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Excellent temperature stability with giant electrostrain in Bi0.5Na0.5TiO3-based ceramics
Scripta Materialia 179 (2020) 70–74
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Excellent temperature stability with giant electrostrain in Bi0.5 Na0.5 TiO3 -based ceramics Yichen Wu a,c, Genshui Wang a,b,d,∗, Zheng Jiao c, Xianlin Dong a,c,d a
Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China c School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China d State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 6 December 2019 Accepted 17 December 2019
Keywords: BNT-based Electrostrain Thermally stability
a b s t r a c t In this work, we report a giant strain (0.42%) in lead-free ceramics of (1–x) (0.79Bi0.5 Na0.5 TiO3 –0.20 Bi0.5 K0.5 TiO3 –0.01NaNbO3 )–xSrTiO3 with x=0.03. Most importantly, ultra-high temperature stability was simultaneously achieved under this giant strain with the variation less than 10% in the range of 20 °C and 140 °C. Systematic temperature dependent Raman spectra measurements and Ginzburg–Landau– Devonshire thermodynamic theory analysis revealed that the intrinsic lattice strain and electric field induced relaxor-ferroelectric phase transition provide reverse strain response with the increase of temperature, jointly conducive to the ultra-high temperature stability of strain property. These materials are extremely competitive in practical application of actuators. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Piezoelectric materials have been an active research topic because of their exceptional capability to interconvert mechanical energy and electrical energy [1–3]. As an indispensable part in smart piezoelectric actuators, they have been put into wide applications in ultrasonic motors, cellular phone terminals, etc. [4,5]. For widely studied materials, the strain level induced by external electric field is a pivotal indicator in the performance of piezoelectric actuators. In practical applications, these investigated ceramics are often exposed into extremely environment [6,7]. However, their strain properties show dramatic attenuation at high temperature and exhibit poor thermal stability. Therefore, fabricating novel piezoelectric ceramics with both large electrostrain and favorable thermal stability are basically demanded for actuator applications. The traditional lead-based {Pb(Zr,Ti)O3 , PZT} piezoelectric ceramics with market-dominating status have an edge on large strain value by means of constructing morphotropic phase boundary (MPB) region [8,9]. The crystal structure is extremely active in the MPB region, conducive to enhanced piezoelectric properties [2]. Furthermore, the MPB region in PZT-based ceramics depends only on the composition rather than temperature, which accounts for their outstanding thermal stability of strain [10]. Nevertheless, due
∗ Corresponding author at: Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 20 0 050, China. E-mail address: [email protected] (G. Wang).
https://doi.org/10.1016/j.scriptamat.2019.12.022 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
to the toxicity of Pb element, searching novel lead-free ceramics with excellent comprehensive properties is an open question [11–14]. As a representative for lead-free candidates of piezoelectric materials, potassium sodium niobium-based {(K0.5 Na0.5 )NbO3 , KNN} ceramics are prominent in large electromechanical response through constructing the polymorphic phase transition (PPT) region toward room temperature [6,15,16]. Unfortunately, this special multiphase coexistence region is highly dependent on the temperature due to the tilted PPT boundary, resulting in temperature sensitivity of strain properties. Consequently, exploring lead-free piezoelectric ceramics with simultaneously large strain properties as well as good temperature stability is an urgent issue in this commercial society. In recent years, BNT-based ceramics have attracted considerable research interests due to the exceptionally giant recoverable strain achieved through chemical modifications [1,2,17–19]. For instance, Lou et al. developed the currently largest electrostrain (~0.72%) in (1–x)(0.8Bi0.5 Na0.5 TiO3 -0.2Bi0.5 K0.5 TiO3 )xSr0.8 Bi0.1 00.1 Ti0.8 Zr0.2 O2.95 ceramics, which is the largest strain value in BNT-based ceramics so far [20]. It is worth noting that this ultrahigh strain behavior is benefited from the combined contribution of the morphotropic relaxor boundary (MRB) which has different local symmetries but the same macroscopic symmetry and the relaxor ferroelectrics (RFEs) to ferroelectrics (FEs) phase transition. On the one hand, the MRB in BNT-based ceramics serves as an intermediate medium to provide a low energy
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Fig. 1. (a) The XRD patterns of ST modified BNKTN ceramics and the inset of (a) exhibit the enlarged diffraction peaks around 40° and 45°. (b) and (c) The selected area electron diffraction (SEAD) patterns with [110] and [111] zones axis for x=0.03 ceramics. (d) The temperature- dependent dielectric constant (ε r ) and loss tangent (tanδ ) for the poled ST modified BNKTN ceramics samples in the range of RT and 400 °C and (e) corresponding diffuseness degrees as a function of ST content. (f) The PFM image for x=0.03 ceramics.
