Effect of BaTiO3 particles with different shape on electrical properties of (Bi0.5Na0.5)TiO3 piezoceramics

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Effect of BaTiO3 particles with different shape on electrical properties of (Bi0.5Na0.5)TiO3 piezoceramics ⁎⁎

Lili Lia, Bing Zhoua, Hongbin Yuana,b, Fei Wena, Zhuo Xua,c, , Gaofeng Wanga,

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a

Key Lab of RF Circuits and Systems of the Ministry of Education, College of Electronics and Information, Hangzhou Dianzi University, Hangzhou 310018, China Qianjiang College, Hangzhou Normal University, Hangzhou 310012, China c Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: (Bi0.5Na0.5)TiO3-based ceramics Piezoelectricity Electromechanical response Lead-free materials

Lead-free (Bi0.5Na0.5)TiO3-based materials are becoming a viable alternative to lead-based material in developing piezoelectricity strategies, due to its high piezoelectric performance as well as large strain response. In the present work, three kind of micro-sized, nano-sized, and plate-like BaTiO3 particles were employed to tailor the electrical properties of (Bi0.5Na0.5)TiO3 piezoceramics. The piezoelectric, strain properties, and phase transition temperature could be improved when employing BT nano-sized particles instead of BT micro-sized and plate-like particles as the raw material. Besides, addition of BT nano-sized particles could also yield a decreased critical poling filed of 22 kV/cm, compared with BT micro-sized (24 kV/cm) and plate-like (28 kV/cm) particles. Most intriguingly, an enhanced frequency-dependent unipolar strain in relative to other lead-free ceramics was also realized in the ceramics with BT nano-sized particles. In addition, the fatigue response of plate-like BT particlesmodified ceramics was superior to that of nano- and micro-sized ones, which was mainly attributed to the lower defect density. This study demonstrates the superiority of uniform and nano-scale modifier for BNT-based ceramics, and also provides new opportunities to tailor and control piezoelectric activity for actuator applications.

1. Introduction Developing lead-free piezoceramics is an imperative asset in today's society with increasingly serious environmental problems [1–3]. Among the lead-free ceramics today available, ABO3 perovskite type structure lead-free ceramics behave the more omnipotent and efficient choice. In particular, (Bi0.5Na0.5)TiO3 (BNT)-based solid solution have embodied the outstanding performing electron component, such as actuators and sensors and so on. This is caused by their large ferroelectricity, high piezoelectricity, and prominent field-induced strain response [4–15]. Nevertheless, the comprehensive performance of BNTbased ceramics is still inferior compared to the lead-based materials, and thus needed to be further optimized. The modifiers with different shape and size have various influences on resulting piezoelectric and electromechanical response for lead-free ceramics. For instance, introducing nano-sized ZnO into Bi0.5Na0.5TiO3BaTiO3 (BNT-BT) to form composite ceramics can successfully tailor the

thermal stability [16–20]. Recent study also validated that different size has distinctive effects on the stability of long-range ferroelectric order [21]. In BNT-based solid solution, (1-x)BNT-xBT represented excellent piezoelectric properties at morphotropic phase boundary with approximately 7 mol% BT [22]. Detailed knowledge of electrical properties have confirmed that piezoelectric performance of BNT-BT is closely associated with the amount of BT content [23,24]. At present, differently shaped and sized BT such as nanoparticles, plate, nanowires, and so forth have been synthesized, and the corresponding applications in piezoelectric response and energy storage have been reported [25,26]. Especially, Cao et al. [27] reported that using BT nanowires rather than BT solution can effectively shift the depolarization temperature (Td) of 0.90BNT-0.05BKT-0.05BT ceramics to higher temperature. Furthermore, Bai et al. investigated the impacts of different templates with plate-like shape on structure and electromechanical response of BNTbased ceramics, and the results demonstrated that different plate-like templates shows different effects on resulting strain response [25]. This

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Corresponding author. Corresponding author at: Key Lab of RF Circuits and Systems of the Ministry of Education, College of Electronics and Information, Hangzhou Dianzi University, Hangzhou 310018, China. E-mail addresses: [email protected] (Z. Xu), [email protected] (G. Wang). ⁎⁎

https://doi.org/10.1016/j.ceramint.2018.10.090 Received 13 September 2018; Received in revised form 10 October 2018; Accepted 11 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Li, L., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.090

