Evaluation of high strain response in lead-free BNBTFS-xNb ceramics by structure and ferroelectric characterizations

Journal Pre-proof Evaluation of high strain response in lead-free BNBTFS-xNb ceramics by structure and ferroelectric characterizations Muhammad Habib, Muhammad Munir, Salman Ali Khan, Tae Kwon Song, Myong-Ho Kim, Muhammad Javid Iqbal, Ibrahim Qazi, Ali Hussain PII:

S0022-3697(18)33312-2

DOI:

https://doi.org/10.1016/j.jpcs.2019.109230

Reference:

PCS 109230

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 5 December 2018 Revised Date:

12 October 2019

Accepted Date: 14 October 2019

Please cite this article as: M. Habib, M. Munir, S.A. Khan, T.K. Song, M.-H. Kim, M.J. Iqbal, I. Qazi, A. Hussain, Evaluation of high strain response in lead-free BNBTFS-xNb ceramics by structure and ferroelectric characterizations, Journal of Physics and Chemistry of Solids (2019), doi: https:// doi.org/10.1016/j.jpcs.2019.109230. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Evaluation of high strain response in lead-free BNBTFS-xNb ceramics by structure and ferroelectric characterizations Muhammad Habib1, Muhammad Munir1, Salman Ali Khan1, Tae Kwon Song1, Myong-Ho Kim1, Muhammad Javid Iqbal2, Ibrahim Qazi3, and Ali Hussain3,* 1

School of Materials Science and Engineering, Changwon National University, Changwon 51140, Republic of Korea. 2

3

Department of Physics, University of Peshawar, Peshawar 25120, Pakistan.

Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan.

Abstract Lead-free piezoelectric ceramics of the form (Bi0.5Na0.5)0.945Ba0.055[Ti0.98(Fe0.5Sb0.5)0.02](1x)NbxO3

(BNBTFS-xNb with x = 0.00–0.03) in the vicinity of the morphotropic phase

boundary were synthesized by a conventional mixed oxide route. The crystal structure, microstructure, and electromechanical properties were investigated as a function of different Nb5+ concentrations. Increasing amount of Nb5+ disrupts the long-range ferroelectric order and induces a relaxor phase with pseudocubic symmetry, which results in a large strain (Smax = 0.39%, equivalent to 666 pm/V) for a critical composition (x = 0.01). Furthermore, ex situ X-ray diffraction of the poled and unpoled ceramics ( = 0.00 and 0.01) was performed, focusing on the origin of the high strain response. The results suggest that such high strain response originates from the compositionally induced phase transition and electric field– induced reversible phase transition from the ergodic relaxor phase to a temporary long-range metastable (non-ergodic) ferroelectric phase. Keywords: Lead-free piezoceramics, Ergodic, Non-ergodic, Polar nanoregions _____________________________________________ *Corresponding author. Tel: +92-51-9075747 Fax: +92-51-927-3310 Email Address: [email protected] (Ali Hussain)

1. Introduction Lead-based piezoelectric materials such as Pb(Zr,Ti)O3 have technological importance because of their excellent piezoelectric performance and their use in practical applications such as sensors, transducers, and actuators [1-4]. These ceramics are facing restrictions in many countries because of their hazardous nature, whereby they damage human health and their disposal pollutes the environment [5]. To look for alternatives, many researchers around the world are actively studying lead-free materials that are compatible with lead-based ceramics [6-11]. Among lead-free ceramics, Bi0.5Na0.5TiO3, (BNT) is very interesting because it has a perovskite structure with rhombohedral symmetry (R3c) at room temperature, has a high Curie temperature (TC = 320 °C), has a large remnant polarization (Pr = 38 µC/cm2), and has good electromechanical properties, having the potential to replace lead-based materials [12, 13]. However, the main drawbacks associated with pure BNT are the low depolarization temperature (Td), high conductivity, high coercive field (Ec), and difficulties in electrical poling, which hinder its use in practical applications [14, 15]. Large flexibility in the piezoelectric and ferroelectric properties are possible around the morphotropic phase boundary (MPB) [16, 17]. Various compositional modifications such as (K0.5Na0.5)NbO3 [18], SrTiO3 [19], BaZrO3 [20] and Ba(Al0.5Sb0.5)O3 [21] have been introduced into pure BNT ceramics. Analogously to lead-based materials, some improvements in the electric field–induced strain (d33* = 560 pm/V) have been achieved [22] but the overall performance is still not remarkable for actuator applications. Among all other lead-free solid solutions, Bi0.5Na0.5TiO3- BaTiO3 ceramic is a promising candidate because of excellent electromechanical properties along with an inherently large strain response around the ferroelectric-relaxor phase transition [23, 24]. The main features that differentiate normal ferroelectric from the relaxor ferroelectric are frequency-dependent dielectric constant, diffuse phase transition, and the absence of spontaneous polarization [25]. Basically, the relaxor phase originates from the chemical inhomogeneity of (A/B-site) on the