pathway for polarization extension and rotation [21,22]. On the other hand, the large volume difference between RFE and FE phase contributes to the ultrahigh strain performance [5]. However, these large strain cannot maintain in a wide temperature range because of the unstable phase ratio between FEs and RFEs [23]. Very recently, a number of works have focused on balancing the temperature stability and strain value of BNT- based ceramics [24]. However, it is inevitable to sacrifice the strain level for improving the thermal stable properties. For instance, Zhai et al. investigated the remarkably thermal-stable electrostrictive strain behavior in BaTiO3 nanowire modified BNT ceramics with the strain variation less than 7% from 27 °C to 120 °C; [23] Bai et al. designed a novel (Bi0.5 Na0.5 )TiO3 –(Bi0.5 K0.5 )TiO3 -Ba(Zr0.05 Ti0.95 )O3 ternary solid solution with the strain fluctuation value of merely 6% in the range of 25 °C and 120 °C [3]. Nevertheless, these thermally stable strain values are usually less than 0.2%, which cannot satisfy the increasing demands of actual application. Therefore, the systematical investigations on how the intrinsic and (or) extrinsic structure contribution affecting the strain and its temperature stability are necessary and are helpful to obtain ideal materials with both thermal-stable stain and large strain value. In this work, temperature-insensitive giant strain behavior was acquired in NaNbO3 (NN) and SrTiO3 (ST) co-modified Bi0.5 Na0.5 TiO3 –Bi0.5 K0.5 TiO3 (BNT-BKT) ceramics. Here, 0.8BNT– 0.2BKT component lying in the MPB was chosen as the host composition [25]. 1mol% NN composition was added to reduce the electric field needed to require giant strain. Different content of ST was introduced to form relaxor ferroelectrics. As a result, x=0.03 (abbreviated as 3ST) composition exhibits enhanced unipolar strain of 0.42% at 8 kV/mm. Most importantly, excellent temperature stability with the strain variation less than 10% is achieved from 20 to 140 °C under the giant strain value of 0.42%.
Systematic temperature dependent Raman spectra measurements and Ginzburg-Landau-Devonshire thermodynamic theory analysis illustrated that the field-induced intrinsic lattice contribution and electric field induced relaxor- ferroelectric phase transition provide inverse strain response with the increase of temperature in this composition. Thus the excellent thermal stability appears in this ceramics, demonstrating its high application potential in a wide temperature range. The experimental details can be found in our previous work [26]. Structural quantification of as-prepared ceramics was measured by the powder X-ray diffraction (XRD, Bruker AXS, Karlsruhe, ˚ radiation at room temperaGermany) using CuKα (λ = 1.5406 A) ture. Microstructure was observed by scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan). TEM characterization was performed using a JEM- 2100F electron microscope (JEOL, Japan). The polarization-electric hysteresis (P−E) loops and strain curves were measured using a ferroelectric measurement system (aixACCT TF Analyzer 20 0 0, Aachen, Germany). Raman spectrum was obtained using a micro-Raman system (LaRAM HR 800, Horiba Jobin Yvon, Paris, France). Fig. 1(a) shows the room-temperature XRD patterns of the unpoled xST ceramics. All the samples exhibit pure polycrystalline perovskite structure without any secondary phases, indicating that the ST has been completely diffused into the host lattice. No splitting of the (111) and (200) diffraction peaks illustrates a highly symmetric pseudocubic phase for all compositions. The substitution of ST shifts the peaks position to lower angles (inset of ˚ successfully Fig. 1(a)), ascribed to the large radius Sr2+ ions (1.45A) ˚ and Bi3+ ions (1.36 A) ˚ replacing the small radius Na+ ions (1.39 A) at A- sites. Despite the obvious lack of long-range noncubic distortion, it is reasonable to presume that the local scale distortion exits in the high symmetry phase, which fails to be detected by the XRD
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Fig. 2. (a) Composition dependent bipolar P−E loops, (b) bipolar S-E curves and (c) unipolar S-E curves for ST modified BNKTN ceramics. (d) The temperature dependent P−E loops and (e) unipolar S-E curves for x=0.03 ceramics. (f) The Hysteresis and remnant strain value of x=0.03 ceramics as a function of temperature.
