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Large piezoelectric coefficient with enhanced thermal stability in Nb5+-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics
Journal Pre-proof 5+ Large piezoelectric coefficient with enhanced thermal stability in Nb -doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics Liqiang He, Yuanchao Ji, Shuai Ren, Luo Zhao, Hanyu Luo, Chang Liu, Yanshuang Hao, Le Zhang, Lixue Zhang, Xiaobing Ren PII:
S0272-8842(19)32877-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.028
Reference:
CERI 23096
To appear in:
Ceramics International
Received Date: 19 July 2019 Revised Date:
2 October 2019
Accepted Date: 3 October 2019
Please cite this article as: L. He, Y. Ji, S. Ren, L. Zhao, H. Luo, C. Liu, Y. Hao, L. Zhang, L. 5+ Zhang, X. Ren, Large piezoelectric coefficient with enhanced thermal stability in Nb -doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.10.028. 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.
Large piezoelectric coefficient with enhanced thermal stability in Nb5+-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics Liqiang Hea, Yuanchao Ji,a,* Shuai Rena, Luo Zhaoa, Hanyu Luoa, Chang Liua, Yanshuang Haoa, Le Zhang,a,c* Lixue Zhang,a,* Xiaobing Rena,b a
Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, China b
Center for Functional Materials, National Institute for Materials Science, Tsukuba, 305-0047,
Ibaraki, Japan c
School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW
2052, Australia
Abstract Chemical doping is an indispensable tool to tailor the properties of the commercial piezoelectric materials. However, a high piezoelectric coefficient with enhanced thermal stability is rarely achieved by one dopant in some high-performance ferroelectrics, e.g., the recently discovered eco-friendly (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics. In order to optimize the piezoelectric property in BCZT system by a simple way, we investigated the doping effect of Fe3+, Nb5+ and Bi3+ cations in BCZT ceramics respectively. The results indicate that only Nb5+-doped BCZT ceramics display a combination of large piezoelectric coefficient and enhanced thermal stability, compared with others. Moreover, the established phase diagrams and in-situ transmission electron microscope (TEM) observations reveal that such optimized piezoelectric properties after Nb5+ doping originates from (i) the low polarization anisotropy near the ambient tetragonal (T)-orthorhombic (O) phase transition and (ii) the easy domain wall motion of persistent miniaturized ferroelectric domains upon heating. *
Corresponding author:
E-mail addresses: [email protected] (Y. Ji)
[email protected] (L. Zhang)
[email protected] (L. Zhang) 1
Keywords: Piezoelectric ceramics; Thermal stability; Phase transition; Nanodomains; Domain wall motion
2
1. Introduction Piezoelectric materials have been widely used in sensors, actuators and transducers [1, 2]. For more than half a century, the huge market has been dominated by the lead zirconate titanate (PZT) families. In order to fulfill various application requirements, different dopants have been incorporated into PZT systems to modulate the ferroelectric and piezoelectric properties [2]. Unfortunately, Lead is now facing a worldwide legislative restriction due to its toxicity concern, which has triggered intense research interests for lead-free systems [3, 4]. In the past decade, a large piezoelectric coefficient of d33~500-620 pC/N, comparable with that of the commercial soft-PZT [5], has been achieved in lead-free Ba(X, Ti)O3-(Ba, Ca)TiO3 (X=Zr, Sn, Hf) ceramics [6-8] and complex doped (K,,Na)NbO3 ceramics [9, 10]. It is reported that the high piezoelectric coefficient in (Ba, Ca)(Zr, Ti)O3 system occurs in the proximity of the tetragonal (T)-orthorhombic (O) phase transition region due to the easy polarization extension and rotation [11]. However, (Ba, Ca)(Zr, Ti)O3 system shows a poor piezoelectric thermal stability upon heating [8, 12], which deeply restricts the correlated device designing and manufacturing. In order to optimize piezoelectric properties, the compound doping or co-doping methods have been introduced into the lead-free (Ba, Ca)(Zr, Ti)O3 solid solutions [1]. For example, Chao et al. introduced a tungsten bronze structure compound, i.e., Ca0.28Ba0.72Nb2O6, into (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) lattice, which can result in a higher piezoelectric coefficient of d33 with enhanced thermal stability [13]. The complex compound doping can also be utilized to modify piezoelectric properties in other ferroelectric systems, including (1-x)Bi0.5Na0.5TiO3-x(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 [14],
(1-x)(K0.48Na0.52)NbO3-(x/5.15)K2.9Li1.95Nb5.15O15.3
[15]
and
(1-x-y)K1-wNawNb1-zSbzO3-yBaZrO3-xBi0.5K0.5HfO3 [9]. Besides, Ding, et al. reported that Sn4+ and Sr2+ co-doped (Ba0.84Ca0.15Sr0.01)(Ti0.9Zr0.09Sn0.01)O3 system displayed the thermal reliable piezoelectric coefficient [16].