unit cell level which creates polar clusters at nanometer level known as polar nanoregions (PNRs) [26, 27]. The existence of PNRs have been confirmed through piezoresponse force microscopy (PFM) [28] and transmission electron microscopy (TEM) [29, 30]. It is well documented that the origin of the good electromechanical properties in BNT-based ceramic is attributed to the electric field induced PNRs growth and field dependence phase transition [31-35]. The ergodic phase transforms to a temporary long-range ferroelectric phase while the non-ergodic phase reverts to a permanent long-range ferroelectric phase under the application of an external electric field [36, 37]. Recently, Li et al. [38] studied the dielectric, ferroelectric, and electric field–induced strain response of lead-free (Fe, Sb)-modified (Bi0.5Na0.5)0.935Ba0.065TiO3 ceramics and investigated very high strain (Smax = 0.37%) at the MPB composition. To examine this material

system

in

detail,

we

selected

a

hard-type

base

composition

(Bi0.5Na0.5)0.945Ba0.055[Ti0.98(Fe0.5Sb0.5)0.02]O3 (BNBTFS; rhombohedral side of the MPB) to obtain further insight and possibly enhance its electric field–induced strain. We developed (Bi0.5Na0.5)0.945Ba0.055[Ti0.98(Fe0.5Sb0.5)0.02](1-x)NbxO3 (BNBTFS-xNb with x = 0.000, 0.005, 0.010, 0.020, 0.025, and 0.030) ceramics with the aim of achieving good electromechanical properties. It was expected that donor ion substitution (Nb5+) would induce a compositiondependent phase transition from a ferroelectric phase to a relaxor phase that will result in a high strain response for a specific composition. Analyzing the data obtained from the crystal structure and the ferroelectric, dielectric, and piezoelectric properties, a schematic phase diagram was constructed to gain better insight into the electric field–induced strain response of BNT-based ceramics. 2. Experimental procedure Lead-free piezoelectric ceramics of the form BNBTFS- Nb with

= 0.000, 0.005,

0.010, 0.020, 0.025, and 0.030 were produced by a conventional solid-state reaction method.

All initial ingredients, such as Bi2O3, Na2CO3, BaCO3, TiO2, Fe2O3, Sb2O3, and Nb2O5 (99.9%, Sigma-Aldrich), were used as the starting precursors according to their stoichiometric formula. All these initial precursors were wet-mixed in ethanol and ball-milled for 24 h in an epicycle rotating bottle containing zirconia balls as a milling medium. The slurries were dried and subsequently calcined at 850 °C for 2 h. The calcined powders were ball-milled for 24 h, dried, and mixed with 5 mol% polyvinyl alcohol as a binding agent. The polyvinyl alcohol–mixed powder was dried at 120 °C and passed through a 150 µm mesh sieve. The disk-shaped samples were prepared with 10 mm diameter by application of 100 MPa uniaxial pressure. The unfired green pellets were sintered in a crucible at 1150 °C for 2 h. To control Bi3+ and Na+ evaporation, all pellets were embedded in powder of the same composition. The crystal structure was confirmed by X-ray diffraction (XRD; Miniflex II, Rigaku), while the surface morphology of the as-sintered samples was examined with a scanning electron microscope (JP/JSM5200, JEOL, Japan). Both sides of the pellets were silver-pasted and then fired at 650 °C for 30 min to make the electrode for the measurements of the electromechanical properties. The pellets were poled in a silicone oil bath with a DC electric field of 50 kV/cm for 20 min, and the static piezoelectric constant (

) was measured with a

d33 meter (ZJ-6B, HC Materials). All characterizations were performed after aging for at least 24 h. Ferroelectric hysteresis loops (P-E) were measured with a ferroelectric test system (Precision LC, Radiant) at 10 Hz in the silicone oil bath. The strain versus electric field (S-E) curves were measured with a linear variable differential transformer (Millimar 1240, Mahr) in triangular wave mode at 50 mHz. The temperature-dependent dielectric constant and dielectric loss were measured by an impedance analyzer (HP4194A, Agilent Technologies) attached to a computerized controlled chamber. Results and discussions