for the low detection accuracy [27]. Thus all the XRD patterns of xST ceramics exhibit the same cubic symmetry. The selected area electron diffraction (SAED) patterns of 3ST composition were measured to probe the local phase structure, as shown in Fig. 1(b) and (c). It can be clearly observed that the 1/2(ooo) and 1/2(ooe) type (o and e stand for odd and even Miller indices, respectively) superlattice diffraction spots highlighted by circles appear in the [110] and [111] zone axis [28], respectively, indicating that the 3ST composition is at the MRB region with the different local symmetries of rhombohedral (R3c) and tetragonal (P4bm) phases. The temperature dependence of dielectric constant (ε r) and loss tangent (tanδ ) for the poled xST samples were measured from 25 °C to 400 °C at 1 kHz, as exhibited in Fig. 1(d). All the samples possess the diffuse phase transition (DPT) feature typical of a relaxor ferroelectric [29]. Here, the TF−R denote the phase transition temperature of FEs to RFEs, coupled with significant dielectric loss peak at this temperature. As shown in Fig. 1(d), the TF−R decreases sharply with the increase of ST content and eventually disappears from the ε −T spectrum at x=0.03. The phenomenon indicates that the 3ST ceramics have evolved into RFEs at room temperature [30]. The diffuseness degrees (γ ) of different components are characterized to assess the relaxor behavior based on the modified Curie– Weiss relationship
1
εr
−
1
εm
=
(T − Tm )γ C
,
where C denotes the Curie constant and γ varies between 1 (for the typical FEs) and 2 (for the complete RFEs) [31]. All the fitted results are presented in Fig. 1(e). The calculated γ value increases from 1.86 to 1.96 as ST content increases from 0 to 5%. The enhanced relaxor behavior mainly origins from the improved local random electric/elastic fields through the modification by ST. Furthermore, the PFM phase image of 3ST ceramics manifest no obvious discrepancy, demonstrating its weak ferroelectricity and extremely strong relaxor characteristic, [32] as exhibited in Fig. 1(f). Fig. 2(a)–(c) displays the composition dependence of the P−E loops, bipolar and unipolar S-E curves under 8 kV/mm, respectively. The P−E loops change from the typical saturated shape to the pinched ones as the ST content increases, indicating that the
long-range ferroelectric order of the host lattice is disrupted and the phase structure transforms into an ergodic relaxor (ER) with the ST modifying, consistent with the dielectric curves and PFM results. Actually, the ergodic relaxor ferroelectrics originate from the local random electric field and elastic field due to the incorporation of non- isovalent ions into the host BNT-based lattice. The formation of the ER phase can boost relatively high strain behavior, as shown in Fig. 2(b) [1,4]. The strain value raises from 0.1% to 0.42% on increasing ST content from 0 to 3%, beyond which the strain exhibits descending trend. Here, the variation of strain of different components is related to the change of PNRs’ size [33], which shows the opposite trend to the incorporation of ST content. Normally, when the ceramics are exposed into electric field, the PNRs with smaller size would transform into macroscale ferroelectric domains more easily due to the flattened energy barriers [34]. Thus the strain value exhibits increasing trend on increasing of ST content until the x=0.03. Nevertheless, when the size of PNRs is less than the critical value, the small-sized PNRs will nucleate firstly and then grow into long-growth ferroelectric order under the electric field with the large nucleation driving force, resulting in declined strain value [33]. Therefore, the maximum unipolar strain of 0.42% was obtained at x=0.03, as exhibited in Fig. 2(c). The temperature dependent P−E loops and unipolar strain curves are depicted in Fig. 2(d)–(e) in the range of 20–140 °C. As shown in Fig. 2(d), the shape of P−E loops for 3ST ceramics does not exhibit significant changes at different temperature except for the reducing hysteresis. The maximum polarization (Pm ) and remnant polarization (Pr ) of 3ST ceramics reduce from 34.7 μC/cm2 and 18.7 μC/cm2 to 27.2 μC/cm2 and 2.4 μC/cm2 respectively in the range of 20–140 °C, indicating the strong reversible under high temperatures. As displayed in Fig. 2(e), one can see that the large unipolar strain value of 3ST ceramics exhibits excellent thermally stable behavior with the variation less than 10% in the range of 20–140 °C, both of which are superior than many other lead-free systems which will be compared and discussed below. In addition, the hysteresis and remnant strain value (Srem) reduce significantly with the increased temperature, as illustrated in Fig. 2(f), which may be related to the variation of microdomain structure of PNRs with the temperature changes [35].
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Fig. 3. (a) The temperature depdend in situ Raman spectra for x=0.03 ceramics. (b) Schematic field- induced lattice strain as a function of temperature. (c) Schematic Landau potential curves for x=0.03 ceramics under different temperature.
