- Email: [email protected]
Enhanced piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 lead-free piezoceramics with high Curie temperature by optimizing Bi self-compensation
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 lead-free piezoceramics with high Curie temperature by optimizing Bi selfcompensation Bo-Wei Xun, Ning Wang, Bo-Ping Zhang , Xin-Yue Chen, Yue-Qi Zheng, Wen-Shuai Jin, Rui Mao, Kai Liang ⁎
The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, 100083, Beijing, China
ARTICLE INFO
ABSTRACT
Keywords: 0.7BF-0.3BT Domain switching Bi self-compensation Piezoelectricity
Excess Bi was added to 0.7Bi1+xFeO3-0.3BaTiO3 (B1+xF-0.3BT; x = 0.00–0.05) ceramics to compensate for the volatilization of Bi2O3 during sintering. The domain structure and the domain switching of the B1+xF-0.3BT ceramics were systematically studied by the characterization of electrical properties and the use of piezoresponse force microscopy (PFM) and transmission electron microscopy (TEM). XRD patterns showed that the phase structure of all of the ceramics was in the morphotropic phase boundary (MPB), consisting of rhombohedral and pseudocubic phases. Appropriate compensation with Bi content can promote domain switching due to the precise control of MPB, which plays a vital role in obtaining high piezoelectric properties. Therefore, the outstanding piezoelectric properties of d33 = 214 pC/N, TC = 528 °C, kp = 0.325 and Sp-p = 0.375% were obtained in B1.02F-0.3BT ceramics, the properties of which are superior to those obtained in previously reported studies of BF-BT based ceramics. The present investigation of the compensating Bi content will inspire further research to develop BF-BT based ceramics suitable for practical use.
1. Introduction
phase structures, resulting in the shift away from the MPB and the deterioration in the piezoelectric performance. Recently, there has been a surge in the number of studies focusing on the volatilization of Bi2O3, including investigations of the modifications of the preparation process and elemental doping. Cheng et al. reported the outstanding electrical properties of d33 = 210 pC/N, TC = 507 °C for 0.7BF-0.3BT ceramic by optimizing the calcination temperature (Tcal) [7]. They believed that the ceramic at Tcal = 800 °C exhibited the lowest Bi3+ deficiency, which contributed to the low amount of the oxygen vacancies (V•• O ) and the appropriate MPB. Zhu et al. investigates the effects of dwell time (td) and the sintering temperature (TS) in the 0.7BF-0.3BT system. As the TS or td increases, the grain size of the ceramics increases and the porosity decreases which is beneficial for the dipole alignment. On the other hand, excess TS or td are unfavorable for piezoelectric properties due to the volatilization of Bi2O3 [8,9]. In addition, BF-BT ceramics modified by Li2CO3 used as a sintering aid can enhance the ceramics properties due to the decrease in Bi2O3 volatilization [10]. Another method to suppress the volatilization of Bi2O3 is doping by similar ions. Song et al. reported that the substitution of Bi3+ by Ho3+ is an effective method for controlling Bi2O3 volatilization due to the suppression of V•• O
BiFeO3 (BF) has been considered to be a superior high temperature lead-free piezoelectric material due to its high TC (830 °C) and its excellent theoretical spontaneous polarization (Pr = 90–100 μC/cm2), which is derived from the hybridization between the Bi3+ 6s2 and O2− 2p orbitals [1,2]. Unfortunately, the development of pure BiFeO3 ceramics is hampered due to the presence of impurity phases and high leakage current. It is generally accepted that Bi loss (evaporation or formation of impurity phase) is always a serious concern during the sintering process in BF system. Bi loss will inevitably cause an increase 3+ in the content of oxygen vacancies (V•• → O ) and a valence change of Fe Fe2+, leading to a high leakage current density [3,4]. Therefore, the loss of Bi content is highly detrimental to properties of BF-based ceramics. It should be noted that the (1-x)BiFeO3-xBaTiO3 (BF-BT) solid solution near the morphotropic phase boundary (MPB) shows relatively good piezoelectricity [5,6], because the various ferroelectric switching paths can help reduce the internal energy barrier during domain switching. However, Bi loss will inevitably give rise to changes in the
⁎
Corresponding author. E-mail address: [email protected] (B.-P. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.157 Received 20 July 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Bo-Wei Xun, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.157
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
formation [11]. Bi-site substitution by lanthanide ions (Eu3+,Gd3+, La3+, Sm3+) [12] can eliminate the secondary phase and the structural phase transition. Therefore, it is necessary to elucidate the underlying structure-property relationships of BF-BT ceramics caused by Bi2O3 volatilization. Although the studies of the effect of the compensating Bi content on the piezoelectricity of BF-BT ceramics have been carried out [13–15], excellent piezoelectric properties were not obtained (d33 = 114 pC/N [14] and d33 = 142 pC/N [15]). Meanwhile, there have been reports on the use of another modifier (MnO2) to increase the insulation in excess Bi content BF-BT ceramics. The introduction of Mn will cause the formation of chemical heterogeneities and the change of phase structure, which is not beneficial for understanding the underlying mechanism of the effects of compensating Bi content. In addition, there have been few studies on 0.7BF-0.3BT ceramics focusing on domain switching, which is an important underlying mechanism for high piezoelectric properties. In this study, a series of 0.7Bi1+xFeO3-0.3BaTiO3 (abbreviated as B1+xF-0.3BT; 0.00 ≤ x≤0.05) ceramics were prepared by conventional solid-state reaction. Excess Bi content was added to compensate the volatilization of Bi2O3 during sintering. The variations in the effect of the compensating Bi content on electrical properties, domain switching and phase structure of BF-0.3BT ceramics were studied systematically by XRD, P-E, I-E, S-E, PFM and TEM. XRD patterns showed that all of the samples were located in an MPB between the rhombohedral and pseudocubic phases. Compensating Bi content can accurately adjust the ratio of the two phases, promoting domain switching due to the precise control of the MPB. Therefore, the ceramic with high piezoelectric properties of d33 = 214 pC/N, kp = 0.325, Sp-p = 0.375%, TC = 528 °C was obtained at x = 0.02. The underlying structure-property relationship necessary for high-performance piezoelectric ceramics was also revealed.
Fig. 1. (a) Piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp) and mechanical quality factor (Qm) as a function of x for B1+xF-0.3BT ceramics. (b) The previously reported d33 and Tc of BF-BT based ceramics.
2. Experimental section A series of B1+xF-0.3BT (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) ceramics were prepared by the conventional solid-state reaction method. The raw powders (Bi2O3, Fe2O3 (> 99%) and BaTiO3 (> 99.9%)) weighed according to the nominal stoichiometric ratio of B1+xF-0.3BT were ball-milled for 12 h at 300 rpm. The dried powders were calcined at 800 °C for 6 h and then remilled for 12 h at 300 rpm. The powders were pressed into disks with a diameter of 10 mm and a thickness of 1.0 mm under 100 MPa using polyvinyl alcohol (2 wt%) as the binder. The disks were heated to 650 °C for 2 h in order to remove the binder and then heated to 1000 °C for 6 h. The crystal structure was characterized by X-ray diffraction (XRD: DMAX-RB, Japan). Well-polished ceramics were chemically etched by adding the etchant solution to the surface for 10 s to observe the morphology by field scanning electron microscopy (FESEM: SUPRATM55, Germany). Transmission electron microscopy (TEM: JEOL 2100, Japan) and piezoresponse force microscopy (PFM: MFP-3D, USA) were employed to observe the microscopic domain structure. X-ray photoelectron spectroscopy (XPS: ESCALAB 250Xi, USA) was used to characterize the surface chemical states of the well-polished ceramics. Silver was pasted on both surfaces of the ceramics and then the sample were sintered at 650 °C for 20 min. The ceramics were poled under a DC field of 3.5 kV/mm for 15 min in a silicone oil bath at 100 °C. Piezoelectric coefficient (d33) was measured using a quasi-static d33 testing meter (ZJ-3A, China). The planar electromechanical coupling coefficient (kp) and the mechanical quality factor (Qm) were measured using a precision impedance analyzer (Agilent 4294A, Hewlett-Packard, Palo Alto, CA). The electric-field-induced strains, polarization hysteresis loops (P-E, I-E) were measured at 0.5 Hz using a ferroelectric-measurement system (TF ANALYZER 1000, Germany). An insulation test instrument (AT683, China) was used to measure the leakage current density. Temperature dependent dielectric spectra were measured using an LCR analyzer (TH2828S) at 1 kHz.
3. Results and discussions Fig. 1(a) shows the piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp) and the mechanical quality factor (Qm) for B1+xF-0.3BT ceramics. The average d33 first increases from 158 pC/N at x = 0.00–214 pC/N at x = 0.02, and then decreases to 172 pC/N at x = 0.05. kp as a function of x shows behavior similar to that of d33. By compensating trace Bi content (x) in BF-0.3BT ceramic, we achieved a high d33 = 214 pC/N (Fig. 1(a)) along with a high TC = 528 °C, which is superior to the previously reported studies of BFBT based ceramics [13–28] (Fig. 1(b)). It is generally accepted that domain structure markedly affects the piezoelectric/ferroelectric properties of ceramics. Domain switching and domain wall movement are the mesoscopic origin of the polarization-electrical field (P-E) loops and the current-electrical field (I-E) loops of normal ferroelectric ceramics [29]. Fig. 2 shows the P-E and I-E loops under 50 kV/cm electric field measured at room temperature for the B1+xF-0.3BT ceramics. All of the samples show typical ferroelectric hysteresis loops and only a single polarization current peak near the coercive field (EC) resulting from domain switching during electric loading. The sharpness of the current peaks first increases and then decreases, indicating that the number of domains reaches the maximum value at x = 0.02 (Fig. 2(h)). Fig. 2(g) shows the remnant polarization (Pr) and EC of B1+xF-0.3BT ceramics. Pr first increases from 17.21 to 19.61 μC/cm2 as x increases from 0.00 to 0.02, and then decreases to 17.75 μC/cm2 at x = 0.05. The EC-x relationship shows the opposite trend from that of the Pr-x curve (Fig. 2(g)). This result suggests that compensating trace Bi content in BF-BT ceramics is beneficial for domain switching. Fig. 3 shows the bipolar and unipolar electric-field-induced strain 2
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 2. (a–f) Ferroelectric properties of P-E and I-E loops under 50 kV/cm electric field, (g) 2Pr and 2Ec and (h) the highest peak current value for B1+xF-0.3BT ceramics (x = 0–0.05).