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gives an indication that BT with different shape has significant roles in generating piezoelectric response for BNT-related ceramics. However, very little information is known about effects of BT with different shape on piezoelectric and electromechanical response of BNT ceramics. Herein, we investigate the roles of 7 mol% BT micro-sized, nanosized, and plate-like particles on resulting piezoelectric performance and electromechanical response of BNT ceramics. Results show that BT particles with different shape have significant influences on the related piezoelectric properties and electromechanical behavior. 2. Experimental section The 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 (BNT-7BT) ceramics were fabricated by traditional solid state reaction approach. BNT matrix and BT micro-sized particles were synthesized by solid state reaction, and BT plate-like particles were prepared by topochemical microcrystal conversion method. The detailed synthesis process are described in previous work [25]. The microstructures of the achieved BT particles with different shape are shown in the insets of Fig. 2(a)-(c). Afterwards, the BT micro-sized particles, nano-sized particles (60 nm, Shandong Guotao S&T Ltd, China), and plate-like particles were added into the singlephase BNT powder to gain mixtures. Subsequently, obtained mixture was dried, pressed and sintered at sintering temperature of 1130–1150 °C for 2 h. Silver electrodes were formed on the circular surfaces of polished samples, and then fired at 600 °C for 30 min. Polarization was performed in silicone oil bath with a dc field of 50 kV/cm at room temperature for 15 min. The phase structures were examined by X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with Cu Ka radiation. The microstructures of the specimens were taken by a scanning electron microscopy (SEM) (JSM, EMP-800; JEOL, Tokyo, Japan). The Raman spectra were measured by a LABRAM microprobe system (Horiba/JobinYvon, Villeneuve d′Ascq, France) with the 532 nm light. Dielectric properties including temperature-dependent dielectric constant and loss were tested using a high-precision LCR meter (HP 4284A; Agilent, Palo Alto, CA). Polarization hysteresis loops and corresponding strain curves were simultaneously done at 10 Hz using a ferroelectric test system (aixACCT TF Analtzer 2000, Germany). Room-temperature piezoelectric constant (d33) of poled samples was determined via quasi-static d33 meter (Institute of acoustic, Chinese Academic Society, ZJ-3A, Beijing, China).

Fig. 1. (a) XRD patterns and (b) Raman spectra for the ceramics with different shape of BT.

microstructures. There are relatively uniform morphologies composing of only “small” grains (Fig. 2(a) and (b)) for the samples using MN- and NN- shaped BT. However, inhomogeneous morphology was observed after doping BT with PN, as characterized by consisting of “large” and “small” grains (Fig. 2(c)). To intuitively observe the change of grain size, the grain size distribution was performed using Nano Measurer software, as shown in Fig. 2(d)-(f). Relatively uniform grain size distribution appeared in the ceramics with MN- and NN-shaped BT, as presented in Fig. 2(d) and (e). Note that, this is not the case in the sample with PN-shaped BT. Obviously, doping PN-shaped BT produced the increasement of grain size (1.32 µm) in relative to the samples with MN- and NN-shaped BT (1.01 µm and 0.99 µm). This observed microstructure primarily caused by the added PN-shaped BT, which possesses broad faces and relatively high aspect ratio (the inset of Fig. 2(c)) [22]. The observed different microstructures with the addition of BT with different shape may has an implication on resulting piezoelectric and electromechanical performance. Fig. 3(a) and (b) show the polarization hysteresis loops and bipolar strain curves of BNT-7BT ceramics with different BT shape, measured at room temperature with a fixed field of 60 kV/cm, respectively. All samples behaved typical room temperature ferroelectric feature, as characterized by well-saturated polarization loops and butterfly-shaped bipolar strain curves. Furthermore, room temperature ferroelectric nature could also be confirmed by two discernible current peaks (seeing the inset of Fig. 3(a)) associated with domain switching. In general, the appearance of current peak as the applied field reached the corresponding coercive field Ec derived from the contributions of dielectric constant and domain switching. To clarify the BT shape effects on domain wall mobility and domain switching, unipolar strain response of the ceramics with different BT shape was plotted in Fig. 3(c). All unipolar strain curves exhibited a quasi-linear behavior, further indicating the nature of ferroelectricity for all samples. Apparently, the NN sample

3. Results and discussion XRD patterns and Raman spectra of the BNT-7BT ceramics [(microsized particles, MN), (nano-particles, NN), (plate-like particles, PN)] are shown in Fig. 1(a) and (b), respectively. Diffraction peaks of all studied ceramics could be indexed by a single-phase perovskite structure, and no secondary phase appeared. In addition, all ceramics possessed pseudo-cubic phase featured by single (111) and (200) peaks. This indicates that the phase structure of BNT-7BT has not been changed in spite of the introduction of BT particles with different shape. Similar pseudo-cubic structure was also reported in previous BNT-BT solid solutions [6,14,22,23,28]. To further explore the structure of the ceramics with different BT shape, Raman spectroscopy were measured and presented in Fig. 1(b). Consistent with the previous report for BNT-based ceramics [6,29,30], there is three main regions in the range of wavenumber, as denoted in Fig. 1(b). As marked by dotted line, two diacritical splitting bands near 260 cm−1 occurred in all samples, which is indicative of rhombohedral and tetragonal phase coexistence [28,31]. Obviously, with the addition of BT particles with different shape, there was little difference in the corresponding Raman spectra. Combined with XRD and Raman results (Fig. 1), it can draw a conclusion that addition of BT with different shape scarcely influences the phase structure of the ceramics. Fig. 2(a)-(c) provides surface morphologies of the ceramics with different shape of BT. All the three specimens exhibited dense 2

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Fig. 2. SEM images of the ceramics with (a) MN, (b) NN, and (c) PN, and grain size distribution of the sample: (d) MN, (e) NN, and (f) PN.