Normally, there exits four different mechanisms of strain in ferroelectrics. (1) the converse piezoelectric effects; (2) the non180°domain switching and rotation; (3) the lattice distortion induced by electric field and (4) the field-induced phase transition [36]. It is worth noting that the piezoelectric effect and non-180° domain switching in 3ST ceramics have no significant contribution to the strain property because of its strong relaxor characteristics which is demonstrated by the dielectric curves and PFM image. Thus the field-induced phase transition as well as the fieldinduced lattice distortion are the main sources of the strain value of the 3ST ceramics. Temperature dependent Raman spectra of the 3ST ceramics were measured to investigate the field-induced intrinsic lattice strain under different temperatures, where eight Raman spectral deconvolutions alongside were composed by a collection of Lorentzian curves, as exhibited in Fig. 3(a). Note that the Raman spectroscopy data was corrected according to the Bose–Einstein formula:
n(w ) + 1 = 1/[1 − exp(−hw/kBT ], where h and kB stand for Planck’s and Boltzmann’s constant, respectively [2]. The Raman spectra can be divided into three main regions: (1) the 10 0–20 0 cm–1 host modes related to the vibrations of the perovskite A-site cations; (2) the modes between 200 and 400 cm–1 associated with Ti-O vibrations; (3) the modes ≥ 400 cm–1 assigned to the vibrations of BO6 octahedra [2]. It can be obviously observed that the peaks at ~260 cm–1 and 526 cm–1 shift to lower wavenumber with the increase of temperature, the softening of which result in the decrease of the unit cell polarity [37]. It has been reported that the volume variation V/V between RFE and FE phase is proportional to the polarization. Therefore, the decrease of the unit cell polarity corresponds to the declined strain value, i.e. the intrinsic lattice strain has negative correlation with the temperature, as illustrated in Fig. 3(b). On the other hand, a simplified Ginzburg–Landau–Devonshire thermodynamic theory was employed to describe the phase transition under varied temperature, as illustrated in Fig. 3(c). Generally, the free energy G of the ferroelectric systems depends on
polarization P:
G(P ) = G0 + α /2P2 − β /4P4 + γ /6P6 − EP(α , β , γ > 0 ) Where G0 denotes the free energy of the RFE phase, E presents the external electric field, and α , β , γ are constant [33]. Here, G presents the energy difference between relaxor ferroelectrics and electric field induced ferroelectrics. When the system is exposed into a low temperature T1 , the free energy of RFE (O) is much smaller than that of FE (O’), revealing that only a small amount of RFE would be induced into FE under electric field. However, as the temperature rises to a higher temperature T2 , the free energy difference between RFE and FE (A and A’) decreases significantly. Accordingly, more nonpolar structure can be induced into polar structure under electric field, leading to enhanced strain level [38]. As the temperature rises to a higher level, the strain origin from phase transition will continue to raise until reaching the maximum at the critical temperature. Therefore, based on the analysis above, we propose that the thermal stability of the strain property is due to a combination of the decrease in the intrinsic lattice strain V/V and the increase in the contribution of electric field induced RFEFE phase transition to the strain in this BNT-based systems. To further evaluate the thermal stability of the strain in the 3ST component, we compared the temperature dependent strain characteristics of our ceramics with other promising piezoelectric strain ceramics reported in recent years as shown in Fig. 4 [39–44]. It can be found as shown in Fig. 4 that the 3ST ceramics exhibit outstanding temperature stability over a wide temperature range. Enhanced strain temperature stability without reducing strain values is a long- standing scientific problem. Here, our systems made great progress in balancing large strain values and excellent thermal stability in piezoelectric strain ceramics, demonstrating the wide application prospect. In summary, the comprehensive properties of the BNT-BKTNN-ST ceramics were investigated in detail. The phase structure changed from FEs into RFEs with the increase of ST content. The giant strain of 0.42% was obtained in the 3ST ceramics, attributed to the extremely relaxor behavior. In addition, in-situ temperature dependent S-E curves demonstrated the excellent temperature stable strain behaviors of the 3ST ceramics, which is attributed to the combined result of the decrease in the intrinsic lattice strain
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Fig. 4. A comparison of temperature dependence of strain property of BNT-based ceramics with other perovskite ceramics.