3
However, the thermally reliable high piezoelectric coefficient is rarely achieved through doping single aliovalent or isovalent cation in both lead and lead-free ferroelectrics. In PZT ceramics, acceptor cations, e.g., Fe3+ replacing Ti4+/Zr4+ [17, 18], can result in an increase of the thermal stability of piezoelectric coefficient but with a decrease of the piezoelectric coefficient [19], while donor cations, e.g., Bi3+ replacing Pb2+ [20] or Nb5+ replacing Ti4+/Zr4+, can lead to an increase of piezoelectric coefficient but with a decrease of the thermal stability [21]. Besides, among recent studies in lead-free systems [22-24], aliovalent cation, such as Tb3+ [25] or Mn3+ [26] was used to enhance the thermal stability of piezoelectric coefficient in (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3
and
(Ba0.838Ca0.162)(Ti0.908Zr0.092)O3
compositions
respectively, but the piezoelectric coefficient of d33 is only about 410 pC/N from room temperature to 80 °C, lower than the best value. Furthermore, by adjusting isovalent Zr4+ or Ca2+ contents in (Ba, Ca)(Zr, Ti)O3 materials [27], the thermal stability of piezoelectric coefficient can also be enhanced to some extent, but the piezoelectric coefficient is sharply reduced to 250 pC/N. Most single dopants either develop a single phase region by removing the ferroelectric-ferroelectric phase boundary [28] or form a diffuse phase transition in a wide temperature range [29] to improve the thermal stability of piezoelectricity, but the high energy barrier between the polarization variants in the single phase and the low polarization response in the diffuse-phase-transition phase would reduce the magnitude of piezoelectric coefficient significantly. In
the
present
study,
we
shows
that
Fe3+-doped
and
Bi3+-doped
(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics display enhanced thermal stability of piezoelectric coefficient but lower piezoelectric coefficients, whereas Nb5+-doped BCZT ceramics exhibit the thermally reliable high piezoelectric coefficient. The phase diagram analysis and TEM results reveal that this is mainly attributed to two reasons, (i) Nb5+ doping keeps the temperature of tetragonal (T)-orthorhombic (O) phase transition unchanged (near room temperature), which will result in large 4
piezoelectric coefficient at the ambient temperature and (ii) the minimized domain pattern with the low polarization anisotropy persists upon heating, leading to enhanced thermal stability of piezoelectric coefficient. The dissimilar effect of the selected dopants on the phase transition behavior and domain sizes was also briefly discussed. 2. Experimental details 2.1. Material fabrication Fe3+-doped, Nb5+-doped and Bi3+-doped BCZT ceramics were fabricated by the conventional solid-state reaction method with starting chemicals of BaCO3 (99.8 wt%, Alfa Aesar), BaZrO3 (99 wt%, Alfa Aesar), TiO2 (99.9 wt%, Alfa Aesar), CaCO3 (99.9 wt%, Alfa Aesar), Fe2O3 (99.9 wt%, Alfa Aesar), Nb2O5 (99.9 wt%, Alfa Aesar) and Bi2O3 (99.9 wt%, Alfa Aesar) (the Alfa Aesar reports in Fig. S5 and Fig. S6). Their chemical formulas are (Ba0.85Ca0.15)((Zr0.1Ti0.9)1-x/100Fex/100)O3-δ (BCZT-xFe), (Ba0.85Ca0.15)((Zr0.1Ti0.9)1-1.25x/100Nbx/100)O3
(BCZT-xNb)
and
((Ba0.85Ca0.15)1-1.5x/100Bix/100)(Zr0.1Ti0.9)O3 (BCZT-xBi), where x is the molar percent of each dopant. The powder chemicals were ball-milled using the alcohol medium for 8 h in a nylon pot with zirconium dioxide balls inside (the wear rate of zirconium dioxide balls is 0.03 ppm/h shown in Fig. S7). Then the mixed powders were calcined at 1300 °C for 3 h in air. After that the calcined products were ground into powders and ball-milled again. And the calcined powders were shaped into pellets after adding some binder (5 wt% Polyvinyl Alcohol (PVA) aqueous solution). Finally the ceramic discs were fabricated by sintering at 1450 °C for 6 h. 2.2. Characterization The surface morphology images of the samples were obtained by field emission scanning electron microscope (Hitachi SU6600). X-ray diffraction (XRD) patterns were collected by Shimadzu XRD7000 under a current of 30 mA and the voltage of
5
40 kV with CuKα radiation (λ=1.5406 Å). The permittivity versus temperature curves were measured by a HIOKI LCR meter from -100 °C to 150 °C with a heating and cooling rate of 2 °C/min. The LCR frequencies are 100 Hz, 1 kHz, 10 kHz, and 100 kHz, and the oscillation voltage is 2.5 V in dielectric measurement. The polarization-electric field (P-E) loops of samples were tested by a Premier Ⅱ ferroelectric test system at 10 Hz and 30 kV/cm. The cylinder specimens with silver paste were poled under a DC-field of 10 kV/cm for 40 minutes at room temperature and then the temperature dependence of the piezoelectric coefficient d33 for all specimens were measured by a piezoelectric d33-meter (Model ZJ-3A, Chinese Academy of Sciences) with a self-assembled temperature chamber [30]. The local microstructure evolution and corresponding diffraction pattern were acquired by JEOL-2100F high resolution transmission electron microscope (TEM) equipped with double tilt heating specimen stages (from Gatan). The Digital Micrograph software was used to analyze the domain images observed in TEM. 3. Results and discussion The XRD results in Fig. 1(a) show that all BCZT specimens manifest the pure perovskite structure without any secondary phases or impurities, indicating that all dopants have completely diffused into the crystal lattice. The SEM morphologies in Fig. S1 further confirm the homogeneity of each specimen. As shown in Fig. 1(b)-(d), the splitting of (002) peaks of Fe3+-doped, Nb5+-doped and Bi3+-doped BCZT samples almost disappears at 80 °C as close to the Curie temperature. When the temperature cools down to 50 °C, the (002) peak splitting indicates the existence of tetragonal symmetry [31]. Table S1 and Fig. S2 show that the presence of Fe3+, Nb5+ and Bi3+ cations induces a slight fluctuation in the lattice parameters of the tetragonal phase. The c/a ratios of BCZT-0.25Fe (abbreviated as 0.25Fe), BCZT-0.25Nb (abbreviated as 0.25Nb) and BCZT-0.25Bi (abbreviated as 0.25Bi) are 1.0031, 1.0039 and 1.0044 respectively, by fitting the (002) peak splitting at 50 °C (shown in Fig. 1 (b)-(d)). 6
However, when the temperature approaches to the T-O phase boundary of each dopant composition (confirmed by the anomalies in temperature dependent dielectric constants in Fig. 2) upon further cooling, a splitting of (002) peak of tetragonal structure becomes unconspicuous due to the low polarization anisotropy at T - O phase boundary [32].
