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Structure and electrical properties of PMZN–PZT quaternary ceramics for piezoelectric transformers
Sensors and Actuators A 116 (2004) 455–460
Structure and electrical properties of PMZN–PZT quaternary ceramics for piezoelectric transformers Yu-Dong Hou a,∗ , Man-Kang Zhu a , Chang-Sheng Tian b , Hui Yan a a
b
The Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing 100022, China Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China Received 27 August 2003; received in revised form 9 May 2004; accepted 17 May 2004 Available online 21 July 2004
Abstract Pb(Mn1/3 Nb2/3 )O3 –Pb(Zn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMZN–PZT) quaternary ceramics with various contents of Pb(Mn1/3 Nb2/3 )O3 from 5 to 20 mol% were prepared by the columbite two-stage process and the effects of composition on the microstructure and piezoelectric properties were investigated. The results revealed that the quaternary ceramic can be densified at 1250–1300 ◦ C, and its crystal structure was transformed from the tetragonal to the cubic with increasing amount of Pb(Mn1/3 Nb2/3 )O3 . The optimized results of kp (0.55), tan δ (0.003) and Qm (2528) were obtained at 10 mol% Pb(Mn1/3 Nb2/3 )O3 , which is a new promising material for piezoelectric transformer application. © 2004 Elsevier B.V. All rights reserved. Keywords: PMZN–PZT quaternary system; Piezoelectric; Mechanical quality factor; Piezoelectric transformers
1. Introduction Piezoelectric ceramics have been extensively studied for many years. However, their applications for piezoelectric transformers were vigorously studied only in recently years for application of lighting liquid crystal display (LCD) backlights [1–3]. The properties of these materials should combine a high mechanical quality factor (Qm ) with high electromechanical coupling factor (kp ) and low dielectric loss (tan δ) simultaneously because the piezoelectric transformer operated at its resonant frequency in transformation between electrical and mechanical energy [4,5]. Until now, many ternary and quaternary systems, such as Pb(Ni1/3 Nb2/3 )O3 –PZT, Pb(Y2/3 W1/3 )O3 –PZT, Pb(Mn1/3 Sb2/3 )O3 –PZT, Pb(Mg1/3 Nb2/3 )O3 –Pb(Ni1/3 Nb2/3 )O3 – PZT, Pb(Ni1/2 W1/2 )O3 –Pb(Mn1/3 Nb2/3 )O3 –PZT, etc. [4,6–9] have been synthesized by modifications and/or substitutions to satisfy the requirements of practical applications of piezoelectric transformer. However, as far as the mechanical quality factor is concerned, there are few sys-
∗ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392412. E-mail addresses: [email protected], [email protected] (Y.-D. Hou).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.05.012
tems that can give a Qm larger than 2000. Because Qm is a reference parameter of the dissipation of vibrational energy due to internal friction effects, in the case of low Qm , energy loss will leads to the temperature rise of ceramic devices and the deterioration of piezoelectric properties. So, it is necessary to fabricate a piezoelectric ceramic that gives a high Qm while kp and tan δ are well situated to satisfy the requirements for practical application in piezoelectric transformers. Since Pb(Zn1/3 Nb2/3 )O3 –PZT ternary system exhibits high electromechanical coupling factor [10] and since Pb(Mn1/3 Nb2/3 )O3 –PZT ternary system can offer a high mechanical quality factor [11], it is believed that Pb(Mn1/3 Nb2/3 )O3 –Pb(Zn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMZN– PZT) by combining Pb(Mn1/3 Nb2/3 )O3 –PZT with Pb(Zn1/3 Nb2/3 )O3 –PZT should give a high Qm and suitable kp . In this study, PMZN–PZT quaternary ceramics with different contents of Pb(Mn1/3 Nb2/3 )O3 were prepared and the effects of Pb(Mn1/3 Nb2/3 )O3 content on the microstructure and piezoelectric properties were investigated. The electrical measurement shows that the PMZN–PZT quaternary ceramic with 10 mol% content of Pb(Mn1/3 Nb2/3 )O3 gives large Qm suitable for the application of high-power piezoelectric transformer.