the piezoelectric and electromechanical property of BNT ceramics. In detail, compared to the d33, Smax/Emax and small signal d33 values of MN and PN sample (156pC/N, 310 pm/V, and 157 pm/V for MN, 129pC/N, 297 pm/V and 135 pm/V for PN sample), enhanced values of d33 = 171 pC/N, Smax/Emax = 343 pm/V, and d33 * = 165 pm/V were realized in the sample with NN-shaped BT. The generated enhancements in piezoelectric response mainly originated from the vast polarization rotation in the sample with NN-shaped BT [27,36–38]. In detail, the dipole movements can be accelerated by the uniformly distributed microstructure (Fig. 2(b)) driven by the NN-shaped BT, giving rise to the improvements of piezoelectricity activity. On the contrary, for the case of PN-shaped BT, the inhomogeneous microstructure (Fig. 2(c)) makes the domain rotation more harder and greatly hinders the polarization rotation, which brings about the deterioration of related piezoelectric performance. On the other hand, an intriguing behavior was observed in Fig. 4, that is, the value of small signal d33 is much lower than that of Smax/Emax. This observation is closely related to the constitution of

demonstrated a relatively large unipolar strain response (~0.204%) as compared to MN (~0.186%) and PN (~0.179%) ones. The small signal hysteresis loops of field-dependent d33(E), dielectric constant, and loss for BNT-7BT ceramics are provided in Fig. 3(d)-(f), respectively. The small signal piezoelectric coefficient d33 can be extracted from the value at zero field in d33(E) loop [32,33]. As demonstrated in polarization and strain measurements (Fig. 3(a)-(c)),typical ferroelectric characteristics are perceptible, as evidenced by the shape of d33(E) and anisotropy in dielectric constant and loss loops [34,35]. Particularly, this behavior is notable for the sample with PN-shaped BT. From these measurements, it is safe to say that different shape BT particles has different effects on electrical properties. Based on the piezoelectric measurements, strain loop (Fig. 3(c)), and small signal d33(E) (Fig. 3(d)), the corresponding piezoelectric and electromechanical parameters including d33, Smax/Emax, and small signal d33 of differently shape BT particles are summarized in Fig. 4. Obviously, the shape and size of added BT particles depended largely on 3

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Fig. 3. (a) Polarization hysteresis loops, (b) bipolar strain curves, (c) unipolar strain curves, (d) electric field-dependent small signal piezoelectric coefficient d33(E) loops, (e) field-dependent dielectric constant, and (f) dielectric loss for BNT-7BT ceramics with different BT shape. The inset of Fig. 3(a) presents the field-current curves for the ceramics.

piezoelectric effect including intrinsic and extrinsic contributions [39]. Generally speaking, the intrinsic contribution to piezoelectric effect was equivalent in spite of different measuring method. However, the extrinsic influences on piezoelectric effect is sensitive to external excitation. As far as the applied electric field is concerned, the field-dependent d33(E) was measured under low external field. Inversely, large driving field (above the Ec) was used to conduct the Smax/Emax measurement. This is to say, in relative to the intrinsic contribution, piezoelectric strain measurement mainly derived from the extrinsic contribution. Consequently, larger Smax/Emax over small signal d33(E) appeared (see Fig. 4). The temperature dependence of dielectric properties of all poled ceramics are comparatively presented in Fig. 5(a), measured at 1 kHz. All the curves exhibited two obvious dielectric anomalies in the investigated temperature range, designating as dielectric constant-maximum temperature Tm and phase transition temperature from

Fig. 4. Summarization of d33, Smax/Emax, small signal d33 for the indicated compositions. 4

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Fig. 5. (a) Temperature dependence of dielectric constant and loss, and (b) change in TF-R for the indicated ceramics.

TF-R (Fig. 5(b)) and enhancement of room temperature d33 (Fig. 4) were simultaneously achieved in the ceramics with NN-shaped BT, which is of great importance for practical application in BNT-based ceramics. Fig. 6(a) illustrates the room temperature d33 under different applied poling electric field for the ceramics with different shape of BT. As demonstrated in other BNT-based ceramics [30,44,45], there was a jump in d33 with increasing the poling field. In detail, the d33 value of all samples first increases slowly, and then ascends rapidly and finally varies slightly. According to previous works [23,45], the field where a sudden change of d33 appeared is defined as critical poling field Ecp, as marked by bar in Fig. 6(a)-(c). In BNT-based ceramics, two factors including local structural fluctuation and polarization reorientation mainly generate an abrupt increase of d33 at Ecp, beyond which fielddriven ordered polarization can be formed [37]. Fig. 6(d) displays the values of Ecp for the BNT-7BT ceramics. It is unambiguous that Ecp of

ferroelectric to relaxor phase TF-R [40–42]. The TF-R can be determined from the dielectric loss peak for poled samples, as labelled with ellipse in Fig. 5(a). The values of TF-R for the ceramics is listed in Fig. 5(b). Apparently, the sample with NN-shaped BT has a higher TF-R in comparison with that of ones with MN- and PN-shaped BT, implying the enhanced thermal stability by means of NN-shaped BT introduction. This reveals that addition of BT with different shape generates obvious impacts on the phase transition temperature. In the present study, compared to BT with MN and PN shape, employing BT with NN shape impedes ferroelectric domain switching to a great extent, thus resulting in the increase of phase transition temperature, as shown in Fig. 5(b). The phenomenon of TF-R shown in Fig. 5(a) and (b) coincides with the ferroelectric and strain measurements (Fig. 3(a) and (b)). For BNTbased materials, the improved TF-R was usually at the cost of the reduction of room temperature d33 [12,43]. Interestingly, the increase of