V/V and the increase in the contribution of electric field induced RFE- FE phase transition to the strain. Very few materials have been found with both large strain and superior temperature stability, thus the ultra-high temperature stability with giant strain value make 3ST ceramics a competitive material for practical application. Declaration of Competing Interest 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. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 11774366, 51831010) and Natural Science Foundation of Shanghai (Grant no. 18ZR1444900). References [1] A. Ayrikyan, O. Prach, N.H. Khansur, S. Keller, S. Yasui, M. Itoh, O. Sakata, K. Durst, K.G. We bber, Acta Mater. 148 (2018) 432–441. [2] X. Liu, S. Xue, F. Li, J. Ma, J. Zhai, B. Shen, F. Wang, X. Zhao, H. Yan, J. Mater. Chem. C 6 (4) (2018) 814–822. [3] W. Bai, D. Chen, Y. Huang, P. Zheng, J. Zhong, M. Ding, Y. Yuan, B. Shen, J. Zhai, Z. Ji, Ceram. Int. 42 (6) (2016) 7669–7680. [4] W. Bai, L. Li, W. Li, B. Shen, J. Zhai, H. Chen, J. Alloys Compd. 603 (2014) 149–157. [5] J. Hao, W. Li, J. Zhai, H. Chen, Mater. Sci. Eng. R 135 (2019) 1–57. [6] Q. Liu, Y. Zhang, L. Zhao, J. Gao, Z. Zhou, K. Wang, X. Zhang, L. Li, J.-F. Li, J. Mater. Chem. C 6 (39) (2018) 10618–10627.
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Excellent temperature stability with giant electrostrain in Bi0.5 Na0.5 TiO3 -based ceramics Yichen Wu a,c, Genshui Wang a,b,d,∗, Zheng Jiao c, Xianlin Dong a,c,d a
Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China c School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China d State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 6 December 2019 Accepted 17 December 2019
Keywords: BNT-based Electrostrain Thermally stability
a b s t r a c t In this work, we report a giant strain (0.42%) in lead-free ceramics of (1–x) (0.79Bi0.5 Na0.5 TiO3 –0.20 Bi0.5 K0.5 TiO3 –0.01NaNbO3 )–xSrTiO3 with x=0.03. Most importantly, ultra-high temperature stability was simultaneously achieved under this giant strain with the variation less than 10% in the range of 20 °C and 140 °C. Systematic temperature dependent Raman spectra measurements and Ginzburg–Landau– Devonshire thermodynamic theory analysis revealed that the intrinsic lattice strain and electric field induced relaxor-ferroelectric phase transition provide reverse strain response with the increase of temperature, jointly conducive to the ultra-high temperature stability of strain property. These materials are extremely competitive in practical application of actuators. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Piezoelectric materials have been an active research topic because of their exceptional capability to interconvert mechanical energy and electrical energy [1–3]. As an indispensable part in smart piezoelectric actuators, they have been put into wide applications in ultrasonic motors, cellular phone terminals, etc. [4,5]. For widely studied materials, the strain level induced by external electric field is a pivotal indicator in the performance of piezoelectric actuators. In practical applications, these investigated ceramics are often exposed into extremely environment [6,7]. However, their strain properties show dramatic attenuation at high temperature and exhibit poor thermal stability. Therefore, fabricating novel piezoelectric ceramics with both large electrostrain and favorable thermal stability are basically demanded for actuator applications. The traditional lead-based {Pb(Zr,Ti)O3 , PZT} piezoelectric ceramics with market-dominating status have an edge on large strain value by means of constructing morphotropic phase boundary (MPB) region [8,9]. The crystal structure is extremely active in the MPB region, conducive to enhanced piezoelectric properties [2]. Furthermore, the MPB region in PZT-based ceramics depends only on the composition rather than temperature, which accounts for their outstanding thermal stability of strain [10]. Nevertheless, due
∗ Corresponding author at: Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 20 0 050, China. E-mail address: [email protected] (G. Wang).
https://doi.org/10.1016/j.scriptamat.2019.12.022 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
to the toxicity of Pb element, searching novel lead-free ceramics with excellent comprehensive properties is an open question [11–14]. As a representative for lead-free candidates of piezoelectric materials, potassium sodium niobium-based {(K0.5 Na0.5 )NbO3 , KNN} ceramics are prominent in large electromechanical response through constructing the polymorphic phase transition (PPT) region toward room temperature [6,15,16]. Unfortunately, this special multiphase coexistence region is highly dependent on the temperature due to the tilted PPT boundary, resulting in temperature sensitivity of strain properties. Consequently, exploring lead-free piezoelectric ceramics with simultaneously large strain properties as well as good temperature stability is an urgent issue in this commercial society. In recent years, BNT-based ceramics have attracted considerable research interests due to the exceptionally giant recoverable strain achieved through chemical modifications [1,2,17–19]. For instance, Lou et al. developed the currently largest electrostrain (~0.72%) in (1–x)(0.8Bi0.5 Na0.5 TiO3 -0.2Bi0.5 K0.5 TiO3 )xSr0.8 Bi0.1 00.1 Ti0.8 Zr0.2 O2.95 ceramics, which is the largest strain value in BNT-based ceramics so far [20]. It is worth noting that this ultrahigh strain behavior is benefited from the combined contribution of the morphotropic relaxor boundary (MRB) which has different local symmetries but the same macroscopic symmetry and the relaxor ferroelectrics (RFEs) to ferroelectrics (FEs) phase transition. On the one hand, the MRB in BNT-based ceramics serves as an intermediate medium to provide a low energy
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Fig. 1. (a) The XRD patterns of ST modified BNKTN ceramics and the inset of (a) exhibit the enlarged diffraction peaks around 40° and 45°. (b) and (c) The selected area electron diffraction (SEAD) patterns with [110] and [111] zones axis for x=0.03 ceramics. (d) The temperature- dependent dielectric constant (ε r ) and loss tangent (tanδ ) for the poled ST modified BNKTN ceramics samples in the range of RT and 400 °C and (e) corresponding diffuseness degrees as a function of ST content. (f) The PFM image for x=0.03 ceramics.
