundaries in BFO materials using site engineering, and thus, the piezoelectric constant (d33) is less than 50 pC/N [82]. However, it is relatively easy to form phase boundaries and enhance the electrical properties by introducing ABO3 (i.e., PbTiO3, BaTiO3, etc.), and the largest reported piezoelectricity reached 402 pC/N [16]. Therefore, the electrical properties in lead-free piezoelectric materials can be effectively promoted by the formation of multiple coexisting phases due to the facile polarization rotation among different symmetries [6–9,14,16,23]. Of course, further explorations into the construction of new phase boundaries, the relationship between phase boundaries and property enhancement, and the true origin of the phase boundary are still required. 2.2.3.3. Temperature stability of piezoelectricity. Temperature stability is an important factor for practical applications. However, the imbalance between piezoelectricity and temperature stability is still not resolved in most lead-free piezoelectric materials because of the characteristics of the PPB [62,83,84]. Fortunately, recent studies have demonstrated that enhancement in the temperature stability of the piezoelectric properties can be obtained in lead-free piezoelectric ceramics through some methods [5,9,85]. For example, an enhanced temperature coefficient of strain (the deviation value of d33* is less than 6.5% from room temperature to 160 °C) can be obtained in textured KNN-based ceramics [5]. Fig. 6(b) shows the temperature stability of d33 or d33* in lead-free KNN, BT, BNT, and BFO-based ceramics. Similar to relaxor-PbTiO3 ferroelectrics, the d33 or d33* in KNN, BNT and BT -based ceramics reaches the largest value at the phase transition region (room temperature). Once the temperature deviates from the phase boundary region, a decreased piezoelectric response can be observed. The physical mechanism for the temperature stability will be discussed in Section 3. Nevertheless, positive temperature dependence of d33* can be observed in BFO-based ceramics, which is totally different from other piezoelectric materials. This phenomenon inspires the underlying physical mechanism exploration of lead-free piezoelectric materials with high temperature reliability. 2.3. Background of domain configurations 2.3.1. Ferroelectric domains Ferroelectric domains, i.e., regions with uniform polarization, have a close relationship with the crystal structure and ferroelectric-related properties of perovskite materials [86]. Domain walls, i.e., the interface between two domains, play an important role in extrinsic contributions to property enhancement [86]. A large number of previous experimental results have shown that reduced domain sizes are responsible for the enhancement in the piezoelectricity. In addition, the domain patterns are strongly dependent on the phase structure of the ferroelectric material. For example, lamellar domains and wedge-shaped domains are frequently detected in tetragonal (T) and/or rhombohedral (R) phases, respectively. In addition, according to crystallographic analysis, 90° and 180° domain walls always occur in materials in the T phase, and the spontaneous polarization is along the 〈0 0 1〉 direction. The coexistence of 60°, 90°, 120°, and 180° domain walls appears in materials in the O phase, and the spontaneous polarization is along the 〈1 1 0〉 direction. However, R-phase materials have coexisting 71°, 109°, and 180° domain walls, and the spontaneous polarization is along the 〈1 1 1〉 direction [86]. Therefore, the domain patterns and domain types reflect the crystal structure of a material. Currently, technologies for the detection of the domain configuration mainly include chemical etching, transmission electron microscopy (TEM), scanning electron microscopy (SEM), piezoresponse force microscopy (PFM), polarized light microscopy (PLM), etc. Chemical etching is the oldest technique for the observation of domains; however, it is destructive and unfavorable for in situ domain dynamic studies. TEM is the most precise technique due to its ultrahigh resolution, which can not only reflect the domain size and domain pattern but also the in situ dynamic domain evolution under external electric field and/or temperature. In particular, the domain types as well as crystal structures can be well identified by TEM combined with selected-area electron diffraction (SAED) or convergent-beam electron diffraction (CBED). 560

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2.3.2. Domain engineering Domain engineering has become one of the most important techniques for the property optimization of ferroelectric materials, in which a desired domain configuration can be obtained [87]. Among domain-engineering techniques, the concept of an engineered domain configuration was initially proposed based on the crystallographic anisotropy in ferroelectric PZN-PT single crystals [88]. As early as 1997, Park and Shrout reported both an ultrahigh piezoelectricity (d33 over 2500 pC/N, k33 over 93%) and hysteresis-free S-E curves in [0 0 1]C-oriented rhombohedral PZN-PT single crystals [44]. However, both poor piezoelectricity (d33 ≈ 83 pC/N and k33 ≈ 38%) and S-E curves with large hysteresis were obtained with this single crystal along the [1 1 1]C polar direction [44]. In addition, in situ domain observation further indicated that the hysteresis-free S-E behavior along the nonpolar direction should be attributed to the stable domain structure and the inhibited domain wall motion [88]. Therefore, an engineered domain configuration has three characteristics: (1) higher piezoelectricity along the nonpolar direction than along the polar direction, (2) hysteresis-free SE curves due to the inhibited domain wall motion, and (3) change in the macroscopic symmetry of the crystal [87,88]. Using the technique of engineered domain configuration, similar electrical behavior was also observed in BT single crystals [87,89,90]. Therefore, in addition to phase-boundary construction, the electrical properties can also be improved in ferroelectric single crystals by controlling the domain configurations using the crystallographic orientation. To illustrate this strong anisotropic piezoelectricity, the Landau theory and first-principles calculations were carried out, which concluded that the largest piezoelectric response along the nonpolar direction can be interpreted by the facilitated polarization rotation induced by the anisotropic instability of the Gibbs free energy [28]. In addition to theoretical verification, Zhang et al. quantitatively studied the contributions of polar nanoregions to the dielectric/piezoelectric responses using both cryogenic experiments and phase-field simulations, and the origin of ultrahigh piezoelectricity in relaxor-PbTiO3 single crystals can be attributed to the facilitated polarization rotation caused by the polar nanoregions [91,92]. 2.3.3. Domain-wall engineering As mentioned above, the enhanced piezoelectricity along the nonpolar direction was assigned to the facilitated polarization rotation in the relaxor-PT or BT single crystals [91,92]. Here, polarization rotation is mainly associated with the lattice distortion of a material and is considered to be an intrinsic effect to promote property enhancement. However, the domain-wall motion is often related to lattice rotation (90° rotation in T crystals; 71° and 109° in R crystals; and 60°, 120°, 90° in O crystals), which is usually thought to be an extrinsic effect to promote property improvement [86]. Similarly, the facilitated domain-wall motion can greatly improve the piezoelectric activity in ferroelectric materials, and thus, the technique of domain-wall engineering was also proposed. For example, by domain-wall engineering using patterning electrodes or a special poling process, higher piezoelectric activity was obtained in PMN-PT or BT single crystals because of the increased domain-wall density [93]. Therefore, a composite containing normal domain regions and domain-wall regions is required to obtain an enhanced piezoelectric response. For lead-free ferroelectric materials, commonly employed approaches to domain engineering or domain-wall engineering are composition, temperature, and electric field-induced domain configuration evolutions, and detailed reviews are given in Section 5. 3. Piezoelectric effects in lead-free piezoelectric perovskite ceramics Lead-free piezoelectric ceramics can be divided into three categories: perovskite, tungsten bronze, and bismuth layered structures. The ABO3-type family of perovskites is one of the most well-known classes of lead-free piezoelectric materials, and includes K0.5Na0.5NbO3 (KNN), Bi1/2Na1/2TiO3 (BNT), BaTiO3 (BT), and BiFeO3 (BFO). These materials are considered to be promising candidates for sensors, actuators, and transducers due to their unique properties. However, some drawbacks still exist for these leadfree piezoelectric perovskite materials, such as the poor temperature stability of KNN [83], low depolarization temperature (Td) and large hysteresis of BNT [14], low Curie temperature (TC) of BT [11], and high leakage current of BFO [22]. However, phase engineering greatly promotes the electrical properties of lead-free piezoelectric perovskite materials, which shows promise. As a result, we briefly review the importance and role of phase boundaries in the piezoelectric effects of lead-free piezoelectric perovskite materials. 3.1. K0.5Na0.5NbO3-based ceramics Among the lead-free piezoelectric perovskite materials, (K,Na)NbO3 (KNN) has been extensively investigated due to its good comprehensive performance (large d33 and high TC). Since the discovery of the high d33 of textured KNN-based ceramics in 2004, [5] some significant advances in piezoelectricity have been attained [7–9,85,94]. In 2015, Wu et al. systematically discussed the past, present, and future of phase boundaries in KNN-based materials [23], but recent developments were not included. For example, a new promotion in piezoelectricity was achieved in 2016, and a higher d33 of 525–570 pC/N was shown in KNN-based ceramics with a new phase boundary [7–9]. In addition, since 2014, the temperature stability and the related physical mechanisms for property enhancement have been improved and clearly illuminated in terms of the microstructure [7–9]. Therefore, in this review, we focus on recent advances in the properties and mechanisms of KNN-based ceramics, and the evolution of the piezoelectric properties with different phase boundaries is also addressed. 3.1.1. Phase boundary in KNN-based ceramics The investigation of the O-T phase boundary was the earliest and widest in KNN-based ceramics [63,83,95–126]. Some additives, such as Li, Ag, Bi, Zr, Hf, Sb, and Ta, can quickly decrease TO-T to room temperature, thus leading to the formation of an O-T phase 561

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Fig. 7. Design idea for new phase boundaries in KNN materials [23].

boundary and relatively good piezoelectricity (d33 = 200–400 pC/N) [63,83,95–126]. Similarly, by increasing TR-O to room temperature, an R-O phase boundary can also be constructed. Unfortunately, it is very difficult to construct the R-O phase boundary, and poor piezoelectricity (d33 ≤ 200 pC/N) is always obtained [127–131]. After years of exploration, a new phase boundary design idea was proposed by Wu’ research group, as shown in Fig. 7 [23]. By applying the appropriate additives to the KNN matrix, TO-T and TR-O can be simultaneously shifted to room temperature, thus leading to the formation of a new phase boundary. Excitingly, ultrahigh piezoelectricity (d33 = 490–570 pC/N) was achieved in ceramics containing the new R-T phase boundary [6-9]. 3.1.2. Piezoelectric and strain properties Fig. 8(a) shows the historical evolution of the d33 of KNN-based ceramics with different phase boundaries, in which the highest d33 values of each year are presented. One can see from Fig. 8(a) that the ceramics with an O-T phase boundary were investigated the earliest and most widely. As early as 2004, Saito et al. reported a large piezoelectric constant (d33 ≈ 416 pC/N) for textured KNNbased ceramics by the construction of an O-T phase boundary using a reactive template grain growth method. [5] To meet requirements for practical applications, it is necessary to promote the piezoelectricity of KNN-based ceramics prepared by the conventional solid-state method. In 2004–2005, Guo et al. reported the enhancement of the piezoelectric constant (d33 = 200–235 pC/N) of LiTaO3 or LiNbO3-modified KNN ceramics by constructing an O-T phase boundary [63,96], and Hollenstein et al. observed a large d33 in Li and Li/Ta co-modified KNN ceramics [97]. After that, ion substitution (Li, Ta, and Sb) [98–110] or the addition of a second member (LiNbO3 [111–115], LiTaO3 [116–120], and LiSbO3 [83,121–126]) was extensively employed to construct an O-T phase boundary in KNN ceramics. In 2011, the largest piezoelectricity of 413 pC/N was obtained in Li, Ta, and Sb co-modified KNN ceramics [110]. However, there has not been great progress in the piezoelectricity of KNN-based ceramics with an O-T phase boundary. Although ten years of hard work was given to the investigation of KNN-based ceramics, their d33 values are still inferior to those reported by Saito. Correspondingly, the piezoelectricity is also seriously inferior to that of PZT ceramics, which is not beneficial to practical applications. Therefore, it is urgent to further improve the piezoelectricity of KNN-based materials. It is well known that property enhancement is related to the phase boundaries, regardless of whether the material is lead-based or lead-free. In addition, high piezoelectricity cannot occur without phase boundaries, which was even shown in Saito’s work in 2004. It is urgent to design new material systems with new phase boundaries.

Fig. 8. (a) Historical evolution in d33 of KNN-based ceramics with different phase boundaries, and (b) their d33 and TC values as a function of different phase boundaries. 562

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Fig. 9. (a and b) Convergent beam electron diffraction (CBED) patterns of KNNS-BZ-BKH ceramics from [111] incidence; (c and d) STEM HAADF (high angle annular dark field) lattice images of KNNS-BF-BNZ ceramics; (e and f) temperature dependence of Raman spectra and dielectric constant of KNNS-BNKH ceramics [7–9]. Reproduced from [8], with the permission from Wiley. Reprinted with permission from [7]. Copyright (2016) American Chemical Society. [9]-Reproduced by permission of the Royal Society of Chemistry.

Inspired by the MPB concept of PZT, a similar R-T phase boundary in KNN-based ceramics can be expected by shifting both TR-O and TO-T to near room temperature. Based on this designed idea, our research group successfully fabricated a series of high-performance KNN-based ceramics through thousands of experiments. Subsequently, we briefly introduced the development of piezoelectricity with new phase boundaries [6–9,65,66,80,94,132–150]. In 2013, a large d33 of 425 pC/N was obtained in KNLNS-BZ ceramics with an R-T phase boundary [139], which is the first material comparable to KNN-based textured ceramics. Since 2014, great improvement in piezoelectricity (d33 = 490–570 pC/N) have been continuously achieved in KNN-based ceramics [6–9], mainly due to the formation of an R-T phase boundary. In addition, accompanied by an increase in TR-O and decrease in TO-T, the O phase was gradually suppressed. However, if the O phase was not completely suppressed by the composition modification, a R-O-T phase boundary may appear near room temperature [79,80,151–154]. As shown in Fig. 8(a), the R-O-T phase boundary was proposed on the basis of the R-T phase boundary, though its piezoelectricity is poorer than that of ceramics with an R-T phase boundary. Fig. 8(b) summarizes the d33 and TC values of KNN-based ceramics with different phase boundaries. A poor piezoelectricity is found in ceramics with an R-O phase boundary [127–131], while the O-T phase boundary promoted the piezoelectricity, and ceramics with an R-T phase boundary possessed the best piezoelectricity. Overall, the order of piezoelectricity for the different phase boundaries is as follows: R-T > R-O-T > O-T > R-O [23]. Therefore, the construction of an R-T phase boundary is highly expected to improve the piezoelectricity of KNN-based ceramics. To confirm the formation of an R-T phase boundary, a series of advanced techniques can be adopted, such as temperature-dependent Raman spectroscopy, CBED, and spherical aberration TEM, as shown in Fig. 9 [7–9]. All these results verified the existence of an R-T phase boundary. Unipolar strain is a key parameter in actuator applications. In the past, it was thought that the strain behavior of KNN-based ceramics could be comparable to those of PZT ceramics [5]. As shown in Fig. 10(a), the similar strain curve can be observed in KNN and PZT ceramics [5]. Therefore, the strain behaviors in KNN-based ceramics have attracted much attention in recent years, as shown in Table 3 [5,9,78,83,85,94,149,155–157]. It was found that the strain behavior of KNN-based ceramics can be improved by composition modification or structural optimization. For example, in 2017, Wang et al. reported enhanced unipolar strain properties (S = 0.19%) in Mn-modified KNN-based ceramics, indicating that the strain properties can be successfully improved by composition modification [94]. In addition, structure optimization, such as core-shell structure [158] and textured structure [10], can also greatly improve the strain response. For example, an exceptionally large strain (d33* > 1000 pm/V) was reported in KNNT-CZ ceramics with core-shell structure [see Fig. 10(b)], which is much higher than that of commercial soft PZT ceramics [158]. As a result, to further promote the development of actuator applications, much efforts should be made to improve the strain response in KNN-based ceramics by providing theoretical direction.

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Fig. 10. (a) Temperature dependences of electric-field-induced longitudinal strain for the textured (LF4T), non-textured (LF4), and PZT-4 ceramics. Insert is electric-field-induced strain curve for LF4T, LF4 and PZT-4 at 25 °C [5]. (b) Strain properties of KNLT-CZ ceramics with core-shell structure [158]. Reproduced from [5], with the permission from Springer Nature. Reprinted with permission from [158]. Copyright (2012) American Chemical Society. Table 3 Unipolar strain properties of KNN-based ceramics. Material system

Phase structure

d33* (pm/V)

Strain (%)

Emax (kV/mm)

Year

Ref.

KNN-LT-LS (LF4T) KNN-LT-LS (LF4) KNN-LS KNN-BLT-BZ6 KNNS-BNKH KNNS-BAH KNNS-BKZS KNN-LT-CZ (CZ5) KNNS-BNKZ KNN-BLT-BZ6-Mn KNLNT-CaZrO3 Textured KNNS-CZ-BKH

O-T O-T O-T R-T R-T O-T R-T O-T R-T R-T

750 400 355 530 465 507 480 357 355 475 1030 980

0.15 0.08 0.07 0.106 0.093 0.152 0.144 0.143 0.142 0.19 0.36 0.196

2 2 2 2 2 3 3 4 4 4 3.5 2

2004 2004 2006 2015 2017 2016 2015 2013 2016 2017 2012 2017

[5] [5] [83] [156] [9] [78] [157] [155] [149] [94] [158] [10]

3.1.3. Temperature stability As mentioned above, the piezoelectric effect (d33 and strain) in KNN-based ceramics can be well modulated by the introduction of phase boundaries. However, temperature stability is very important for practical applications. Therefore, in addition to investigations into the piezoelectric and strain properties, increasing effort has been given to the temperature stability of the piezoelectric effect in KNN-based ceramics [9,83,85,155,159]. Previously, it was thought that KNN-based ceramics exhibited a piezoelectric effect with poor temperature stability due to the characteristics of the involved phase boundaries [83]. For example, as early as 2006, Zhang et al. studied the temperature stability of d33* in LiSbO3-modified KNN ceramics with an O-T phase boundary and found poor temperature stability (355 pm/V at 15 °C to 255 pm/V at 50 °C) [83]. After that, piezoelectricity with improved temperature stability was achieved by the addition of CaTiO3, but deteriorated piezoelectricity was exhibited because the O-T phase transition temperature was shifted to below room temperature [159]. Recently, composition modification and phase engineering have been proposed to balance the temperature stability and piezoelectric effect of KNN-based ceramics [9,85]. For example, both a large d33 of 300 pC/N and temperature insensitivity of d33* (less than 10% from room temperature to 175 °C) were realized in CaZrO3-modified KNN ceramics (CZ5) despite the presence of an O-T phase boundary [155], which was attributed to the high electric field-induced diffused phase transition [85], as shown in Fig. 11. In particular, we reported both a giant d33 of 525 pC/N and the good temperature stability of d33* in KNNS-BNKH ceramics with a R-T phase boundary under a low driving electric field, and the physical origin of the enhanced temperature stability can be ascribed to the stability of the nanodomains, as shown in Fig. 12 [9]. As a result, the temperature stability of the piezoelectric effect in KNN-based ceramics can be improved by composition modification and modulation of the phase boundaries.

3.2. Bi1/2Na1/2TiO3-based ceramics Bismuth sodium titanate (Bi1/2Na1/2TiO3, BNT) materials have a rhombohedral structure, which was first discovered by Smolenskii et al. in 1960 [4]. After that, BNT materials have attracted much attention due to their large remnant polarization (Pr = 38 μC/cm2). However, it is very difficult to sufficiently pole a pure BNT ceramic due to the high conductivity and large coercive field (EC = 73 kV/cm), which often leads to undesirable piezoelectricity (d33 = 73–95 pC/N) [160–163]. Therefore, chemical 564

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Fig. 11. (a) Temperature stability of normalized d33* to its room temperature value d33RT* of CZ5 ceramics under electric fields ranging from 0.01 to 5 kV mm−1; (b) PPT point TO-T and the range of PPT TO-T90% as a function of electric fields [85]. Reproduced from [85], with the permission from Wiley.

Fig. 12. Normalized d33* and vertical piezoresponse force microscopy (VPFM) amplitude images of 0.96 K0.48Na0.52Nb0.95Sb0.050.04Bi0.5(Na0.82K0.18)0.5HfO3 ceramics at various temperatures [9]. [9]-Reproduced by permission of the Royal Society of Chemistry.

modification (ion substitution or solid solutions) is widely employed to promote the piezoelectric behavior of BNT ceramics by the construction of phase boundaries. In this part, we mainly review the piezoelectric/strain properties of BNT-based ceramics according to the development of phase boundaries and then emphasize some difficulties (e.g., limited temperature usage range and large hysteresis). 3.2.1. Phase boundary in BNT-based ceramics For BNT-based ceramics, there are two kinds of phase boundaries. One was defined as MPB(I) and consisted of ferroelectric R and T phases, resulting in the largest piezoelectricity. Typical material systems of MPB(I) include BNT-BT and BNT-BKT [164,165]. In addition, the relaxor (nonpolar)-to-ferroelectric (polar) phase transition was called MPB(II), which can lead to the maximum strain response. For example, the giant strain (S = 0.45%) in BNT-BT-KNN materials was attributed to the formation of MPB(II) [13]. Subsequently, we introduce the development of two kinds of phase boundaries in BNT-based ceramics by composition modification. 3.2.1.1. Binary systems. Previously, it was reported that pure BNT ceramic has poor piezoelectricity due to the unsuccessful poling process caused by the high coercive field. To improve its electrical properties, some additives were used to substitute the A and/or B sites of BNT [166–180]. Unfortunately, it is hard to realize the great advance in piezoelectricity due to the absence of phase boundaries, and therefore, other attempts should be conducted. Two kinds of typical BNT binary ceramics will be introduced next. 3.2.1.1.1. BNT-BT system. Although the sintering behavior and electrical properties of BNT ceramics can be partly improved by ion substitution, it is still difficult to achieve the great advance in their piezoelectricity [166–180]. Phase boundaries have been employed to promote the piezoelectric properties of BNT-based ceramics. For example, in 1991, the enhancement of piezoelectricity (d33 = 125 pC/N) was observed in (1 − x)BNT-xBT (x = 0.06) ceramics due to the formation of an R-T phase boundary [164]. After 565

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Table 4 Electrical properties of BNT-BT ceramics. Material system

Phase structure

d33 (pC/N)

kp

Tm (°C)

0.94Bi1/2Na1/2TiO3-0.06BaTiO3 0.94Bi1/2Na1/2TiO3-0.06BaTiO3 0.94Bi1/2Na1/2TiO3-0.06BaTiO3 0.94Bi1/2Na1/2TiO3-0.06BaTiO3 0.93Bi1/2Na1/2TiO3-0.07BaTiO3 0.925Bi1/2Na1/2TiO3-0.075BaTiO3 0.935Bi1/2Na1/2TiO3-0.065BaTiO3 0.94Bi1/2Na1/2TiO3-0.06BaTiO3

R-T R-T R-T R-T R-T R-T R-T R-T

125 122 155 148 134 186 150 174

0.2 0.29 0.367 – – – 0.51 (kt) 0.28

288 225

Td (°C)

100 105

305

165

Ref. [164] [181] [193] [188] [194] [192] [195] [196]

that, many investigations on BNT-BT were carried out [181–197], and some significant results in the properties and physical mechanisms were realized. For example, Chu et al. investigated the relationship between the piezoelectric constant and depolarization temperature of BNT-BT ceramics, confirming that the lower the depolarization temperature, the higher the piezoelectric constant [181]. Zhang et al. studied the temperature-dependent electrical properties of BNT-BT ceramics, and the largest unipolar strain of ∼0.42% can be observed at 100 °C because of the appearance of antiferroelectric order [182]. Jo et al. further investigated the electric field-induced phase transformation, strain properties, and thermal evolution of BNT-BT ceramics and provided the concept of a “poling-induced” morphotropic phase boundary [183–187]. In addition, some authors have examined the origin of high strain and piezoelectricity in BNT-BT ceramics [188–192]. For example, Simos et al. attributed the significant enhancement in strain in BNT-BT ceramics to the electric field-induced structure transition [188], and the large recoverable strain at elevated temperatures was associated with the reversible nature of the phase transformation [189]. Similar to PZT ceramics, an intermediate monoclinic phase was also discovered in BNT-BT ceramics with the MPB composition, facilitating polarization rotation and thus piezoelectric enhancement [190]. Table 4 summarizes the electrical properties of BNT-BT ceramics. Although the same phase boundary is observed in the composition range of 0.06–0.075, different properties are exhibited. However, enhanced piezoelectricity can be obtained in BNT-BT ceramics with the MPB composition. To further improve the piezoelectricity of BNT-BT ceramics, some ions, such as Zr [198–203], Hf [204], Ca [205,206], and Sr [207], were utilized to substitute the Ti or Ba ions, as shown in Table 5. It can be found that all the ceramics have a ferroelectric R-T phase boundary, and the peak piezoelectric constant can be observed in the region of the phase boundary, indicating that the construction of phase boundaries is an effective method to improve the piezoelectric response of BNT. 3.2.1.1.2. BNT-BKT system. In the binary system, the BNT-BKT ceramic system is another promising material with a wide range of investigations [165,209–213]. Table 6 shows the electrical properties of the BNT-BKT binary system with the same R-T phase boundary. Similar to the MPB -induced property enhancement in BNT-BT ceramics, a phase boundary concerning the R and T phases was also constructed, and then, an enhancement in the electrical properties was obtained. In particular, the d33 value is often superior to the reported results in BNT-BT ceramics, as shown in Tables 4–6. A MPB (R and T) was reported in (1 − x)BNT-xBKT ceramics with x = 0.16–0.20 as early as 1999 [165]. After that, improved piezoelectricity was obtained by composition modification and process improvement [165,209–213]. For example, an enhanced piezoelectricity (d33 ≈ 192 pC/N) was achieved in BNT-BKT ceramics by optimizing the sintering temperature [210], and a higher piezoelectric constant (d33 ≈ 207 pC/N) was also reported for Bicompensated BNT-BKT ceramics [211]. As discussed above, an enhancement in the electrical properties can be achieved in BNKT ceramics by the formation of a ferroelectric R-T phase boundary, and some interesting phenomena can be observed [14,70–76,214–216]. In Table 7, we list the phase structure and electrical properties of BNKT ceramics with ion substitutions. The ion (Hf, Zr, Sn, Nb, and Ta) substituted in BNKT can decrease the depolarization temperature (Td) and induce a phase transition from a polar (ferroelectric R-T) phase to a non-polar (pseudocubic) phase, finally resulting in improved strain (0.34–0.47%) [72–76]. However, their P-E loops often became slim, and d33 is simultaneously degraded. A similar electric field-induced phase transition can also be observed in Li-Sn- or Li-Ta-modified BNKT ceramics [215,216]. In addition, the effects of ion types on the properties were also investigated; that is, a comparison of Li- and Ladoped BNKT ceramics was made [214]. It was found that the addition of Li can result in saturated P-E loops and typical butterflyshaped S-E curves, while La induces a ferroelectric-relaxor phase transition, which leads to increased strain. More importantly, both giant strain (∼0.7%) and d33* (∼1400 pm/V) were obtained in Sr and Nb co-doped BNKT ceramics (BNT-2.5Nb) because of the electric field-induced relaxor-to-ferroelectric phase transition [14]. Therefore, the phase structure and electrical behavior in BNT-

Table 5 Electrical properties of BNT-BT ceramics with ion substitutions. Material system

Phase structure

d33 (pC/N)

kp

0.95Bi1/2Na1/2TiO3-0.05Ba(Zr0.25Ti0.75)O3 0.92Bi1/2Na1/2TiO3-0.08Ba(Hf0.05Ti0.95)O3 0.91Bi1/2Na1/2TiO3-0.09(Ba0.7Ca0.3)TiO3 0.92Bi1/2Na1/2TiO3-0.08(Ba0.7Sr0.3)TiO3 0.94Bi1/2Na1/2TiO3-0.04(Ba0.98Ca0.02)(Ti0.94Sn0.06)O3

R-T R-T R-T R-T R-T

131 136 125 160 170

0.23 – 0.33 0.31 0.33

566

Td (°C)

Tm (°C)

180 100

270 200

Ref. [203] [204] [205] [207] [208]

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Table 6 Electrical properties of BNT-BKT ceramics. Material system

Phase structure

d33 (pC/N)

kp

Tm (°C)

Td (°C)

Ref.

0.80Bi1/2Na1/2TiO3-0.20Bi1/2K1/2TiO3 0.78Bi1/2Na1/2TiO3-0.22Bi1/2K1/2TiO3 0.77Bi1/2Na1/2TiO3-0.23Bi1/2K1/2TiO3 0.82Bi1/2Na1/2TiO3-0.18Bi1/2K1/2TiO3 0.80Bi1/2Na1/2TiO3-0.20Bi1/2K1/2TiO3

R-T R-T R-T R-T R-T

157 192 207 144 195

0.56 (k33) 0.32 0.35 0.29 0.27

270

174

284

150 100

[209] [210] [211] [212] [213]

Table 7 Electrical properties of BNKT ceramics with ion substitution. Material system Bi1/2(Na0.7K0.2Li0.1)1/2TiO3 Bi1/2(Na0.78K0.2Ag0.02)1/2TiO3 Bi1/2(Na0.78K0.22)1/2(Ti0.97Hf0.03)O3 Bi1/2(Na0.78K0.22)1/2(Ti0.97Zr0.03)O3 Bi1/2(Na0.82K0.18)1/2(Ti0.97Nb0.03)O3 Bi1/2(Na0.82K0.18)1/2(Ti0.98Ta0.02)O3 Bi1/2(Na0.78K0.22)1/2(Ti0.95Sn0.05)O3 [Bi1/2(Na0.82K0.18)1/2]0.97Li0.03TiO3 [Bi1/2(Na0.82K0.18)1/2]1−xLaxTiO3 (Bi0.5Na0.78K0.18Li0.04)0.5(Ti0.95Sn0.05)O3 (Bi0.5Na0.385K0.09Li0.025)(Ti0.975Ta0.025)O3 {[Bi0.5(Na0.84K0.16)0.5]0.96Sr0.04}Ti0.975Nb0.025O3

Phase structure

d33 (pC/N)

R

231 180

Strain (%)

0.38 0.43 0.47 0.34 0.35

31 RT-Pc RT-Pc T-Pc 110 172 (x = 0.02) T-Pc RT-Pc R3c-P4bm

0.39 0.436 0.70

d33* (pm/V)

Ref.

475 614 641 566 585 250 650 (x = 0.03) 646 727 1400

[70] [71] [73] [72] [76] [75] [74] [214] [214] [215] [216] [14]

based ceramics are closely related to the chemical composition. For example, when ions are employed to substitute BNKT, the addition of some ions (e.g., Li and Ag) substituting at the A site can maintain the ferroelectric phase structure, saturated P-E loops, and typical butterfly-shaped S-E curves [70,71]. However, some ions (e.g., Hf, Zr, Sn, Nb, Ta) substituting at the B site can disrupt the long-range ferroelectric phase and induce a phase transition from the ferroelectric R-T phase to the relaxor pseudocubic phase [72–76]. As a result, ion substitution can modify the type of phase boundary in BNT-BKT materials, where the R-T phase boundary promotes d33 and enhancements in the strain properties can be observed with the coexistence of polar and nonpolar phases or with the electric field-induced polar-to-nonpolar phase transition. 3.2.1.1.3. Other binary systems. As discussed before, a typical ferroelectric R-T phase boundary can be established by introducing ABO3 (BaTiO3 or Bi0.5K0.5TiO3) to the BNT matrix, finally resulting in enhanced piezoelectricity. Inspiring by the construction of the Table 8 Electrical properties of BNT-based binary systems with ABO3. Material system

Phase structure

d33 (pC/N)

0.94Bi1/2Na1/2TiO3-0.06K0.5Na0.5NbO3 0.97Bi1/2Na1/2TiO3-0.03BaSnO3 0.9625Bi1/2Na1/2TiO3-0.0375Bi(Zn0.5Ti0.5)O3 0.98Bi1/2Na1/2TiO3-0.02Bi(Zn0.5Hf0.5)O3 0.9875Bi1/2Na1/2TiO3-0.0125Bi0.8La0.2FeO3 0.96Bi1/2Na1/2TiO3-0.04Bi(Mg0.5Ti0.5)O3 0.97Bi1/2Na1/2TiO3-0.03Bi(Al0.5Ga0.5)O3 0.993Bi1/2Na1/2TiO3-0.007Bi(Mg2/3Nb1/3)O3 0.98Bi1/2Na1/2TiO3-0.02Li0.12Na0.88NbO3 0.72Bi1/2Na1/2TiO3-0.28SrTiO3 0.75Bi1/2Na1/2TiO3-0.25SrTiO3 0.74Bi1/2Na1/2TiO3-0.26SrTiO3 0.75Bi1/2Na1/2TiO3-0.25SrTiO3 (1 − x)[Bi1/2(Na0.8K0.2)1/2TiO3]-xBiMeO3 (Me: Fe, Sc, Mn, Al) 0.96[Bi1/2(Na0.84K0.16)1/2(Ti0.9825Nb0.0175)O3]–0.04SrTiO3 (1 − x)Bi1/2Na1/2TiO3-xNaNbO3 (1 − x)Bi1/2Na1/2TiO3-xKNbO3 (1 − x)Bi1/2Na1/2TiO3-xK0.47Na0.47Li0.06Nb0.74Sb0.06Ta0.2O3 (1 − x)Bi1/2Na1/2TiO3-xSrZrO3 (1 − x)Bi1/2Na1/2TiO3-xBaZrO3 (1 − x)Bi1/2Na1/2TiO3-xBi(Ni0.5Ti0.5)O3 (1 − x)Bi1/2Na1/2TiO3-xBa(Ni0.5Nb0.5)O3 (1 − x)Bi1/2Na1/2TiO3-xBa(Al0.5Sb0.5)O3 (1 − x)Bi1/2Na1/2TiO3-xBa(Al0.5Ta0.5)O3 (1 − x)Bi1/2Na1/2TiO3-xBa(Al0.5Nb0.5)O3

R-T R-T R-T R-Pc R-O R R R R R-Pc R-Pc R-Pc R-Pc RT-Pc RT-Pc

94 93 92 92 128 108 93 94 113

Strain (%)

d33* (pm/V)

0.29 0.29 0.38

488 483 1100

0.34–0.36 0.438 0.22 (x = 0.10) 0.40 (x = 0.08) 0.39 (x = 0.08) 0.24 (x = 0.09) 0.4 (x = 0.055) 0.22 (x = 0.14) 0.3 (x = 0.05) 0.48 (x = 0.045) 0.36 (x = 0.045) 0.40 (x = 0.055)

425–450 876 259 498 487 340 500 249 428 532 448 533

367

83 (x = 0.06) 152 (x = 0.07) 169 (x = 0.03) 102 (x = 0.05) 112 (x = 0.04) 102 (x = 0.06) 121 (x = 0.045) 118 (x = 0.035) 127 (x = 0.04) 133 (x = 0.05)

567

Ref. [217] [218] [219] [220] [237] [221] [222] [223] [238] [224] [225] [226] [227] [228] [239] [229] [229] [230] [231] [232] [233] [234] [235] [62] [236]

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Fig. 13. (a) electric field-temperature phase diagram in 0.75Bi0.5Na0.5TiO3-0.25SrTiO3, and (b) temperature dependence of piezoelectric constant under various electric fields [227]. Reproduced from [227], with the permission from Wiley.

R-T phase boundary, some researchers have attempted to modify BNT with other ABO3. In Table 8, the electrical properties of BNT binary systems with ABO3 are listed. Generally, three types of binary systems are obtained. (1) Piezoelectricity enhancement: for example, some ABO3 can be employed to promote the piezoelectric properties, such as K0.5Na0.5NbO3 [217], BaSnO3 [218], Bi (Zn0.5Ti0.5)O3 [219], Bi(Zn0.5Hf0.5)O3 [220], Bi(Mg0.5Ti0.5)O3 [221], Bi(Al0.5Ga0.5)O3 [222], and Bi(Mg2/3Nb1/3)O3 [223]. (2) Strain enhancement: the strain response can be enhanced because of the formation of a ferroelectric-relaxor phase boundary, and the doped additives include SrTiO3 [224–227], and BiMeO3 (Me: Sc, Mn, Al) [228]. In particular, the BNT-SrTiO3 ceramic is considered to be a typical binary system with a ferroelectric-to-relaxor pseudocubic phase boundary [224–227], and in 2016, the novel concept of a “critical point” in 0.75Bi0.5Na0.5TiO3-0.25SrTiO3 relaxor ceramics was provided to tailor the piezoelectric and electrocaloric properties (Fig. 13) [227]. By this method, a small d33 can increase from 218 pC/N (room temperature) to 367 pC/N (higher temperature ∼90 °C) [227], providing a new approach to further facilitate the development of piezoelectric materials. (3) Composition dependence of strain/piezoelectric properties: ferroelectric order can be maintained at low doping contents, and a ferroelectric-to-relaxor pseudocubic phase transition can appear with a further increase in the doping content. Therefore, the maximum piezoelectricity and strain can be tailored by controlling the doping content in the chemical composition [62,229–236]. For example, a maximum d33 of ∼152 pC/N can be observed in (1 − x)Bi0.5Na0.5TiO3-xKNbO3 ceramics with x = 0.07, and a large reversible strain of S = 0.40% (d33* = 498 pm/V) can be achieved due to the coexistence of rhombohedral and relaxor pseudocubic phases as the KNbO3 content continues to increase [229]. This phenomenon can also be found in other ABO3-modified BNT ceramics [62,230–236]. More interestingly, a giant strain response was always accompanied by a decrease in Td or the ferroelectric-to-relaxor transition temperature (TF-R), together with the appearance of slim or pinched P-E loops. Therefore, the strain can be greatly improved in BNT-based ceramics by reducing Td or TF-R to room temperature because of the involvement of a ferroelectric-relaxor or polar-nonpolar phase boundary. In addition, such phase boundary with giant strain is obviously different from a R-T phase boundary with improved d33. Generally, a maximum d33 can be achieved in BNT-BT and BNT-BKT systems thought the construction of a ferroelectric R-T phase boundary, which is denoted MPB(I). However, the largest strain is shown at the phase boundary consisting of polar and nonpolar phases, such as BNT-SrTiO3 and BNT-KNbO3 systems, which is defined MPB(II). 3.2.1.2. Ternary systems 3.2.1.2.1. BNT-BT-KNN and BNT-BKT-KNN systems. Since 2007, BNT-BT-KNN systems have been extensively investigated due to the observation of giant strain [13]. In 2007, Zhang et al. first reported a new 0.92Bi1/2Na1/2TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3 ceramic with giant strain (S = 0.45%) comparable to that of lead-based antiferroelectric materials [13]. Some unique properties are shown by this ceramic system [Fig. 14], as follows: (1) Giant strain with large hysteresis can be obtained under a large electric field of 8 kV/mm. (2) The long-range ferroelectric order was disrupted by doping KNN, after which ferroelectric domains become invisible and the piezoelectricity nearly vanishes. (3) The electric field-induced antiferroelectric-to-ferroelectric transition is responsible for the giant strain. After that, a series of studies attempted to improve the strain behavior or related electrical properties in BNT-based ceramics by the construction of phase boundaries [240–252]. In particular, the physical mechanisms for the enhanced strain were discussed in detail. Initially, the giant strain was mainly ascribed to the lattice change by the formation of an antiferroelectric-to-ferroelectric phase transition induced by the electric field [13,241]. However, the electric field-induced reversible nonpolar-to-polar phase transition was considered to be responsible for the high strain, according to the report of Jo et al. [244]. Due to the observation of the BNT-BT-KNN system and the in-depth studies about the physical mechanisms, BNT-based ceramics were the most promising candidate for actuator applications until now. In addition to BNT-BT-KNN, BNT-BKT-KNN is another widely studied material system [253–256]. Similar to BNT-BT-KNN, a large strain can also be found in BNT-BKT-KNN because of the formation of MPB(II). For example, the ferroelectric order can be 568

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Fig. 14. (a) Electric field-induced strain loops in (0.94 − x)Bi1/2Na1/2TiO3-0.06BaTiO3-xK0.5Na0.5NbO3 ceramics, and (b) d33* and d33 as a function of x [13]. Reproduced from [13], with the permission from AIP publishing.

significantly disrupted with the addition of KNN, greatly improving the unipolar strain (S ≈ 0.48%, d33* ≈ 600 pm/V) [253]. In 2013, the switching characteristics of MPB(II) were studied in BNT-BKT-KNN ceramics, indicating that the addition of KNN can change the structure from MPB(I) to MPB(II) [254]. A large electric field-induced strain accompanied by a degraded d33 was achieved for compositions near MPB(II) because TF-R was shifted to room temperature [254]. As a result, the addition of KNN in BNT-BT or BNT-BKT matrixes can effectively disrupt the long-range ferroelectric order and shift Td or TF-R to room temperature, finally leading to the formation of MPB(II) and the enhancement of the strain properties. 3.2.1.2.2. Other ternary systems. The composition design for ternary ceramics mainly focused on the chemical modification of BNT-BT, BNT-BKT, and BNT-ST to enhance their electrical properties. In addition to KNN, researchers have also doped BNT ternary systems with other ABO3, as shown in Table 9. Subsequently, we briefly introduce the development of three kinds of BNT-based ternary system from the view of phase boundary construction. Generally, the improved properties of BNT-BT are mainly assigned to the formation of phase boundaries. It was found that Bi0.5M0.5TiO3 (M: Li, and Ag) can improve the piezoelectricity of BNT-BT [257,258]. For example, a large d33 (208 pC/N) can be achieved in Bi0.5Li0.5TiO3-modified BNT-BT ceramics due to the formation of MPB(I) [257]. In addition, Bi0.5Ni0.5TiO3 [259], LiNbO3 [260], ABO3 (A: Sr, Ba; B: Zr, Sn) [261–263], and BiAlO3 [264] can also be employed to enhance the strain properties of BNT-BT ceramics due to the construction of MPB(II). For example, strain enhancement (S = 0.6%, d33* = 857 pm/V) can be observed in LiNbO3-doped BNT-BT ceramics [260]. Similarly, ABO3 oxides have been widely used to construct phase boundaries and promote the electrical properties of BNT-BKT ceramics, including BaTiO3, [265–267] SrTiO3 [268], Bi0.5Li0.5TiO3 [269,270], BiMeO3 (Me: Fe, Cr) [271,272], and ANbO3 (A: K, Li) [273,274]. As shown in Table 9, it was found that only SrTiO3 is beneficial for the improvement of the strain properties, owing to the formation of MPB(II) [268], while the addition of other oxides can only induce the formation of MPB(I), thus promoting the

Table 9 Electrical properties in BNT-based ternary systems by other ABO3. Material system

Phase structure

d33 (pC/N)