Fig. 6. d33 versus applied poling filed for the ceramics with (a) MN, (b) NN, and (c) PN. (d) Summarization of Ecp for the indicated ceramics. 5

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Fig. 7. Frequency-dependent (a) polarization hysteresis loops, (b) bipolar strain curves, and (c) unipolar strain for the ceramics. (d) A comparison of normalized unipolar strain of NN-shaped ceramics against frequency with that of other reported lead-based and lead-free KNN- and BNT-based ferroelectric ceramics [46–48].

marked by strip shown in Fig. 7(d). Compared to the reported ceramics like soft PZT [46], the sample with NN-shaped BT presents comparable frequency-dependent strain characteristic. What's more, the sample also shows insensitive frequency-dependent strains with increasing frequency than the Li-modified KNN, BNT-BT-AN, and BNT-ST ceramics [46–48,51]. This characteristic validates the advantage of NN-shaped BT in generation of strain response for BNT piezoceramics. For understanding of fatigue feature of the ceramics with different shape of BT, the polarization and strain loops were characterized, as shown in Fig. 8. The shape of obtained curves had no obvious change, indicating that typical ferroelectric response was still be observed when they are subjected to cycles after 105 times. Note that, the fluctuation of the ferroelectricity and strain response could be found with the application of cycle numbers. To quantify the detailed impact of cycle numbers on the change of ferroelectricity and strain response, the maximum polarization Pm, remnant polarization Pr and negative strain ( the definition is given in the inset of Fig. 8(b)) were summarized in Fig. 8(c), (f) and (i). It is interesting to note that the ceramics using PNshaped BT exhibited good fatigue response on the ferroelectric and strain properties in relative to the MN- and NN-shaped ones after being cycled for 105. In detail, the variation of Pm, Pr and Sneg were within 3%, 7% and 15% of the original values for the PN-shaped modified BNT ceramics (see Fig. 8(i)), which were superior to these changes for MN(13% for Pm, 22.1% for Pr, and 20.3% for Sneg, see Fig. 8(c)) and NNshaped (13.5% for Pm, 22.5% for Pr, and 38.7% for Sneg, see Fig. 8(f)) modified ones. This suggests that the introduction of BT with PN shape can tailor the fatigue response of ferroelectricity and strain while maintaining the ferroelectric long-range order structure. The achieved speciality is beneficial to the actuator applications. It is well known that the Sneg is associated with the domain switching in piezoelectric materials, during which the ferroelectricity has been changed and the butterfly-shaped bipolar strain curves generated upon the applied electric field direction [46]. As shown in Fig. 2, the ceramics with PNshaped BT exhibited obviously inhomogeneous microstructure as compared to the ones with MN- and NN-shaped BT. With increasing

the sample using NN-shaped BT is lower than that of MN- and PNshaped BT-doped BNT-BT ceramics. Based on the measurements shown in Fig. 6(a)-(c), the low Ecp of 22 kV/cm can be achieved in the ceramic with NN-shaped BT, which is lower than those values (24 kV/cm for MN and 28 kV/cm for PN). This achieved peculiarity in NN-shaped BTmodified ceramics offers a feasible strategy to design novel piezoelectric ceramics that demand low poling fields. Fig. 7(a)-(c) illustrate polarization hysteresis loops, bipolar strain curves, and unipolar strain for the ceramics with NN-shaped BT particles, measured at frequency range from 0.1 Hz to 20 Hz. In general, the extrinsic contribution was dominant for the frequency-dependent piezoelectric response in piezoelectric ceramics [49]. In typical ferroelectric materials, the strain was mainly governed by ferroelectric domain switching, during which the spontaneously polarized states have been altered along the imposed electric field direction [50]. The domain wall motion has been delayed with increasing the measuring frequency, producing small change to strain level. Recently, Zuo et al. [46] reported frequency-insensitive strain response in BMT-PT-BNT ternary solid solution. It can be ascribed to extremely large dynamics and small size of ergodic polar nanoregions, and thus very fast response of domain wall motion to applied field. In this study, the ceramics possess traditional ferroelectric phases rather than ergodic relaxor ferroelectrics at room temperature, which can be clearly proved by the ferroelectric (Fig. 3(a)) and dielectric(Fig. 5(a)) measurements. For ferroelectric materials, upon the application of field, the ferroelectric phase would exhibit a relatively slow response to the field as compared to the relaxor phase. As a consequence, increasing frequency generated the changed values of ferroelectricity and strain values (see Fig. 7(a)(c)) to some extent. Based on the unipolar strain measurements shown in Fig. 7(c), a comparison of normalized unipolar strain of NN-shaped ceramics against frequency with that of other reported lead-based and lead-free ferroelectric ceramics was presented in Fig. 7(d). Apparently, a relatively good frequency-strain behavior was observed in the NNmodified sample. Quantitatively, the normalized strain value ranges smaller than 5% within the investigated frequency range 0.1–20 Hz, as 6

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Fig. 8. Fatigue behavior tested under 60 kV/cm and 10 Hz for [(a),(d),(g)] polarization hysteresis, [(b),(e),(h)] strain hysteresis and [(c),(f), (i)] change in Pm, Pr, and Sneg of the ceramics with [(a)-(c)] MN, [(d)-(f)] NN, and [(g)-(i)] PN with the cycles range from 10−1 to 105.