Fig. 1(a) shows the XRD patterns of Nb5+-modified BNBTFS ceramics measured at room temperature in the 2θ range from 20° to 70°. All compositions show a perovskite structure within the detection limit of the XRD instrument. According to previous reports, Nb5+ solubility is limited to approximately 3 mol% in BNT-based ceramics [37, 39]. However, in the present work, a small amount of added Nb5+ causes a significant change in the crystal structure of BNBTFS ceramics. Peak fitting deconvolution is a good and easy method to analyze the crystal structure of perovskite ceramics [40-44]. For better understanding of the crystal structure, peak fitting deconvolution was performed on the (111) peak at around 40° with use of a Lorentzian function as shown in Fig. 1(b). In the pure ceramic (x = 0.00) a predominant rhombohedral phase exists as can be seen from the relatively small intensity (area) of the (111)T peak as compared with the (111)R and (111)R peaks, where R and T represent the rhombohedral and tetragonal phases. On the other hand, the ceramic with x = 0.005 showed relatively equivalent intensity (area) of all three peaks, suggesting the coexistence of rhombohedral and tetragonal phases. However, with increasing amount of Nb5 (x ≥ 0.01), the (111)T and (111)R peaks are suppressed, while the (111)R peak becomes dominant. The (111)R peak resembles a cubic peak, indicative of a compositiondependent phase transformation to a pseudocubic symmetry. On the unit cell level, this crystal structure transformation may be due to the inhomogeneity of the Nb5+ donor ion. Similar phase transformations were observed previously in Nb5+-modified BNT-based ceramics [37, 39, 45, 46]. For further analysis, the software program MAUD was used for crystal structure refinement. Fig. 2 exemplary shows the characteristic plots of the pure ceramic (x = 0.00) and the ceramic with x = 0.01. The pure ceramic fits with rhombohedral (R3c, JCPDF no. 2103297) and tetragonal (P4mm, JCPDF no. 1507756) phases. However, the ceramic with x

= 0.01 matches a single phase of pseudocubic symmetry (

3 , JCPDF no. 1542140). The

goodness of fit was calculated with Eq. (1):

χ2 = (Rwp/Rexp)2,

(1)

where Rwp and Rexp represent the weighted R factor and the expected R factor, respectively [47]. χ2 gradually decreases with increasing of Nb5+ concentration (x ≥ 0.01), indicating that the pseudocubic phase fraction is becoming more dominant. The complete detail for Rwp, Rexp, and χ are given in Table 1.

Fig 3 shows the change of the tetragonal lattice constants ( lattice constant (

), tetragonality ( /

and

), rhombohedral

), and rhombohedral lattice distortion (90° − αR)

with increasing Nb5+ content. It can be seen that

and a decrease and

increases with 0.5

mol% Nb5+ doping. However, above this critical amount of the chemical modifier, the crystal structure changes to a cubic-like phase; as a result,

decreases, while both

increase. The highest crystal structure distortion (90° − αR = 0.102° and

/

and = 1.004) is

observed for the x = 0.005 composition. The structural distortion gradually decreases for the x ≥ 0.01 compositions, and results in a phase transition to a cubic-like phase. The increasing and decreasing trend in the crystal structure distortion and lattice parameter show good agreement with the XRD peak fitting profile. Fig. 4 presents scanning electron micrographs of the as-sintered surface of the Nb5+modified BNBTFS ceramics (x = 0.00–0.03). In piezoelectric ceramics, the grain size plays an important role in changing the electromechanical properties [48, 49]. All the ceramics were well sintered and have similar grain morphologies. Image J was used to find the average