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2. Experimental The general formula of the materials studied was (Pb0.95 Sr0.05 )(Mn1/3 Nb2/3 )x (Zn1/3 Nb2/3 )0.20−x Ti0.42 Zr0.38 O3 + 0.25 wt.% CeO2 , where x = 0.05, 0.10, 0.15 and 0.20, respectively (PMZN–PZT). The samples were prepared by the two-stage method [12]. Reagent-grade oxide powders, Pb3 O4 , ZrO2 , TiO2 , SrCO3 , ZnO, MnCO3 , Nb2 O5 and CeO2 were used as the starting materials. In the first stage, the powders of columbite precursor, ZnNb2 O6 and MnNb2 O6 , were prepared by reactions of ZnO and MnCO3 , respectively, with Nb2 O5 at 1100 ◦ C for 4 h. In the second stage, the columbite precursors with other oxides including Pb3 O4 , ZrO2 , TiO2 , SrCO3 and CeO2 were weighed and mixed through the use of a polyethylene jar and ZrO2 milling media. The mixture was then calcined at 850 ◦ C for 2 h in air, remilled, pressed into discs of 12.0 mm in diameter at around 120 MPa, and then sintered at 1050–1300 ◦ C for 2 h in a sealed alumina crucible with PbZrO3 powder atmosphere. The bulk density was measured using the Archimedean method and linear shrinkage was determined manually. Fig. 1 shows the variations of bulk density and the radial shrinkage ratio of PMZN–PZT specimens at different sintering temperature. It can be seen from Fig. 1 that the bulk densities for PMZN–PZT change as the sintering temperature in the same way of radial shrinkage ratio. Both curves are in S shape and can be divided into three regions, i.e. low-, intermediate- and high-temperature regions. In the low sintering temperature region from 1050 to 1150 ◦ C, both bulk densities and the radial shrinkage ratios increase slightly. In the intermediate temperature region from 1150 to 1250 ◦ C, bulk densities and the radial shrinkage ratios increase steeply, which shows the densification process of PMZN–PZT ceramics mainly carried in this temperature region. Further increasing the sintering temperature above 1250 ◦ C, the curves of bulk density and radial shrinkage ratio show the saturation trend. According to the above results,
Fig. 1. Bulk density and radial shrinkage ratio of PMZN–PZT specimens at different sintering temperature.
the suitable sintering temperature region of PMZN–PZT is from 1250 to 1300 ◦ C. The crystal structure of the samples was analyzed using an X-ray diffractometry (XRD; Model DMX-C, Japan) with a 2θ range from 15◦ to 70◦ . A step scan with a step size of 0.02◦ was used with a counting time of 1 s/step. Microstructural evolution was observed using a scanning electron microscopy (SEM; Model Hitachi S-570, Japan). The sintered discs were lapped and electroded with a silver paste. The specimens for the piezoelectric property measurements were poled in a silicone oil bath at 120 ◦ C by applying a dc electric field of 3 kV/mm for 30 min. The specimens were aged for 24 h prior to testing. The piezoelectric constant (d33 ) was measured using a quasi-static piezoelectric d33 meter (Model ZJ-3d, Institute of Acoustics Academic Sinica, China). The planar coupling coefficient (kp ) and mechanical quality factor (Qm ) were determined by the resonance and anti-resonance technique using an impedance analyzer (Model HP4194A, Hewlett-Packard, CA). The Curie temperature (Tc ) was determined by temperature dependence of the dielectric constant at 1 kHz.
3. Results and discussion 3.1. Phase characterization Figs. 2 and 3 show XRD patterns of the powders calcined at 850 ◦ C and ceramics sintered at 1275 ◦ C, respectively. Clearly seen from Fig. 2, all calcined powders consisted of pyrochlore phase formed along with perovskite phase. The amount of pyrochlore phase increased with increasing Pb(Mn1/3 Nb2/3 )O3 content. It is known that Pb(Mn1/3 Nb2/3 )O3 is an unstable relaxor ferroelectric compared to others, such as Pb(Mg1/3 Nb2/3 )O3 , Pb(Ni1/3 Nb2/3 )O3 , etc. Hence, the more Pb(Mn1/3 Nb2/3 )O3 contents were consisted in PMZN–PZT system, the more contents of pyrochlore phase appeared. In addition, the
Fig. 2. XRD patterns of PMZN–PZT powders calcined at 850 ◦ C.