0.865Bi1/2Na1/2TiO3-0.06BaTiO3-0.075Bi0.5Li0.5TiO3 0.88Bi1/2Na1/2TiO3-0.06BaTiO3-0.06Bi0.5Ag0.5TiO3 0.905Bi1/2Na1/2TiO3-0.06BaTiO3-0.035Bi0.5Ni0.5TiO3 (0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-0.025LiNbO3 as0.98(0.935Bi1/2Na1/2TiO3-0.065BaTiO3)-0.02SrZrO3 0.97(0.935Bi1/2Na1/2TiO3-0.065BaTiO3)-0.03BaZrO3 0.98(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-0.02BaSnO3 0.92Bi1/2Na1/2TiO3-0.06BaTiO3-0.02BiAlO3 0.854Bi1/2Na1/2TiO3-0.12Bi1/2K1/2TiO3-0.026BaTiO3 0.852[Bi1/5(Na0.9Li0.1)1/5]TiO3-0.110(Bi1/2K1/2)TiO3-0.038BaTiO3 0.88Bi1/2Na1/2TiO3-0.08Bi1/2K1/2TiO3-0.04BaTiO3 0.7Bi1/2Na1/2TiO3-0.2Bi1/2K1/2TiO3-0.1Bi1/2Li1/2TiO3 0.72(Bi1/2Na1/2TiO3-0.2(Bi1/2K1/2)TiO3-0.08(Bi1/2Li1/2)TiO3 0.79Bi1/2Na1/2TiO3-0.18Bi1/2K1/2TiO3-0.03BiFeO3 0.805Bi1/2Na1/2TiO3-0.18Bi1/2K1/2TiO3-0.015BiCrO3 0.82Bi1/2Na1/2TiO3-0.16Bi1/2K1/2TiO3-0.02KNbO3 0.83Bi1/2Na1/2TiO3-0.11Bi1/2K1/2TiO3-0.06Ba0.85Ca0.15Ti0.9Zr0.1O3 0.85(0.83Bi1/2Na1/2TiO3-0.17Bi1/2K1/2TiO3)-0.15SrTiO3 0.90Bi1/2Na1/2TiO3-0.05SrTiO3-0.05KNbO3 0.79Bi1/2Na1/2TiO3-0.14SrTiO3-0.07NaNbO3 0.87Bi1/2Na1/2TiO3-0.06SrTiO3-0.07K0.5Na0.5NbO3

R-T R-T R-Pc R-Pc T-Pc T-Pc RT-Pc R-Pc R-T R-T R-T R-T R-T R-T R-T R-T R-T R-Pc R-Pc R-Pc R-Pc

208 172

569

Strain (%)

d33* (pm/V)

0.35 0.6 0.39 0.38 0.4 0.37

590 857 722 542 669 533

0.38 0.34 0.22 0.28

626 486 314 397

295 235 181 231 190 170 168 215 198

Ref. [257] [258] [259] [260] [261] [262] [263] [264] [265] [266] [267] [269] [270] [271] [272] [273] [276] [268] [275] [275] [275]

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piezoelectric response. For example, a large strain (S = 0.38%) was reported for SrTiO3-doped BNT-BKT ceramics [268], and a different piezoelectric constant (d33 = 181–295 pC/N) was found in the BaTiO3-doped BNT-BKT ceramics with the same R-T phase boundary [265–267]. However, MPB(II) can be easily formed in BNT-ST ceramics. For example, MeNbO3 (Me = K, Na, K0.5Na0.5) was added to modify the BNT-ST matrix [275], and MPB(II) can be driven simply by tuning the composition, finally resulting in strain enhancement. In conclusion, we find that BNT-BT-based ternary material systems present great advantages in strain applications, while BNT-BKTbased ternary systems show excellent piezoelectric characteristics. 3.2.2. Piezoelectric and strain properties As discussed before, two kinds of phase boundaries {MPB(I) and MPB(II)} can typically be constructed in BNT-based ceramics by chemical modification. Here, we provide an overview of the relationship between the type of phase boundary and the electrical properties (piezoelectricity and strain) in BNT-based ceramics. For pure BNT ceramics, it is difficult to realize enhanced piezoelectricity due to the lack of phase boundaries [160–163]. In addition, the high conductivity and large coercive field also limits its development. However, the construction of MPB(I) improved the piezoelectricity of BNT-based ceramics. Two typical materials are represented by BNT-BT and BNT-BKT, which exhibit a ferroelectric R-T phase boundary [164,165]. However, two shortcomings hinder the development of piezoelectricity. The piezoelectricity (d33 ≤ 367 pC/N) is still inferior to that of PZT ceramics [227], and a low depolarization temperature limits the temperature range for practical applications [181,193,205,212]. Therefore, these two tough issues must be resolved before the application of these materials in piezoelectric devices. Strain is the most advantageous property in the field of lead-free piezoelectric materials. As discussed before, MPB(II) can be employed to promote the strain properties of BNT-based materials. For example, the addition of KNN can induce the formation of MPB(II) in BNT-BT or BNT-BKT ceramics, finally leading to giant strain [13,253]. However, a slim or pinched P-E loop, nearly vanishing piezoelectric response and large hysteresis can be exhibited simultaneously. A giant strain of 0.45% was first reported in 0.92Bi1/2Na1/2TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3 ceramics in 2007 [13]. After many studies, various chemical compositions were employed to further tune the strain properties. Generally, enhanced strain can be realized in the vicinity of the depolarization temperature (Td). In Fig. 15(a), we show the temperature dependence of d33* in BNT-BT ceramics [182]. The maximum d33* appeared when the measurement temperatures approached Td. Therefore, if composition modifications can drop TF-R or Td to/near room temperature, MPB(II) can be induced, and a large strain can be obtained. One can see from Fig. 15(b) that the addition of SrTiO3 can decrease Td to room temperature and result in the formation of MPB(II), and correspondingly, the largest d33* appeared when Td was around room temperature [275]. To illustrate this viewpoint, the strain vs. Td or TF-R of BNT-based materials was assessed, as shown in Fig. 16. The strain can be promoted by a decrease in Td or TF-R, and the best strain properties can be achieved in materials with Td or TF-R around room temperature. As a result, the chemical compositions can be reasonably employed to shift their Td or TF-R values to/near room temperature, and then, the construction of MPB(II) finally results in high strain performance in BNT-based systems. 3.2.3. Temperature usage range Although piezoelectricity in BNT-based ceramics can be promoted by the construction of MPB(I), the narrow temperature usage range caused by the depolarization temperature (Td) is a longstanding obstacle for practical applications [181,193,205,212]. To broaden the temperature usage range of BNT-based ceramics, composition modification was used to improve the temperature stability [277–280]. In 2014, Zhang et al. found that the introduction of semiconducting ZnO particles into the BNT-BT matrix can effectively increase Td and even remove it, thus leading to temperature-insensitive piezoelectric and strain properties, as shown in Fig. 17 [277]. The experimental phenomenon of increased Td by composition modification has also been observed in Mn-doped BNTbased ceramics [280]. These results not only help elucidate the mechanism of depolarization but also pave the way for practical applications. However, we must note that the piezoelectric constant remains low even when the temperature usage range is widened. The existence of Td or Tf-r in BNT-based ceramics with MPB(II) is beneficial to strain enhancement, but temperature-sensitive

Fig. 15. (a) Temperature dependence of d33* in 0.94Bi1/2Na1/2TiO3-0.06BaTiO3 ceramics [182]; (b) Td and d33* values of (1 − x)Bi1/2Na1/2TiO30.05KNbO3-xSrTiO3 material system with different ST contents [275]. Reproduced from [182,275], with the permission from Wiley. 570

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Fig. 16. strain vs Td or TF-R of some typical materials.

Fig. 17. (a) The retained d33 of BNT-6BT and BNT-6BT:0.3ZnO as a function of annealing temperature; and (b) temperature dependence of unipolar strain of BNT-6BT and BNT-6BT:0.3ZnO [277]. Reproduced from [277], with the permission from Springer Nature.

strain can also be observed, as shown in Fig. 18 [62,281–286]. With an increase in the measurement temperature, a gradually decreasing strain can be observed in most of the samples. However, relatively good temperature stability of the strain behavior can still be obtained in some ceramics by composition modification [281–283]. For example, enhanced temperature stability can be achieved (e.g., △d33* ≈ 10% from room temperature to 100 °C) in SrTiO3- and/or BiFeO3-modified BNT-BKT ceramics [282]. More excitingly, temperature-insensitive strain (d33* = 800 pm/V) under a low driving electric field (E = 5 kV/mm) can be achieved in LiNb-modified BNKT-ST ceramics, which was attributed to the almost temperature-insensitive nature of electrostriction [281]. All these results indicate that a broadened temperature usage range can be obtained in BNT-based ceramics by composition modification.

Fig. 18. Temperature-dependent normalized d33* of BNT-based ceramics [62,281-286]. 571

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Fig. 19. d33* vs. hysteresis of BNT-based ceramics [14,62,72,241,253,259,275,283,285,287–296].

3.2.4. Hysteresis Although MPB(II) can greatly promote the strain response of BNT-based ceramics, the large hysteresis caused by domain-wall motion seriously hinders their actuator application. Fig. 19 summarizes the d33* vs. hysteresis of some reported BNT-based ceramics [14,62,72,241,253,259,275,283,285,287–296]. It can be seen that the unbalanced development of strain and hysteresis was frequently observed [14,62,72,241,253,259,275,283,285,287–295]. For example, a giant d33* of 1400 pm/V can be obtained in BNT2.5Nb ceramics; however, the large hysteresis (H = 68.9%) is detrimental to practical applications [14]. Although an ultra-low hysteresis (H = 17.2%) can be achieved in textured BKT-BT-BNT ceramics, a relatively poor d33* of 400 pm/V is found [290]. In 2017, both giant strain (S = 0.72%, d33* ≈ 916 pm/V) and low hysteresis (H = 36.2%) were simultaneously obtained in lead-free BNKT-SBTZ6 ceramics by introducing A sites and oxygen vacancies [296]. It was proposed that the local polarization field induced by vacancies smears the transition between the relaxor and ferroelectric state, thus leading to a narrow hysteresis loop [296]. As a result, it seems that defect engineering can promote the balanced development of strain and hysteresis in BNT-based ceramics. However, there are still a few methods to decrease the hysteresis, and thus, more effort should be made to apply BNT-based ceramics to actuators. 3.3. BiFeO3-based ceramics BiFeO3-based ceramics are considered promising materials in high-temperature applications due to their high Curie temperature (TC) and relatively good electrical properties [22,26,297–302]. In the past, many investigations into the electrical properties have been carried out, and unfortunately, some obstacles were discovered, including high leakage current, low resistivity, and the formation of impure phases [26,297]. In this section, we briefly review the relationship between composition modification and the phase boundary in BFO-based ceramics and provide effective methods for property enhancement. 3.3.1. Phase boundary in BiFeO3-based ceramics In BFO-based ceramics with ion substitution, there is no effective phase boundary to benefit piezoelectricity enhancement. However, for BFO-based binary and/or ternary material systems, a temperature-independent phase boundary can be easily constructed, and improved electrical properties can be achieved. Here, we systematically discuss the development of phase boundaries in BFO-based ceramics from the view of composition modification. 3.3.1.1. Ion substitution. Ion substitution is very beneficial to property enhancement in BFO-based ceramics due to the improved resistivity. As is well known, ion substitution can be classified into three categories: substitution at the Bi site [82,303–322], substitution at the Fe site [323–331], and substitution at both the Bi and Fe sites [320,327,332–347]. 3.3.1.1.1. Ion substitution at the Bi site. Table 10 presents the electrical properties of BFO-based ceramics with ion substitutions at the Bi site. It can be found that the addition of rare earth elements promote the piezoelectricity and ferroelectricity, such as Sm [82,303–306,322], La [308–310,322], Nd [311,322], Dy [312,313,322], Eu [314,322], Yb [315,320], Y [320], Ce [319] and Ho [317,320]. For example, a large piezoelectricity (d33 ≈ 50 pC/N) can be achieved in Sm and La co-doped BFO ceramics due to suppression of the leakage current [82], and a high remnant polarization (Pr = 31 or 40 μC/cm2) was also observed in BFO ceramics substituted with Dy [313] or Sm [305]. In these reported articles, the enhancement in the electrical properties was mainly attributed to the suppression of impure phases and the reduction in leakage current. However, the relationship between the phase structure and electrical properties was rarely discussed. Therefore, two crucial questions need to be considered through the summarization of the previous results. One question is whether phase boundaries are formed in BFO ceramics upon ion doping, and another is whether phase boundaries can enhance the electrical properties of BFO. According to the previous literature, the phase structure does not greatly affect the electrical properties of BFO ceramics with ion substitution. For example, the phase structure of Bi1−xLaxFeO3 ceramics can change from rhombohedral R3c symmetry (x = 0) to triclinic P1 symmetry (x = 0.05–0.15) and then to pseudotetragonal (x = 0.20–0.25) [309], while its piezoelectricity decreases 572

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Table 10 Electrical properties of BFO ceramics with ion substitutions at the Bi site. Material system

Phase structure

d33 (pC/N)

Pr (μC/cm2)

EC (kV/cm)

Mr (emu/g)

Ref.

Bi0.875Sm0.125FeO3 Bi1−xSmxFeO3 (x = 0–0.15) Bi0.875Sm0.125FeO3 Bi0.88Sm0.12FeO3 Bi0.925La0.05Sm0.025FeO3 (Eu, Gd, Tb, Dy)-doped Bi0.85La0.15FeO3 Bi1−xLaxFeO3 (x = 0–0.20) Bi0.7La0.3FeO3 Bi0.975Nb0.025FeO3 Bi1−xDyxFeO3 (x ≤ 0.08) Bi0.95Dy0.05FeO3 Bi0.9Eu0.1FeO3 Bi0.85Yb0.15FeO3 Bi0.8Pr0.2FeO3 Bi0.9Ho0.1FeO3 Bi0.7Sr0.3FeO3 Bi0.85Ce0.15FeO3 (Sm, Yb, Y, Ho)-doped Bi0.95-xLa0.05TbxFeO3 (Eu, Pr, Sm, La, Dy)-doped

T R-O R R T R Pc R-T-Pc R-O M R R – R R R T R T

29 – 45 – 50 ∼48 27.7 > 25 – 28 > 35

15.09 – ∼40 40–50 – 21–34 12.08 > 9.8 11.2 – > 18 31 11 8.5

90 – ∼150 130 – 106–117 60

0.071 0.078 – – – 0.016–0.044 0.0744 0.2 0.02 –

13

63

8.4 – 16.2 12–20

70 – ~100

[303] [304] [305] [306] [82] [307] [308] [309] [310] [311] [312] [313] [314] [315] [316] [317] [318] [319] [320] [321] [322]

–

40–50

– – 110 120 74.5 ∼50

0.021 0.0347 – 0.0051 0.028 0.02 – 0.3 > 40

almost linearly with an increase in La content, indicating that the phase transition does not enhance the piezoelectricity. Although a saturated P-E loop (Pr = 11.2 μC/cm2) can be found in La-doped BFO ceramics with an R-O phase boundary [310], the enhanced polarization was mainly attributed to the quenching technique. In addition, the rhombohedral perovskite phase was found in Smdoped BFO ceramics (Table 10) [305,306], and the enhancement in the electrical properties was mainly attributed to the lack of impure phases and the decrease in leakage current, regardless of the mixed phases. In particular, a series of BFO ceramics with different ions (Eu, Gd, Tb, and Dy) were also investigated, and large piezoelectricity (d33 ≈ 48 pC/N) and good ferroelectricity (Pr = 21–34 μC/cm2) can be achieved in BFO ceramics with an R phase [307]. Similar results can also be attained in BFO ceramics doped with Eu, Pr, Sm, La, and Dy, as shown in Fig. 20 [322]. These results indicated that enhancements in the ferroelectric and piezoelectric properties can be achieved in ceramics with a R phase with respect to those with the coexistence of multiple phases, and the enhanced electrical properties mainly originate from the disappearance of secondary phases and the decrease in leakage current. Therefore, it is very difficult to improve the electrical properties of BFO ceramics with ion substitution by constructing phase boundaries – that is, phase boundaries do not play a critical role in enhancing the electrical properties of BFO ceramics with ion substitution at the Bi site, even though enhanced electromechanical response has been reported in the morphotropic phase boundary region driven by epitaxial strain or rare earth elements in BFO thin films [348,349]. As a result, it is more effective to promote the electrical properties of BFO ceramics with ion substitution by modification of the processing methods instead of the construction of phase boundaries. 3.3.1.1.2. Ion substitution at the Fe site. Table 11 shows the electrical properties of BFO ceramics with ion substitution at the Fe site. Most results in the field of ferro/piezoelectrics were disappointing [323–326,328–331], and only Sc-doped BFO ceramics show enhanced electrical properties (d33 ≈ 46 pC/N and Pr ≈ 15 μC/cm2) [327]. In addition, it was found that the phase structure remained almost unchanged in BFO ceramics upon ion substitution at the Fe site [323,324,326,327]. As a result, a phase boundary cannot be constructed in BFO ceramics with ion substitution at the Fe site, and thus, the electrical properties cannot be greatly enhanced. 3.3.1.1.3. Ion substitution at both the Bi and Fe site. As mentioned above, the relationship between the phase structure and electrical properties in BFO ceramics with ion substitution at the Fe site is unsatisfactory. Here, we introduce the electrical properties of BFO ceramics with ion substitution at both the Bi and Fe site, as shown in Table 12. One can find that most of the BFO ceramics exhibited poor piezoelectricity and ferroelectricity; however their magnetic properties were greatly improved. For example, a large remnant magnetization can be attained in Y + Mn [339], Ba + Ni [341], and Ca + Mg [344] co-doped ceramics. In addition, one can find from Table 12 that the phase structure of BFO ceramics is almost unchanged upon doping ion pairs such as La+(Co, Nb, Ti, Zr) [334,335], Pr+(Ti, Zr) [337,338], Y+(Mn, Zr) [339,340], Ca+(Nb, Mn) [343,344], Ho + Ni [345], and Sm + Sc [327]. A phase transition even occurred in La + Ti co-doped BFO ceramics, but unfortunately, the phase transition did not effectively enhance the electrical properties [335]. In our previous reports, a series of BFO ceramics with La, Sm, and M (Sc, In, Al, and Ga) were fabricated by modifying the processing methods, in which the variations in the piezoelectricity (15–47 pC/N) were not related to the phase structure [320]. Even though enhanced electrical properties can be obtained in Sm + Sc-doped BFO ceramics due to the reduced leakage current, the phase structure still maintained the R phase, as shown in Fig. 21 [327]. All these results indicated that ion substitution cannot improve the electrical properties of BFO ceramics by the formation of phase boundaries, even though few phase boundaries were constructed. As a result, it is not feasible to enhance the electrical properties of BFO ceramics by constructing phase boundaries.

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Fig. 20. Phase structure and electrical properties of Bi1−xAxFeO3 ceramics doped with rare earth elements of Eu, Pr, Sm, La, and Dy [322]. Reproduced from [322], with the permission from Elsevier. Table 11 Electrical properties of BFO ceramics with ion substitutions at the Fe site. Material system

Phase structure

BiFe0.75Ti0.25O3 BiFe0.7Mn0.3O3 BiFe1−xTaxO3+x BiFe0.85Ho0.15O3 BiFe0.95Sc0.05O3 Bi1.05Fe0.9Ti0.1O3

R R R R

d33 (pC/N)

46

Pr (μC/cm2)

EC (kV/cm)

Mr (emu/g)

Ref.

0.081 3.99 0.22 8.4 15.1 4.70

2.571 19.79

0.025

[323] [324] [325] [326] [327] [331]

74 54.8 85

0.06 0.0258

3.3.1.2. Addition of ABO3. As discussed before, it is difficult to promote the electrical properties of BFO ceramics by ion substitution (d33 ≤ 50 pC/N) due to the absence of phase boundaries [82,327,350]. Therefore, much attention has been given to the investigation of BFO ceramics with the addition of ABO3 to further improve their electrical properties. Typical candidate materials mainly include BiFeO3-PbTiO3 [351–360] and BiFeO3-BaTiO3 [361,362]. Although relatively good piezoelectricity can be achieved in BiFeO3PbTiO3 ceramics by the construction of a R-T phase boundary, less attention has been given to this material due to the presence of the hazardous lead element [351–360]. In addition, other ABO3 have also been used to improve the electrical properties of BFO ceramics, though poor ferro/piezoelectric properties were often found due to the high leakage current, for example, in BiFeO3-SrTiO3 [363], BiFeO3-CaTiO3 [364], BiFeO3-NaNbO3 [365], BiFeO3-K0.5Na0.5NbO3 [366], and BiFeO3-Bi0.5K0.5TiO3 [367]. Therefore, we next introduce the development of BiFeO3-BaTiO3. 574

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Table 12 Electrical properties of BFO ceramics with ion substitutions at both Bi and Fe site. Material system

Phase structure

Bi0.9La0.1Fe0.97Co0.03O3 Bi0.80La0.20Fe0.97Nb0.03O3 Bi0.9La0.1Fe0.98Zr0.02O3 Bi0.85La0.15Fe0.9Ti0.1O3 Bi1-xNdxFe1-xMnxO3 (x = 0.05–0.1) (Bi0.94Pr0.06)(Fe0.94Ti0.06)O3 (Bi0.94Pr0.06)(Fe0.94Zr0.06)O3 Bi0.9Y0.1Fe0.93Mn0.07O3 Bi0.85Y0.15Fe0.95Zr0.05O3 Bi0.75Ba0.25Fe0.975Ni0.025O3 Bi0.8Ba0.2Fe0.9Nb0.1O3 Bi0.82Ca0.18Fe0.91Nb0.09O3 Bi0.8Ca0.2Fe0.9Mg0.1O3 Bi0.9Ho0.1Fe0.97Ni0.03O3 Bi0.9Gd0.1Fe0.9Ti0.1O3 Bi0.9Zn0.1Fe0.9Ni0.1O3 Bi0.95Sm0.05Fe0.95Sc0.05O3 Bi0.925La0.05Sm0.025Fe0.95M0.05O3 (M:Sc, In, Al, Ga)

R R R R-T R R R R R

d33 (pC/N)

Pr (μC/cm2)

0.37 0.22 0.63

R R R R R R R T

Mr (emu/g)

Ref.

0.23 0.1970 0.025 0.25 0.033 0.1824 0.1234 0.62 0.4149 0.7 0.03933

[332] [333] [334] [335] [336] [337] [338] [339] [340] [341] [342] [343] [344] [345] [346] [347] [327] [320]

120

51 15–47

0.1 10

0.6038 0.2280 ∼0.02 1.5

Fig. 21. (a) Unit cell of BFO with rhombohedrally distorted perovskite structure; (b) X-ray photoelectron spectroscopy of Fe 2p in Bi0.95Sm0.05Fe0.95Sc0.05O3 ceramics; (c) P-E loops of the Bi0.95Sm0.05Fe0.95Sc0.05O3 ceramics, measured at 100 °C and 10 Hz; and (d) d33 of the Bi0.95Sm0.05Fe0.95Sc0.05O3 ceramics as a function of poling dwell time under Ep = 6 kV/mm and Tp = 100 °C [327]. [327]-Reproduced by permission of the Royal Society of Chemistry.

3.3.1.2.1. BiFeO3-BaTiO3 binary systems. The (1 − x)BiFeO3-xBaTiO3 system has a high Curie temperature (TC), good piezoelectricity (d33), and lead-free characteristics. Kumar et al. reported that the phase structure changes from rhombohedral to pseudocubic and then to tetragonal with an increase in the BT content, forming an R-Pc phase boundary at x = 0.30 [361]. After that, a similar phase structure transition was also reported by other researchers. As shown in Table 13, a phase boundary (R-Pc or R-T) always appears in the composition range of x = 0.275–0.33 [371–374,385]. Compared with BFO ceramics with ion substitution, the electrical properties can be greatly promoted due to the enhanced resistivity and presence of phase boundaries. For example, enhanced piezoelectricity (d33 = 124–240 pC/N) can be observed in (1 − x)BiFeO3-xBaTiO3 ceramics [371–374] due to the involvement of phase boundaries. Recently, ion substitutions (Nd, La, Ga, Al, Sc, Co, etc.) were employed to further modify the 575

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Table 13 Electrical properties of BFO-BTO binary systems. Material system

Phase structure

d33 (pC/N)

0.8BiFeO3–0.2BaTiO3 + SiO2 0.8BiFeO3–0.2BaTiO3(N2) 0.75BiFeO3-0.25BaTiO3 + MnO2 0.725BiFeO3-0.275BaTiO3 0.725BiFeO3-0.275BaTiO3 + MnO2 0.70BiFeO3-0.30BaTiO3 0.675BiFeO3-0.325BaTiO3 + MnO2 + CuO 0.67BiFeO3-0.33BaTiO3 0.67BiFeO3-0.33BaTiO3 + MnO2 0.65BiFeO3-0.35BaTiO3 0.725Bi(Fe0.98Sc0.02)O3–0.275BaTiO3 + MnO2 0.71Bi(Fe0.985Ga0.015)O3–0.29BaTiO3 0.71BiFe0.994Co0.006O3–0.29BaTiO3 0.75Bi0.95Nd0.05FeO3-0.25BaTiO3 + MnO2 0.725BiFe0.96Sc0.04O3-0.275BaTiO3 + MnO2 0.72Bi(Fe0.99Al0.01)O3–0.28BaTiO3 0.70Bi0.98La0.02FeO3-0.30BaTiO3 + MnO2 0.70Bi1.05Fe0.97A0.03O3-0.30BaTiO3 (A: Sc, Ga, Al) 0.67Bi1.05Fe0.97Ga0.03O3-0.33BaTiO3

R R R R-Pc R-Pc R-Pc R-Pc R-T Pc Pc R Pc Pc R-M R-M R-Pc R-T R-Pc R-T

86 98 116 124 136 180 170 240 104 127 157 167 121 143 151 121 160–180 402

Pr (μC/cm2)

EC (kV/cm)

TC (°C)

Ref.

25.7 22.9

74.6 39.3

628 632 619

15 25 ∼20 ∼20 39 30.6 ∼20 8

30 23 ∼30 ∼25 ∼25 27.9 ∼50 23

17.5 17.6 14.41 25 18–22 20

40 33.31 33 ∼30 24

[368] [369] [370] [371] [372] [373] [374] [16] [375] [376] [377] [378] [379] [380] [381] [382] [383] [384] [16]

485 506 485 456 420 400 636 464 488 482 596 450 550 470–500 454

BFO-BTO ceramics, which can partly enhance the piezoelectricity due to the presence of phase boundaries together with the improved microstructure [16,377–384]. For instance, the addition of Ga2O3 [378], Sc2O3 [377], and Co2O3 [379] can effectively promote grain growth, and the enhanced piezoelectricity is mainly ascribed to the dense microstructure and large grain size. In addition, doping with Nd2O3 [380], Al2O3 [382], La2O3 [383] or Ga2O3 [16] can induce phase transitions, and the improved electrical properties were mainly attributed to the formation of phase boundaries. In particular, in our previous work, a series of BFOBTO ceramics doped with Sc, Ga, Al, In, Co, or Ni were employed to investigate the relationship between the phase structure and electrical properties. Fig. 22 shows the electrical properties of 0.7Bi1.05Fe0.97A0.03O3-0.3BaTiO3 (A: Sc, Ga, Al, In, Co, Ni) ceramics as a function of the A element. Saturated P-E loops and large piezoelectricity (d33 = 160–180 pC/N) can be attained simultaneously in ceramics with Sc, Ga, and Al due to the presence of a R-Pc phase boundary [384]. In particular, in 2015, a large piezoelectricity was found in BiGaO3-modified BFO-BTO ceramics prepared by the conventional solid-state method together with quenching sintering,

Fig. 22. (a) P-E loops, (b) Pr and EC, (c) d33, (d) ɛr and tan δ of 0.7Bi1.05Fe0.97A0.03O3-0.3BaTiO3 ceramics as a function of A elements [384]. [384]Reproduced by permission of the Royal Society of Chemistry. 576

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Table 14 Electrical properties of BFO-BTO ternary systems. Material system

Phase structure

d33 (pC/N)

Pr (μC/cm2)

EC (kV/cm)

TC (°C)

Ref.

0.97(0.67Bi1.05FeO3-0.33BaTiO3)-0.03Bi1.05(Zn0.5Ti0.5)O3 0.715BiFeO3-0.275BaTiO3-0.01Bi(Mg0.5Zr0.5)O3+MnO2 0.705BiFeO3-0.275BaTiO3-0.02Bi0.5Na0.5TiO3+MnO2 0.725BiFeO3-0.25BaTiO3-0.025Bi0.5K0.5TiO3+MnO2 0.69BiFeO3-0.29BaTiO3-0.02Bi(Mg0.5Ti0.5)O3+MnO2 0.71BiFe0.97(Mg0.5Ti0.5)0.03O3-0.29BaTiO3+MnO2 0.71BiFe0.97(Ni0.5Ti0.5)0.03O3-0.29BaTiO3+MnO3 0.925BiFeO3–0.275Ba0.85Ca0.15Ti0.90Zr0.10O3+MnO2

R-T R-T R-T R-Pc Pc Pc Pc R

324 130 140 134 115 155 156 106

24 ∼9 18.4 16.52 24.5 ∼17 22.5 24.5

26 ∼32

466 575

4.59 40 ∼25 28.2 5.93

509 425 431 425

[16] [386] [387] [388] [389] [390] [391] [392]

that is, a large d33 of 402 pC/N and a high TC of 454 °C were obtained simultaneously due to the formation of an R-T phase boundary [16]. 3.3.1.2.2. BiFeO3-BaTiO3 ternary systems. As shown in Table 14, Another effective way to tailor the electrical properties of BFOBTO ceramics is to introduce a third component, such as Bi(Mg0.5Zr0.5)O3 [386], Bi(Mg0.5Ti0.5)O3 [389,390], Bi(Zn0.5Ti0.5)O3 [16], Bi0.5Na0.5TiO3 [387], Bi0.5K0.5TiO3 [388], and Bi(Ni0.5Ti0.5)O3 [391]. However, it was found that the addition of Bi(Mg0.5Ti0.5)O3 and Bi(Ni0.5Ti0.5)O3 does not result in phase transition, and the enhanced piezoelectricity is mainly attributed to the dense microstructure and large grain size [389–391]. More interestingly, different phase boundaries can be constructed by optimizing the compositions of ABO3. For example, a rhombohedral-tetragonal (R-T) phase boundary can be formed by doping Bi(Zn0.5Ti0.5)O3 [16], Bi(Mg0.5Zr0.5) O3 [386] or Bi0.5Na0.5TiO3 [387] into the BFO-BTO matrix, and the rhombohedral-pseudocubic (R-Pc) phase boundary has been reported in BFO-BTO-Bi0.5K0.5TiO3 ceramics [388]. In particular, a large d33 of 324 pC/N was obtained in BFO-BTO-BZT ceramics with an R-T phase boundary [16]. As a result, enhanced electrical properties can be mainly assigned to the phase boundary, and other factors, including dense microstructure and high resistivity, were partly responsible for the improved properties. 3.3.2. Temperature stability of strain properties In recent years, strain investigations have been extensively carried out in lead-free piezoelectric materials, and increasing attention has been paid to BFO-based ceramics due to both high strain response and distinctive temperature dependence [16,393–399]. Although typical butterfly-shaped bipolar S-E curves and good strain properties (S < 0.15%) can be measured in pure or ionsubstituted BFO ceramics, the extra high driving electric field (> 100 kV/cm) is unfavorable [393,394]. By the efforts of the researchers, the strain properties were greatly enhanced in BFO-BTO ceramics by composition modification [16,395–399]. It was found that the addition of BaZrO3 can modify the strain properties of BFO-BTO ceramics [395,396]. For example, a significantly improved strain of ∼0.37% without negative strain can be attained in BFO-BTO ceramics by tuning the BaZrO3 content [396], which is similar to the situation in BNT-based ceramics [13]. Therefore, the strain enhancement can be attributed to the phase transition in BFO-BTObased ceramics, which is similar to MPB(II) in BNT-based ceramics. More importantly, the phase structure in BFO-based ceramics is only dependent on the composition [16,61], while MPB(II) in BNT-based ceramics is sensitive to not only the composition but also the temperature. Therefore, the positive temperature dependence of strain properties can always be achieved in BFO-based ceramics [16,61,400]. For example, d33* can reach 117% in Nd-doped

Fig. 23. in situ temperature dependence of (a) bipolar polarization hysteresis and (b) unipolar strain loops for BF-BT and BN0.02F-BT; (c) Pr, EC and d33* as a function of temperatures [61]. Journal of European Ceramic Society, DOI 10.1016/j.jeurceramsoc.2016.10.027; Used in accordance with the Creative Commons Attribution (CC BY) license. 577

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BFO-BTO ceramics when the temperature is increased to 150 °C, as shown in Fig. 23 [61]. The excellent strain (d33 = 525 pm/V at 200 °C) can also be observed in Mn-modified BFO-BTO ceramics measured at high temperature [400]. The origin of enhanced temperature stability should be attributed to the facile electric field-induced phase transition from the nonergodic relaxor state to the ferroelectric state, thermally activated domain wall motion, intrinsic lattice strain and so on [61,400]. Therefore, BFO-BTO-based ceramics can be considered promising candidates for high-temperature piezoelectric actuators if the high leakage current density can be overcome. In the future, more effort should be made to determine the relationships between composition modification, strain properties, and temperature stability in BFO-based ceramics. 3.3.3. Relationship between piezoelectricity/strain and phase boundaries In this work, we discussed the evolution of the strain/piezoelectric properties in BFO-based ceramics by studying the relationship between the phase boundaries and electrical properties. Some results are as follows: Ion substitutions cannot induce the formation of effective phase boundaries, which can improve the piezoelectric properties. Even if composition modifications and optimized preparation processes are widely applied, their piezoelectric constant was still limited to 50 pC/N due to the lack of real MPB [82,320,350]. However, for BFO-BTO systems, several types of phase boundaries (e.g., Rpseudocubic, R-T, R-monoclinic, etc.) can be successfully constructed by chemical modification, and enhanced piezoelectricity (d33 = 100–402 pC/N) can be observed. [16,381,384,388] Except for the formation of phase boundaries, optimization of the composition is also critical to determine the piezoelectricity of BFO-based ceramics. A strong strain response can be found in BFO-BTO-based ceramics. Similar to BNT-based ceramics, the phase transition between the ferroelectric state and the relaxor state can be used to promote the strain properties in BFO-BTO-based ceramics. More importantly, two types of phase boundaries are real MPB, including the ferroelectric phase boundary for piezoelectric enhancement and ferroelectric-relaxor phase transition for strain enhancement. Therefore, the strain behavior is very similar to that of BNT ceramics, and the strain increased with an increase in temperature, making it the most promising candidate for lead-free actuators operated at high temperature. 3.4. BaTiO3-based ceramics 3.4.1. Phase boundary in BT-based ceramics Among four kinds of perovskite oxides, barium titanate (BaTiO3) was the first lead-free piezoelectric material to be found. Three phase transition temperatures can be found: 120 °C for TC, 0 °C for TO-T, and −90 °C for TR-O. Although the piezoelectricity of pure BT ceramics can be greatly improved by advanced preparation methods (Fig. 24) [401–406], the conventional solid-state method is still the most feasible tool due to its economic efficiency and industrialization. As is well known, site engineering is an effective way to improve the electrical properties of piezoelectric ceramics [67,69,407–410]. Previously, it was confirmed that the addition of Zr, Hf, Sn or Ca affects the phase transitions in BT ceramics [67,69,407]. Fig. 26 summarizes the effects of doping elements (Zr, Hf, Sn, and Ca) on the phase transition temperatures of BT ceramics. As shown in Fig. 25, Zr, Hf or Sn substitution for Ti can decrease TC and increase both TR-O and TO-T [69], while the addition of Ca has a slight effect on TC and decreases both TR-O and TO-T [67,407]. Based on this finding, different phase boundaries of BT can be designed by shifting TR-O and/or TO-T to room temperature through ion substitution. Similar to KNN-based ceramics, the reported phase boundary types mainly include R-T, O-T, and R-O-T. In general, the piezoelectricity of BT-based ceramics with R-T and R-O-T phase boundaries is much better than those with O-T phase boundaries, and the R-O phase boundary was rarely considered due to the poor piezoelectricity. Therefore, the construction of the coexistence of R and T phases can effectively enhance the piezoelectricity in BT-based ceramics. 3.4.2. Composition vs. phase boundary 3.4.2.1. (Ba, Ca)(Ti, Zr)O3. As discussed above, ion substitution is important to tune the phase transition temperature of BT-based ceramics [67,69,407-410]. In 2009, Ren et al. achieved a major advance in the piezoelectric constant of 50Ba(Ti0.8Zr0.2)O3-

Fig. 24. Piezoelectricity of pure BT ceramics fabricated by different methods [401–406]. 578

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Fig. 25. Phase diagrams of (a) BaTi1−xZrxO3, (b) BaTi1−xHfxO3, (c) BaTi1−xSnxO3, and (d) CaxBa1−xTiO3 ceramics [67,69]. Reproduced from [69], with the permission from AIP publishing. Reproduced from [67], with the permission from The American Physical Society.

50(Ba0.7Ca0.3)TiO3 (BZT-xBCT) ceramics by constructing a “MPB” starting from a tricritical triple point [11]. One can find from Fig. 26 that a surprisingly large d33 of 620 pC/N was first reported, [11] and the high piezoelectric constant was mainly ascribed to the nearly vanishing polarization anisotropy and facilitated polarization rotation between the R and T states because of the involvement of MPB composition close to the tricritical triple point [11]. After that, the in-depth investigations into property modification and the physical mechanisms were also conducted [84,411–421]. Subsequently, we introduced the development of BZT-

Fig. 26. (a) Phase diagrams of Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics [11]; (b) composition dependence of d33 during composition-induced RMPB-T along horizontal line (red) in (a) [411]; (c) temperature dependence of d33 during temperature-induced R-MPB-T transition along vertical line (blue) in (a) [411]; (d) TEM observation of BZT-0.5BCT ceramics; (d1) and (d2) the CBED images obtained from one domain lamella in (d), indicating coexistence of R and T symmetry in MPB region [411]. Reproduced from [11], with the permission from the American Physical Society. Reproduced from [411], with the permission from AIP publishing. 579

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Fig. 27. (a) Revised phase diagram of Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics, orthorhombic Amm2 phase persisting to the phase convergence region [413]; (b) both ƐrPr and d33 vs. x were measured at room temperature. Two peaks at x = 50 and x = 44 respectively observed in T-O and O-R. T-O phase boundary at x = 50 shows a higher piezoelectric response than the O-R phase boundary at x = 44 [416]. Reproduced from [413,416], with the permission from AIP publishing.

xBCT. It is highly expected to develop R-T phase boundary to improve the piezoelectric properties of BT ceramics, since the R-T phase boundary was found in BZT-xBCT ceramics. In 2011, Xue et al. investigated the elastic, piezoelectric, and dielectric properties of the ceramics at the MPB region, and the excellent piezoelectricity was attributed to the low polarization anisotropy as well as elastic softening at the MPB [84]. In 2012, a giant strain value (d33* ∼ 1310 pm/V) was also obtained for 0.55BZT-0.45BCT ceramics due to the presence of an R-T phase boundary [412]. It is well known that the existence of intermediate phases is also an inevitable topic for lead-based or lead-free piezoelectric materials. Contrary to the previous reports, an intermediate orthorhombic phase was clearly observed in the R-T phase boundary region, and a revised phase diagram was also proposed, as shown in Fig. 27(a) [413–417,419–421]. In addition, the intermediate O phase affected the electrical properties of the ceramics [415], and a maximum d33 was found for the O-T phase boundary due to the facile polarization rotation, larger lattice softening, and reduced anisotropic energy [Fig. 27(b)] [416,417]. Therefore, the modification of phase compositions is very important to modulate the electrical properties in such a ceramic. As discussed above, a giant piezoelectricity of 620 pC/N was first reported in 0.5BZT-0.5BCT ceramics with R-T phase boundary at the tricritical triple point [11]. After that, the researchers expected to obtain an enhanced piezoelectric effect in BT-based ceramics by tuning compositions or processing modification. Table 15 shows the phase structure and electrical properties of (Ba, Ca)(Ti, Zr)O3 ceramics. It can be seen that a much better piezoelectricity can be obtained in the (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics with R-T or RO-T phase boundaries. For example, excellent d33 of 673 pC/N, large kp of 0.56, and improved TC of 110 °C were simultaneously obtained in CeO2-modified BZT-BCT ceramics [422]. In addition, the optimized preparation, sintering or poling conditions can also result in improved electrical properties. For example, improved piezoelectricity (d33 = 650 pC/N) was reported in BCTZ ceramics by optimizing the calcination and sintering temperatures [423]. High piezoelectric coefficient d33 of 637 pC/N, large kp of 0.596, a large strain of 0.157%, and a high piezoelectric voltage coefficient g33 of 29 mV m/N have been obtained in the BCTZ ceramics by optimizing poling conditions [424]. Especially, a very high piezoelectricity (d33 = 755 pC/N) and an extremely large piezoelectric strain Table 15 Electrical properties of (Ba,Ca)(Ti,Zr)O3 ceramics. Material system

Phase boundary

d33 (pC/N)

kp

TC (oC)

Ref.