Fig. 1. (a) XRD patterns of doped and undoped BCZT samples at room temperature. The (002) diffraction peak splitting of (b) BCZT-0.25Fe at 15 °C, 50 °C and 80 °C; (c) BCZT-0.25Nb at 20 °C, 50 °C and 80 °C; (d) BCZT-0.25Bi at 0 °C, 50 °C and 80 °C. TC, TT-O and TO-R corresponding to the Curie temperature and the temperatures of T-O and O-R phase transitions, are obtained from temperature-dependent dielectric curves (Fig. 2(a1-c1)) and loss behaviors (Fig. 2(a2-c2)). The related phase diagrams are established in Fig. 2(a3-c3). After doping Fe3+ cations (see Fig. 2(a3)), TC and TT-O in BCZT ceramics both drop down, while TC and TT-O of Nb5+-doped ceramics remain nearly unchanged (see Fig. 2(b3)). Different from the Nb5+-doped and Fe3+-doped cases, Bi3+-doped ceramics perform the decrease of TT-O while maintaining TC, as 7
shown in Fig. 2(c3). This dissimilar effect on the phase transitions for each dopant mainly derives from the dopant size effect and electronegativity features [33-36]. Due to the close ionic radius and electronegativity of Nb5+ cations to B-site of BCZT matrix [34], the changes in crystal lattice parameters and chemical bond of BCZT ceramics are not so much. In this respect, the temperature of phase transition is almost unchanged in Nb5+-doped BCZT ceramics. But for the Fe3+ and Bi3+ doped samples, the degree of strain heterogeneity and electronegativity difference are larger [34]. Besides, Bi3+ ions have 6s2 lone-pair electrons hybridized with the surrounding oxygen anions, which has the potential to manipulate the ferroelectricity [35, 36]. It is necessary to make a further deep research on the piezoelectric or ferroelectric properties of three doped BCZT ceramics.
Fig. 2. Temperature dependence of (a1)-(c1) dielectric constants (ε) and (a2)-(c2) dielectric loss (tanδ) of doped and undoped BCZT ceramics at 100 Hz. (a3)-(c3) The corresponding phase diagrams of BCZT-xFe, BCZT-xNb, and BCZT-xBi ceramics. Fig. 3(a)-(c) show ambient polarization-electric field (P-E) loops of the Fe3+-doped BCZT, Nb5+-doped BCZT and Bi3+-doped BCZT ceramics respectively, which are also compared with the P-E loop of the undoped BCZT ceramics. As summarized in Fig. 3(d), Fe3+ dopants will slightly lessen the maximum polarization 8
Pm and the remnant polarization Pr at room temperature. In contrast, the Pm and the Pr of Nb5+-doped and Bi3+-doped BCZT ceramics increase when the molar percent of dopants locates at 0.25 and then decreases at the molar percent ~0.5. When the dopant concentration is 0.75%, an obvious frequency-dispersion dielectric permittivity and the further decreased ferroelectric properties (see Fig. S3). The explanation for above phenomena is that the increase of dopant concentration above 0.25% may introduce more compositional disorders, local stress field and free charges, which will lead to the transformation of normal ferroelectric state to relaxor ferroelectric state, and possible high leakage [37]. Besides, Fig. 3(d) shows that doping Fe3+ cations or Nb5+ cations can decrease EC of BCZT, while doping Bi3+ causes the increase of EC. Therefore, due to the ambient high polarization and low coercive field (near T-O phase transition), the largest piezoelectric coefficient at room temperature prefers to exist in Nb5+-doped BCZT ceramics rather than Fe3+ or Bi3+-doped cases [38].
Fig. 3. P-E Hysteresis loops of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics, in comparison with P-E loops of undoped BCZT ceramics. (d) Comparison of maximum polarization Pm, remnant polarization Pr and coercive field EC among all specimens at room temperature. The temperature dependence of d33 values of each BCZT samples are shown in Fig. 4. The undoped BCZT ceramics just exhibit one piezoelectric coefficient peak of 9
d33 ~540 pC/N near T-O phase transition and then sharply decreases to around 290 pC/N when the temperature is close to TC [6]. But after doping Fe3+, Nb5+ and Bi3+ cations, the decrease of piezoelectric coefficient slows down upon heating, indicating an improved thermal stability which can be seen from the value of d33/d33
293 K
depicted in Fig. S4. Among three doped BCZT ceramics, the 0.25% Nb5+ doped sample is the only one rendering the large piezoelectric coefficient and thermal reliability.
Fig. 4. Temperature dependence of piezoelectric coefficient of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics. (d) Comparison of piezoelectric coefficient at 50 °C of each specimen. The
microstructure
evolutions
of
selected
BCZT-0.25Fe
ceramics,
BCZT-0.25Nb ceramics and BCZT-0.25Bi ceramics with heating are obtained by in-situ bright field (BF) TEM (see Fig. 5(b1-b3)(c1-c3)(d1-d3)). As a comparison, Fig. 5(a1) shows the domain structure of the undoped BCZT ceramics at room temperature. The grown lamella nanodomains inside the large domains [39] can be clearly seen in the inset of Fig. 5 (a1). Upon heating to 50 °C, the domain structure of undoped BCZT samples transforms to the pure tetragonal structure without coexisting 10
nanodomains, which results in the observed poor piezoelectric coefficient in Fig. 4. But for doped specimens, nanodomain structure patterns can keep stable in the domain morphology from 20 °C to 50 °C (see Fig. 5(b1)(b2), (c1)(c2), (d1)(d2)). When heating samples to 80 °C, the domains disappear due to near Curie temperature (see Fig. 5(b3)-(d3)). The in-situ TEM observation reveals that the enhanced thermal stability of piezoelectric coefficient for doped specimens is attributed to the optimized temperature dependent behavior of ferroelectric nanodomains (see Fig. 5).