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In this experiment, PMZN–PZT ceramics with 5 and 10 mol% PMN content have tetragonal structure and the ceramics with 15 and 20 mol% PMN content have cubic structure. 3.2. Microstructure
Fig. 3. XRD patterns of PMZN–PZT ceramics sintered at 1275 ◦ C.
pyrochlore phase has stable structure and is formed ahead of perovskite phase. In the low calcining temperature, the compositions diffusion is difficult and insufficient, which results in the formation of pyrochlore phase. When the temperature increased, the amount of pyrochlore was eliminated gradually because the high sintering temperature is favor to the compositions diffusion and the phase transformation from pyrochlore to perovskite. So, for sintered samples, only pure perovskite phase exists and no pyrochlore phase can be detected, as can be seen from Fig. 3. Moreover, PMZN–PZT systems are transformed from the tetragonal to the cubic structure with the increasing amount of Pb(Mn1/3 Nb2/3 )O3 , as revealed by incorporation of (0 0 2) and (2 0 0) peaks as the addition of Pb(Mn1/3 Nb2/3 )O3 in Fig. 3. The lattice parameters (a, c) of the perovskite phase have been evaluated from the diffraction patterns and are plotted in Fig. 4. Pb(Zr0.52 Ti0.48 )O3 has tetragonal structure (Powder Diffraction File No. 33-784) while Pb(Mn1/3 Nb2/3 )O3 has cubic structure (Powder Diffraction File No. 39-1007), so the increase of Pb(Mn1/3 Nb2/3 )O3 content leads to the phase transformation of PMZN–PZT.
Fig. 4. Lattice constant of PMZN–PZT as a function of Pb(Mn1/3 Nb2/3 )O3 content.
For the application of piezoelectric transformer operating at high vibration velocity, it is favorable to obtain the fine grain ceramics which possess higher strength. Fig. 5 shows the SEM micrographs of PMZN–PZT ceramics with 5–20 mol% Pb(Mn1/3 Nb2/3 )O3 sintered at 1275 ◦ C. All specimens have uniform and fine microstructure and the largest grain size is below 1.70 m. The structure of fine grains are mainly attributed to the CeO2 addition as it is believed that the addition of Ce ions will segregate at grain boundaries and inhibit the grain growth [13]. Moreover, it can be seen from Fig. 5 that the different amount of Pb(Mn1/3 Nb2/3 )O3 shows little impact on the grain growth and densification in PMZN–PZT ceramics, which is different from the observation in Pb(Mg1/3 Nb2/3 )O3 –Pb(Mn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMMN–PZT) system reported by Chen et al. [11]. It is implied that in PMZN–PZT ceramics, addition of CeO2 become more dominant than the effects of Pb(Mn1/3 Nb2/3 )O3 on the grain growth. 3.3. Electrical properties The poled and unpoled dielectric constant as a function of Pb(Mn1/3 Nb2/3 )O3 content measured at room temperature and frequency of 1 kHz are plotted in Fig. 6. It can be seen that dielectric constant decreases monotonically with increasing Pb(Mn1/3 Nb2/3 )O3 content. Pb(Mn1/3 Nb2/3 )O3 has little solubility (<5 mol%) in the PZT composition matrix and excess Pb(Mn1/3 Nb2/3 )O3 beyond the solubility limit is believed to segregate at grain boundaries [4], which attributes to the increase in the extent of space-charge polarization at the grain boundaries, resulting in domains bound within grains and decrease of dielectric constant. In addition, Fig. 6 also shows that the difference of dielectric constants before and after poling vary with Pb(Mn1/3 Nb2/3 )O3 content. K = (Kpole − Kunpole ) is positive for the amount of Pb(Mn1/3 Nb2/3 )O3 below 10 mol% and is negative for the amount of Pb(Mn1/3 Nb2/3 )O3 above 15 mol%. This transition from positive K for tetragonal phase to negative K for cubic phase has also been previously observed in Pb(Mg,Nb)O3 –PbZrO3 –PbTiO3 system [14]. Fig. 7 shows the variation of dielectric constant with temperature for the composition containing 10 and 20 mol% Pb(Mn1/3 Nb2/3 )O3 . Compared with the high Curie temperature (Tc = 140 ◦ C) of Pb(Zn1/3 Nb2/3 )O3 , Pb(Mn1/3 Nb2/3 )O3 has a low Curie temperature (Tc = 20 ◦ C) [11]. So with increasing Pb(Mn1/3 Nb2/3 )O3 addition, the Curie temperature of PMZN–PZT becomes lower and consequently the peak of dielectric spectrum corre-
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Fig. 5. SEM micrographs for the composition containing (a) 5, (b) 10, (c) 15 and (d) 20 mol% Pb(Mn1/3 Nb2/3 )O3 .