0.5Ba(Zr0.20Ti0.80)O3-0.5(Ba0.70Ca0.30)TiO3 (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 0.5Ba(Zr0.20Ti0.80)O3-0.5(Ba0.70Ca0.30)TiO3+0.08 wt% CeO2 (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 Ba0.85Ca0.15Ti0.90Zr0.10O3–0.08 wt% ZnO 0.5Ba(Zr0.20Ti0.80)O3-0.5(Ba0.70Ca0.30)TiO3 0.5Ba(Zr0.20Ti0.80)O3-0.5(Ba0.70Ca0.30)TiO3 Textured (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (Ba0.92Ca0.08)(Ti0.95Zr0.05)O3 (Ba0.85Ca0.15)(Ti0.95Zr0.05)O3 (Ba0.9Ca0.1)(Ti0.98Zr0.02)O3 (Ba0.94Ca0.06)(Ti0.925Zr0.075)O3

R-T R-T R-T R-T R-T R-T R-T R-O-T R-O-T O-T O-T O-T R-O

620 572 673 650 603 637 630 755 572 365 458 375 360

– 0.57 0.56 0.53 0.56 0.596 0.56

∼90 ∼82 ∼110 ∼85 ∼85 ∼90 ∼90 ∼90 ∼82 ∼110 ∼85 ∼115 ∼100

[11] [426] [422] [423] [427] [424] [428] [12] [429] [425] [430] [431] [432]

580

0.57 0.485 – 0.44 0.5

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Fig. 28. Structure and properties of textured BCTZ ceramics [12]. Reprinted with permission from [12], Copyright (2017) American Chemical Society.

coefficient (d33* = 2027 pm/V) along with an ultralow strain hysteresis (H = 4.1%) were simultaneously obtained in the [0 0 1]c grain-oriented (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 ceramics, as shown in Fig. 28 [12]. The exceptionally large piezoelectric response was ascribed to the piezoelectric anisotropy and the smaller sized domains in the BCTZ textured ceramics. However, the change of Ca/Zr ratio can result in different phase boundaries, such as O-T and R-O. Correspondingly, its electrical properties have a decrease. For example, a much lower electrical properties (d33 = 365 pC/N and kp = 0.458) were reported in the (Ba0.92Ca0.08)(Ti0.95Zr0.05)O3 ceramics with O-T phase boundary [425]. As a result, it is very effective to improve electrical properties of (Ba,Ca)(Ti,Zr)O3 ceramics by constructing R-T or R-O-T phase boundary and modifying processing conditions. 3.4.2.2. (Ba, Ca)(Ti, Sn)O3. In addition to (Ba, Ca)(Ti, Zr)O3, (Ba, Ca)(Ti, Sn)O3 ceramic is another important system with good electrical properties, as shown in Table 16. Similarity to (Ba, Ca)(Ti, Zr)O3 ceramics, much better electrical properties can be also achieved in these ceramics with R-T or R-O-T phase boundary. For example, the Li2O-modified (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 ceramics with R-T phase boundary possess a large d33 of 578 pC/N [433], and R-O-T phase boundary results in a much higher d33 of 630 pC/N and kp of 0.52 in 0.55(Ba0.9Ca0.1)TiO3-0.45Ba(Sn0.2Ti0.8)O3 ceramics [434]. Especially, the variation of Ca/Sn contents can result in different electrical properties even though the same O-T phase boundary was constructed. For example, the (Ba0.95Ca0.05) (Ti0.92Sn0.08)O3 ceramics exhibited a superior piezoelectricity of 568 pC/N than other ones [435]. As a result, the phase boundary types are very sensitive to the chemical compositions, finally resulting in different piezoelectricity. 3.4.2.3. (Ba, Ca)(Ti, Hf)O3. As mentioned above, the enhanced electrical properties can be achieved in the (Ba, Ca)(Ti, M)O3 (M = Zr, Sn) ceramics with R-T or R-O-T phase boundaries. Similarly, improved electrical properties were also expected to be achieved in the (Ba, Ca)(Ti, Hf)O3 ceramics due to their similar ionic radii and valence state of Zr, Sn, and Hf. Table 17 summarizes Table 16 Electrical properties of (Ba, Ca)(Ti, Sn)O3 ceramics. Material system

Phase boundary

d33 (pC/N)

kp

TC (oC)

Ref.

(Ba0.95Ca0.05)(Ti0.90Sn0.10)O3+4%Li2O (1 − x)BaTiO3-x(0.4CaTiO3-0.6BaSnO3) (x = 0.16) (1 − x)(Ba0.9Ca0.1)TiO3-xBa(Sn0.2Ti0.8)O3 (x = 0.45) (Ba0.94Ca0.06)(Ti1−xSnx)O3 (x = 0.10) (Ba0.95Ca0.05)(Ti1−xSnx)O3 (x = 0.09) (Ba1−xCax)(Ti0.95Sn0.05)O3 (x = 0.02) (Ba1−xCax)(Ti0.96Sn0.04)O3 (x = 0.02) (Ba1−xCax)(Ti0.94Sn0.06)O3 (x = 0.03) (Ba1−xCax)(Ti0.92Sn0.08)O3 (x = 0.05) (Ba0.90Ca0.10)(Ti1-xSnx)O3 (x = 0.10) (Ba0.91Ca0.09−xSrx)(Ti0.92Sn0.08)O3 (x = 0.01)

R-T R-O-T R-O-T R-O-T Pc-O O-T O-T O-T O-T O-T O-T

578 570 630 600 670 464 510 440 568 521 583

0.38 0.52 0.52 0.50 0.50 0.43 0.48 0.45 0.477 0.455 0.40

50 ∼60 ∼60 ∼50 ∼60 ∼75 ∼80 ∼60 ∼60 ∼50 ∼70

[433] [436] [434] [432] [437] [438] [439] [440] [435] [441] [442]

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Table 17 Electrical properties of (Ba, Ca)(Ti, Hf)O3 ceramics. Material system

Phase boundary

d33 (pC/N)

kp

TC (oC)

Ref.

(Ba0.7Ca0.3)TiO3-Ba(Ti0.8Hf0.2)O3 (Ba0.85Ca0.15)(Ti0.9Hf0.1)O3 0.52Ba(Hf0.16Ti0.84)O3-0.48(Ba0.7Ca0.3)TiO3 0.47Ba(Hf0.20Ti0.80)O3-0.53(Ba0.7Ca0.3)TiO3 (Ba0.94Ca0.06)(Ti0.93Hf0.07)O3

R-T R-O-T O-T O-T R-O

550 540 410 448 355

– 0.52 0.47 – 0.45

∼50 ∼85 ∼106 ∼80 ∼100

[443] [446] [444] [445] [432]

the electrical properties of (Ba, Ca)(Ti, Hf)O3 ceramics with different phase structure. It can be seen that their electrical properties were also greatly affected by the Hf contents due to the different phase boundaries. For example, the Ba(Ti0.8Hf0.2)O3(Ba0.7Ca0.3)TiO3 ceramics with a triple-point morphotropic phase boundary (TMPB) was fabricated, and then a high d33 of 550 pC/N can be realized due to the low energy barrier of TMPB [443]. In addition, a relatively low d33 of 410–448 pC/N can be observed in these (Ba, Ca)(Ti, Hf)O3 ceramics with O-T phase boundary [444,445]. However, much poorer d33 of 355 pC/N was reported in the (Ba, Ca)(Ti, Hf)O3 ceramics with R-O phase boundary [432]. As a result, the phase boundaries as well as the doped elements and contents can seriously affect the electrical properties of BTbased ceramics. In general, ceramics with O-T or R-O phase boundaries possess poorer piezoelectric behavior than those of the ones with R-T or R-O-T. The R-T or R-O-T phase boundary can greatly improve the piezoelectric activity because of the different anisotropy for polarization rotation and the different domain-wall contribution. 3.4.3. Other dominated factors of phase boundaries We have described that the chemical compositions can greatly influence the phase boundary of BT-based ceramics, and then different electrical properties can be achieved. However, it is not enough to consider the relationship between composition and phase boundary of BT-based ceramics, because its original phase structure can be easily influenced by other factors, such as temperature [447], electric field [448–450], and grain size [451,452]. Fig. 30 shows the temperature dependence of dielectric constant and XRD patterns of BCTZ materials [447]. It can be found that the phase transition temperatures (TR-O and TO-T) are very close to each other [Fig. 29(a)], and the temperature fluctuation can easily cause the variations of phase structure [Fig. 29(b)]. In addition, electric field can also affect phase structure of BT materials [448–450]. As early as 1999, is-situ domain observation and Raman measurement of BT single crystal with engineered domain configuration confirmed that the applied electric fields can induce a phase transition from tetragonal to monoclinic under ∼10 kV/cm, and then the further increased electric field (30 kV/cm) results in the transition from monoclinic to rhombohedral phase [448]. In addition, the largest piezoelectric coefficient can be obtained in the monoclinic BT single crystal. In 2009, high precision XRD also indicated the electric field-induced a monoclinic MC

Fig. 29. Temperature dependence of dielectric constant and XRD patterns of BCTZ [447]. Reproduced from [447], with the permission from The American Physical Society. 582

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Table 18 Piezoelectricity in BT-based ceramics with oxides under low temperature sintering. Material system

Phase structure

d33 (pC/N)

kp

TC (oC)

TS (oC)

Ref.

Ba0.98Ca0.02Zr0.02Ti0.98O3 + 0.3 mol% MnO2 (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3 + 0.08 wt%Ga2O3 (Ba0.7Ca0.3)TiO3-Ba(Zr0.2Ti0.8)O3 + 0.04 wt%CuO (Ba0.95Ca0.05)(Ti0.9Sn0.1)O3 − 2%CuO (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 + 0.04 wt%CeO2 (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 + 0.8 wt%Ba(Cu0.5W0.5)O3 (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 + 3%Li2CO3 (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 + 4%Li2O Ba0.85Ca0.15Ti0.9Zr0.1O3 + 0.5 wt%Li2CO3 (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3 + 0.1 wt%HfO2 (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3 + 0.1 wt%ZnO (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 + 0.06 wt%Pr2O3

O-T O-T R-T R-O-T R-T R-T R-T R-T R-O-T O T T

308 440 510 683 600 541 485 578 493 430 428 460

0.56 0.45 0.55 0.51 0.49 0.39 0.38 0.45 0.48 0.53 0.47

120 120 95 50 90 78 50 50 70 80 89 80

1400 1350 1350 1250 1350 1220 1300 1350 1350 1400 1350 1400

[453] [454] [455] [460] [456] [457] [458] [433] [461] [462] [459] [463]

phase in BT single crystal [449]. Ranjan et al. also conducted a deep investigation about the study of structural evolution in BT-based ceramics under electric fields [450]. For example, three phases coexistences (R + O + T) was proposed in the typical (Ba0.85Ca0.15) (Ti0.90Zr0.10)O3 (BCTZ) ceramic at room temperature through the structure analysis of XRD. The XRD results of unpoled and poled samples reveal that electric field can transform partial T phase to R + O at room temperature, which is associated with the giant piezoelectric response. Besides the application of temperatures and electric fields can easily lead to the variations of phase structure, grain size can also transfer the phase structure and influence corresponding electrical properties. For example, the Curie temperature and c/a of BaTiO3 ceramics will gradually decrease with the decreased grain sizes, demonstrating that BT becomes less tetragonal at smaller grain sizes [451,452]. 3.4.4. High sintering temperature and low Curie temperature It was also reported that both the calcination and sintering temperatures are closely related with the piezoelectricity of BT-based ceramics. For example, an improved d33 of 650 pC/N can be obtained in (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics when calcined at 1300 °C and sintered at 1540 °C [423]. In general, high-performance BT-based ceramics can only be prepared by applying high sintering temperatures of ≥1450 °C, which limits the practical applications. In addition, it is necessary to avoid compositional fluctuation in BT-based ceramics by lowering the sintering temperature, and some sintering aids (e.g., MnO2 [453], Ga2O3 [454], CuO [455], CeO2 [456], Ba(Cu0.5W0.5)O3 [457], Li2CO3 [458], Li2O [433], ZnO [459], etc.) have been used to form liquid phases during the sintering process. Table 18 shows the phase structure, electrical properties, and sintering temperatures of BT-based ceramics with different sintering aids. The addition of some sintering aids can greatly lower the sintering temperature (TS ≤ 1400 °C), improve the sinterability, and finally enhance the piezoelectricity of BT-based ceramics [433,455,456]. For example, ultrahigh piezoelectric coefficient d33 of 683 pC/N and a large kp of 0.55 can be obtained in the CuO modified BCTS ceramics [460]. Although the addition of oxides as sintering aids can lower the sintering temperatures and improve the piezoelectricity to a certain extent, their Curie temperature always deteriorates. For example, the TC of BCTZ ceramics reduced linearly with an increase in the Ba(Cu0.5W0.5) content [457]. High-performance BZT-BCT ceramics reported by Ren et al. also had a low TC of 93 °C [11]. It is thought that the low Curie temperature is the biggest obstacle to the practical application of BT-based ceramics. However, the Curie temperature can be increased by doping with some oxides. For example, Xu et al. found that the TC of BT-based ceramics can be partly promoted by the introduction of Dy2O3, Er2O3, and Y2O3 additives [464–466]. Unfortunately, no great progress into the improvement of the Curie temperature has been made. As a result, a balance between the piezoelectricity and Curie temperature still needs to be obtained for BT. 3.5. Summary of the development of lead-free piezoelectric perovskite ceramics Perovskite-structured ferroelectrics include (K,Na)NbO3, (Bi1/2Na1/2)TiO3, BiFeO3, BaTiO3, and their solid solutions. Recently, some advances in their electrical properties have been achieved, which can gradually approach or match those of PZT. In particular, perovskite-type ferroelectrics are of interest as replacements for PZT since relatively high dielectric and piezoelectric properties were found in this crystal structure. Considerable studies and developments have been obtained for lead-free piezoelectric ceramics to overcome these issues, and some progress has been made. In this part, we briefly summarize the development of lead-free piezoelectric perovskite ceramics. Potassium sodium niobate {(K,Na)NbO3, KNN} is considered to be one of the leading lead-free candidates among these materials due to its high Curie temperature and good piezoelectric properties, and thus, this material has received much attention (Fig. 30) [23]. It is well known that pure KNN exhibits a poor piezoelectricity of ∼80 pC/N; however, its piezoelectricity has been greatly increased to 570 pC/N in the past fifteen years [8]. Several progresses in d33 are as follows. In 2004, a high d33 (∼416 pC/N) was obtained in textured (K, Na, Li)(Nb, Ta, Sb)O3 ceramics fabricated by the reactive templated grain growth (RTGG) method [5]. In this study, the d33 value can be increased to several times that of pure KNN. In 2017, much higher piezoelectric properties with piezoelectric coefficients (d33 = 700 pC/N, d33* = 980 pm/V) and planar electromechanical coupling factor (kp = 0.76) were reported in highly textured KNN-based ceramics [10]. Since 2014, conventional sintered lead-free KNN-based ceramics with d33 ≈ 490–570 pC/ 583

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Fig. 30. Comparison of (a) d33 vs. TC and (b) strain vs. d33* in lead-based and lead-free materials.

N were developed by our group through the construction of a new rhombohedral-tetragonal (R-T) phase boundary [6–9], which has further inspired research interest into KNN-based materials. Bismuth sodium titanate (Bi1/2Na1/2TiO3, BNT) materials exhibit a rhombohedral structure, which was first discovered by Smolenskii et al. in 1960 [4]. One of the most important features is that two kinds of morphotropic phase boundaries (MPB) can be established in BNT by chemical modification, including a R-T phase boundary {MPB(I)} and a FE-relaxor or polar-nonpolar {MPB(II)} phase boundary. Different types of phase boundaries induce different physical properties in BNT-based materials – that is, high d33 and large strain can be found in MPB(I) and MPB(II), respectively. For example, a high d33 of ∼230 pC/N can be attained in BNTbased ceramics due to the formation of R-T phase boundaries [70], and a giant strain (0.45%) in 0.92BNT-0.06BT-0.02KNN ceramics with MPB(II) was first observed in 2007 [13], which is the first discovery of strains comparable to those of lead-based materials. In particular, a series of studies on the strain properties has been reported in recent years, and a giant strain of ∼0.70% can be realized via optimization of the composition [14]. The main limitation of BNT in piezoelectric devices is the presence of a depolarization temperature (Td). Although the Td can be removed by the addition of ZnO [277], a low d33 was obtained. As a result, strain investigations will be at the core of BNT-based materials in the future. Bismuth ferrite (BiFeO3, BFO) is one of very few single-phase multiferroic compounds for high-temperature applications. A great discovery in the ferroelectric properties (Pr = 60 μC/cm2) of strained BFO thin films was realized in 2003 [15]. Since then, some interesting findings have been reported. For example, BFO-based thin films exhibit a giant Pr of > 150 μC/cm2 by modification of the crystal structure, [467] and high d33 (∼50 pC/N) and high TC (> 700 °C) can be attained in BFO ceramics by ion substitution [82,350]. More importantly, the enhancement in d33 ≈ 402 pC/N (TC ≈ 454 °C) was obtained in BiFeO3-BaTiO3-BiGaO3 ceramics by forming a R-T phase boundary [16]. Due to these new findings, enthusiasm for research into BFO-based materials for piezoelectric devices remains undiminished in recent years and will continue in the future. Barium titanate (BaTiO3) was the first material used to fabricate high-performance lead-free piezoceramics. However, BaTiO3 is considered to be a dielectric material because a poor d33 of ∼190 pC/N is often observed in pure BT ceramics fabricated by the conventional solid-state method [401]. In 2009, Ren et al. achieved a major evolvement in piezoelectricity by constructing a MPB starting at a tricritical triple point, and a surprisingly large d33 of 620 pC/N was obtained in 50Ba(Ti0.8Zr0.2)O3-50(Ba0.7Ca0.3)TiO3 ceramics [11]. In 2017, much higher piezoelectric properties (d33 = 755 pC/N, d33* = 2027 pm/V) were obtained in the highly textured BCTZ ceramics [12]. However, the working temperature range of BT is too narrow for actual piezoelectric applications because of its low Curie temperature (TC ≈ 120 °C), and the multiple polymorphic phase transitions cause significant temperaturedependent properties. Although these two shortcomings have limited the practical application of this material in piezoelectric devices, the design of a phase boundary can benefit the development of other lead-free material systems. Recently, other properties, such as the electrocaloric effect, have also inspired the development of BT-based materials. For more than 60 years, lead zirconate titanate (PZT) and related lead-based perovskite materials have been the mainstay for a wide range of piezoelectric applications due to their superior piezoelectric and electromechanical properties. In the last fifteen years, research studies have become more concerned with the development of lead-free piezoelectric materials with properties comparable to those of PZT owing to environmental regulations, as shown in Fig. 30. Over several decades of effort, great progress has been made in lead-free piezoelectric materials. The key progress in material properties that are typically considered for piezoelectric applications include piezoelectric coefficients (dij), electromechanical coupling coefficients (kij), strain coefficients (Sij), ferroelectric remnant polarization (Pr), and dielectric permittivity (εr). In addition, new developments in energy storage and electrocaloric effect were also exhibited. For perovskite-structured ferroelectrics, the construction of phase boundaries dramatically promotes the development of the piezoelectric properties. For example, the piezoelectric constant of BT and KNN-based ceramics can match or exceed that of PZT ceramics; BFO solid solutions with ABO3 can simultaneously realize high Curie temperature and large piezoelectricity, which is comparable to BiScO3-PbTiO3. Large unipolar strain (0.7%) was first observed in polycrystalline lead-free piezoelectric materials, which is superior to or comparable with that of some antiferroelectric PZT-based ceramics. As a result, we believe that advances in lead-free piezoelectric materials will allow practical application in many types of electronic devices in the future.

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Table 19 Electrical properties of lead-free KNN, BNT, and BFO single crystals prepared by different methods. Material system

Preparation method

Phase structure

K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.622Na0.378NbO3 K0.252Na0.75NbO3 K0.8Na0.2NbO3 K0.47Na0.53NbO3 K0.5Na0.5NbO3 + 0.5%MnO2 K0.5Na0.5NbO3 + 0.5%MnO2 Na0.5K0.5Nb0.995Mn0.005O3 K0.53Na0.47 Nb0.996Mn0.004Oy Na0.5K0.5Nb0.995Mn0.005O3 0.95K0.5Na0.5NbO3-0.05LiNbO3 Bi0.5Na0.5TiO3 Bi0.5Na0.5TiO3 0.96Na0.5Bi0.5TiO3-0.04BaTiO3 0.95Na0.5Bi0.5TiO3-0.05BaTiO3 0.9(Bi,Na)TiO3–0.1BaTiO3 + MnO2 0.88(Bi,Na)TiO3–0.12BaTiO3 0.95Na0.5Bi0.5TiO3-0.05K0.5Bi0.5TiO3 0.92Na0.5Bi0.5TiO3-0.08K0.5Bi0.5TiO3 0.87Na0.5Bi0.5TiO3-0.13K0.5Bi0.5TiO3 0.85Na0.5Bi0.5TiO3-0.15K0.5Bi0.5TiO3 Na0.5Bi0.5TiO3-Bi(Zn0.5Ti0.5)TiO3 BiFeO3 BiFeO3 BiFeO3 + Zn + Mn BiFeO3 BiFeO3

Flux Flux TSSG TSSG TSSG TSSG Flux Slow-cooling FZM Flux Flux Bridgman Flux Flux TSSG SSCG TSSG TSSG TSSG TSSG TSSG TSSG TSSG Flux Flux Flux Flux LDFZ

O O O O O O O O O O O O R R R R T T R R R R R R R R R R

Pr (μC/cm2)

18 12 7.2 27

106 40 52 9 44 54

45.3 54 12.6 8.1 13.8 15 21.5 35 60 36 70 40 ∼ 50

d33 (pC/N)

kt

Ref.

160 148 161 145 104 220 270 350

0.45 – – 0.51 0.67 0.702 – – – – 0.64(k33) 0.61 – – – 0.5(k33) 0.56 – 0.52 0.52 0.51 0.54 –

[468] [469] [470] [471] [472] [473] [474] [475] [476] [477] [478] [479] [480] [481] [482] [483] [484] [485] [486] [486] [486] [487] [488] [489] [490] [491] [492] [493]

161 405

146 207 483 147 175 205 238 121

4. Piezoelectric effect in lead-free piezoelectric perovskite single crystals In studies, single crystals present a great advantage for investigating the intrinsic nature of a material because they are free from the randomly oriented grains found in polycrystalline ceramics. In addition, inspired by the improvement in the electrical properties of textured ceramics, many researchers expected that superior properties could also be obtained in single crystals by domain engineering and controlling the crystal orientation. However, as is well known, it is difficult to synthesize high-quality single crystals of complex components due to limitations in preparation techniques or the obstacle of high leakage current. Below, we present progress into the research of lead-free KNN-, BNT-, BFO-, and BT-based single crystals, considering the processing methods, phase boundaries, and defect engineering. 4.1. Processing methods vs. electrical properties Similarity to the polycrystalline ceramics, the electrical properties of lead-free perovskite piezoelectric single crystals have a close relationship with processing methods too. The processing methods for single crystal mainly include flux method, solid-state crystal growth (SSCG), and top seeded solution growth (TSSG) method. In this part, we mainly introduced the relationship between processing methods and electrical properties in KNN, BNT, and BFO lead-free single crystals, as shown in Table 19. 4.1.1. KNN-based single crystal For pure KNN single crystals prepared by different methods, the d33 value can change from 80 to 220 pC/N, [468–473,494] which was determined by fabrication technique. For Mn-modified KNN single crystals, the enhancement of electrical properties can be also achieved by improving the processing techniques, that is, a large d33 of 350 pC/N can be observed by a slow-cooling method [475]. Interestingly, a large remnant polarization (Pr ∼ 52 μC/cm2 and 106 μC/cm2) can also be obtained in these KNN-Mn single crystals grown by flux method and floating zone method (FZM), respectively [476,478]. It is worth noting that a much larger piezoelectric coefficient (d33 ∼ 405 pC/N) was found in the LN-modified KNN single crystals grown by Bridgman method [479]. Anyway, these d33 values are much higher than that of the corresponding ceramics. 4.1.2. BNT-based single crystal As shown in Table 19, higher remnant polarization can be obtained in pure BNT single crystals grown by flux method [480,481]. However, there is a decreased ferroelectricity and no obvious improvement in piezoelectricity when the BNT-BT and BNT-BKT compositions deviated from phase boundaries [482,483]. Fortunately, a giant d33 of 483 pC/N together with a high Pr of 45.3 μC/cm2 was simultaneously achieved in the Mn-modified 0.90BNT-0.10BT single crystals prepared by the TSSG method [484], and a much higher Pr (∼54 μC/cm2) was also found in 0.88BNT-0.12BT single crystals grown by the same TSSG method [485]. At the same time, 585

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the TSSG method was also widely employed in the fabrication of BNT-BKT single crystals [486,487]. According to the phase structure of (1 − x)BNT-xBKT ceramics, the R-T phase boundary always locates in the composition range of 0.18 ≤ x ≤ 0.22, and therefore the phase structure of (1 − x)BNT-xBKT (x ≤ 0.15) single crystals should be R phase. Excitedly, the enhancement of ferroelectric and piezoelectric properties can be observed when the chemical composition is near the phase boundary region, indicating that the phase boundary may play a more important role in the property enhancement. 4.1.3. BFO-based single crystal It is difficult to prepare high-quality BFO ceramics due to high leakage current. Flux method was widely used to grow BFO single crystals, as shown in Table 19. Compared with BFO ceramics, the ferroelectricity of single crystals can be greatly improved due to the enhanced electrical resistivity, which is almost comparable to the corresponding films [489–492]. For example, a large remnant polarization (Pr ∼ 60 μC/cm2) was shown in BFO single crystals prepared by the flux method [490]. In addition, some new fabrication technique was also employed to fabricate the high-quality BFO single crystals. For example, highly insulating bulk BFO single crystals can be also grown by a newly developed method called the laser –diode heating floating zone (LDFZ), and different ferroelectric behavior (Pr = 40–50 μC/cm2) can be obtained when an electric field was applied in different directions [493]. Previously, it was thought that large ferroelectric response was mainly attributed from the structure transformation-induced by the strain between films and substrates. However, the ferroelectric measurements on single crystals indicate that the large polarization is an intrinsic characteristic for BiFeO3 materials. 4.2. Phase boundaries vs. electrical properties Apart from the effects of the processing method on the electrical behavior of lead-free piezoelectric perovskite single crystals, the phase boundaries were the main factor influencing the electrical performance. Here, we summarize the development of lead-free KNN- and BNT-based single crystals from the view of the phase boundary. 4.2.1. KNN-based single crystal Table 20 presents the electrical properties of KNN-based single crystals. It is clearly shown that the phase boundaries still dominated the piezoelectricity of KNN-based single crystals [496,498,501]. One can see from Table 20 that property enhancement can be observed when an O-T phase boundary was constructed by composition modification. For example, the addition of Li can decrease TO-T, and both a giant d33 of 689 pC/N and a high TC of 432 °C were reported for (K0.45Na0.55)0.96Li0.04NbO3 single crystals [496]. For Ta or Li-Ta co-doped KNN single crystals, the O-T phase boundary can also lead to larger d33 values of 480 pC/N and 630 pC/N and higher k33 values of 0.836 and 0.95, respectively [498,501]. Therefore, the phase boundary greatly affected the piezoelectricity of KNN-based materials, regardless of whether they were in the form of single crystals or ceramics. The d33 vs. TO-T of KNN-based single crystals are presented in Fig. 31(a) to illuminate this view. One conclusion can be made: the lower the TO-T, the higher the d33. In addition, the piezoelectric constant of some typical KNN-based ceramics and single crystals with similar compositions is presented in Fig. 31(b). One can find that the single crystals exhibit a much higher piezoelectric coefficient, and the enhancement of the electrical properties can be achieved by the formation of phase boundaries. As a result, selecting a composition with a phase boundary is crucial for the enhancement of the electrical properties in KNN-based single crystals. 4.2.2. BNT-based single crystal Similar to BNT-based ceramics, the regulation of MPB(I) and MPB(II) can also govern the piezoelectric and strain response in BNTbased single crystals, as shown in Table 21. Superior piezoelectric constants (d33 = 360–590 pC/N) can be obtained in BNT-BT single crystals with an R-T phase boundary [502–504], and an ultrahigh electromechanical response with d33* = 2500 pm/V was found in [0 0 1]-oriented BNT-BT single crystals near the Td due to the polarization extension [505]. In particular, for BNT-BT-KNN single crystals with MPB(II), excellent strain values of 0.57–0.83% and d33* > 1000 pm/V can also be observed [506–508]. These results strongly confirmed the importance of phase boundaries in property enhancement. The phase boundary vs. electrical properties in lead-free single crystals has been systematically discussed, and the enhancement of the electrical properties is closely related to the formation of phase boundaries. For KNN and BNT-based single crystals, superior piezoelectric and strain behaviors can be obtained by driving the phase boundary through tuning the chemical composition. However, few reports have considered the phase boundary in BT- and BFO-based single crystals due to technical issues. As a result, it Table 20 Electrical properties of KNN-based single crystals. Material system

Preparation method

Pr (μC/cm2)

d33 (pC/N)

kt

TO-T (oC)

TC (oC)

Ref.

(K0.5Na0.5)0.98Li0.02NbO3 (K0.45Na0.55)0.96Li0.04NbO3 (K,Na)(Nb,Ta)O3 (K,Na)(Nb,Ta)O3 (K, Na, Li)(Nb,Ta)O3 (K, Na, Li)(Nb,Ta)O3 (K, Na, Li)(Nb,Ta)O3:Mn

TGG SFSSCG TSSG TSSG TSSG TSSG TSSG

21.4 24.1

280 689 200 480 255 354 630

– 0.646 0.836(k33) 0.65 0.451 0.95(k33)

167 83 120 39 79 49 30

430 432 300 161 276 230 235

[495] [496] [497] [498] [499] [500] [501]

6.59 2.67 3.45

586

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Fig. 31. (a) d33 vs. TO-T of KNN-based single crystals; (b) d33 of some typical KNN-based ceramics and single crystals with a similar composition. Table 21 Electrical properties of BNT-based single crystals. Material system

Phase structure

Pr (μC/cm2)

d33 (pC/N)

0.94Na0.5Bi0.5TiO3-0.06BaTiO3 0.93Na0.5Bi0.5TiO3-0.07BaTiO3 0.95Na0.5Bi0.5TiO3-0.05BaTiO3 + Fe 0.944Na0.5Bi0.5TiO3-0.056BaTiO3 0.91BNT-0.06BT-0.03KNN 0.90BNT-0.05BT-0.05KNN 0.92BNT-0.06BT-0.02KNN

R-T R-T R-T PC-T

16.02 41 53.2

360 450 590

PC-T

d33* (pm/V)

Ref.

928 2500 1050 1670 2964

[502] [503] [504] [505] [506] [507] [508]

is expected that exciting results will be obtained if high-quality (Ba,Ca)(Ti,Zr)O3 and BFO-BTO single crystals with multiple phase boundaries can be fabricated. 4.3. Defect engineering vs. electrical properties For BaTiO3 single crystals, the piezoelectric properties are always influenced by the engineering domain configuration [87,89,90]. Similar to relaxor-PT ferroelectric single crystals, much better electrical properties are observed in BT single crystals along the nonpolar direction [87,89,90]. In particular, an extremely high d33 of over 2000 pC/N together with a large strain of 0.6% under a low electric field was attained in 〈7 2 0〉 -oriented BT single crystals [509]. In addition to the engineering domain configuration, Ren et al. reported a giant recoverable electrostrain (S = 0.75%) under a low field of 200 V/mm in ferroelectric aged BT single crystals doped with Fe ions, which is larger than the strain of Pb-based materials, as shown in Fig. 32(a) [510]. A similar effect was later reported in aged Mn-doped BT single crystals with a maximum strain of 0.4% and a double hysteresis loop, while a normal hysteresis loop and butterfly-shaped irrecoverable electrostrain behavior was observed in an un-aged sample, as shown in Fig. 32(b) [511]. Combined with the in situ domain observation under an electric field, the defect symmetry principle was proposed to explain the giant recoverable electrostrain and ferroelectric aging effect, as shown in Fig. 32(c) [511]. All these results indicate that the giant recoverable electrostrain originates from the defect-mediated reversible domain switching mechanism [511], and the volume effect refers to the reorientation of point defects with respect to the PS direction, where the whole volume is the governing mechanism for aging in the hysteresis loop [512]. In conclusion, defect engineering was proved to be an effective method to modulate the electrical behavior in lead-free single crystals. 5. Domain configuration in bulk lead-free piezoelectric perovskites In Sections 3 and 4, we mainly reviewed the piezoelectric/strain properties of lead-free piezoelectric ceramics/single crystals from the view of the macroscopic phase structure. To elaborate the structural and physical mechanisms of property enhancement, local structural information should be carefully understood from the view of the microscopic domain configuration, because the domain configuration can determine the ferroelectric and related properties. Although the piezoelectric activity can be greatly enhanced by engineering the domain configuration in single crystals, this technique is not applicable to lead-free ceramics due to the absence of grain anisotropy. To date, commonly employed approaches for engineering domains or domain walls include composition, temperature, and electric field-induced domain evolution. Thus, in this chapter, we mainly consider the relationship among the domain configuration and electrical properties in bulk lead-free piezoelectric perovskites from three aspects. 587

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Fig. 32. (a) The non-linear electrostain in aged point-defect-doped BaTiO3 single crystal [510]; (b) Left: direct evidence for reversible domainswitching process and its relation to double hysteresis loop in aged Mn-BaTiO3 single-crystal, Right: irreversible domain-switching process and its relation to normal hysteresis loop in un-aged Mn-BaTiO3 single crystal sample [511]; (c) the mechanism is given regarding defect symmetry and related crystal symmetry in perovskite BaTiO3 structure doped with Mn3+ ions at Ti4+ sites [511]. Reproduced from [510], with the permission from Springer Nature. Reproduced from [511], with the permission from The American Physical Society.

5.1. Composition-induced domain evolution In recent years, the domain configurations in lead-free piezoelectric perovskite ceramics have been clearly observed, and distinct domain configuration evolutions and phase structure transformations can be induced by composition modification. For example, the composition dependence of the domain configuration and crystal structure in unpoled BNT-xBT ceramics was investigated by TEM and SAED, and the phase diagram is displayed in Fig. 33(a) [513]. This phase diagram is obviously different from that proposed by

Fig. 33. Phase diagrams for unpoled (1 − x)BNT-xBT ceramics reported by (a) Tan XL [513] and (b) Takenaka [164]. Reproduced from [513], with the permission from AIP Publishing. Copyright [164] 1991 The Japan Society of Appplied Physics. 588

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Fig. 34. TEM images of (1 − x − y)K1-wNawNb1-zSbzO3-yBaZrO3-xBi0.5K0.5HfO3 ceramics with (a) x = 0.03, y = 0.02, w = 0.55, z = 0; and (b) x = 0.03, y = 0.02, z = 0.035, w = 0.4; and (c) d33 vs domain size in KNN-based ceramics [8]. Reproduced from [8], with the permission from Wiley.

Takenaka et al. in 1991, who reported a ferroelectric R-T phase boundary [Fig. 33(b)] [164]. As shown in Fig. 33(a), for compositions with a R phase (x = 0.04), complex ferroelectric domains with sizes of ∼100 nm were dominant in the grains. For compositions with x = 0.06, ∼60% of the grains are composed of nanodomains with short-range AFE order, while a core-shell structure was found in ∼40% of the grains. The core and shell present complex domains and nanodomains, respectively, indicating the formation of coexisting R3c and P4bm phases. For ceramics with x = 0.07 and 0.09, the ferroelectric complex domains disappeared, and only nanodomains were detected, demonstrating that the ferroelectric R3c phase was completely transformed to the relaxor P4bm phase. With a further increase in the BT content (x = 0.11), grains with both lamellar ferroelectric domains (P4mm symmetry) and small nanodomains (P4bm symmetry) can be observed. Finally, the nanodomains disappeared, and only lamellar ferroelectric domains were observed in samples with x > 0.11, indicating the complete transformation from the relaxor P4bm phase to the ferroelectric P4mm phase. The evolution of the domain configuration and crystal structure successfully explains the frequency dispersion in BNTbased ceramics. That is, ferroelectric domains lead to minimum frequency dispersion with a dielectric constant of x ≤ 0.06 or x ≥ 0.11, while relaxor nanodomains result in strong frequency dispersion with 0.07 < x < 0.11. Composition-induced variations in the domain configuration and crystal structure can also be observed in KNN-based ceramics [8,120]. For example, complicated ferroelectric domain patterns with irregularly shaped domain boundaries can be found in (1 − x) KNNS-xLT ceramics in the O phase (x = 0.02). Many nanodomains with a width of 20–50 nm were arranged within microdomains with an increase in x (0.035), and the coexistence of O and T phases in the local structure are confirmed in the CBED pattern. However, typical 90° parallel stripes (lamellar patterns in three-dimensional space) were observed in the composition in the T phase (x = 0.05). The nanodomains in the phase boundary can be believed to provide the main contribution to high piezoelectricity [120]. A similar phenomenon was also reported in other material systems. For example, O-T KNNS-BKH-BZ ceramics with poor piezoelectricity have only a few ferroelectric domains with large domain size [Fig. 34(a)], while the R-T compositions with ultrahigh piezoelectricity (d33 = 570 pC/N) possess many ferroelectric nanodomains with a size of approximately 1.8 nm [Fig. 34(b)] [8]. In addition, the reported data statistics about domain size vs. d33 strongly reflect the importance of a miniaturized domain size, as shown in Fig. 34(c). Domain size engineering is also widely used to modulate the electrical properties of single crystals [474,475]. Generally, enhanced piezoelectricity can be obtained in single crystals with smaller domain sizes. For example, Zhang et al. observed that the addition of Mn is responsible for the decrease in the domain size of KNN single crystals, and its d33 increased from 160 pC/N to 270 pC/N when the domain size decreased from 25 μm to 9 μm [474]. By further optimizing the poling conditions (poling temperature and poling electric field) in Mn-KNN single crystals, a higher d33 (∼350 pC/N) was achieved when the domain size was reduced to 2 μm, as shown in Fig. 35(a) [475]. A similar experimental phenomenon was also reported in BT-based single crystals. For tetragonal BT single crystals with 90° domain walls, different domain sizes can be obtained by a change in the poling process (poling electric field and temperature), and the domain size dependence of the piezoelectric properties is shown in Fig. 35(b) [87]. In particular, Wada et al. suggested that lead-free single crystals with d33 > 1000 pC/N can be achieved when the domain size is decreased below 1 μm, as shown in Fig. 35(c) [90]. As a result, engineering of the domain size is crucial to determine the piezoelectricity of lead-free piezoelectric single crystals. As discussed before, many nanodomains were arranged within the microdomains of (1 − x)KNNS-xLT ceramics (x = 0.035), thus leading to the maximization of the piezoelectric properties [120]. The specific domain configuration was called a “hierarchical nanodomain architecture” and has been observed in other materials, including BZT-BCT and KNNS-BNKH [9,411]. For example, a composition-induced micron-nano-micron evolution in the domain configuration can be observed in BZT-BCT ceramics with a RMPB-T phase transition, and the largest piezoelectricity (d33 ≈ 600 pC/N) comes from the miniaturized hierarchical nanodomains [411]. In addition, the domain configurations of KNNS-BNKH ceramics with a giant d33 of 525 pC/N were also observed by PFM, HRTEM, and CBED, as shown in Fig. 36 [9]. It can be seen from Fig. 36(a) and (b) that lamellar domains with widths of hundreds of 589

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Fig. 35. domain size dependence of d33 for (a) KNN-Mn single crystal [475] and (b) [111] oriented tetragonal BaTiO3 single crystal [87]; and (c) relationship between d33 and domain size (WD) in BaTiO3 single crystals by calculations [90]. Reproduced from [90], with the permission from Taylor & Francis.

Fig. 36. (a) and (b) Lateral piezoresponse force microscopy (LPFM) images for amplitude and phase of the KNNS-BNKH ceramics; (c) TEM brightfield image under the two-beam condition showing the hierarchical nanodomain structure; (d) Enlarged image showing slim nanodomains with a width of 10–30 nm; (e) and (f) CBED pattern showing 4 mm symmetry and 3 m symmetry [9]. [9]-Reproduced by permission of the Royal Society of Chemistry.

nanometers and lengths of several micrometers dominated in the samples. The TEM results further confirm the formation of submicron sized domains [Fig. 36(c)]. Interestingly, parallel stripe nanodomains with widths of 10–30 nm and lengths of 100–300 nm can be observed inside the sub-micron domains, thus resulting in the formation of the hierarchical nanodomain architecture [Fig. 36(d)]. Finally, CBED was employed to further verify the local crystal symmetries inside the hierarchical nanodomains, which showed that (1 1 0)-type 90° tetragonal nanotwins and (1 1 0)-type 71° rhombohedral nanotwins simultaneously appear in the nanodomains. Derived from the coexisting nanoscale structure, the physical origins of the high performance can be ascribed to the facilitated polarization rotation between different states under an external electric field induced by the low domain-wall energy and 590

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Fig. 37. (a) A schematic phase diagram for structural phase transitions in unpoled (1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 ceramics; (b–d) The bright field micrograph for (1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 ceramics with x = 0.06, measured at 25 °C, 140 °C, and 190 °C, respectively; (e–g) The bright field micrograph for (1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 ceramics with x = 0.11, measured at 25 °C, 200 °C, and 250 °C, respectively [514]. Reproduced from [514], with the permission from Wiley.

nearly vanishing polarization anisotropy. As a result, the appearance of miniaturized nanodomains is always accompanied by the formation of macroscopic phase boundaries, and composition-induced nanodomains and phase boundaries were proved to be excellent approaches for property enhancement.

5.2. Temperature-induced domain evolution In addition to the composition, the temperature can also induce the phase structure and domain configuration evolution of ferroelectric materials. As discussed in part 3.2, the piezoelectricity of BNT-based ceramics almost vanishes around Td. To illustrate this issue on the microscale, the temperature-induced domain variation and phase structure evolution were examined by hot-stage TEM and SAED. As shown in Fig. 37(a) [514], the unpoled (1 − x)BNT-xBT ceramics with x = 0.06 possess a mixture of R3c symmetry with complex ferroelectric domains and P4bm symmetry with relaxor nanodomains at room temperature. With an increase in temperature, the ferroelectric domains begin to shrink and nanodomains with P4bm symmetry occupy the whole grain after reaching the depolarization temperature (Td), thus resulting in a temperature-induced phase transition from R3c to P4bm, as shown in Fig. 37(b)–(d). Similar results can also be observed in ceramics with x = 0.11, whose phase boundary evolves into P4mm symmetry in the form of ferroelectric lamellar domains and P4bm symmetry in the form of relaxor nanodomains. During the heating process, the P4bm nanodomains increase at the expense of lamellar domains, demonstrating the P4mm-to-P4bm phase transition, as shown in Fig. 37(e)–(g). The temperature-induced ferroelectric-to-nanodomains evolution explains the deteriorated piezoelectricity around the Td. Temperature-driven domain evolution was also studied in BZT-BCT ceramics with a high piezoelectric response [515]. At room temperature, both wedge-shaped domains (typical pattern for R3c symmetry) and lamellar domains (typical pattern for P4mm symmetry) can be observed in the samples. In particular, many nanodomains with an average width of 10 nm were assembled into the wedge-shaped microdomains. When the temperature increases to 60 °C, the wedge-shaped microdomains gradually disappear, and nanodomains inside the microdomains begin to merge and transform into lamellar domains, indicating the R-T phase transition. With a further increase in temperature, the domain contrast was sharply reduced and then completely disappears at a high temperature (120 °C). In conclusion, the largest piezoelectric response can be obtained in samples with multiple coexisting ferroelectric domains, which explains why the optimum poling temperature is near the corresponding phase transition temperature.