Fig. 5. In-situ TEM observations of domain evolution under bright field (BF) mode. The bright field image of (a1-a3) the undoped BCZT ceramics; (b1-b3) BCZT-0.25Fe ceramics; (c1-c3) BCZT-0.25Nb ceramics; (d1-d3) BCZT-0.25Bi ceramics. The distribution of domain sizes at 50 °C for (a4) the undoped BCZT ceramics, (b4) BCZT-0.25Fe ceramics, (c4) BCZT-0.25Nb ceramics and (d4) BCZT-0.25Bi ceramics respectively. Furthermore, the undoped BCZT ceramics at 50 °C are much larger than the doped specimens in the average domain size (see Fig. 5(b4)-(d4)). In addition, the average domain size (ranged from 50 nm to 80 nm) of BCZT-0.25Nb ceramics is the 11
smallest when compared with that of BCZT-0.25Fe ceramics (ranged from 60 nm to 130 nm) and BCZT-0.25Bi ceramics (ranged from 80 nm to 180 nm). This is possibly because for Nb5+-doped samples, higher temperature region (below TC) is relatively closer to T-O phase boundary [39]. The smallest domains at the higher temperature can also flatten the free energy profile between different oriented domains and significantly decrease the activation energy of domain wall motion [40]. So the persisting miniaturized domains is considered as the main origin for the thermal stability of large piezoelectric coefficient in Nb5+ doped case at the high temperature [10]. And for Nb5+-doped specimens, due to the temperature of T-O phase transition (close to room temperature), there is a low polarization anisotropy between <001>T and <011>O around the T-O phase transition, starting from a tricritical point, which causes a low energy barrier of polarization rotation from orthorhombic (PO) to tetragonal (PT) state [41]. So the piezoelectric coefficient is high at room temperature. But for BCZT-0.25Fe ceramics and BCZT-0.25Bi ceramics, the temperature of the T-O phase transition shifts to 15 °C and 0 °C (see Fig. 2(a2)(c2)) respectively, away from room temperature. Thus the energy barrier of polarization rotation rises at room temperature, resulting in the decrease of ambient piezoelectric coefficient (see Fig. 4(a)(c)) [42]. When the temperature increases to 50 °C, the miniaturized nanodomains of BCZT-0.25Fe ceramics can also reduce the domain wall energy and promote the domain wall motion. Therefore, the piezoelectric coefficient of BCZT-0.25Fe ceramics can exhibit a higher value than the undoped one but lower piezoelectric coefficient than Nb5+-doped specimens at the elevated temperature (see Fig. 4(d)). For BCZT-0.25Bi ceramics, due to the larger domain size and a much lower temperature of T-O phase transition, a lower piezoelectric coefficient is observed at the high temperature. 4. Conclusion In conclusion, Nb5+-doped BCZT ceramics show a combination of high piezoelectric coefficient and enhanced thermal stability. This thermally reliable high 12
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[28] W. Li, Z. Xu, R. Chu, P. Fu, G. Zang, M. Narayanan, Temperature Stability in Dy-Doped (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3 Lead-Free Ceramics with High Piezoelectric Coefficient, J. Am. Ceram. Soc., 94 (2011) 3181-3183. [29] M. Fang, Y. Ji, Z. Zhang, Y. Yang, C. Liu, D. Wang, L. Zhang, J. Gao, X. Ren, Re-entrant relaxor–ferroelectric composite showing exceptional electromechanical properties, NPG Asia Mater., 10 (2018) 1029-1036. [30] M. Fang, S. Rajput, Z. Dai, Y. Ji, Y. Hao, X. Ren, Understanding the mechanism of thermal-stable high-performance piezoelectricity, Acta Mater., 169 (2019) 155-161. [31] C. Zhou, X. Ke, Y. Yao, S. Yang, Y. Ji, W. Liu, Y. Yang, L. Zhang, Y. Hao, S. Ren, L. Zhang, X. Ren, Evolution from successive phase transitions to “morphotropic phase boundary” in BaTiO3-based ferroelectrics, Appl. Phys. Lett., 112 (2018). [32] L. Zhang, M. Zhang, L. Wang, C. Zhou, Z. Zhang, Y. Yao, L. Zhang, D. Xue, X. Lou, X. Ren, Phase transitions and the piezoelectricity around morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 lead-free solid solution, Appl. Phys. Lett., 105 (2014) 162908. [33] R. K. Zheng, J. Wang, X. G. Tang, Y. Wang, H. L. W. Chan, C. L. Choy, X.G. Li, Effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba0.4Sr0.6-xCaxTiO3 (0⩽x⩽0.3) perovskites, J Appl. Phys., 98 (2005) 084108. [34] A. L. Allred, Electronegativity values from thermochemical data, J. Inorg. Nucl. Chem., 17 (1961) 215-221. [35] Z. Liu, A. R. Paterson, H. Wu, P. Gao, W. Ren, Z. G. Ye, Synthesis, structure and piezo-/ferroelectric properties of a novel bismuth-containing ternary complex perovskite solid solution, J. Mater. Chem. C, 5 (2017) 3916-3923. [36] X. Huang, H. Hao, S. Zhang, H. Liu, W. Zhang, Q. Xu, M. Cao, Structure and Dielectric Properties of BaTiO3-BiYO3 Perovskite Solid Solutions, J. Am. Ceram. Soc., 97 (2014) 1797-1801. [37] V. V. Shvartsman, D. C. Lupascu, Lead-Free Relaxor Ferroelectrics, J. Am. 16
Ceram. Soc., 95 (2012) 1-26. [38] L. Jin, F. Li, S. Zhang, D.J. Green, Decoding the Fingerprint of Ferroelectric Loops: Comprehension of the Material Properties and Structures, J. Am. Ceram. Soc., 97 (2014) 1-27. [39] J. Gao, D. Xue, Y. Wang, D. Wang, L. Zhang, H. Wu, S. Guo, H. Bao, C. Zhou, W. Liu, S. Hou, G. Xiao, X. Ren, Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics, Appl. Phys. Lett., 99 (2011) 092901. [40] J. Gao, X. Hu, L. Zhang, F. Li, L. Zhang, Y. Wang, Y. Hao, L. Zhong, X. Ren, Major
contributor
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17
Figure captions Fig. 1. (a) XRD patterns of doped and undoped BCZT samples at room temperature. The (002) diffraction peak splitting of (b) BCZT-0.25Fe at 15 °C, 50 °C and 80 °C; (c) BCZT-0.25Nb at 20 °C, 50 °C and 80 °C; (d) BCZT-0.25Bi at 0 °C, 50 °C and 80 °C. Fig. 2. Temperature dependence of (a1)-(c1) dielectric constants (ε) and (a2)-(c2) dielectric loss (tanδ) of doped and undoped BCZT ceramics at 100 Hz. (a3)-(c3) The corresponding phase diagrams of BCZT-xFe, BCZT-xNb, and BCZT-xBi ceramics. Fig. 3. P-E Hysteresis loops of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics, in comparison with P-E loops of undoped BCZT ceramics. (d) Comparison of maximum polarization Pm, remnant polarization Pr and coercive field EC among all specimens at room temperature. Fig. 4. Temperature dependent of piezoelectric coefficient of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics. (d) Comparison of piezoelectric coefficient at 50 °C of each specimen. Fig. 5. In-situ TEM observations of domain evolution under bright field (BF) mode. The bright field image of (a1-a3) the undoped BCZT ceramics; (b1-b3) BCZT-0.25Fe ceramics; (c1-c3) BCZT-0.25Nb ceramics; (d1-d3) BCZT-0.25Bi ceramics. The distribution of domain sizes at 50 °C for (a4) the undoped BCZT ceramics, (b4) BCZT-0.25Fe ceramics, (c4) BCZT-0.25Nb ceramics and (d4) BCZT-0.25Bi ceramics respectively.
