- Email: [email protected]
Structure evolution and enhanced piezoelectric properties of (K0.5Na0.5)NbO3–0.06LiTaO3–SrZrO3 lead-free ceramics
Journal of Alloys and Compounds 653 (2015) 523e527
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structure evolution and enhanced piezoelectric properties of (K0.5Na0.5)NbO3e0.06LiTaO3eSrZrO3 lead-free ceramics Xiaobin Yan a, *, Biaolin Peng b, Xuefeng Lu a, Qizheng Dong a, Wensheng Li a a
School of Materials Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China b Department of Applied Physics, the Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2015 Received in revised form 4 September 2015 Accepted 8 September 2015 Available online 10 September 2015
(K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) lead-free piezoelectric ceramics with rhombohedraletetragonal (ReT) phase boundary have been designed and prepared by the conventional solid-state reaction method, the phase transitional behavior and composition-dependent piezoelectric properties of KNNeLTexST lead-free system were investigated. The ReT phase boundary is identified in the KNNeLT exST ceramics with a composition of 0.04 x 0.06. The ceramics in the vicinity of the ReT phase coexistence zone exhibit a strong compositional dependence and significantly enhanced piezoelectric properties. A comprehensive performance of d33 (222e254 pC/N) and Tc (282e312 C) could be obtained in the KNNeLTexSZ ceramics with 0.04 x 0.06 by tailoring the SrZrO3 content. The ceramic with x ¼ 0.045 shows the optimum electrical properties: d33 ¼ 254 pC/N, kp ¼ 28%, εr ¼ 758, tand ¼ 1.7%, Pr ¼ 8.98 mC/cm2 and Tc ¼ 300 C. The existence of an ReT phase boundary and therefore the involvement of the more polarization states should be responsible for the enhanced piezoelectric properties of the KNNeLTexSZ ceramics. The good piezoelectric properties together with a high cubicetetragonal phase transition temperatures (Tc) make the KNNeLTexSZ ceramics showing the promising lead-free piezoelectric material system for the practical applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lead-free ceramics Phase transitions Piezoelectricity Rhombohedraletetragonal phase boundary
1. Introduction Piezoelectric ceramics as a functional material, which are widely used in actuators, sensors, transducers, and other electronic devices, have become essential for modern society, especially in the fields of information and communications, industrial automation, medical diagnostics, piezoelectric energy harvesting etc. [1e3]. Pb(Zr, Ti)O3 (PZT) ceramics are now widely used piezoelectric materials owing to their excellent piezoelectric performance. However, the high toxicity of the lead contained more than 60 wt.% in PZT-based ceramics will cause crucial environmental pollution during their processing and recycling. Therefore, the development of lead-free piezoelectric ceramics has become one of worldwide materials topics in recent years due to the restriction on use of lead based toxic materials in several counties and as they represent an environmentally friendly alternative to the PZT-based ceramics [4,5].
* Corresponding author. E-mail address: [email protected] (X. Yan). http://dx.doi.org/10.1016/j.jallcom.2015.09.055 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Over the past few years, extensive studies on lead-free materials, including the perovskites barium titanate BaTiO3 (BT)-, bismuth sodium titanate (Na0.5Bi0.5)TiO3 (NBT)-, potassium sodium niobate (K0.5Na0.5)NbO3 (KNN)-based ceramics and their various modified solid solutions, have been made [2,3]. Among them (K0.5Na0.5)NbO3 (KNN) has been considered a good candidate for lead-free piezoelectric ceramics because of its outstanding piezoelectric properties of textured (Li, Sb, Ta)-modified KNN ceramics was reported by Saito in 2004 [5]. After that, significant attention has been to the KNN based ceramics and extensive research by using chemical modification and processing optimization has been made [6,7]. In an effort to search for a high d33 in KNN, the phase boundary always plays a major and constructive role [8e18]. Due to the significantly enhanced polarizability in the proximity of multiple ferroelectric phases coexistence zone, such as a polymorphic phase transition (PPT) and a morphotropic phase boundary (MPB), which leads to maxima in dielectric and piezoelectric properties as in PZT, PMN-PT and Ba(Ti0.8Zr0.2) O3e(Ba0.7Ca0.3)TiO3 piezoelectric ceramics [19,20]. In this work, we designed a new lead-free material system of (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) prepared by
524
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
a conventional solid-state reaction method, and the rhombohedraletetragonal (ReT) phase boundary was constructed by tailoring SrZrO3 contents, attempting the enhanced piezoelectric properties and a high Tc were obtained in KNNeLTexST lead-free ceramics. Effects of SrZrO3 content on the phase structure, microstructure and electrical properties of KNNeLTexST ceramics were importantly investigated. At last, a large d33 of 254 pC/N and a high Tc of 300 C were simultaneously observed in the ceramics with x ¼ 0.045 by forming the ReT phase boundary, showing a promising candidate for the lead-free piezoelectric material system for the practical applications. 2. Experimental procedures The (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) leadfree ceramics, where x is the molar percent of SZ, were prepared by a conventional ceramic processing route, where the KNNeLT and ST powders were first synthesized respectively by a solid-state reaction method, the oxides or carbonates of the respective elements, namely Na2CO3 (99.8%), K2CO3 (99.0%), Li2CO3 (99.9%), Ta2O5 (99.99%), Nb2O5 (99.99%), SrCO3 (99%), and ZrO2 (99.95%) were used as the raw materials in this work. Specimens were sintered in air by covering with their powders at 1080e1200 C for 2 h. The electric poling was performed in a silicone oil bath by applying a dc electric field of 0.5e4 kV/mm at 80e150 C for 20 min. The crystal phase structure of the ceramics were characterized by X-ray diffraction (Bruker-AXS D5005, Siemens, Munich, Germany) using a CueKa radiation (l ¼ 1.5406 Å). The morphologies of the sintered ceramics were observed using FESEM (JSM-6701F, Japan Electron Co., Tokyo, Japan). Piezoelectric coefficient d33 values were measured by a quasi-static d33 m (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). Planar electromechanical coupling factor (kp) of the poled ceramics were measured by resonanceeantiresonance method using an impedance analyzer (HP 4294A, Palo Alto, CA). Temperature dependence of dielectric constant (εr) and dielectric loss (tand) of the poled ceramics at 103e106 Hz was measured using an LCR meter (WK P6505) from 200 C to 500 C. Ferroelectric hysteresis loops (P-E) were measured by a ferroelectric tester (RT66A; Radiant Technologies Inc., Albuquerque, NM). 3. Results and discussion The effect of SrZrO3 on phase structure of (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 lead-free ceramics was shown in Fig. 1, which indicates that all the samples exhibited the pure perovskite phase, demonstrating that SrZrO3 has completely dissolved in (K0.5Na0.5)NbO3e0.06LiTaO3 lattices to form a single homogeneous solid solution for all compositions in this work. Expanded XRD patterns were also conducted in the range of 2q from 21 to 23 and
Fig. 1. The XRD patterns of the KNNeLTexSZ ceramics in the 2q range of (a) 20 e60 , (b) 21 e23 , and (c) 55 e58 , respectively.
from 55 to 58 respectively for characterizing the phase structure evolution of (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 ceramics, as shown in Fig. 1(b) and (c) respectively. It can be seen that the ceramics with x < 0.04 shows tetragonal phase at room temperature characterized by the relative intensities of (001)/(100) diffraction peaks and the number of diffraction peaks near 57 [21]. As the SrZrO3 content further increases, the splitting of the peaks decreases and at last the splitting peaks merged into one peak correspondingly for the ceramics with x > 0.06, revealed a characteristic of rhombohedral phase were formed in the ceramics with x > 0.06 [9]. Accordingly, the coexistence of tetragonal and rhombohedral ferroelectric phases in the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics was identified in the composition range of 0.04 x < 0.06. A similar phenomenon has also been achieved with additions of BaZrO3-modified KNN ceramics [9,14,22]. Fig. 2 shows the variation in lattice parameters of the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics as a function of SrZrO3 content x. As can be seen that, in the tetragonal zone, the lattice constants c and a ¼ b gradually show close values with increasing x, and the c/a ratio, characterizing the tetragonality of the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics, was found to decrease with further increasing x and finally becomes unity for the ceramic with x ¼ 0.07, implying a TeR phase transition has accomplished. So a coexistence of tetragonal and rhombohedral ferroelectric phases in the (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 ceramics should exist approximately at 0.04 x < 0.06. The phase transition of KNNeLTexSZ ceramics with SrZrO3 modification is also confirmed by the temperature-dependent capacitance behavior in the measurement temperature from 200 C to 100 C as shown in Fig. 3. It can be seen that with SrZrO3 content increase, the capacitance peak was observed near room temperature for the samples with x ¼ 0.04e0.06, which confirmed the ferroelectriceferroelectric phase transition and therefore the coexistence of tetragonal and rhombohedral ferroelectric phases at room temperature shown by the XRD analysis and result for these samples. Similar phase transition behavior was also observed in BaZrO3 modified KNN-based ceramics [22]. To investigate the effect of the SrZrO3 on morphologies of KNNeLTexSZ lead-free ceramics, the SEM studies on the surface morphologies of the KNNeLTexSZ ceramics was carried out and the result was shown in Fig. 4. It can be clearly observed that the variation of the grains size and distribution was insignificant for the ceramic with x 0.06, while the grains became distinctly smaller and more uniform when the SrZrO3 content x > 0.06, means a certain amount of SrZrO3 doped in KNNeLT ceramic has an evident effect on grain size reduction. The result are similar to the previous
Fig. 2. Variation in lattice parameters of the KNNeLTexSZ ceramics as a function of SrZrO3 content x.
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
525
Fig. 3. Temperature dependence of the capacitance of KNNeLTexSZ ceramics measured at 10 KHz and in the temperature range of 200e100 C.
Fig. 4. SEM patterns of KNNeLTexSZ ceramics with x ¼ 0.02, x ¼ 0.04, x ¼ 0.06, x ¼ 0.08, respectively.
reports that the introduced certain amount of ABO3 perovskites component in KNN-based ceramics usually lead to grain refinement [10,22]. The dielectric permittivity (εr) versus temperature curves for the KNNeLTexSZ ceramics in the temperature range of room
temperature to 400 C at 10 KHz are shown in Fig. 5(a). The Curite temperature Tc almost linearly decreases with SrZrO3 content as seen from Fig. 5(a), but it is still above 248 C for the ceramic with x ¼ 0.08, indicating that the c/a ratio finally becomes unity for the ceramic with SrZrO3 content x > 0.06 is a ferroelectric
526
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
Fig. 5. (a) Temperature and (b) SZ content dependence of the dielectric constant (εr) of the KNNeLTexSZ ceramics measured at 10 kHz. The inset is the SZ content dependence of Tc of the KNNeLTexSZ ceramics.
rhombohedral phase (R) rather than a paraelectric cubic phase (C) at room temperature, moreover, a relative high Curite temperature Tc of 282 C ~ 312 C was obtained for the ceramics in the coexistence of tetragonal and rhombohedral phases zone with x ¼ 0.04e0.06. It is reported that the tolerance factor and the mass of the A ion all affect the ferroelectric phase transition temperatures of perovskite ABO3 experimentally, usually, ferroelectric phase transition is favored by a large tolerance factor and a large mass of the A ion, because SrZrO3 is a paraelectrics, when the SrZrO3 is introduced to (K0.5Na0.5)NbO3e0.06LiTaO3, according to Vegard's rule, the Tc will decreases, and since the atomic mass mSr > m(K0.5Na0.5Li0.06), therefore the above factors lead to a decrease in Tc of KNNeLTexSZ ceramics [23]. Fig. 5(b) shows the room temperature relative dielectric permittivity (εr) and loss (tand) of the KNNeLTexSZ ceramics as a function of SrZrO3 content x at 10 KHz. The relative dielectric permittivity εr increases with x increases and reaches a maximum value 901 at x ¼ 0.04, then slightly decreases when x smaller than 0.05, with x further increases the εr markedly increases when x 0.06. The enhancement of dielectric properties of the KNNeLTexSZ ceramics at x ¼ 0.04 is mainly attributed to the formation of the ReT phase boundary [1], a further increase in dielectric permittivity εr should be ascribed to the decrease of Tc of the KNNeLTexSZ ceramics with x 0.06 as BaZrO3 modified KNN-base ceramics [9,22]. While the dielectric loss tangent tand slightly decreases with x increases and keeps a relatively low value of 0.015e0.034. The dielectric loss tangent tand firstly decreases with x increased to 0.045, which maybe involved the more spontaneous polarization in the vicinity of the ReT phase boundary, the microstructures of fine grain and the decrease of spontaneous polarization are mainly responsible for the increase of
dielectric loss tangent tand of the ceramics with x > 0.06 [1,9]. Fig. 6 shows the P-E hysteresis loops of the KNNeLTexSZ ceramics as a function of SrZrO3 content x, measured at f ¼ 10 Hz and room temperature. It can be seen from Fig. 6(a) that all the samples show a typical P-E ferroelectric hysteresis loops. To accurately evaluate the effect of SrZrO3 content on the ferroelectric properties of the KNNeLTexSZ ceramics, the remnant polarization (Pr) and coercive field (Ec) as a function of SrZrO3 content x shown in Fig. 6(b). It can be found that a low concentration of SrZrO3 with x 0.045 will lead to the enhancement of ferroelectric property of the ceramics, the Pr increases with x increases and reaches a maximum value 8.98 mC/cm2 at x ¼ 0.045, then the Pr slightly decline when x increases, with x further increases from 0.05 to 0.07 the Pr increases and followed a decline with x increases from 0.07 to 0.08, suggesting a certain amount of SrZrO3 introduced in KNNeLTexSZ ceramic has an enhancement effect on the ferroelectricity of the ceramics, which related the involvement of the more polarization states formed in the ReT phase boundary [24]. The Pr of ceramic with x ¼ 0.08 drops, compared with ceramics x ¼ 0.06, the grain size distinctly smaller in the KNNeLTe0.08SZ ceramic, because the grain size has a close relationship with the ferroelectric properties of piezoelectric ceramics [25e27], accordingly, the smaller grain size degrade the ferroelectric properties of KNNeLTexSZ ceramics. In Fig. 6(b) the Ec decline gradually with x increase and decreases sharply at x ¼ 0.045, it is the reduced energy barriers for polarization rotation in the ReT phase coexistence zone and the no-90 C domains in the R ferroelectric phase zone that would be mainly responsible for the change of Ec [9,24]. Piezoelectric properties of poled KNNeLTexSZ ceramics as a function of SrZrO3 content x are shown in Fig. 7. It is shown that
Fig. 6. (a) P-E hysteresis loops of the KNNeLTexSZ ceramics and (b) the remnant polarization (Pr) and coercive field (Ec) of the KNNeLTexSZ ceramics.
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
527
properties together with a relative high cubicetetragonal phase transition temperatures (Tc) make the KNNeLTexSZ ceramics showing the promising lead-free piezoelectrics for the practical applications in different devices.
Acknowledgments
Fig. 7. Piezoelectric properties of KNNeLTexSZ ceramics as a function of SZ content x.
piezoelectric constant d33 and planar electromechanical coefficient kp increases with the increase of SrZrO3 content x, exhibit optimum values of d33 ¼ 254 pC/N and kp ¼ 28% at x ¼ 0.045 where the ReT ferroelectric phases coexist, and then it decline with further increase of x. The possible underlying physical mechanism for the enhanced piezoelectric properties should be related to the ReT phase boundary formed in the KNNeLTexSZ ceramics, it is the more polarization states formed in the ReT phase boundary that is beneficial to the rotation ferroelectric domains and therefore achievement of enhanced ferroelectricity of the ceramics, which proved by the enhanced remnant polarization (Pr) in the Fig. 6. Therefore, the construction of the ReT phase boundary in the KNNeLTexSZ ceramics should be mainly responsible for the enhanced piezoelectric properties in this work. 4. Conclusions (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexSZ) lead-free ceramics with good piezoelectric properties have been developed by the normal sintering. The compositional dependence of phase structure and electrical properties of the ceramics was studies. It was found that the tetragonalerhombohedral phase coexistence boundary in the composition range of 0.04 x 0.06 of the KNNeLTexSZ ceramics has been identified at room temperature and confirmed by the XRD patterns. A comprehensive performance of d33 (222e254 pC/N) and Tc (282e312 C) could be obtained in the KNNeLTexSZ ceramics with 0.04 x 0.06 by tailoring the SrZrO3 content. The ceramic with x ¼ 0.045 shows the optimum electrical properties: d33 ¼ 254 pC/N, kp ¼ 28%, εr ¼ 758, tand ¼ 1.7%, Pr ¼ 8.98 mC/cm2 and Tc ¼ 300 C. The existence of a TeR phase boundary and therefore the involvement of the more polarization states should be responsible for the enhanced piezoelectric properties of the KNNeLTexSZ ceramics. The good piezoelectric
Authors gratefully acknowledge the supports of Lanzhou University of Technology, China. This work was supported by the National Natural Science Foundation of China (Grants 51402196, 51402142, 21301084), the China Postdoctoral Science Foundation (Grant 2014M552229, 2015M572615), the Gansu Provincial Youth Science and Technology Fund Projects (1310RJYA006), the Program for Hongliu Young Teachers in Lanzhou University of Technology (Q201401) and Key Laboratory Open Fund in Lanzhou University of Technology.
References [1] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 111e124. €del, W. Jo, K.T.P. Seifert, E. Anton, T. Granzow, J. Am. Ceram. Soc. 92 (2009) [2] J. Ro 1153e1177. [3] P.K. Panda, B. Sahoo, Ferroelectrics 474 (2015) 128e143. [4] E. Cross, Nature 432 (2004) 24e25. [5] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84e87. [6] J.F. Li, K. Wang, F.Y. Zhu, L.Q. Cheng, F.Z. Yao, J. Am. Ceram. Soc. 96 (2013) 3677e3696. [7] J.G. Wu, D.Q. Xiao, J.G. Zhu, Chem. Rev. 115 (2015) 2559e2595. [8] D. Lin, K.W. Kwok, H.W.L. Chan, Appl. Phys. Lett. 91 (2007), 143513-1-3. [9] R.Z. Zuo, J. Fu, J. Am. Ceram. Soc. 94 (2011) 1467e1470. [10] X.B. Yan, F. Gao, Z.T. Liu, J. Mater. Sci. Mater. Electron. 25 (2014) 5576e5580. [11] K. Wang, F.Z. Yao, W. Jo, D. Gobeljic, V.V. Shvartsman, D.C. Lupascu, J.F. Li, € del, Adv. Funct. Mater. 23 (2013) 4079e4086. J. Ro [12] J.G. Wu, D.Q. Xiao, J. Appl. Phys. 102 (2007), 114113-1-5. [13] Y. Guo, K. Kakimoto, H. Ohsato, Mater. Lett. 59 (2005) 241e244. [14] B. Zhang, J. Wu, X. Cheng, X. Wang, D. Xiao, J. Zhu, X. Wang, X. Lou, ACS Appl. Mater. Interfaces 5 (2013) 7718e7725. [15] T. Zheng, Y. Zu, J. Wu, J. Alloys Compd. 647 (2015) 927e934. [16] H.T. Li, Q. Cao, F. Wang, M.H. Zhang, Q. Yu, R.Y. Dong, J. Alloys Compd. 634 (2015) 163e167. [17] X.P. Wang, J.G. Wu, D.Q. Xiao, J.G. Zhu, X.J. Cheng, T. Zheng, B.Y. Zhang, X.J. Lou, X.J. Wang, J. Am. Chem. Soc. 136 (2014) 2905e2910. [18] T. Zheng, J.G. Wu, D.Q. Xiao, J.G. Zhu, Scr. Mater. 94 (2015) 25e27. [19] D.E. Cox, B. Noheda, G. Shirane, Y. Uesu, K. Fujishiro, Y. Yamada, Appl. Phys. Lett. 79 (2001), 400-1-3. [20] W.F. Liu, X.B. Ren, Phys. Rev. Lett. 103 (2009), 257602-1-4. [21] K. Wang, J.F. Li, Appl. Phys. Lett. 91 (2007), 262902-1-3. [22] W.F. Liang, W.J. Wu, D.Q. Xiao, J.G. Zhu, J.G. Wu, J. Mater. Sci. 46 (2011) 6871e6876. [23] R. Wang, H. Bando, M. Kidate, Y. Nishihara, M. Itoh, Jpn. J. Appl. Phys. 50 (2011), 09ND10-1-5. [24] D. Damjanovic, J. Am. Ceram. Soc. 88 (2005) 2663e2676. [25] K. Okazaki, K. Nagata, J. Am. Ceram. Soc. 56 (1973) 82e86. [26] X.B. Yan, F. Gao, B.L. Peng, Z.T. Liu, J.Mater. Sci. Mater. Electron. 25 (2014) 2220e2226. [27] R.Z. Zuo, J. Fu, D.Y. Lv, Y.J. Liu, J. Am. Ceram. Soc. 93 (2010) 2783e2787.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structure evolution and enhanced piezoelectric properties of (K0.5Na0.5)NbO3e0.06LiTaO3eSrZrO3 lead-free ceramics Xiaobin Yan a, *, Biaolin Peng b, Xuefeng Lu a, Qizheng Dong a, Wensheng Li a a
School of Materials Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China b Department of Applied Physics, the Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2015 Received in revised form 4 September 2015 Accepted 8 September 2015 Available online 10 September 2015
(K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) lead-free piezoelectric ceramics with rhombohedraletetragonal (ReT) phase boundary have been designed and prepared by the conventional solid-state reaction method, the phase transitional behavior and composition-dependent piezoelectric properties of KNNeLTexST lead-free system were investigated. The ReT phase boundary is identified in the KNNeLT exST ceramics with a composition of 0.04 x 0.06. The ceramics in the vicinity of the ReT phase coexistence zone exhibit a strong compositional dependence and significantly enhanced piezoelectric properties. A comprehensive performance of d33 (222e254 pC/N) and Tc (282e312 C) could be obtained in the KNNeLTexSZ ceramics with 0.04 x 0.06 by tailoring the SrZrO3 content. The ceramic with x ¼ 0.045 shows the optimum electrical properties: d33 ¼ 254 pC/N, kp ¼ 28%, εr ¼ 758, tand ¼ 1.7%, Pr ¼ 8.98 mC/cm2 and Tc ¼ 300 C. The existence of an ReT phase boundary and therefore the involvement of the more polarization states should be responsible for the enhanced piezoelectric properties of the KNNeLTexSZ ceramics. The good piezoelectric properties together with a high cubicetetragonal phase transition temperatures (Tc) make the KNNeLTexSZ ceramics showing the promising lead-free piezoelectric material system for the practical applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lead-free ceramics Phase transitions Piezoelectricity Rhombohedraletetragonal phase boundary