5.3. Electric field-induced domain evolution In recent years, some exciting studies into the electric field-induced domain evolution in lead-free ceramics have been carried out [149,516–527]. For example, the domain configurations of KNN-based ceramics before and after poling were investigated by the chemical etching technique [149,516–521]. In general, complicated domains with many 180° watermarks and 90° herringbones can be observed in the unpoled samples, while simple domains with parallel stripes are always obtained in the ceramics after poling [149,516–521]. Although the domain configuration evolution can be reflected by this technique, it is not conceivable to achieve in situ domain observation under an electric field. Thus, in situ TEM was employed to study the electric field-induced domain evolution. 591

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Fig. 38. The electric field in situ TEM study on the P4bm-to-P4mm phase transition in the 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 ceramic: (a) E = 0 kV/ cm, (b) E = 5 kV/cm, (c) E = 7.5 kV/cm, (d) E = 10 kV/cm, (e) E = 12.5 kV/cm, (f) E = 25 kV/cm [522]. Reproduced from [522], with the permission from Wiley.

5.3.1. Irreversible domain evolution The electric field-induced domain variation and phase transition during poling was observed by in situ TEM in BNT-based materials [522]. As shown in Fig. 38(a), unpoled (1 − x)BNT-xBT ceramics with x = 0.07 are in the relaxor P4bm phase and have nanodomains. With an increase in the electric field (E ≤ 7.5 kV/cm), P4mm lamellar domains formed and increased at the expense of the P4bm nanodomains, indicating that the electric field induced the relaxor P4bm-to-ferroelectric P4mm phase transition [Fig. 38(b) and (c)]. When an electric field of 10 kV/cm was applied, the electric field-induced phase transition continues, and a large ferroelectric domain forms because many of the domain walls disappeared [Fig. 38(d)]. At 12.5 kV/cm, the P4bm-to-P4mm phase transition was complete, and the large ferroelectric domain grew enormously [Fig. 38(e)]. At a much higher electric field (E = 25 kV/ cm), a single domain dominates the whole grain, and only a few 90° lamellar domains can be observed in the grain boundary regions [Fig. 38(f)]. When the electric field was removed, the induced ferroelectric P4mm phase still remained, and the original relaxor P4bm phase could not be detected, indicating that the P4bm-to-P4mm phase transition is irreversible. After that, more detailed investigations into the relationship among electric field-induced domain evolution, phase transition and piezoelectricity in (1 − x)BNT-xBT ceramics with different x contents were performed, as shown in Fig. 39 [523]. For unpoled ceramics with x = 5.5%, the R3c/P4bm phase boundary was obtained, and an increase in the electric field can induce an irreversible phase transition from P4bm to R3c, thus resulting in a poor d33 of 120 pC/N in the R phase [Fig. 39(a)]. For unpoled ceramics with

Fig. 39. The corresponding piezoelectric property d33 as a function of poling field Epol is displayed for (a) x = 5.5%, (b) x = 6%, (c) x = 7%. (d) d33 as a function of composition at Epol = 6.5 kV/mm. (e) The proposed Epol vs x phase diagram for (1 − x)Bi1/2Na1/2TiO3-xBaTiO3 [523]. Reproduced from [523], with the permission from The American Physical Society. 592

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x = 0.06, a mixture of complex ferroelectric domains with R3c symmetry and nanodomains with P4bm symmetry were observed. When an electric field of 3.2 kV/mm was applied, the original P4bm nanodomains completely transformed to ferroelectric P4mm lamellar domains, thus leading to the formation of a new phase boundary of wedge-shaped domains with R3c symmetry and lamellar domains with P4mm symmetry. Correspondingly, the largest piezoelectricity (d33 ≈ 130 pC/N) was seen in the R3C-P4mm phase boundary region. With a further increase in the electric field, the R3c wedge-shaped domain expanded to the whole grain, indicating that the electric field induced the P4mm-to-R3c phase transition, and a decreased piezoelectricity was also observed, as shown in Fig. 39(b). This high electric field-induced P4mm-to-R3c phase transition can also be observed in the sample with x = 0.07, and the greater d33 (∼167 pC/N) can be attributed the resulting ferroelectric R3c/P4mm phase boundary, as shown in Fig. 39(c). The excellent structure-property relationship can also be confirmed by examining d33 vs x with the same poling electric field (6.5 kV/mm). It can be seen from Fig. 39(d) that the ceramics with x = 0.07 have the largest piezoelectricity under Epol = 6.5 kV/mm due to the formation of an R3c-P4mm phase boundary. As a result, the in situ TEM results show that the phase boundary in BNT-BT ferroelectric materials can be destroyed, created, or even replaced by another phase boundary during electric poling, and the enhancement in the piezoelectricity is closely related to the formation of ferroelectric wedge-shaped domains with R3c symmetry and lamellar domains with P4mm symmetry. Similar experimental results can also be observed in KNN-LS ceramics [524]. Therefore, the high piezoelectric activity in KNN-LS ceramics was considered to result from the formation of the monoclinic phase, which originated from the destruction of PPB under high poling field [524].

5.3.2. Reversible domain evolution For (1 − x)BNT-xBT ceramics with x = 0.07, a moderate electric field can induce the formation of a single domain, and this electric field-induced domain transition is irreversible [522]. However, a reversible domain evolution was observed in 0.7BZT0.3BCT ceramics, even though a single domain can also be formed by electric poling [525]. That is, multidomains can reappear with the removal of the electric field. Fig. 40 shows the scheme of the domain evolution as a function of electric field in 0.7BZT-0.3BCT ceramics with a pure R phase at room temperature [525]. With an increase in the electric field, multidomain state (A) reversibly transformed to the nanodomain state and then to a single-domain state. A further increase in the electric field leads to the reappearance of multidomain state (B). This electric field-induced transition of multidomains to a single domain and then to multidomains was also observed in (1 − x)BZT-xBCT ceramics with x = 0.5, which has a R-T phase boundary at room temperature [526]. As a result, it seems that the different piezoelectricity of the BZT-BCT material system can not only be interpreted by the extrinsic contribution of domain switching, the intrinsic contribution of polarization rotation should be also considered. Electric field-induced reversible phase transition was also reported in BNT-based ceramics with strong strain response [14,250]. For example, for the BNT-BT-KNN ceramic system with a high strain value of 0.45% under 8 kV/mm, no domains are visible in the unpoled sample. An increase in the poling electric field can induce the formation of ferroelectric lamellar domains. When the electric field was removed, the lamellar domains disappeared [250]. This electric field-induced reversible phase transition from the nonpolar state to the ferroelectric state was also observed in Sr and Nb co-doped BNKT ceramics, which currently has the highest strain (S ≈ 0.7%) among BNT-based ceramics, as shown in Fig. 41 [14]. The domain configuration evolution with electric field variation in BNT-2.5Nb ceramics can be summarized as follows: relaxor nanodomains (Z0) to ferroelectric lamellar domains (Z2) and then to relaxor nanodomains (Z4). As a result, an in situ TEM study further revealed that the strong strain response originated from the

Fig. 40. Scheme of the domain evolution as a function of electric field in the 0.7BZT-0.3BCT ceramics [525]. Reproduced from [525], with the permission from AIP Publishing. 593

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Fig. 41. Electric field–induced phase transition revealed by strain measurement and in situ TEM observation in BNT-2.5Nb ceramics [14]. (a) Unipolar strain curve; (b–g) In situ TEM bright field images of a representative grain oriented along the [112] zone axis corresponding to conditions Z(0) through Z(5), respectively. The selected area electron diffraction patterns are displayed in (h) for the virgin state (Z0) and in (i) at the peak field (Z2). Reproduced from [14], with the permission from Wiley.

electric field-induced reversible relaxor-to-ferroelectric phase transition. 5.4. Domain configurations vs. electrical properties We systematically reviewed the approaches for domain engineering and/or domain wall engineering in lead-free ceramics, including composition, temperature, and electric field-induced domain configuration evolution. The structural and physical origins for property enhancement have been elaborated effectively from microscopy domain configurations due to the close relationship between domain configuration and phase structure. For example, wedge-shaped domains were commonly observed in grains with ferroelectric R phase (R3c symmetry), while parallel stripe domains (lamellar domains in three dimensional space) or herringbone domains frequently appeared in the samples with ferroelectric T phase (P4mm symmetry), and the nanodomains with short range AFE orders were the typical characteristics for relaxor antiferroelectrics with P4bm symmetry. Here, we will mainly pay attention to the relationship between domain configurations and electrical properties in BNT, KNN, and BT-based ceramics according to the results mentioned above. 5.4.1. BNT-based ceramics According to the phase diagram of (1 − x)BNT-xBT ceramics reported by Tan et al, we can obviously find that composition can induce the domain evolution from complex ferroelectric domains with R3c symmetry to relaxor nanodomains with P4bm symmetry and then to lamellar ferroelectric domains with P4mm symmetry, thus leading to the formation of R3c/P4bm and P4bm/P4mm phase boundary (x = 0.06 and 0.11). Normally, the formation of R3c/P4bm phase boundary in the ceramic with x = 0.06 should contribute to the largest piezoelectric coefficient. However, electric field was proved to induce the domain configuration and crystal structure evolution irreversably. That is, a much higher piezoelectric coefficient (d33 ∼ 167 pC/N) can be obtained in the 0.93BNT-0.07BT ceramic with a newly created ferroelectric P4mm/R3c phase boundary during high field poling. Unfortunately, the 0.94BNT-0.06BT ceramics with R3c/P4bm phase boundary in its virgin state have a lower piezoelectricity (d33 ∼ 121 pC/N) due to the formation of a pure R phase under high field poling. As a result, excellent piezoelectricity can be obtained in the poled ceramics with ferroelectric phase boundaries. In addition, for BNT-based ceramics with giant strain response, reversable electric field-induced domain evolution or phase transition can be obtained, which was the main reason for the strain enhancement. 5.4.2. KNN-based ceramics In recent years, a series of interesting experimental results in piezoelectricity have been achieved in KNN-based ceramics. Microstructure analysis demonstrated that miniaturized ferroelectric nanostructure can be only formed in the phase boundary regions. Especially, the “hierarchical nanodomain architecture” in R-T phase boundary was the structural origin for their high piezoelectric behavior. And the physical origin can be attributed to be the low domain wall energy and nearly vanishing polarization anisotropy, facilitating easy polarization rotation among different states. Therefore, the domain size engineering seems to be an important approach to achieve performance improvement in KNN-based ceramics. Unfortunately, there are few reports about the electric field-induced domain configuration and crystal structure evolution. Some exciting results may be observed by further 594

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investigation in those high performance KNN -based ceramics under field poling. 5.4.3. BT-based ceramics Composition or temperature can also induce the formation of “hierarchical nanodomain architecture” in BT-based ceramics with phase boundary, which was responsible for the large piezoelectric activity. Interestingly, in situ TEM results indicate that a single domain can be observed in the 0.5BZT-0.5BCT ceramics under a certain electric field, indicating that the ultrahigh piezoelectricity may be associated to the domain wall-free state. However, the reversable electric field-induced domain evolution between multidomain and single domain was also reported in the low performance 0.7BZT-0.3BCT ceramics with a pure R phase. Therefore, the contribution of intrinsic effect to the piezoelectricity should be dominated. 6. Physical mechanisms of high piezoelectricity In Sections 3-5, we systematically reviewed the relationship among piezoelectricity, phase structure, and domain configurations in lead-free piezoelectric bulk materials and found that enhanced piezoelectricity can be obtained in the phase boundary regions. However, the origins of the enhanced piezoelectricity for lead-free materials have been debated over the years. Especially, one critical question about why R-T boundaries should be preferred to R-O or O-T boundaries has plagued us for a long time. To deepen our understanding of those lead-free materials and develop a feasible theory to further improve the electrical properties that may then be converted into practical applications, it is essential to explore the underlying physical mechanisms of high piezoelectricity. 6.1. Polarization rotation As early as 2000, Fu and Cohen et al. reported a first principle study of BT single crystal and proposed that an electric fieldinduced polarization rotation can drive a large piezoelectric response [30]. Afterward, the polarization rotation mechanism has been widely used to explain the physical origins of enhanced piezoelectricity for both lead-based and lead-free materials [7,9–11,91,92,528,529]. In general, polarization rotation is closely related to the local phase structure and energy barriers. That is, ultralow energy barriers in multiphase coexistence regions can facilitate the polarization rotation and then lead to improved piezoelectricity. Fig. 42 presents the Landau free energy modeling for a typical BTZ-BCT system with a tricritical-point (TCP) [529]. It can be clearly seen that phase structure can greatly influence the free energy. For example, for the compositions far away from the TCP (x < xTCP and x > xTCP), an anisotropic free energy surface can be obtained, as shown in Fig. 42(a1, b1) and (a3, b3). However, for the composition in the vicinity of the TCP (x = xTCP), the energy barrier between T/O/R and C phases nearly vanishes, and then, an isotropic energy profile can be achieved, as shown in Fig. 42(a2) and (b2). Therefore, a phase coexistence can result in an ultralow energy barrier, which is beneficial to facile polarization rotation and enhanced piezoelectric response. Apart from the BTZ-BCT system, similar phenomena can also be observed in high-performance KNN-based ceramics with an R-T phase boundary by utilizing CBED or spherical aberration (Cs)-corrected STEM [7,9]. The intimate coexistence of R and T phases can lead to nearly vanishing

Fig. 42. (a1), (a2) and (a3) respectively showing the (1 1 0) projection of free energy profiles for the BZT-BCT ceramics with the composition of x < xTCP, x ≈ xTCP and x > xTCP closing to their Curie temperatures. (b1), (b2), and (b3) showing their corresponding 2D free energy profiles [529]. Reproduced from [529], with the permission from Elsevier. 595

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Fig. 43. Domain configurations of (a) BCTZ ceramics [411] and (b) KNN -based ceramics [8,9,80] with different phase structure. Reproduced from [411], with the permission from AIP Publishing. Reproduced from [8], with the permission from Wiley. [9]-Reproduced by permission of the Royal Society of Chemistry. Reprinted with permission from [80], Copyright (2016) Americal Chemical Society.

polarization anisotropy, and then, facilitate easy polarization rotation among different states. As a result, a multiphase coexistence and flat free energy are the dominant factors for high piezoelectric response. 6.2. Domain wall motion Polarization rotation under electric fields or mechanical stress was considered to be an intrinsic contribution to piezoelectricity, which is closely related to lattice distortion. In contrast to polarization rotation, the domain wall motion was deemed as an extrinsic contribution, which is closely related to the lattice rotation in space. It is well accepted that domain wall motion also plays a critical role in piezoelectric ceramics [8,10,12,515,527,530,531]. In general, a minimum domain wall energy barrier induced by miniaturized nanodomains in compositions with multiple coexisting phases can facilitate domain wall motion, and then, result in enhanced electrical properties. Fig. 44 compares the domain configurations of BT and KNN-based ceramics with different phase structures. For BCTZ ceramics, the composition can induce the R-MPB-T phase transition and micro-nano-micro domain evolution. Additionally, the MPB composition with the largest piezoelectricity has a domain hierarchy with miniaturized nanodomains structures [Fig. 43(a)] [411]. Similarly, KNN-based ceramics with the coexistence of multiple phases also possess a smaller domain size and larger piezoelectricity [Fig. 43(b)] [8,9,80]. Therefore, an improved piezoelectricity can be obtained in those piezoelectric ceramics with an R-T phase boundary because of the miniaturized nanodomains, which can greatly reduce the domain wall energy and promote domain wall motion. It is worth noting that the domain wall motion not only appears in compositions with a phase boundary but also in compositions deviating from a phase boundary. For example, the evolution of ferroelectric domains in BZT-xBCT ceramics were investigated by in situ TEM under an applied electric field [525,532]. All studied compositions showed similar electric field-induced domain transformations: a multiple-domain state (A) → nanodomain state (A) → single-domain state → nanodomain state (B) → multiple-domain state (B) [532]. According to these results, it is not appropriate to just consider extrinsic or intrinsic contributions individually. The polarization rotation and domain wall motion may totally have a contribution to the piezoelectric response. The proportion of intrinsic and extrinsic contributions can be estimated by Rayleigh calculations [530]. 6.3. Electric field-induced phase transition In the middle 1990s, Park and Shrout reported that an electric field-induced rhombohedral (R) to tetragonal (T) phase transition is responsible for the large strain of relaxor-PT-based single crystals [44]. In their opinions, the increase in electric field can induce the spontaneous polarization of R crystals along the 〈1 1 1〉 direction inclined to the [0 0 1] direction, thus, leading to the phase transition from R to T [44]. For lead-free piezoelectric materials, the electric field-induced phase transition was also confirmed by in situ 596

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Fig. 44. (a) Evolution of (2 0 0), (2 2 0), and (2 2 2) pseudocubic reflections for NKNSy (y = 0.07, 0.09, and 0.12) samples under different external electric fields. The inset is the sketch of the crystallographic relationship between R and O phase. (b) Phase diagram of poled and unpoled NKNSy ceramics and the inset is the composition dependent piezoelectric properties of NKNSy ceramics [533]. Reproduced from [533], with the permission from AIP Publishing.

TEM and high-power energy XRD techniques. The effect of the electric field on the phase structure of BT materials was systematically discussed in Section 3.4.3. For BNT-based materials, Tan et al. found that an electric field can induce a phase transition from the relaxor P4bm to ferroelectric P4mm or R3c, leading to an improvement in the piezoelectricity of BNT-BT ceramics with ferroelectric phase boundary regions [see Section 5.3]. In particular, the giant strain of BNT-based materials is commonly attributed to the electric field-induced reversible phase transition from the relaxor to ferroelectric state [14]. In addition, an electric field-induced intermediate phase (monoclinic) has also been proposed to play an important role in polarization rotation [40,42,47]. Especially in lead-based materials, the monoclinic phase is considered as a bridge between R and T phases, and the origin of high piezoelectricity can be attributed to the facilitated polarization rotation because of the existence of a monoclinic phase at the R-T phase boundary [40,42,47]. For lead-free piezoelectric materials, the intermediate monoclinic phase has also been observed [533–536]. Especially, Zuo et al. observed that an electric field can induce the monoclinic phase in KNN-based ceramics close to the R-O, O-T, or R-T [533–535]. For example, for (Na0.52K0.48)(Nb1-ySby)O3 ceramics with R-O, ex situ and in situ synchrotron X-ray diffraction and dielectric measurements indicate that an intermediate monoclinic (M) phase was irreversibly induced from the O phase during the poling process, leading to a modified polarization rotation path along R-M-O and improved piezoelectricity (Fig. 44) [533]. For (Na, K)(Nb, Sb)O3-LiTaO3 ceramics with an O-T phase boundary, an electric field can irreversibly transform an initial O phase to a low-symmetry Mc phase and reversibly transform Mc to a T phase. Therefore, both the Mc-T phase coexistence and the phase instability-induced by the reversible Mc-T phase transition lead to an easier polarization rotation, and then, contribute to the enhanced piezoelectric activity in these compositions [534]. As a result, the phase structure evolution under electric fields can also explain the enhanced electrical properties in the compositions with coexisting multiple phases. 6.4. Defect effect The defect effect can cause some interesting experimental phenomena, such as the enhancement of electrical properties, asymmetrical S-E or P-E loops, and so on [537]. As mentioned in Section 4.3, a large strain (S = 0.75%) can be obtained in aged BT single crystals by point-defect-mediated reversible domain switching [510]. Wang et al. also reported the enhancement of piezoelectricity in KNN-based ceramics by an aging and re-poling technique [538]. In addition, a high strain (S = 0.72%) and low hysteresis (H = 36.2%) induced by the defect effect have also been reported in BNKT-SBTZ ceramics, as confirmed by Fig. 45 [296]. The existence of an A vacancy (VA) in the atom columns can be observed through atomic-resolution HAADF-STEM images [Fig. 45(a) and (b)]. Furthermore, a model containing (I) the defect-free state and (II) the state with defects has been proposed [Fig. 45(c)], and the corresponding Landau free energy curves as a function of the order parameter (P) at different electric fields have been provided [Fig. 45(d) and (e)]. It can be found that the state with defects has a lower free energy, which can facilitate the nucleation and growth of the ferroelectric phases and help reduce the magnitude of the electric field required for the relaxor to ferroelectric transition. Furthermore, the existence of defects can also induce a randomly distributed local polarization field, which can smear the transition between relaxor and ferroelectric states, thus leading to a narrow hysteresis loop. As a result, the defect effect seems to be able to change electrical properties. However, it is still very complex to clarify the correlations among defects, the internal electric field, polarization rotation, domain switching, and macroscopic electrical behavior in piezoelectric materials. In conclusion, the universally accepted physical mechanisms for high piezoelectricity were provided, including polarization rotation, domain wall motion, the electric field-induced phase transition, and the defect effect. According to the experimental results 597

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Fig. 45. (a) Atomic-resolution HAADF-STEM image of BNKT-SBTZ6 ceramics along the [0 0 1] zone axis, showing the existence of A-site vacancies (VA) marked by yellow circle. (b) The representative intensity trace along the yellow rectangular frame in (a). (c) Structure model for the BNKTSBTZ6 ceramic along the [0 0 1] zone axis, including (I) the defect-free state and (II) the state with defects. (d) and (e) Landau potential corresponding to two different states, including (I) the defect free state and (II) the state with defects [296]. Reproduced from [296], with the permission from Elsevier.

and a theoretical analysis, the principle of lowest free energy induced by a phase instability or domain wall motion is the most important factor for high piezoelectricity, while the electric field-induced phase transition is the critical factor for the high strain response of BNT-based materials. In general, a phase coexistence is required to achieve the lowest free energy. Especially, the promoted polarization rotation and facilitated domain wall motion usually appear in piezoelectric ceramics with an R-T phase boundary, resulting in the largest piezoelectric response. Therefore, constructing a phase boundary, especially the R-T phase coexistence, is very important to improve the electrical properties of lead-free piezoelectric materials. 7. Applications Recently, several electronic devices have been fabricated using lead-free piezoelectric materials, and especially, some properties can be comparable or superior to the lead-based devices. Previously, a figure of merit analysis for key devices was given out by Rödel and Jo et al. [21,539]. Therefore, in this part, we briefly note the key parameters (d, g, kp, Qm, etc), some tough issues for practical applications (temperature/frequency/fatigue stability), and the development and prospective of some electronic devices. 7.1. Material parameters of concern Depending on the application type, several key parameters as figures of merit are reviewed, such as the piezoelectric charge coefficient (d), piezoelectric strain coefficient (d*), piezoelectric voltage coefficient (g), electromechanical coupling factor (k), and mechanical quality factor (Qm). As follows, we elaborate on these material parameters in terms of different operation modes (nonresonance and resonance). 7.1.1. FOM of non-resonance applications In the non-resonance mode, the piezoelectric coefficients (d, g, d·g, and d*) are usually considered as the figures of merit. Fig. 47 summarizes these related parameters of different materials as a function of Curie temperature and/or depolarization temperature [21]. The piezoelectric charge coefficient (d), the most widely investigated parameter, is very important for piezoelectric sensors because the output signal is proportional to it. In addition, the piezoelectric voltage coefficient (g) can reflect the sensitivity of sensors; the product of d and g is a critical parameter for non-resonant transducer/sensor applications, because it can refer to the energy density of a device. Therefore, KNN and BT-based ceramics have become the candidates showing the most potential for hightemperature and low-temperature piezoelectric sensor/transducer applications, as shown in Fig. 46(a) and (b). At last, BNT and BT -based materials have attracted an abundance of attention in recent years because of their large strain response, where d33* is a figure of merit for actuator applications, as shown in Fig. 46(c). 598

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Fig. 46. (a) d33, (b) d·g, and (c) d33* of KNN, BNT, BT, and PZT-based ceramics as a function of Curie temperature or depolarization temperature [21]. Reproduced from [21], with the permission from Elsevier.

7.1.2. FOM of resonance applications For resonance applications, the resonance frequency is exactly the working frequency. The electromechanical coupling factor (k) and mechanical quality factor (Qm) or the product of k2 and Qm are usually considered as the figures of merit for resonance applications. First, the electromechanical coupling factor (k) can reflect the transfer efficiency between electrical and mechanical energy, which is an important parameter for resonance transducer applications. Generally, the higher the electromechanical coupling factor, the better the performance for resonance applications. The other parameter (Qm) is the opposite of mechanical loss (tan δm) and is important for resonance modes. However, another indicator (Qe), the reverse of dielectric loss (tan δE), is critical for non-resonance modes. Therefore, a higher Qm value can benefit high-power applications because of less energy loss during operation. Finally, the product of k2 and Qm is usually utilized to characterize the performance of a material and to prevent any numerical misleading, which is an important indicator for ultrasonic sensors.

7.1.3. Full set of matrix properties To comprehensively understand the electrical behaviors of lead-free piezoelectric materials, Tables 22 and 23 compare the piezoelectric, dielectric, elastic coefficient, and electromechanical coupling factors between some typical lead-based and lead-free piezoelectric bulk materials. As shown in Table 21, excellent electrical properties can be observed in those relaxor-PT single crystals with an engineering domain configuration [540–542]. For example, a PZN-PT single crystal has a giant piezoelectric coefficient (d33 ≈ 2890 pC/N and d31 ≈ −1455 pC/N), high dielectric constant (ɛ33T ≈ 7700), large elastic compliance constant (s33E ≈ 141), and large electromechanical coupling factors (k33 ≈ 94%) [540]. However, higher piezoelectric voltage coefficients (g33, g15, and g31) can be obtained in Li and Ta co-modified KNN single crystals [500]. In addition, compared with Table 23, the single crystals show Table 22 Electrical parameters of some typical lead-based and lead-free single crystals. Lead-based single crystals Electrical parameters

Lead-free single crystals

PZN-PT [540]

PMN-PT [541]

PIN-PMN-PT [542]

KNN-Li, Ta [500]

BNT-BT [545]

BT [546] 90

dij (pC/N)

d33 d15 d31

2890 159 −1455

2820 146 −1330

2742 232 −1337

354 171 −163

360 162 −113

gij (10−3 Vm/N)

g33 g15 g31

42 6 −21

40 10 −18

43 2.6 −13

50.6 17.5 −23

40 16.7 −12.5

984 2720 7700 2900

680 1434 8200 1600

659 4736 7244 1081

234 887 790 1100

489 877 1021 1099

56 2200 129 4380

c33E c33D

11.51 14.3

10.3 17.4

11.2 17.1

12.5 15.7

8.1 11.4

151 24

sij (10−10 m2/N)

s33E s33D

141 18.5

119.6 11.1

77.8 11.3

27 9.08

27.7 13.3

13.1 5.6

k

kt k33 k31 k15

45% 94% 60% 25%

64% 94% 59% 32%

59% 95% 65% 20%

45% 81% 47% 44%

54% 72% 34% 45%

ɛ

ɛ33

S

ɛ11S ɛ33 ɛ11

cij (10

10

2

N/m )

T T

599

−33.4

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Table 23 Electrical parameters of some typical lead-based and lead-free piezoelectric ceramics. Electrical parameters

Lead-based ceramics

Lead-free ceramics

PZT-8 [544]

KNN-based

PZT-4 [544]

BT-based

BNT-based

KNN-KCN [544]

KNN-Li, Ta [543]

BT [84]

BCTZ [84]

BNT-6BT [164]

BNT-20BKT [165,213]

TC (°C)

TC

300

328

410

320

120

90

288

284

dij (pC/N)

d33 d15 d31

225 330 −97

289 496 −126

90 125 −32

174 200 −50

191 270 −79

546 453 −231

125 194

195

g33 g15 g31

25.4 29 −10.9

25.1 38 −10.7

34.9 35.7 −12.3

20.6 30 −5.9

11.4 18.8 −4.7

15.3 31 −6.5

580 900 1000 1290

635 730 1300 1475

190 310 292 395

361 424 956 776

1419 1269 1898 1622

2390 1652 4050 2732

gij (10

−3

Vm/N)

ɛ

ɛ33

S

ɛ11S ɛ33 ɛ11

T T

cij (1010N/m2)

c33E c33D

12.3 16.1

11.5 15.9

12.9 17.2

13.7 15.5

16.2 18.9

11.3 13.3

sij (10−12 m2/N)

s33E s33D

13.5 8.0

15.5 7.9

9.3 6.4

11 7.4

8.93 6.76

19.7 11.4

k

kp kt k33 k31 k15

0.51 0.48 0.64 0.30 0.55

0.58 0.51 0.70 0.33 0.71

0.36 0.5 0.55 0.21 0.45

0.38 0.34 0.57 0.16 0.43

0.35

0.53 0.42 0.65 0.31 0.48

loss

tan δ

0.4%

0.4%

0.6%

Q

Qm

1000

500

1500

0.49 0.21 0.48

−39

580 733

1030

0.27 0.42 0.55 0.19 0.49 1.3%

0.16 5% 109

much better electrical behaviors (d33 ∼ 354, g33 ∼ 50.6, and k33 ∼ 81%) than those of the KNN-LT ceramics (d33 ∼ 174, g33 ∼ 20.6, and k33 ∼ 57%) [543] and PZT-8 ceramics (d33 ∼ 225, g33 ∼ 25.4, and k33 ∼ 64%) [544]. Therefore, KNN-based single crystals can be considered as one of the most promising candidates in piezoelectric sensors and ultrasonic transducer applications. Table 23 summarizes the full set of matrix properties of some typical lead-based and lead-free ceramics. Compared with hard PZT ceramics, KCN-doped KNN “hard” ceramics possess comparable values of electromechanical coupling factors (k), mechanical quality factor (Qm), and dielectric loss, which are suitable to high-power applications [544]. It is well accepted that acceptor dopants can result in the increase of Qm and the decrease of d, k, and ɛ in some piezoelectric materials, such as Fe- or Mn-doped BNT-based ceramics [547]. Furthermore, BCTZ ceramics with “MPB” have substantially higher dielectric constants and comparable electromechanical coupling factors with respect to the “hard” PZT ceramics and can transfer them into low-temperature piezoelectric sensors, ultrasonic transducers, and supercapacitors [84]. A low piezoelectric response and poor electromechanical properties limit the application of piezoelectric sensor and transducer applications in BNT-based materials (BNT-BT or BNT-BKT) [164,165,213]. However, BNT-based incipient materials have become the most promising candidates in actuator applications because of its large strain response [Section 3.2]. In conclusion, KNN-based materials can become competitive for piezoelectric sensors and ultrasonic transducers in the intermediate temperature regime (100–400 °C), (Ba,Ca)(Ti,Zr) may be suitable to fabricate sensors and actuators in the low temperature regime (T < 100 °C), and BNT-based incipient piezoelectric materials can be employed to fabricate the actuators only. 7.2. Additional considerations For real-world applications, some other functional properties cannot be ignored, such as temperature/fatigue/frequency stability, reproducibility of properties, scale-up production, and so on. 7.2.1. Temperature stability To evaluate the temperature stability, the temperature stability between lead-based and lead-free piezoelectric ceramics was compared [Fig. 47]. Excellent temperature stability can be obtained in PZT-based ceramics because of MPB characteristics [5,52]. However, a temperature sensitivity always appeared in lead-free piezoceramics because of the structural and domain issues [11,156]. For KNN and BT-based ceramics, the maximum properties can be achieved near the phase transition temperature, such as TR-T and TOT, while degraded electrical properties can be observed when deviating from the phase transition temperature. Especially, BT-based ceramics have a very poor temperature stability because of its low Curie temperature, leading to a narrow temperature usage range for piezoelectric devices [11]. In addition, the temperature sensitivity of strain properties can also be found in BNT-based ceramics 600

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Fig. 47. Temperature stability of (a) d33, (b) d33*, and (c) kp or k33 in KNN [156], BT [11,84,436], BNT [62], and PZT [5,52]-based ceramics.

because of the strong effect of the depolarization temperature [62]. In recent years, some research groups have focused on the temperature stability of lead-free piezoelectric ceramics, pointing out that the variations of phase structure and domain configuration under different temperatures are the main factors affecting temperature stability [9,85]. Although the texturing technique [5], composition modification [9], and high external electric field [85] can partly improve the temperature stability of large piezoelectric coefficient (d33*), there is still a long way to go. 7.2.2. Fatigue stability In addition to temperature stability, the reliability during electrical cycling is also a major concern for industrial applications. The fatigue effects (the degradation of electrical properties), the appearance of an internal bias field (Ebias) and the asymmetrical S-E/P-E/ ɛ-E curves during electrical cycling can be commonly observed in piezoelectric materials. According to the literature, fatigue behavior can be affected by some factors, such as measurement conditions (temperature, electric field, frequency, electrodes, and so on) and the material itself (crystallographic structure, microstructure, and so on) [552]. According to different devices requirements, there are two kinds of electrical loading regimes: unipolar loading for multilayer actuators and bipolar loading for ferroelectric memory devices. Fig. 48 compares the bipolar fatigue stability of ferroelectricity in some lead-based and lead-free piezoelectric ceramics. Compared with temperature stability, fatigue-free behavior can be obtained in modified BT and KNN-based ceramics up to 106 cycles, which is superior to soft PZT ceramics, as shown in Fig. 48(a) and (c) [548,551]. However, poor fatigue behavior was obtained in those BNT-based ceramics, as shown in Fig. 49(b) [549,550]. 7.2.3. Frequency stability The frequency stability of piezoelectric materials is also an important indicator for practical applications. In general, electrical properties can be deteriorated with the increase in frequency, resulting in frequency-sensitive ferroelectric or strain behaviors [327]. Fig. 49(a) presents the frequency dependence of P-E loops for BFO ceramics [327]. One can find that the remnant polarization (Pr) and coercive electric field (EC) dramatically decreased with increasing frequency. This phenomenon can be interpreted as follows: the domain wall motion does not have enough time to respond, and then, its contribution to the electrical properties are reduced with increasing frequency. However, the enhancement of frequency stability can also be obtained in some modified lead-free piezoelectric materials, as shown in Fig. 49(b) [115,553]. The physical mechanism for improved frequency stability will be further explored to meet practical applications. 7.2.4. Other conditions Up to now, a lack of reproducibility and large-scale production of lead-free piezoelectric materials have made it difficult to satisfy the demands of electronic devices because that electrical properties are sensitive to compositions, sintering conditions and poling conditions. As we know, the largest property parameters are always found in very narrow composition and sintering temperature ranges, regardless of KNN, BT, BNT, and BFO-based materials. Furthermore, the availability of raw powders is also a key factor and

Fig. 48. Normalized polarization as a function of number of bipolar cycles: (a) PIC 151 and BCTZ [548], (b) BNT-BT [549] and BNT-BT-KNN [550], (c) KNNLT (CZ0) and KNNLT-CZ (CZ5) ceramics [551]. 601

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Fig. 49. (a) Frequency dependence of P-E loops of BFO ceramics [327]; (b) frequency dependence of d33* of PZT and KNN -based ceramics [115,553]. [327]-Reproduced by permission of the Royal Society of Chemistry. Reproduced from [553], with the permission from Wiley. Copyright [115] publication 2011 The Japan Society of Applied Physics.

can greatly influence the reproducibility of lead-free piezoceramics. To assure reproducibility and large-scale production, some issues should be solved before use in practical applications. On the one hand, it is highly expected that high-performance piezomaterials with weak sensitivity to material compositions can be manufactured. One the other hand, it is necessary to ascertain the relationships among final performance (electrical properties and microstructure analysis), raw materials (purity and morphology), and synthesis processes (reaction kinetics and sintering mechanisms). 7.3. Development and prospective of electronic devices Although PZT-based piezoelectrics have dominated the device markets since the 1960s, lead-free piezoelectric materials have the potential applications for use in some electronic devices because of the recent advances in electrical properties. In this section, we address the development and prospective of some electronic devices fabricated by lead-free piezoelectrics. 7.3.1. Piezoelectric sensor applications 7.3.1.1. Piezoelectric energy harvesting (PEH). Piezoelectric energy harvesters are one of the most typical devices using the direct piezoelectric effect. Therefore, the piezoelectric charge coefficient (d) and piezoelectric voltage coefficient (g) can determine the performance of PEH under non-resonance frequencies. The energy density (μ) of PEH can be calculated by the following equation:

μ=

1 F 2 ·(dij ·gij ) ⎛ ⎞ 2 ⎝ A⎠

(13)

where F is the applied force, and A is the area. Therefore, a higher d · g indicates a higher energy density, which is the figure of merit for PEH. In addition, a high k and Qm are also desired by considering the energy convergence efficiency. Therefore, a dimensionless figure of merit (DFOM) for PEH has been proposed, as follows:

dij ·gij ⎤ kij 2·Qm ⎤ ×⎡ DFOM = ⎡ ⎢ sjj E ⎥ ⎢ tan δ ⎥ ⎣ ⎦off ⎣ ⎦on

(14)

In this review, we mainly focus on the PEH fabricated by lead-free bulk materials. In general, KNN and BT-based materials are often employed to fabricate lead-free piezoelectric energy harvesting because of the high electromechanical conversion coefficient (d · g), as shown in Fig. 46(b). In 2014, Byeon et al. investigated the electrical properties of (NaxK1−x)0.96Li0.04(Nb0.90Ta0.10)0.998Zn0.005O3 ceramics by varying the sodium concentration, realizing an optimal d33xg33 value of 10.47 pm2/N (x = 0.56) [554]. In 2016, Hou et al. fabricated Mn-modified KNNL ceramics with a transduction coefficient of 9.31 pm2/N by the sol-gel method, which is comparable to the reported values in KNN single crystals [555]. In addition to KNN, an enhanced transduction coefficient can be observed in BT-based ceramics [556]. For example, a large d33xg33 value of 11.59 pm2/N was reported in Pr-doped BCTZ ceramics [556]. As a result, composition modification and process optimization have been proved to be beneficial to the enhancement of the transduction coefficient. Here, we give out two typical examples of PEH devices fabricated by lead-free KNN and BT-based ceramics (Table 24). The addition of CuO can promote the DFOM because of the larger Qm and lower tan δ of KNN ceramics; then, a high power density of 12 mW/cm3 (93 Hz) under a load resistance of 250 kΩ can be found, which is comparable to a similar sized PZT-based energy harvester [557]. In addition, a high output voltage of 8 V and power of 70 μW can be obtained in the cantilever-type energy harvester [Fig. 50(a)] fabricated by the BCTZ-Cu ceramics under a low resonance frequency of 90 Hz and acceleration of 10 m/s2 [558]. By further increasing acceleration (50 m/s2), a significantly higher output voltage of 25 V and power of 700 μW can be achieved, which is superior to some lead-based ceramics (Table 24) [558]. Therefore, we believe that lead-based PEHs can be effectively replaced by 602

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Table 24 Comparison of power generation characteristics in lead-free and lead-based PEHs. Materials

d33 × g33 (pm2/N)

KNN-Cu BZCT-BCT PZT PZT BS-PT

4.9 4.3

Acceleration (m/s2)

Frequency (Hz)

Load resistance (KΩ)

Output voltage (V)

Output power (μW)

Ref.

10 0.981 5 10

93 90 100 234.5 40

250 500 11.8 220 600

8 8

12 mW/cm3 70 1.6 66.75 13

[557] [558] [559] [560] [561]

Fig. 50. Photographs of (a) piezoelectric energy harvester based on BCTZ-Cu ceramics [558] and (b) AE sensor based on KNN-based ceramics [563]. Reproduced from [558], with the permission from Elsevier. Reproduced from [563], with the permission from AIP publishing.

lead-free KNN and BT-based materials in the future; however, it has to be acknowledged that a long road is ahead should to go to achieve this objective because of some tough issues, such as temperature stability, repeatability, and large-scale production. 7.3.1.2. Other sensor applications. In addition to mechanical energy harvesting, acoustic energy harvesting has attracted some attention in recent years. For example, a resonator fabricated by lead-free K0.475Na0.475Li0.05(Nb0.92Ta0.05Sb0.03)O3 ceramics for acoustic energy harvesting was reported in 2017 [562]. When an acoustic sound pressure of 1 dB was used, the maximum output voltage and power can reach 16.3 V and 0.033 mW, respectively [562]. In addition, a sudden deformation induced by stress can generate a transient elastic wave, which is called as acoustic emission (AE). AE sensors can convert the elastic wave into an electrical signal. Similar to PEH, the piezoelectric properties (d and g) are the most important parameters to determine the performance of AE sensors. In 2007, lead-free AE sensors fabricated by both soft and hard KNN-based ceramics were reported by Lin et al. [Fig. 50(b)] [563]. It was observed that the sensitivity of lead-free AE sensors can be comparable to those of lead-based sensors. For example, the thickness-shear-mode AE sensor based on (Na,K,Li)(Nb,Ta,Zn)O3 ceramics was also reported, and its peak sensitivity can reach 75.61 dB under 68.62 kHz [564]. Therefore, KNN-based ceramics can become the most promising competitors for lead-free piezoelectric sensor applications. 7.3.2. Piezoelectric actuator applications Piezoelectric actuators can be used in many applications, such as precise positioning, active damping control, etc. The piezoelectric coefficient (d33*) as a figure of merit for non-resonance piezoelectric actuator applications can dominate the performance of actuator devices. However, for resonance actuator applications (e.g., ultrasonic motors), the strain (χ) of a piezoelectric material is proportional to the figure of merit d·Q, according to Equation below:

χ=

8 Qm d·E π2

(15)

Therefore, except for a high piezoelectric strain (d), the mechanical quality factor (Qm) is also very important for resonance actuators. 7.3.2.1. Non-resonance actuators. There are several types of non-resonance piezoelectric actuators, such as in-plate actuators, stack actuators, and flextensional actuators. Lead-based materials dominate the actuator market because of their superior electrical properties. For example, several different types of piezoelectric actuators were fabricated using relaxor-PT single crystals, and then, their excellent performance can be realized for various applications [565]. After years of development, the strain properties of leadfree materials have been promoted. In addition, some attempts using lead-free actuators have been conducted, including multilayer ceramic actuators and cymbal actuators. 7.3.2.2. Multilayer ceramic actuators (MLCA). It is commonly considered that an enhanced displacement of piezoelectric actuators can be realized by a multilayer structure. As far as the multilayer technology is concerned, a low applied voltage can generate a high electric field because the thickness of the ceramic layers can be reduced to below 100 μm. For multilayer ceramic actuators, some 603

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Table 25 Performance of some MLCAs based on lead-free materials. Materials

d33* (pm/V)

Inner electrode

Size (mm3)

Displacement (µm)

Input voltage (V)

Ref.