sponding to the Curie temperature moves toward room temperature as shown in Fig. 7. The piezoelectric modulus (d33 ), planar coupling factor (kp ), dielectric loss (tan δ) and mechanical quality
factor (Qm ) versus the content of Pb(Mn1/3 Nb2/3 )O3 in PMZN–PZT are shown in Fig. 8. It is observed in Fig. 8 that as the increase of Pb(Mn1/3 Nb2/3 )O3 , Qm present a peak of 2528 at 10% Pb(Mn1/3 Nb2/3 )O3 ; meanwhile, the increasing
Fig. 6. Dielectric constant of poled and unpoled PMZN–PZT as a function of Pb(Mn1/3 Nb2/3 )O3 content.
Fig. 7. Temperature dependence of dielectric constants for PMZN–PZT containing 10 and 20 mol% Pb(Mn1/3 Nb2/3 )O3 .
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Fig. 8. Qm , d33 , kp and tan δ as a function of Pb(Mn1/3 Nb2/3 )O3 content.
of Pb(Mn1/3 Nb2/3 )O3 leads to the decrease of d33 , kp and tan δ. These variation in electrical properties can be contributed to both effects of compensation of oxygen vacancies for the charge deficits at B sites [11] and segregation at grain boundaries of excess Pb(Mn1/3 Nb2/3 )O3 which inhibit the movement of domain walls [4]. In particular, it should be noted that although the composition containing 10 mol% Pb(Mn1/3 Nb2/3 )O3 shows d33 and kp slightly less than that containing 5 mol% Pb(Mn1/3 Nb2/3 )O3 , the former has much larger Qm and lower tan δ, which is promising as an optimal material for high-power piezoelectric transformer.
4. Conclusions PMZN–PZT ceramics of pure perovskite structure were prepared via the two-stage processing route. The appropriate range of sintering temperature was from 1250 to 1300 ◦ C and lattice structure of system was transformed from the tetragonal to the cubic with the increasing amount of Pb(Mn1/3 Nb2/3 )O3 . The composition containing 10 mol% Pb(Mn1/3 Nb2/3 )O3 gave the desirable piezoelectric properties, i.e. Qm (2528), kp (0.55), and tan δ (0.003), which is a good candidate for piezoelectric transformer application.
Acknowledgements The authors are grateful to Science & Technology Development Project of Beijing Education Committee for financial support. References [1] Y. Fuda, K. Kumasaka, M. Katsuno, H. Sato, Y. Ino, Piezoelectric transformer for cold cathode fluorescent lamp inverter, Jpn. J. Appl. Phys. 36 (1997) 3050–3052. [2] J.H. Hu, H.L. Li, H.L.W. Chan, C.L. Choy, A ring-shaped piezoelectric transformer operating in the third symmetric extensional vibration mode, Sens. Actuators A 88 (2001) 79–86.