18
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:
S0272-8842(19)32877-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.028
Reference:
CERI 23096
To appear in:
Ceramics International
Received Date: 19 July 2019 Revised Date:
2 October 2019
Accepted Date: 3 October 2019
Please cite this article as: L. He, Y. Ji, S. Ren, L. Zhao, H. Luo, C. Liu, Y. Hao, L. Zhang, L. 5+ Zhang, X. Ren, Large piezoelectric coefficient with enhanced thermal stability in Nb -doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.10.028. 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.
Large piezoelectric coefficient with enhanced thermal stability in Nb5+-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics Liqiang Hea, Yuanchao Ji,a,* Shuai Rena, Luo Zhaoa, Hanyu Luoa, Chang Liua, Yanshuang Haoa, Le Zhang,a,c* Lixue Zhang,a,* Xiaobing Rena,b a
Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, China b
Center for Functional Materials, National Institute for Materials Science, Tsukuba, 305-0047,
Ibaraki, Japan c
School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW
2052, Australia
Abstract Chemical doping is an indispensable tool to tailor the properties of the commercial piezoelectric materials. However, a high piezoelectric coefficient with enhanced thermal stability is rarely achieved by one dopant in some high-performance ferroelectrics, e.g., the recently discovered eco-friendly (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics. In order to optimize the piezoelectric property in BCZT system by a simple way, we investigated the doping effect of Fe3+, Nb5+ and Bi3+ cations in BCZT ceramics respectively. The results indicate that only Nb5+-doped BCZT ceramics display a combination of large piezoelectric coefficient and enhanced thermal stability, compared with others. Moreover, the established phase diagrams and in-situ transmission electron microscope (TEM) observations reveal that such optimized piezoelectric properties after Nb5+ doping originates from (i) the low polarization anisotropy near the ambient tetragonal (T)-orthorhombic (O) phase transition and (ii) the easy domain wall motion of persistent miniaturized ferroelectric domains upon heating. *
Corresponding author:
E-mail addresses: [email protected] (Y. Ji)
[email protected] (L. Zhang)
[email protected] (L. Zhang) 1
Keywords: Piezoelectric ceramics; Thermal stability; Phase transition; Nanodomains; Domain wall motion
2
1. Introduction Piezoelectric materials have been widely used in sensors, actuators and transducers [1, 2]. For more than half a century, the huge market has been dominated by the lead zirconate titanate (PZT) families. In order to fulfill various application requirements, different dopants have been incorporated into PZT systems to modulate the ferroelectric and piezoelectric properties [2]. Unfortunately, Lead is now facing a worldwide legislative restriction due to its toxicity concern, which has triggered intense research interests for lead-free systems [3, 4]. In the past decade, a large piezoelectric coefficient of d33~500-620 pC/N, comparable with that of the commercial soft-PZT [5], has been achieved in lead-free Ba(X, Ti)O3-(Ba, Ca)TiO3 (X=Zr, Sn, Hf) ceramics [6-8] and complex doped (K,,Na)NbO3 ceramics [9, 10]. It is reported that the high piezoelectric coefficient in (Ba, Ca)(Zr, Ti)O3 system occurs in the proximity of the tetragonal (T)-orthorhombic (O) phase transition region due to the easy polarization extension and rotation [11]. However, (Ba, Ca)(Zr, Ti)O3 system shows a poor piezoelectric thermal stability upon heating [8, 12], which deeply restricts the correlated device designing and manufacturing. In order to optimize piezoelectric properties, the compound doping or co-doping methods have been introduced into the lead-free (Ba, Ca)(Zr, Ti)O3 solid solutions [1]. For example, Chao et al. introduced a tungsten bronze structure compound, i.e., Ca0.28Ba0.72Nb2O6, into (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) lattice, which can result in a higher piezoelectric coefficient of d33 with enhanced thermal stability [13]. The complex compound doping can also be utilized to modify piezoelectric properties in other ferroelectric systems, including (1-x)Bi0.5Na0.5TiO3-x(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 [14],
(1-x)(K0.48Na0.52)NbO3-(x/5.15)K2.9Li1.95Nb5.15O15.3
[15]
and
(1-x-y)K1-wNawNb1-zSbzO3-yBaZrO3-xBi0.5K0.5HfO3 [9]. Besides, Ding, et al. reported that Sn4+ and Sr2+ co-doped (Ba0.84Ca0.15Sr0.01)(Ti0.9Zr0.09Sn0.01)O3 system displayed the thermal reliable piezoelectric coefficient [16].