grain size. The pure ceramic and the ceramics with a small amount of Nb5+ exhibit a relatively wide grain size distribution and large grain size. However, a higher amount of Nb5+ inhibits grain growth and results in a narrow grain size distribution. The B-site Nb5+ donor ion in BNT-based ceramics usually creates cation vacancies, which are thermodynamically more stable at the grain boundary relative to the interior of the grain [50]. These charged defects would be pinned at the grain boundaries, weakening the mass transport during sintering and resulting in relatively small grains size [51]. A similar grain growth–inhibiting behavior was observed previously in Nb5+-substituted BNT-based ceramics [50-53]. Fig. 5 presents the average grain size and density as a function of Nb5+ content. For the x = 0.000 and x = 0.005 ceramics, the average grain was 2.34 µm and 2.23 µm, respectively. However, for the x = 0.010, x = 0.020, x = 0.025, and x = 0.030 ceramics, the grain size decreases to 1.68 µm, 1.64 µm, 1.58 µm, and 1.52 µm, respectively. The density of all the ceramics was measured on the basis of Archimedes’ principle, with use of xylene oil. The lower density ( = 5.74 g/cm2) for the x = 0.00 composition is associated with the low densification, and shows good agreement with the scanning electron micrographs. For the optimum composition (x = 0.010) a nearly pore-free microstructure was obtained with relatively high density ( = 5.85 g/cm2), which decreases slightly for higher Nb5+ content. This suggests that a small amount of Nb5+ is a more effective chemical modifier for achieving high densification. The densities obtained in the current work are consistent with previous results for BNT-based ceramics [54]. The composition-dependent electric field–induced polarization (P-E) loops of the Nb5+-modified BNBTFS ceramics (with x = 0.00, 0.005, 0.010, 0.020, 0.025, and 0.030) were measured as shown in Fig. 6(a). The corresponding maximum polarization (Pmax), remnant polarization (Pr), and coercive field (Ec) of the P-E loops are given in Fig. 6(b). The selected base composition without Nb5+ doping exhibits a typical ferroelectric behavior and a wellsaturated P-E hysteresis loop with relatively large Pr ≈ 35 μC/cm2. However, the long-range

ferroelectric order is disrupted with Nb5+ addition, and as a result Pr decreases. The 0.5 mol% Nb–modified BNBTFS ceramic illustrates the combined effect of the ferroelectric phase and the ergodic phase as evident from the pinched-type P-E hysteresis loop [55, 56]. Similar pinching behavior was observed before in Nb-modified BNT-based ceramics [37, 39, 45]. In the light of the current results and previous reports, a possible explanation for the pinching behavior of the P-E loop is the presence of an incipient nonpolar state (termed an ergodic state) at zero field, which reverts to a polar state (termed a nonergodic state) on the application of an external electric field. In contrast, for higher Nb5+ content, the P-E loops become slimmer, where both polarization and Ec decrease drastically. The Nb5+ donor substitution disrupts the long-range ferroelectric order, leaving a nonpolar phase at zero electric field that transforms back to a ferroelectric phase on the application of an external electric field. According to the preceding discussion, the P-E hysteresis loop profile shows good agreement with the XRD plots in Figs. 1, 2, and 3. The temperature-dependent P-E hysteresis loop data were collected for the = 0.005 and = 0.01 compositions under an applied electric field of 50 kV/cm and are provided in Fig. 7. At room temperature, the non-ergodic ceramic ( = 0.005) has a pinched loop with relatively high Pr = 15 µC/cm2 and Ec = 13 kV/cm. However, Pmax, Pr, and Ec progressively reduce as the temperature increases. The pinching phenomenon is retained up to 75 °C (TF-R), and then completely vanishes because of the development of an ergodic phase, which is consistent with the findings of previous studies. A similar phenomenon was noticed before in other BNT-based ceramics [57]. For the x = 0.010 composition, the room-temperature loop exhibits an ergodic relaxor characteristic, and a very small change was observed in Pr and Ec with increasing temperature as shown in Fig. 7(b). In piezoelectric materials, the electric field–induced strain is an important factor for actuator applications. For the enhancement of the electric field–induced strain, it is important