[3] L.T. Li, Z.L. Gui, Fabrication of low firing piezoelectric ceramics and their applications, Ferroelectrics 262 (2001) 3–10. [4] J.H. Yoo, Y.W. Lee, K.H. Yoon, S.M. Hwang, S.J. Suh, J.S. Kim, J.S. Kim, C.S. Yoo, Microstructural, electrical properties and temperature stability of resonant frequency in Pb(Ni1/2 W1/2 )O3 –Pb(Mn1/3 Nb2/3 )O3 –Pb(Zr, Ti)O3 ceramics for high power piezoelectric transformer, Jpn. J. Appl. Phys. 40 (2001) 3256–3259. [5] L.T. Li, W.T. Deng, J.H. Chai, Z.L. Gui, X.W. Zhang, Lead zirconate titanate ceramics and monolithic piezoelectric transformer of low firing temperature, Ferroelectrics 101 (1990) 193–200. [6] M. Kobune, Y. Tomoyoshi, A. Mineshige, S. Fujii, Effects of MnO2 addition on piezoelectric and ferroelectric properties of PbNi1/3 Nb2/3 O3 –PbTiO3 –PbZrO3 ceramics, J. Ceram. Soc. Jpn. 108 (2000) 633–637. [7] S.J. Yoon, A. Joshi, K. Uchino, Effect of additives on the electromechanical properties of Pb(Zr, Ti)O3 –Pb(Y2/3 W1/3 )O3 ceramics, J. Am. Ceram. Soc. 80 (1997) 1035–1039. [8] Y.K. Gao, Y.H. Chen, J.H. Ryu, K.J. Uchino, D. Viehland, Eu and Yb substituent effects on the properties of Pb(Zr0.52 Ti0.48 )O3 –Pb(Mn1/3 Sb2/3 )O3 ceramics: development of a new high-power piezoelectric with enhanced vibrational velocity, Jpn. J. Appl. Phys. 40 (2001) 687–693. [9] Z.L. Gui, L.T. Li, H.Q. Lin, X.W. Zhang, Low temperature sintering of lead magnesium nickel niobate zirconate titanate (PMN–PNN–PZT) piezoelectric ceramic, with high performances, Ferroelectrics 101 (1990) 93–99. [10] H.Q. Fan, H.E. Kim, Effect of lead content on the structure and electrical properties of Pb((Zn1/3 Nb2/3 )0.5 (Zr0.47 Ti0.53 )0.5 )O3 ceramics, J. Am. Ceram. Soc. 84 (2001) 636–638. [11] H.Y. Chen, X.B. Guo, Z.Y. Meng, Processing and properties of PMMN–PZT quaternary piezoelectric ceramics for ultrasonic motors, Mater. Chem. Phys. 75 (2002) 202–206. [12] S.L. Swartz, T.R. Shrout, Fabrication of perovskite lead magnesium niobate, Mater. Res. Bull. 17 (1982) 1245–1250. [13] L.T. Li, Y.J. Yao, Z.H. Mu, Piezoelectric ceramic transformer, Ferroelectrics 28 (1980) 403–406. [14] J.C. Shaw, K.S. Liu, I.N. Lin, Modification of piezoelectric characteristics of the Pb(Mg, Nb)O3 –PbZrO3 –PbTiO3 ternary system by aliovalent additives, J. Am. Ceram. Soc. 78 (1995) 178–182.
Biographies Yu-Dong Hou was born in China in November 1974. He received BS and MS degrees in inorganic chemistry from Northwest University, and PhD
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degree in materials science from Northwestern Polytechnical University, China, in 1997, 1999, and 2002, respectively. He has been a postdoctoral research fellow at the Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing. His main research interest is the preparation, structure and properties of ferroelectric, piezoelectric and dielectric ceramic materials and their applications for transformers, resonators and actuators. Chang-Sheng Tian was born in China in 1938. He received BS and MS degrees in materials science from Northwestern Polytechnical University, China, in 1961 and 1965, respectively. He is currently a professor of functional materials in Northwestern Polytechnical University and led a group to make creative studies on multilayer ceramic capacitor, piezoelectric transformer, relaxor ferroelectric ceramics and ferroelectric oxide thin films. He has published over 100 papers and has co-authored three books and dictionaries.