KNN-LT KNN-Li, Ta, Sb-Cu KNNT BNT-BLT-BKT BKT BNKLTN-ST BCTZ BCTS

292

70Ag-30Pd 70Ag-30Pd

240 140 600 S = 0.43% S = 0.156%

Pt 70Ag-30Pd Ag Ag Ag

10 × 10 × 1 30 × 3 × 1 4.5 × 10 × 10 5×5×2 7 × 7 × 1.6

1 36 7 2.1 1 16 2.3 1.73

150 20 150 E = 70 kV/cm E = 100 kV/cm E = 45 kV/cm 300 300

[570] [571] [572] [573] [568] [281] [574] [575]

features of actuator materials have to be considered, such as strain, temperature dependence of strain, displacement, blocking force, and the interaction with electrodes [566]. The strain behaviors of lead-free materials were reviewed systematically in Section 3. In general, a stronger strain response can result in a larger displacement. However, it is still not enough for the operation of an actuator just considering the displacement alone. The blocking force, defined as the maximum force an actuator can generate in a clamped state, is another important parameter to determine the performance and work output of piezoelectric actuators. Dittmer et al. investigated the blocking force in lead-free BNT-BT and BNT-BT-KNN ceramics and found that the largest blocking force, maximum free strain, and the highest work-outputdensity can be obtained in a limited temperature range near the depolarization temperature [567]. Therefore, the high temperature dependence of blocking force and free strain is a tough issue to overcome for actuator applications. Except for strain and force, the interaction with electrodes is also an important concern for multilayer ceramic actuators [568,569]. There are some kinds of electrode materials, such as expensive Pt inner electrodes, as well as silver (Ag) and palladium (Pd) alloy, nickel (Ni) and copper (Cu) metal electrodes. In the assembly process of the devices, it is critical to improve the physical properties of ceramics and the total performance of final devices by depressing the diffusion of electrode material into the ceramic and the formation of secondary phases. Table 25 summarizes the performance of MLCA using lead-free materials as active elements. As early as 2009, an MLCA using a LT-doped KNN ceramic as the active element was fabricated, and a displacement of 1 µm can be obtained under an input voltage of 150 V [570]. In 2017, a substantially higher total displacement of 36 µm was achieved in an MLCA prepared by CuO-doped KNN-Li/ Ta/Sb ceramics [571]. However, it is difficult to get both large displacements and large forces in a small actuator. To solve this issue, researchers from Japan improved their assembly technologies and fabricated tailor-made multilayer piezoelectric actuators (TAMPAs) using KNNT ceramics , and a displacement of ∼7 μm and a force of ∼0.2 N can be observed [see Table 25] [572]. In the future, it is possible to further improve the performance of TAMPAs by developing a double-sized adhesive tape and make it more suitable for practical applications. In addition to KNN-based materials, BNT-based ones are also promising for MLCA because of their large strain properties. In 2010, an MLCA using BNT-BLT-BKT ceramics as active layers and Pt as inner electrodes was fabricated, yielding an electric field-induced strain and longitudinal dynamic displacement of 0.17% and 2.1 µm under 70 kV/cm, respectively. With further increases in the input electric field (E = 80 kV/cm), a larger displacement of 7.74 µm can be obtained [573]. In 2013, a multilayer ceramic actuator device with BKT active layers was fabricated, and the diffusion between ceramic and Ag/Pd electrodes cannot be found; a strain of 0.14% and the longitudinal displacement of 1 µm at 100 kV/cm can be achieved [568]. In 2015, a significantly larger strain (d33* = 600 pm/V), lower hysteresis and displacement of 16 µm under a low electric field of 45 kV/cm were reported in a 10 layered stack-type multilayer actuators using BNKLTN-ST ceramics as active layers, indicating that BNT-based materials are promising for MLCAs in the future [281]. At last, BT-based materials with large strain properties were also utilized to attempt MLCA fabrication. For example, MLCAs fabricated by BCTZ and BCTS ceramics were reported by Yi et al, and the displacement has a close relationship with the active layer and input voltage [574,575]. The schematic diagram of MLCA can be seen in Fig. 51(a) As mentioned previously, KNLNT-CZ ceramics with a core-shell structure have a very gigantic electrostrain, which is promising for actuator applications [158]. Fig. 51(b) shows the photograph of two cantilever-type actuators using KNLNT-CZ and soft PZT ceramics. When an electric field of 1 kV/mm was applied to the KNN cantilever-type actuators, a few millimeters moving distance can be detected, which is longer than that from the PZT cantilever actuator [158]. Therefore, lead-free KNN, BT and BNT-based materials have the potential for use in actuators in the near future. 7.3.2.3. Cymbal actuators. Cymbal actuators (class V flextensional actuator devices) fabricated by lead-free piezoelectric materials have received considerable attention because of their high displacement and moderate generative force. The effective coupling coefficient (keff) and fastest response time (tFRT) generally determine the performance of the cymbal actuator. The former keff, describing the conversion efficiency between electrical and mechanical signals, can be calculated through the resonance frequency (fr) and anti-resonance frequency (fa), as shown below: 2

keff =

f 1−⎜⎛ r ⎟⎞ f ⎝ a⎠

(16)

The later tFRT, defined as the time for a device to achieve the precise response without over-shooting and ringing, is inversely 604

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Fig. 51. Photographs of (a) multilayer ceramic actuator based on BCTS ceramic [575], (b) cantilever-type actuator based on KNLNT-CZ ceramics with core-shell structure [158], (c) cymbal actuator based on BNT-BKT-BT ceramics [577], and (d) ultrasonic motor based on CZ5 [582]. Reproduced from [575,577], with the permission from Elsevier. Reprinted with the permission from [158], Copyright (2012) American Chemical Society. Reproduced from [582], with the permission from The Journal of Sensor Science and Technology.

proportional to fr. Table 26 shows the compared performances of lead-free and lead-based cymbal actuators. Compared with the lead-based PKI802 cymbal actuator, a higher fundamental resonance frequency (fr) of 107.1 kHz, a faster response time (tFRT) of 9.3 µs, and a comparable effective coupling coefficient (keff) of 0.15 can be obtained in the KNN-Li cymbal actuator [576]. In addition, a lead-free cymbal actuator was also prepared using a BNT-BKT-BT ceramic as the driving element [Fig. 51(c)] [577]. It is worth noting that the performance of lead-free cymbal actuators can be comparable to that of lead-based ones using a hard PZT ceramic as the driving element, as shown in Table 26. These results indicate that lead-free piezoelectrics have the potential to become next generation actuator materials. As a result, lead-free bulk materials can become promising candidates for piezoelectric actuators if the tough issues are completely solved, such as low strain response, large hysteresis, high external electric field, and so on.

7.3.2.4. Ultrasonic motors. Ultrasonic motors (USMs) are a type of resonant actuator that can be widely used in mobile equipment, such as digital still cameras and cellular phones. As mentioned before, a high-performance USM should possess both a high piezoelectric coefficient (d) and a large mechanical quality factor (Qm). In addition, some indicators are generally used to characterize the performance of USMs, including revolution speed (Ω0), torque (T0), and efficiency (η). To date, only a few reports regarding USMs driven by lead-free materials have been reported because of the difficulty of simultaneously meeting a large d and high Q. For example, Doshida et al. reported a miniature cantilever-type ultrasonic motor fabricated by lead-free (Sr, Ca)2NaNb5O15 (SCNN) multilayer piezoelectric ceramics (MLPCs), whose performance is comparable to Table 26 Performances of lead-free and lead-based cymbal actuators. Materials

fr (kHz)

keff

tFRT (µs)

Ref.

KNLN8 PKI802

107.1 85.1

0.15 0.16

9.3 11.8

[576]

BNT-BKT-BT5 PZT802 PZT552

94.2 85.1 85.9

0.17 0.16 0.17

10.3 11.8 11.6

[577]

605

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Table 27 Material constants and mechanical properties of ultrasonic motor. Driving elements

SCNN KNN-LN-Cu LF4-BF (high-d USM) KNNL-Cu (high-Qm USM) CZ5

Material constants

Mechanical properties

Ref.

d15

Qm

f (kHz)

Ω0 (rpm)

T0 μN m

η (%)

98 1396 46 1124 103

13.8 34.5 18.1 20.1 45.1

517 486 416 313 5–7

1.4

7

207 328 155 85

41.5 19.6 50

0.6 1.6

[578] [580] [581] [581] [582]

that of an USM made using a PZT-MLPC [578]. Tamura et al. reported a miniature ultrasonic motor using a piezoelectric LiNbO3 single crystal and observed that the crystal vibrator can retain superior performance even under a large vibrational velocity [579]. Although excellent performance can be obtained in such two ultrasonic motors, the increased cost and size of the motors decrease the advantage of ultrasonic motors. Therefore, it is highly anticipated to be able to prepare ultrasonic motors made of a single plate of lead-free ceramics. For example, a shear-mode ultrasonic motor using lead-free KNN-LN-Cu ceramics with a high Qm as the driving element was developed, and the highest revolution speed of 486 rpm was obtained under 34.5 kHz (Table 27) [580]. Takeda et al. fabricated two kinds of rod-type piezoelectric ultrasonic motors (high-d USM and high-Qm USM) using the shear-mode of KNN-based ceramics, and the large-d USM shows a slightly better performance: a revolution speed of 416 rpm, a torque of 41.5 μN m, and an efficiency of 0.6% (Table 26) [581]. In particular, a ring-type USM in an auto-focusing module of a digital camera was fabricated by the shear-mode of a KNN-based ceramic (CZ5) [Fig. 51(d)] [582]. Although the performance of lead-free USMs is far away from those of SCNN and lead-based ones, there is promising of utilizing KNN-based ceramics in ultrasonic motors in the future. 7.3.3. Transducer applications 7.3.3.1. Electroacoustic transducer for non-resonance applications. An electroacoustic transducer is a device that converts an electric signal into an acoustic signal, which can be applied in smart mobile phones, laptops, and so on. As a non-resonance application, higher piezoelectric properties are critical for the performance of electroacoustic transducers. Therefore, multilayer technology has also been used to fabricate electroacoustic transducers [583–585]. For example, a piezoelectric speaker based on 6-layer KNN-Cu multilayered ceramics was fabricated, and an average sound pressure level (SPL) of 75 dB can be obtained in the frequency range of 300 Hz to 20 kHz, which is comparable to a loud stereo radio [583]. In 2014, a flat panel micro speaker was fabricated from three layers of 30-μm-thick KNN-based ceramics by a tape casting and cofiring process, and a higher average SPL of 87 dB can be detected for f = 100 Hz to 20 kHz, which is even better than those of lead-based ceramics under the same conditions [584]. In 2015, a prototype sound speaker based on BNKT-Cu multilayered ceramics was also reported, and the SPL value is similar to the lead-based speaker for 2–20 kHz [585]. In addition to speakers, a buzzer based on KNNLS ceramics was fabricated by our group, and a maximum SPL of 88.8 dB can be obtained under 2.81 kHz [586]. All these results indicate that lead-free KNN and BNT-based materials are suitable to fabricate piezoelectric micro speakers and buzzers. 7.3.3.2. Ultrasound transducer for resonance applications. Ultrasounds are termed as sound waves with frequencies higher than 20 kHz. Ultrasonic transducers (converting an electric signal into an ultrasonic signal) are one kind of important electronic device in ultrasonic diagnostic techniques, which is more effective and non-invasive than computed tomography (CT), X-ray, and magnetic resonance imaging (MRI). Based on different operating frequencies, ultrasonic transducers can be categorized into low-frequency, high frequency (> 20 MHz), and ultrahigh frequency (> 100 MHz). An abundance of attention has been given to lead-free piezoelectric materials for ultrasonic transducer applications because of the high phase transition temperature, large electromechanical coupling coefficient (k) and relatively small acoustic impedance (Z). For transducer applications, bandwidth (BW) and insertion loss (IL) are usually used to characterize the device performance. In general, ultrasonic transducers with a better resolution always exhibit a broader BW and smaller IL. According to the frequency spectrum, the -6 dB BW can be calculated by:

BW = [(fu −f1 )/ fc ] × 100%

(17)

where fu and fl are upper and lower −6 dB frequencies at which the pulse-echo response is one half (−6 dB) the maximum response in the frequency spectrum. fc is the center frequency, which can be calculated by: fc=(fu + fl)/2. The insertion loss (IL), which reflects the sensitivity of the transducers can be calculated by: IL = 20 log(Vi/Vo) (Vi and Vo are the input and output voltages). Table 28 compares the critical property parameters of active elements and their performance indicators as ultrasonic transducers in lead-based and lead-free materials. At the same time, we give out some pictures of ultrasonic transducers with different frequencies [Fig. 52]. Here, we systematically introduced the development of lead-free ultrasonic transducers because they are the most promising alternatives for transducer applications. 7.3.3.2.1. Low-frequency ultrasonic transducer. In 2014, PI Ceramic GmbH prepared a high performance BNT-based lead-free ceramic (PIC 700) and converted it into commercial mass-production [604]. PIC 700 , as an ultrasonic transducer operating in the MHz frequency range, has a strong anisotropic behavior in electromechanical coupling factors and similar transducer performance compared with the conventional material PIC 255, offering suitable conditions for sensor and transducer applications. Apart from PIC 700, low-frequency ultrasonic transducers fabricated by BFO [587], BNT [588], and KNN [589] -based materials have also been 606

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Table 28 Property parameters of active elements and devices performance in LF, HF, and UHF ultrasonic transducers. Active elements

LF transducer

HF transducer

HUF transducer

Property parameters

Devices performance

kt

Z (MRayl)

fc (MHz)

BW (%)

BFO-BTO-Mn ceramic BNT-BKT-BLT-BT disc KNN-LT-LS + Ba PZT-5H

0.375 0.524 0.369

29.8

2.54 2 5.51 5.83

53 10 50.4 59.2

KN, 1–3 compositie KNNLT, 1–3 composite KNN-Mn single crystal KNN-Ta single crystal KNNS-BNKZ ceramic BNT-BKT-BLT ceramic BNT-BT single crystal BCTZ ceramic BSZT ceramic PMN-PT single crystal PZN-PT single crystal PIN-PMN-PT single crystal

0.47 0.655 0.64 0.646 0.55 0.45 0.52 0.41 0.45 0.58 0.53 0.58

30 29 51.8 45.4 37 23 25.39 30 42.2 44 43 60

48.5 89.7 70.2 57.6 56.8 55 46.16 53 76.4 45 45 47

KNN/BNT composite film PZT-5H film PZT-5H sheet

0.34 0.55

170–320 120 128

35–64 60 40

26.85

6.6 26.5 30.2 31.9 30 26.7 36.9 33.9 32.6

Ref. IL (dB)

14

[587] [588] [589] [589]

16 32 31.89 18.7 26 15 17 17

[590] [591] [592] [593] [594] [595] [596] [597] [598] [599] [600] [601]

50–60 41 28

[602] [603] [603]

25.1 91.2

Fig. 52. Photographs of lead free ultrasonic transducers fabricated by: (a) BNT piezoceramic [588], (b) KNNS-BNKZ ceramics [594], and (c) KNN/ BNT composite thick film [602]. Reproduced from [588,602], with the permission from Elsevier. Reproduced from [594], with the permission from Springer Nature.

reported, and the corresponding property parameters are shown in Table 28. Especially, the BW of BFO and KNN-based materials can be comparable to that of PZT-5H, indicating that lead-free piezoelectric materials are promising candidates for LF ultrasonic transducers. Fig. 52(a) compares the photo graphes of low-frequency ultrasonic transducers fabricated by PZT -based (APC 840) and BNT -based ceramics. The BNT -based transducer, when fitted with titanium front and back plates, having axial vibration similar to that of PZT transducer [588]. 7.3.3.2.2. High-frequency ultrasonic transducer. For high-frequency transducers, the sample density is an important parameter because the resonance frequency is inversely proportional to the density. Therefore, a low density means a high working frequency, which can benefit a broader BW and better resolution for the ultrasonic transducer. Lead-free piezoelectrics become promising candidates for high-frequency transducers because of their low density and large k. It can be found from Table 28 that single-crystal KN has a high kt (∼0.47), high phase transition temperature (∼220 °C), low density (∼4.6 g/cm3), and good temperature stability; this indicates the material is a promising candidate for high-frequency transducers. For example, a high-frequency linear-array transducer (30 MHz) based on a KN, 1–3 composite (BW ∼ 50%) was fabricated to acquire high resolution images of the human skin in vivo [590]. Compared with the probe based on PZT ceramics, a comparable performance can be achieved. Besides, the image resolution and penetration depth are superior to the PZT-based probe when operating under 20 MHz. In addition, a high-frequency ultrasonic transducer was fabricated by a 1–3 composite (kt ∼ 0.655) consisting of a KNN-based ceramic and expoxy polymer [Table 28]; the BW at −6 dB is nearly 90% at a center frequency of 29 MHz, which is higher than that of relaxor-PT single crystals [591]. In particular, the KNNS-BNKZ ceramics with large d and k were employed to fabricate a 37-MHz high-frequency needle transducer, and a high BW and IL comparable to lead-based transducers can be found [Fig. 52(b)] [594]. As a result, lead-free piezoelectrics are promising alternatives for high-frequency ultrasonic transducers.

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7.3.3.3. Ultrahigh-frequency ultrasonic transducer. The working frequencies of ultrahigh frequency ultrasonic transducers are more than 100 MHz, making it is very difficult to prepare the corresponding devices out of lead-free materials. As early as 2009, very-highfrequency PZT kerfless linear arrays were fabricated using a PZT-5H film and sheet, and the devices’ performance is shown in Table 27 [603]. In 2013, a single element ultrasonic transducer with significantly higher frequencies (170–320 MHz) was fabricated with KNN/BNT composite thick films [see Fig. 52(c)], and the measured bandwidth can change from 35% to 64%, while a higher insertion loss was obtained [602]. Therefore, more effort should be made to convert lead-free piezoelectric materials into ultrahighfrequency ultrasonic transducers. In conclusion, lead-free piezoelectric materials have been widely investigated to prepare ultrasonic transducers, and commercial mass production was also achieved. We believe that lead-free candidates for high-frequency ultrasonic transducers will completely replace PZT materials in the near future. 7.3.4. High-power applications The figure of merit for high-power applications is the mechanical quality factor, Qm; therefore, “hard” PZT ceramics or relaxor-PT single crystals with high Q can be applied in some high-power applications, such as ultrasonic cleaning [605]. Previously, lead-based materials were generally utilized at a vibration velocity below 1.0 m/s because the mechanical quality factor (Qm) and resonance frequency (fr) can be greatly reduced with an increase in vibration velocity. In contrast, lead-free piezoelectrics, such as bismuth layer-structured ferroelectrics and BNT-based materials, can maintain a large mechanical quality factor, more stable resonance frequency, and smaller dissipation power density up to a higher vibration velocity, resulting in a higher output power density [606–608]. Although lead-free piezoelectric materials have better high-power characteristics than those of lead-based ones, few reports have focused on lead-free high-power devices because of the reliability issue. We believe that lead-free materials with a high Qm will become a promising candidate in high-power applications. Subsequently, we summarize the development and prospective of lead-free bulk materials as piezoelectric sensors, actuators, and transducers, as well as use in high-power applications. (1). Completely replacing PZT ones. Lead-free transducers have attracted great attention, and the corresponding performance can be comparable to that of PZT-based devices. Especially, the mass production of some devices has been realized at present. We believe that lead-free piezoelectric materials for transducer applications can completely replace PZT ones in the near future. (2). Potential for replacing PZT ones. For piezoelectric sensors, lead-free KNN and BT-based ceramics are the potential candidates to replace lead-based ones in terms of piezoelectric energy harvesting if the low values of d·g and the inferior temperature stability can be solved. In addition, high Qm materials are the most promising competitive substances for frequency devices, such as resonators. Especially, bismuth layer-structured ferroelectrics with high Qm and TC have been instituted in commercial applications. As a result, it can be considered that high piezoelectric coefficient and high Qm materials have the potential to replace PZT ones in PEH and frequency device applications in the near future. (3). Cannot currently replace PZT ones. For actuator applications, some devices cannot be replaced by lead-free piezoelectric ceramics in a short time because of some tough issues, such as low strain response, large hysteresis, high external electric field, and inferior temperature stability. Researcher should pay more attention to those tough issues to achieve industry applications in the future. 8. Challenges and outlook In this review, we systematically discussed recent advances in bulk lead-free piezoelectric perovskites, including the piezoelectric effect, physical mechanisms, and potential applications, and the exciting progress has been achieved over the decades. Undoubtedly, the phase boundaries and domain configurations determine the piezoelectric effects of most lead-free piezoelectric perovskite materials. However, some properties of lead-free piezoelectric materials are still inferior to those of lead-based piezoelectrics, even though unique properties were exhibited. Some shortcomings are as follows: (1). Temperature sensitivity of electrical properties For lead-free piezelectrics (KNN, BNT, BFO and BT), the temperature sensitivity of the electrical properties is not completely solved, which is not beneficial for practical applications. Especially, there are only a few papers involved in in-situ temperature stability of small singal d33. Therefore, future work should focus on balancing the piezoelectricity and temperature stability. Some attempts may be considered, such as the construction of “MPB” similar to PZT, modification of domain configuration, etc. (2). Large hysteresis and high driving electric field BNT-based ceramics have a giant strain value with respect to lead-based or other lead-free piezoelectric. Its giant strain originating from MPB(II) shows the preferred candidate for actuator applications. However, the large hysteresis and high driving electric field still hinder their practical applications. For other lead-free piezoelectrics, the strain value should be further improved. 608

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(3). High leakage current Due to the fluctuation of Fe valence, high leakage current always limited the development of BFO-based ceramics. Few advances in piezoelectricity can be found even if the phase boundary can be designed, and therefore the optimization of phase boundaries should be further explored. Fortunately, it is promising for BFO composite transforming into applications by its magnetoelectric effect.

(4). Low Curie temperature or depolarization temperature Although BTO ceramics have a giant d33, the low Curie temperature (TC ≤ 120 °C) limited the practical piezoelectric application. In addition, poor thermal stability also degraded the corresponding piezoelectricity. Similarity to BTO, the lower depolarization temperature, the higher piezoelectric constant. Therefore, the development of BNT-based materials as piezoelectrics was also hindered. Although the shortcoming can be partly solved, no obvious progress can be realized.

(5). Electronic devices By the development of decades, some advances have been observed in lead-free piezoelectric materials, and the corresponding applications are transferring. In some fields, lead-free piezoelectric materials have the potential to replace PZT ones in some electronic devices, such as transducers and PEHs. However, it is still a long way to go to completely achieve the mass-production because of the existing issues, such as temperature/fatigue reliability, repeatability, and so on. In addition to the gap between lead-based and lead-free piezoelectric materials, other crucial scientific problems need to be considered: (1) Intrinsic characteristics of phase boundaries: The intrinsic characteristics of phase boundaries in lead-free piezoelectric materials remain under debate, thus leading to the vague understanding of the physical mechanism of enhanced piezoelectricity. Consequently, it is critical to determine the intrinsic characteristics of the phase boundaries as well as the different phase boundary types (e.g., R-O, O-T, R-T, etc.) in lead-free materials. (2) Structural/physical origins of high piezoelectricity: For lead-based piezoelectric materials with morphotropic phase boundaries, the concepts of polarization rotation and free energy instability have been proposed to interpret the enhancement in the piezoelectric properties. For lead-free piezoelectric materials, phase boundary and domain configuration were considered to be the dominate factors determining the property behavior. However, further investigation into the local structural analysis needs to be carried out. Thus, experiments providing theoretical guidance should be realized. (3) Development of new lead-free piezoelectric materials: Currently, novelty in the design of a material system is not enough, and a gap in comprehensive electrical properties remains between lead-free and lead-based materials. New material systems should be designed to further improve the comprehensive performance of lead-free materials through the design of new phase boundaries similar to those in PZT. Although those shortcomings still exist in lead-free materials, we had to admit that great advances have been achieved after many years development, such as the enhancement of piezoelectricity, strain response, physical mechanism elaboration, and the preparation of some prototype electronic devices. In the further research, we should pay much more attention on the enhancement of comprehensive properties, property/structure evolution under external fields, detailed performance characterization of electronic devices, and so on. We believe that lead-free materials have promising potentials to achieve practical applications in the further.

9. Conclusions From the point of view of the relationships among the phase boundaries, domain configurations and electrical properties, recent advances in lead-free piezoelectric perovskite materials have been reviewed in detail. The piezoelectricity/strain can be greatly affected by the types of phase boundary and domain configuration. However, serious shortcomings still exist in these materials. In particular, some electronic devices have been designed and fabricated according to the characteristics of lead-free piezoelectrics and some of which can be comparable to the lead-based ones. We believe that this review will guide researchers to further improve the electrical behavior from the perspective of phase boundary construction and domain configuration modification and achieve the practical applications of lead-free materials finally.

Acknowledgement We gratefully acknowledge the support of the National Science Foundation of China (NSFC Nos. 51722208, 51472169, 51332003), the Key Technologies Research and Development Program of Sichuan Province (No. 2018JY0007), and the Fundamental Research Funds for the Central Universities (2012017yjsy111). 609

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50]

Jaffe B, Cook WR, Jaffe H. Piezoelectric ceramics. New York: Academic New York; 1971. Cady WG. Piezoelectricity. New York: McGraw-Hill; 1946. p. 1–20. Jaffe B, Roth RS, Marzullo S. Piezoelectric properties of lead zirconate-lead titanate solid-solution ceramics. J Appl Phys 1954;25(6):809–10. Smolenskii GA, Isupov VA, Agranovskaya AI, Krainik NN. New ferroelectrics of complex composition IV. Sov Phys Solid State 1961;2:2651. Saito Y, Takao H, Tani T, Nonoyama T, Takatori K, Homma T, et al. Lead-free piezoceramics. Nature 2004;432(7013):84. Wang X, Wu J, Xiao D, Zhu J, Cheng X, Zheng T, et al. Giant piezoelectricity in potassium-sodium niobate lead-free ceramics. J Am Chem Soc 2014;136(7):2905–10. Wu B, Wu H, Wu J, Xiao D, Zhu J, Pennycook SJ. Giant piezoelectricity and high Curie temperature in nanostructured alkali niobate lead-free piezoceramics through phase coexistence. J Am Chem Soc 2016;138(47):15459. Xu K, Li J, Lv X, Wu J, Zhang X, Xiao D, et al. Superior piezoelectric properties in potassium-sodium niobate lead-free ceramics. Adv Mater 2016;28(38):8519–23. Zheng T, Wu H, Yuan Y, Lv X, Li Q, Men T, et al. The structural origin of enhanced piezoelectric performance and stability in lead free ceramics. Energy Environ Sci 2017;10(2):528–37. Li P, Zhai JW, Shen B, Zhang SJ, Li XL, Zhu FY, et al. Ultrahigh piezoelectric properties in textured (K, Na)NbO3-based lead-free ceramics. Adv Mater 2017;1705171. Liu W, Ren X. Large piezoelectric effect in Pb-free ceramics. Phys Rev Lett 2009;103(25):257602. Liu Y, Chang Y, Li F, Yang B, Sun Y, Wu J, et al. Exceptionally high piezoelectric coefficient and low strain hysteresis in grain-oriented (Ba, Ca)(Ti, Zr)O3 through integrating crystallographic texture and domain engineering. ACS Appl Mater Inter 2017;9(35):29863. Zhang S, Kounga AB, Aulbach E, Ehrenberg H, Rödel J. Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 system. Appl Phys Lett 2007;91(11):112906. Liu X, Tan X. Giant strains in non-textured (Bi1/2Na1/2)TiO3-based lead-free ceramics. Adv Mater 2016;28(3):574–8. Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003;299:14. Lee MH, Kim DJ, Park JS, Kim SW, Song TK, Kim MH, et al. High-performance lead-free piezoceramics with high curie temperatures. Adv Mater 2015;27(43):6976–82. Ibn-Mohammed T, Koh LSC, Reaney IM, Acquaye A, Wang D, Taylor S, et al. Integrated hybrid life cycle assessment and supply chain environmental profile evaluations of lead-based (lead zirconate titanate) versus lead-free (potassium sodium niobate) piezoelectric ceramics. Energy Environ Sci 2016;9. Cross E. Materials science: lead-free at last. Nature 2004;432(7013):24. Jo W, Dittmer R, Acosta M, Zang J, Groh C, Sapper E, et al. Giant electric-field-induced strains in lead-free ceramics for actuator applications-status and perspective. J Electroceram 2012;29(1):71–93. Rödel J, Jo W, Seifert KTP, Anton E, Granzow T, Damjanovic D. Perspective on the development of lead-free piezoceramics. J Am Ceram Soc 2009;92(6):1153–77. Rödel J, Webber KG, Dittmer R, Jo W, Kimura M, Damjanovic D. Transferring lead-free piezoelectric ceramics into application. J Eur Ceram Soc 2015;35(6):1659–81. Rojac T, Bencan A, Malic B, Tutuncu G, Jones JL, Daniels JE, et al. BiFeO3 ceramics: processing, electrical, and electromechanical properties. J Am Ceram Soc 2014;97(7):1993–2011. Wu J, Xiao D, Zhu J. Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries. Chem Rev 2015;115(7):2559–95. Shrout TR, Zhang SJ. Lead-free piezoelectric ceramics: alternatives for pzt? J Electroceram 2007;19(1):113–26. Li J, Wang K, Zhu F, Cheng L, Yao F. (K, Na)NbO3-based lead-free piezoceramics: fundamental aspects, processing technologies, and remaining challenges. J Am Ceram Soc 2013;96(12):3677–96. Wu J, Fan Z, Xiao D, Zhu J, Wang J. Multiferroic bismuth ferrite-based materials for multifunctional applications: ceramic bulks, thin films and nanostructures. Prog Mater Sci 2016;84:335–402. Acosta M, Novak N, Rojas V, Patel S, Vaish R, Koruza J, et al. BaTiO3-based piezoelectrics: fundamentals, current status, and perspectives. Appl Phys Rev 2017;4:41305. Damjanovic D. Contributions to the piezoelectric effect in ferroelectric single crystals and ceramics. J Am Ceram Soc 2005;88(10):2663–76. Davis M, Budimir M, Damjanovic D, Setter N. Rotator and extender ferroelectrics: importance of the shear coefficient to the piezoelectric properties of domainengineered crystals and ceramics. J Appl Phys 2007;101(5):54112. Fu H, Cohen RE. Polarization rotation mechanism for ultrahigh electromechanical response in single-crystal piezoelectrics. Nature 2000;403(6767):281. Shirane G, Takeda A. Phase transitions in solid solutions of PbZrO3 and PbTiO3 (i) small concentrations of PbTiO3. J Phys Soc Jpn 1952;7(1):5–11. Gerson R. Variation in ferroelectric characteristics of lead zirconate titanate ceramics due to minor chemical modifications. J Appl Phys 1960;31(1):188–94. Takahashi S. Effects of impurity doping in lead zirconate-titanate ceramics. Ferroelectrics 1982;41(1):143–56. Amin A, Newnham RE, Cross LE. Effect of elastic boundary conditions on morphotropic Pb(Zr, Ti)O3 piezoelectrics. Phys Rev B Condens Matter 1986;34(3):1595–8. Cao WW, Cross LE. Theoretical model for the morphotropic phase boundary in lead zirconate-lead titanate solid solution. Phys Rev B 1993;47:9. Takahashi S, Hirose S, Uchino K. Stability of PZT piezoelectric ceramics under vibration level change. J Am Ceram Soc 2010;77(9):2429–32. Mishra SK, Pandey D, Singh AP. Effect of phase coexistence at morphotropic phase boundary on the properties of Pb(ZrxTi1-x)O3 ceramics. Appl Phys Lett 1996;69(12):1707–9. Randall CA, Kim N, Kucera JP, Cao W, Shrout TR. Intrinsic and extrinsic size effects in fine-grained morphotropic-phase-boundary lead zirconate titanate ceramics. J Am Ceram Soc 2010;81(3):677–88. Noheda B, Cox DE, Shirane G, Gonzalo JA, Cross LE, Park S. A monoclinic ferroelectric phase in the Pb(Zr1-xTix)O3 solid solution. Appl Phys Lett 1999;74(14):2059–61. Noheda B, Gonzalo JA, Cross LE, Guo R, Park SE, Cox DE, et al. Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr0.52Ti0.48O3. Phys Rev B 2000;61(13):8687–95. Bellaiche L, Garcia A, Vanderbilt D. Finite-temperature properties of Pb(Zr1-xTix)O3 alloys from first principles. Phys Rev Lett 2000;84(23):5427–30. Guo R, Cross LE, Park SE, Noheda B, Cox DE, Shirane G. Origin of the high piezoelectric response in PbZr1-xTixO3. Phys Rev Lett 2000;84(23):5423–6. Damjanovic D. Comments on origins of enhanced piezoelectric properties in ferroelectrics. IEEE Trans Ultrason Ferroelectr Freq Control 2009;56(8):1574. Park S, Shrout TR. Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals. J Appl Phys 1997;82(4):1804–11. Nomura S, Takahashi T, Yokomizo Y. Ferroelectric properties in the system Pb(ZnNb)O3-PbTiO3. J Phys Soc Jpn 1969;27. Shrout TR, Chang ZP, Kim N, Markgraf S. Dielectric behavior of single crystals near the (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 morphotropic phase boundary. Ferroelectrics Lett 1990;3:63–9. Noheda B, Cox DE, Shirane G, Park SE, Cross LE, Zhong Z. Polarization rotation via a monoclinic phase in the piezoelectric 92% PbZn1/3Nb2/3O3-8% PbTiO3. Phys Rev Lett 2001;86(17):3891–4. Ye ZG, Noheda B, Dong M, Cox D, Shirane G. Monoclinic phase in the relaxor-based piezoelectric/ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 system. Phys Rev B 2001;64(18):184114. La-Orauttapong D, Noheda B, Ye ZG, Gehring PM, Toulouse J, Cox DE, et al. Phase diagram of the relaxor ferroelectric (1–x)Pb(Zn1/3Nb2/3)O3-xPbTiO3. Phys Rev B 2002;65(14):144101. Haun MJ, Furman E, Jang SJ, Cross LE. Thermodynamic theory of the lead zirconate-titanate solid solution system, part v: theoretical calculations.

610

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

Ferroelectrics 1989;99(1):63–86. [51] Burianova L, Hana P, Pustka M, Prokopova M, Nosek J. Non-linear properties of PZT ceramics in the wide temperature range. J Eur Ceram Soc 2005;25(12):2405–9. [52] Miclea C, Tanasoiu C, Amarande L, Miclea CF, Plavitu C, Cioangher M, et al. Effect of temperature on the main piezoelectric parameters of a soft PZT ceramic. Rom J Inf Sci Tech 2007;10(3):243–50. [53] Li F, Xu Z, Wei X, Yao X. Temperature- and dc bias field-dependent piezoelectric effect of soft and hard lead zirconate titanate ceramics. J Electroceram 2010;24(4):294–9. [54] Hooker MW. Properties of PZT-based piezoelectric ceramics between 150 and 250 degrees celsius. NASA Langley Technical Report Server; 1998. [55] Sabat RG, Ren W, Yang G, Mukherjee BK. Temperature dependence of the dielectric, elastic and piezoelectric material constants of lead zirconate titanate (PZT) ceramics. Proc SPIE 2006;6170:61700A-1. [56] Viehland D, Amin A, Li JF. Piezoelectric instability in < 011 > -oriented Pb(B1/3IB2/3II)O3-PbTiO3 crystals. Appl Phys Lett 2001;79(7):1006–8. [57] Li F, Zhang S, Xu Z, Wei X, Luo J, Shrout TR. Investigation of electromechanical properties and related temperature characteristics in domain-engineered tetragonal Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals. J Am Ceram Soc 2010;93(9):2731–4. [58] Ren W, Liu S, Mukherjee BK. Piezoelectric properties and phase transitions of < 001 > -oriented Pb(Zn1/3Nb2/3)O3-PbTiO3 single crystals. Appl Phys Lett 2002;80(17):3174–6. [59] Zhang S, Lee SM, Kim DH, Lee HY, Shrout TR. Characterization of high Tc Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 single crystals fabricated by solid state crystal growth. Appl Phys Lett 2007;90(23):232911. [60] Luo N, Zhang S, Li Q, Yan Q, He W, Zhang Y, et al. PMN-PT based quaternary piezoceramics with enhanced piezoelectricity and temperature stability. Appl Phys Lett 2014;104(18):182911. [61] Wang D, Khesro A, Murakami S, Feteira A, Zhao Q, Reaney IM. Temperature dependent, large electromechanical strain in Nd-doped BiFeO3-BaTiO3 lead-free ceramics. J Eur Ceram Soc 2017;37(4):1857–60. [62] Bai W, Bian Y, Hao J, Shen B, Zhai J. The composition and temperature-dependent structure evolution and large strain response in (1–x)(Bi0.5Na0.5)TiO3-xBa (Al0.5Ta0.5)O3 ceramics. J Am Ceram Soc 2013;96(1):246–52. [63] Guo Y, Kakimoto K. Ohsato H. Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3-LiNbO3 ceramics. Appl Phys Lett 2004;85(18):4121–3. [64] Lei C, Ye ZG. New lead-free piezoelectric ceramics derived from the K0.5Na0.5NbO3-AgNbO3 solid solution system. Appl Phys Lett 2008;93(4):183–241. [65] Zheng T, Wu J, Xiao D, Zhu J, Wang X, Lou X. Composition-driven phase boundary and piezoelectricity in potassium-sodium niobate-based ceramics. ACS Appl Mater Inter 2015;7(36):20332–41. [66] Wang X, Wu J, Xiao D, Cheng X, Zheng T, Zhang B, et al. Large in (K, Na)(Nb, Ta, Sb)O3-(Bi, Na, K)ZrO3 lead-free ceramics. J Mater Chem A 2014;2(12):4122–6. [67] Mitsui T, Westphal WB. Dielectric and x-ray studies of CaxBa1-xTiO3 and CaxSr1-xTiO3. Phys Rev 1961;124(5):1354–9. [68] Cheng X, Shen M. Enhanced spontaneous polarization in Sr and Ca co-doped BaTiO3 ceramics. Solid State Commun 2007;141(11):587–90. [69] Kalyani AK, Brajesh K, Senyshyn A, Ranjan R. Orthorhombic-tetragonal phase coexistence and enhanced piezo-response at room temperature in Zr, Sn, and Hf modified BaTiO3. Appl Phys Lett 2014;104(25):252906. [70] Lin D, Xiao D, Zhu J, Yu P. Piezoelectric and ferroelectric properties of [Bi0.5(Na1-x-yKxLiy)0.5]TiO3 lead-free piezoelectric ceramics. Appl Phys Lett 2006;88(6):62901. [71] Liao Y, Xiao D, Lin D, Zhu J, Yu P, Wu L, et al. Synthesis and properties of Bi0.5(Na1-x-yKxAgy)0.5TiO3 lead-free piezoelectric ceramics. Ceram Int 2007;33(8):1445–8. [72] Hussain A, Chang WA, Lee JS, Ullah A, Kim IW. Large electric-field-induced strain in Zr-modified lead-free Bi0.5(Na0.78K0.22)0.5TiO3 piezoelectric ceramics. Sens Actuat A Phys 2010;158(1):84–9. [73] Hussain A, Chang WA, Ullah A, Lee JS, Kim IW. Effects of hafnium substitution on dielectric and electromechanical properties of lead-free Bi0.5(Na0.78K0.22)0.5(Ti1-xHfx)O3 ceramics. Jpn J Appl Phys 2010;49(4):41504. [74] Lee JS, Pham KN, Han HS, Lee HB, Tran VDN. Strain enhancement of lead-free Bi1/2(Na0.82K0.18)1/2TiO3 ceramics by Sn doping. J Kor Phys Soc 2012;60(2):212–5. [75] Do N, Lee H, Yoon C, Kang J, Lee J, Kim I. Effect of ta-substitution on the ferroelectric and piezoelectric properties of Bi0.5(Na0.82 K0.18)0.5TiO3 ceramics. Trans Electr Electron Mater 2011;12(2):64–7. [76] Pham KN, Han BL, Han HS, Kang JK, Lee JS, Ullah A, et al. Dielectric, ferroelectric, and piezoelectric properties of Nb-substituted Bi1/2(Na 0.82K0.18)1/2TiO3 lead-free ceramics. J Kor Phys Soc 2012;60(2):207–11. [77] Eitel RE, Randall CA, Shrout TR, Rehrig PW. New high temperature morphotropic phase boundary piezoelectrics based on Bi(Me)O3-PbTiO3 ceramics. Jpn J Appl Phys 2014;40(10):5999. [78] Zheng T, Wu W, Wu J, Zhu JG, Xiao DQ. Balanced development of piezoelectricity, Curie temperature, and temperature stability in potassium-sodium niobate lead-free ceramics. J Mater Chem C 2016;4(41). [79] Lv X, Li Z, Wu J, Xi J, Gong M, Xiao D, et al. Enhanced piezoelectric properties in potassium-sodium niobate-based ternary ceramics. Mater Des 2016;109:609–14. [80] Xiang L, Wu J, Shuang Y, Xiao D, Zhu J. Identification of phase boundaries and electrical properties in ternary potassium-sodium niobate-based ceramics. ACS Appl Mater Inter 2016;8(29):18943. [81] Cheng X, Wu J, Lou X, Wang X, Wang X, Xiao D, et al. Achieving both giant d33 and high TC in patassium-sodium niobate ternary system. ACS Appl Mater Interf 2014;6(2):750–6. [82] Zheng T, Wu J. Enhanced piezoelectric activity in high-temperature Bi1-x-ySmxLayFeO3 lead-free ceramics. J Mater Chem C 2015;3(15):3684–93. [83] Zhang S, Xia R, Shrout TR, Zang G, Wang J. Piezoelectric properties in perovskite 0.948(K0.5Na0.5)NbO3-0.052LiSbO3 lead-free ceramics. J Appl Phys 2006;100(10):104108. [84] Xue D, Zhou Y, Bao H, Zhou C, Gao J, Ren X. Elastic, piezoelectric, and dielectric properties of Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 Pb-free ceramic at the morphotropic phase boundary. J Appl Phys 2011;109(5):54110. [85] Yao F, Wang K, Jo W, Webber KG, Comyn TP, Ding J, et al. Diffused phase transition boosts thermal stability of high-performance lead-free piezoelectrics. Adv Funct Mater 2016;26(8):1217–24. [86] Zhang S, Li F. High performance ferroelectric relaxor-PbTiO3 single crystals: status and perspective. J Appl Phys 2012;111(3):31301. [87] Wada S. Domain wall engineering in piezoelectric crystals with engineered domain configuration. Handbook of Advanced Dielectric, Piezoelectric and Ferroelectric Materials-Synthesis, Characterization and Applications. Cambridge, England: Woodhead; 2008. p. 266–303. [88] Wada S, Park S, Cross LE, Shrout TR. Engineered domain configuration in rhombohedral PZN-PT single crystals and their ferroelectric related properties. Ferroelectrics 1999;221(1):147–55. [89] Yako K, Kakemoto H, Tsurumi T, Wada S. Domain size dependence of d33 piezoelectric properties for barium titanate single crystals with engineered domain configurations. Mat Sci Eng B 2005;120(1–3):181–5. [90] Yako K. Domain wall engineering in barium titanate single crystals for enhanced piezoelectric properties. Ferroelectrics 2006;334(1):17–27. [91] Li F, Zhang S, Xu Z, Wei X, Shrout TR. Critical property in relaxor-PbTiO3 single crystals-shear piezoelectric response. Adv Funct Mater 2011;21(11):2118–28. [92] Li F, Zhang S, Yang T, Xu Z, Zhang N, Liu G, et al. The origin of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution crystals. Nat Commun 2016;7:13807. [93] Yako K. Enhanced piezoelectric property of barium titanate single crystals by domain wall engineering using patterning electrode. Key Eng Mater 2006;301(1):17–24. [94] Zhang M, Wang K, Du Y, Dai G, Sun W, Li G, et al. High and temperature-insensitive piezoelectric strain in alkali niobate lead-free perovskite. J Am Chem Soc