that the base composition intrinsically has a long-range ferroelectric phase. The chemical modifier disrupts the long-range ferroelectric phase and stabilizes the short-range ergodic relaxor phase, with corresponding fragmentation of domains, resulting in an electrostriction effect [50]. For a low amount of Nb5+ (x < 0.01), the alignment of long-range ferroelectric domains is preserved after the removal of the applied electric field, as is evident from the noticeable negative strain (Sneg = 0.25%) of the bipolar S-E curve as shown in Figs. 8(a) and 9(a). The bipolar S-E curves (for x ≥ 0.01) transform into sprout-shaped curves with significant increase in the positive strain (Smax ≈ 0.39%). Such a drastic increase in the strain is associated with the reversible electric field–induced phase transformation from the nonergodic relaxor to the metastable ergodic state owing to their comparable free energies. The Nb5+ donor ion substitution in the present case obviously creates some charge defects inside the grains. The chemical heterogeneity disrupts the long-range ferroelectric phase and stabilizes the short-range ergodic relaxor phase, with the corresponding fragmentation of domains [50]. The alignment of dynamically fluctuating PNRs in the ergodic state cannot be preserved after removal of the applied electric field, and as a result a large strain is obtained for the critical composition (x = 0.01). The disruption of the long-range ferroelectric phase and the formation of PNRs is evident from the pinching behavior of the P-E hysteresis loops [Fig. 6(a)]. Similar behavior was observed before in other Nb5+-modified BNT-based ceramics [51, 58]. Recently Liu et al. [29] confirmed the electric field–induced growth of micrometer size domains from nanometer size domains by in situ transmission electron microscopy analysis. Fig. 9(a) shows the composition dependence of the normalized unipolar (d33*= Smax/Emax) strain and the bipolar butterfly curves. It is interesting to observe a sudden drop in Sneg and a simultaneous increase in the positive strain response with increasing amount of Nb5+. The reason for the high strain response and reduction in Sneg may possibly be due to electric field–induced ferroelectric micrometer-sized domains that are preserved and cannot

revert to nanometer-sized domains after the removal of the applied electric field [25, 35, 46]. The shape and trend of the bipolar strain and unipolar strain show good agreement with previous results for BNT-based ceramics [16-21]. The large d33* = 666 pm/V obtained at a relatively low electric field (60 kV/cm) is best than previously reported values as listed in Table 2. Furthermore, besides Pr and d33*, the static piezoelectric constant (d33) drastically drops for compositions with x ≥ 0.010 as shown in Fig. 9(b). Such a drastic decrease in d33 can be explained through the thermodynamic theory of ferroelectrics i.e., d33 =



,

where Q11 represents the electrostrictive coefficient and remains constant for an ABO3 perovskite structure). This relation shows that large Pr and εr leads to a high static piezoelectric constant, which is also observed in the current work. High values of d33 = 168 pC/N and d33 = 161 pC/N were obtained for the x = 0.000 and x = 0.005 compositions because of the large polarization, such as Pr = 34.34 µC/cm2 and Pr = 31.33 µC/cm2. According to previous reports, an applied electric field changes the non-ergodic state into an irreversible/permanently long-range ferroelectric state, yielding high d33 ≈ 220 pC/N and large Pr ≈ 32 µC/cm2 [23]. In contrast, the ergodic state changes temporarily into a longrange ferroelectric state under the applied electric field, but cannot sustain its ferroelectricity and reverts to its original state after removal of the applied electric field [31, 36, 59]. Therefore, in the current work the x = 0.00 and x = 0.005 compositions can be considered as having rhombohedral symmetry with a non-ergodic state that exhibits high d33 ≥ 160 pC/N and large Pr ≥ 30 µC/cm2 as is evident from Figs. 6 and 9. On the other hand, the compositions with x > 0.010 can be regarded as having a pseudocubic phase with an ergodic state as evident from the small d33 ≤ 60 pC/N, Pr ≤ 6 µC/cm2, and Ec ≤ 9 kV/cm. Particularly for the x = 0.010 composition, the ergodic and nonergodic states coexist, which become the origin of the high strain response.

For practical applications, fatigue characteristics are desirable to gauge the reliability of long-term stability during electrical field cycling [60]. Fig. 10 shows the variation of the unipolar strain for the x = 0.010 composition at 60 kV/cm for different numbers of cycles. The normalized piezoelectric constant, d33*, shows only a 2.7% decrease after 104 cycles. A variation in the electromechanical strain with electric field cycling was observed before in BNT-based ceramics. This fatigue-free behavior can be attributed to the reversible electric field–induced phase transition between the long-range ferroelectric phase and the short-range ergodic relaxor ferroelectric phase [61]. To further clarify whether the origin of the high strain is due to the electric field– induced phase transformation, the electrical poling effect on the crystal structure was investigated. According to previous reports, the poling field brings about a significant change in the XRD profile of polar phases (rhombohedral or tetragonal) and has no effect on the nonpolar cubic phase [62, 63]. Therefore, XRD of the poled and unpoled ceramic was measured for the