δ = δ0 exp(−Ea/ κT )

cycle numbers, this inhomogeneous microstructure confined the ferroelectric domain switching along the driving field direction, generating the fact that the domain motion lags and its response to the cycle numbers also slows down. Thus, the change of Sneg is relatively small in PN-shaped BT modified BNT ceramics in relative to ones modified with MN- and NN-shaped BT, as provided in Fig. 8. In addition, the obvious fluctuation in Sneg appeared shown in Fig. 8, which may be a consequence of the poling effect. As is well-known, appropriate poling conditions including optimal poling filed, time, and temperature can produce the improvement of piezoelectricity in piezoelectric materials [52]. Moreover, DC field poling on lead-based and lead-free piezoceramics shares similarities with cycling in our present work [34,53]. Considering the situation that defects strongly affect the fatigue behavior for the perovskite ferroelectric system [17,54], complex impedance spectra were measured to explore the defect type. The corresponding complex impedance spectra are shown in Fig. 9(a)-(c). A single semicircle was observed for all samples, which is indicative of little importance difference of thermal-activated electrical properties between grains and grain boundaries. The bulk resistance can be estimated by means of extrapolating the low-frequency intercept of the real axis using the Z-View software [17]. Moreover, the resistance of the ceramics shows a negative temperature coefficient, as evidenced by decreased diameter of semicircle upon the increase in temperature. The activation energies Ea can be calculated to confirm the defect type of the investigated samples with the Arrhenius law:

(1)

where σ0 corresponds to a constant, κ is the Boltzmann constant, and T represents the absolute temperature. Fig. 9(d) plots the logarithms of resistance R as a function of reciprocal temperature 1000/T. Based on the Eq. (1), the Ea can be calculated with the assistance of the best leastsquares fitting. The corresponding Ea was determined to be ~ 1.54 eV for MN, 1.50 eV for NN, and 1.62 eV for PN, respectively, which obeys the Arrhenius law. It was found that the magnitude of Ea in these samples is close to half the band gap of BNT-BT (3.25–3.4 eV), demonstrating that the present materials behave intrinsic electronic conduction closely associated with electronic defects [55,56]. As far as the oxygen vacancies is concerned, Ea changes from 0.5 to 2 eV, depending on the doping content. Hence, it is reasonable to propose that the electronic defects mainly contributed by oxygen vacancies, which dominates the electronic conduction for all the ceramics at high temperature. The oxygen vacancies largely derived from the volatility of Na+ at high sintering temperature. From the Fig. 9(d), the Ea for PNshaped ceramics was enhanced as compared to that of MN- and NNshaped ones, suggesting reduced oxygen vacancies. Furthermore, the lower concentration of oxygen vacancies would generate a lower defect density. Fundamentally, the lower defect density was the main cause for the generation of improved fatigue behavior in PN-shaped samples as compared to MN- and NN-shaped ones, as presented in Fig. 8.

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Fig. 9. Complex impedance spectra for the ceramics measured at 460–600 °C and 100 Hz-1 MHz: (a) MN, (b) NN, and (c) PN. (d) ln(1/R) against temperature for the specimens in the forms of Arrhenius plots.

4. Conclusions

[2] J. Rödel, K.G. Webber, R. Dittmer, W. Jo, M. Kimura, D. Damjanovic, Transferring lead-free piezoelectric ceramics into application, J. Eur. Ceram. Soc. 35 (2015) 1659–1681. [3] J. Wu, D. Xiao, J. Zhu, Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries, Chem. Rev. 115 (2015) 2559–2595. [4] 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 (2012) 71–93. [5] S.-T. Zhang, A.B. Kounga, E. Aulbach, H. Ehrenberg, Jr Rödel, Giant strain in leadfree piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system, Appl. Phys. Lett 91 (2007), p. 112906. [6] F. Wang, M. Xu, Y. Tang, T. Wang, W. Shi, C.M. Leung, J. Rödel, Large strain response in the ternary Bi0.5Na0.5TiO3-BaTiO3-SrTiO3 solid solutions, J. Am. Ceram. Soc. 95 (2012) 1955–1959. [7] A. Ullah, C. Won Ahn, A. Ullah, I. Won Kim, Large strain under a low electric field in lead-free bismuth-based piezoelectrics, Appl. Phys. Lett. 103 (2013) 022906. [8] D. Maurya, Y. Zhou, Y. Wang, Y. Yan, J. Li, D. Viehland, S. Priya, Giant strain with ultra-low hysteresis and high temperature stability in grain oriented lead-free K0.5Bi0.5TiO3-BaTiO3-Na0.5Bi0.5TiO3 piezoelectric materials, Sci. Rep. 5 (2015) 8595. [9] X. Liu, X. Tan, Giant strains in non-textured (Bi1/2Na1/2)TiO3 -based lead-free ceramics, Adv. Mater. 28 (2016) 574–578. [10] H.-L. Li, Q. Liu, J.-J. Zhou, K. Wang, J.-F. Li, H. Liu, J.-Z. Fang, Grain size dependent electrostrain in Bi1/2Na1/2TiO3-SrTiO3 incipient piezoceramics, J. Eur. Ceram. Soc. 36 (2016) 2849–2853. [11] H. Jigong, X. Zhijun, C. Ruiqing, L. Wei, D. Juan, L. Guorong, Ultrahigh strain response with fatigue-free behavior in (Bi0.5Na0.5)TiO3 -based lead-free piezoelectric ceramics, J. Phys. D Appl. Phys. 48 (2015) 472001. [12] P. Peng, H. Nie, Z. Liu, W. Ren, F. Cao, G. Wang, X. Dong, Enhanced ferroelectric properties and thermal stability of Mn-doped 0.96(Bi0.5Na0.5)TiO3-0.04BiAlO3 ceramics, J. Am. Ceram. Soc. 100 (2017) 1030–1036. [13] C. Groh, D.J. Franzbach, W. Jo, K.G. Webber, J. Kling, L.A. Schmitt, H.-J. Kleebe, S.J. Jeong, J.-S. Lee, J. Rödel, Relaxor/ferroelectric composites: a solution in the quest for practically viable lead-free incipient piezoceramics, Adv. Funct. Mater. 24 (2014) 356–362. [14] W. Bai, D. Chen, P. Zheng, B. Shen, J. Zhai, Z. Ji, Composition- and temperaturedriven phase transition characteristics and associated electromechanical properties in Bi0.5Na0.5TiO3-based lead-free ceramics, Dalton Trans. 45 (2016) 8573–8586. [15] W. Bai, D. Chen, P. Zheng, J. Xi, Y. Zhou, B. Shen, J. Zhai, Z. Ji, NaNbO3 templatesinduced phase evolution and enhancement of electromechanical properties in < 00l > grain oriented lead-free BNT-based piezoelectric materials, J. Eur. Ceram. Soc. 37 (2017) 2591–2604. [16] J. Zhang, Z. Pan, F.F. Guo, W.C. Liu, H. Ning, Y.B. Chen, M.H. Lu, B. Yang, J. Chen, S.T. Zhang, X. Xing, J. Rodel, W. Cao, Y.F. Chen, Semiconductor/relaxor 0-3 type

In summary, this study presented investigations of microstructure, phase structure and related electrical properties of BNT-BT piezoceramics with different shape of micro-sized, nano-sized, and plate-like BT as the raw material. Results showed that the shape of BT had a different roles on resulting piezoelectric and strain performance of BNTBT ceramics. Compared to micro-sized and plate-like particles, the introduction of BT nano-sized particles into BNT was identified to have a positive influence to piezoelectric, strain properties, and phase transition temperature. Detailed measurements of poling field-dependent piezoelectric constant also uncovered that addition of BT nano-sized particles can effectively decrease critical field as compared to added micro-sized and plate-like ones, as characterized by the decreased critical field from 28 kV/cm for BT plate-like particles, 24 kV/cm for BT micro-sized particles to 22 kV/cm for BT nano-sized particles. More importantly, improved frequency-dependent feature of unipolar strain was achieved in the ceramic added BT nano-sized particles, which was comparable to traditional PZT ceramics. In addition, the ceramics with plate-like BT particles behaved the enhanced fatigue response in relative to the nano- and micro-sized ones, which mainly originated from the lower defect density. As a consequence, the present work offers some guidelines to tune piezoelectric and electromechanical response of lead-free ceramics for actuator application. Acknowledgements This work was financially supported by Key Research and Development Projects of Zhejiang Province (Grant No. 2017C01056). References [1] J. Koruza, A.J. Bell, T. Frömling, K.G. Webber, K. Wang, J. Rödel, Requirements for the transfer of lead-free piezoceramics into application, J. Mater. 4 (2018) 13–26.

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[17]

[18]

[19]

[20]

[21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

piezoelectric Na0.5Bi0.5TiO3-BaTiO3 ceramics, Appl. Phys. Lett. 100 (2012) 172906. [37] P.-Y. Chen, C.-S. Chen, C.-S. Tu, P.-H. Chen, J. Anthoniappen, Effects of texture on microstructure, Raman vibration, and ferroelectric properties in 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 ceramics, J. Eur. Ceram. Soc. 36 (2016) 1613–1622. [38] Y. Liu, Y. Chang, F. Li, B. Yang, Y. Sun, J. Wu, S. Zhang, R. Wang, W. Cao, Exceptionally high piezoelectric coefficient and low strain hysteresis in grain-oriented (Ba, Ca)(Ti, Zr)O3 through integrating crystallographic texture and domain engineering, ACS Appl. Mater. Inter. 9 (2017) 29863–29871. [39] W. Bai, L. Wang, P. Zheng, F. Wen, L. Li, J. Zhai, Z. Ji, Electromechanical response and piezoelectric properties in (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 piezoceramics using nano-sized AlN modification, Ceram. Int. 44 (2018) 16040–16050. [40] K. Wang, A. Hussain, W. Jo, J. Rödel, D.D. Viehland, Temperature-dependent properties of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-SrTiO3 lead-free piezoceramics, J. Am. Ceram. Soc. 95 (2012) 2241–2247. [41] W. Jo, J. Daniels, D. Damjanovic, W. Kleemann, J. Rödel, Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2Na1/2)TiO30.06BaTiO3, Appl. Phys. Lett 102 (2013), p. 192903. [42] E.-M. Anton, W. Jo, D. Damjanovic, J. Rödel, Determination of depolarization temperature of (Bi1/2Na1/2) TiO3-based lead-free piezoceramics, J. Appl. Phys. 110 (2011) 094108. [43] Y. Jie, Z. Chunlin, Z. Yuxing, W. Jiagang, Composition‐induced phase transitions and enhanced electrical properties in bismuth sodium titanate ceramics, J. Am. Ceram. Soc. 100 (2017) 5601–5609. [44] W. Bai, P. Zheng, F. Wen, J. Zhang, D. Chen, J. Zhai, Z. Ji, Lead-free BNT-based composite materials: enhanced depolarization temperature and electromechanical behavior, Dalton Trans. 46 (2017) 15340–15353. [45] H. Guo, C. Ma, X. Liu, X. Tan, Electrical poling below coercive field for large piezoelectricity, Appl. Phys. Lett. 102 (2013) 092902. [46] W. Zhao, R. Zuo, D. Zheng, L. Li, Dielectric relaxor evolution and frequency-insensitive giant strains in (Bi0.5Na0.5)TiO3-modified Bi(Mg0.5Ti0.5)O3–PbTiO3 ferroelectric ceramics, J. Am. Ceram. Soc. 97 (2014) 1855–1860. [47] H. Zhang, P. Xu, E. Patterson, J. Zang, S. Jiang, J. Rödel, Preparation and enhanced electrical properties of grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics, J. Eur. Ceram. Soc. 35 (2015) 2501–2512. [48] M. Acosta, W. Jo, J. Rödel, D.C. Lupascu, Temperature- and frequency-dependent properties of the 0.75Bi1/2Na1/2TiO3-0.25SrTiO3 lead-free incipient piezoceramic, J. Am. Ceram. Soc. 97 (2014) 1937–1943. [49] A.J. Masys, W. Ren, G. Yang, B.K. Mukherjee, Piezoelectric strain in lead zirconate titante ceramics as a function of electric field, frequency, and dc bias, J. Appl. Phys. 94 (2003) 1155–1162. [50] D. Zheng, R. Zuo, S. Zhang, A novel BiFeO3-BaTiO3-BaZrO3 lead-free relaxor ferroelectric ceramic with low-hysteresis and frequency-insensitive large strains, J. Am. Ceram. Soc. 98 (2015) 3670–3672. [51] Z. Jia‐Jun, W. Ke, L. Fu, L. Jing‐Feng, Z. Xiao‐Wen, W. Qing‐Ming, High and frequency‐insensitive converse piezoelectric coefficient obtained in AgSbO3‐modified (Li, K, Na)(Nb,Ta)O3 lead‐free piezoceramics, J. Am. Ceram. Soc. 96 (2013) 519–523. [52] Q. Li, M.-H. Zhang, Z.-X. Zhu, K. Wang, J.-S. Zhou, F.-Z. Yao, J.-F. Li, Poling engineering of (K,Na)NbO3-based lead-free piezoceramics with orthorhombic–tetragonal coexisting phases, J. Mater. Chem. C 5 (2017) 549–556. [53] 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 (2014) 114102. [54] C. Tian, F. Wang, X. Ye, Y. Xie, T. Wang, Y. Tang, D. Sun, W. Shi, Bipolar fatigueresistant behavior in ternary Bi0.5Na0.5TiO3-BaTiO3-SrTiO3 solid solutions, Scr. Mater. 83 (2014) 25–28. [55] J. Zang, M. Li, D.C. Sinclair, W. Jo, J. Rödel, S. Zhang, Impedance spectroscopy of (Bi1/2Na1/2)TiO3-BaTiO3 ceramics modified with (K0.5Na0.5)NbO3, J. Am. Ceram. Soc. 97 (2014) 1523–1529. [56] N. Kumar, E.A. Patterson, T. Frömling, D.P. Cann, Conduction mechanisms in BaTiO3-Bi(Zn1/2Ti1/2)O3 ceramics, J. Am. Ceram. Soc. 99 (2016) 3047–3054.

composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics, Nat. Commun. 6 (2015) 6615. X.-Y. Geng, J. Zhang, R.-X. Wang, X.-Z. Deng, L. Sun, Z.-B. Gu, S.-T. Zhang, ZnOenhanced electrical properties of Bi0.5Na0.5TiO3-based incipient ferroelectrics, J. Am. Ceram. Soc. 100 (2017) 5659–5667. L. Li, M. Zhu, K. Zhou, Q. Wei, M. Zheng, Y. Hou, Delayed thermal depolarization of Bi0.5Na0.5TiO3-BaTiO3 by doping acceptor Zn2+ with large ionic polarizability, J. Appl. Phys. 122 (2017) 204104. A. Mahajan, H. Zhang, J. Wu, E.V. Ramana, M.J. Reece, H. Yan, Effect of phase transitions on thermal depoling in lead-free 0.94(Bi0.5Na0.5TiO3)–0.06(BaTiO3) based piezoelectrics, J. Phys. Chem. C 121 (2017) 5709–5718. J. Zhang, Z. Pan, P.-X. Nie, Y.-S. Cui, B. Yang, J. Chen, S.-T. Zhang, Bi0.5Na0.5TiO3:zno lead-free piezoelectric composites with deferred thermal depolarization, Appl. Phys. Lett. 106 (2015) 232904. J. Koruza, P. Groszewicz, H. Breitzke, G. Buntkowsky, T. Rojac, B. Malič, Grain-sizeinduced ferroelectricity in NaNbO3, Acta Mater. 126 (2017) 77–85. D. Maurya, Y. Zhou, Y. Yan, S. Priya, Synthesis mechanism of grain-oriented leadfree piezoelectric Na0.5Bi0.5TiO3–BaTiO3 ceramics with giant piezoelectric response, J. Mater. Chem. C 1 (2013) 2102. C. Ma, H. Guo, S.P. Beckman, X. Tan, Creation and destruction of morphotropic phase boundaries through electrical poling: a case study of lead-free(Bi1/2Na1/ 2)TiO3-BaTiO3 piezoelectrics, Phys. Rev. Lett. 109 (2012) 107602. C. Ma, H. Guo, X. Tan, A new phase boundary in (Bi1/2Na1/2)TiO3−BaTiO3 revealed via a novel method of electron diffraction analysis, Adv. Funct. Mater. 23 (2013) 5261–5266. W. Bai, J. Xi, J. Zhang, B. Shen, J. Zhai, H. Yan, Effect of different templates on structure evolution and large strain response under a low electric field in < 00l > textured lead-free BNT-based piezoelectric ceramics, J. Eur. Ceram. Soc. 35 (2015) 2489–2499. Z. Yao, Z. Song, H. Hao, Z. Yu, M. Cao, S. Zhang, M.T. Lanagan, H. Liu, Homogeneous/inhomogeneous-structured dielectrics and their energy-storage performances, Adv. Mater. 29 (2017) 1601727. W.P. Cao, W.L. Li, Y. Feng, D. Xu, W. Wang, Y.F. Hou, T.D. Zhang, W.D. Fei, Enhanced depolarization temperature in 0.90NBT–0.05KBT–0.05BT ceramics induced by BT nanowires, J．Phys. Chem. Solids 78 (2015) 41–45. Q. Yao, F. Wang, F. Xu, C.M. Leung, T. Wang, Y. Tang, X. Ye, Y. Xie, D. Sun, W. Shi, Electric field-induced giant strain and photoluminescence-enhancement effect in rare-earth modified lead-free piezoelectric ceramics, ACS Appl. Mater. Inter. 7 (2015) 5066–5075. E. Aksel, J.S. Forrester, B. Kowalski, M. Deluca, D. Damjanovic, J.L. Jones, Structure and properties of Fe-modified Na0.5Bi0.5TiO3 at ambient and elevated temperature, Phys. Rev. B 85 (2012) 024121. D. Maurya, A. Pramanick, M. Feygenson, J.C. Neuefeind, R.J. Bodnar, S. Priya, Effect of poling on nanodomains and nanoscale structure in A-site disordered leadfree piezoelectric Na0.5Bi0.5TiO3-BaTiO3, J. Mater. Chem. C 2 (2014) 8423–8431. W. Bai, L. Li, W. Li, B. Shen, J. Zhai, H. Chen, Phase diagrams and electromechanical strains in lead‐free BNT‐based ternary perovskite compounds, J. Am. Ceram. Soc. 97 (2014) 3510–3518. F.-Z. Yao, Q. Yu, K. Wang, Q. Li, J.-F. Li, Ferroelectric domain morphology and temperature-dependent piezoelectricity of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics, RSC Adv. 39 (2014) 20062–20068. J.-S. Zhou, K. Wang, F.-Z. Yao, T. Zheng, J. Wu, D. Xiao, J. Zhu, J.-F. Li, Multi-scale thermal stability of niobate-based lead-free piezoceramics with large piezoelectricity, J. Mater. Chem. C 3 (2015) 8780–8787. F.-Z. Yao, K. Wang, Y. Shen, J.-F. Li, Robust CaZrO3-modified (K, Na)NbO3-based lead-free piezoceramics: high fatigue resistance insensitive to temperature and electric field, J. Appl. Phys. 118 (2015) 134102. M.-H. Zhang, K. Wang, J.-S. Zhou, J.-J. Zhou, X. Chu, X. Lv, J. Wu, J.-F. Li, Thermally stable piezoelectric properties of (K, Na)NbO3 -based lead-free perovskite with rhombohedral-tetragonal coexisting phase, Acta Mater. 122 (2017) 344–351. D. Maurya, A. Pramanick, K. An, S. Priya, Enhanced piezoelectricity and nature of electric-field induced structural phase transformation in textured lead-free

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