3
However, the thermally reliable high piezoelectric coefficient is rarely achieved through doping single aliovalent or isovalent cation in both lead and lead-free ferroelectrics. In PZT ceramics, acceptor cations, e.g., Fe3+ replacing Ti4+/Zr4+ [17, 18], can result in an increase of the thermal stability of piezoelectric coefficient but with a decrease of the piezoelectric coefficient [19], while donor cations, e.g., Bi3+ replacing Pb2+ [20] or Nb5+ replacing Ti4+/Zr4+, can lead to an increase of piezoelectric coefficient but with a decrease of the thermal stability [21]. Besides, among recent studies in lead-free systems [22-24], aliovalent cation, such as Tb3+ [25] or Mn3+ [26] was used to enhance the thermal stability of piezoelectric coefficient in (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3
and
(Ba0.838Ca0.162)(Ti0.908Zr0.092)O3
compositions
respectively, but the piezoelectric coefficient of d33 is only about 410 pC/N from room temperature to 80 °C, lower than the best value. Furthermore, by adjusting isovalent Zr4+ or Ca2+ contents in (Ba, Ca)(Zr, Ti)O3 materials [27], the thermal stability of piezoelectric coefficient can also be enhanced to some extent, but the piezoelectric coefficient is sharply reduced to 250 pC/N. Most single dopants either develop a single phase region by removing the ferroelectric-ferroelectric phase boundary [28] or form a diffuse phase transition in a wide temperature range [29] to improve the thermal stability of piezoelectricity, but the high energy barrier between the polarization variants in the single phase and the low polarization response in the diffuse-phase-transition phase would reduce the magnitude of piezoelectric coefficient significantly. In
the
present
study,
we
shows
that
Fe3+-doped
and
Bi3+-doped
(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics display enhanced thermal stability of piezoelectric coefficient but lower piezoelectric coefficients, whereas Nb5+-doped BCZT ceramics exhibit the thermally reliable high piezoelectric coefficient. The phase diagram analysis and TEM results reveal that this is mainly attributed to two reasons, (i) Nb5+ doping keeps the temperature of tetragonal (T)-orthorhombic (O) phase transition unchanged (near room temperature), which will result in large 4
piezoelectric coefficient at the ambient temperature and (ii) the minimized domain pattern with the low polarization anisotropy persists upon heating, leading to enhanced thermal stability of piezoelectric coefficient. The dissimilar effect of the selected dopants on the phase transition behavior and domain sizes was also briefly discussed. 2. Experimental details 2.1. Material fabrication Fe3+-doped, Nb5+-doped and Bi3+-doped BCZT ceramics were fabricated by the conventional solid-state reaction method with starting chemicals of BaCO3 (99.8 wt%, Alfa Aesar), BaZrO3 (99 wt%, Alfa Aesar), TiO2 (99.9 wt%, Alfa Aesar), CaCO3 (99.9 wt%, Alfa Aesar), Fe2O3 (99.9 wt%, Alfa Aesar), Nb2O5 (99.9 wt%, Alfa Aesar) and Bi2O3 (99.9 wt%, Alfa Aesar) (the Alfa Aesar reports in Fig. S5 and Fig. S6). Their chemical formulas are (Ba0.85Ca0.15)((Zr0.1Ti0.9)1-x/100Fex/100)O3-δ (BCZT-xFe), (Ba0.85Ca0.15)((Zr0.1Ti0.9)1-1.25x/100Nbx/100)O3
(BCZT-xNb)
and
((Ba0.85Ca0.15)1-1.5x/100Bix/100)(Zr0.1Ti0.9)O3 (BCZT-xBi), where x is the molar percent of each dopant. The powder chemicals were ball-milled using the alcohol medium for 8 h in a nylon pot with zirconium dioxide balls inside (the wear rate of zirconium dioxide balls is 0.03 ppm/h shown in Fig. S7). Then the mixed powders were calcined at 1300 °C for 3 h in air. After that the calcined products were ground into powders and ball-milled again. And the calcined powders were shaped into pellets after adding some binder (5 wt% Polyvinyl Alcohol (PVA) aqueous solution). Finally the ceramic discs were fabricated by sintering at 1450 °C for 6 h. 2.2. Characterization The surface morphology images of the samples were obtained by field emission scanning electron microscope (Hitachi SU6600). X-ray diffraction (XRD) patterns were collected by Shimadzu XRD7000 under a current of 30 mA and the voltage of
5
40 kV with CuKα radiation (λ=1.5406 Å). The permittivity versus temperature curves were measured by a HIOKI LCR meter from -100 °C to 150 °C with a heating and cooling rate of 2 °C/min. The LCR frequencies are 100 Hz, 1 kHz, 10 kHz, and 100 kHz, and the oscillation voltage is 2.5 V in dielectric measurement. The polarization-electric field (P-E) loops of samples were tested by a Premier Ⅱ ferroelectric test system at 10 Hz and 30 kV/cm. The cylinder specimens with silver paste were poled under a DC-field of 10 kV/cm for 40 minutes at room temperature and then the temperature dependence of the piezoelectric coefficient d33 for all specimens were measured by a piezoelectric d33-meter (Model ZJ-3A, Chinese Academy of Sciences) with a self-assembled temperature chamber [30]. The local microstructure evolution and corresponding diffraction pattern were acquired by JEOL-2100F high resolution transmission electron microscope (TEM) equipped with double tilt heating specimen stages (from Gatan). The Digital Micrograph software was used to analyze the domain images observed in TEM. 3. Results and discussion The XRD results in Fig. 1(a) show that all BCZT specimens manifest the pure perovskite structure without any secondary phases or impurities, indicating that all dopants have completely diffused into the crystal lattice. The SEM morphologies in Fig. S1 further confirm the homogeneity of each specimen. As shown in Fig. 1(b)-(d), the splitting of (002) peaks of Fe3+-doped, Nb5+-doped and Bi3+-doped BCZT samples almost disappears at 80 °C as close to the Curie temperature. When the temperature cools down to 50 °C, the (002) peak splitting indicates the existence of tetragonal symmetry [31]. Table S1 and Fig. S2 show that the presence of Fe3+, Nb5+ and Bi3+ cations induces a slight fluctuation in the lattice parameters of the tetragonal phase. The c/a ratios of BCZT-0.25Fe (abbreviated as 0.25Fe), BCZT-0.25Nb (abbreviated as 0.25Nb) and BCZT-0.25Bi (abbreviated as 0.25Bi) are 1.0031, 1.0039 and 1.0044 respectively, by fitting the (002) peak splitting at 50 °C (shown in Fig. 1 (b)-(d)). 6
However, when the temperature approaches to the T-O phase boundary of each dopant composition (confirmed by the anomalies in temperature dependent dielectric constants in Fig. 2) upon further cooling, a splitting of (002) peak of tetragonal structure becomes unconspicuous due to the low polarization anisotropy at T - O phase boundary [32].