(S-E) curves for the unpoled and poled B1+xF-0.3BT ceramics under 50 kV/cm electric field. The negative strain (Sneg) and the peak-to-peak strain (Sp-p) are obtained from the bipolar S-E loops [30,31]. The d*33 is calculated as d*33 = Smax/Emax; where Emax is the maximum electric field and Smax corresponds to the maximum strain obtained from the unipolar S-E loops [32]. It is well-known that the origin of the high strain for piezoceramics is ascribed mainly to the field-induced antiferroelectric-to-ferroelectric phase transition, relaxor-to-normal ferroelectric phase transition and the enhancement of domain switching and domain wall movement under the applied electric field [33]. The large Sneg in all of the typical butterfly shaped S-E loops (Fig. 3(a1, a3)) suggests that the large strain is mainly related to a classic non-180° domain switching behavior rather than to the field-induced phase transition (antiferroelectric-to-ferroelectric and relaxor-to-normal phase transition). Furthermore, irreversible domain switching is the mesoscopic origin of the Sneg that represents the degree of the non-180° domain switching [34]. |Sneg| achieves the maximum value at x = 0.02 (Fig. 3(a3)). The results are consistent with the P-E and I-E research, indicating that under the applied electric field the highest irreversible domain switching is obtained for the B1.02F-0.3BT ceramics. Similar to
the variation of |Sneg|, d*33 also reaches the maximum value (208 pm/ V) at x = 0.02. The butterfly shaped S-E loops for poled ceramics are very different from those of the unpoled ceramics as shown by the comparison of Fig. 3(a1) and 3(b1). The domain switching process corresponding to the special position of the butterfly curve is shown in Fig. 4. For unpoled B1+xF-0.3BT ceramics, the defect dipoles (DD) located at the domain boundaries are randomly oriented and do not reorient as the applied electric field increases [35,36]. Therefore, the strain in the S-E loops will decrease due to the pinning of the domains by the irreversible defect dipoles. However, under the long-term and the large directcurrent electric field, the DD and most of the domains can be reoriented and arranged linearly, resulting in the establishment of an internal bias electric field (Ei) in the grains. In addition, small domains can coalesce into large domains while the domain boundaries decrease, so that the domains are easier to switch when the electric field is applied (Fig. 4). The aligned DD partially contributes to the high strain [36]. Therefore, Sp-p and |Sneg| are enhanced by almost 2 times after poling (Fig. 3(a3b3)). The maximum Sp-p value of 0.375% and the Smax value of 0.13% under 50 kV/cm are obtained at x = 0.02. Moreover, the d*33 values of 3
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 3. (a1) Bipolar electromechanical strain (S–E) and (b1) unipolar strains loops of unpoled B1+xF-0.3BT ceramics; (a2) unipolar strains loops of unpoled and (b2) poled B1+xF-0.3BT ceramics under 50 kV/cm electric field. Sp-p, Sneg, d*33 of (a3) unpoled and (b3) poled B1+xF-0.3BT ceramics.
Fig. 4. Illustration of the domain switching process corresponding to the special position of the butterfly curve. The red arrows represent spontaneous polarization direction(PS) and the green arrows represent defect dipole polarization direction (PD). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 5. Room temperature impedance |Z| and phase angle θ as a function of frequency for B1+xF-0.3BT ceramics. (a) x = 0.00; (b) x = 0.01; (c) x = 0.02; (d) x = 0.03; (e) x = 0.04; (f) x = 0.05.
Fig. 6. PFM scanning results of out-of-plane ((a) x = 0.00; (b) x = 0.02; (c) x = 0.05) amplitude and ((d) x = 0.00; (e) x = 0.02; (f) x = 0.05) phase for B1+xF-0.3BT ceramics measured at 25 °C.
all of the samples have also been greatly improved after poling, peaking at 262 pm/V when x = 0.02. Therefore, domain structure is a vital factor for improving the properties of piezoelectric ceramics. The maximum phase angle (θmax) that ranges between −90° and 90° is an important indicator for the evaluation of the polarization degree (reorientation degree of domains) of piezoceramics. θmax should be equal to −90° for unpoled piezoceramics and the θmax is supposed to 90° for ideal poling piezoceramics among the frequency range between the resonance (fr) and anti-resonance frequency (fa) [37,38]. A greater
the θmax corresponds to, a stronger degree of orientation degree of domains. The impedance (|Z|) and phase angle (θ) measured as a function of frequency for B1+xF-0.3BT ceramics are shown in Fig. 5 θmax is approximately 11.36° at x = 0 and reaches 28.66° for x = 0.02 (Fig. 5(a–f)). However, θmax decreased to 23.62° at x = 0.05 (Fig. 5(a–f)). Therefore, it is thought that the B1.02F-0.3BT ceramic has the highest poling state due to the largest contribution of domain switching. To explore the origin of the high piezoelectricity, V-PFM amplitude 5
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 7. Representative bright-field transmission electron microscope (TEM) images with the magnification of (a) domain configuration and (b) enlarged nanodomains.
Disrupted strip-like domains that are the characteristic domains in the PC symmetry are also observed (red dotted circle). Therefore, the complex domain structure of the 0.7B1.02F-0.3BT ceramic is composed of mostly regular strip-like domains and a small percentage of disrupted strip-like domains. The large variety of domain sizes indicated that the mobility of domains had a broad distribution [39]. In addition to domain switching, the large insulation resistance (Re) is also beneficial for enhanced piezoelectric properties. Fig. 8 plots the electric field dependence of (a) Re and (b) the leakage current (J) of the B1+xF-0.3BT ceramics. The Re values of all of the samples are in the range of 6.48 × 109–1.10 × 1011 Ω cm (Fig. 8(a)) and the leakage current density values are in the range of 2.58 × 10−9–1.48 × 10−10 A/cm2 (Fig. 8(b)) under a 1 kV/cm field. The leakage current density reaches the minimum value at x = 0.02, suggesting that the B1.02F0.3BT ceramic has the minimal amount of defects. It was reported that the formation of V•• O resulted in the high J in BF-BT ceramics [40]. Excess Bi (0.01 ≤ x ≤ 0.02) may enter into the lattice and decrease V•• O. Fig. 9(a1-a4) show the XRD patterns for the B1+xF-0.3BT calcined powders and nano-BaTiO3 (BT) powders. The vertical lines are the standard diffraction peaks of BaTiO3 with a cubic (CBT) symmetry (PDF#75–0461, Pm-3m) and BiFeO3 with a rhombohedral (R) symmetry (PDF#71–2494, R3c). All calcined powders (Fig. 9(a1)) show a main perovskite phase and a small amount of the Bi25FeO40 impurity phase (★, PDF#46–0416). The perovskite phase includes the mechanical mixture of the Rα-R3c and the CBT phase, as found from the clear splitting of the peaks near 32° and 39° in Fig. 9(a1-a3) and a shoulder peak on the left side of the (104)R3c peak (Fig. 9(a3)). Therefore, three phases are present in all of the B1+xF-0.3BT powders, namely: the Rα-R3c phase, the CBT phase and the Bi25FeO40 impurity phase. The reaction equation can be expressed as: Fig. 8. (a) The insulation resistance (Re) and (b) the leakage current density (J) of B1+xF-0.3BT ceramics.
Bi2O3 + Fe2 O3 + BaTiO3
800°C
BiFeO3 R
R3c + BaTiO3 CBT + Bi25FeO40
(1)
In addition, when x increases, the Rα-R3c phase and the Bi25FeO40 impurity phase diffraction peaks first shift toward a lower angle and then toward a higher angle as shown in Fig. 9(a2-a4), reaching the minimum values at x = 0.02. The slight lattice enlargement at 0.00 ≤ x ≤ 0.02 indicates that the excess Bi has entered the lattice and compensated the volatilization of Bi2O3 in the Rα-R3c phase and the Bi25FeO40 impurity phase, simultaneously. The lattice shrinkage of the Rα-R3c phase at 0.03 ≤ x ≤ 0.05 may be due to the formation of a greater amount of the Bi25FeO40 impurity phase (Fig. 9(a4)), leading to the lack of Bi content in the Rα-R3c phase. The Bi2O3 volatilization in the Bi25FeO40 impurity phase is significant, as verified by the high offset of the impurity phase peak at 0.03 ≤ x ≤ 0.05 (Fig. 9(a2-a4)). The Bi2O3 volatilization in the Bi25FeO40 impurity phase can be expressed as:
and phase images (Fig. 6(a)-(f)) were employed to determine the domain structures and the local piezoresponse of the B1+xF-0.3BT ceramics. The BF-0.3BT ceramic has heterogeneous and sparse stripy domains. The domain morphology is clearly changed upon the addition of compensating trace Bi content. As x increases to 0.02, the amount and density of the domains increase, which is consistent with the I-E loops results, indicating that the amount of the domains is the highest at x = 0.02. However, the inhomogeneous block domains are observed at x = 0.05. Fig. 7 presents the representative images of the bright-field transmission electron microscope (TEM) for the x = 0.02 sample. Complex domain patterns composed of dense strip-like domains are observed as shown in Fig. 7(a). Coarse herringbone-type domains separated by 71° domain walls are also observed as shown in Fig. 7(b) [32], which is the characteristic domain in the R symmetry. The distribution of the domain sizes is uneven, with the domain size varying from 100 to 600 nm.