pathway for polarization extension and rotation [21,22]. On the other hand, the large volume difference between RFE and FE phase contributes to the ultrahigh strain performance [5]. However, these large strain cannot maintain in a wide temperature range because of the unstable phase ratio between FEs and RFEs [23]. Very recently, a number of works have focused on balancing the temperature stability and strain value of BNT- based ceramics [24]. However, it is inevitable to sacrifice the strain level for improving the thermal stable properties. For instance, Zhai et al. investigated the remarkably thermal-stable electrostrictive strain behavior in BaTiO3 nanowire modified BNT ceramics with the strain variation less than 7% from 27 °C to 120 °C; [23] Bai et al. designed a novel (Bi0.5 Na0.5 )TiO3 –(Bi0.5 K0.5 )TiO3 -Ba(Zr0.05 Ti0.95 )O3 ternary solid solution with the strain fluctuation value of merely 6% in the range of 25 °C and 120 °C [3]. Nevertheless, these thermally stable strain values are usually less than 0.2%, which cannot satisfy the increasing demands of actual application. Therefore, the systematical investigations on how the intrinsic and (or) extrinsic structure contribution affecting the strain and its temperature stability are necessary and are helpful to obtain ideal materials with both thermal-stable stain and large strain value. In this work, temperature-insensitive giant strain behavior was acquired in NaNbO3 (NN) and SrTiO3 (ST) co-modified Bi0.5 Na0.5 TiO3 –Bi0.5 K0.5 TiO3 (BNT-BKT) ceramics. Here, 0.8BNT– 0.2BKT component lying in the MPB was chosen as the host composition [25]. 1mol% NN composition was added to reduce the electric field needed to require giant strain. Different content of ST was introduced to form relaxor ferroelectrics. As a result, x=0.03 (abbreviated as 3ST) composition exhibits enhanced unipolar strain of 0.42% at 8 kV/mm. Most importantly, excellent temperature stability with the strain variation less than 10% is achieved from 20 to 140 °C under the giant strain value of 0.42%.
Systematic temperature dependent Raman spectra measurements and Ginzburg-Landau-Devonshire thermodynamic theory analysis illustrated that the field-induced intrinsic lattice contribution and electric field induced relaxor- ferroelectric phase transition provide inverse strain response with the increase of temperature in this composition. Thus the excellent thermal stability appears in this ceramics, demonstrating its high application potential in a wide temperature range. The experimental details can be found in our previous work [26]. Structural quantification of as-prepared ceramics was measured by the powder X-ray diffraction (XRD, Bruker AXS, Karlsruhe, ˚ radiation at room temperaGermany) using CuKα (λ = 1.5406 A) ture. Microstructure was observed by scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan). TEM characterization was performed using a JEM- 2100F electron microscope (JEOL, Japan). The polarization-electric hysteresis (P−E) loops and strain curves were measured using a ferroelectric measurement system (aixACCT TF Analyzer 20 0 0, Aachen, Germany). Raman spectrum was obtained using a micro-Raman system (LaRAM HR 800, Horiba Jobin Yvon, Paris, France). Fig. 1(a) shows the room-temperature XRD patterns of the unpoled xST ceramics. All the samples exhibit pure polycrystalline perovskite structure without any secondary phases, indicating that the ST has been completely diffused into the host lattice. No splitting of the (111) and (200) diffraction peaks illustrates a highly symmetric pseudocubic phase for all compositions. The substitution of ST shifts the peaks position to lower angles (inset of ˚ successfully Fig. 1(a)), ascribed to the large radius Sr2+ ions (1.45A) ˚ and Bi3+ ions (1.36 A) ˚ replacing the small radius Na+ ions (1.39 A) at A- sites. Despite the obvious lack of long-range noncubic distortion, it is reasonable to presume that the local scale distortion exits in the high symmetry phase, which fails to be detected by the XRD
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Fig. 2. (a) Composition dependent bipolar P−E loops, (b) bipolar S-E curves and (c) unipolar S-E curves for ST modified BNKTN ceramics. (d) The temperature dependent P−E loops and (e) unipolar S-E curves for x=0.03 ceramics. (f) The Hysteresis and remnant strain value of x=0.03 ceramics as a function of temperature.