Fig. 1. (a) XRD patterns of doped and undoped BCZT samples at room temperature. The (002) diffraction peak splitting of (b) BCZT-0.25Fe at 15 °C, 50 °C and 80 °C; (c) BCZT-0.25Nb at 20 °C, 50 °C and 80 °C; (d) BCZT-0.25Bi at 0 °C, 50 °C and 80 °C. TC, TT-O and TO-R corresponding to the Curie temperature and the temperatures of T-O and O-R phase transitions, are obtained from temperature-dependent dielectric curves (Fig. 2(a1-c1)) and loss behaviors (Fig. 2(a2-c2)). The related phase diagrams are established in Fig. 2(a3-c3). After doping Fe3+ cations (see Fig. 2(a3)), TC and TT-O in BCZT ceramics both drop down, while TC and TT-O of Nb5+-doped ceramics remain nearly unchanged (see Fig. 2(b3)). Different from the Nb5+-doped and Fe3+-doped cases, Bi3+-doped ceramics perform the decrease of TT-O while maintaining TC, as 7
shown in Fig. 2(c3). This dissimilar effect on the phase transitions for each dopant mainly derives from the dopant size effect and electronegativity features [33-36]. Due to the close ionic radius and electronegativity of Nb5+ cations to B-site of BCZT matrix [34], the changes in crystal lattice parameters and chemical bond of BCZT ceramics are not so much. In this respect, the temperature of phase transition is almost unchanged in Nb5+-doped BCZT ceramics. But for the Fe3+ and Bi3+ doped samples, the degree of strain heterogeneity and electronegativity difference are larger [34]. Besides, Bi3+ ions have 6s2 lone-pair electrons hybridized with the surrounding oxygen anions, which has the potential to manipulate the ferroelectricity [35, 36]. It is necessary to make a further deep research on the piezoelectric or ferroelectric properties of three doped BCZT ceramics.
Fig. 2. Temperature dependence of (a1)-(c1) dielectric constants (ε) and (a2)-(c2) dielectric loss (tanδ) of doped and undoped BCZT ceramics at 100 Hz. (a3)-(c3) The corresponding phase diagrams of BCZT-xFe, BCZT-xNb, and BCZT-xBi ceramics. Fig. 3(a)-(c) show ambient polarization-electric field (P-E) loops of the Fe3+-doped BCZT, Nb5+-doped BCZT and Bi3+-doped BCZT ceramics respectively, which are also compared with the P-E loop of the undoped BCZT ceramics. As summarized in Fig. 3(d), Fe3+ dopants will slightly lessen the maximum polarization 8
Pm and the remnant polarization Pr at room temperature. In contrast, the Pm and the Pr of Nb5+-doped and Bi3+-doped BCZT ceramics increase when the molar percent of dopants locates at 0.25 and then decreases at the molar percent ~0.5. When the dopant concentration is 0.75%, an obvious frequency-dispersion dielectric permittivity and the further decreased ferroelectric properties (see Fig. S3). The explanation for above phenomena is that the increase of dopant concentration above 0.25% may introduce more compositional disorders, local stress field and free charges, which will lead to the transformation of normal ferroelectric state to relaxor ferroelectric state, and possible high leakage [37]. Besides, Fig. 3(d) shows that doping Fe3+ cations or Nb5+ cations can decrease EC of BCZT, while doping Bi3+ causes the increase of EC. Therefore, due to the ambient high polarization and low coercive field (near T-O phase transition), the largest piezoelectric coefficient at room temperature prefers to exist in Nb5+-doped BCZT ceramics rather than Fe3+ or Bi3+-doped cases [38].
Fig. 3. P-E Hysteresis loops of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics, in comparison with P-E loops of undoped BCZT ceramics. (d) Comparison of maximum polarization Pm, remnant polarization Pr and coercive field EC among all specimens at room temperature. The temperature dependence of d33 values of each BCZT samples are shown in Fig. 4. The undoped BCZT ceramics just exhibit one piezoelectric coefficient peak of 9
d33 ~540 pC/N near T-O phase transition and then sharply decreases to around 290 pC/N when the temperature is close to TC [6]. But after doping Fe3+, Nb5+ and Bi3+ cations, the decrease of piezoelectric coefficient slows down upon heating, indicating an improved thermal stability which can be seen from the value of d33/d33
293 K
depicted in Fig. S4. Among three doped BCZT ceramics, the 0.25% Nb5+ doped sample is the only one rendering the large piezoelectric coefficient and thermal reliability.
Fig. 4. Temperature dependence of piezoelectric coefficient of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics. (d) Comparison of piezoelectric coefficient at 50 °C of each specimen. The
microstructure
evolutions
of
selected
BCZT-0.25Fe
ceramics,
BCZT-0.25Nb ceramics and BCZT-0.25Bi ceramics with heating are obtained by in-situ bright field (BF) TEM (see Fig. 5(b1-b3)(c1-c3)(d1-d3)). As a comparison, Fig. 5(a1) shows the domain structure of the undoped BCZT ceramics at room temperature. The grown lamella nanodomains inside the large domains [39] can be clearly seen in the inset of Fig. 5 (a1). Upon heating to 50 °C, the domain structure of undoped BCZT samples transforms to the pure tetragonal structure without coexisting 10
nanodomains, which results in the observed poor piezoelectric coefficient in Fig. 4. But for doped specimens, nanodomain structure patterns can keep stable in the domain morphology from 20 °C to 50 °C (see Fig. 5(b1)(b2), (c1)(c2), (d1)(d2)). When heating samples to 80 °C, the domains disappear due to near Curie temperature (see Fig. 5(b3)-(d3)). The in-situ TEM observation reveals that the enhanced thermal stability of piezoelectric coefficient for doped specimens is attributed to the optimized temperature dependent behavior of ferroelectric nanodomains (see Fig. 5).