2Bi25FeO40
2BiFeO3 + 24Bi2O3
+ O2
(2)
The XRD patterns of B1+xF-0.3BT ceramics and the standard diffraction peaks cited by BT with a cubic symmetry (PDF#75–0461, Pm3m) and BF with a R3m symmetry (PDF#72–2112) are exhibited in 6
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 9. XRD patterns of (a) calcined powders and (b) ceramics for B1+xF-0.3BT samples. (b4) The Rietveld refinement of XRD patterns of B1+xF-0.3BT ceramics. Table 1 Rietveld refined lattice parameters and phase ratio of the BFScx-0.3BT ceramics. Composition
BT BF 0.00 0.01 0.02 0.03 0.04 0.05
Phase structure
PC-PDF#75-0461 R-PDF#72-2112 PC R PC R PC R PC R PC R PC R
Space group
Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m
Lattice parameters
Reliability factor
CR/CPC (%)
a = b = c(Å)
α = β = γ(°)
Rwp%
(%)
4.012 3.952 4.126 4.107 4.018 3.982 3.998 3.988 3.999 4.007 4.001 4.000 4.014 4.012
90.00 89.60 90.00 89.99 90.00 90.26 90.00 89.86 90.00 89.77 90.00 89.89 90.00 90.01
– – 9.06
– – 32/68
11.52
77/23
10.45
68/32
9.12
65/34
8.203
58/42
6.65
40/59
Fig. 9(b1-b3). Compared with the XRD patterns of powders, there are distinct changes in the XRD patterns of ceramics. First, the Bi25FeO40 impurity phase disappears completely at 0.00 ≤ x ≤ 0.02 and reduces obviously at 0.03 ≤ x ≤ 0.05 (Fig. 9(b1)). Second, all peaks shift toward the standard cards of BaTiO3 and the shoulder peak on the (104)R3c peak's left has disappeared, indicating that BT has completely
dissolved into the BF matrix. Third, the B1+xF-0.3BT ceramics locate in the MPB, consisting of Rβ-R3m-PC phases due to the misfit between the individual standard diffraction peaks and the XRD patterns. The reaction equation can be expressed as:
7
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
x = 0.02. It has been reported that the nonstoichiometric defects (the bismuth vacancy (V B" i ) and the oxygen vacancy (VO••)) deriving from the volatilization of Bi2O3 in 0.75BF-0.25BT-Mn ceramics may enlarge the lattice due to the charge repulsion [14]. The defect equations of B1+xF0.3BT ceramics can be expressed as:
2BiFeO3 × 2Fe×Fe + OO
R3c + BaTiO3 CBT + Bi25 FeO40
1000°C
(Bi,Ba) Fe,Ti)O3 R
R3m + PC
× + 2Fe×Fe + 2VBi + 3V•• O + 3OO
2FeFe + V•• O +
1 O2 2
(4) (5)
Compensating trace Bi content (0.00 ≤ x ≤ 0.02) may weaken the charge repulsion in the Rβ-R3m phase and decrease the lattice parameters due to charge compensation. In other words, the B1.02F-0.3BT ceramics have the least amount of V"Bi and the V•• O . The slight lattice enlargement at 0.03 ≤ x ≤ 0.05 may be due to the charge repulsion because of the greater volatilization of Bi2O3 in the powders (Fig. 9(a2-a4)) and the formation of the Bi25FeO40 impurity phase (Fig. 9(b1)). In addition to the shift of the peaks with x, a change in the phase symmetry was also observed. Although all of the samples have the RβR3m phase coexisting with the PC phase, the samples show a clear difference regarding their Rβ-R3m/PC (CR/CPC) ratio (Fig. 9(b3)). The intensity of the (111)PC peak is higher than that of the (1 1 1) R3m peak, indicating that the PC phase content is greater than the Rβ-R3m phase content in the BF-0.3BT ceramic. It is possible that a greater amount of vacancies gave rise to cell expansion, facilitating the transformation from the Rβ-R3m phase (R3m, a = b = c = 3.952 Å, α = β = γ = 89.6°) to the PC (Pm-3m, a = b = c = 4.012 Å, α = β = γ = 90°) phase. Trace Bi self-compensation (0.01 ≤ x ≤ 0.02) may enter into the lattice and fill the vacancies, leading to the increase in the Rβ-R3m phase content. The intensity of the (111)PC peaks in the 0.03 ≤ x ≤ 0.05 samples increased, indicating an increase in the PC phase amount. The change of CR/CPC has a strong influence on the piezoelectric properties. It should be noted that a piezoceramic near the morphotropic phase boundary (MPB) exhibits relatively good ferroelectricity [5,6], because of the various ferroelectric switching paths that can help to reduce the internal energy barrier during domain switching. However, Bi loss in BFBT will inevitably give rise to a change in the phase structures, resulting in the shift away from the MPB and the deterioration in the piezoelectric performance. Compensating with an appropriate Bi content can fill the vacancies, which is beneficial for approaching the MPB and reducing VO••. Therefore, appropriate Bi self-compensation plays a vital role in obtaining the high piezoelectric properties. The results of the Xray Rietveld refinement are plotted in Fig. 9(b4) and the corresponding lattice parameters and CR/CPC values are listed in Table 1.
Fig. 10. BS-SEM images of chemically etched surfaces of B1+xF-0.3BT ceramics. (a) x = 0.00, (b) x = 0.02, (c) x = 0.05.
BiFeO3 R
Bi2 O3
(3)
In addition, the peaks near 32° first shift toward a higher angle and then toward a lower angle with x (Fig. 9(b3)), reaching the maximum at
Fig. 11. SEM images of the fracture surface microstructure for B1+xF-0.3BT ceramics. (a) x = 0.00, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05. 8
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 12. XPS spectra of (a) Fe 2p, (b) Fe 2p, (c) Bi 4f, (d) Ba 3d, (e) Ti 2p and (f) O 1s for the B1+xF-0.3BT ceramics.
atomic percentage of the constituent elements is listed in the images. The Bi content in the B1+xF-0.3BT grains reaches maximum values at x = 0.02, indicating that the B1.02F-0.3BT ceramic has the highest Bi content, which is in agreement with the XRD analysis results. Fig. 11 shows the grain morphologies of the B1+xF-0.3BT ceramics. Grain size histograms are shown in the top right corner of each SEM images. The average grain size (d ) of B1+xF-0.3BT ceramics is approximately 7.02, 6.96, 7.04, 6.53, 5.62 and 5.36 μm at x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. The excess Bi content (0.00 ≤ x ≤ 0.02) has no obvious effect on the grain sizes and grain morphology of the ceramics. However, the grain size of the samples at 0.03 ≤ x ≤ 0.05 gradually decreases with increasing the Bi content (0.03 ≤ x ≤ 0.05). Fig. 12 shows the XPS spectra of all of the elements in the B1+xF0.3BT ceramics. Symmetric peaks are observed at ca. 164.15 and 158.85 eV, 794.70 and 779.90 eV and 464.04 and 457.87 eV corresponding to Bi3+ 4f7/2 and 4f5/2, Ba2+ 3d5/2 and 3d3/2 and Ti 2p3/2 and 2p1/2, respectively. However, the asymmetric Fe 2p peaks indicate the coexistence of a mixture of two valences (Fe3+ and Fe2+) in the B1+xF0.3BT ceramics (Fig. 12(a)) that can be separated into the 2p3/2 and 2p1/2 states of Fe3+ and Fe2+ (Fig. 12(b)). The patterns of Fe 2p peaks for B1+xF-0.3BT ceramics are identical, indicating that the relative contents of Fe2+ are basically similar in all B1+xF-0.3BT ceramics.
Fig. 13. Temperature dependences of dielectric constant εr and dielectric loss tanδ measured at 1 kHz for B1+xF-0.3BT ceramics.
Fig. 10 shows the BS-SEM images of well-polished and chemically etched B1+xF-0.3BT ceramics. Energy-dispersive spectroscopy (EDS) was performed to investigate the elemental content in the grains. The 9
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 13 shows the dielectric constant as a function of temperature for the B1+xF-0.3BT ceramics. All of the ceramics exhibit a single sharp dielectric peak, indicating the typical transition from the ferroelectric to the paraelectric phase (Fig. 13(a)). The samples show high TC values that are greater than 495 °C. The maximum (TC = 528 °C) is obtained at x = 0.02.