Man-Kang Zhu was born in China in 1963. He received BS degree from Zhejiang University, China, in 1983, and MS degree from Wuhan University of Technology, China in 1986, both in materials science and engineering. He has devoted to ferroelectric, piezoelectric and dielectric ceramic materials and their applications. Now, he is an associate professor in materials institute, Beijing University of Technology, and a PhD candidate in subject of physics and chemistry of materials. Hui Yan was born in China in 1963. He received BS degree in semiconductor physics from Lanzhou University of China and PhD degree in solid electronics from Kanazawa University of Japan in 1984 and 1993, respectively. Dr. Yan worked as a postdoctoral at the electronic department of the Chinese university of Hong Kong from 1993 to 1996. He is currently a professor materials science and condensed state physics at Beijing University of Technology.
Structure and electrical properties of PMZN–PZT quaternary ceramics for piezoelectric transformers Yu-Dong Hou a,∗ , Man-Kang Zhu a , Chang-Sheng Tian b , Hui Yan a a
b
The Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing 100022, China Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China Received 27 August 2003; received in revised form 9 May 2004; accepted 17 May 2004 Available online 21 July 2004
Abstract Pb(Mn1/3 Nb2/3 )O3 –Pb(Zn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMZN–PZT) quaternary ceramics with various contents of Pb(Mn1/3 Nb2/3 )O3 from 5 to 20 mol% were prepared by the columbite two-stage process and the effects of composition on the microstructure and piezoelectric properties were investigated. The results revealed that the quaternary ceramic can be densified at 1250–1300 ◦ C, and its crystal structure was transformed from the tetragonal to the cubic with increasing amount of Pb(Mn1/3 Nb2/3 )O3 . The optimized results of kp (0.55), tan δ (0.003) and Qm (2528) were obtained at 10 mol% Pb(Mn1/3 Nb2/3 )O3 , which is a new promising material for piezoelectric transformer application. © 2004 Elsevier B.V. All rights reserved. Keywords: PMZN–PZT quaternary system; Piezoelectric; Mechanical quality factor; Piezoelectric transformers
1. Introduction Piezoelectric ceramics have been extensively studied for many years. However, their applications for piezoelectric transformers were vigorously studied only in recently years for application of lighting liquid crystal display (LCD) backlights [1–3]. The properties of these materials should combine a high mechanical quality factor (Qm ) with high electromechanical coupling factor (kp ) and low dielectric loss (tan δ) simultaneously because the piezoelectric transformer operated at its resonant frequency in transformation between electrical and mechanical energy [4,5]. Until now, many ternary and quaternary systems, such as Pb(Ni1/3 Nb2/3 )O3 –PZT, Pb(Y2/3 W1/3 )O3 –PZT, Pb(Mn1/3 Sb2/3 )O3 –PZT, Pb(Mg1/3 Nb2/3 )O3 –Pb(Ni1/3 Nb2/3 )O3 – PZT, Pb(Ni1/2 W1/2 )O3 –Pb(Mn1/3 Nb2/3 )O3 –PZT, etc. [4,6–9] have been synthesized by modifications and/or substitutions to satisfy the requirements of practical applications of piezoelectric transformer. However, as far as the mechanical quality factor is concerned, there are few sys-
∗ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392412. E-mail addresses: [email protected], [email protected] (Y.-D. Hou).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.05.012
tems that can give a Qm larger than 2000. Because Qm is a reference parameter of the dissipation of vibrational energy due to internal friction effects, in the case of low Qm , energy loss will leads to the temperature rise of ceramic devices and the deterioration of piezoelectric properties. So, it is necessary to fabricate a piezoelectric ceramic that gives a high Qm while kp and tan δ are well situated to satisfy the requirements for practical application in piezoelectric transformers. Since Pb(Zn1/3 Nb2/3 )O3 –PZT ternary system exhibits high electromechanical coupling factor [10] and since Pb(Mn1/3 Nb2/3 )O3 –PZT ternary system can offer a high mechanical quality factor [11], it is believed that Pb(Mn1/3 Nb2/3 )O3 –Pb(Zn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMZN– PZT) by combining Pb(Mn1/3 Nb2/3 )O3 –PZT with Pb(Zn1/3 Nb2/3 )O3 –PZT should give a high Qm and suitable kp . In this study, PMZN–PZT quaternary ceramics with different contents of Pb(Mn1/3 Nb2/3 )O3 were prepared and the effects of Pb(Mn1/3 Nb2/3 )O3 content on the microstructure and piezoelectric properties were investigated. The electrical measurement shows that the PMZN–PZT quaternary ceramic with 10 mol% content of Pb(Mn1/3 Nb2/3 )O3 gives large Qm suitable for the application of high-power piezoelectric transformer.