611

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139]

2017;139(10):3889–95. Guo Y, Kakimoto K. Ohsato H. Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics. Solid State Commun 2004;129(5):279–84. Guo Y, Kakimoto K. Ohsato H. (Na0.5K0.5)NbO3-LiTaO3 lead-free piezoelectric ceramics. Mater Lett 2005;59(2–3):241–4. Hollenstein E, Davis M, Damjanovic D, Setter N. Piezoelectric properties of Li- and Ta-modified (K0.5Na0.5)NbO3 ceramics. Appl Phys Lett 2005;87(18):182905. Zang G, Wang J, Chen H, Su W, Wang C, Qi P, Ming B, Du J, Zheng L, Zhang S, Shrout TR. Perovskite (Na0.5K0.5)1-x(LiSb)xNb1-xO3 lead-free piezoceramics. Appl Phys Lett 2006;88(21):212908. Lin D, Kwok KW, Chan HLW. Structure, piezoelectric and ferroelectric properties of Li- and Sb-modified K0.5Na0.5NbO3 lead-free ceramics. J Appl Phys 2007;102(7):2183–236. Wu J, Wang Y, Xiao D, Zhu J, Pu Z. Effects of ag content on the phase structure and piezoelectric properties of (K0.4-xNa0.52Li0.04Agx)(Nb0.91Ta0.05Sb0.04)O3 leadfree ceramics. Appl Phys Lett 2007;91(13):132914. Lin D, Kwok KW, Chan HLW. Microstructure, phase transition, and electrical properties of (K0.5Na0.5)1-xLix(Nb1-yTay)O3 lead-free piezoelectric ceramics. J Appl Phys 2007;102(3):1183–489. Zhao P, Zhang B, Li J. Enhancing piezoelectric d33 coefficient in Li/Ta-codoped lead-free (Na, K)NbO3 ceramics by compensating Na and K at a fixed ratio. Appl Phys Lett 2007;91(17):172901. Yang Z, Chang Y, Wei L. Phase transitional behavior and electrical properties of lead-free (K0.44Na0.52Li0.04)(Nb0.96-xTaxSb0.04)O3 piezoelectric ceramics. Appl Phys Lett 2007;90(4):42911. Ming B, Wang J, Qi P, Zang G. Piezoelectric properties of (Li, Sb, Ta) modified (Na, K)NbO3 lead-free ceramics. J Appl Phys 2007;101(5):54103. Wu J, Xiao D, Wang Y, Wu W, Zhang B, Zhu J, et al. Microstructure and electrical properties of (Li, Ag, Ta, Sb)-modified (K0.50Na0.50)NbO3 lead-free ceramics with good temperature stability. J Phys D Appl Phys 2008;41(12):125405. Zhang JL, Zong XJ, Wu L, Gao Y, Zheng P, Shao SF. Polymorphic phase transition and excellent piezoelectric performance of (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 lead-free ceramics. Appl Phys Lett 2009;95(2):22909. Hao J, Bai W, Shen B, Zhai J. Improved piezoelectric properties of (KxNa1-x)0.94Li0.06NbO3 lead-free ceramics fabricated by combining two-step sintering. J Alloy Compd 2012;534:13–9. Wongsaenmai S, Ananta S. Yimnirun R. Effect of Li addition on phase formation behavior and electrical properties of (K0.5Na0.5)NbO3 lead free ceramics. Ceram Int 2012;38(1):147–52. Zuo R, Fu J, Lv D. Phase transformation and tunable piezoelectric properties of lead-free (Na0.52K0.48-xLix)(Nb1-x-ySbyTax)O3 system. J Am Ceram Soc 2009;92(1):283–5. Gao Y, Zhang J, Qing Y, Tan Y. Zhang Z, Hao X. Remarkably strong piezoelectricity of lead-free (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 ceramic. J Am Ceram Soc 2011;94(9):2968–73. Du H, Zhou W, Luo F, Zhu D, Qu S, Pei Z. An approach to further improve piezoelectric properties of (K0.5Na0.5)NbO3-based lead-free ceramics. Appl Phys Lett 2007;91(20):202907. Zhao P, Zhang B, Li J. High piezoelectric d33 coefficient in Li-modified lead-free (Na, K)NbO3 ceramics sintered at optimal temperature. Appl Phys Lett 2007;90(24):242909. Wang K, Li J, Liu N. Piezoelectric properties of low-temperature sintered li-modified (Na, K)NbO3 lead-free ceramics. Appl Phys Lett 2008;93(9):92904. Wang K, Li J. Low-temperature sintering of Li-modified (K, Na)NbO3 lead-free ceramics: sintering behavior, microstructure, and electrical properties. J Am Ceram Soc 2010;93(4):1101–7. Wang K, Li J, Zhou J. High normalized strain obtained in Li-modified (K, Na) NbO3 lead-free piezoceramics. Appl Phys Exp 2011;4(6):061501. Dai Y, Zhang X, Zhou G. Phase transitional behavior in K0.5Na0.5NbO3-LiTaO3 ceramics. Appl Phys Lett 2007;90(26):262903. Fu J, Zuo R, Fang X, Liu K. Lead-free ceramics based on alkaline niobate tantalate antimonate with excellent dielectric and piezoelectric properties. Mater Res Bull 2009;44(5):1188–90. Lin D, Kwok KW, Lam KH, Chan HLW. Fast track communication: phase structure and electrical properties of K0.5Na0.5(Nb0.94Sb0.06)O3-LiTaO3 lead-free piezoelectric ceramics. J Phys D Appl Phys 2008;41(5):052002. Fu J, Zuo RZ, Wang XH, Li L. Polymorphic phase transition and enhanced piezoelectric properties of LiTaO3-modified (Na0.52K0.48)(Nb0.93Sb0.07)O3 lead-free ceramics. J Phys D Appl Phys 2009;42:12006. Fu J, Zuo R, Xu Z. High piezoelectric activity in (Na, K)NbO3 based lead-free piezoelectric ceramics: contribution of nanodomains. Appl Phys Lett 2011;99(6):62901. Lin D, Kwok KW, Lam KH, Chan HLW. Structure and electrical properties of K0.5Na0.5NbO3-LiSbO3 lead-free piezoelectric ceramics. J Appl Phys 2007;101(7):74111. Wu J, Xiao D, Wang Y, Zhu J, Yu P, Jiang Y. Compositional dependence of phase structure and electrical properties in (K0.42Na0.58)NbO3-LiSbO3 lead-free ceramics. J Appl Phys 2007;102(11):114113. Zhang S, Xia R, Shrout TR, Zang G, Wang J. Characterization of lead free (K0.5Na0.5)NbO3-LiSbO3 piezoceramic. Solid State Commun 2007;141(12):675–9. Wu J, Wang Y, Xiao D, Zhu J, Yu P, Wu L, et al. Piezoelectric properties of LiSbO3-modified (K0.48Na0.52)NbO3 lead-free ceramics. Jpn J Appl Phys 2007;46(11):7375–7. Wu J, Xiao D, Wang Y, Zhu J, Yu P. Effects of k content on the dielectric, piezoelectric, and ferroelectric properties of 0.95(KxNa1-x)NbO3-0.05LiSbO3 lead-free ceramics. J Appl Phys 2008;103(2):24102. Palei P, Sonia Kumar P. Dielectric ferroelectric and piezoelectric properties of (1–x)[K0.5Na0.5NbO3]-xLiSbO3 ceramics. J Phys Chem Solids 2012;73(7):827–33. Zuo R, Ye C, Fang X. Dielectric and piezoelectric properties of lead free Na0.5K0.5NbO3-BiScO3 ceramics. Jpn J Appl Phys 2007;46:6733–6. Zuo R, Ye C, Fang X. Na0.5K0.5NbO3-BiFeO3 lead-free piezoelectric ceramics. J Phys Chem Solids 2008;69(1):230–5. Zuo R, Fu J, Lv D, Liu Y. Antimony tuned rhombohedral-orthorhombic phase transition and enhanced piezoelectric properties in sodium potassium niobate. J Am Ceram Soc 2010;93(9):2783–7. Zhang B, Wu J, Wang X, Cheng X, Zhu J, Xiao D. Rhombohedral-orthorhombic phase coexistence and electrical properties of Ta and BaZrO3 co-modified (K, Na) NbO3 lead-free ceramics. Curr Appl Phys 2013;13(8):1647–50. Wu J, Hong T, Yuan Y, Xiang L, Wang X, Lou X. Role of antimony in the phase structure and electrical properties of potassium-sodium niobate lead-free ceramics. Rsc Adv 2015;5(19):14575–83. Zuo R, Fu J. Rhombohedral-tetragonal phase coexistence and piezoelectric properties of (NaK)(NbSb)O3-LiTaO3-BaZrO3 lead-free ceramics. J Am Ceram Soc 2011;94(5):1467–70. Cheng X, Wu J, Wang X, Zhang B, Lou X, Wang X, et al. Mediating the contradiction of d33 and TC in potassium-sodium niobate lead-free piezoceramics. ACS Appl Mater Inter 2013;5(21):10409. Cheng X, Wu J, Wang X, Zhang B, Zhu J, Xiao D, et al. Lead-free piezoelectric ceramics based on (0.97-x)K0.48Na0.52NbO3-0.03Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3xB0.5Na0.5TiO3 ternary system. J Appl Phys 2013;114(12):124107. Cheng X, Wu J, Wang X, Zhang B, Zhu J, Xiao D, et al. Giant d33 in (K, Na)(Nb, Sb)O3-(Bi, Na, K, Li)ZrO3 based lead-free piezoelectrics with hightc. Appl Phys Lett 2013;103(5):52906. Wang X, Wu J, Xiao D, Cheng X, Zheng T, Lou X, et al. New potassium-sodium niobate ceramics with a giant d33. ACS Appl Mater Inter 2014;6(9):6177–80. Wu J, Wang Y, Wang H. Phase boundary, poling conditions, and piezoelectric activity and their relationships in (K0.42Na0.58)(Nb0.96Sb0.04)O3(Bi0.5K0.5)0.90Zn0.10ZrO3 lead-free ceramics. RSC Adv 2014;4(110):64835–42. Zheng T, Wu J, Cheng X, Wang X, Zhang B, Xiao D, et al. Wide phase boundary zone, piezoelectric properties, and stability in 0.97(K0.4Na0.6)(Nb1-xSbx) O3–0.03Bi0.5Li0.5ZrO3 lead-free ceramics. Dalton Trans 2014;43(25):9419–26. Zhang B, Wu J, Cheng X, Wang X, Xiao D, Zhu J, et al. Lead-free piezoelectrics based on potassium-sodium niobate with giant d33. ACS Appl Mater Inter 2013;5(16):7718–25.

612

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[140] Zheng T, Wu J, Xiao D, Zhu J, Wang X, Lou X. Potassium-sodium niobate lead-free ceramics: modified strain as well as piezoelectricity. J Mater Chem A 2015;3(5):1868–74. [141] Wu J, Xiao J, Zheng T, Wang X, Cheng X, Zhang B, et al. Giant piezoelectricity of (K, Na)(Nb, Sb)O3-(Bi, Na, K, Pb)ZrO3 ceramics with rhombohedral–tetragonal (R-T) phase boundary. Scripta Mater 2014;88:41–4. [142] Zheng T, Wu J, Cheng X, Wang X, Zhang B, Xiao D, et al. X. High strain in (K0.40Na0.60)(Nb0.955Sb0.045)O3-Bi0.5Na0.50ZrO3 lead-free ceramics with large piezoelectricity. J Mater Chem C 2014;2(41):8796–803. [143] Zheng T, Wu J, Cheng X, Wang X, Zhang B, Xiao D, et al. New potassium-sodium niobate material system: a giant-d33 and high-TC lead-free piezoelectric. Dalton Trans 2014;43(3):11759–66. [144] Rubio-Marcos F, López-Juárez R, Rojas-Hernandez RE, Del Campo A, Razo-Pérez N, Fernandez JF. Lead-free piezoceramics: revealing the role of the rhombohedral-tetragonal phase coexistence in enhancement of the piezoelectric properties. ACS Appl Mater Inter 2015;7(41):23080–8. [145] Yuan Y, Wu J, Tao H, Lv X, Wang X, Lou X. Composition design and electrical properties in (1-y)(K0.40Na0.60)0.985Li0.015(Nb1-xSbx)O3-yBi0.5Na0.5ZrO3 lead-free ceramics. J Appl Phys 2015;117(8):84103. [146] Zheng T, Wu J, Xiao D, Zhu J, Wang X, Xin L, Lou X. Strong piezoelectricity in (1–x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free binary system: identification and role of multiphase coexistence. ACS Appl Mater Inter 2015;7(10):5927–37. [147] Zheng T, Wu J. Enhanced piezoelectricity over wide sintering temperature (400–1050 oC) range in potassium sodium niobate-based ceramics by two step sintering. J Mater Chem A 2015;3(13):6772–80. [148] Tao H, Wu J, Zheng T, Wang X, Lou X. New (1-x)K0.45Na0.55Nb0.96Sb0.04O3-xBi0.5Na0.5HfO3 lead-free ceramics: phase boundary and their electrical properties. J Appl Phys 2015;118(4):44102. [149] Qin Y, Zhang J, Yao W, Lu C, Zhang S. Domain configuration and thermal stability of (K0.48Na0.52)(Nb0.96Sb0.04)O3-Bi0.50(Na0.82 K0.18)0.50ZrO3 piezoceramics with high d33 coefficient. ACS Appl Mater Inter 2016;8(11):7257–65. [150] Zheng T, Wu J. Relationship between poling characteristics and phase boundaries of potassium-sodium niobate ceramics. ACS Appl Mater Inter 2016;8(14):9242. [151] Wang Z, Xiao D, Wu J, Xiao M, Li F, Zhu J. New lead-free (1-x)(K0.5Na0.5)NbO3-x(Bi0.5Na0.5)ZrO3 ceramics with high piezoelectricity. J Am Ceram Soc 2014;97(3):688–90. [152] Cheng X, Wu J, Wang X, Zhang B, Zhu J, Xiao D, et al. New lead-free piezoelectric ceramics based on (K Na)(Nb Ta)O3-Bi(Na K Li)ZrO3. Dalton Trans 2014;43(9):3434–42. [153] Wu B, Wu J, Xiao D, Zhu J. Modification of both d33 and TC in a potassium-sodium niobate ternary system. Dalton Trans 2015;44(48):21141. [154] Lv X, Wu J, Xiao D, Tao H, Yuan Y, Zhu J, et al. (1–x)(K Na)(Nb Ta Sb)O3-Bi (Na K)ZrO3 lead-free ceramics: composition dependence of the phase boundaries and electrical properties. Dalton Trans 2015;44(10):4440–8. [155] Wang K, Yao F, Jo W, Gobeljic D, Shvartsman VV, Lupascu DC, et al. Temperature-insensitive (K, Na)NbO3 -based lead-free piezoactuator ceramics. Adv Funct Mater 2013;23(33):4079–86. [156] Wang R, Wang K, Yao F, Li J, Schader FH, Webber KG, et al. Temperature stability of lead-free niobate piezoceramics with engineered morphotropic phase boundary. J Am Ceram Soc 2015;98(7):2177–82. [157] Zhou JS, Wang K, Yao FZ, Zheng T, Wu J, Xiao D, et al. Multi-scale thermal stability of niobate-based lead-free piezoceramics with large piezoelectricity. J Mater Chem C 2015;3(34):8780–7. [158] Choi SY, Jeong SJ, Lee DS, Kim MS, Lee JS, Cho JH, et al. Gigantic electrostrain in duplex structured alkaline niobates. Chem Mater 2012;24(17):3363–9. [159] Zhang S, Xia R, Shrout TR. Modified (K0.5Na0.5)NbO3 based lead-free piezoelectrics with broad temperature usage range. Appl Phys Lett 2007;91(13):132913. [160] Hiruma Y, Nagata H, Takenaka T. Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics. J Appl Phys 2009;105(8):84112. [161] Sung YS, Kim JM, Cho JH, Song TK, Kim MH, Park TG. Effects of bi nonstoichiometry in (Bi0.5+xNa)TiO3 ceramics. Appl Phys Lett 2011;98(1):12902. [162] Zuo R, Su S, Wu Y, Fu J, Wang M, Li L. Influence of a-site nonstoichiometry on sintering, microstructure and electrical properties of (Bi0.5Na0.5)TiO3 ceramics. Mater Chem Phys 2008;110(2–3):311–5. [163] Naderer M, Kainz T, Schütz D, Reichmann K. The influence of ti-nonstoichiometry in Bi0.5Na0.5TiO3. J Eur Ceram Soc 2014;34(3):663–7. [164] Takenaka T, Maruyama K, Sakata K. (Bi1/2Na1/2)TiO3-BaTiO3 system for lead-free piezoelectric ceramics. Jpn J Appl Phys 1991;30:2236–9. [165] Sasaki A, Chiba T, Mamiya Y, Otsuki E. Dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3 systems. Jpn J Appl Phys 1999;38(9). [166] Herabut A, Safari A. Effects of substitution in A- and B-site cations of Bi0.5Na0.5TiO3. In: Applications of ferroelectrics ISAF'96 proceedings of the tenth IEEE international symposium on IEEE, vol. 1996. p. 775–8. [167] Herabut A, Safari A. Processing and electrochemical properties of (Bi0.5Na0.5)1–1.5xLaxTiO3 ceramics. J Am Ceram Soc 1997;80(11):2954–8. [168] Watanabe Y, Hiruma Y, Nagata H, Takenaka T. Electrical properties and phase transition temperatures of lanthanoid substituted (Bi1/2Na1/2)TiO3 ceramics. Ferroelectrics 2007;358(1):139–43. [169] Liu X, Guo H, Tan X. Evolution of structure and electrical properties with lanthanum content in [(Bi1/2Na1/2)0.95Ba0.05]1-xLaxTiO3 ceramics. J Eur Ceram Soc 2014;34(12):2997–3006. [170] Watanabe Y, Hiruma Y, Nagata H, Takenaka T. Phase transition temperatures and electrical properties of divalent ions (Ca2+, Sr2+ and Ba2+) substituted (Bi1/ 2Na1/2)TiO3 ceramics. Ceram Int 2008;34(4):761–4. [171] Pal V, Dwivedi RK. Effect of rare earth gadolinium substitution on the structural, microstructure and dielectric properties of lead free BNT ceramics. Adv Mater Res 2012;2012(585):200–4. [172] Sun H, Zhang Q, Wang X, Zhang Y. The photoluminescence and electrical properties of lead-free (Bi0. 5Na0.5)TiO3: Pr ceramics. Ceram Int 2014;40(10):15669–75. [173] Watcharapasorn A, Jiansirisomboon S. Grain growth kinetics in Dy-doped Bi0.5Na0.5TiO3 ceramics. Ceram Int 2008;34(4):769–72. [174] Pal V, Thakur OP, Dwivedi RK. Effect of co-substitution of La/Li on structure, microstructure, dielectric and piezoelectric behavior of Bi0.50Na0.50TiO3 ceramics. Ind J Phys 2015;89(2):123–30. [175] Nagata H, Takenaka T. Lead-free piezoelectric ceramics of (BiNa)TiO3-1/2(Bi2O3·Sc2O3) system. Jpn J Appl Phy Reg Pap Short Notes 1997;36:6055–7. [176] Watcharapasorn A, Jiansirisomboon S, Tunkasiri T. Sintering of Fe-doped Bi0.5Na0.5TiO3 at < 1000 °C. Mater Lett 2007;61(14–15):2986–9. [177] Zuo R, Wang H, Ma B, Li L. Effects of Nb5+ doping on sintering and electrical properties of lead-free (Bi0.5Na0.5)TiO3 ceramics. J Mater Sci: Mater Electron 2009;20(11):1140–3. [178] Zhou C, Liu X. Piezoelectric properties and dielectric behavior of Bi1/2Na1/2[Ti1-x(Sb1/2Nb1/2)x]O3 lead-free piezoelectric ceramics. J Electroceram 2007;19(2–3):237–40. [179] Zhou C, Liu X. Effect of b-site substitution of complex ions on dielectric and piezoelectric properties in (Bi1/2Na1/2)TiO3 piezoelectric ceramics. Mater Chem Phys 2008;108(2–3):413–6. [180] Park S, Hong KS. Variations of structure and dielectric properties on substituting a-site cations for Sr2+ in (Na1/2Bi1/2)TiO3. J Mater Res 1997;12(08):2152–7. [181] Chu BJ, Chen DR, Li GR, Yin QR. Electrical properties of Na1/2Bi1/2TiO3-BaTiO3 ceramics. J Eur Ceram Soc 2002;22(13):2115–21. [182] Zhang S, Kounga AB, Aulbach E, Deng Y. Temperature-dependent electrical properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics. J Am Ceram Soc 2008;91(12):3950–4. [183] Daniels JE, Jo W, Rödel J, Jones JL. Electric-field-induced phase transformation at a lead-free morphotropic phase boundary: case study in a 93%(Bi0.5Na0.5)TiO3-7%BaTiO3 piezoelectric ceramic. Appl Phys Lett 2009;95(3):32904. [184] Jo W, Daniels JE, Jones JL, Tan X, Thomas PA, Damjanovic D, et al. Evolving morphotropic phase boundary in lead-free (Bi1/2Na1/2)TiO3-BaTiO3 piezoceramics. J Appl Phys 2011;109(1):14110. [185] Jo W, Schaab S, Sapper E, Schmitt LA, Kleebe H, Bell AJ, et al. On the phase identity and its thermal evolution of lead free (Bi1/2 Na1/2)TiO3-6mol% BaTiO3. J Appl Phys 2011;110(7):74106. [186] Jo W, Rödel J. Electric-field-induced volume change and room temperature phase stability of (Bi1/2Na1/2)TiO3-xmol.%BaTiO3 piezoceramics. Appl Phys Lett

613

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

2011;99(4):42901. [187] Jo W, Daniels J, Damjanovic D, Kleemann W, Rödel J. Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2 Na1/2)TiO30.06BaTiO3. Appl Phys Lett 2013;102(19):192903. [188] Simons H, Daniels J, Jo W, Dittmer R, Studer A, Avdeev M, et al. Electric-field-induced strain mechanisms in lead-free 94%(Bi1/2Na1/2)TiO3-6%BaTiO3. Appl Phys Lett 2011;98(8):82901. [189] Simons H, Daniels JE, Glaum J, Studer AJ, Jones JL, Hoffman M. Origin of large recoverable strain in 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 near the ferroelectricrelaxor transition. Appl Phys Lett 2013;102(6):62902. [190] Maurya D, Murayama M, Pramanick A, Reynolds WT, An K, Priya S. Origin of high piezoelectric response in a-site disordered morphotropic phase boundary composition of lead-free piezoelectric 0.93(Na0.5Bi0.5)TiO3-0.07BaTiO3. J Appl Phys 2013;113(11):114101. [191] Zeng W, Zhou X, Chen J, Liao J, Zhou C, Cen Z, et al. Origin of high piezoelectric activity in perovskite ferroelectric ceramics. Appl Phys Lett 2014;104(24):242910. [192] Anthoniappen J, Lin CH, Tu CS, Chen PY, Chen CS, Chiu SJ, et al. Enhanced piezoelectric and dielectric responses in 92.5%(Bi0.5Na0.5)TiO3-7.5%BaTiO3 ceramics. J Am Ceram Soc 2014;97(6):1890–4. [193] Xu C, Lin D, Kwok KW. Structure, electrical properties and depolarization temperature of (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoelectric ceramics. Solid State Sci 2008;10(7):934–40. [194] Parija B, Badapanda T, Panigrahi S, Sinha TP. Ferroelectric and piezoelectric properties of (1–x)(Bi0.5Na0.5)TiO3-xBaTiO3 ceramics. J Mater Sci: Mater Electron 2013;24(1):402–10. [195] Lidjici H, Lagoun B, Berrahal M, Rguitti M, Hentatti MA, Khemakhem H. XRD, raman and electrical studies on the (1–x)(Na0.5Bi0.5)TiO3-xBaTiO3 lead free ceramics. J Alloy Compd 2015;618:643–8. [196] Kim B, Han S, Kim J, Lee J, Ahn B, Xu Q. Electrical properties of (1–x)(Bi0.5Na0.5)TiO3-xBaTiO3 synthesized by emulsion method. Ceram Int 2007;33(3):447–52. [197] Eerd WV, Damjanovic D, Klein N, Setter N, Trodahl J. Structural complexity of (Na0.5Bi0.5)TiO3-BaTiO3 as revealed by raman spectroscopy. Phys Rev B 2010;82(10):104112. [198] Peng C, Li J, Gong W. Preparation and properties of (Bi1/2Na1/2)TiO3-Ba(Ti, Zr)O3 lead-free piezoelectric ceramics. Mater Lett 2005;59(12):1576–80. [199] Lee W, Lee Y, Tseng M, Huang C, Crystal WuY. structure and ferroelectric properties of (Bi0.5Na0.5)TiO3-Ba(Zr0.05Ti0.95)O3 piezoelectric ceramics. J Am Ceram Soc 2009;92(5):1069–73. [200] Glaum J, Simons H, Acosta M, Hoffman M. Tailoring the piezoelectric and relaxor properties of (Bi1/2Na1/2)TiO3-BaTiO3 via zirconium doping. J Am Ceram Soc 2013;96(9):2881–6. [201] Glaum J, Simons H, Hudspeth J, Acosta M, Daniels JE. Temperature dependent polarization reversal mechanism in 0.94(Bi1/2Na1/2)TiO3-0.06Ba(Zr0.02Ti0.98)O3 relaxor ceramics. Appl Phys Lett 2015;107(23):232906. [202] Glaum J, Zakhozheva M, Acosta M, Aksel E, Kleebe H, Hoffman M, et al. Influence of b-site disorder on the properties of unpoled Bi1/2 Na1/2TiO3-0.06Ba (ZrxTi1-x)O3 piezoceramics. J Am Ceram Soc 2016;99(8):2801–8. [203] Parija B, Badapanda T, Panigrahi S. Morphotropic phase boundary in BNT-BZT solid solution: a study by raman spectroscopy and electromechanical parameters. J Ceram Process Res 2015;16(5):565–71. [204] Tian HY, Wang DY, Lin DM, Zeng JT, Kwok KW, Chan HLW. Diffusion phase transition and dielectric characteristics of Bi0.5Na0.5TiO3-Ba(Hf, Ti)O3 lead-free ceramics. Solid State Commun 2007;142(1–2):10–4. [205] Yang J, Liu P, Bian X, Jing H, Wang Y, Zhang Y, et al. Dielectric, ferroelectric and piezoelectric properties of Bi0.5Na0.5TiO3-(Ba0.7Ca0.3)TiO3 ceramics at morphotropic phase boundary composition. Mat Sci Eng B 2011;176(3):260–5. [206] Jan SU, Zeb A, Milne SJ. Electrical properties of ca-modified Na0.5Bi0.5TiO3-BaTiO3 ceramics. Ceram Int 2014;40(10):15439–45. [207] Lee W, Huang C, Tsao L, Wu Y. Chemical composition and tolerance factor at the morphotropic phase boundary in (Bi0.5Na0.5)TiO3-based piezoelectric ceramics. J Eur Ceram Soc 2009;29(8):1443–8. [208] Wu B, Xiao D, Wu W, Zhu J, Chen Q, Wu J. Microstructure and electrical properties of (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3-modified Bi0.51Na0.50TiO3 lead-free ceramics. Ceram Int 2012;38(7):5677–81. [209] Hiruma Y, Nagata H, Takenaka T. Electrical properties and depolarization temperature of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics. Jpn J Appl Phys 2006;45(12):4493–6. [210] Zhang YR, Li J, Zhang BP. Enhancing electrical properties in NBT-KBT lead-free piezoelectric ceramics by optimizing sintering temperature. J Am Ceram Soc 2008;91(8):2716–9. [211] Zhang Y, Li J, Zhang B, Peng C. Piezoelectric and ferroelectric properties of bi-compensated (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics. J Appl Phys 2008;103(7):74109. [212] Yang Z, Liu B, Wei L, Hou Y. Structure and electrical properties of (1–x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 ceramics near morphotropic phase boundary. Mater Res Bull 2008;43(1):81–9. [213] Moosavi A, Bahrevar MA, Aghaei AR, Ramos P. Algueró M, Amorín H. High-field electromechanical response of Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 across its morphotropic phase boundary. J Phys D Appl Phys 2014;47(47):1–7. [214] Dinh TH, Bafandeh MR, Kang J, Hong C, Jo W, Lee J. Comparison of structural, ferroelectric, and strain properties between A-site donor and acceptor doped Bi1/2(Na0.82K0.18)1/2TiO3 ceramics. Ceram Int 2015;41:S458–63. [215] Nguyen V, Hong C, Lee H, Kong YM, Lee J, Ahn K. Enhancement in the microstructure and the strain properties of Bi1/2(Na, K)1/2TiO3-based lead-free ceramics by Li substitution. J Korean Phys Soc 2012;61(6):895–8. [216] Nguyen V, Han H, Kim K, Dang D, Ahn K, Lee J. Strain enhancement in Bi1/2(Na0.82K0.18)1/2TiO3 lead-free electromechanical ceramics by co-doping with Li and. Ta. J Alloy Compd 2012;511(1):237–41. [217] Kounga AB, Zhang S, Jo W, Granzow T, Rödel J. Morphotropic phase boundary in (1-x)Bi0.5Na0.5TiO3-xK0.5Na0.5NbO3 lead-free piezoceramics. Appl Phys Lett 2008;92(22):222902. [218] Jiao L, Guo F, Wang J, Gu Z, Zhang S, Yang B. Morphotropic phase boundary and electric properties in (1–x)Bi0.5Na0.5TiO3-xBaSnO3 lead-free piezoelectric ceramics. J Mater Sci: Mater Electron 2013;24(10):4080–4. [219] Zhang S, Yan F, Yang B. Morphotropic phase boundary and electrical properties in (1-x)Bi0.5Na0.5TiO3−xBi(Zn0.5Ti0.5)O3 lead-free piezoceramics. J Appl Phys 2010;107(11):114110. [220] Patterson EA, Cann DP. Relaxor to ferroelectric transitions in (Bi1/2 Na1/2)TiO3-Bi(Zn1/2Ti1/2)O3 solid solutions. J Am Ceram Soc 2012;95(11):3509–13. [221] Ullah A, Ishfaq M, Won Ahn C, Ullah A, Ehsan Awan S, Won Kim I. Relaxor behavior and piezoelectric properties of Bi(Mg0.5Ti0.5)O3-modified Bi0.5Na0.5TiO3 lead-free ceramics. Ceram Int 2015;41(9):10557–64. [222] Wang Q, Chen J, Fan L, Song H, Gao W, Rong Y, et al. Preparation and electric properties of Bi0.5Na0.5TiO3-Bi(Al0.5 Ga0.5)O3 lead-free piezoceramics. J Am Ceram Soc 2013;96(12):3793–7. [223] Zhou C, Liu X. Dielectric and piezoelectric properties of bismuth-containing complex perovskite solid solution of Bi1/2Na1/2TiO3−Bi(Mg2/3Nb1/3)O3. J Mater Sci 2008;43(3):1016–9. [224] Hiruma Y, Imai Y, Watanabe Y, Nagata H, Takenaka T. Large electrostrain near the phase transition temperature of (Bi0.5Na0.5)TiO3-SrTiO3 ferroelectric ceramics. Appl Phys Lett 2008;92(26):262904. [225] Krauss W, Schütz D, Mautner FA, Feteira A, Reichmann K. Piezoelectric properties and phase transition temperatures of the solid solution of (1–x) (Bi0.5Na0.5)TiO3-xSrTiO3. J Eur Ceram Soc 2010;30(8):1827–32. [226] Li H, Liu Q, Zhou J, Wang K, Li J, Liu H, et al. Grain size dependent electrostrain in Bi1/2Na1/2TiO3-SrTiO3 incipient piezoceramics. J Eur Ceram Soc 2016;36(11):2849–53. [227] Weyland F, Acosta M, Koruza J, Breckner P, Rödel J, Novak N. Criticality: concept to enhance the piezoelectric and electrocaloric properties of ferroelectrics. Adv Funct Mater 2016;26(40):7326–33.

614

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[228] Hao J, Shen B, Zhai J, Chen H. Effect of BiMeO3 on the phase structure, ferroelectric stability, and properties of lead-free Bi0.5(Na0.80 K0.20)0.5TiO3 ceramics. J Am Ceram Soc 2014;97(6):1776–84. [229] Hiruma Y, Nagata H, Takenaka T. Phase diagrams and electrical properties of (Bi1/2Na1/2)TiO3-based solid solutions. J Appl Phys 2008;104(12):124106. [230] Singh A, Chatterjee R. 0.40% bipolar strain in lead free BNT-KNN system modified with Li, Ta and Sb. J Am Ceram Soc 2013;96(2):509–12. [231] Maqbool A, Hussain A, Rahman JU, Park JK, Park TG, Song JS, et al. Ferroelectric and piezoelectric properties of SrZrO3-modified Bi0.5Na0.5TiO3 lead-free ceramics. T Nonferr Metal Soc 2014;24:146–51. [232] Rahman JU, Hussain A, Maqbool A, Song TK, Kim WJ, Kim SS, et al. Dielectric, ferroelectric and field-induced strain response of lead-free BaZrO3-modified Bi0.5Na0.5TiO3 ceramics. Curr Appl Phys 2014;14(3):331–6. [233] Bai W, Liu F, Li P, Shen B, Zhai J, Chen H. Structure and electromechanical properties in Bi0.5Na0.5TiO3-based lead-free piezoceramics with calculated endmember Bi(Ni0.5Ti0.5)O3. J Eur Ceram Soc 2015;35(13):3457–66. [234] Li F, Zuo R, Zheng D, Li L. Phase-composition-dependent piezoelectric and electromechanical strain properties in (Bi1/2Na1/2)TiO3-Ba(Ni1/2Nb1/2)O3 lead-free ceramics. J Am Ceram Soc 2015;98(3):811–8. [235] Hiruma Y, Nagata H, Takenaka T. Detection of morphotropic phase boundary of (Bi1/2Na1/2)TiO3-Ba(Al1/2Sb1/2)O3 solid-solution ceramics. Appl Phys Lett 2009;95(5):52903. [236] Hiruma Y, Nagata H, Takenaka T. Formation of morphotropic phase boundary and electrical properties of (Bi1/2Na1/2)TiO3-Ba(Al1/2Nb1/2)O3 solid solution ceramics. Jpn J Appl Phys 2009;48(9):8K–9K. [237] Wang L, Zhang C, Ji W, Zhang S, Chen Y. Morphotropic phase boundary in (1–x)Bi0.5Na0.5TiO3-x(Bi0.8La0.2)FeO3 with improved depolarization temperature. Phys Status Solidi (RRL) - Rapid Res Lett 2009;3(7–8):245–7. [238] Chen T, Zhang T, Wang G, Zhou J, Zhang J, Liu Y. Effect of Li0.12Na0.88NbO3 content on the electrical properties of Bi0.50Na0.50TiO3 lead-free piezoelectric ceramics. J Alloy Compd 2012;520:7–10. [239] Malik RA, Kang JK, Hussain A, Ahn CW, Han HS, Lee JS. High strain in lead-free Nb-doped Bi1/2(Na0.84K0.16)1/2TiO3-SrTiO3 incipient piezoelectric ceramics. Appl Phys Expr 2014;7(6):61502. [240] Zhang S, Kounga AB, Jo W, Jamin C, Seifert K, Granzow T, et al. High-strain lead-free antiferroelectric electrostrictors. Adv Mater 2009;21(46):4716–20. [241] Zhang S, Kounga AB, Aulbach E, Granzow T, Jo W, Kleebe H, et al. Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3K0.5Na0.5NbO3. I. Structure and room temperature properties. J Appl Phys 2008;103(3):34107. [242] Zhang S, Wang L, Chen Y, Kounga AB. Phase characteristics and piezoelectric properties in the Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 system. J Am Ceram Soc 2010;93(6):1561–4. [243] Zhang S, Yan F, Yang B, Cao W. Phase diagram and electrostrictive properties of Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 ceramics. Appl Phys Lett 2010;97(12):122901. [244] Jo W, Granzow T, Aulbach E, Rödel J, Damjanovic D. Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics. J Appl Phys 2009;105(9):94102. [245] Daniels JE, Jo W, Rödel J, Honkimäki V, Jones JL. Electric-field-induced phase-change behavior in (Bi0.5Na0.5)TiO3-BaTiO3-(K0.5Na0.5)NbO3: a combinatorial investigation. Acta Mater 2010;58(6):2103–11. [246] Dittmer R, Jo W, Rödel J, Kalinin S, Balke N. Nanoscale insight into lead-free BNT-BT-xKNN. Adv Funct Mater 2012;22(20):4208–15. [247] Groh C, Jo W, Rödel J. Tailoring strain properties of (0.94-x)Bi1/2 Na1/2TiO3-0.06BaTiO3-xK0.5Na0.5NbO3 ferroelectric/relaxor composites. J Am Ceram Soc 2014;97(5):1465–70. [248] Gao F, Dong X, Mao C, Zhang H, Cao F, Wang G. Poling temperature tuned electric-field-induced ferroelectric to antiferroelectric phase transition in 0.89Bi0.5Na0.5TiO3-0.06BaTiO3-0.05K0.5Na0.5NbO3 ceramics. J Appl Phys 2011;110(9):94109. [249] Dul Kin E, Mojaev E, Roth M, Jo W, Granzow T. Acoustic emission study of domain wall motion and phase transition in (1-x-y)Bi0.5Na0.5TiO3-xBaTiO3yK0.5Na0.5NbO3 lead-free piezoceramics. Scripta Mater 2009;60(4):251–3. [250] Kling J, Tan X, Jo W, Kleebe H, Fuess H, Rödel J. In situ transmission electron microscopy of electric field-triggered reversible domain formation in Bi-based lead-free piezoceramics. J Am Ceram Soc 2010;93(9):2452–5. [251] Hao J, Ye C, Shen B, Zhai J. Enhanced electrostricitive properties and thermal endurance of textured (Bi0.5Na0.5)TiO3-BaTiO3-(K0.5Na0.5)NbO3 ceramics. J Appl Phys 2013;114(5):54101. [252] Gao F, Dong X, Mao C, Liu W, Zhang H, Yang L, et al. Energy-storage properties of 0.89Bi0.5Na0.5TiO3-0.06BaTiO3-0.05K0.5Na0.5NbO3 lead-free anti-ferroelectric ceramics. J Am Ceram Soc 2011;94(12):4382–6. [253] Seifert KTP, Jo W, Rödel J. Temperature-insensitive large strain of (Bi1/2 Na1/2)TiO3-(Bi1/2K1/2)TiO3-(K0.5Na0.5)NbO3 lead-free piezoceramics. J Am Ceram Soc 2010;95(3):1392–6. [254] Hao J, Shen B, Zhai J, Liu C, Li X, Gao X. Switching of morphotropic phase boundary and large strain response in lead-free ternary (Bi0.5 Na0.5)TiO3(K0.5Bi0.5)TiO3-(K0.5Na0.5)NbO3 system. J Appl Phys 2013;113(11):114106. [255] Dittmer R, Anton E, Jo W, Simons H, Daniels JE, Hoffman M, et al. Rödel J. A high-temperature-capacitor dielectric based on K0.5 Na0.5NbO3-modified Bi1/2Na1/ 2TiO3-Bi1/2K1/2TiO3. J Am Ceram Soc 2012;95(11):3519–24. [256] Singh A, Chatterjee R. Structural, electrical, and strain properties of stoichiometric 1-x-y(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5TiO3−y(Na0.5K0.5)NbO3 solid solutions. J Appl Phys 2011;109(2):24105. [257] Lin D, Kwok K, Chan H. Structure and electrical properties of Bi0.5Na0.5TiO3-BaTiO3-Bi0.5Li0.5TiO3 lead-free piezoelectric ceramics. Solid State Ionics 2008;178(37):1930–7. [258] Wu L, Xiao D, Zhou F, Teng Y, Li Y. Microstructure, ferroelectric, and, piezoelectric properties of (1-x-y)Bi0.5Na0.5TiO3-xBaTiO3-yBi0.5Ag0.5TiO3 lead-free ceramics. J Alloy Compd 2011;509(2):466–70. [259] Bai W, Li P, Li L, Zhang J, Shen B, Zhai J. Structure evolution and large strain response in BNT-BT lead-free piezoceramics modified with Bi(Ni0.5Ti0.5)O3. J Alloy Compd 2015;649:772–81. [260] Chen J, Wang Y, Zhang Y, Yang Y, Jin R. Giant electric field-induced strain at room temperature in LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3. J Eur Ceram Soc 2017;37(6):2365–71. [261] Maqbool A, Hussain A, Ur Rahman J, Kwon Song T, Kim W, Lee J, et al. Enhanced electric field-induced strain and ferroelectric behavior of (Bi0.5Na0.5)TiO3BaTiO3-SrZrO3 lead-free ceramics. Ceram Int 2014;40(8):11905–14. [262] Rahman JU, Hussain A, Maqbool A, Ryu GH, Song TK, Kim W, et al. Field induced strain response of lead-free BaZrO3-modified Bi0.5Na0.5TiO3-BaTiO3 ceramics. J Alloy Compd 2014;593:97–102. [263] Janbua J, Niemchareon S, Muanghlua R, Vittayakorn N. High strain response of the (1–x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBaSnO3 lead free piezoelectric ceramics system. Ferroelectrics 2016;490(1):13–22. [264] Bai W, Chen D, Huang Y, Shen B, Zhai J, Ji Z. Electromechanical properties and structure evolution in BiAlO3-modified Bi0.5Na0.5TiO3-BaTiO3 lead-free piezoceramics. J Alloy Compd 2016;667:6–17. [265] Shieh J, Wu KC, Chen CS. Switching characteristics of mpb compositions of (Bi0.5Na0.5)TiO3-BaTiO3-(Bi0.5K0.5)TiO3 lead-free ferroelectric ceramics. Acta Mater 2007;55(9):3081–7. [266] Dai Y, Zhang S, Shrout TR, Zhang X. Piezoelectric and ferroelectric properties of li-doped (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 lead-free piezoelectric ceramics. J Am Ceram Soc 2010;93(4):1108–13. [267] Hiruma Yuji, Nagata H, Takenaka T. Phase transition temperatures and piezoelectric properties of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3 lead-free piezoelectric ceramics. Jpn J Appl Phys 2006;45:7409–12. [268] Bai W, Li L, Li W, Shen B, Zhai J, Chen H. Effect of SrTiO3 template on electric properties of textured BNT-BKT ceramics prepared by templated grain growth process. J Alloy Compd 2014;603:149–57. [269] Lin D, Zheng Q, Xu C, Kwok KW. Structure, electrical properties and temperature characteristics of Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-Bi0.5Li0.5TiO3 lead-free