[13] T. Zheng, Y. Ding, J. Wu, Bi nonstoichiometry and composition engineering in (1−x)Bi1+yFeO3+3y/2-xBaTiO3 ceramics, RSC Adv. 6 (93) (2016) 90831–90839. [14] J. Chen, J. Cheng, Enhanced thermal stability of lead-free high temperature 0.75BiFeO3-0.25BaTiO3 ceramics with excess Bi content, J. Alloy. Comp. 589 (2014) 115–119. [15] C. Zhou, H. Yang, Q. Zhou, G. Chen, W. Li, H. Wang, Effects of Bi excess on the structure and electrical properties of high-temperature BiFeO3-BaTiO3 piezoelectric ceramics, J. Mater. Sci. Mater. Electron. 24 (5) (2013) 1685–1689. [16] K. Tong, C. Zhou, Q. Li, J. Wang, L. Yang, J. Xu, G. Chen, C. Yuan, G. Rao, Enhanced piezoelectric response and high-temperature sensitivity by site-selected doping of BiFeO3-BaTiO3 ceramics, J. Eur. Ceram. Soc. 38 (4) (2018) 1356–1366. [17] D. Wang, Z. Fan, D. Zhou, A. Khesro, S. Murakami, A. Feteira, Q. Zhao, X. Tan, I.M. Reaney, Bismuth ferrite-based lead-free ceramics and multilayers with high recoverable energy density, J. Mater. Chem. 6 (9) (2018) 4133–4144. [18] D. Wang, A. Khesro, S. Murakami, A. Feteira, Q. Zhao, I.M. Reaney, Temperature dependent, large electromechanical strain in Nd-doped BiFeO3-BaTiO3 lead-free ceramics, J. Eur. Ceram. Soc. 37 (4) (2017) 1857–1860. [19] G.H. Ryu, A. Hussain, M.H. Lee, R.A. Malik, T.-K. Song, W.-J. Kim, M.-H. Kim, Leadfree high performance Bi(Zn0.5Ti0.5) O3-modified BiFeO3-BaTiO3 piezoceramics, J. Eur. Ceram. Soc. 38 (13) (2018) 4414–4421. [20] F. Akram, R.A. Malik, T.K. Song, S. Lee, M.-H. Kim, Thermally-stable high dielectric properties of (1-x)(0.65Bi1.05FeO3-0.35BaTiO3)–xBiGaO3 piezoceramics, J. Eur. Ceram. Soc. 39 (7) (2019) 2304–2309. [21] C. Zhou, H. Yang, Q. Zhou, Z. Cen, W. Li, C. Yuan, H. Wang, Dielectric, ferroelectric and piezoelectric properties of La-substituted BiFeO3-BaTiO3 ceramics, Ceram. Int. 39 (4) (2013) 4307–4311. [22] L.-F. Zhu, B.-P. Zhang, S. Li, G.-L. Zhao, Large piezoelectric responses of Bi (Fe,Mg,Ti)O3-BaTiO3 lead-free piezoceramics near the morphotropic phase boundary, J. Alloy. Comp. 727 (2017) 382–389. [23] Q. Li, J. Wei, T. Tu, J. Cheng, J. Chen, Remarkable piezoelectricity and stable high‐temperature dielectric properties of quenched BiFeO3-BaTiO3 ceramics, J. Am. Ceram. Soc. 100 (12) (2017) 5573–5583. [24] S.O. Leontsev, R.E. Eitel, Dielectric and piezoelectric properties in Mn‐modified (1x)BiFeO3-xBaTiO3 ceramics, J. Am. Ceram. Soc. 92 (12) (2009) 2957–2961. [25] Z. Yao, C. Xu, H. Liu, H. Hao, M. Cao, Z. Wang, Z. Song, W. Hu, A. Ullah, Greatly reduced leakage current and defect mechanism in atmosphere sintered BiFeO3BaTiO3 high temperature piezoceramics, J. Mater. Sci. Mater. Electron. 25 (11) (2014) 4975–4982. [26] Y. Wei, X. Wang, J. Jia, X. Wang, Multiferroic and piezoelectric properties of 0.65BiFeO3-0.35BaTiO3 ceramic with pseudo-cubic symmetry, Ceram. Int. 38 (4) (2012) 3499–3502. [27] W. Gao, J. Lv, X. Lou, Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics, J. Am. Ceram. Soc. 101 (8) (2018) 3383–3392. [28] Y. Wei, X. Wang, J. Zhu, X. Wang, J. Jia, Dielectric, ferroelectric, and piezoelectric properties of BiFeO3-BaTiO3 ceramics, J. Am. Ceram. Soc. 96 (10) (2013) 3163–3168. [29] C. Zhao, B. Wu, K. Wang, J.-F. Li, D. Xiao, J. Zhu, J. Wu, Practical high strain with superior temperature stability in lead-free piezoceramics through domain engineering, J. Mater. Chem. 6 (46) (2018) 23736–23745. [30] F.-Z. Yao, K. Wang, W. Jo, J.-S. Lee, J.-F. Li, Effect of poling temperature on piezoelectricity of CaZrO3-modified (K,Na) NbO3-based lead-free ceramics, J. Appl. Phys. 116 (11) (2014) 114102. [31] W. Jo, T. Granzow, E. Aulbach, J. Rödel, D. Damjanovic, Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics, J. Appl. Phys. 105 (9) (2009) 094102. [32] D. Wang, G. Wang, S. Murakami, Z. Fan, A. Feteira, D. Zhou, S. Sun, Q. Zhao, I.M. Reaney, BiFeO3-BaTiO3: a new generation of lead-free electroceramics, J. Adv. Dielectr. 8 (2018) 1–35, https://doi.org/10.1142/S2010135X18300049. [33] L. Jin, F. Li, S. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension of the material properties and structures, J. Am. Ceram. Soc. 97 (1) (2014) 1–27. [34] W. Jo, R. Dittmer, M. Acosta, J. Zang, C. Groh, E. Sapper, K. Wang, J. Rödel, Giant electric-field-induced strains in lead-free ceramics for actuator applications–status and perspective, J. Electroceram. 29 (1) (2012) 71–93. [35] M.H. Lee, D.J. Kim, H.I. Choi, M.H. Kim, T.K. Song, W.J. Kim, J.S. Park, D. Do, Low sintering temperature for lead‐free BiFeO3‐BaTiO3 ceramics with high piezoelectric performance, J. Am. Ceram. Soc. 102 (5) (2019) 2666–2674. [36] J. Zeng, K. Zhao, X. Shi, X. Ruan, L. Zheng, G. Li, Large strain induced by the alignment of defect dipoles in (Bi3+,Fe3+) co-doped Pb(Zr,Ti)O3 ceramics, Scr. Mater. 142 (2018) 20–22. [37] D. Lin, K. Kwok, H. Chan, Piezoelectric and ferroelectric properties of KxNa1−xNbO3 lead-free ceramics with MnO2 and CuO doping, J. Alloy. Comp. 461 (1–2) (2008) 273–278. [38] H.-Y. Park, I.-T. Seo, M.-K. Choi, S. Nahm, H.-G. Lee, H.-W. Kang, B.-H. Choi, Microstructure and piezoelectric properties of the CuO-added (Na0.5K0.5) (Nb0.97Sb0.03)O3 lead-free piezoelectric ceramics, J. Appl. Phys. 104 (3) (2008) 034103. [39] Q. Liu, Y. Zhang, J. Gao, Z. Zhou, H. Wang, K. Wang, X. Zhang, L. Li, J.-F. Li, Highperformance lead-free piezoelectrics with local structural heterogeneity, Energy Environ. Sci. 11 (12) (2018) 3531–3539. [40] B.-W. Xun, Y.-C. Tang, J.-Y. Chen, B.-P. Zhang, Enhanced resistance in Bi(Fe1-xScx) O3-0.3BaTiO3 lead-free piezoelectric ceramics: facile analysis and reduction of oxygen vacancy, J. Eur. Ceram. Soc. 39 (14) (2019) 4085–4095.
4. Conclusions The effects of compensating Bi content on the electrical properties, domain structure and phase structure in the 0.7BF-0.3BT ceramic was studied systematically by P-E, I-E, S-E, PFM, TEM and XRD. These results indicate that compensating with an appropriate Bi content promotes domain switching due to the precise control of the MPB. The complex domain structure was composed mostly of regular strip-like domains together with a small percentage of disrupted strip-like domains in the 0.7B1.02F-0.3BT ceramic. Meanwhile, inhomogeneous block domains are observed in the 0.7BF-0.3BT and 0.7B1.05F-0.3BT ceramics. |Sneg| reached the maximum value at x = 0.02, indicating the largest non-180° domain switching. Therefore, the excellent piezoelectric properties of d33 = 214 pC/N, kp = 0.325, TC = 528 °C and Spp = 0.375% were achieved. The investigation of the compensating Bi content will inspire further research to develop BF-BT based ceramics suitable for practical use. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no.51472026). References [1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Epitaxial BiFeO3 multiferroic thin film heterostructures, Science 299 (5613) (2003) 1719–1722. [2] J.B. Neaton, C. Ederer, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, First-principles study of spontaneous polarization in multiferroic BiFeO3, Phys. Rev. B 71 (1) (2005) 014113. [3] Z. Dai, Y. Akishige, Electrical properties of multiferroic BiFeO3 ceramics synthesized by spark plasma sintering, J. Phys. D Appl. Phys. 43 (44) (2010) 445403. [4] Q. Zhang, X. Zhu, Y. Xu, H. Gao, Y. Xiao, D. Liang, J. Zhu, J. Zhu, D. Xiao, Effect of La3+ substitution on the phase transitions, microstructure and electrical properties of Bi1−xLaxFeO3 ceramics, J. Alloy. Comp. 546 (2013) 57–62. [5] M.M. Kumar, A. Srinivas, S.V. Suryanarayana, Structure property relations in BiFeO3/BaTiO3 solid solutions, J. Appl. Phys. 87 (2) (2000) 855–862. [6] M.H. Lee, D.J. Kim, J.S. Park, S.W. Kim, T.K. Song, M.H. Kim, W.J. Kim, D. Do, I.K. Jeong, High‐performance lead‐free piezoceramics with high curie temperatures, Adv. Mater. 27 (43) (2015) 6976–6982. [7] S. Cheng, B.-P. Zhang, L. Zhao, K.-K. Wang, Enhanced insulating and piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 lead-free ceramics by optimizing calcination temperature: analysis of Bi3+ volatilization and phase structures, J. Mater. Chem. C 6 (15) (2018) 3982–3989. [8] L.-F. Zhu, B.-P. Zhang, J.-Q. Duan, B.-W. Xun, N. Wang, Y.-C. Tang, G.-L. Zhao, Enhanced piezoelectric and ferroelectric properties of BiFeO3-BaTiO3 lead-free ceramics by optimizing the sintering temperature and dwell time, J. Eur. Ceram. Soc. 38 (10) (2018) 3463–3471. [9] Y. Lee, J. Kim, S. Han, H.-W. Kang, H.-G. Lee, C.I. Cheon, Effect of sintering temperature on the piezoelectric properties in BiFeO3-BaTiO3 ceramics, J. Korean Phys. Soc. 61 (6) (2012) 947–950. [10] S. Guan, H. Yang, Y. Zhao, R. Zhang, Effect of Li2CO3 addition in BiFeO3-BaTiO3 ceramics on the sintering temperature, electrical properties and phase transition, J. Alloy. Comp. 735 (2018) 386–393. [11] G. Song, Y. Song, J. Su, X. Song, N. Zhang, T. Wang, F. Chang, Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material, J. Alloy. Comp. 696 (2017) 503–509. [12] K. Nalwa, A. Garg, A. Upadhyaya, Effect of samarium doping on the properties of solid-state synthesized multiferroic bismuth ferrite, Mater. Lett. 62 (6–7) (2008) 878–881.
10
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 lead-free piezoceramics with high Curie temperature by optimizing Bi selfcompensation Bo-Wei Xun, Ning Wang, Bo-Ping Zhang , Xin-Yue Chen, Yue-Qi Zheng, Wen-Shuai Jin, Rui Mao, Kai Liang ⁎
The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, 100083, Beijing, China
ARTICLE INFO
ABSTRACT
Keywords: 0.7BF-0.3BT Domain switching Bi self-compensation Piezoelectricity
Excess Bi was added to 0.7Bi1+xFeO3-0.3BaTiO3 (B1+xF-0.3BT; x = 0.00–0.05) ceramics to compensate for the volatilization of Bi2O3 during sintering. The domain structure and the domain switching of the B1+xF-0.3BT ceramics were systematically studied by the characterization of electrical properties and the use of piezoresponse force microscopy (PFM) and transmission electron microscopy (TEM). XRD patterns showed that the phase structure of all of the ceramics was in the morphotropic phase boundary (MPB), consisting of rhombohedral and pseudocubic phases. Appropriate compensation with Bi content can promote domain switching due to the precise control of MPB, which plays a vital role in obtaining high piezoelectric properties. Therefore, the outstanding piezoelectric properties of d33 = 214 pC/N, TC = 528 °C, kp = 0.325 and Sp-p = 0.375% were obtained in B1.02F-0.3BT ceramics, the properties of which are superior to those obtained in previously reported studies of BF-BT based ceramics. The present investigation of the compensating Bi content will inspire further research to develop BF-BT based ceramics suitable for practical use.
1. Introduction
phase structures, resulting in the shift away from the MPB and the deterioration in the piezoelectric performance. Recently, there has been a surge in the number of studies focusing on the volatilization of Bi2O3, including investigations of the modifications of the preparation process and elemental doping. Cheng et al. reported the outstanding electrical properties of d33 = 210 pC/N, TC = 507 °C for 0.7BF-0.3BT ceramic by optimizing the calcination temperature (Tcal) [7]. They believed that the ceramic at Tcal = 800 °C exhibited the lowest Bi3+ deficiency, which contributed to the low amount of the oxygen vacancies (V•• O ) and the appropriate MPB. Zhu et al. investigates the effects of dwell time (td) and the sintering temperature (TS) in the 0.7BF-0.3BT system. As the TS or td increases, the grain size of the ceramics increases and the porosity decreases which is beneficial for the dipole alignment. On the other hand, excess TS or td are unfavorable for piezoelectric properties due to the volatilization of Bi2O3 [8,9]. In addition, BF-BT ceramics modified by Li2CO3 used as a sintering aid can enhance the ceramics properties due to the decrease in Bi2O3 volatilization [10]. Another method to suppress the volatilization of Bi2O3 is doping by similar ions. Song et al. reported that the substitution of Bi3+ by Ho3+ is an effective method for controlling Bi2O3 volatilization due to the suppression of V•• O
BiFeO3 (BF) has been considered to be a superior high temperature lead-free piezoelectric material due to its high TC (830 °C) and its excellent theoretical spontaneous polarization (Pr = 90–100 μC/cm2), which is derived from the hybridization between the Bi3+ 6s2 and O2− 2p orbitals [1,2]. Unfortunately, the development of pure BiFeO3 ceramics is hampered due to the presence of impurity phases and high leakage current. It is generally accepted that Bi loss (evaporation or formation of impurity phase) is always a serious concern during the sintering process in BF system. Bi loss will inevitably cause an increase 3+ in the content of oxygen vacancies (V•• → O ) and a valence change of Fe Fe2+, leading to a high leakage current density [3,4]. Therefore, the loss of Bi content is highly detrimental to properties of BF-based ceramics. It should be noted that the (1-x)BiFeO3-xBaTiO3 (BF-BT) solid solution near the morphotropic phase boundary (MPB) shows relatively good piezoelectricity [5,6], because the various ferroelectric switching paths can help reduce the internal energy barrier during domain switching. However, Bi loss will inevitably give rise to changes in the
⁎
Corresponding author. E-mail address: [email protected] (B.-P. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.157 Received 20 July 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Bo-Wei Xun, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.157
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
formation [11]. Bi-site substitution by lanthanide ions (Eu3+,Gd3+, La3+, Sm3+) [12] can eliminate the secondary phase and the structural phase transition. Therefore, it is necessary to elucidate the underlying structure-property relationships of BF-BT ceramics caused by Bi2O3 volatilization. Although the studies of the effect of the compensating Bi content on the piezoelectricity of BF-BT ceramics have been carried out [13–15], excellent piezoelectric properties were not obtained (d33 = 114 pC/N [14] and d33 = 142 pC/N [15]). Meanwhile, there have been reports on the use of another modifier (MnO2) to increase the insulation in excess Bi content BF-BT ceramics. The introduction of Mn will cause the formation of chemical heterogeneities and the change of phase structure, which is not beneficial for understanding the underlying mechanism of the effects of compensating Bi content. In addition, there have been few studies on 0.7BF-0.3BT ceramics focusing on domain switching, which is an important underlying mechanism for high piezoelectric properties. In this study, a series of 0.7Bi1+xFeO3-0.3BaTiO3 (abbreviated as B1+xF-0.3BT; 0.00 ≤ x≤0.05) ceramics were prepared by conventional solid-state reaction. Excess Bi content was added to compensate the volatilization of Bi2O3 during sintering. The variations in the effect of the compensating Bi content on electrical properties, domain switching and phase structure of BF-0.3BT ceramics were studied systematically by XRD, P-E, I-E, S-E, PFM and TEM. XRD patterns showed that all of the samples were located in an MPB between the rhombohedral and pseudocubic phases. Compensating Bi content can accurately adjust the ratio of the two phases, promoting domain switching due to the precise control of the MPB. Therefore, the ceramic with high piezoelectric properties of d33 = 214 pC/N, kp = 0.325, Sp-p = 0.375%, TC = 528 °C was obtained at x = 0.02. The underlying structure-property relationship necessary for high-performance piezoelectric ceramics was also revealed.
Fig. 1. (a) Piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp) and mechanical quality factor (Qm) as a function of x for B1+xF-0.3BT ceramics. (b) The previously reported d33 and Tc of BF-BT based ceramics.
2. Experimental section A series of B1+xF-0.3BT (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) ceramics were prepared by the conventional solid-state reaction method. The raw powders (Bi2O3, Fe2O3 (> 99%) and BaTiO3 (> 99.9%)) weighed according to the nominal stoichiometric ratio of B1+xF-0.3BT were ball-milled for 12 h at 300 rpm. The dried powders were calcined at 800 °C for 6 h and then remilled for 12 h at 300 rpm. The powders were pressed into disks with a diameter of 10 mm and a thickness of 1.0 mm under 100 MPa using polyvinyl alcohol (2 wt%) as the binder. The disks were heated to 650 °C for 2 h in order to remove the binder and then heated to 1000 °C for 6 h. The crystal structure was characterized by X-ray diffraction (XRD: DMAX-RB, Japan). Well-polished ceramics were chemically etched by adding the etchant solution to the surface for 10 s to observe the morphology by field scanning electron microscopy (FESEM: SUPRATM55, Germany). Transmission electron microscopy (TEM: JEOL 2100, Japan) and piezoresponse force microscopy (PFM: MFP-3D, USA) were employed to observe the microscopic domain structure. X-ray photoelectron spectroscopy (XPS: ESCALAB 250Xi, USA) was used to characterize the surface chemical states of the well-polished ceramics. Silver was pasted on both surfaces of the ceramics and then the sample were sintered at 650 °C for 20 min. The ceramics were poled under a DC field of 3.5 kV/mm for 15 min in a silicone oil bath at 100 °C. Piezoelectric coefficient (d33) was measured using a quasi-static d33 testing meter (ZJ-3A, China). The planar electromechanical coupling coefficient (kp) and the mechanical quality factor (Qm) were measured using a precision impedance analyzer (Agilent 4294A, Hewlett-Packard, Palo Alto, CA). The electric-field-induced strains, polarization hysteresis loops (P-E, I-E) were measured at 0.5 Hz using a ferroelectric-measurement system (TF ANALYZER 1000, Germany). An insulation test instrument (AT683, China) was used to measure the leakage current density. Temperature dependent dielectric spectra were measured using an LCR analyzer (TH2828S) at 1 kHz.
3. Results and discussions Fig. 1(a) shows the piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp) and the mechanical quality factor (Qm) for B1+xF-0.3BT ceramics. The average d33 first increases from 158 pC/N at x = 0.00–214 pC/N at x = 0.02, and then decreases to 172 pC/N at x = 0.05. kp as a function of x shows behavior similar to that of d33. By compensating trace Bi content (x) in BF-0.3BT ceramic, we achieved a high d33 = 214 pC/N (Fig. 1(a)) along with a high TC = 528 °C, which is superior to the previously reported studies of BFBT based ceramics [13–28] (Fig. 1(b)). It is generally accepted that domain structure markedly affects the piezoelectric/ferroelectric properties of ceramics. Domain switching and domain wall movement are the mesoscopic origin of the polarization-electrical field (P-E) loops and the current-electrical field (I-E) loops of normal ferroelectric ceramics [29]. Fig. 2 shows the P-E and I-E loops under 50 kV/cm electric field measured at room temperature for the B1+xF-0.3BT ceramics. All of the samples show typical ferroelectric hysteresis loops and only a single polarization current peak near the coercive field (EC) resulting from domain switching during electric loading. The sharpness of the current peaks first increases and then decreases, indicating that the number of domains reaches the maximum value at x = 0.02 (Fig. 2(h)). Fig. 2(g) shows the remnant polarization (Pr) and EC of B1+xF-0.3BT ceramics. Pr first increases from 17.21 to 19.61 μC/cm2 as x increases from 0.00 to 0.02, and then decreases to 17.75 μC/cm2 at x = 0.05. The EC-x relationship shows the opposite trend from that of the Pr-x curve (Fig. 2(g)). This result suggests that compensating trace Bi content in BF-BT ceramics is beneficial for domain switching. Fig. 3 shows the bipolar and unipolar electric-field-induced strain 2
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 2. (a–f) Ferroelectric properties of P-E and I-E loops under 50 kV/cm electric field, (g) 2Pr and 2Ec and (h) the highest peak current value for B1+xF-0.3BT ceramics (x = 0–0.05).