for the low detection accuracy [27]. Thus all the XRD patterns of xST ceramics exhibit the same cubic symmetry. The selected area electron diffraction (SAED) patterns of 3ST composition were measured to probe the local phase structure, as shown in Fig. 1(b) and (c). It can be clearly observed that the 1/2(ooo) and 1/2(ooe) type (o and e stand for odd and even Miller indices, respectively) superlattice diffraction spots highlighted by circles appear in the [110] and [111] zone axis [28], respectively, indicating that the 3ST composition is at the MRB region with the different local symmetries of rhombohedral (R3c) and tetragonal (P4bm) phases. The temperature dependence of dielectric constant (ε r) and loss tangent (tanδ ) for the poled xST samples were measured from 25 °C to 400 °C at 1 kHz, as exhibited in Fig. 1(d). All the samples possess the diffuse phase transition (DPT) feature typical of a relaxor ferroelectric [29]. Here, the TF−R denote the phase transition temperature of FEs to RFEs, coupled with significant dielectric loss peak at this temperature. As shown in Fig. 1(d), the TF−R decreases sharply with the increase of ST content and eventually disappears from the ε −T spectrum at x=0.03. The phenomenon indicates that the 3ST ceramics have evolved into RFEs at room temperature [30]. The diffuseness degrees (γ ) of different components are characterized to assess the relaxor behavior based on the modified Curie– Weiss relationship
1
εr
−
1
εm
=
(T − Tm )γ C
,
where C denotes the Curie constant and γ varies between 1 (for the typical FEs) and 2 (for the complete RFEs) [31]. All the fitted results are presented in Fig. 1(e). The calculated γ value increases from 1.86 to 1.96 as ST content increases from 0 to 5%. The enhanced relaxor behavior mainly origins from the improved local random electric/elastic fields through the modification by ST. Furthermore, the PFM phase image of 3ST ceramics manifest no obvious discrepancy, demonstrating its weak ferroelectricity and extremely strong relaxor characteristic, [32] as exhibited in Fig. 1(f). Fig. 2(a)–(c) displays the composition dependence of the P−E loops, bipolar and unipolar S-E curves under 8 kV/mm, respectively. The P−E loops change from the typical saturated shape to the pinched ones as the ST content increases, indicating that the
long-range ferroelectric order of the host lattice is disrupted and the phase structure transforms into an ergodic relaxor (ER) with the ST modifying, consistent with the dielectric curves and PFM results. Actually, the ergodic relaxor ferroelectrics originate from the local random electric field and elastic field due to the incorporation of non- isovalent ions into the host BNT-based lattice. The formation of the ER phase can boost relatively high strain behavior, as shown in Fig. 2(b) [1,4]. The strain value raises from 0.1% to 0.42% on increasing ST content from 0 to 3%, beyond which the strain exhibits descending trend. Here, the variation of strain of different components is related to the change of PNRs’ size [33], which shows the opposite trend to the incorporation of ST content. Normally, when the ceramics are exposed into electric field, the PNRs with smaller size would transform into macroscale ferroelectric domains more easily due to the flattened energy barriers [34]. Thus the strain value exhibits increasing trend on increasing of ST content until the x=0.03. Nevertheless, when the size of PNRs is less than the critical value, the small-sized PNRs will nucleate firstly and then grow into long-growth ferroelectric order under the electric field with the large nucleation driving force, resulting in declined strain value [33]. Therefore, the maximum unipolar strain of 0.42% was obtained at x=0.03, as exhibited in Fig. 2(c). The temperature dependent P−E loops and unipolar strain curves are depicted in Fig. 2(d)–(e) in the range of 20–140 °C. As shown in Fig. 2(d), the shape of P−E loops for 3ST ceramics does not exhibit significant changes at different temperature except for the reducing hysteresis. The maximum polarization (Pm ) and remnant polarization (Pr ) of 3ST ceramics reduce from 34.7 μC/cm2 and 18.7 μC/cm2 to 27.2 μC/cm2 and 2.4 μC/cm2 respectively in the range of 20–140 °C, indicating the strong reversible under high temperatures. As displayed in Fig. 2(e), one can see that the large unipolar strain value of 3ST ceramics exhibits excellent thermally stable behavior with the variation less than 10% in the range of 20–140 °C, both of which are superior than many other lead-free systems which will be compared and discussed below. In addition, the hysteresis and remnant strain value (Srem) reduce significantly with the increased temperature, as illustrated in Fig. 2(f), which may be related to the variation of microdomain structure of PNRs with the temperature changes [35].