615

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

piezoelectric ceramics. Appl Phys A 2008;93(2).. [270] Hiruma Y, Nagata H, Takenaka T. Depolarization temperature and piezoelectric properties of (Bi1/2Na1/2)TiO3-(Bi1/2Li1/2)TiO3-(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics. Ceram Int 2009;35(1):117–20. [271] Zhou C, Liu X, Li W. Dielectric and piezoelectric properties of BiFeO3 modified Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 lead-free piezoelectric ceramics. Mat Sci Eng B 2008;153(1–3):31–5. [272] Zhou C, Liu X, Li W, Yuan C. Dielectric and piezoelectric properties of Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-BiCrO3 lead-free piezoelectric ceramics. J Alloy Compd 2009;478(1–2):381–5. [273] Fan G, Lu W, Wang X, Liang F. Morphotropic phase boundary and piezoelectric properties of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-KNbO3 lead-free piezoelectric ceramics. Appl Phys Lett 2007;91(20):202908. [274] Zhou C, Liu X, Li W, Yuan C. Microstructure and electrical properties of Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-LiNbO3 lead-free piezoelectric ceramics. J Phys Chem Solids 2009;70(3–4):541–5. [275] Bai W, Li L, Li W, Shen B, Zhai J, Chen H. Phase diagrams and electromechanical strains in lead-free BNT-based ternary perovskite compounds. J Am Ceram Soc 2014;97(11):3510–8. [276] Wu B, Han C, Xiao D, Wang Z, Zhu J, Wu J. Investigation of a new lead-free (0.89-x)(Bi0.5Na0.5)TiO3-0.11(Bi0.5K0.5)TiO3-xBa0.85Ca0.15Ti0.90Zr0.10O3 ceramics. Mater Res Bull 2012;47(11):3937–40. [277] Zhang J, Pan Z, Guo F, Liu W, Ning H, Chen YB, et al. Semiconductor/relaxor 0-3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based leadfree piezoceramics. Nat Commun 2015;6:6615. [278] Zhang J, Pan Z, Nie P, Cui Y, Yang B, Chen J, et al. Bi0.5 Na0.5TiO3: ZnO lead-free piezoelectric composites with deferred thermal depolarization. Appl Phys Lett 2015;106(23):232904. [279] Zhang J, Sun L, Geng XY, Zhang BB, Yuan GL, Zhang ST. Temperature dependent structures and properties of Bi0.5Na0.5TiO3-based lead free piezoelectric composite. Dalton Trans 2016;45(27):10891–6. [280] Peng P, Nie H, Liu Z, Ren W, Cao F, Wang G, et al. Enhanced ferroelectric properties and thermal stability of Mn-doped 0.96(Bi0.5 Na0.5)TiO3-0.04BiAlO3 ceramics. J Am Ceram Soc 2017;100(3):1030–6. [281] Malik RA, Hussain A, Maqbool A, Zaman A, Ahn C, Rahman JU, et al. Temperature-insensitive high strain in lead-free Bi0.5 (Na0.84K0.16)0.5TiO3-0.04SrTiO3 ceramics for actuator applications. J Am Ceram Soc 2015;98(12):3842–8. [282] Yu J, Chu S, Song X, Gao H, Pan J, Hao J. Temperature-insensitive strain behavior in 0.99[(1–x)Bi0.5(Na0.80K0.20)0.5TiO3-x BiFeO3]-0.01Ta lead-free piezoelectric ceramics. Int J Appl Ceram Tec 2017;14(4):623–9. [283] Wang K, Hussain A, Jo W, Rödel J. Temperature-dependent properties of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-SrTiO3 lead-free piezoceramics. J Am Ceram Soc 2012;95(7):2241–7. [284] Bai W, Xi J, Zhang J, Shen B, Zhai J, Yan H. Effect of different templates on structure evolution and large strain response under a low electric field in < 00l > textured lead-free bnt-based piezoelectric ceramics. J Eur Ceram Soc 2015;35(9):2489–99. [285] Zhang H, Xu P, Patterson E, Zang J, Jiang S, Rödel J. Preparation and enhanced electrical properties of grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics. J Eur Ceram Soc 2015;35(9):2501–12. [286] Zhang ST, Kounga AB, Aulbach E, Jo W, Torsten G, Helmut E, et al. Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3K0.5Na0.5NbO3 ii. Temperature dependent properties. J. Appl. Phys 2008;103:34108. [287] Wang F, Xu M, Tang Y, Wang T, Shi W, Leung CM. Large strain response in the ternary Bi0.5Na0.5TiO3-BaTiO3-SrTiO3 solid solutions. J Am Ceram Soc 2012;95(6):1955–9. [288] Li Y, Wang F, Ye X, Xie Y, Tang Y, Sun D, et al. Large strain response and fatigue-resistant behavior in ternary Bi0.5 Na0.5TiO3-BaTiO3-Bi(Zn0.5Ti0.5)O3 solid solutions. J Am Ceram Soc 2014;97(11):3615–23. [289] Ullah A, Malik RA, Ullah A, Lee DS, Jeong SJ, Lee JS, et al. Electric-field-induced phase transition and large strain in lead-free Nb-doped BNKT-BST ceramics. J Eur Ceram Soc 2014;34(1):29–35. [290] Maurya D, Zhou Y, Wang Y, Yan Y, Li J, Viehland D, et al. Giant strain with ultra-low hysteresis and high temperature stability in grain oriented lead-free K0.5Bi0.5TiO3-BaTiO3-Na0.5Bi0.5TiO3 piezoelectric materials. Sci Rep 2015;5(1). [291] Hao J, Xu Z, Chu R, Li W, Du J, Li G. Ultrahigh strain response with fatigue-free behavior in (Bi0.5Na0.5)TiO3-based lead-free piezoelectric ceramics. J Phys D Appl Phys 2015;48(47):472001. [292] Yao Q, Wang F, Xu F, Leung CM, Wang T, Tang Y, et al. Electric field-induced giant strain and photoluminescence-enhancement effect in rare-earth modified lead-free piezoelectric ceramics. ACS Appl Mater Inter 2015;7(9):5066–75. [293] Dittmer R, Jo W, Daniels J, Schaab S, Rödel J. Relaxor characteristics of morphotropic phase boundary (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 modified with Bi(Zn1/ 2Ti1/2)O3. J Am Ceram Soc 2011;94(12):4283–90. [294] Dong G, Fan H, Shi J, Li M. Composition- and temperature-dependent large strain in (1–x)(0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3)-xNaNbO3 ceramics. J Am Ceram Soc 2015;98(4):1150–5. [295] Bai W, Chen D, Huang Y, Zheng P, Zhong J, Ding M, et al. Temperature-insensitive large strain response with a low hysteresis behavior in BNT-based ceramics. Ceram Int 2016;42(6):7669–80. [296] Li T, Lou X, Ke X, Cheng S, Mi S, Wang X, et al. Giant strain with low hysteresis in A-site-deficient (Bi0.5Na0.5)TiO3-based lead-free piezoceramics. Acta Mater 2017;128:337–44. [297] Yuan GL, Or SW, Wang YP, Liu ZG, Liu JM. Preparation and multi-properties of insulated single-phase BiFeO3 ceramics. Solid State Commun 2006;138(2):76–81. [298] Chen F, Zhang QF, Li JH, Qi YJ, Lu CJ, Chen XB, et al. Sol-gel derived multiferroic BiFeO3 ceramics with large polarization and weak ferromagnetism. Appl Phys Lett 2006;89(9):92910. [299] Shvartsman VV, Kleemann W, Haumont R, Kreisel J. Large bulk polarization and regular domain structure in ceramic BiFeO3. Appl Phys Lett 2007;90(17):475. [300] Su WN, Wang DH, Cao QQ, Han ZD, Yin J, Zhang JR, et al. Large polarization and enhanced magnetic properties in BiFeO3 ceramic prepared by high-pressure synthesis. Appl Phys Lett 2007;91(9):92905. [301] Rojac T, Kosec M, Budic B, Setter N, Damjanovic D. Strong ferroelectric domain-wall pinning in BifFeO3 ceramics. J Appl Phys 2010;108(7):74107. [302] Song SH, Zhu QS, Weng LQ, Mudinepalli VR. A comparative study of dielectric, ferroelectric and magnetic properties of BiFeO3 multiferroic ceramics synthesized by conventional and spark plasma sintering techniques. J Eur Ceram Soc 2015;35(1):131–8. [303] Yuan GL, Or SW. Multiferroicity in polarized single-phase Bi0.875Sm0.125FeO3 ceramics. J Appl Phys 2006;100(2):24109. [304] Dai HY, Chen ZP, Li T, Wang CM, Li Y, Guo YS. Influence of samarium substitution on structural and multiferroic properties of bismuth ferrite ceramics. Mater Res Innov 2013;17(2):62–6. [305] Chen X, Wang J, Yuan G, Wu D, Liu J, Yin J, et al. Structure, ferroelectric and piezoelectric properties of multiferroic Bi0.875Sm0.125FeO3 ceramics. J Alloy Compd 2012;541:173–6. [306] Walker J, Budic B, Bryant P, Kurusingal V, Sorrell C, Bencan A, et al. Robust polarization and strain behavior of Sm-modified BiFeO3 piezoelectric ceramics. IEEE Trans Ultrason Ferr Freq Control 2015;62(1):83–7. [307] Yao Y, Liu W, Chan Y, Leung C, Mak C, Ploss B. Studies of rare-earth-doped BiFeO3 ceramics. Int J Appl Ceram Tec 2011;8(5):1246–53. [308] Yuan GL, Baba-Kishi KZ, Liu JM, Wing Or S, Wang YP, Liu ZG. Multiferroic properties of single-phase Bi0.85La0.15FeO3 lead-free ceramics. J Am Ceram Soc 2006;89(10):3136–9. [309] Yuan GL, Or S Wing, Chan H Lai Wa. Structural transformation and ferroelectric-paraelectric phase transition in Bi1-xLaxFeO3 (x=0–0.25) multiferroic ceramics. J Phys D Appl Phys 2007;40(4):1196–200. [310] Zhang S, Pang L, Zhang Y, Lu M, Chen Y. Preparation, structures, and multiferroic properties of single phase Bi1−xLaxFeO3 (x=0–0.40) ceramics. J Appl Phys 2006;100(11):114108.

616

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[311] Yuan GL, Or SW. Enhanced piezoelectric and pyroelectric effects in single-phase multiferroic Bi1−xNdxFeO3 (x=0–0.15) ceramics. Appl Phys Lett 2006;88(6):62905. [312] Sun C, Wang Y, Yang Y, Yuan G, Yin J, Liu Z. Multiferroic properties of Bi1−xDyxFeO3 (x=0-0.2) ceramics at various temperatures. Mater Lett 2012;72:160–3. [313] Zhang S, Wang L, Chen Y, Wang D, Yao Y, Ma Y. Observation of room temperature saturated ferroelectric polarization in Dy substituted BiFeO3 ceramics. J Appl Phys 2012;111(7):818. [314] Uniyal P, Yadav KL. Room temperature multiferroic properties of Eu doped BiFeO3. J Appl Phys 2009;105(7):7D–914D. [315] Yan Z, Wang KF, Qu JF, Wang Y, Song ZT, Feng SL. Processing and properties of Yb-doped BiFeO3 ceramics. Appl Phys Lett 2007;91(8):82906. [316] Singh V, Sharma S, Dwivedi RK, Kumar M, Kotnala RK. Multiferroic and optical properties of Pr-substituted bismuth ferrite ceramics. Phys Status Solidi (a) 2013;210(7):1442–7. [317] Jeon N, Rout D, Kim IW, Kang SL. Enhanced multiferroic properties of single-phase BiFeO3 bulk ceramics by ho doping. Appl Phys Lett 2011;98(7):72901. [318] Wang LY, Wang DH, Huang HB, Han ZD, Cao QQ, Gu BX, et al. The magnetic properties of polycrystalline Bi1−xSrxFeO3 ceramics. J Alloy Compd 2009;469(1–2):1–3. [319] Pradhan SK, Roul BK. Electrical behavior of high resistivity Ce-doped BiFeO3 multiferroic. Physica B 2012;407(13):2527–32. [320] Zheng T, Wu J. Effects of site engineering and doped element types on piezoelectric and dielectric properties of bismuth ferrite lead-free ceramics. J Mater Chem C 2015;3(43):11326–34. [321] Jiang Q, Lin Y, Nan C, Shen Z. Ferroelectric and ferromagnetic properties of hot-pressed Bi0.95-xLa0.05TbxFeO3 ceramics. J Am Ceram Soc 2007;90(5):1444–7. [322] Tao H, Lv J, Zhang R, Xiang R, Wu J. Lead-free rare earth-modified BiFeO3 ceramics: phase structure and electrical properties. Mater Design 2017;120:83–9. [323] Kumar M, Yadav KL. Study of room temperature magnetoelectric coupling in Ti substituted bismuth ferrite system. J Appl Phys 2006;100(7):6694. [324] Kumar M, Yadav KL. Rapid liquid phase sintered Mn doped BiFeO3 ceramics with enhanced polarization and weak magnetization. Appl Phys Lett 2007;91(24):6694. [325] Jun Y, Lee SB, Kim M, Hong S, Kim JW, Kim KH. Dielectric and magnetic properties in Ta-substituted BiFeO3 ceramics. J Mater Res 2007;22(12):3397–403. [326] Pradhan SK. Influence of iron deficiency on electric and magnetic behavior of Ho doped BiFeO3 electroceramic. J Mater Sci: Mater Electron 2013;24(5):1720–6. [327] Lv J, Lou X, Wu J. Defect dipole-induced poling characteristics and ferroelectricity of quenched bismuth ferrite-based ceramics. J Mater Chem C 2016;4(25). [328] Xu Q, Zai H, Wu D, Tang YK, Xu MX. The magnetic properties of BiFeO3 and Bi(Fe0.95Zn0.05)O3. J Alloy Compd 2009;485(1–2):13–6. [329] Luo B, Chen C, Xu Z, Xie Q. Effect of Cr substitution on the multiferroic properties of BiFe1−xCrxO3 compounds. Phys Lett A 2010;374(41):4265–8. [330] Sen K, Singh K, Gautam A, Singh M. Study of dielectric and ferroelectric properties of multiferroic BiCoxFe1-xO3 ceramic. Integr Ferroelectr 2010;120(1):122–30. [331] Zhang CD, Yu J, Zhang DM, Yang B, Wu YY, Wang LH, et al. Processing and ferroelectric properties of Ti-doped BiFeO3 ceramics. Integr Ferroelectr 2007;94(1):31–6. [332] Wang S, Feng Y, Liu W, Yu D, Li D. Effects of Co doping on electronic structure and electric/magnetic properties of La0.1Bi0.9FeO3 ceramics. Sci China Phys, Mech Astron 2013;56(10):1861–5. [333] Singh H, Yadav KL. Dielectric, magnetic and magnetoelectric properties of La and Nb codoped bismuth ferrite. J Phys Condens Matter 2011;23(38):385901. [334] Lan C, Jiang Y, Yang S. Magnetic properties of La and (La, Zr) doped BiFeO3 ceramics. J Mater Sci 2011;46(3):734–8. [335] Cui YF, Zhao YG, Luo LB, Yang JJ, Chang H, Zhu MH, et al. Dielectric, magnetic, and magnetoelectric properties of La and Ti codoped BiFeO3. Appl Phys Lett 2010;97(22):222904. [336] Khomchenko VA, Pereira LCJ, Paixão JA. Structural and magnetic phase transitions in Bi1-xNdxFe1-xMnxO3 multiferroics. J Appl Phys 2014;115(3):34102. [337] Sati PC, Arora M, Chauhan S, Kumar M, Chhoker S. Structural, magnetic, vibrational and impedance properties of Pr and Ti codoped BiFeO3 multiferroic ceramics. Ceram Int 2014;40(6):7805–16. [338] Chandra Sati P, Arora M, Chauhan S, Chhoker S, Kumar M. Structural, magnetic, and optical properties of Pr and Zr codoped BiFeO3 multiferroic ceramics. J Appl Phys 2012;112(9):94102. [339] Kumar V, Gaur A, Kotnala RK. Anomalous dielectric response with suppression in Neel temperature of Bi0.9Y0.1Fe1-xMnxO3 (0⩽x⩽0.07) ceramics. J Alloy Compd 2013;551:410–4. [340] Xu J, Xie D, Yin C, Feng T, Zhang X, Li G, et al. Enhanced dielectric and multiferroic properties of single-phase Y and Zr co-doped BiFeO3 ceramics. J Appl Phys 2013;114(15):154103. [341] Castañeda R, Rojas-George G, Silva J, Fuentes-Montero ME, Matutes-Aquino JA, Reyes-Rojas A, et al. Effects of Ni doping on ferroelectric and ferromagnetic properties of Bi0.75Ba0.25FeO3. Ceram Int 2013;39(7):8527–30. [342] Makhdoom AR, Akhtar MJ, Rafiq MA, Siddique M, Iqbal M, Hasan MM. Enhancement in the multiferroic properties of BiFeO3 by charge compensated aliovalent substitution of Ba and Nb. Aip Adv 2014;4(3):37113. [343] Troyanchuk IO, Tereshko NV, Karpinsky DV, Kholkin AL, Kopcewicz M, Bärner K. Enhanced piezoelectric and magnetic properties of Bi1-x CaxFe1−x/2Nbx/2O3 solid solutions. J Appl Phys 2011;109(11):114102. [344] Xu J, Xie D, Yin C, Feng T, Zhang X, Zhao H, et al. Bi0.8Ca0.2FeO3 with enhanced ferromagnetic properties. Mater Lett 2014;122:139–42. [345] Park JS, Yoo YJ, Hwang JS, Kang JH, Lee BW, Lee YP. Enhanced ferromagnetic properties in Ho and Ni co-doped BiFeO3 ceramics. J Appl Phys 2014;115(1):13904. [346] Basith MA, Kurni O, Alam MS, Sinha BL, Ahmmad B. Room temperature dielectric and magnetic properties of Gd and Ti co-doped BiFeO3 ceramics. J Appl Phys 2014;115(2):24102. [347] Chaudhari YA, Singh A, Mahajan CM, Jagtap PP, Abuassaj EM, Chatterjee R, et al. Multiferroic properties in Zn and Ni co-doped BiFeO3 ceramics by solution combustion method (SCM). J Magn Magn Mater 2013;347:153–60. [348] Zeches RJ, Rossell MD, Zhang JX, Hatt AJ, He Q, Yang CH, et al. A strain-driven morphotropic phase boundary in BiFeO3. Science 2009;326(5955):977–80. [349] Kan D, Pálová L, Anbusathaiah V, Cheng CJ, Fujino S, Nagarajan V, et al. Universal behavior and electric-field-induced structural transition in rare-earthsubstituted BiFeO3. Adv Funct Mater 2010;20(7):1108–15. [350] Lv J, Wu J, Wu W. Enhanced electrical properties of quenched (1–x)Bi 1-ySmyFeO3-xBiScO3 lead-free ceramics. J Phys Chem C 2015;119(36):21105–15. [351] Woodward DI, Reaney IM, Eitel RE, Randall CA. Crystal and domain structure of the BiFeO3-PbTiO3 solid solution. J Appl Phys 2003;94(5):3313–8. [352] Cheng J, Li N, Cross LE. Structural and dielectric properties of Ga-modified BiFeO3-PbTiO3 crystalline solutions. J Appl Phys 2003;94(8):5153. [353] Cheng J, Cross LE. Effects of la substituent on ferroelectric rhombohedral/tetragonal morphotropic phase boundary in (1-x)(Bi, La)(Ga0.05Fe0.95)O3-xPbTiO3 piezoelectric ceramics. J Appl Phys 2003;94(8):5188. [354] Bhattacharjee S, Tripathi S, Pandey D. Morphotropic phase boundary in (1–x)BiFeO3-xPbTiO3: phase coexistence region and unusually large tetragonality. Appl Phys Lett 2007;91(4):42903. [355] Bhattacharjee S, Pandey D. Stability of the various crystallographic phases of the multiferroic (1–x)BiFeO3-xPbTiO3 system as a function of composition and temperature. J Appl Phys 2010;107(12):124112. [356] Bhattacharjee S, Pandey D. Effect of stress induced monoclinic to tetragonal phase transformation in the multiferroic (1–x)BiFeO3-xPbTiO3 system on the width of the morphotropic phase boundary and the tetragonality. J Appl Phys 2011;110(8):84105. [357] Amorín H, Correas C, Ramos P, Hungría T, Castro A, Algueró M. Very high remnant polarization and phase-change electromechanical response of BiFeO3PbTiO3 at the multiferroic morphotropic phase boundary. Appl Phys Lett 2012;101(17):172908. [358] Fan L, Chen J, Li S, Kang H, Liu L, Fang L, et al. Enhanced piezoelectric and ferroelectric properties in the BaZrO3 substituted BiFeO3-PbTiO3. Appl Phys Lett 2013;102(2):22905. [359] Sharma S, Singh V, Dwivedi RK, Ranjan R, Anshul A, Amritphale SS, et al. Phase transformation, improved ferroelectric and magnetic properties of (1-x)BiFeO3xpb(Zr0.52Ti0.48)O3 solid solutions. J Appl Phys 2014;115(22):224106. [360] Zhuang J, Wu H, Ren W, Ye Z. Local polar structure and multiferroic properties of (1-x)Bi0.9Dy0.1FeO3-xPbTiO3 solid solution. J Appl Phys 2014;116(6):66809. [361] Kumar MM, Srinivas A, Suryanarayana SV. Structure property relations in BiFeO3/BaTiO3 solid solutions. J Appl Phys 2000;87(2):855–62.

617

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[362] Zheng T, Wu J. Quenched bismuth ferrite-barium titanate lead-free piezoelectric ceramics. J Alloy Compd 2016;676:505–12. [363] Ma ZZ, Tian ZM, Li JQ, Wang CH, Huo SX, Duan HN, et al. Enhanced polarization and magnetization in multiferroic (1–x)BiFeO3-xSrTiO3 solid solution. Solid State Sci 2011;13(12):2196–200. [364] Wang QQ, Zhuo W, Xiao QL, Xiang MC. Improved structure stability and multiferroic characteristics in CaTiO3-modified BiFeO3 ceramics. J Am Ceram Soc 2012;95(2):670–5. [365] Ma Y, Ming X. Enhanced multiferroic characteristics in NaNbO3-modified BiFeO3 ceramics. J Appl Phys 2009;105(5):54107. [366] Huo SX, Yuan SL, Qiu Y, Ma ZZ, Wang CH. Crystal structure and multiferroic properties of BiFeO3-Na0.5K0.5NbO3 solid solution ceramics prepared by pechini method. Mater Lett 2012;68:8–10. [367] Morozov MI, Einarsrud M, Grande T. Control of conductivity and electric field induced strain in bulk Bi0.5K0.5TiO3-BiFeO3 ceramics. Appl Phys Lett 2014;104(12):122905. [368] Yao Z, Xu C, Wang Z, Song Z, Zhang Y, Hu W, et al. Microstructure, ferro-piezoelectric and thermal stability of SiO2 modified BiFeO3-BaTiO3 high temperature piezoceramics. J Mater Sci: Mater Electron 2015;26(1):479–84. [369] Yao Z, Xu C, Liu H, Hao H, Cao M, Wang Z, et al. Greatly reduced leakage current and defect mechanism in atmosphere sintered BiFeO3-BaTiO3 high temperature piezoceramics. J Mater Sci: Mater Electron 2014;25(11):4975–82. [370] Leontsev SO, Eitel RE. Dielectric and piezoelectric properties in Mn-modified (1–x)BiFeO3-xBaTiO3 ceramics. J Am Ceram Soc 2009;92(12):2957–61. [371] Yang S, Kumar A, Petkov V, Priya S. Room-temperature magnetoelectric coupling in single-phase BaTiO3-BiFeO3 system. J Appl Phys 2013;113(14):144101. [372] Yang H, Zhou C, Liu X, Zhou Q, Chen G, Wang H, et al. Structural, microstructural and electrical properties of BiFeO3-BaTiO3 ceramics with high thermal stability. Mater Res Bull 2012;47(12):4233–9. [373] Zheng T, Ding Y, Wu J. Bi nonstoichiometry and composition engineering in (1–x)Bi1+yFeO3-xBaTiO3 piezoceramics. RSC Adv 2016;6(93). [374] Yang H, Zhou C, Liu X, Zhou Q, Chen G, Li W, et al. Piezoelectric properties and temperature stabilities of Mn- and Cu-modified BiFeO3-BaTiO3 high temperature ceramics. J Eur Ceram Soc 2013;33(6):1177–83. [375] Futakuchi T, Kakuad T, Sakai Y. Multiferroic properties of BiFeO3-BaTiO3 based ceramics. J Ceram Soc Jpn 2014;122(1426):464–8. [376] Wei Y, Wang X, Jia J, Wang X. Multiferroic and piezoelectric properties of 0.65BiFeO3-0.35BaTiO3 ceramic with pseudo-cubic symmetry. Ceram Int 2012;38(4):3499–502. [377] Wan Y, Li Y, Li Q, Zhou W, Zheng Q, Wu X, et al. Microstructure, ferroelectric, piezoelectric, and ferromagnetic properties of Sc-modified BiFeO3-BaTiO3 multiferroic ceramics with MnO2 addition. J Am Ceram Soc 2014;97(6):1809–18. [378] Zhou Q, Zhou C, Yang H, Yuan C, Chen G, Cao L, et al. Piezoelectric and ferroelectric properties of Ga modified BiFeO3-BaTiO3 lead-free ceramics with high curie temperature. J Mater Sci: Mater Electron 2014;25(1):196–201. [379] Zhou C, Cen Z, Yang H, Zhou Q, Li W, Yuan C, et al. Structure, electrical properties of Bi(Fe, Co)O3-BaTiO3 piezoelectric ceramics with improved curie temperature. Physica B 2013;410:13–6. [380] Zheng Q, Luo L, Lam KH, Jiang N, Guo Y, Lin D. Enhanced ferroelectricity, piezoelectricity, and ferromagnetism in Nd-modified BiFeO3-BaTiO3 lead-free ceramics. J Appl Phys 2014;116(18):184101. [381] Guo Y, Xiao P, Wen R, Wan Y, Zheng Q, Shi D, et al. Critical roles of Mn-ions in enhancing insulation, piezoelectricity and multiferroicity of BiFeO3-based leadfree high temperature ceramics. J Mater Chem C 2015;3(22):5811–24. [382] Cen Z, Zhou C, Yang H, Zhou Q, Li W, Yan C, et al. Remarkably high-temperature stability of Bi(Fe1-xAlx)O3-BaTiO3 solid solution with near-zero temperature coefficient of piezoelectric properties. J Am Ceram Soc 2013;96(7):2252–6. [383] Wu X, Tian M, Guo Y, Zheng Q, Luo L, Lin D. Phase transition, dielectric, ferroelectric and ferromagnetic properties of La-doped BiFeO3-BaTiO3 multiferroic ceramics. J Mater Sci: Mater Electron 2015;26(2):978–84. [384] Zheng T, Jiang Z, Wu J. Enhanced piezoelectricity in (1–x)Bi1.05Fe1-yAyO3-xBaTiO3 lead-free ceramics: site engineering and wide phase boundary region. Dalton Trans 2016;45(28):11277–85. [385] Unruan S, Unruan M, Monnor T, Priya S, Yimnirun R. Local structure investigation in multiferroic BiFeO3-BaTiO3 ceramics by xas technique and their relevant properties. J Am Ceram Soc 2015;98(10):1172–84. [386] Luo L, Jiang N, Lei F, Guo Y, Zheng Q, Lin D. Phase transition, ferroelectric and piezoelectric properties of Bi(Mg0.5Zr0.5)O3-modified BiFeO3-BaTiO3 lead-free ceramics. J Mater Sci: Mater Electron 2014;25(4):1736–44. [387] Li Y, Guo Y, Zheng Q, Lam KH, Zhou W, Wan Y, et al. Enhancement in multiferroic and piezoelectric properties of BiFeO3-BaTiO3-Bi0.5Na0.5TiO3 lead-free ceramics with MnO2 addition by optimizing sintering temperature and dwell time. Mater Res Bull 2015;68:92–9. [388] Lin D, Zheng Q, Li Y, Wan Y, Li Q, Zhou W. Microstructure, ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3-BaTiO3 lead-free ceramics with high curie temperature. J Eur Ceram Soc 2013;33(15–16):3023–36. [389] Lin Y, Yu J. Ferroelectric and piezoelectric properties of high temperature perovskite-type 0.69BiFeO3-0.02Bi(Mg1/2Ti1/2)O3–0.29BaTiO3 ceramics. J Mater Sci: Mater Electron 2014;25(12):5462–6. [390] Zhou C, Feteira A, Shan X, Yang H, Zhou Q, Cheng J, Li W, Wang H. Remarkably high-temperature stable piezoelectric properties of Bi(Mg0.5 Ti0.5)O3 modified BiFeO3-BaTiO3 ceramics. Appl Phys Lett 2012;101(3):32901. [391] Zhou Q, Zhou C, Yang H, Chen G, Li W, Wang H. Dielectric, ferroelectric, and piezoelectric properties of Bi(Ni1/2Ti1/2)O3-modified BiFeO3-BaTiO3 ceramics with high curie temperature. J Am Ceram Soc 2012;95(12):3889–93. [392] Yi J, Tian Y, Wei L, Li J, Liang P, Shi P, et al. Yang Z. Structure, dielectric, ferroelectric, and magnetic properties of (1–x)BiFeO3-x (Ba0.85Ca0.15)(Zr0.10Ti0.90)O3 ceramics. Mater Res Bull 2015;66:132–9. [393] Rojac T, Kosec M, Damjanovic D. Large electric-field induced strain in BiFeO3 ceramics. J Am Ceram Soc 2011;94(12):4108–11. [394] Walker J, Bryant P, Kurusingal V, Sorrell C, Kuscer D, Drazic G, et al. Synthesis-phase-composition relationship and high electric-field-induced electromechanical behavior of samarium-modified BiFeO3 ceramics. Acta Mater 2015;83:149–59. [395] Chen J, Cheng J. High electric-induced strain and temperature-dependent piezoelectric properties of 0.75BF-0.25BZT lead-free ceramics. J Am Ceram Soc 2016;99(2):536–42. [396] Zheng D, Zuo R. A novel BiFeO3-BaTiO3-BaZrO3 lead-free relaxor ferroelectric ceramic with low-hysteresis and frequency-insensitive large strains. J Am Ceram Soc 2015;98(12):3670–2. [397] Zheng D, Zuo R, Zhang D, Li Y. Novel BiFeO3-BaTiO3 -Ba(Mg1/3Nb2/3)O3 lead-free relaxor ferroelectric ceramics for energy-storage capacitors. J Am Ceram Soc 2015;98(9):2692–5. [398] Fan Q, Zhou C, Zeng W, Cao L, Yuan C, Rao G, et al. Normal-to-relaxor ferroelectric phase transition and electrical properties in Nb-modified 0.72BiFeO30.28BaTiO3 ceramics. J Electroceram 2016;36(1–4):1–7. [399] Zheng T, Ding Y, Wu J. Effects of oxide additives on structure and properties of bismuth ferrite-based ceramics. J Mater Sci: Mater Electron 2017;5613:1–9. [400] Li Q, Wei J, Cheng J, Chen J. High temperature dielectric, ferroelectric and piezoelectric properties of Mn-modified BiFeO3-BaTiO3 lead-free ceramics. J Mater Sci 2017;52(1):229–37. [401] Bechmann R. Elastic, piezoelectric, and dielectric constants of polarized barium titanate ceramics and some applications of the piezoelectric equations. J Acoust Soc Am 1956;28(3):347–50. [402] Takahashi H, Numamoto Y, Tani J, Tsurekawa S. Piezoelectric properties of BaTiO3 ceramics with high performance fabricated by microwave sintering. Jpn J Appl Phys 2006;45(9):7405–8. [403] Shen ZY, Li JF. Enhancement of piezoelectric constant d33 in BaTiO3 ceramics due to nano-domain structure. Jpn J Ceram Soc 2010;118(1382):940–3. [404] Karaki T, Yan K, Adachi M. Barium titanate piezoelectric ceramics manufactured by two-step sintering. Jpn J Appl Phys 2007;46(10B):7035–8. [405] Zhang JL, Ji PF, Wu YQ, Zhao X, Tan YQ, Wang CL. Strong piezoelectricity exhibited by large-grained BaTiO3 ceramics. Appl Phys Lett 2014;104(22):L30. [406] Wada S, Takeda K, Muraishi T, Kakemoto H, Tsurumi T, Kimura T. Preparation of [110] grain oriented barium titanate ceramics by templated grain growth method and their piezoelectric properties. Jpn J Appl Phys 2007;46:7039–43.

618

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[407] Zhu XN, Zhang W, Chen XM. Enhanced dielectric and ferroelectric characteristics in Ca-modified BaTiO3 ceramics. Aip Adv 2013;3(8):82125. [408] Yu Z, Ang C, Guo R, Bhalla AS. Piezoelectric and strain properties of Ba(Ti1-xZrx)O3 ceramics. J Appl Phys 2002;92(3):1489–93. [409] Brajesh K, Kalyani AK, Ranjan R. Ferroelectric instabilities and enhanced piezoelectric response in Ce modified BaTiO3 lead-free ceramics. Appl Phys Lett 2015;106(1):257602. [410] Abdelkefi H, Khemakhem H, Vélu G, Carru JC, Von der Mühll R. Dielectric properties and ferroelectric phase transitions in BaxSr1−xTiO3 solid solution. J Alloy Compd 2005;399(1–2):1–6. [411] Gao J, Xue D, Wang Y, Wang D, Zhang L, Wu H, et al. Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics. Appl Phys Lett 2011;99(9):92901. [412] Ehmke MC, Ehrlich SN, Blendell JE, Bowman KJ. Phase coexistence and ferroelastic texture in high strain (1-x)Ba(Zr0.2 Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 piezoceramics. J Appl Phys 2012;111(12):124110. [413] Keeble DS, Benabdallah F, Thomas PA, Maglione M, Kreisel J. Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3). Appl Phys Lett 2013;102(9):92903. [414] Xue D, Gao J, Zhou Y, Ding X, Sun J, Lookman T, Ren X. Phase transitions and phase diagram of Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 Pb-free system by anelastic measurement. J Appl Phys 2015;117(12):124107. [415] Cordero F, Craciun F, Dinescu M, Scarisoreanu N, Galassi C, Schranz W, Soprunyuk V. Elastic response of (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (x=0.45-0.55) and the role of the intermediate orthorhombic phase in enhancing the piezoelectric coupling. Appl Phys Lett 2014;105(23):232904. [416] Zhang L, Zhang M, Wang L, Zhou C, Zhang Z, Yao Y, Zhang L, Xue D, Lou X, Ren X. Phase transitions and the piezoelectricity around morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 lead-free solid solution. Appl Phys Lett 2014;105(16):162908. [417] Acosta M, Khakpash N, Someya T, Novak N, Jo W, Nagata H, Rossetti Jr GA, Rödel J. Origin of the large piezoelectric activity in (1-x) Ba(Zr0.2Ti0.8)O3-x (Ba0.7Ca0.3)TiO3 ceramics. Phys Rev B 2015;91(10):104108. [418] Ehmke MC, Glaum J, Hoffman M, Blendell JE, Bowman KJ. The effect of electric poling on the performance of lead-free (1–x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 piezoceramics. J Am Ceram Soc 2013;96(12):3805–11. [419] Acosta M, Novak N, Jo W, Rödel J. Relationship between electromechanical properties and phase diagram in the Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 lead-free piezoceramic. Acta Mater 2014;80:48–55. [420] Damjanovic D, Biancoli A, Batooli L, Vahabzadeh A, Trodahl J. Elastic, dielectric, and piezoelectric anomalies and raman spectroscopy of 0.5Ba(Ti0.8 Zr0.2)O30.5(Ba0.7Ca0.3)TiO3. Appl Phys Lett 2012;100(19):192907. [421] Bjørnetun Haugen A, Forrester JS, Damjanovic D, Li B, Bowman KJ, Jones JL. Structure and phase transitions in 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2Ti0.8)O3 from -100 °C to 150 °C. J Appl Phys 2013;113(1):14103. [422] Chandrakala E, Paul Praveen J, Kumar A, James AR, Das D. Strain-induced structural phase transition and its effect on piezoelectric properties of (BZT-BCT)(CeO2) ceramics. J Am Ceram Soc 2016;99(11):3659–69. [423] Wang P, Li Y, Lu Y. Enhanced piezoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free ceramics by optimizing calcination and sintering temperature. J Eur Ceram Soc 2011;31(11):2005–12. [424] Praveen JP, Karthik T, James AR, Chandrakala E, Asthana S, Das D. Effect of poling process on piezoelectric properties of sol-gel derived BZT-BCT ceramics. J Eur Ceram Soc 2015;35(6):1785–98. [425] Li W, Xu Z, Chu R, Fu P, Zang G. Piezoelectric and dielectric properties of (Ba1-xCax)(Ti0.95Zr0.05)O3 lead-free ceramics. J Am Ceram Soc 2010;93(10):2942–4. [426] Tian Y, Chao X, Wei L, Liang P. Phase transition behavior and electrical properties of lead-free (Ba1-xCax)(Zr0.1Ti0.9)O3 piezoelectric ceramics. J Appl Phys 2013;113(18):184107. [427] Zhao Z, Li X, Ji H, Dai Y, Li T. Microstructure and electrical properties in zn-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 piezoelectric ceramics. J Alloy Compd 2015;637:291–6. [428] Su S, Zuo R, Lu S, Xu Z, Wang X, Li L. Poling dependence and stability of piezoelectric properties of Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics with huge piezoelectric coefficients. Curr Appl Phys 2011;11(3):S120–3. [429] Tian Y, Wei L, Chao X, Liu Z. Yang Z. Phase transition behavior and large piezoelectricity near the morphotropic phase boundary of lead-free (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics. J Am Ceram Soc 2013;96(2):496–502. [430] Zhang Y, Glaum J, Groh C, Ehmke MC, Blendell JE, Bowman KJ, et al. Correlation between piezoelectric properties and phase coexistence in (Ba, Ca)(Ti, Zr)O3 ceramics. J Am Ceram Soc 2014;97(9):2885–91. [431] Li W, Xu Z, Chu R, Fu P, Zang G. High piezoelectric d33 coefficient in (Ba1−xCax)(Ti0.98Zr0.02)O3 lead-free ceramics with relative high curie temperature. Mater Lett 2010;64(21):2325–7. [432] Zhao C, Wang H, Xiong J, Wu J. Composition-driven phase boundary and electrical properties in (Ba0.94Ca0.06)(Ti1-xMx)O3 (M=Sn, Hf, Zr) lead-free ceramics. Dalton Trans 2016;45(15):6466. [433] Zhao L, Zhang B, Zhou P, Zhu L, Wang N. Piezoelectric and ferroelectric properties of (Ba, Ca)(Ti, Sn)O3 lead-free ceramics sintered with Li2O additives: analysis of point defects and phase structures. Ceram Int 2016;42(1):1086–93. [434] Zhu L, Zhang B, Zhao L, Li S, Zhou Y, Shi X, et al. Large piezoelectric effect of (Ba, Ca)TiO3-xBa(Sn, Ti)O3 lead-free ceramics. J Eur Ceram Soc 2016;36(4):1017–24. [435] Zhu L, Zhang B, Zhao X, Zhao L, Zhou P, Li J. Enhanced piezoelectric properties of (Ba1−xCax)(Ti0.92Sn0.08)O3 lead-free ceramics. J Am Ceram Soc 2013;96(1):241–5. [436] Zhu L, Zhang B, Zhao L, Li J. High piezoelectricity of BaTiO3-CaTiO3-BaSnO3 lead-free ceramics. J Mater Chem C 2014;2:4764. [437] Zhu LF, Zhang BP, Zhao XK, Zhao L. Phase transition and high piezoelectricity in (Ba, Ca)(Ti1-xSnx)O3 lead-free ceramics. Appl Phys Lett 2013;103(7):72905. [438] Chen M, Xu Z, Chu R, Qiu H, Li M, Liu Y, et al. Li G. Enhanced piezoelectricity in broad composition range and the temperature dependence research of (Ba1-xCax)(Ti0.95Sn0.05)O3 piezoceramics. Physica B 2014;433:43–7. [439] Li W, Xu Z, Chu R, Fu P, Zang G. Large piezoelectric coefficient in (Ba1-xCax)(Ti0.96Sn0.04)O3 lead-free ceramics. J Am Ceram Soc 2011;94(12):4131–3. [440] Li W, Xu Z, Chu R, Fu P, Zang G. Enhanced ferroelectric properties in (Ba1-xCax)(Ti0.94Sn0.06)O3 lead-free ceramics. J Eur Ceram Soc 2012;32(3):517–20. [441] Lin D, Kwok KW, Chan HLW. Structure, dielectric and piezoelectric properties of Ba0.90Ca0.10Ti1-xSnxO3 lead-free ceramics. Ceram Int 2014;40(5):6841–6. [442] Wang X, Chao X, Liang P, Wei L, Yang Z. Polymorphic phase transition and enhanced electrical properties of (Ba0.91Ca0.09-xSrx)(Ti0.92Sn0.08)O3 lead-free ceramics. Ceram Int 2014;40(7):9389–94. [443] Zhou C, Liu W, Xue D, Ren X, Bao H, Gao J, Zhang L. Triple-point-type morphotropic phase boundary based large piezoelectric Pb-free material-Ba(Ti0.8Hf0.2) O3-(Ba0.7Ca0.3)TiO3. Appl Phys Lett 2012;100(22):222910. [444] Wang D, Jiang Z, Yang B, Zhang S, Zhang M, Guo F, et al. Phase transition behavior and high piezoelectric properties in lead-free BaTiO3-CaTiO3-BaHfO3 ceramics. J Mater Sci 2014;49(1):62–9. [445] Wang D, Jiang Z, Yang B, Zhang S, Zhang M, Guo F, et al. Phase diagram and enhanced piezoelectric response of lead-free BaTiO3-CaTiO3-BaHfO3 system. J Am Ceram Soc 2014;97(10):3244–51. [446] Zhao C, Wu W, Wang H, Wu J. Site engineering and polarization characteristics in (Ba1−yCay)(Ti1−xHfx)O3 lead-free ceramics. J Appl Phys 2016;119(2):24108. [447] Brajesh K, Abebe M. Ranjan R. Structural transformations in morphotropic-phase-boundary composition of the lead-free piezoelectric system Ba (Ti0.8Zr0.2)O3(Ba0.7Ca0.3)TiO3. Phys Rev B 2016;94(10). [448] Wada S, Suzuki S, Noma T, Suzuki T, Osada M, Kakihana M, et al. Enhanced piezoelectric property of barium titanate single crystals with engineered domain configurations. Jpn J Appl Phys 1999;38:5505–11. [449] Cao H, Devreugd CP, Ge W, Li J, Viehland D, Luo H, et al. Monoclinic mc phase in (001) field cooled BaTiO3 single crystals. Appl Phys Lett 2009;94(3):2059. [450] Brajesh K, Tanwar K, Abebe M, Ranjan R. Relaxor ferroelectricity and electric-field-driven structural transformation in the giant lead-free piezoelectric (Ba, Ca) (Ti, Zr)O3. Phys Rev B 2015;92(22):224112. [451] Zhao Z, Buscaglia V, Viviani M, Buscaglia MT, Mitoseriu L, Testino A, et al. Grain-size effects on the ferroelectric behavior of dense nanocrystalline BaTiO3. Phys