= 0.00 and x = 0.01 compositions (Fig. 11). For the x = 0.000 ceramic, a

significant variation in the (111) peak of the XRD pattern after poling suggests a permanent switching of the ferroelectric domains. Therefore, the observed high d33 = 168 pC/N of the pure sample is due to its dominant polar rhombohedral phase as indicated in Fig. 9(b). On the other hand, for the x = 0.010 ceramic no noticeable change was observed in the XRD pattern of the poled ceramic. Hence, it is suggested that Nb5+ donor substitution inhibits the grain growth and creates a PNR inside the grain. The applied poling field is not able to overcome the random fields established by the thermodynamically fluctuating PNRs [50]. Under the applied poling field, a temporary phase transition occurs from the nonferroelectric cubic phase with high-entropy PNR domains to the ferroelectric phase with low-entropy PNR domains. However, after removal of the applied poling field, the metastable ferroelectric state will revert to the initial short-range ergodic phase (high-entropy PNR domains), which

becomes the origin of the high d33* = 666 pm/V given in Fig. 9(a). A similar explanation was provided before for the origin of the high strain response [64, 65]. Fig. 12 shows the temperature dependence of the dielectric constant ( ) and the corresponding dielectric loss (tan ) of Nb-modified BNBTFS ceramics (with x = 0.00, 0.01, 0.02, and 0.03). Two diffuse transition peaks can be observed in the dielectric constant and dielectric loss curves. The low-temperature shoulder is known as the depolarization temperature (Td) and the frequency-dispersive high-temperature dielectric constant peak is named as Tm. According to recent reports, Td is related to the relaxation of PNRs of the rhombohedral phase, while Tm is associated with the tetragonal PNRs emerging from the rhombohedral PNRs [51, 66, 67]. It is clear that the high-temperature dielectric constant peak becomes broader with increase of Nb5+ concentration. The low-temperature dielectric constant peak is also referred to a phase transition from the ferroelectric phase to the relaxor phase (TF-R) that shifts down to room temperature with increasing amount of Nb5+, analogously to results obtained previously for other BNT-based ceramics [46, 67, 68]. The increase in diffuseness in the dielectric constant strongly suggests a relaxor phase reported in other studies in BNT-based ceramics [69-71]. The sudden increase in the dielectric loss above 350 °C in BNT-based ceramics is usually related to the space charge conduction mechanism of the ionic species, such as bismuth (

) and oxygen (

••

) vacancies [46].

Conclusions Nb-modified lead-free BNBTFS piezoelectric ceramics were successfully synthesized by a solid-state reaction method. The effect of Nb5+ addition on the structural, ferroelectric, and piezoelectric properties was investigated. The long-range ferroelectric order is disrupted with increasing Nb5+ concentration due to a phase transition from a rhombohedral-tetragonal mixed phase to a relaxor pseudocubic phase. This structural transition causes a significant reduction in Pr (from 34.3 C/cm2 to 1.9 C/cm2) and Ec (from 24 kV/cm to 5.3 kV/cm)

when the Nb5+ concentration is changed from 0.00 to 0.03. In addition, the static piezoelectric constant of the pure BNBTFS ceramic decreased from 168 pC/N to 19 pC/N for 3 mol% Nb5+ substitution. However, a significant enhancement occurred in the field-induced strain (Smax = 0.39%) and the corresponding dynamic piezoelectric constant (d33* = 666 pm/V) for the x = 0.01 composition. Such a high strain response is attributed to the combined effect of the composition-dependent phase transition from an ergodic relaxor to a nonergodic state and the electric field–induced temporary reversible structural phase transition from the dominant nonferroelectric phase to the long-range ferroelectric phase.

Acknowledgments This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012-0009457) and the Ministry of Education (2016R1A2B2016348), South Korea.

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Table 1. Refinement factors for BNTBTFS-xNb ceramics with x = 0.00–0.03. !"

χ2

534.56936

263.27795

2.0304373

1507756

521.33265

263.27795

1.9801607

R3c

2103295

369.48271

174.07528

2.1225456

P4mm

1507756

363.17096

174.07528

2.0862869

JCPDF no.