Fig. 5. In-situ TEM observations of domain evolution under bright field (BF) mode. The bright field image of (a1-a3) the undoped BCZT ceramics; (b1-b3) BCZT-0.25Fe ceramics; (c1-c3) BCZT-0.25Nb ceramics; (d1-d3) BCZT-0.25Bi ceramics. The distribution of domain sizes at 50 °C for (a4) the undoped BCZT ceramics, (b4) BCZT-0.25Fe ceramics, (c4) BCZT-0.25Nb ceramics and (d4) BCZT-0.25Bi ceramics respectively. Furthermore, the undoped BCZT ceramics at 50 °C are much larger than the doped specimens in the average domain size (see Fig. 5(b4)-(d4)). In addition, the average domain size (ranged from 50 nm to 80 nm) of BCZT-0.25Nb ceramics is the 11
smallest when compared with that of BCZT-0.25Fe ceramics (ranged from 60 nm to 130 nm) and BCZT-0.25Bi ceramics (ranged from 80 nm to 180 nm). This is possibly because for Nb5+-doped samples, higher temperature region (below TC) is relatively closer to T-O phase boundary [39]. The smallest domains at the higher temperature can also flatten the free energy profile between different oriented domains and significantly decrease the activation energy of domain wall motion [40]. So the persisting miniaturized domains is considered as the main origin for the thermal stability of large piezoelectric coefficient in Nb5+ doped case at the high temperature [10]. And for Nb5+-doped specimens, due to the temperature of T-O phase transition (close to room temperature), there is a low polarization anisotropy between <001>T and <011>O around the T-O phase transition, starting from a tricritical point, which causes a low energy barrier of polarization rotation from orthorhombic (PO) to tetragonal (PT) state [41]. So the piezoelectric coefficient is high at room temperature. But for BCZT-0.25Fe ceramics and BCZT-0.25Bi ceramics, the temperature of the T-O phase transition shifts to 15 °C and 0 °C (see Fig. 2(a2)(c2)) respectively, away from room temperature. Thus the energy barrier of polarization rotation rises at room temperature, resulting in the decrease of ambient piezoelectric coefficient (see Fig. 4(a)(c)) [42]. When the temperature increases to 50 °C, the miniaturized nanodomains of BCZT-0.25Fe ceramics can also reduce the domain wall energy and promote the domain wall motion. Therefore, the piezoelectric coefficient of BCZT-0.25Fe ceramics can exhibit a higher value than the undoped one but lower piezoelectric coefficient than Nb5+-doped specimens at the elevated temperature (see Fig. 4(d)). For BCZT-0.25Bi ceramics, due to the larger domain size and a much lower temperature of T-O phase transition, a lower piezoelectric coefficient is observed at the high temperature. 4. Conclusion In conclusion, Nb5+-doped BCZT ceramics show a combination of high piezoelectric coefficient and enhanced thermal stability. This thermally reliable high 12
piezoelectric coefficient is considered being benefited from remained low energy barriers of the polarization rotation and the easy domain wall motion of ferroelectric nanodomains from room temperature to the high temperature. Our result may provide a routine to obtain large piezoelectric coefficient with enhanced thermal stability by doping cations in lead-free piezoelectric ceramics. Acknowledgement The present work was financially supported by the National Natural Science Foundation of China (51701150, 51431007, 51621063, 51831006 and 51772242), Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R85), China Postdoctoral Science Foundation (2017M610637), and 111 Project 2.0 (BP2018008). References [1] B. Jaffe, Piezoelectric Ceramics, J. P. Roberts, New York, 1971. [2] G. H. Haertling, Ferroelectric Ceramics History and Technology, J. Am. Ceram. Soc., 82 (1999). [3] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Lead-free piezoelectrics, Nature, 432 (2004) 84-87. [4] E. cross, Materials science: lead-free at last, Nature, 432 (2004) 24-25. [5] T. R. Shrout, S. J. Zhang, Lead-free piezoelectric ceramics: Alternatives for PZT?, J. Electroceram., 19 (2007) 113-126. [6] W. Liu, X. Ren, Large piezoelectric effect in Pb-free ceramics, Phys. Rev. Lett., 103 (2009) 257602. [7] Y. Yao, C. Zhou, D. Lv, D. Wang, H. Wu, Y. Yang, X. Ren, Large piezoelectricity and dielectric permittivity in BaTiO3-xBaSnO3system: The role of phase coexisting, EPL-Europhys Lett, 98 (2012) 27008. [8] M. Acosta, N. Novak, V. Rojas, S. Patel, R. Vaish, J. Koruza, G.A. Rossetti, J. Rödel, BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives, 13
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Figure captions Fig. 1. (a) XRD patterns of doped and undoped BCZT samples at room temperature. The (002) diffraction peak splitting of (b) BCZT-0.25Fe at 15 °C, 50 °C and 80 °C; (c) BCZT-0.25Nb at 20 °C, 50 °C and 80 °C; (d) BCZT-0.25Bi at 0 °C, 50 °C and 80 °C. Fig. 2. Temperature dependence of (a1)-(c1) dielectric constants (ε) and (a2)-(c2) dielectric loss (tanδ) of doped and undoped BCZT ceramics at 100 Hz. (a3)-(c3) The corresponding phase diagrams of BCZT-xFe, BCZT-xNb, and BCZT-xBi ceramics. Fig. 3. P-E Hysteresis loops of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics, in comparison with P-E loops of undoped BCZT ceramics. (d) Comparison of maximum polarization Pm, remnant polarization Pr and coercive field EC among all specimens at room temperature. Fig. 4. Temperature dependent of piezoelectric coefficient of (a) Fe3+-doped, (b) Nb5+-doped, and (c) Bi3+-doped BCZT ceramics. (d) Comparison of piezoelectric coefficient at 50 °C of each specimen. Fig. 5. In-situ TEM observations of domain evolution under bright field (BF) mode. The bright field image of (a1-a3) the undoped BCZT ceramics; (b1-b3) BCZT-0.25Fe ceramics; (c1-c3) BCZT-0.25Nb ceramics; (d1-d3) BCZT-0.25Bi ceramics. The distribution of domain sizes at 50 °C for (a4) the undoped BCZT ceramics, (b4) BCZT-0.25Fe ceramics, (c4) BCZT-0.25Nb ceramics and (d4) BCZT-0.25Bi ceramics respectively.
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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: