- Email: [email protected]
Effect of grain size on the phase structure and electrical properties of PZT–PNZN quaternary systems
Journal of Alloys and Compounds 617 (2014) 222–227
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of grain size on the phase structure and electrical properties of PZT–PNZN quaternary systems Zhirong Ai, Yudong Hou ⇑, Mupeng Zheng, Mankang Zhu College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 25 June 2014 Received in revised form 23 July 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: Piezoelectric ceramics Sintering temperature Phase stability Grain size
a b s t r a c t The quaternary systems 0.74Pb(Zr0.47Ti0.53)O3–0.26Pb[(Ni0.6Zn0.4)1/3Nb2/3]O3(PZT–PNZN) have been prepared using the conventional oxide mixing method in a wide temperature region between 1050 and 1250 °C. All specimens present high relative density above 95%, indicating a wide sintering window for this system. Investigation of the microstructure reveals a continuous increase in grain size from 1.07 to 2.77 lm when the sintering temperature is increased from 1050 to 1250 °C, which obeys the common grain-growth law well. However, unlike compositional component PZN–PZT and PNN–PZT, the phase structure of quaternary PZN–PNZN is not sensitive to the grain size. The specimens with different grain size retain the morphotropic phase boundary (MPB) structure and the size induced phase transition does not appear. Excluding the influence of the phase structure, the size effect plays a dominant role in improving the piezoelectric properties, which facilitates the motion of domain walls due to the decreasing number of grain boundary. The optimum piezoelectric parameters are obtained at 1150 °C: d33 = 307 pC/N, kp = 0.56. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Relaxor ferroelectric materials with compositions near morphotropic phase boundary (MPB) exhibit anomalously high piezoelectric and dielectric properties, which open a wide application field for the use in multilayer capacitors, piezoelectric transducer and electrostrictive actuators [1]. By introducing various dopants or adding other component to the relaxor ferroelectrics, piezoelectric and dielectric properties can be tailored to a wide extent [2–4]. On the other hand, adjusting the process parameters such as sintering temperature is also an effective means of materials modification [5,6]. As we known, sintering temperature is an important process parameter to endow ferroelectric polycrystals with proper microscopic structure. Wagner et al. made a detailed study on the variation of the grain size, phase composition, and electric properties with sintering temperature in the relaxor ferroelectric system Pb(Zr, Ti)O3–Pb(Ni1/3Nb2/3)O3 (PZT–PNN) [7]. They have found that increasing the sintering temperature leads to grain growth, which in turn increases the tetragonal phase content significantly, although the composition of the initial PZT–PNN powder was kept constant. Samples with a tetragonal phase content of 70% and a corresponding grain size of 1.5–2.5 lm show good piezoelectric
⇑ Corresponding author. E-mail address: [email protected] (Y. Hou). http://dx.doi.org/10.1016/j.jallcom.2014.07.181 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
properties. Later, Chang et al. investigated the phase transition and dielectrical response as a function of sintering temperature in Pb(Zr, Ti)O3–Pb(Zn1/3Nb2/3)O3 (PZT–PZN) [8]. They reported that the increasing sintering temperature not only induced the increase of grain size and tetragonal phase content, but also caused the weakening of the dielectric relaxor behavior. The latter can be attributed to that increasing the sintering temperature gives rise to compositional fluctuation and could stabilize the negatively charged ordered microdomains, leading to the degree of the short-range B-site order to increase. Considering the similar relaxor ferroelectric behaviors of PZT–PNN and PZT–PZN, a quaternary PZT–PNZN solid solution, the combination of the two ternary compositions, has been constructed and some related research has been reported. Chao et al. prepared piezoelectric ceramics bimorph using phase-pure PZT–PNZN ceramics with Zr/Ti ratio = 0.985, which exhibits a large displacement and meets the needs of the needle selecting mechanism of electronic jacquard [9]. Liu et al. have found that due to high Curie temperature and small grain size, the PZT–PNZN ceramics with MPB composition maintained a high d33 level after depoling treatment, revealing a superior strain capacity for high-temperature application [10]. Recently, Sahn Nahm et al. have conducted series research on PZT–PNZN systems for energy harvesting device application [11,12]. They found that through tailoring the microstructure and phase composition, the transduction
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
coefficient d33 g33 can be optimized to a high value, which provides high energy density materials for application to energy harvesting devices. During previous research one important question was raised: how does the sintering condition influence the properties of the quaternary PZT–PNZN ceramics and is it similar to the variation trend of ternary PZT–PNN and PZT–PZN? In the present paper, as an extension to the intrinsic study on the PZT–PNZN quaternary system, sintering temperature was selected to modulate the microstructure and the electrical properties of the PZT–PNZN system. Emphasis was placed on the grain size dependences of phase structure, ferroelectric and piezoelectric properties. 2. Experimental procedure The specimens were prepared using a conventional mixed-oxide process. The compositions used in this study are as follows: 0.74Pb(Zr0.47Ti0.53)O3–0.26Pb[(Ni0.6 Zn0.4)1/3Nb2/3]O3 (PZT–PNZN). Reagent-grade oxide powders (Pb3O4, ZrO2, TiO2, ZnO, NiO, and Nb2O5) were used as starting materials. The powders were weighed and mixed through ball milling, with partially stabilized zirconia balls as media, in alcohol for 12 h. After drying, the mixture was calcined in a covered alumina crucible at 850 °C for 2 h. The calcined powders were remilled for 12 h and then pressed into disks of 11.5 mm in diameter at around 100 MPa. The green disks were sintered at 50 °C temperature intervals between 1050 and 1250 °C for 2 h in a sealed alumina crucible. To minimize PbO loss, a PbO-rich atmosphere was maintained by placing powders of PbZrO3 inside the crucible used as packing powders. The densities of the sintered specimens were measured by a water-immersion method using Archimedes-principle. The crystal structures of the samples were examined by XRD (Bruker D8 Advance, Karlsruhe, Germany) in the h–2h configuration using Cu Ka radiation. Raman scattering spectra were recorded at room temperature from a Raman spectrometer (Model T64000; Jobin–Yvon, Paris, France) under back scattering geometry. Excitation was taken as the 488 nm line of an Ar+ laser with a 50 mW output power. Micromorphology was detected on a thermally etched surface by SEM (Hitachi S4800, Japan) and the mean grain size was calculated by the line intercept method. To measure the electrical properties, silver paste was printed on the lapped surfaces of the sintered pellets and then fired at 560 °C for 30 min to form electrodes. The dielectric property was measured using a multi-frequency inductance capacitance resistance (LCR) analyzer (Agilent E4980A, Santa Clara, CA). Impedance analysis was conducted using impedance analyzer (Novocontrol Technologies, Hundsangen, Germany) at 300 °C in the frequency range from 1 MHz to 10 MHz. Ferroelectric behavior was studied using a ferroelectric tester (Premier II, Radiant Technologies Inc, Albuquerque, NM) at 1 Hz before poling treatment. Prior to the testing of piezoelectric properties, the specimens were poled in a silicone oil bath at 120 °C by applying a DC field of 30 kV/cm for 30 min and then aged for 24 h. The piezoelectric constant d33 was measured by a piezoelectric d33 meter (ZJ-6A, Institute of Acoustics, Academic Sinica, China) at 100 Hz. The electromechanical coupling factor kp was determined by a precision impedance analyzer (4294A; Agilent Technologies, Santa Clara, CA) through the resonance-anti-resonance method based on IEEE standards.
3. Results and discussion 3.1. Microstructure evolution Fig. 1 presents the SEM micrographs of five samples sintered between 1050 and 1250 °C for 2 h. All specimens retain a high relative density above 95% and show a homogenous grain-size distribution, thus demonstrating that PZT–PNZN system has a wide sintering window. As expected, grain coarsening is observed with increasing sintering temperature. The quantitative grain-size analysis shows a significant increase of the mean grain size from d50 = 1.07 lm at 1050 °C to d50 = 2.77 lm at 1250 °C. These results, shown in Fig. 1(f), could be successfully fitted by using a common grain-growth law, considering non-isothermal kinetics [7,13]:
Q n n t d d0 ¼ k exp RT
ð1Þ
In this equation, n is the grain growth exponent, T is the sintering temperature, t is the sintering time, d is the grain size at time t and temperature T, d0 is the initial grain size, Q is the activation energy, and k is a grain-growth constant. The fit procedure yields a grain growth exponent of n = 1 and an activation energy of
223
Q = 301 kJ/mol, which are in the middle of the reported fit data of PZT–PNN [7] (Q = 557 kJ/mol) and PZT–PZN [8] (Q = 125 kJ/mol). In our work, the experimental results indicate that a single kinetics of grain growth in this temperature interval is active. It also suggests that a further decrease of temperature would result in a reduction of the average grain size into the submicrometer range. Unfortunately, dense specimen cannot be obtained below 1050 °C from this approach. Previously, the grain size induced phase transition phenomena has been reported in many binary and ternary relaxor ferroelectric systems, such as PMN–PT [14], PNN–PZT [7] and PZN–PZT [8]. These works have indicated that in the micrometer range, the enlarged grain size is in favor of inducing phase transition to tetragonal side. Considering the similar distribution range of grain size to those reported systems, it is interesting to verify that whether the grain size induced phase transition behavior occurs in the quaternary PZT–PNZN system. Fig. 2(a) shows the X-ray diffraction patterns of PZT–PNZN ceramics sintered at various temperatures. A complete perovskite structure is formed, and no detectable traces of the pyrochlore or other impurities are observed. As we known, the shape of peaks near 45° can be used to judge the phase structure roughly for PZT based ceramic. Normally, rhombohedral structure shows only one maximum while tetragonal structure presents two pronounced maxima. However, these characters are not observed in fine scanning XRD patterns (Fig. 2(b)), and for all PZT–PNZN [14] specimens, the peaks exhibit similar broadening feature. To quantitatively investigate the influence of sintering temperature and, therefore, grain size on phase composition, the tetragonal phase content (TP) was calculated using Eq. (2) [4,7]:
TPð%Þ ¼
Ið200ÞT þ Ið002ÞT 100% Ið200ÞT þ Ið002ÞT þ Ið200ÞR
ð2Þ
where I(200)R is the integral intensity of rhombohedral (2 0 0) reflection, and I(002)T and I(200)T are the integral intensities of tetragonal (0 0 2) and (2 0 0) reflections, respectively. The peaks are separated by fitting the Gaussian–Lorentz line shape, and the positions of the reflections are fixed using the least squares method. The results of the calculation using Eq. (2) are 48–53% for PZT–PNZN systems with variant grain size, indicating no grain size induced phase transition occurred. Especially, considering the very close amounts of rhombohedral and tetragonal phases, it is no doubt that all specimens have the MPB structure [15,16]. Previously, Wagner et al. explained the change of the phase content in PZT–PNN samples with the variation of the grain size [7]. They indicated that stress relaxation induced by the enlarged grain size was responsible for the phase transition phenomena from a rhombohedral to a tetragonal structure. Later, this mechanism has also been applied to explain the size induced phase transition phenomena in PZT–PZN [8] and PMN–PT [14]. However, it can not be simply extended to quaternary PZT–PNZN systems. In this case, it is believed that the initial complicated MPB structure can resist the stress variation aroused from enlarged grain size. During cooling after sintering, internal stress was introduced into the lattice for the reason of phase transition across the Curie temperature. The induced internal stresses could be alleviated, but not relieved completely, for the complicated MPB structure [17]. Therefore, though the grain size varied in a wide range, the so called ‘‘grain size induced phase transition’’ does not occur in PZT–PNZN systems. It is well-known that Raman scattering is an effective method in studies of the microscopic features of materials and could be taken as an effective tool for the quantitative analysis on the phase coexistence of ferroelectric perovskite other than XRD measurement. To further resolve the phase structure of PZT–PNZN ceramics sintered at different temperatures, room-temperature Raman scattering
224
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
Fig. 1. SEM micrographs of PZT–PNZN ceramics sintered at (a) 1050 °C, (b) 1100 °C, (c) 1150 °C, (d) 1200 °C, (e) 1250 °C and (f) grain size vs sintering temperature with a fitted grain growth law.
generated longitude modes of T1u(3) below the Curie temperature. Hence, the intensity relationship between the tetragonal and rhombohedral modes could reflect the phase coexistence of tetragonal and rhombohedral phases in the perovskite [18,19]. Through peak fitting and analyzing the behavior of the relative intensity of these vibration modes, the content of tetragonal phase Itetra (%) was calculated using Eq. (3) [20]:
Itetra ð%Þ ¼
Fig. 2. (a) XRD patterns of PZT–PNZN ceramics sintered at 1050 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, and (b) the corresponding fine scanning XRD patterns of 2h = 43°–46°.
patterns were recorded, and the result are shown in Fig. 3. Normally, different phase structures of a PZT-based system present different vibration modes and Raman shifts in the Raman scattering spectrum. Among them, the tetragonal A1(3TO), located at about 600 cm1, tetragonal E(4TO) at 530 cm1, and the rhombohedral R1 mode at 560 cm1 are all the generated transverse modes from the cubic T1u(3) transformation below the Curie temperature, whereas the tetragonal A1(3LO) and E(4LO) modes at 825 and 700 cm1 and the rhombohedral Rh mode at 765 cm1 are the
IA1ð3LOÞ þ IA1ð3TOÞ þ IEð4TOÞ þ IEð4LOÞ 100% IA1ð3LOÞ þ IA1ð3TOÞ þ IEð4TOÞ þ IEð4LOÞ þ IR1 þ IRh
ð3Þ
here, IA1(3TO) and IA1(3LO) are the Raman scattering intensities of the tetragonal A1(3TO) and A1(3LO) vibrations, IE(4TO) and IE(4LO) are the Raman scattering intensities of the tetragonal E(4TO) and E(4LO) vibrations, and IR1and IRh are the intensities of the rhombohedral Rl and Rh vibrations, respectively. The results of the calculation using Eq. (3) are 53–55% for PZT–PNZN systems sintered at different temperatures, which agrees well with those derived from XRD patterns. Thus, it can be safely concluded that in the studied sintering temperature range, the phase structure of PZT–PNZN is not sensitive to the grain size variation. 3.2. Impedance, ferroelectric and piezoelectric properties To examine the effect of sintering temperature on the resistance of the samples, we have performed an ac impedance analysis [21]. The complex impedance patterns presented in Fig. 4 show only one semicircle feature, which was caused by a conducting mechanism at the bulk perovskite grains [22]. The bulk electrical resistivity for
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
225
Fig. 4. Complex impedance patterns of PZT–PNZN ceramics sintered at different temperatures.
Fig. 3. Raman scattering spectra of PZT–PNZN ceramics sintered at 1050 °C; 1100 °C; 1150 °C, 1200 °C and 1250 °C.
a given semicircle was obtained from the intercept of the semicircle with the Z0 -axis, which showed a gradual decrease with increasing sintering temperature. Here, it should pointed out that although PbZrO3 packing powder was used as an atmosphere protection, the volatilization of PbO in PZT–PNZN ceramics was inevitable due to the high sintering temperature. A corresponding amount of lead vacancy should form in the ceramic samples after sintering, whereas oxygen vacancy also appears to neutralize the electric charge. Accordingly, the observed decrease in the electrical resistivity that is caused by increasing sintering temperature can be attributed to the generation of mobile oxygen vacancies in a metal oxide. The room temperature P–E hysteresis loops for PZT–PNZN sintered at different temperature are shown in Fig. 5(a). For comparison the hysteresis loops were measured for all the ceramics at the same maximum electric field of 30 kV/cm. All P–E hysteresis loops present saturated ferroelectric nature. Moreover, the coercive field Ec is nearly independent of sintering temperature. Previously, Kungl and Hoffmann stated that the rhombohedral PZT has a lower coercive field than that of tetragonal PZT [23]. However, in our work, due to the similar phase structure of PZT–PNZN sintered at different temperature, the variation of Ec is slight. In contrast, the influence of sintering temperature, therefore, grain size on remanent polarization Pr is distinct. Fig. 5(b) represents the dependence of Pr on sintering temperature. An increase of sintering temperature and, therefore, an increase of grain size lead to an increase of the remanent polarization Pr. After passing a maximum at a temperature of 1150 °C, Pr decreases. Considering the relationship of grain size with grain boundary, it can be derived that the larger grain size correlates with lower volume of grain boundaries, which is benefit in weakening the constrain force clamping the domain walls and optimizing polarization orientations, resulting in the increase of Pr value [24]. Here, it should point that for PZT based ceramic, the sintering temperature can regulate the Pr by controlling the evaporation of lead oxide [25]. In this case, the optimal
Fig. 5. (a) The polarization–electric field (P–E) loops, and (b) remnant polarization Pr as a function of sintering temperature for PZT–PNZN ceramics.
sintering temperature is 1150 °C and after that, Pr showed the descending trend due to the extensive evaporation of lead oxide. Fig. 6 shows the variation of the dielectric constant er at different sintering temperatures. It can be seen that with
226
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
where e0 is the vacuum permittivity, Q11 is the electrostriction coefficient, and Pr is the remanent polarization. Distinctly, the improvement of d33 can be achieved by an increase of the relative permittivity er or remnant polarization Pr, according to Eq. (4) [29]. As seen in Figs. 5 and 6, for 1150 °C sintered specimen, Pr is maximum while er possesses relative high value, the latter comes from a slow decreasing of dielectric constant in the middle range between 1100 °C and 1200 °C. Thus, both factors contribute to obtain superior d33 at 1150 °C according to Eq. (4). When temperature exceeded 1200 °C, the d33 value considerably decreased due to both permittivity and remanent polarization Pr value decreased rapidly.
4. Conclusions
Fig. 6. The dielectric constant er for PZT–PNZN ceramics sintered at different temperatures.
Increasing the sintering temperature leads to grain growth monotonously in the PZT–PNZN with a grain growth exponent of n = 1. Unlike other reported binary or ternary relaxor ferroelectric system, it is interesting to find that sintering temperature and, consequently, grain size shows negligible effect on the phase compositions and a stable MPB configuration was retained in a wide sintering temperature region. However, the electric properties have close relationship with grain size. The largest values of remnant polarization Pr, piezoelectric constant d33 and electromechanical coupling factor kp are obtained simultaneously at the median grain size of 1.58 lm, whereas the maximum permittivity is found at lower grain size of 1.07 lm. This work indicates that an optimum choice of the grain size can leads to maximum enhanced electric properties. Acknowledgements
Fig. 7. The piezoelectric constant d33 and electromechanical coupling factor kp for PZT–PNZN ceramics sintered at different temperatures.
decreasing sintering temperature, dielectric constant increased continuously. This phenomenon is common for polycrystalline ferroelectric ceramics. In the past decades, effect of grain size on the relative permittivity has been extensively reported and different theoretical models have been introduced to describe the physical origin of this effect [26]. In BaTiO3, for example, it is now widely accepted that er increases with decreasing grain size, reaching a value of 5000, or higher, as the grain size approaches 1 lm. Below 1 lm, however, er of BaTiO3 decreases markedly with further decreasing grain size [27]. In our work, the maximum value (1710) of er was obtained for specimen with mean grain size of 1.07 lm sintered at 1050 °C. However, during the sizes scales in this case, the transition point of er does not occur. Further lowering the sintering temperature to obtain smaller grain size is impossible, because the sample is difficult to be densified. Fig. 7 shows the variation in piezoelectric constant d33 and electromechanical coupling factor kp as a function of sintering temperatures. Both kp and d33 show a similar variation trend with increasing temperatures. The optimized values of kp (0.56) and d33 (307 pC/N) were obtained at 1150 °C, and they are superior to the reported values (kp = 0.50 and d33 = 278 pC/N) in a Li2CO3-doped PZT–PZN system [28]. It is known that the evolution of piezoelectric responses can be explained by the conventional LandauDevonshire relation [4]:
d33 ¼ 2er e0 Q 11 P r
ð4Þ
This work was supported by the National Natural Science Foundation of China (Nos. 51072008 & 51172006), the Natural Science Foundation of Beijing (No. 2132007), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20131103110031). References [1] L.E. Cross, Ferroelectrics 151 (1994) 305–320. [2] Y.D. Hou, M.K. Zhu, F. Gao, H. Wang, B. Wang, H. Yan, C.S. Tian, J. Am. Ceram. Soc. 161 (2004) 455–460. [3] Y.D. Hou, M.K. Zhu, C.S. Tian, H. Yan, Sensor Actuator A: Phys. 116 (2004) 455– 460. [4] M.P. Zheng, Y.D. Hou, F.Y. Xie, J. Chen, M.K. Zhu, H. Yan, Acta. Mater. 61 (2013) 1489–1498. [5] Y. Hou, Mater. Lett. 58 (2004) 1508–1512. [6] T.M. Kamel, G. de With, J. Eur. Ceram. Soc. 28 (2008) 851–861. [7] S. Wagner, D. Kahraman, H. Kungl, M.J. Hoffmann, C. Schuh, K. Lubitz, J.A. Schmid, J. Appl. Phys. 98 (2005) 024102. [8] L.M. Chang, Y.D. Hou, M.K. Zhu, H. Yan, J. Appl. Phys. 101 (2007) 034101. [9] X. Chao, L. Yang, H. Pan, Z. Yang, Ceram. Int. 38 (2012) 3377–3382. [10] Q. Liu, Q. Sun, W. Ma, M. Li, Q. Xu, Q. Zhang, J. Eur. Ceram. Soc. 34 (2014) 1181– 1189. [11] I.T. Seo, Y.J. Cha, I.Y. Kang, J.H. Choi, S. Nahm, T.H. Seung, J.H. Paik, S. Priya, J. Am. Ceram. Soc. 94 (2011) 3629–3631. [12] Y.H. Jeong, K.B. Kim, Y.J. Lee, J.H. Cho, B.I. Kim, J.H. Paik, S. Nahm, Jpn. J. Appl. Phys. 51 (2012) 09MD04. [13] H. Amorín, J. Ricote, R. Jiménez, J. Holc, M. Kosec, Scr. Mater. 58 (2008) 755– 758. [14] N.N. Wu, Y.D. Hou, C. Wang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 105 (2009) 084107. [15] H. Fan, H.E. Kim, J. Appl. Phys. 91 (2002) 317. [16] L.Y. Zhao, Y.D. Hou, L.M. Chang, M.K. Zhu, H. Yan, J. Mater. Res. 24 (2011) 2029–2034. [17] N. Wu, X. Song, Y. Hou, M. Zhu, C. Wang, H. Yan, Chinese. Sci. Bull. 54 (2009) 1267–1274. [18] M.K. Zhu, P.X. Lu, Y.D. Hou, X.M. Song, H. Wang, H. Yan, J. Am. Ceram. Soc. 89 (2006) 3739–3744. [19] Y.D. Hou, L.M. Chang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 102 (2007) 084507–084512.
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227 [20] M.P. Zheng, Y.D. Hou, S. Wang, C.H. Duan, M.K. Zhu, H. Yan, J. Am. Ceram. Soc. 96 (2013) 2486–2492. [21] M.P. Zheng, Y.D. Hou, J. Fu, X.W. Fan, M.K. Zhu, H. Yan, Mater. Chem. Phys. 138 (2013) 358–365. [22] K.M. Lee, H.M. Jang, J. Am. Ceram. Soc. 81 (1998) 2586–2596. [23] H. Kungl, M.J. Hoffmann, J. Appl. Phys. 107 (2010) 054111. [24] M.P. Zheng, Y.D. Hou, M.K. Zhu, M. Zhang, H. Yan, J. Eur. Ceram. Soc. 34 (2014) 2275–2283. [25] M.P. Zheng, Y.D. Hou, H.Y. Ge, M.K. Zhu, H. Yan, Phys. Status Solid (a) 210 (2013) 261–266.
227
[26] M. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Z. Zhao, M. Nygren, C. Harnagea, D. Piazza, C. Galassi, Phys. Rev. B 73 (2006). [27] D. Ghosh, A. Sakata, J. Carter, P.A. Thomas, H. Han, J.C. Nino, J.L. Jones, Adv. Funct. Mater. 24 (2014) 885–896. [28] Y.D. Hou, L.M. Chang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 102 (2007) 084507. [29] Y. Yan, Y. Xu, Y. Feng, Ceram. Int. 40 (2014) 5897–5903.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of grain size on the phase structure and electrical properties of PZT–PNZN quaternary systems Zhirong Ai, Yudong Hou ⇑, Mupeng Zheng, Mankang Zhu College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 25 June 2014 Received in revised form 23 July 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: Piezoelectric ceramics Sintering temperature Phase stability Grain size
a b s t r a c t The quaternary systems 0.74Pb(Zr0.47Ti0.53)O3–0.26Pb[(Ni0.6Zn0.4)1/3Nb2/3]O3(PZT–PNZN) have been prepared using the conventional oxide mixing method in a wide temperature region between 1050 and 1250 °C. All specimens present high relative density above 95%, indicating a wide sintering window for this system. Investigation of the microstructure reveals a continuous increase in grain size from 1.07 to 2.77 lm when the sintering temperature is increased from 1050 to 1250 °C, which obeys the common grain-growth law well. However, unlike compositional component PZN–PZT and PNN–PZT, the phase structure of quaternary PZN–PNZN is not sensitive to the grain size. The specimens with different grain size retain the morphotropic phase boundary (MPB) structure and the size induced phase transition does not appear. Excluding the influence of the phase structure, the size effect plays a dominant role in improving the piezoelectric properties, which facilitates the motion of domain walls due to the decreasing number of grain boundary. The optimum piezoelectric parameters are obtained at 1150 °C: d33 = 307 pC/N, kp = 0.56. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Relaxor ferroelectric materials with compositions near morphotropic phase boundary (MPB) exhibit anomalously high piezoelectric and dielectric properties, which open a wide application field for the use in multilayer capacitors, piezoelectric transducer and electrostrictive actuators [1]. By introducing various dopants or adding other component to the relaxor ferroelectrics, piezoelectric and dielectric properties can be tailored to a wide extent [2–4]. On the other hand, adjusting the process parameters such as sintering temperature is also an effective means of materials modification [5,6]. As we known, sintering temperature is an important process parameter to endow ferroelectric polycrystals with proper microscopic structure. Wagner et al. made a detailed study on the variation of the grain size, phase composition, and electric properties with sintering temperature in the relaxor ferroelectric system Pb(Zr, Ti)O3–Pb(Ni1/3Nb2/3)O3 (PZT–PNN) [7]. They have found that increasing the sintering temperature leads to grain growth, which in turn increases the tetragonal phase content significantly, although the composition of the initial PZT–PNN powder was kept constant. Samples with a tetragonal phase content of 70% and a corresponding grain size of 1.5–2.5 lm show good piezoelectric
⇑ Corresponding author. E-mail address: [email protected] (Y. Hou). http://dx.doi.org/10.1016/j.jallcom.2014.07.181 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
properties. Later, Chang et al. investigated the phase transition and dielectrical response as a function of sintering temperature in Pb(Zr, Ti)O3–Pb(Zn1/3Nb2/3)O3 (PZT–PZN) [8]. They reported that the increasing sintering temperature not only induced the increase of grain size and tetragonal phase content, but also caused the weakening of the dielectric relaxor behavior. The latter can be attributed to that increasing the sintering temperature gives rise to compositional fluctuation and could stabilize the negatively charged ordered microdomains, leading to the degree of the short-range B-site order to increase. Considering the similar relaxor ferroelectric behaviors of PZT–PNN and PZT–PZN, a quaternary PZT–PNZN solid solution, the combination of the two ternary compositions, has been constructed and some related research has been reported. Chao et al. prepared piezoelectric ceramics bimorph using phase-pure PZT–PNZN ceramics with Zr/Ti ratio = 0.985, which exhibits a large displacement and meets the needs of the needle selecting mechanism of electronic jacquard [9]. Liu et al. have found that due to high Curie temperature and small grain size, the PZT–PNZN ceramics with MPB composition maintained a high d33 level after depoling treatment, revealing a superior strain capacity for high-temperature application [10]. Recently, Sahn Nahm et al. have conducted series research on PZT–PNZN systems for energy harvesting device application [11,12]. They found that through tailoring the microstructure and phase composition, the transduction
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
coefficient d33 g33 can be optimized to a high value, which provides high energy density materials for application to energy harvesting devices. During previous research one important question was raised: how does the sintering condition influence the properties of the quaternary PZT–PNZN ceramics and is it similar to the variation trend of ternary PZT–PNN and PZT–PZN? In the present paper, as an extension to the intrinsic study on the PZT–PNZN quaternary system, sintering temperature was selected to modulate the microstructure and the electrical properties of the PZT–PNZN system. Emphasis was placed on the grain size dependences of phase structure, ferroelectric and piezoelectric properties. 2. Experimental procedure The specimens were prepared using a conventional mixed-oxide process. The compositions used in this study are as follows: 0.74Pb(Zr0.47Ti0.53)O3–0.26Pb[(Ni0.6 Zn0.4)1/3Nb2/3]O3 (PZT–PNZN). Reagent-grade oxide powders (Pb3O4, ZrO2, TiO2, ZnO, NiO, and Nb2O5) were used as starting materials. The powders were weighed and mixed through ball milling, with partially stabilized zirconia balls as media, in alcohol for 12 h. After drying, the mixture was calcined in a covered alumina crucible at 850 °C for 2 h. The calcined powders were remilled for 12 h and then pressed into disks of 11.5 mm in diameter at around 100 MPa. The green disks were sintered at 50 °C temperature intervals between 1050 and 1250 °C for 2 h in a sealed alumina crucible. To minimize PbO loss, a PbO-rich atmosphere was maintained by placing powders of PbZrO3 inside the crucible used as packing powders. The densities of the sintered specimens were measured by a water-immersion method using Archimedes-principle. The crystal structures of the samples were examined by XRD (Bruker D8 Advance, Karlsruhe, Germany) in the h–2h configuration using Cu Ka radiation. Raman scattering spectra were recorded at room temperature from a Raman spectrometer (Model T64000; Jobin–Yvon, Paris, France) under back scattering geometry. Excitation was taken as the 488 nm line of an Ar+ laser with a 50 mW output power. Micromorphology was detected on a thermally etched surface by SEM (Hitachi S4800, Japan) and the mean grain size was calculated by the line intercept method. To measure the electrical properties, silver paste was printed on the lapped surfaces of the sintered pellets and then fired at 560 °C for 30 min to form electrodes. The dielectric property was measured using a multi-frequency inductance capacitance resistance (LCR) analyzer (Agilent E4980A, Santa Clara, CA). Impedance analysis was conducted using impedance analyzer (Novocontrol Technologies, Hundsangen, Germany) at 300 °C in the frequency range from 1 MHz to 10 MHz. Ferroelectric behavior was studied using a ferroelectric tester (Premier II, Radiant Technologies Inc, Albuquerque, NM) at 1 Hz before poling treatment. Prior to the testing of piezoelectric properties, the specimens were poled in a silicone oil bath at 120 °C by applying a DC field of 30 kV/cm for 30 min and then aged for 24 h. The piezoelectric constant d33 was measured by a piezoelectric d33 meter (ZJ-6A, Institute of Acoustics, Academic Sinica, China) at 100 Hz. The electromechanical coupling factor kp was determined by a precision impedance analyzer (4294A; Agilent Technologies, Santa Clara, CA) through the resonance-anti-resonance method based on IEEE standards.
3. Results and discussion 3.1. Microstructure evolution Fig. 1 presents the SEM micrographs of five samples sintered between 1050 and 1250 °C for 2 h. All specimens retain a high relative density above 95% and show a homogenous grain-size distribution, thus demonstrating that PZT–PNZN system has a wide sintering window. As expected, grain coarsening is observed with increasing sintering temperature. The quantitative grain-size analysis shows a significant increase of the mean grain size from d50 = 1.07 lm at 1050 °C to d50 = 2.77 lm at 1250 °C. These results, shown in Fig. 1(f), could be successfully fitted by using a common grain-growth law, considering non-isothermal kinetics [7,13]:
Q n n t d d0 ¼ k exp RT
ð1Þ
In this equation, n is the grain growth exponent, T is the sintering temperature, t is the sintering time, d is the grain size at time t and temperature T, d0 is the initial grain size, Q is the activation energy, and k is a grain-growth constant. The fit procedure yields a grain growth exponent of n = 1 and an activation energy of
223
Q = 301 kJ/mol, which are in the middle of the reported fit data of PZT–PNN [7] (Q = 557 kJ/mol) and PZT–PZN [8] (Q = 125 kJ/mol). In our work, the experimental results indicate that a single kinetics of grain growth in this temperature interval is active. It also suggests that a further decrease of temperature would result in a reduction of the average grain size into the submicrometer range. Unfortunately, dense specimen cannot be obtained below 1050 °C from this approach. Previously, the grain size induced phase transition phenomena has been reported in many binary and ternary relaxor ferroelectric systems, such as PMN–PT [14], PNN–PZT [7] and PZN–PZT [8]. These works have indicated that in the micrometer range, the enlarged grain size is in favor of inducing phase transition to tetragonal side. Considering the similar distribution range of grain size to those reported systems, it is interesting to verify that whether the grain size induced phase transition behavior occurs in the quaternary PZT–PNZN system. Fig. 2(a) shows the X-ray diffraction patterns of PZT–PNZN ceramics sintered at various temperatures. A complete perovskite structure is formed, and no detectable traces of the pyrochlore or other impurities are observed. As we known, the shape of peaks near 45° can be used to judge the phase structure roughly for PZT based ceramic. Normally, rhombohedral structure shows only one maximum while tetragonal structure presents two pronounced maxima. However, these characters are not observed in fine scanning XRD patterns (Fig. 2(b)), and for all PZT–PNZN [14] specimens, the peaks exhibit similar broadening feature. To quantitatively investigate the influence of sintering temperature and, therefore, grain size on phase composition, the tetragonal phase content (TP) was calculated using Eq. (2) [4,7]:
TPð%Þ ¼
Ið200ÞT þ Ið002ÞT 100% Ið200ÞT þ Ið002ÞT þ Ið200ÞR
ð2Þ
where I(200)R is the integral intensity of rhombohedral (2 0 0) reflection, and I(002)T and I(200)T are the integral intensities of tetragonal (0 0 2) and (2 0 0) reflections, respectively. The peaks are separated by fitting the Gaussian–Lorentz line shape, and the positions of the reflections are fixed using the least squares method. The results of the calculation using Eq. (2) are 48–53% for PZT–PNZN systems with variant grain size, indicating no grain size induced phase transition occurred. Especially, considering the very close amounts of rhombohedral and tetragonal phases, it is no doubt that all specimens have the MPB structure [15,16]. Previously, Wagner et al. explained the change of the phase content in PZT–PNN samples with the variation of the grain size [7]. They indicated that stress relaxation induced by the enlarged grain size was responsible for the phase transition phenomena from a rhombohedral to a tetragonal structure. Later, this mechanism has also been applied to explain the size induced phase transition phenomena in PZT–PZN [8] and PMN–PT [14]. However, it can not be simply extended to quaternary PZT–PNZN systems. In this case, it is believed that the initial complicated MPB structure can resist the stress variation aroused from enlarged grain size. During cooling after sintering, internal stress was introduced into the lattice for the reason of phase transition across the Curie temperature. The induced internal stresses could be alleviated, but not relieved completely, for the complicated MPB structure [17]. Therefore, though the grain size varied in a wide range, the so called ‘‘grain size induced phase transition’’ does not occur in PZT–PNZN systems. It is well-known that Raman scattering is an effective method in studies of the microscopic features of materials and could be taken as an effective tool for the quantitative analysis on the phase coexistence of ferroelectric perovskite other than XRD measurement. To further resolve the phase structure of PZT–PNZN ceramics sintered at different temperatures, room-temperature Raman scattering
224
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
Fig. 1. SEM micrographs of PZT–PNZN ceramics sintered at (a) 1050 °C, (b) 1100 °C, (c) 1150 °C, (d) 1200 °C, (e) 1250 °C and (f) grain size vs sintering temperature with a fitted grain growth law.
generated longitude modes of T1u(3) below the Curie temperature. Hence, the intensity relationship between the tetragonal and rhombohedral modes could reflect the phase coexistence of tetragonal and rhombohedral phases in the perovskite [18,19]. Through peak fitting and analyzing the behavior of the relative intensity of these vibration modes, the content of tetragonal phase Itetra (%) was calculated using Eq. (3) [20]:
Itetra ð%Þ ¼
Fig. 2. (a) XRD patterns of PZT–PNZN ceramics sintered at 1050 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, and (b) the corresponding fine scanning XRD patterns of 2h = 43°–46°.
patterns were recorded, and the result are shown in Fig. 3. Normally, different phase structures of a PZT-based system present different vibration modes and Raman shifts in the Raman scattering spectrum. Among them, the tetragonal A1(3TO), located at about 600 cm1, tetragonal E(4TO) at 530 cm1, and the rhombohedral R1 mode at 560 cm1 are all the generated transverse modes from the cubic T1u(3) transformation below the Curie temperature, whereas the tetragonal A1(3LO) and E(4LO) modes at 825 and 700 cm1 and the rhombohedral Rh mode at 765 cm1 are the
IA1ð3LOÞ þ IA1ð3TOÞ þ IEð4TOÞ þ IEð4LOÞ 100% IA1ð3LOÞ þ IA1ð3TOÞ þ IEð4TOÞ þ IEð4LOÞ þ IR1 þ IRh
ð3Þ
here, IA1(3TO) and IA1(3LO) are the Raman scattering intensities of the tetragonal A1(3TO) and A1(3LO) vibrations, IE(4TO) and IE(4LO) are the Raman scattering intensities of the tetragonal E(4TO) and E(4LO) vibrations, and IR1and IRh are the intensities of the rhombohedral Rl and Rh vibrations, respectively. The results of the calculation using Eq. (3) are 53–55% for PZT–PNZN systems sintered at different temperatures, which agrees well with those derived from XRD patterns. Thus, it can be safely concluded that in the studied sintering temperature range, the phase structure of PZT–PNZN is not sensitive to the grain size variation. 3.2. Impedance, ferroelectric and piezoelectric properties To examine the effect of sintering temperature on the resistance of the samples, we have performed an ac impedance analysis [21]. The complex impedance patterns presented in Fig. 4 show only one semicircle feature, which was caused by a conducting mechanism at the bulk perovskite grains [22]. The bulk electrical resistivity for
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
225
Fig. 4. Complex impedance patterns of PZT–PNZN ceramics sintered at different temperatures.
Fig. 3. Raman scattering spectra of PZT–PNZN ceramics sintered at 1050 °C; 1100 °C; 1150 °C, 1200 °C and 1250 °C.
a given semicircle was obtained from the intercept of the semicircle with the Z0 -axis, which showed a gradual decrease with increasing sintering temperature. Here, it should pointed out that although PbZrO3 packing powder was used as an atmosphere protection, the volatilization of PbO in PZT–PNZN ceramics was inevitable due to the high sintering temperature. A corresponding amount of lead vacancy should form in the ceramic samples after sintering, whereas oxygen vacancy also appears to neutralize the electric charge. Accordingly, the observed decrease in the electrical resistivity that is caused by increasing sintering temperature can be attributed to the generation of mobile oxygen vacancies in a metal oxide. The room temperature P–E hysteresis loops for PZT–PNZN sintered at different temperature are shown in Fig. 5(a). For comparison the hysteresis loops were measured for all the ceramics at the same maximum electric field of 30 kV/cm. All P–E hysteresis loops present saturated ferroelectric nature. Moreover, the coercive field Ec is nearly independent of sintering temperature. Previously, Kungl and Hoffmann stated that the rhombohedral PZT has a lower coercive field than that of tetragonal PZT [23]. However, in our work, due to the similar phase structure of PZT–PNZN sintered at different temperature, the variation of Ec is slight. In contrast, the influence of sintering temperature, therefore, grain size on remanent polarization Pr is distinct. Fig. 5(b) represents the dependence of Pr on sintering temperature. An increase of sintering temperature and, therefore, an increase of grain size lead to an increase of the remanent polarization Pr. After passing a maximum at a temperature of 1150 °C, Pr decreases. Considering the relationship of grain size with grain boundary, it can be derived that the larger grain size correlates with lower volume of grain boundaries, which is benefit in weakening the constrain force clamping the domain walls and optimizing polarization orientations, resulting in the increase of Pr value [24]. Here, it should point that for PZT based ceramic, the sintering temperature can regulate the Pr by controlling the evaporation of lead oxide [25]. In this case, the optimal
Fig. 5. (a) The polarization–electric field (P–E) loops, and (b) remnant polarization Pr as a function of sintering temperature for PZT–PNZN ceramics.
sintering temperature is 1150 °C and after that, Pr showed the descending trend due to the extensive evaporation of lead oxide. Fig. 6 shows the variation of the dielectric constant er at different sintering temperatures. It can be seen that with
226
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227
where e0 is the vacuum permittivity, Q11 is the electrostriction coefficient, and Pr is the remanent polarization. Distinctly, the improvement of d33 can be achieved by an increase of the relative permittivity er or remnant polarization Pr, according to Eq. (4) [29]. As seen in Figs. 5 and 6, for 1150 °C sintered specimen, Pr is maximum while er possesses relative high value, the latter comes from a slow decreasing of dielectric constant in the middle range between 1100 °C and 1200 °C. Thus, both factors contribute to obtain superior d33 at 1150 °C according to Eq. (4). When temperature exceeded 1200 °C, the d33 value considerably decreased due to both permittivity and remanent polarization Pr value decreased rapidly.
4. Conclusions
Fig. 6. The dielectric constant er for PZT–PNZN ceramics sintered at different temperatures.
Increasing the sintering temperature leads to grain growth monotonously in the PZT–PNZN with a grain growth exponent of n = 1. Unlike other reported binary or ternary relaxor ferroelectric system, it is interesting to find that sintering temperature and, consequently, grain size shows negligible effect on the phase compositions and a stable MPB configuration was retained in a wide sintering temperature region. However, the electric properties have close relationship with grain size. The largest values of remnant polarization Pr, piezoelectric constant d33 and electromechanical coupling factor kp are obtained simultaneously at the median grain size of 1.58 lm, whereas the maximum permittivity is found at lower grain size of 1.07 lm. This work indicates that an optimum choice of the grain size can leads to maximum enhanced electric properties. Acknowledgements
Fig. 7. The piezoelectric constant d33 and electromechanical coupling factor kp for PZT–PNZN ceramics sintered at different temperatures.
decreasing sintering temperature, dielectric constant increased continuously. This phenomenon is common for polycrystalline ferroelectric ceramics. In the past decades, effect of grain size on the relative permittivity has been extensively reported and different theoretical models have been introduced to describe the physical origin of this effect [26]. In BaTiO3, for example, it is now widely accepted that er increases with decreasing grain size, reaching a value of 5000, or higher, as the grain size approaches 1 lm. Below 1 lm, however, er of BaTiO3 decreases markedly with further decreasing grain size [27]. In our work, the maximum value (1710) of er was obtained for specimen with mean grain size of 1.07 lm sintered at 1050 °C. However, during the sizes scales in this case, the transition point of er does not occur. Further lowering the sintering temperature to obtain smaller grain size is impossible, because the sample is difficult to be densified. Fig. 7 shows the variation in piezoelectric constant d33 and electromechanical coupling factor kp as a function of sintering temperatures. Both kp and d33 show a similar variation trend with increasing temperatures. The optimized values of kp (0.56) and d33 (307 pC/N) were obtained at 1150 °C, and they are superior to the reported values (kp = 0.50 and d33 = 278 pC/N) in a Li2CO3-doped PZT–PZN system [28]. It is known that the evolution of piezoelectric responses can be explained by the conventional LandauDevonshire relation [4]:
d33 ¼ 2er e0 Q 11 P r
ð4Þ
This work was supported by the National Natural Science Foundation of China (Nos. 51072008 & 51172006), the Natural Science Foundation of Beijing (No. 2132007), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20131103110031). References [1] L.E. Cross, Ferroelectrics 151 (1994) 305–320. [2] Y.D. Hou, M.K. Zhu, F. Gao, H. Wang, B. Wang, H. Yan, C.S. Tian, J. Am. Ceram. Soc. 161 (2004) 455–460. [3] Y.D. Hou, M.K. Zhu, C.S. Tian, H. Yan, Sensor Actuator A: Phys. 116 (2004) 455– 460. [4] M.P. Zheng, Y.D. Hou, F.Y. Xie, J. Chen, M.K. Zhu, H. Yan, Acta. Mater. 61 (2013) 1489–1498. [5] Y. Hou, Mater. Lett. 58 (2004) 1508–1512. [6] T.M. Kamel, G. de With, J. Eur. Ceram. Soc. 28 (2008) 851–861. [7] S. Wagner, D. Kahraman, H. Kungl, M.J. Hoffmann, C. Schuh, K. Lubitz, J.A. Schmid, J. Appl. Phys. 98 (2005) 024102. [8] L.M. Chang, Y.D. Hou, M.K. Zhu, H. Yan, J. Appl. Phys. 101 (2007) 034101. [9] X. Chao, L. Yang, H. Pan, Z. Yang, Ceram. Int. 38 (2012) 3377–3382. [10] Q. Liu, Q. Sun, W. Ma, M. Li, Q. Xu, Q. Zhang, J. Eur. Ceram. Soc. 34 (2014) 1181– 1189. [11] I.T. Seo, Y.J. Cha, I.Y. Kang, J.H. Choi, S. Nahm, T.H. Seung, J.H. Paik, S. Priya, J. Am. Ceram. Soc. 94 (2011) 3629–3631. [12] Y.H. Jeong, K.B. Kim, Y.J. Lee, J.H. Cho, B.I. Kim, J.H. Paik, S. Nahm, Jpn. J. Appl. Phys. 51 (2012) 09MD04. [13] H. Amorín, J. Ricote, R. Jiménez, J. Holc, M. Kosec, Scr. Mater. 58 (2008) 755– 758. [14] N.N. Wu, Y.D. Hou, C. Wang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 105 (2009) 084107. [15] H. Fan, H.E. Kim, J. Appl. Phys. 91 (2002) 317. [16] L.Y. Zhao, Y.D. Hou, L.M. Chang, M.K. Zhu, H. Yan, J. Mater. Res. 24 (2011) 2029–2034. [17] N. Wu, X. Song, Y. Hou, M. Zhu, C. Wang, H. Yan, Chinese. Sci. Bull. 54 (2009) 1267–1274. [18] M.K. Zhu, P.X. Lu, Y.D. Hou, X.M. Song, H. Wang, H. Yan, J. Am. Ceram. Soc. 89 (2006) 3739–3744. [19] Y.D. Hou, L.M. Chang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 102 (2007) 084507–084512.
Z. Ai et al. / Journal of Alloys and Compounds 617 (2014) 222–227 [20] M.P. Zheng, Y.D. Hou, S. Wang, C.H. Duan, M.K. Zhu, H. Yan, J. Am. Ceram. Soc. 96 (2013) 2486–2492. [21] M.P. Zheng, Y.D. Hou, J. Fu, X.W. Fan, M.K. Zhu, H. Yan, Mater. Chem. Phys. 138 (2013) 358–365. [22] K.M. Lee, H.M. Jang, J. Am. Ceram. Soc. 81 (1998) 2586–2596. [23] H. Kungl, M.J. Hoffmann, J. Appl. Phys. 107 (2010) 054111. [24] M.P. Zheng, Y.D. Hou, M.K. Zhu, M. Zhang, H. Yan, J. Eur. Ceram. Soc. 34 (2014) 2275–2283. [25] M.P. Zheng, Y.D. Hou, H.Y. Ge, M.K. Zhu, H. Yan, Phys. Status Solid (a) 210 (2013) 261–266.
227
[26] M. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Z. Zhao, M. Nygren, C. Harnagea, D. Piazza, C. Galassi, Phys. Rev. B 73 (2006). [27] D. Ghosh, A. Sakata, J. Carter, P.A. Thomas, H. Han, J.C. Nino, J.L. Jones, Adv. Funct. Mater. 24 (2014) 885–896. [28] Y.D. Hou, L.M. Chang, M.K. Zhu, X.M. Song, H. Yan, J. Appl. Phys. 102 (2007) 084507. [29] Y. Yan, Y. Xu, Y. Feng, Ceram. Int. 40 (2014) 5897–5903.