(S-E) curves for the unpoled and poled B1+xF-0.3BT ceramics under 50 kV/cm electric field. The negative strain (Sneg) and the peak-to-peak strain (Sp-p) are obtained from the bipolar S-E loops [30,31]. The d*33 is calculated as d*33 = Smax/Emax; where Emax is the maximum electric field and Smax corresponds to the maximum strain obtained from the unipolar S-E loops [32]. It is well-known that the origin of the high strain for piezoceramics is ascribed mainly to the field-induced antiferroelectric-to-ferroelectric phase transition, relaxor-to-normal ferroelectric phase transition and the enhancement of domain switching and domain wall movement under the applied electric field [33]. The large Sneg in all of the typical butterfly shaped S-E loops (Fig. 3(a1, a3)) suggests that the large strain is mainly related to a classic non-180° domain switching behavior rather than to the field-induced phase transition (antiferroelectric-to-ferroelectric and relaxor-to-normal phase transition). Furthermore, irreversible domain switching is the mesoscopic origin of the Sneg that represents the degree of the non-180° domain switching [34]. |Sneg| achieves the maximum value at x = 0.02 (Fig. 3(a3)). The results are consistent with the P-E and I-E research, indicating that under the applied electric field the highest irreversible domain switching is obtained for the B1.02F-0.3BT ceramics. Similar to
the variation of |Sneg|, d*33 also reaches the maximum value (208 pm/ V) at x = 0.02. The butterfly shaped S-E loops for poled ceramics are very different from those of the unpoled ceramics as shown by the comparison of Fig. 3(a1) and 3(b1). The domain switching process corresponding to the special position of the butterfly curve is shown in Fig. 4. For unpoled B1+xF-0.3BT ceramics, the defect dipoles (DD) located at the domain boundaries are randomly oriented and do not reorient as the applied electric field increases [35,36]. Therefore, the strain in the S-E loops will decrease due to the pinning of the domains by the irreversible defect dipoles. However, under the long-term and the large directcurrent electric field, the DD and most of the domains can be reoriented and arranged linearly, resulting in the establishment of an internal bias electric field (Ei) in the grains. In addition, small domains can coalesce into large domains while the domain boundaries decrease, so that the domains are easier to switch when the electric field is applied (Fig. 4). The aligned DD partially contributes to the high strain [36]. Therefore, Sp-p and |Sneg| are enhanced by almost 2 times after poling (Fig. 3(a3b3)). The maximum Sp-p value of 0.375% and the Smax value of 0.13% under 50 kV/cm are obtained at x = 0.02. Moreover, the d*33 values of 3
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 3. (a1) Bipolar electromechanical strain (S–E) and (b1) unipolar strains loops of unpoled B1+xF-0.3BT ceramics; (a2) unipolar strains loops of unpoled and (b2) poled B1+xF-0.3BT ceramics under 50 kV/cm electric field. Sp-p, Sneg, d*33 of (a3) unpoled and (b3) poled B1+xF-0.3BT ceramics.
Fig. 4. Illustration of the domain switching process corresponding to the special position of the butterfly curve. The red arrows represent spontaneous polarization direction(PS) and the green arrows represent defect dipole polarization direction (PD). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 5. Room temperature impedance |Z| and phase angle θ as a function of frequency for B1+xF-0.3BT ceramics. (a) x = 0.00; (b) x = 0.01; (c) x = 0.02; (d) x = 0.03; (e) x = 0.04; (f) x = 0.05.
Fig. 6. PFM scanning results of out-of-plane ((a) x = 0.00; (b) x = 0.02; (c) x = 0.05) amplitude and ((d) x = 0.00; (e) x = 0.02; (f) x = 0.05) phase for B1+xF-0.3BT ceramics measured at 25 °C.
all of the samples have also been greatly improved after poling, peaking at 262 pm/V when x = 0.02. Therefore, domain structure is a vital factor for improving the properties of piezoelectric ceramics. The maximum phase angle (θmax) that ranges between −90° and 90° is an important indicator for the evaluation of the polarization degree (reorientation degree of domains) of piezoceramics. θmax should be equal to −90° for unpoled piezoceramics and the θmax is supposed to 90° for ideal poling piezoceramics among the frequency range between the resonance (fr) and anti-resonance frequency (fa) [37,38]. A greater
the θmax corresponds to, a stronger degree of orientation degree of domains. The impedance (|Z|) and phase angle (θ) measured as a function of frequency for B1+xF-0.3BT ceramics are shown in Fig. 5 θmax is approximately 11.36° at x = 0 and reaches 28.66° for x = 0.02 (Fig. 5(a–f)). However, θmax decreased to 23.62° at x = 0.05 (Fig. 5(a–f)). Therefore, it is thought that the B1.02F-0.3BT ceramic has the highest poling state due to the largest contribution of domain switching. To explore the origin of the high piezoelectricity, V-PFM amplitude 5
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 7. Representative bright-field transmission electron microscope (TEM) images with the magnification of (a) domain configuration and (b) enlarged nanodomains.
Disrupted strip-like domains that are the characteristic domains in the PC symmetry are also observed (red dotted circle). Therefore, the complex domain structure of the 0.7B1.02F-0.3BT ceramic is composed of mostly regular strip-like domains and a small percentage of disrupted strip-like domains. The large variety of domain sizes indicated that the mobility of domains had a broad distribution [39]. In addition to domain switching, the large insulation resistance (Re) is also beneficial for enhanced piezoelectric properties. Fig. 8 plots the electric field dependence of (a) Re and (b) the leakage current (J) of the B1+xF-0.3BT ceramics. The Re values of all of the samples are in the range of 6.48 × 109–1.10 × 1011 Ω cm (Fig. 8(a)) and the leakage current density values are in the range of 2.58 × 10−9–1.48 × 10−10 A/cm2 (Fig. 8(b)) under a 1 kV/cm field. The leakage current density reaches the minimum value at x = 0.02, suggesting that the B1.02F0.3BT ceramic has the minimal amount of defects. It was reported that the formation of V•• O resulted in the high J in BF-BT ceramics [40]. Excess Bi (0.01 ≤ x ≤ 0.02) may enter into the lattice and decrease V•• O. Fig. 9(a1-a4) show the XRD patterns for the B1+xF-0.3BT calcined powders and nano-BaTiO3 (BT) powders. The vertical lines are the standard diffraction peaks of BaTiO3 with a cubic (CBT) symmetry (PDF#75–0461, Pm-3m) and BiFeO3 with a rhombohedral (R) symmetry (PDF#71–2494, R3c). All calcined powders (Fig. 9(a1)) show a main perovskite phase and a small amount of the Bi25FeO40 impurity phase (★, PDF#46–0416). The perovskite phase includes the mechanical mixture of the Rα-R3c and the CBT phase, as found from the clear splitting of the peaks near 32° and 39° in Fig. 9(a1-a3) and a shoulder peak on the left side of the (104)R3c peak (Fig. 9(a3)). Therefore, three phases are present in all of the B1+xF-0.3BT powders, namely: the Rα-R3c phase, the CBT phase and the Bi25FeO40 impurity phase. The reaction equation can be expressed as: Fig. 8. (a) The insulation resistance (Re) and (b) the leakage current density (J) of B1+xF-0.3BT ceramics.
Bi2O3 + Fe2 O3 + BaTiO3
800°C
BiFeO3 R
R3c + BaTiO3 CBT + Bi25FeO40
(1)
In addition, when x increases, the Rα-R3c phase and the Bi25FeO40 impurity phase diffraction peaks first shift toward a lower angle and then toward a higher angle as shown in Fig. 9(a2-a4), reaching the minimum values at x = 0.02. The slight lattice enlargement at 0.00 ≤ x ≤ 0.02 indicates that the excess Bi has entered the lattice and compensated the volatilization of Bi2O3 in the Rα-R3c phase and the Bi25FeO40 impurity phase, simultaneously. The lattice shrinkage of the Rα-R3c phase at 0.03 ≤ x ≤ 0.05 may be due to the formation of a greater amount of the Bi25FeO40 impurity phase (Fig. 9(a4)), leading to the lack of Bi content in the Rα-R3c phase. The Bi2O3 volatilization in the Bi25FeO40 impurity phase is significant, as verified by the high offset of the impurity phase peak at 0.03 ≤ x ≤ 0.05 (Fig. 9(a2-a4)). The Bi2O3 volatilization in the Bi25FeO40 impurity phase can be expressed as:
and phase images (Fig. 6(a)-(f)) were employed to determine the domain structures and the local piezoresponse of the B1+xF-0.3BT ceramics. The BF-0.3BT ceramic has heterogeneous and sparse stripy domains. The domain morphology is clearly changed upon the addition of compensating trace Bi content. As x increases to 0.02, the amount and density of the domains increase, which is consistent with the I-E loops results, indicating that the amount of the domains is the highest at x = 0.02. However, the inhomogeneous block domains are observed at x = 0.05. Fig. 7 presents the representative images of the bright-field transmission electron microscope (TEM) for the x = 0.02 sample. Complex domain patterns composed of dense strip-like domains are observed as shown in Fig. 7(a). Coarse herringbone-type domains separated by 71° domain walls are also observed as shown in Fig. 7(b) [32], which is the characteristic domain in the R symmetry. The distribution of the domain sizes is uneven, with the domain size varying from 100 to 600 nm.