0.000

Space group R3c

2103295

0.000

P4mm

0.005 0.005

Composition (x)

0.010

3

1542140

449.40023

166.15842

2.7046492

0.020

3

1542140

417.90934

171.30061

2.4396255

0.025

3

1542140

385.04204

160.81157

2.3943678

0.030

3

1542140

417.41726

178.54451

2.337889

Table. 1. Comparison of the static and dynamic piezoelectric constants and remnant polarization with the applied electric field of Bi0.5Na0.5TiO3 (BNT) or (Bi,Na,K)TiO3 (BNKT)-based piezoelectric ceramics. d33*

d33

(pm/V)

(pC/N)

BNKT-xNb

641

152

70

18.5

[9]

BNKT-(K,Na)SbO3

513

-

80

35

[55]

BNT-xBa(Al,Ta)O3

448

127

80

30

[56]

BNT-Bi(Zn,Ti)O3

550

165

60

24

[72]

BNKT-(Ba,Sr)TiO3

650

-

60

32

[73]

BNBTFS-xNb

666

168

60

35

This work

Composition

Ea

Pr

Reference

(kV/cm) (μC/cm2)

Figures Fig. 1. X-ray diffraction pattern of BNBTFS-xNb ceramics with

= 0.00, 0.005, 0.01, 0.02,

0.025, and 0.03 measured at room temperature in the 2# range from 20° to 70°. Fig. 2. Rietveld-refined X-ray diffraction patterns for BNBTFS-xNb ceramics with (a) x = 0.00 and (b) x = 0.01. Fig. 3. (a) The lattice constants ( and tetragonality ( /

,

, and

$ ),

and (b) rhombohedral distortion (90° − αR)

) of BNBTFS-xNb ceramics as function of Nb content.

Fig. 4. Scanning electron micrographs of the as-sintered BNBTFS-xNb ceramics: (a) x = 0.00, (b) x = 0.005, (c) x = 0.01, (d) x = 0.02, (e) x = 0.025, and (f) x = 0.03. Fig. 5. Grain size and density of BNBTFS- Nb ceramics (with

= 0.00, 0.005, 0.01, 0.02,

0.025, and 0.03) as a function of Nb5+ doping level. Fig. 6. (a) Ferroelectric hysteresis (P-E) loops of BNBTFS-xNb ceramics (x = 0.00, 0.005, 0.01, 0.02, 0.025, and 0.03) measured at room temperature under an applied electric field of 60 kV/cm. (b) Composition dependence of Pmax, Pr, and Ec. Fig. 7. P-E hysteresis loops measured at an applied electric field of 50 kV/cm from room temperature to 125 °C in 25 °C increments of BNBTFS-xNb ceramics for

= 0.005 and x =

0.01samples. Fig. 8. Bipolar and unipolar hysteresis curves of BNBTFS-xNb ceramics (x = 0.00–0.03) measured at room temperature under an applied electric field of 60 kV/cm. Fig. 9. (a) Normalized piezoelectric constant (d33*) and negative strain of the bipolar loops and (b) static piezoelectric constant (d33) for BNBTFS- Nb ceramics (x = 0.00–0.03). FE, ferroelectric; R, rhombohedral; T, tetragonal. Fig. 10. (a) Unipolar strain for the x = 0.010 ceramic with different fatigue cycles and (b) normalized piezoelectric constant (d33*) for the respective cycles. Fig. 11. X-ray diffraction patterns of the BNBTFS-xNb ceramics before and after the electric poling process for (a) x = 0.00 and (b) x = 0.01, and schematic phase diagrams for the possible nonreversible electric field–induced phase transformation in long-range ferroelectric ceramics and the reversible phase transformation in short-range ferroelectric ceramics. FE, ferroelectric.

Fig. 12. Temperature-dependent dielectric constant and dielectric loss of BNBTFS-xNb ceramics (x = 0.00, 0.01, 0.02, and 0.03).

Fig.1

Fig. 2

Fig. 3

Fig.4

Fig.5

Fig.6

Fig.7

Fig.8

Fig.9

Fig.10

Fig.11

Fig.12

Highlights •

A detailed electromechanical study of lead-free ceramics of the form (Bi0.5Na0.5)0.945Ba0.055[Ti0.98(Fe0.5Sb0.5)0.02](1-x)NbxO3 was attempted.

•

The unmodified (without Nb) ceramic has a long-range ferroelectric phase with a large static piezoelectric constant, d33 = 168 pC/N.

•

Nb substitution disrupts the long-range ferroelectric phase, and the crystal structure transforms to short-range pseudocubic symmetry.

•

High electric field–induced strain (Smax = 0.39%) with normalized piezoelectric constant d33* = 666 pm/V was obtained for the x = 0.010 composition.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

The authors declare no competing financial interest