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2. Experimental The general formula of the materials studied was (Pb0.95 Sr0.05 )(Mn1/3 Nb2/3 )x (Zn1/3 Nb2/3 )0.20−x Ti0.42 Zr0.38 O3 + 0.25 wt.% CeO2 , where x = 0.05, 0.10, 0.15 and 0.20, respectively (PMZN–PZT). The samples were prepared by the two-stage method [12]. Reagent-grade oxide powders, Pb3 O4 , ZrO2 , TiO2 , SrCO3 , ZnO, MnCO3 , Nb2 O5 and CeO2 were used as the starting materials. In the first stage, the powders of columbite precursor, ZnNb2 O6 and MnNb2 O6 , were prepared by reactions of ZnO and MnCO3 , respectively, with Nb2 O5 at 1100 ◦ C for 4 h. In the second stage, the columbite precursors with other oxides including Pb3 O4 , ZrO2 , TiO2 , SrCO3 and CeO2 were weighed and mixed through the use of a polyethylene jar and ZrO2 milling media. The mixture was then calcined at 850 ◦ C for 2 h in air, remilled, pressed into discs of 12.0 mm in diameter at around 120 MPa, and then sintered at 1050–1300 ◦ C for 2 h in a sealed alumina crucible with PbZrO3 powder atmosphere. The bulk density was measured using the Archimedean method and linear shrinkage was determined manually. Fig. 1 shows the variations of bulk density and the radial shrinkage ratio of PMZN–PZT specimens at different sintering temperature. It can be seen from Fig. 1 that the bulk densities for PMZN–PZT change as the sintering temperature in the same way of radial shrinkage ratio. Both curves are in S shape and can be divided into three regions, i.e. low-, intermediate- and high-temperature regions. In the low sintering temperature region from 1050 to 1150 ◦ C, both bulk densities and the radial shrinkage ratios increase slightly. In the intermediate temperature region from 1150 to 1250 ◦ C, bulk densities and the radial shrinkage ratios increase steeply, which shows the densification process of PMZN–PZT ceramics mainly carried in this temperature region. Further increasing the sintering temperature above 1250 ◦ C, the curves of bulk density and radial shrinkage ratio show the saturation trend. According to the above results,
Fig. 1. Bulk density and radial shrinkage ratio of PMZN–PZT specimens at different sintering temperature.
the suitable sintering temperature region of PMZN–PZT is from 1250 to 1300 ◦ C. The crystal structure of the samples was analyzed using an X-ray diffractometry (XRD; Model DMX-C, Japan) with a 2θ range from 15◦ to 70◦ . A step scan with a step size of 0.02◦ was used with a counting time of 1 s/step. Microstructural evolution was observed using a scanning electron microscopy (SEM; Model Hitachi S-570, Japan). The sintered discs were lapped and electroded with a silver paste. The specimens for the piezoelectric property measurements were poled in a silicone oil bath at 120 ◦ C by applying a dc electric field of 3 kV/mm for 30 min. The specimens were aged for 24 h prior to testing. The piezoelectric constant (d33 ) was measured using a quasi-static piezoelectric d33 meter (Model ZJ-3d, Institute of Acoustics Academic Sinica, China). The planar coupling coefficient (kp ) and mechanical quality factor (Qm ) were determined by the resonance and anti-resonance technique using an impedance analyzer (Model HP4194A, Hewlett-Packard, CA). The Curie temperature (Tc ) was determined by temperature dependence of the dielectric constant at 1 kHz.