2Bi25FeO40
2BiFeO3 + 24Bi2O3
+ O2
(2)
The XRD patterns of B1+xF-0.3BT ceramics and the standard diffraction peaks cited by BT with a cubic symmetry (PDF#75–0461, Pm3m) and BF with a R3m symmetry (PDF#72–2112) are exhibited in 6
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 9. XRD patterns of (a) calcined powders and (b) ceramics for B1+xF-0.3BT samples. (b4) The Rietveld refinement of XRD patterns of B1+xF-0.3BT ceramics. Table 1 Rietveld refined lattice parameters and phase ratio of the BFScx-0.3BT ceramics. Composition
BT BF 0.00 0.01 0.02 0.03 0.04 0.05
Phase structure
PC-PDF#75-0461 R-PDF#72-2112 PC R PC R PC R PC R PC R PC R
Space group
Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m Pm-3m R-3m
Lattice parameters
Reliability factor
CR/CPC (%)
a = b = c(Å)
α = β = γ(°)
Rwp%
(%)
4.012 3.952 4.126 4.107 4.018 3.982 3.998 3.988 3.999 4.007 4.001 4.000 4.014 4.012
90.00 89.60 90.00 89.99 90.00 90.26 90.00 89.86 90.00 89.77 90.00 89.89 90.00 90.01
– – 9.06
– – 32/68
11.52
77/23
10.45
68/32
9.12
65/34
8.203
58/42
6.65
40/59
Fig. 9(b1-b3). Compared with the XRD patterns of powders, there are distinct changes in the XRD patterns of ceramics. First, the Bi25FeO40 impurity phase disappears completely at 0.00 ≤ x ≤ 0.02 and reduces obviously at 0.03 ≤ x ≤ 0.05 (Fig. 9(b1)). Second, all peaks shift toward the standard cards of BaTiO3 and the shoulder peak on the (104)R3c peak's left has disappeared, indicating that BT has completely
dissolved into the BF matrix. Third, the B1+xF-0.3BT ceramics locate in the MPB, consisting of Rβ-R3m-PC phases due to the misfit between the individual standard diffraction peaks and the XRD patterns. The reaction equation can be expressed as:
7
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
x = 0.02. It has been reported that the nonstoichiometric defects (the bismuth vacancy (V B" i ) and the oxygen vacancy (VO••)) deriving from the volatilization of Bi2O3 in 0.75BF-0.25BT-Mn ceramics may enlarge the lattice due to the charge repulsion [14]. The defect equations of B1+xF0.3BT ceramics can be expressed as:
2BiFeO3 × 2Fe×Fe + OO
R3c + BaTiO3 CBT + Bi25 FeO40
1000°C
(Bi,Ba) Fe,Ti)O3 R
R3m + PC
× + 2Fe×Fe + 2VBi + 3V•• O + 3OO
2FeFe + V•• O +
1 O2 2
(4) (5)
Compensating trace Bi content (0.00 ≤ x ≤ 0.02) may weaken the charge repulsion in the Rβ-R3m phase and decrease the lattice parameters due to charge compensation. In other words, the B1.02F-0.3BT ceramics have the least amount of V"Bi and the V•• O . The slight lattice enlargement at 0.03 ≤ x ≤ 0.05 may be due to the charge repulsion because of the greater volatilization of Bi2O3 in the powders (Fig. 9(a2-a4)) and the formation of the Bi25FeO40 impurity phase (Fig. 9(b1)). In addition to the shift of the peaks with x, a change in the phase symmetry was also observed. Although all of the samples have the RβR3m phase coexisting with the PC phase, the samples show a clear difference regarding their Rβ-R3m/PC (CR/CPC) ratio (Fig. 9(b3)). The intensity of the (111)PC peak is higher than that of the (1 1 1) R3m peak, indicating that the PC phase content is greater than the Rβ-R3m phase content in the BF-0.3BT ceramic. It is possible that a greater amount of vacancies gave rise to cell expansion, facilitating the transformation from the Rβ-R3m phase (R3m, a = b = c = 3.952 Å, α = β = γ = 89.6°) to the PC (Pm-3m, a = b = c = 4.012 Å, α = β = γ = 90°) phase. Trace Bi self-compensation (0.01 ≤ x ≤ 0.02) may enter into the lattice and fill the vacancies, leading to the increase in the Rβ-R3m phase content. The intensity of the (111)PC peaks in the 0.03 ≤ x ≤ 0.05 samples increased, indicating an increase in the PC phase amount. The change of CR/CPC has a strong influence on the piezoelectric properties. It should be noted that a piezoceramic near the morphotropic phase boundary (MPB) exhibits relatively good ferroelectricity [5,6], because of the various ferroelectric switching paths that can help to reduce the internal energy barrier during domain switching. However, Bi loss in BFBT will inevitably give rise to a change in the phase structures, resulting in the shift away from the MPB and the deterioration in the piezoelectric performance. Compensating with an appropriate Bi content can fill the vacancies, which is beneficial for approaching the MPB and reducing VO••. Therefore, appropriate Bi self-compensation plays a vital role in obtaining the high piezoelectric properties. The results of the Xray Rietveld refinement are plotted in Fig. 9(b4) and the corresponding lattice parameters and CR/CPC values are listed in Table 1.
Fig. 10. BS-SEM images of chemically etched surfaces of B1+xF-0.3BT ceramics. (a) x = 0.00, (b) x = 0.02, (c) x = 0.05.
BiFeO3 R
Bi2 O3
(3)
In addition, the peaks near 32° first shift toward a higher angle and then toward a lower angle with x (Fig. 9(b3)), reaching the maximum at
Fig. 11. SEM images of the fracture surface microstructure for B1+xF-0.3BT ceramics. (a) x = 0.00, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05. 8
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 12. XPS spectra of (a) Fe 2p, (b) Fe 2p, (c) Bi 4f, (d) Ba 3d, (e) Ti 2p and (f) O 1s for the B1+xF-0.3BT ceramics.
atomic percentage of the constituent elements is listed in the images. The Bi content in the B1+xF-0.3BT grains reaches maximum values at x = 0.02, indicating that the B1.02F-0.3BT ceramic has the highest Bi content, which is in agreement with the XRD analysis results. Fig. 11 shows the grain morphologies of the B1+xF-0.3BT ceramics. Grain size histograms are shown in the top right corner of each SEM images. The average grain size (d ) of B1+xF-0.3BT ceramics is approximately 7.02, 6.96, 7.04, 6.53, 5.62 and 5.36 μm at x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. The excess Bi content (0.00 ≤ x ≤ 0.02) has no obvious effect on the grain sizes and grain morphology of the ceramics. However, the grain size of the samples at 0.03 ≤ x ≤ 0.05 gradually decreases with increasing the Bi content (0.03 ≤ x ≤ 0.05). Fig. 12 shows the XPS spectra of all of the elements in the B1+xF0.3BT ceramics. Symmetric peaks are observed at ca. 164.15 and 158.85 eV, 794.70 and 779.90 eV and 464.04 and 457.87 eV corresponding to Bi3+ 4f7/2 and 4f5/2, Ba2+ 3d5/2 and 3d3/2 and Ti 2p3/2 and 2p1/2, respectively. However, the asymmetric Fe 2p peaks indicate the coexistence of a mixture of two valences (Fe3+ and Fe2+) in the B1+xF0.3BT ceramics (Fig. 12(a)) that can be separated into the 2p3/2 and 2p1/2 states of Fe3+ and Fe2+ (Fig. 12(b)). The patterns of Fe 2p peaks for B1+xF-0.3BT ceramics are identical, indicating that the relative contents of Fe2+ are basically similar in all B1+xF-0.3BT ceramics.
Fig. 13. Temperature dependences of dielectric constant εr and dielectric loss tanδ measured at 1 kHz for B1+xF-0.3BT ceramics.
Fig. 10 shows the BS-SEM images of well-polished and chemically etched B1+xF-0.3BT ceramics. Energy-dispersive spectroscopy (EDS) was performed to investigate the elemental content in the grains. The 9
Ceramics International xxx (xxxx) xxx–xxx
B.-W. Xun, et al.
Fig. 13 shows the dielectric constant as a function of temperature for the B1+xF-0.3BT ceramics. All of the ceramics exhibit a single sharp dielectric peak, indicating the typical transition from the ferroelectric to the paraelectric phase (Fig. 13(a)). The samples show high TC values that are greater than 495 °C. The maximum (TC = 528 °C) is obtained at x = 0.02.
[13] T. Zheng, Y. Ding, J. Wu, Bi nonstoichiometry and composition engineering in (1−x)Bi1+yFeO3+3y/2-xBaTiO3 ceramics, RSC Adv. 6 (93) (2016) 90831–90839. [14] J. Chen, J. Cheng, Enhanced thermal stability of lead-free high temperature 0.75BiFeO3-0.25BaTiO3 ceramics with excess Bi content, J. Alloy. Comp. 589 (2014) 115–119. [15] C. Zhou, H. Yang, Q. Zhou, G. Chen, W. Li, H. Wang, Effects of Bi excess on the structure and electrical properties of high-temperature BiFeO3-BaTiO3 piezoelectric ceramics, J. Mater. Sci. Mater. Electron. 24 (5) (2013) 1685–1689. [16] K. Tong, C. Zhou, Q. Li, J. Wang, L. Yang, J. Xu, G. Chen, C. Yuan, G. Rao, Enhanced piezoelectric response and high-temperature sensitivity by site-selected doping of BiFeO3-BaTiO3 ceramics, J. Eur. Ceram. Soc. 38 (4) (2018) 1356–1366. [17] D. Wang, Z. Fan, D. Zhou, A. Khesro, S. Murakami, A. Feteira, Q. Zhao, X. Tan, I.M. Reaney, Bismuth ferrite-based lead-free ceramics and multilayers with high recoverable energy density, J. Mater. Chem. 6 (9) (2018) 4133–4144. [18] D. Wang, A. Khesro, S. Murakami, A. Feteira, Q. Zhao, I.M. Reaney, Temperature dependent, large electromechanical strain in Nd-doped BiFeO3-BaTiO3 lead-free ceramics, J. Eur. Ceram. Soc. 37 (4) (2017) 1857–1860. [19] G.H. Ryu, A. Hussain, M.H. Lee, R.A. Malik, T.-K. Song, W.-J. Kim, M.-H. Kim, Leadfree high performance Bi(Zn0.5Ti0.5) O3-modified BiFeO3-BaTiO3 piezoceramics, J. Eur. Ceram. Soc. 38 (13) (2018) 4414–4421. [20] F. Akram, R.A. Malik, T.K. Song, S. Lee, M.-H. Kim, Thermally-stable high dielectric properties of (1-x)(0.65Bi1.05FeO3-0.35BaTiO3)–xBiGaO3 piezoceramics, J. Eur. Ceram. Soc. 39 (7) (2019) 2304–2309. [21] C. Zhou, H. Yang, Q. Zhou, Z. Cen, W. Li, C. Yuan, H. Wang, Dielectric, ferroelectric and piezoelectric properties of La-substituted BiFeO3-BaTiO3 ceramics, Ceram. Int. 39 (4) (2013) 4307–4311. [22] L.-F. Zhu, B.-P. Zhang, S. Li, G.-L. Zhao, Large piezoelectric responses of Bi (Fe,Mg,Ti)O3-BaTiO3 lead-free piezoceramics near the morphotropic phase boundary, J. Alloy. Comp. 727 (2017) 382–389. [23] Q. Li, J. Wei, T. Tu, J. Cheng, J. Chen, Remarkable piezoelectricity and stable high‐temperature dielectric properties of quenched BiFeO3-BaTiO3 ceramics, J. Am. Ceram. Soc. 100 (12) (2017) 5573–5583. [24] S.O. Leontsev, R.E. Eitel, Dielectric and piezoelectric properties in Mn‐modified (1x)BiFeO3-xBaTiO3 ceramics, J. Am. Ceram. Soc. 92 (12) (2009) 2957–2961. [25] Z. Yao, C. Xu, H. Liu, H. Hao, M. Cao, Z. Wang, Z. Song, W. Hu, A. Ullah, Greatly reduced leakage current and defect mechanism in atmosphere sintered BiFeO3BaTiO3 high temperature piezoceramics, J. Mater. Sci. Mater. Electron. 25 (11) (2014) 4975–4982. [26] Y. Wei, X. Wang, J. Jia, X. Wang, Multiferroic and piezoelectric properties of 0.65BiFeO3-0.35BaTiO3 ceramic with pseudo-cubic symmetry, Ceram. Int. 38 (4) (2012) 3499–3502. [27] W. Gao, J. Lv, X. Lou, Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics, J. Am. Ceram. Soc. 101 (8) (2018) 3383–3392. [28] Y. Wei, X. Wang, J. Zhu, X. Wang, J. Jia, Dielectric, ferroelectric, and piezoelectric properties of BiFeO3-BaTiO3 ceramics, J. Am. Ceram. Soc. 96 (10) (2013) 3163–3168. [29] C. Zhao, B. Wu, K. Wang, J.-F. Li, D. Xiao, J. Zhu, J. Wu, Practical high strain with superior temperature stability in lead-free piezoceramics through domain engineering, J. Mater. Chem. 6 (46) (2018) 23736–23745. [30] F.-Z. Yao, K. Wang, W. Jo, J.-S. Lee, J.-F. Li, Effect of poling temperature on piezoelectricity of CaZrO3-modified (K,Na) NbO3-based lead-free ceramics, J. Appl. Phys. 116 (11) (2014) 114102. [31] W. Jo, T. Granzow, E. Aulbach, J. Rödel, D. Damjanovic, Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics, J. Appl. Phys. 105 (9) (2009) 094102. [32] D. Wang, G. Wang, S. Murakami, Z. Fan, A. Feteira, D. Zhou, S. Sun, Q. Zhao, I.M. Reaney, BiFeO3-BaTiO3: a new generation of lead-free electroceramics, J. Adv. Dielectr. 8 (2018) 1–35, https://doi.org/10.1142/S2010135X18300049. [33] L. Jin, F. Li, S. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension of the material properties and structures, J. Am. Ceram. Soc. 97 (1) (2014) 1–27. [34] W. Jo, R. Dittmer, M. Acosta, J. Zang, C. Groh, E. Sapper, K. Wang, J. Rödel, Giant electric-field-induced strains in lead-free ceramics for actuator applications–status and perspective, J. Electroceram. 29 (1) (2012) 71–93. [35] M.H. Lee, D.J. Kim, H.I. Choi, M.H. Kim, T.K. Song, W.J. Kim, J.S. Park, D. Do, Low sintering temperature for lead‐free BiFeO3‐BaTiO3 ceramics with high piezoelectric performance, J. Am. Ceram. Soc. 102 (5) (2019) 2666–2674. [36] J. Zeng, K. Zhao, X. Shi, X. Ruan, L. Zheng, G. Li, Large strain induced by the alignment of defect dipoles in (Bi3+,Fe3+) co-doped Pb(Zr,Ti)O3 ceramics, Scr. Mater. 142 (2018) 20–22. [37] D. Lin, K. Kwok, H. Chan, Piezoelectric and ferroelectric properties of KxNa1−xNbO3 lead-free ceramics with MnO2 and CuO doping, J. Alloy. Comp. 461 (1–2) (2008) 273–278. [38] H.-Y. Park, I.-T. Seo, M.-K. Choi, S. Nahm, H.-G. Lee, H.-W. Kang, B.-H. Choi, Microstructure and piezoelectric properties of the CuO-added (Na0.5K0.5) (Nb0.97Sb0.03)O3 lead-free piezoelectric ceramics, J. Appl. Phys. 104 (3) (2008) 034103. [39] Q. Liu, Y. Zhang, J. Gao, Z. Zhou, H. Wang, K. Wang, X. Zhang, L. Li, J.-F. Li, Highperformance lead-free piezoelectrics with local structural heterogeneity, Energy Environ. Sci. 11 (12) (2018) 3531–3539. [40] B.-W. Xun, Y.-C. Tang, J.-Y. Chen, B.-P. Zhang, Enhanced resistance in Bi(Fe1-xScx) O3-0.3BaTiO3 lead-free piezoelectric ceramics: facile analysis and reduction of oxygen vacancy, J. Eur. Ceram. Soc. 39 (14) (2019) 4085–4095.
4. Conclusions The effects of compensating Bi content on the electrical properties, domain structure and phase structure in the 0.7BF-0.3BT ceramic was studied systematically by P-E, I-E, S-E, PFM, TEM and XRD. These results indicate that compensating with an appropriate Bi content promotes domain switching due to the precise control of the MPB. The complex domain structure was composed mostly of regular strip-like domains together with a small percentage of disrupted strip-like domains in the 0.7B1.02F-0.3BT ceramic. Meanwhile, inhomogeneous block domains are observed in the 0.7BF-0.3BT and 0.7B1.05F-0.3BT ceramics. |Sneg| reached the maximum value at x = 0.02, indicating the largest non-180° domain switching. Therefore, the excellent piezoelectric properties of d33 = 214 pC/N, kp = 0.325, TC = 528 °C and Spp = 0.375% were achieved. The investigation of the compensating Bi content will inspire further research to develop BF-BT based ceramics suitable for practical use. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no.51472026). References [1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Epitaxial BiFeO3 multiferroic thin film heterostructures, Science 299 (5613) (2003) 1719–1722. [2] J.B. Neaton, C. Ederer, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, First-principles study of spontaneous polarization in multiferroic BiFeO3, Phys. Rev. B 71 (1) (2005) 014113. [3] Z. Dai, Y. Akishige, Electrical properties of multiferroic BiFeO3 ceramics synthesized by spark plasma sintering, J. Phys. D Appl. Phys. 43 (44) (2010) 445403. [4] Q. Zhang, X. Zhu, Y. Xu, H. Gao, Y. Xiao, D. Liang, J. Zhu, J. Zhu, D. Xiao, Effect of La3+ substitution on the phase transitions, microstructure and electrical properties of Bi1−xLaxFeO3 ceramics, J. Alloy. Comp. 546 (2013) 57–62. [5] M.M. Kumar, A. Srinivas, S.V. Suryanarayana, Structure property relations in BiFeO3/BaTiO3 solid solutions, J. Appl. Phys. 87 (2) (2000) 855–862. [6] M.H. Lee, D.J. Kim, J.S. Park, S.W. Kim, T.K. Song, M.H. Kim, W.J. Kim, D. Do, I.K. Jeong, High‐performance lead‐free piezoceramics with high curie temperatures, Adv. Mater. 27 (43) (2015) 6976–6982. [7] S. Cheng, B.-P. Zhang, L. Zhao, K.-K. Wang, Enhanced insulating and piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 lead-free ceramics by optimizing calcination temperature: analysis of Bi3+ volatilization and phase structures, J. Mater. Chem. C 6 (15) (2018) 3982–3989. [8] L.-F. Zhu, B.-P. Zhang, J.-Q. Duan, B.-W. Xun, N. Wang, Y.-C. Tang, G.-L. Zhao, Enhanced piezoelectric and ferroelectric properties of BiFeO3-BaTiO3 lead-free ceramics by optimizing the sintering temperature and dwell time, J. Eur. Ceram. Soc. 38 (10) (2018) 3463–3471. [9] Y. Lee, J. Kim, S. Han, H.-W. Kang, H.-G. Lee, C.I. Cheon, Effect of sintering temperature on the piezoelectric properties in BiFeO3-BaTiO3 ceramics, J. Korean Phys. Soc. 61 (6) (2012) 947–950. [10] S. Guan, H. Yang, Y. Zhao, R. Zhang, Effect of Li2CO3 addition in BiFeO3-BaTiO3 ceramics on the sintering temperature, electrical properties and phase transition, J. Alloy. Comp. 735 (2018) 386–393. [11] G. Song, Y. Song, J. Su, X. Song, N. Zhang, T. Wang, F. Chang, Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material, J. Alloy. Comp. 696 (2017) 503–509. [12] K. Nalwa, A. Garg, A. Upadhyaya, Effect of samarium doping on the properties of solid-state synthesized multiferroic bismuth ferrite, Mater. Lett. 62 (6–7) (2008) 878–881.
10