619

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

Rev B Condens Matter 2004;70(2):2199–208. [452] Dipankar G, Akito S, Jared C, Thomas PA, Han H, Nino JC, et al. Ferroelectric materials: domain wall displacement is the origin of superior permittivity and piezoelectricity in BaTiO3 at intermediate grain sizes. Adv Funct Mater 2014;24(7):885–96. [453] Sun H, Duan S, Liu X, Wang D, Lead-free Sui H. Ba0.98Ca0.02Zr0.02Ti0.98O3 ceramics with enhanced electrical performance by modifying MnO2 doping content and sintering temperature. J Alloy Compd 2016;670:262–7. [454] Ma J, Liu X, Li W. High piezoelectric coefficient and temperature stability of Ga2O3-doped (Ba0.99Ca0.01)(Zr0.02Ti0.98)O3 lead-free ceramics by low-temperature sintering. J Alloy Compd 2013;581:642–5. [455] Cui Y, Liu X, Jiang M, Hu Y, Su Q, Wang H. Lead-free (Ba0.7Ca0.3)TiO3-Ba(Zr0.2Ti0.8)O3-xwt%CuO ceramics with high piezoelectric coefficient by low-temperature sintering. J Mater Sci: Mater Electron 2012;23(7):1342–5. [456] Cui Y, Liu X, Jiang M, Zhao X, Shan X, Li W, et al. Lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-CeO2 ceramics with high piezoelectric coefficient obtained by lowtemperature sintering. Ceram Int 2012;38(6):4761–4. [457] Chao X, Wang Z, Tian Y, Zhou Y, Yang Z. Ba(Cu0.5W0.5)O3-induced sinterability, electrical and mechanical properties of (Ba0.85Ca0.15Ti0.90Zr0.10)O3 ceramics sintered at low temperature. Mater Res Bull 2015;66:16–25. [458] Zhao L, Zhang B, Zhou P, Zhao X, Zhu L. Phase structure and property evaluation of (Ba, Ca)(Ti, Sn)O3 sintered with Li2CO3 addition at low temperature. J Am Ceram Soc 2014;97(7):2164–9. [459] Wu B, Xiao D, Wu J, Gou Q, Zhu J. Microstructure and electrical properties of (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3-xwt%ZnO lead-free piezoelectric ceramics sintered at lower temperature. J Mater Sci: Mater Electron 2015;26(4):2323–8. [460] Zhou PF, Zhang BP, Zhao L, Zhao XK, Zhu LF, Cheng LQ, et al. High piezoelectricity due to multiphase coexistence in low-temperature sintered (Ba, Ca)(Ti, Sn) O3-CuO ceramics. Appl Phys Lett 2013;103(17):62. [461] Chen X, Li Y, Zeng J, Zheng L, Park CH, Li G. Phase transition and large electrostrain in lead-free Li-doped (Ba, Ca)(Ti, Zr)O3 ceramics. J Am Ceram Soc 2016;99(6):2170–4. [462] Wu J, Wang T, Cheng X, Wang X, Zhang B, Zhu J, et al. Enhanced d33 value in HfO2-modified (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3 ceramics. J Alloy Compd 2013;576:299–301. [463] Han C, Wu J, Pu C, Qiao S, Wu B, Zhu J, et al. High piezoelectric coefficient of Pr2O3-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics. Ceram Int 2012;38(8):6359–63. [464] Li W, Xu Z, Chu R, Fu P, Zang G. Improved piezoelectric property and bright upconversion luminescence in Er doped (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3 ceramics. J Alloy Compd 2014;583:305–8. [465] Chen M, Xu Z, Chu R, Wang Z, Gao S, Yu G, Li W, Gong S, Li G. Y2O3-modified Ba(Ti0.96Sn0.04)O3 ceramics with improved piezoelectricity and raised curie temperature. Mater Res Bull 2014;59:305–10. [466] Li W, Xu Z, Chu R, Fu P, Zang G. Temperature stability in Dy-doped (Ba0.99 Ca0.01)(Ti0.98Zr0.02)O3 lead-free ceramics with high piezoelectric coefficient. J Am Ceram Soc 2011;94(10):3181–3. [467] Yan J, Gomi M, Yokota T, Song H. Phase transition and huge ferroelectric polarization observed in BiFe1-xGaxO3 thin films. Appl Phys Lett 2013;102(22):24113. [468] Lin D, Li Z, Zhang S, Xu Z, Yao X. Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free single crystal. Solid State Commun 2009;149(39–40):1646–9. [469] Gupta S, Priya S. Ferroelectric properties and dynamic scaling of 〈100〉 oriented (K0.5Na0.5)NbO3 single crystals. Appl Phys Lett 2011;98(24):242906. [470] Tian H, Hu C, Meng X, Tan P, Zhou Z, Li J, et al. Top-seeded solution growth and properties of K1-xNaxNbO3 crystals. Cryst Growth Des 2015;15(3):1180–5. [471] Hao D, Zhao X, Zhang H, Chao C, Li X, Di L, et al. Orientation dependence of electrical properties of large-sized sodium potassium niobate lead-free single crystals. Crystengcomm 2014;16(13):2760–5. [472] Tian H, Hu C, Meng X, Zhou Z, Shi G. Dielectric, piezoelectric, and elastic properties of K0.8Na0.2NbO3 single crystals. J Mater Chem C 2015;3(37):9609–14. [473] Hu C, Tian H, Meng X, Shi G, Cao W, Zhou Z. High-quality K0.47Na0.53NbO3 single crystal toward high performance transducer. RSC Adv 2017;7(12):7003–7. [474] Lin D, Li Z, Zhang S. Xu Z, Yao X. Influence of MnO2 doping on the dielectric and piezoelectric properties and the domain structure in (K0.5Na0.5)NbO3 single crystals. J Am Ceram Soc 2010;93(4):941–4. [475] Lin D, Zhang S, Cai C, Liu W. Domain size engineering in 0.5%MnO2 -(K0.5Na0.5)NbO3 lead free piezoelectric crystals. J Appl Phys 2015;117(7):74103. [476] Inagaki Y, Kakimoto K. Kagomiya I. Crystal growth and ferroelectric property of Na0.5K0.5NbO3 and Mn-doped Na0.5K0.5NbO3 crystals grown by floating zone method. J Eur Ceram Soc 2010;30(2):301–6. [477] Kizaki Y, Noguchi Y, Miyayama M. Defect control for low leakage current in Na0.5K0.5NbO3 single crystals. Appl Phys Lett 2006;89(14):142910. [478] Inagaki Y, Kakimoto K. Dielectric and piezoelectric properties of Mn-doped Na0.5K0.5NbO3 single crystals grown by flux method. Appl Phys Expr 2008;1(6):61602. [479] Xu G, Yang D, Chen K, Payne DA, Carroll JF. Growth, domain dynamics and piezoelectric properties of some lead-free ferroelectric crystals. J Electroceram 2010;24(3):226–30. [480] Yamamoto K, Suzuki M, Noguchi Y, High-performance Miyayama M. Bi0.5Na0.5TiO3 single crystals grown by high-oxygen-pressure flux method. Jpn J Appl Phys 2008;47(9):7623–9. [481] Suzuki M, Morishita A, Kitanaka Y, Noguchi Y, Miyayama M. Polarization and piezoelectric properties of high performance bismuth sodium titanate single crystals grown by high-oxygen-pressure flux method. Jpn J Appl Phys 2010;49(9):9M. [482] Ge W, Liu H, Zhao X, Li X, Pan X, Lin D, et al. Orientation dependence of electrical properties of 0.96Na0.5Bi0.5TiO3-0.04BaTiO3 lead-free piezoelectric single crystal. Appl Phys A 2009;95(3):761–7. [483] Moon K, Rout D, Lee H, Kang SL. Solid state growth of Na0.5Bi0.5TiO3-BaTiO3 single crystals and their enhanced piezoelectric properties. J Cryst Growth 2011;317(1):28–31. [484] Zhang Q, Zhang Y, Wang F, Wang Y, Lin D, Zhao X, Luo H, Ge W, Viehland D. Enhanced piezoelectric and ferroelectric properties in Mn-doped Na0.5Bi0.5TiO3BaTiO3 single crystals. Appl Phys Lett 2009;95(10):102904. [485] Onozuka H, Kitanaka Y, Noguchi Y, Miyayama M. Crystal growth and characterization of (Bi0.5Na0.5)TiO3-BaTiO3 single crystals obtained by the top-seeded solution growth method under high-pressure oxygen atmosphere. Key Eng Mater 2013;566(9):7N–9N. [486] Zhang H, Chen C, Zhao X, Deng H, Ren B, Li X, et al. Structure and electrical properties of Na1/2Bi1/2TiO3-xK1/2Bi1/2TiO3 lead-free ferroelectric single crystals. Solid State Commun 2015;201:125–9. [487] Sun R, Wang J, Wang F, Feng T, Kong F, Liu X, et al. Growth and electrical properties of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3 lead-free single crystals by the tssg method. Ceram Int 2016;42(13):14557–64. [488] Chen C, Zhang H, Zhao X, Deng H, Li L, Ren B, et al. Structure, electrical, and optical properties of (Na1/2 Bi1/2)TiO3-1.5at.%Bi(Zn1/2Ti1/2)O3 lead-free single crystal grown by a TSSG technique. J Am Ceram Soc 2014;97(6):1861–5. [489] Lebeugle D, Colson D, Forget A, Viret M, Bonville P, Marucco J, et al. Room temperature coexistence of large electric polarization and magnetic order in BiFeO3 single crystals. Phys Rev B 2007;76:24116. [490] Lebeugle D, Colson D, Forget A, Viret M. Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields. Appl Phys Lett 2007;91(2):22907. [491] Chishima Y, Noguchi Y, Kitanaka Y, Miyayama M. Defect control for polarization switching in BiFeO3 single crystals. IEEE Trans Ultrason Ferroelectr Freq Control 2010;57(10):2233–6. [492] Lu J, Qiao LJ, Fu PZ, Wu YC. Phase equilibrium of Bi2O3-Fe2O3 pseudo-binary system and growth of BiFeO3 single crystal. J Cryst Growth 2011;318(1):936–41. [493] Ito T, Ushiyama T, Yanagisawa Y, Kumai R, Tomioka Y. Growth of highly insulating bulk single crystals of multiferroic BiFeO3 and their inherent internal strains in the domain-switching process. Cryst Growth Des 2011;11(11):5139–43. [494] Uršič H, Benčan A, Škarabot M, Godec M, Kosec M. Dielectric, ferroelectric, piezoelectric, and electrostrictive properties of K0.5Na0.5NbO3 single crystals. J Appl Phys 2010;107(3):33705. [495] Rafiq MA, Costa ME, Vilarinho PM. Pairing high piezoelectric coefficients, d33, with high curie temperature (TC) in lead-free (K, Na)NbO3. ACS Appl Mater Inter

620

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

2016;8(49):33755–64. [496] Yang J, Zhang F, Yang Q, Liu Z, Li Y, Liu Y, et al. Large piezoelectric properties in KNN-based lead-free single crystals grown by a seed-free solid-state crystal growth method. Appl Phys Lett 2016;108(18):182904. [497] Zheng L, Huo X, Wang R, Wang J, Jiang W, Cao W. Large size lead-free (Na, K)(Nb, Ta)O3 piezoelectric single crystal: growth and full tensor properties. Crystengcomm 2013;15(38):7718–22. [498] Tian H, Meng X, Hu C, Tan P, Cao X, Shi G, et al. Origin of giant piezoelectric effect in lead-free K1-xNaxTa1-yNbyO3 single crystals. Sci Rep-Uk 2016;6:25637. [499] Huo X, Zheng L, Zhang S, Zhang R, Liu G, Wang R, et al. Growth and properties of Li, Ta modified (K, Na)NbO3 lead-free piezoelectric single crystals. Phys Status Solidi (RRL) - Rapid Res Lett 2014;8(1):86–90. [500] Huo X, Zheng L, Zhang R, Wang R, Wang J, Sang S, et al. High quality lead-free (Li, Ta) modified (K, Na)NbO3 single crystal and its complete set of elastic, dielectric and piezoelectric coefficients with macroscopic 4mm symmetry. Crystengcomm 2014;16(42):9828–33. [501] Huo X, Zhang R, Zheng L, Zhang S, Wang R, Wang J, et al. (K, Na, Li)(Nb, Ta)O3: Mn lead-free single crystal with high piezoelectric properties. J Am Ceram Soc 2015;98(6):1829–35. [502] Chen C, Jiang X, Li Y, Wang F, Zhang Q, Luo H. Growth and electrical properties of Na1/2Bi1/2TiO3-BaTiO3 lead-free single crystal with morphotropic phase boundary composition. J Appl Phys 2010;108(12):124106. [503] Chen C, Chen P, Tu C. Polar nanoregions and dielectric properties in high-strain lead-free 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 piezoelectric single crystals. J Appl Phys 2014;115(1):14105. [504] Zhang H, Chen C, Deng H, Ren B, Zhao X, Lin D, et al. Ultrahigh ferroelectric response in fe modified 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3 single crystals. J Mater Chem C 2014;2(47):10124–8. [505] Ge W, Luo C, Zhang Q, Devreugd CP, Ren Y, Li J, Luo H, Viehland D. Ultrahigh electromechanical response in (1-x)(Na0.5 Bi0.5)TiO3-xBaTiO3 single-crystals via polarization extension. J Appl Phys 2012;111(9):93508. [506] Park J, Lee H, Kang SL. Solid-state conversion of (Na1/2Bi1/2)TiO3-BaTiO3-(K1/2Na1/2)NbO3 single crystals and their piezoelectric properties. Appl Phys Lett 2014;104(22):222910. [507] Ko S, Park J, Kim I, Won S, Chung S, Kang SL. Improved solid-state conversion and piezoelectric properties of 90(Na1/2Bi1/2)TiO3-5BaTiO3 -5(K1/2Na1/2)NbO3 single crystals. J Eur Ceram Soc 2017;37(1):407–11. [508] Chen C, Zhao X, Wang Y, Zhang H, Deng H, Li X, et al. Giant strain and electric-field-induced phase transition in lead-free (Na1/2Bi1/2)TiO3-BaTiO3-(K1/2Na1/2) NbO3 single crystal. Appl Phys Lett 2016;108(2):22903. [509] Chu R, Xu Z, Li G, Zeng H, Yu H, Luo H, et al. Ultrahigh piezoelectric response perpendicular to special cleavage plane in BaTiO3 single crystals. Appl Phys Lett 2005;86(1):12905. [510] Ren X. Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching. Nat Mater 2004;3(2):91–4. [511] Zhang LX, Ren X. In situ observation of reversible domain switching in aged Mn-doped BaTiO3 single crystals. Phys Rev B 2005;71(17):4108. [512] Zhang L, Ren X. Aging behavior in single-domain mn-doped BaTiO3 crystals: implication for a unified microscopic explanation of ferroelectric aging. Phys Rev B 2006;73(9). [513] Ma C, Tan X, Dul'Kin E, Roth M. Domain structure-dielectric property relationship in lead-free (1–x)(Bi1/2Na1/2)TiO3-xBaTiO3 ceramics. J Appl Phys 2010;108(10):104105. [514] Ma C, Tan X. In situ transmission electron microscopy study on the phase transitionsin lead-free (1–x)(Bi1/2Na1/2)TiO3-xBaTiO3 ceramics. J Am Ceram Soc 2011;94(11):4040–4. [515] Lu S, Xu Z, Su S, Zuo R. Temperature driven nano-domain evolution in lead-free Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 piezoceramics. Appl Phys Lett 2014;105(3):32903. [516] Qin Y, Zhang J, Gao Y, Tan Y, Wang C. Study of domain structure of poled (K, Na)NbO3 ceramics. J Appl Phys 2013;113(20):204107. [517] Zhang J, Gao Y, Qin Y, Yao W, Tian X. Comparative study of two (K, Na)NbO3 -based piezoelectric ceramics. J Appl Phys 2014;116(10):104106. [518] Zhang J, Qin Y, Gao Y, Yao W, Zhao M. Improvement of physical properties for KNN-based ceramics by modified two-step sintering. J. Am. Ceram. Soc 2014;97(3):759–64. [519] Qin Y, Zhang J, Tan Y, Yao W, Wang C, Zhang S. Domain configuration and piezoelectric properties of (K0.50Na0.50)1-xLix(Nb0.80Ta0.20)O3 ceramics. J Eur Ceram Soc 2014;34(16):4177–84. [520] Qin Y, Zhang J, Yao W, Wang C, Zhang S. Domain structure of potassium-sodium niobate ceramics before and after poling. J Am Ceram Soc 2015;98(3):1027–33. [521] López-Juárez R, Novelo-Peralta O, González-García F, Rubio-Marcos F, Villafuerte-Castrejón M. Ferroelectric domain structure of lead-free potassium-sodium niobate ceramics. J Eur Ceram Soc 2011;31(9):1861–4. [522] Tan X, Ma C, Frederick J, Beckman S, Webber KG. The antiferroelectric ↔ ferroelectric phase transition in lead-containing and lead-free perovskite ceramics. J Am Ceram Soc 2011;94(12):4091–107. [523] Ma C, Guo H, Beckman SP, Tan X. Creation and destruction of morphotropic phase boundaries through electrical poling: a case study of lead-free (Bi1/2Na1/ 2)TiO3-BaTiO3 piezoelectrics. Phys Rev Lett 2012;109(10):107602. [524] Guo H, Zhang S, Beckman SP, Tan X. Microstructural origin for the piezoelectricity evolution in (K0.5Na0.5)NbO3 -based lead-free ceramics. J Appl Phys 2013;114(15):154102. [525] Zakhozheva M, Schmitt LA, Acosta M, Jo W, Rödel J, Kleebe HJ. In situ electric field induced domain evolution in Ba(Zr0.2Ti0.8)O3-0.3(Ba0.7Ca0.3)TiO3 ferroelectrics. Appl Phys Lett 2014;105(11):112904. [526] Guo H, Voas BK, Zhang S, Zhou C, Ren X, Beckman SP, et al. Polarization alignment, phase transition, and piezoelectricity development in polycrystalline 0.5Ba (Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3. Phys Rev B 2014;90(1):93–7. [527] Tutuncu G, Li B, Bowman K, Jones JL. Domain wall motion and electromechanical strain in lead-free piezoelectrics: insight from the model system (1-x)Ba (Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 using in situ high-energy x-ray diffraction during application of electric fields. J Appl Phys 2014;115(14):144104. [528] Nahas Y, Akbarzadeh A, Prokhorenko S, Prosandeev S, Walter R, Kornev I, et al. Microscopic origins of the large piezoelectricity of leadfree (Ba, Ca)(Zr, Ti)O3. Nat Commun 2017;8:15944. [529] Gao JH, Xu XH, Wang Y, Liu YB, Zhang LX, Ke XQ, et al. Understanding the mechanism of large dielectric response in Pb-free (1–x)Ba(Zr0.2Ti0.8)O3–x (Ba0.7Ca0.3)TiO3 ferroelectric ceramics. Acta Mater 2017;125:177–86. [530] Gao JH, Hu XH, Zhang L, Li F, Zhang LX, Wang Y, Hao YS, Zhong LS, Ren XB. Major contributor to the large piezoelectric response in (1-x)Ba(Zr0.2Ti0.8)O3-x (Ba0.7Ca0.3)TiO3 ceramics: domain wall motion. Appl Phys Lett 2014(104):252909. [531] Turygin AP, Neradovskiy MM, Naumova NA, Zayats DV, Coondoo I, Kholkin AL, Shur VY. Domain structures and local switching in lead-free piezoceramics Ba0.85Ca0.15Ti0.90Zr0.10O3. J Appl Phys 2015(118):72002. [532] Zakhozheva M, Schmitt LA, Acosta M, Guo H, Jo W, Schierholz R, Kleebe HJ, Tan XL. Wide compositional rangein situelectric field investigations on lead-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 piezoceramic. Phys Rev Appl 2015(3):64018. [533] Fu J, Zuo R, Gao X. Electric field induced monoclinic phase in (Na0.52K0.48)(Nb1-ySbyO3 ceramics close to the rhombohedral-orthorhombic polymorphic phase boundary. Appl Phys Lett 2013;103(18):4825. [534] Zuo R, Fu J, Yin GZ, Li XL, Jiang JZ. Electric field induced phase instability in typical (Na, K)(Nb, Sb)O3-LiTaO3 ceramics near orthorhombic and tetragonal phase boundary. Appl Phys Lett 2012;101(9):84. [535] Fu J, Zuo R, Wu SC, Jiang JZ. Electric field induced intermediate phase and polarization rotation path in alkaline niobate based piezoceramics close to the rhombohedral and tetragonal phase boundary. Appl Phys Lett 2012;100(12):4121. [536] Iamsasri T, Tutuncu G, Uthaisar C, Wongsaenmai S, Pojprapai S, Jones JL. Electric field-induced phase transitions in Li-modified Na0.5K0.5NbO3 at the polymorphic phase boundary. J Appl Phys 2015;117(2):1153. [537] Lv J, Gao W, Li J, Li T, Long C, Lou X, et al. Large strain and strain memory effect in bismuth ferrite lead-free ceramics. J Mater Chem C 2017;5(37):9528–33.

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Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[538] Wang K, Li JF. Domain engineering of lead-free Li-modified (K, Na)NbO3 polycrystals with highly enhanced piezoelectricity. Adv Funct Mater 2010;20(12):1924–9. [539] Hong CH, Kim HP, Choi BY, Han HS, Son JS, Chang WA, et al. Lead-free piezoceramics - where to move on? J Materiomics 2016;2(1):1–24. [540] Rui Z, Bei J, Cao W, Amin A. Complete set of material constants of 0.93Pb(Zn 1/3Nb2/3)O3–0.07PbTiO3 domain engineered single crystal. J Mater Sci Lett 2002;21(23):1877–9. [541] Zhang R, Jiang B, Cao W. Elastic, piezoelectric and dielectric properties of domain engineered 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystal. J Appl Phys 2001;90(7):3471–5. [542] Liu X, Zhang S, Luo J, Shrout TR, Cao W. Complete set of material constants of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal with morphotropic phase boundary composition. J Appl Phys 2009;106(7):74112. [543] Yao FZ, Wang K, Li JF. Comprehensive investigation of elastic and electrical properties of li/ta-modified (K, Na)NbO3 lead-free piezoceramics. J Appl Phys 2013;113(17):174105. [544] Zhang S, Lim JB, Lee HJ, Shrout TR. Characterization of hard piezoelectric lead-free ceramics. IEEE Trans Ultras Ferr Freq Control 2009;56(8):1523. [545] Zheng L, Yi X, Zhang S, Jiang W, Yang B, Zhang R, et al. Complete set of material constants of 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 lead-free piezoelectric single crystal and the delineation of extrinsic contributions. Appl Phys Lett 2013;103(12):3683–5. [546] Zgonik M, Bernasconi P, Duelli M, Schlesser R, Günter P, Garrett MH, et al. Dielectric, elastic, piezoelectric, electro-optic, and elasto-optic tensors of BaTiO3 crystals. Phys Rev B 1994;50(9):5941. [547] Taghaddos E, Hejazi M, Safari A. Electromechanical properties of acceptor-doped lead-free piezoelectric ceramics. J Am Ceram Soc 2014;97(6):1756–62. [548] Zhang Y, Glaum J, Ehmke MC, Blendell JE, Bowman KJ, Hoffman MJ. High bipolar fatigue resistance of BCTZ lead-free piezoelectric ceramics. J Am Ceram Soc 2016;99(1):174–82. [549] Luo Z, Glaum J, Granzow T, Jo W, Dittmer R, Hoffman M, et al. Bipolar and unipolar fatigue of ferroelectric BNT-based lead-free piezoceramics. J Am Ceram Soc 2011;94(2):529–35. [550] Luo Z, Granzow T, Glaum J, Jo W, Rödel J, Hoffman M. Effect of ferroelectric long-range order on the unipolar and bipolar electric fatigue in Bi1/2Na1/2TiO3 -based lead-free piezoceramics. J Am Ceram Soc 2011;94(11):3927–33. [551] Yao FZ, Patterson EA, Wang K, Jo W, Rodel J, Li JF. Enhanced bipolar fatigue resistance in CaZrO3-modified (K, Na)NbO3 lead-free piezoceramics. Appl Phys Lett 2014;104(24):84. [552] Lou XJ. Polarization fatigue in ferroelectric thin films and related materials. J Appl Phys 2009;105(2):177397–601. [553] Zhou JJ, Wang K, Li F, Li J, Zhang XW, Wang QM. High and frequency-insensitive converse piezoelectric coefficient obtained in AgSbO3 -modified (Li, K, Na) (Nb, Ta)O3 lead-free piezoceramics. J Am Ceram Soc 2013;96(2):519–23. [554] Byeon S, Yoo J. Dielectric, piezoelectric properties and temperature stability in modified (Na, K, Li)(Nb, Ta)O3 ceramics for piezoelectric energy harvesting device. J Electroceram 2014;33(3–4):202–7. [555] Zheng M, Hou Y, Zhang L, Zhu M. High energy density lead-free piezoelectric ceramics for energy harvesting and derived from a sol-gel route. Eur J Inorg Chem 2016;2016(19):3072–5. [556] Coondoo I, Panwar N, Amorín H, Ramana VE, Algueró M, Kholkin A. Enhanced piezoelectric properties of praseodymium-modified lead-free (Ba0.85Ca0.15) (Ti0.90Zr0.10)O3 ceramics. J Am Ceram Soc 2015;98(10):3127–35. [557] Seo IT, Choi CH, Song D, Jang MS, Kim BY, Nahm S, et al. Piezoelectric properties of lead-free piezoelectric ceramics and their energy harvester characteristics. J Am Ceram Soc 2013;96(4):1024–8. [558] Yan X, Zheng M, Hou Y, Zhu M. Composition-driven phase boundary and its energy harvesting performance of BCTZ lead-free piezoelectric ceramic. J Eur Ceram Soc 2017;37(7):2583–9. [559] Janphuang P, Lockhart R, Uffer N, Briand D, Rooij NFD. Vibrational piezoelectric energy harvesters based on thinned bulk PZT sheets fabricated at the wafer level. Sensor Actuat A Phys 2014;210(1):1–9. [560] Yu H, Zhou J, Deng L, Wen Z. A vibration-based mems piezoelectric energy harvester and power conditioning circuit. Sensors-Basel 2014;14(2):3323. [561] Wu J, Shi H, Zhao T, Yu Y, Dong S. High Temperature BiScO3-PbTiO3 piezoelectric vibration energy harvester. Adv Funct Mater 2016;26:7186–94. [562] Kumar A, Sharma A, Kumar R, Vaish R. Finite element study on acoustic energy harvesting using lead-free piezoelectric ceramics. J Electron Mater 2017:1–12. [563] Lam KH, Lin DM, Chan HLW. Lead-free acoustic emission sensors. Rev Sci Instrum 2007;78(11):115109. [564] Jeong Y, Byeon S, Park M, Yoo J. Sensitivity properties of acoustic emission sensor using lead-free (Na, K, Li)(Nb, Ta, Zn)O3 system ceramics. Integr Ferroelectr 2012;140(1):123–31. [565] Jiang X, Rehrig PW, Hackenberger WS, Smith E, Dong S, Viehland D, et al. Advanced piezoelectric single crystal based actuators. Proc of SPIE - Int Soc Optical Eng 2005;5761:253–62. [566] Reichmann K, Feteira A, Li M. Bismuth sodium titanate based materials for piezoelectric actuators. Materials 2015;8(12):8467. [567] Robert Dittmer EAWJ. Large blocking force in Bi1/2Na1/2TiO3-based lead-free piezoceramics. Scripta Mater 2012;67:100–3. [568] Nagata H, Tabuchi K, Takenaka T. Fabrication and electrical properties of multilayer ceramic actuator using lead-free (Bi1/2K1/2)TiO3. Jpn J Appl Phys 2013;52(9S1):5K–9K. [569] Morozov MI, Bäurer M, Hoffmann MJ. Interaction of modified (K, Na)NbO3 ceramics with Ag-containing electrodes. J Am Ceram Soc 2011;94(10):3591–5. [570] Kim MS, Jeon S, Lee DS, Jeong SJ, Song JS. Lead-free NKN-5LT piezoelectric materials for multilayer ceramic actuator. J Electroceram 2009;23(2–4):372–5. [571] Lee KS, Yoo J, Hwang L, Lee KS, Yoo J, Hwang L, et al. Electrical properties of (Na, K, Li)(Nb, Sb, Ta)O3 ceramics for multilayer-type piezoelectric actuator. Ferroelectrics 2017;515(1):18–24. [572] Motoo K, Toda N, Fukuda T, Arai F, Kikuta K, Hirano SI, et al. Tailor-made multilayer piezoelectric actuator having large displacements and forces produced from lead-free piezoelectric ceramics 2006;1–6. [573] Nagata H, Hiruma Y, Takenaka T. Electric-field-induced strain for (Bi1/2Na1/2)TiO3-based lead-free multilayer actuator. J Ceram Soc Jap 2010;118(1380):726–30. [574] Long P, Yi Z. Fabrication and properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics and their multilayer piezoelectric actuators. Int J Appl Ceram Tec 2017;14. [575] Long P, Liu X, Long X, Yi Z. Dielectric relaxation, impedance spectra, piezoelectric properties of (Ba, Ca)(Ti, Sn)O3 ceramics and their multilayer piezoelectric actuators. J Alloy Compd 2017;706:234–43. [576] Wang XX, Or SW, Lam KH, Chan HLW, Choy PK, Liu PCK. Cymbal actuator fabricated using (Na0.46K0.46Li0.08)NbO3 lead-free piezoceramic. J Electroceram 2006;16(4):385–8. [577] Lam KH, Wang XX, Chan HLW. Lead-free piezoceramic cymbal actuator. Sensor Actuat A Phys 2006;125(2):393–7. [578] Doshida Y, Kishimoto S, Ishii K, Kishi H, Tamura H, Tomikawa Y, et al. Miniature cantilever-type ultrasonic motor using Pb-free multilayer piezoelectric ceramics. Jpn J Appl Phys 2007;46:4921–5. [579] Tamura H, Iwase M, Hirose S, Aoyagi M, Takano T, Tomikawa Y. Measurement of LiNbO3 rectangular plate under large vibration velocity of the first longitudinal and second flexural modes. Jpn J Appl Phys 2008;47(5):4034–40. [580] Li E, Kakemoto H, Hoshina T, Tsurumi T. A shear-mode ultrasonic motor using potassium sodium niobate-based ceramics with high mechanical quality factor. Jpn J Appl Phys 2008;47(9S):7702–6. [581] Li E, Sasaki R, Hoshina T, Takeda H, Tsurumi T. Miniature ultrasonic motor using shear mode of potassium sodium niobate-based lead-free piezoelectric ceramics. Jpn J Appl Phys 2009;48(9). N/A. [582] Hong CH, Han HS, Lee JS, Wang K, Yao FZ, Li JF, et al. Ring-type rotary ultrasonic motor using lead-free ceramics. J Sens Sci Technol 2015;24(4):228–31. [583] Seo IT, Kang IY, Cha YJ, Choi JH, Nahm S, Sung TH, et al. Piezoelectric properties of CuO-added (Na0.5K0.5)NbO3 ceramic multilayers. J Eur Ceram Soc 2012;32(5):1085–90. [584] Gao R, Chu X, Huan Y, Sun Y, Liu J, Wang X, et al. A study on (K, Na)NbO3 based multilayer piezoelectric ceramics micro speaker. Smart Mater Struct 2014;23(10):105018.

622

Progress in Materials Science 98 (2018) 552–624

T. Zheng et al.

[585] Ahn CW, Kim HS, Woo WS, Won SS, Seog HJ, Chae SA, et al. Chong HH. Low-temperature sintering of Bi0.5(Na, K)0.5TiO3 for multilayer ceramic actuators. J Am Ceram Soc 2015;98(6):1877–83. [586] Wu L, Xiao D, Wu J, Sun Y, Lin D, Zhu J, et al. Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers. J Eur Ceram Soc 2008;28(15):2963–8. [587] Chen Y, Mei K, Wong CM, Lin D, Chan H, Dai J. Ultrasonic transducer fabricated using lead-free BFO-BTO+Mn piezoelectric 1–3 composite. Actuators 2015;4(2):127–34. [588] Edwards GC, Choy SH, Chan HLW, Scott DA, Batten A. Lead-free transducer for non-destructive evaluation. Appl Phys A 2007;88(1):209–15. [589] Hagh NM, Jadidian B, Ashbahian E, Safari A. Lead-free piezoelectric ceramic transducer in the donor-doped K1/2Na1/2NbO3 solid solution system. IEEE Trans Ultrason Ferr Freq Control 2008;55(1):214–24. [590] Bantignies C, Filoux E, Mauchamp P, Dufait R, Thi MP, Rouffaud R, et al. Lead-free high-frequency linear-array transducer (30 MHz) for in vivo skin imaging. IEEE international ultrasonics symposium (IUS). IEEE; 2014. p. 785–8. [591] Shen ZY, Li JF, Chen R, Zhou Q, Shung KK. Microscale 1-3-type (Na, K)NbO3-based Pb-free piezocomposites for high-frequency ultrasonic transducer applications. J Am Ceram Soc 2011;94(5):1346. [592] Ma J, Xue S, Zhao X, Wang F, Tang Y, Duan Z, Wang T, Shi W, Yue Q, Zhou H. High frequency transducer for vessel imaging based on lead-free Mn-doped (K0.44Na0.56)NbO3 single crystal. Appl Phys Lett 2017;111(9):92903. [593] Chen Z, Zheng L, Cao W, Chen X, Chen R, Li R, et al. High-frequency ultrasonic imaging with lead-free (Na, K)(Nb, Ta)O3 single crystal. Ultrason Imag 2017. [1385066630]. [594] Ou-Yang J, Zhu B, Zhang Y, Chen S, Yang X, Wei W. New KNN-based lead-free piezoelectric ceramic for high-frequency ultrasound transducer applications. Appl Phys A 2015;118(4):1177–81. [595] Hejazi MM, Jadidian B, Safari A. Fabrication and evaluation of a single-element Bi0.5Na0.5TiO3-based ultrasonic transducer. IEEE Trans Ultrason Ferr Freq Control 2012;59(8):1840–7. [596] Chen Y, Jiang XP, Luo HS, Dai JY, Chan HL. High-frequency ultrasonic transducer fabricated with lead-free piezoelectric single crystal. IEEE Trans Ultrason Ferr Freq Control 2010;57(11):2601. [597] Yan X, Lam KH, Li X, Chen R, Ren W, Ren X, et al. Lead-free intravascular ultrasound transducer using BZT-50BCT ceramics. IEEE Trans Ultrason Ferr Freq Control 2013;60(6):1272. [598] Lee ST, Lam KH, Zhang XM, Chan HL. High-frequency ultrasonic transducer based on lead-free BSZT piezoceramics. Ultrasonics 2011;51(7):811–4. [599] Zhou Q, Xu X, Gottlieb EJ, Sun L, Cannata JM, Ameri H, et al. Pmn-pt single crystal, high-frequency ultrasonic needle transducers for pulsed-wave doppler application. IEEE Trans Ultrason Ferr Freq Control 2007;54(3):668–75. [600] Zhou Q, Wu D, Jin J, Hu C, Xu X, Williams J, et al. Design and fabrication of PZN-7%PT single crystal high frequency angled needle ultrasound transducers. IEEE Trans Ultrason Ferr Freq Control 2008;55(6):1394–9. [601] Sun P, Zhou Q, Zhu B, Wu D. Design and fabrication of PIN-PMN-PT single-crystal high-frequency ultrasound transducers. IEEE Trans Ultrason Ferr Freq Control 2009;56(12):2760–3. [602] Lam KH, Hong FJ, Fan Z, Wei R, Zhou Q, Shung KK. Development of lead-free single-element ultrahigh frequency (170–320MHz) ultrasonic transducers. Ultrasonics 2013;53(5):1033. [603] Wu DW, Zhou Q, Geng X, Liu CG, Djuth F, Shung KK. Very high frequency (beyond 100 MHz) PZT kerfless linear arrays. IEEE Trans Ultrason Ferr Freq Control 2009;56(10):2304–10. [604] Ditas P, Hennig E, Kynast A. Lead-free piezoceramic materials for ultrasonic applications. Sens Measur Syst 2014:1–4. [605] Tou T, Hamaguti Y, Maida Y, Yamamori H, Takahashi K, Terashima Y. Properties of (Bi0.5Na0.5)TiO3-BaTiO3-(Bi0.5Na0.5)(Mn1/3Nb2/3)O3 lead-free piezoelectric ceramics and its application to ultrasonic cleaner. Jpn J Appl Phys 2009;48(7):3G–7G. [606] Hiruma Y, Watanabe T, Nagata H, Takenaka T. Piezoelectric properties of (Bi1/2Na1/2)TiO3-based solid solution for lead-free high-power applications. Jpn J Appl Phys 2014;47(9):7659–63. [607] Nagata H, Takai K, Nomura Y, Sato S, Hiruma Y, Takenaka T. Vibration velocities under high-power driving on perovskite-type lead-free ferroelectric ceramics. In: IEEE international symposium on the IEEE applications of ferroelectrics (ISAF); 2010. p. 1–4 [608] Nagata H, Seki M, Noumura Y, Hiruma Y, Takenaka T. High-power piezoelectric characteristics of nontextured bismuth layer-structured ferroelectric ceramics. Jpn J Appl Phys 2011;50(9S2):09ND05. Prof. Jiagang Wu has published more than 150 papers in the field of ferroelectric and piezoelectric materials as the first author or corresponding author since 2007 [Chem. Rev. 2015, 115, 2559, Prog. Mater. Sci., 2016, 84, 335, J. Am. Chem. Soc. (2014, 136, 2905, 2016, 138, 15459), Adv. Mater., 2016, 28, 8519, Energy Environ. Sci., 2017, 10, 528, etc]. All the published papers have been cited more than 3100 times, and H index is 36. He received his bachelor degree from Sichuan University in 2003 and Ph. D. degree from Sichuan University in 2008, and worked as the Singapore Millennium Postdoctoral Fellowships (SMF-PDF) at National University of Singapore from 2008.11-2010.11. He was an associate professor at Department of Materials Science of Sichuan University from 2011.2 to 2015.9. Since 2015.9, he has been promoted to professor in Sichuan University. Currently, he is vice dean of the College of Materials Science and Engineering in Sichuan University. His main research interest is the composition design and properties modification of ferroelectric/ piezoelectric materials.

Miss Ting Zheng (First author) received her bachelor degree from Sichuan University in 2014. Currently, she is a PhD candidate in the College of Materials Science and Engineering of Sichuan University. Her main research interest is composition design and phase boundaries modification of ferroelectric/piezoelectric materials. She published more than 15 papers in the field of lead-free piezoelectric materials as the first author [Energy Environ. Sci., 2017, 10, 528, ACS Appl. Mater. Interfaces, J. Mater. Chem. A/C,etc].

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Prof. Dingquan Xiao: He graduated from Sichuan University as an undergraduate in 1968 and as a postgraduate in 1980, studied in Queen Mary College, University of London, UK from 1980 to 1982, and worked as a Visiting Professor in Penn State University, USA in 1990 and in University of Houston, USA in 1998. He has worked at Sichuan University since 1983. He was a member of the National Advanced Materials Committee of China, and a member of the Chinese Materials Research Society and also the Chinese Physics Society. He has published more than 200 papers in English journals since 2000, and all published papers have been cited more than 3000 times. At the 9th International Meeting on Ferroelectricity (August 24–29, 1997, Seoul, Korea), he was aware of the demands of eco-materials or environmental friendly materials, pointed out that ferroelectric materials should comprise non-hazardous substances with a small environmental load, and the manufacturing processes for these materials should also be with a small environmental load, and “the research on lead-free piezoelectric ceramics is a typical example.” [See D. Q. Xiao, J. Korean Phys. Soc. 1998, 32, S1798]. Since then, his group has paid much attention to the research of lead-free ferroelectric/piezoelectric materials, especially for perovskite structure lead-free piezoelectric ceramics.

Prof. Jianguo Zhu received his MS degree at 1987 and PhD degree at 1998 in Physics from Sichuan University, respectively. At 1987~1988, he worked at Department of Physics, Sichuan University as an assistant professor. Since 1988, he has worked at Department of Materials Science, Sichuan University as an assistant professor, associated professor and professor, respectively. His research interests manly focus on the controllable-synthesis, characterization and properties of ferroelectric, piezoelectric and dielectric functional materials. ZHU is the author of over 300 peer reviewed articles, 9 books, over 20 Chinese patents and given about 50 invited lectures in various meetings and conferences.

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