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Fig. 3. (a) The temperature depdend in situ Raman spectra for x=0.03 ceramics. (b) Schematic field- induced lattice strain as a function of temperature. (c) Schematic Landau potential curves for x=0.03 ceramics under different temperature.
Normally, there exits four different mechanisms of strain in ferroelectrics. (1) the converse piezoelectric effects; (2) the non180°domain switching and rotation; (3) the lattice distortion induced by electric field and (4) the field-induced phase transition [36]. It is worth noting that the piezoelectric effect and non-180° domain switching in 3ST ceramics have no significant contribution to the strain property because of its strong relaxor characteristics which is demonstrated by the dielectric curves and PFM image. Thus the field-induced phase transition as well as the fieldinduced lattice distortion are the main sources of the strain value of the 3ST ceramics. Temperature dependent Raman spectra of the 3ST ceramics were measured to investigate the field-induced intrinsic lattice strain under different temperatures, where eight Raman spectral deconvolutions alongside were composed by a collection of Lorentzian curves, as exhibited in Fig. 3(a). Note that the Raman spectroscopy data was corrected according to the Bose–Einstein formula:
n(w ) + 1 = 1/[1 − exp(−hw/kBT ], where h and kB stand for Planck’s and Boltzmann’s constant, respectively [2]. The Raman spectra can be divided into three main regions: (1) the 10 0–20 0 cm–1 host modes related to the vibrations of the perovskite A-site cations; (2) the modes between 200 and 400 cm–1 associated with Ti-O vibrations; (3) the modes ≥ 400 cm–1 assigned to the vibrations of BO6 octahedra [2]. It can be obviously observed that the peaks at ~260 cm–1 and 526 cm–1 shift to lower wavenumber with the increase of temperature, the softening of which result in the decrease of the unit cell polarity [37]. It has been reported that the volume variation V/V between RFE and FE phase is proportional to the polarization. Therefore, the decrease of the unit cell polarity corresponds to the declined strain value, i.e. the intrinsic lattice strain has negative correlation with the temperature, as illustrated in Fig. 3(b). On the other hand, a simplified Ginzburg–Landau–Devonshire thermodynamic theory was employed to describe the phase transition under varied temperature, as illustrated in Fig. 3(c). Generally, the free energy G of the ferroelectric systems depends on
polarization P:
G(P ) = G0 + α /2P2 − β /4P4 + γ /6P6 − EP(α , β , γ > 0 ) Where G0 denotes the free energy of the RFE phase, E presents the external electric field, and α , β , γ are constant [33]. Here, G presents the energy difference between relaxor ferroelectrics and electric field induced ferroelectrics. When the system is exposed into a low temperature T1 , the free energy of RFE (O) is much smaller than that of FE (O’), revealing that only a small amount of RFE would be induced into FE under electric field. However, as the temperature rises to a higher temperature T2 , the free energy difference between RFE and FE (A and A’) decreases significantly. Accordingly, more nonpolar structure can be induced into polar structure under electric field, leading to enhanced strain level [38]. As the temperature rises to a higher level, the strain origin from phase transition will continue to raise until reaching the maximum at the critical temperature. Therefore, based on the analysis above, we propose that the thermal stability of the strain property is due to a combination of the decrease in the intrinsic lattice strain V/V and the increase in the contribution of electric field induced RFEFE phase transition to the strain in this BNT-based systems. To further evaluate the thermal stability of the strain in the 3ST component, we compared the temperature dependent strain characteristics of our ceramics with other promising piezoelectric strain ceramics reported in recent years as shown in Fig. 4 [39–44]. It can be found as shown in Fig. 4 that the 3ST ceramics exhibit outstanding temperature stability over a wide temperature range. Enhanced strain temperature stability without reducing strain values is a long- standing scientific problem. Here, our systems made great progress in balancing large strain values and excellent thermal stability in piezoelectric strain ceramics, demonstrating the wide application prospect. In summary, the comprehensive properties of the BNT-BKTNN-ST ceramics were investigated in detail. The phase structure changed from FEs into RFEs with the increase of ST content. The giant strain of 0.42% was obtained in the 3ST ceramics, attributed to the extremely relaxor behavior. In addition, in-situ temperature dependent S-E curves demonstrated the excellent temperature stable strain behaviors of the 3ST ceramics, which is attributed to the combined result of the decrease in the intrinsic lattice strain
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Fig. 4. A comparison of temperature dependence of strain property of BNT-based ceramics with other perovskite ceramics.
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