1. Introduction Piezoelectric ceramics as a functional material, which are widely used in actuators, sensors, transducers, and other electronic devices, have become essential for modern society, especially in the fields of information and communications, industrial automation, medical diagnostics, piezoelectric energy harvesting etc. [1e3]. Pb(Zr, Ti)O3 (PZT) ceramics are now widely used piezoelectric materials owing to their excellent piezoelectric performance. However, the high toxicity of the lead contained more than 60 wt.% in PZT-based ceramics will cause crucial environmental pollution during their processing and recycling. Therefore, the development of lead-free piezoelectric ceramics has become one of worldwide materials topics in recent years due to the restriction on use of lead based toxic materials in several counties and as they represent an environmentally friendly alternative to the PZT-based ceramics [4,5].
* Corresponding author. E-mail address: [email protected] (X. Yan). http://dx.doi.org/10.1016/j.jallcom.2015.09.055 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Over the past few years, extensive studies on lead-free materials, including the perovskites barium titanate BaTiO3 (BT)-, bismuth sodium titanate (Na0.5Bi0.5)TiO3 (NBT)-, potassium sodium niobate (K0.5Na0.5)NbO3 (KNN)-based ceramics and their various modified solid solutions, have been made [2,3]. Among them (K0.5Na0.5)NbO3 (KNN) has been considered a good candidate for lead-free piezoelectric ceramics because of its outstanding piezoelectric properties of textured (Li, Sb, Ta)-modified KNN ceramics was reported by Saito in 2004 [5]. After that, significant attention has been to the KNN based ceramics and extensive research by using chemical modification and processing optimization has been made [6,7]. In an effort to search for a high d33 in KNN, the phase boundary always plays a major and constructive role [8e18]. Due to the significantly enhanced polarizability in the proximity of multiple ferroelectric phases coexistence zone, such as a polymorphic phase transition (PPT) and a morphotropic phase boundary (MPB), which leads to maxima in dielectric and piezoelectric properties as in PZT, PMN-PT and Ba(Ti0.8Zr0.2) O3e(Ba0.7Ca0.3)TiO3 piezoelectric ceramics [19,20]. In this work, we designed a new lead-free material system of (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) prepared by
524
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
a conventional solid-state reaction method, and the rhombohedraletetragonal (ReT) phase boundary was constructed by tailoring SrZrO3 contents, attempting the enhanced piezoelectric properties and a high Tc were obtained in KNNeLTexST lead-free ceramics. Effects of SrZrO3 content on the phase structure, microstructure and electrical properties of KNNeLTexST ceramics were importantly investigated. At last, a large d33 of 254 pC/N and a high Tc of 300 C were simultaneously observed in the ceramics with x ¼ 0.045 by forming the ReT phase boundary, showing a promising candidate for the lead-free piezoelectric material system for the practical applications. 2. Experimental procedures The (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexST) leadfree ceramics, where x is the molar percent of SZ, were prepared by a conventional ceramic processing route, where the KNNeLT and ST powders were first synthesized respectively by a solid-state reaction method, the oxides or carbonates of the respective elements, namely Na2CO3 (99.8%), K2CO3 (99.0%), Li2CO3 (99.9%), Ta2O5 (99.99%), Nb2O5 (99.99%), SrCO3 (99%), and ZrO2 (99.95%) were used as the raw materials in this work. Specimens were sintered in air by covering with their powders at 1080e1200 C for 2 h. The electric poling was performed in a silicone oil bath by applying a dc electric field of 0.5e4 kV/mm at 80e150 C for 20 min. The crystal phase structure of the ceramics were characterized by X-ray diffraction (Bruker-AXS D5005, Siemens, Munich, Germany) using a CueKa radiation (l ¼ 1.5406 Å). The morphologies of the sintered ceramics were observed using FESEM (JSM-6701F, Japan Electron Co., Tokyo, Japan). Piezoelectric coefficient d33 values were measured by a quasi-static d33 m (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). Planar electromechanical coupling factor (kp) of the poled ceramics were measured by resonanceeantiresonance method using an impedance analyzer (HP 4294A, Palo Alto, CA). Temperature dependence of dielectric constant (εr) and dielectric loss (tand) of the poled ceramics at 103e106 Hz was measured using an LCR meter (WK P6505) from 200 C to 500 C. Ferroelectric hysteresis loops (P-E) were measured by a ferroelectric tester (RT66A; Radiant Technologies Inc., Albuquerque, NM). 3. Results and discussion The effect of SrZrO3 on phase structure of (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 lead-free ceramics was shown in Fig. 1, which indicates that all the samples exhibited the pure perovskite phase, demonstrating that SrZrO3 has completely dissolved in (K0.5Na0.5)NbO3e0.06LiTaO3 lattices to form a single homogeneous solid solution for all compositions in this work. Expanded XRD patterns were also conducted in the range of 2q from 21 to 23 and
Fig. 1. The XRD patterns of the KNNeLTexSZ ceramics in the 2q range of (a) 20 e60 , (b) 21 e23 , and (c) 55 e58 , respectively.
from 55 to 58 respectively for characterizing the phase structure evolution of (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 ceramics, as shown in Fig. 1(b) and (c) respectively. It can be seen that the ceramics with x < 0.04 shows tetragonal phase at room temperature characterized by the relative intensities of (001)/(100) diffraction peaks and the number of diffraction peaks near 57 [21]. As the SrZrO3 content further increases, the splitting of the peaks decreases and at last the splitting peaks merged into one peak correspondingly for the ceramics with x > 0.06, revealed a characteristic of rhombohedral phase were formed in the ceramics with x > 0.06 [9]. Accordingly, the coexistence of tetragonal and rhombohedral ferroelectric phases in the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics was identified in the composition range of 0.04 x < 0.06. A similar phenomenon has also been achieved with additions of BaZrO3-modified KNN ceramics [9,14,22]. Fig. 2 shows the variation in lattice parameters of the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics as a function of SrZrO3 content x. As can be seen that, in the tetragonal zone, the lattice constants c and a ¼ b gradually show close values with increasing x, and the c/a ratio, characterizing the tetragonality of the (K0.5Na0.5) NbO3e0.06LiTaO3exSrZrO3 ceramics, was found to decrease with further increasing x and finally becomes unity for the ceramic with x ¼ 0.07, implying a TeR phase transition has accomplished. So a coexistence of tetragonal and rhombohedral ferroelectric phases in the (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 ceramics should exist approximately at 0.04 x < 0.06. The phase transition of KNNeLTexSZ ceramics with SrZrO3 modification is also confirmed by the temperature-dependent capacitance behavior in the measurement temperature from 200 C to 100 C as shown in Fig. 3. It can be seen that with SrZrO3 content increase, the capacitance peak was observed near room temperature for the samples with x ¼ 0.04e0.06, which confirmed the ferroelectriceferroelectric phase transition and therefore the coexistence of tetragonal and rhombohedral ferroelectric phases at room temperature shown by the XRD analysis and result for these samples. Similar phase transition behavior was also observed in BaZrO3 modified KNN-based ceramics [22]. To investigate the effect of the SrZrO3 on morphologies of KNNeLTexSZ lead-free ceramics, the SEM studies on the surface morphologies of the KNNeLTexSZ ceramics was carried out and the result was shown in Fig. 4. It can be clearly observed that the variation of the grains size and distribution was insignificant for the ceramic with x 0.06, while the grains became distinctly smaller and more uniform when the SrZrO3 content x > 0.06, means a certain amount of SrZrO3 doped in KNNeLT ceramic has an evident effect on grain size reduction. The result are similar to the previous
Fig. 2. Variation in lattice parameters of the KNNeLTexSZ ceramics as a function of SrZrO3 content x.
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
525
Fig. 3. Temperature dependence of the capacitance of KNNeLTexSZ ceramics measured at 10 KHz and in the temperature range of 200e100 C.
Fig. 4. SEM patterns of KNNeLTexSZ ceramics with x ¼ 0.02, x ¼ 0.04, x ¼ 0.06, x ¼ 0.08, respectively.
reports that the introduced certain amount of ABO3 perovskites component in KNN-based ceramics usually lead to grain refinement [10,22]. The dielectric permittivity (εr) versus temperature curves for the KNNeLTexSZ ceramics in the temperature range of room
temperature to 400 C at 10 KHz are shown in Fig. 5(a). The Curite temperature Tc almost linearly decreases with SrZrO3 content as seen from Fig. 5(a), but it is still above 248 C for the ceramic with x ¼ 0.08, indicating that the c/a ratio finally becomes unity for the ceramic with SrZrO3 content x > 0.06 is a ferroelectric
526
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
Fig. 5. (a) Temperature and (b) SZ content dependence of the dielectric constant (εr) of the KNNeLTexSZ ceramics measured at 10 kHz. The inset is the SZ content dependence of Tc of the KNNeLTexSZ ceramics.
rhombohedral phase (R) rather than a paraelectric cubic phase (C) at room temperature, moreover, a relative high Curite temperature Tc of 282 C ~ 312 C was obtained for the ceramics in the coexistence of tetragonal and rhombohedral phases zone with x ¼ 0.04e0.06. It is reported that the tolerance factor and the mass of the A ion all affect the ferroelectric phase transition temperatures of perovskite ABO3 experimentally, usually, ferroelectric phase transition is favored by a large tolerance factor and a large mass of the A ion, because SrZrO3 is a paraelectrics, when the SrZrO3 is introduced to (K0.5Na0.5)NbO3e0.06LiTaO3, according to Vegard's rule, the Tc will decreases, and since the atomic mass mSr > m(K0.5Na0.5Li0.06), therefore the above factors lead to a decrease in Tc of KNNeLTexSZ ceramics [23]. Fig. 5(b) shows the room temperature relative dielectric permittivity (εr) and loss (tand) of the KNNeLTexSZ ceramics as a function of SrZrO3 content x at 10 KHz. The relative dielectric permittivity εr increases with x increases and reaches a maximum value 901 at x ¼ 0.04, then slightly decreases when x smaller than 0.05, with x further increases the εr markedly increases when x 0.06. The enhancement of dielectric properties of the KNNeLTexSZ ceramics at x ¼ 0.04 is mainly attributed to the formation of the ReT phase boundary [1], a further increase in dielectric permittivity εr should be ascribed to the decrease of Tc of the KNNeLTexSZ ceramics with x 0.06 as BaZrO3 modified KNN-base ceramics [9,22]. While the dielectric loss tangent tand slightly decreases with x increases and keeps a relatively low value of 0.015e0.034. The dielectric loss tangent tand firstly decreases with x increased to 0.045, which maybe involved the more spontaneous polarization in the vicinity of the ReT phase boundary, the microstructures of fine grain and the decrease of spontaneous polarization are mainly responsible for the increase of
dielectric loss tangent tand of the ceramics with x > 0.06 [1,9]. Fig. 6 shows the P-E hysteresis loops of the KNNeLTexSZ ceramics as a function of SrZrO3 content x, measured at f ¼ 10 Hz and room temperature. It can be seen from Fig. 6(a) that all the samples show a typical P-E ferroelectric hysteresis loops. To accurately evaluate the effect of SrZrO3 content on the ferroelectric properties of the KNNeLTexSZ ceramics, the remnant polarization (Pr) and coercive field (Ec) as a function of SrZrO3 content x shown in Fig. 6(b). It can be found that a low concentration of SrZrO3 with x 0.045 will lead to the enhancement of ferroelectric property of the ceramics, the Pr increases with x increases and reaches a maximum value 8.98 mC/cm2 at x ¼ 0.045, then the Pr slightly decline when x increases, with x further increases from 0.05 to 0.07 the Pr increases and followed a decline with x increases from 0.07 to 0.08, suggesting a certain amount of SrZrO3 introduced in KNNeLTexSZ ceramic has an enhancement effect on the ferroelectricity of the ceramics, which related the involvement of the more polarization states formed in the ReT phase boundary [24]. The Pr of ceramic with x ¼ 0.08 drops, compared with ceramics x ¼ 0.06, the grain size distinctly smaller in the KNNeLTe0.08SZ ceramic, because the grain size has a close relationship with the ferroelectric properties of piezoelectric ceramics [25e27], accordingly, the smaller grain size degrade the ferroelectric properties of KNNeLTexSZ ceramics. In Fig. 6(b) the Ec decline gradually with x increase and decreases sharply at x ¼ 0.045, it is the reduced energy barriers for polarization rotation in the ReT phase coexistence zone and the no-90 C domains in the R ferroelectric phase zone that would be mainly responsible for the change of Ec [9,24]. Piezoelectric properties of poled KNNeLTexSZ ceramics as a function of SrZrO3 content x are shown in Fig. 7. It is shown that
Fig. 6. (a) P-E hysteresis loops of the KNNeLTexSZ ceramics and (b) the remnant polarization (Pr) and coercive field (Ec) of the KNNeLTexSZ ceramics.
X. Yan et al. / Journal of Alloys and Compounds 653 (2015) 523e527
527
properties together with a relative high cubicetetragonal phase transition temperatures (Tc) make the KNNeLTexSZ ceramics showing the promising lead-free piezoelectrics for the practical applications in different devices.
Acknowledgments
Fig. 7. Piezoelectric properties of KNNeLTexSZ ceramics as a function of SZ content x.
piezoelectric constant d33 and planar electromechanical coefficient kp increases with the increase of SrZrO3 content x, exhibit optimum values of d33 ¼ 254 pC/N and kp ¼ 28% at x ¼ 0.045 where the ReT ferroelectric phases coexist, and then it decline with further increase of x. The possible underlying physical mechanism for the enhanced piezoelectric properties should be related to the ReT phase boundary formed in the KNNeLTexSZ ceramics, it is the more polarization states formed in the ReT phase boundary that is beneficial to the rotation ferroelectric domains and therefore achievement of enhanced ferroelectricity of the ceramics, which proved by the enhanced remnant polarization (Pr) in the Fig. 6. Therefore, the construction of the ReT phase boundary in the KNNeLTexSZ ceramics should be mainly responsible for the enhanced piezoelectric properties in this work. 4. Conclusions (K0.5Na0.5)NbO3e0.06LiTaO3exSrZrO3 (KNNeLTexSZ) lead-free ceramics with good piezoelectric properties have been developed by the normal sintering. The compositional dependence of phase structure and electrical properties of the ceramics was studies. It was found that the tetragonalerhombohedral phase coexistence boundary in the composition range of 0.04 x 0.06 of the KNNeLTexSZ ceramics has been identified at room temperature and confirmed by the XRD patterns. A comprehensive performance of d33 (222e254 pC/N) and Tc (282e312 C) could be obtained in the KNNeLTexSZ ceramics with 0.04 x 0.06 by tailoring the SrZrO3 content. The ceramic with x ¼ 0.045 shows the optimum electrical properties: d33 ¼ 254 pC/N, kp ¼ 28%, εr ¼ 758, tand ¼ 1.7%, Pr ¼ 8.98 mC/cm2 and Tc ¼ 300 C. The existence of a TeR phase boundary and therefore the involvement of the more polarization states should be responsible for the enhanced piezoelectric properties of the KNNeLTexSZ ceramics. The good piezoelectric
Authors gratefully acknowledge the supports of Lanzhou University of Technology, China. This work was supported by the National Natural Science Foundation of China (Grants 51402196, 51402142, 21301084), the China Postdoctoral Science Foundation (Grant 2014M552229, 2015M572615), the Gansu Provincial Youth Science and Technology Fund Projects (1310RJYA006), the Program for Hongliu Young Teachers in Lanzhou University of Technology (Q201401) and Key Laboratory Open Fund in Lanzhou University of Technology.
References [1] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 111e124. €del, W. Jo, K.T.P. Seifert, E. Anton, T. Granzow, J. Am. Ceram. Soc. 92 (2009) [2] J. Ro 1153e1177. [3] P.K. Panda, B. Sahoo, Ferroelectrics 474 (2015) 128e143. [4] E. Cross, Nature 432 (2004) 24e25. [5] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84e87. [6] J.F. Li, K. Wang, F.Y. Zhu, L.Q. Cheng, F.Z. Yao, J. Am. Ceram. Soc. 96 (2013) 3677e3696. [7] J.G. Wu, D.Q. Xiao, J.G. Zhu, Chem. Rev. 115 (2015) 2559e2595. [8] D. Lin, K.W. Kwok, H.W.L. Chan, Appl. Phys. Lett. 91 (2007), 143513-1-3. [9] R.Z. Zuo, J. Fu, J. Am. Ceram. Soc. 94 (2011) 1467e1470. [10] X.B. Yan, F. Gao, Z.T. Liu, J. Mater. Sci. Mater. Electron. 25 (2014) 5576e5580. [11] K. Wang, F.Z. Yao, W. Jo, D. Gobeljic, V.V. Shvartsman, D.C. Lupascu, J.F. Li, € del, Adv. Funct. Mater. 23 (2013) 4079e4086. J. Ro [12] J.G. Wu, D.Q. Xiao, J. Appl. Phys. 102 (2007), 114113-1-5. [13] Y. Guo, K. Kakimoto, H. Ohsato, Mater. Lett. 59 (2005) 241e244. [14] B. Zhang, J. Wu, X. Cheng, X. Wang, D. Xiao, J. Zhu, X. Wang, X. Lou, ACS Appl. Mater. Interfaces 5 (2013) 7718e7725. [15] T. Zheng, Y. Zu, J. Wu, J. Alloys Compd. 647 (2015) 927e934. [16] H.T. Li, Q. Cao, F. Wang, M.H. Zhang, Q. Yu, R.Y. Dong, J. Alloys Compd. 634 (2015) 163e167. [17] X.P. Wang, J.G. Wu, D.Q. Xiao, J.G. Zhu, X.J. Cheng, T. Zheng, B.Y. Zhang, X.J. Lou, X.J. Wang, J. Am. Chem. Soc. 136 (2014) 2905e2910. [18] T. Zheng, J.G. Wu, D.Q. Xiao, J.G. Zhu, Scr. Mater. 94 (2015) 25e27. [19] D.E. Cox, B. Noheda, G. Shirane, Y. Uesu, K. Fujishiro, Y. Yamada, Appl. Phys. Lett. 79 (2001), 400-1-3. [20] W.F. Liu, X.B. Ren, Phys. Rev. Lett. 103 (2009), 257602-1-4. [21] K. Wang, J.F. Li, Appl. Phys. Lett. 91 (2007), 262902-1-3. [22] W.F. Liang, W.J. Wu, D.Q. Xiao, J.G. Zhu, J.G. Wu, J. Mater. Sci. 46 (2011) 6871e6876. [23] R. Wang, H. Bando, M. Kidate, Y. Nishihara, M. Itoh, Jpn. J. Appl. Phys. 50 (2011), 09ND10-1-5. [24] D. Damjanovic, J. Am. Ceram. Soc. 88 (2005) 2663e2676. [25] K. Okazaki, K. Nagata, J. Am. Ceram. Soc. 56 (1973) 82e86. [26] X.B. Yan, F. Gao, B.L. Peng, Z.T. Liu, J.Mater. Sci. Mater. Electron. 25 (2014) 2220e2226. [27] R.Z. Zuo, J. Fu, D.Y. Lv, Y.J. Liu, J. Am. Ceram. Soc. 93 (2010) 2783e2787.