3. Results and discussion 3.1. Phase characterization Figs. 2 and 3 show XRD patterns of the powders calcined at 850 ◦ C and ceramics sintered at 1275 ◦ C, respectively. Clearly seen from Fig. 2, all calcined powders consisted of pyrochlore phase formed along with perovskite phase. The amount of pyrochlore phase increased with increasing Pb(Mn1/3 Nb2/3 )O3 content. It is known that Pb(Mn1/3 Nb2/3 )O3 is an unstable relaxor ferroelectric compared to others, such as Pb(Mg1/3 Nb2/3 )O3 , Pb(Ni1/3 Nb2/3 )O3 , etc. Hence, the more Pb(Mn1/3 Nb2/3 )O3 contents were consisted in PMZN–PZT system, the more contents of pyrochlore phase appeared. In addition, the
Fig. 2. XRD patterns of PMZN–PZT powders calcined at 850 ◦ C.
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In this experiment, PMZN–PZT ceramics with 5 and 10 mol% PMN content have tetragonal structure and the ceramics with 15 and 20 mol% PMN content have cubic structure. 3.2. Microstructure
Fig. 3. XRD patterns of PMZN–PZT ceramics sintered at 1275 ◦ C.
pyrochlore phase has stable structure and is formed ahead of perovskite phase. In the low calcining temperature, the compositions diffusion is difficult and insufficient, which results in the formation of pyrochlore phase. When the temperature increased, the amount of pyrochlore was eliminated gradually because the high sintering temperature is favor to the compositions diffusion and the phase transformation from pyrochlore to perovskite. So, for sintered samples, only pure perovskite phase exists and no pyrochlore phase can be detected, as can be seen from Fig. 3. Moreover, PMZN–PZT systems are transformed from the tetragonal to the cubic structure with the increasing amount of Pb(Mn1/3 Nb2/3 )O3 , as revealed by incorporation of (0 0 2) and (2 0 0) peaks as the addition of Pb(Mn1/3 Nb2/3 )O3 in Fig. 3. The lattice parameters (a, c) of the perovskite phase have been evaluated from the diffraction patterns and are plotted in Fig. 4. Pb(Zr0.52 Ti0.48 )O3 has tetragonal structure (Powder Diffraction File No. 33-784) while Pb(Mn1/3 Nb2/3 )O3 has cubic structure (Powder Diffraction File No. 39-1007), so the increase of Pb(Mn1/3 Nb2/3 )O3 content leads to the phase transformation of PMZN–PZT.
Fig. 4. Lattice constant of PMZN–PZT as a function of Pb(Mn1/3 Nb2/3 )O3 content.
For the application of piezoelectric transformer operating at high vibration velocity, it is favorable to obtain the fine grain ceramics which possess higher strength. Fig. 5 shows the SEM micrographs of PMZN–PZT ceramics with 5–20 mol% Pb(Mn1/3 Nb2/3 )O3 sintered at 1275 ◦ C. All specimens have uniform and fine microstructure and the largest grain size is below 1.70 m. The structure of fine grains are mainly attributed to the CeO2 addition as it is believed that the addition of Ce ions will segregate at grain boundaries and inhibit the grain growth [13]. Moreover, it can be seen from Fig. 5 that the different amount of Pb(Mn1/3 Nb2/3 )O3 shows little impact on the grain growth and densification in PMZN–PZT ceramics, which is different from the observation in Pb(Mg1/3 Nb2/3 )O3 –Pb(Mn1/3 Nb2/3 )O3 –PbZrO3 –PbTiO3 (PMMN–PZT) system reported by Chen et al. [11]. It is implied that in PMZN–PZT ceramics, addition of CeO2 become more dominant than the effects of Pb(Mn1/3 Nb2/3 )O3 on the grain growth. 3.3. Electrical properties The poled and unpoled dielectric constant as a function of Pb(Mn1/3 Nb2/3 )O3 content measured at room temperature and frequency of 1 kHz are plotted in Fig. 6. It can be seen that dielectric constant decreases monotonically with increasing Pb(Mn1/3 Nb2/3 )O3 content. Pb(Mn1/3 Nb2/3 )O3 has little solubility (<5 mol%) in the PZT composition matrix and excess Pb(Mn1/3 Nb2/3 )O3 beyond the solubility limit is believed to segregate at grain boundaries [4], which attributes to the increase in the extent of space-charge polarization at the grain boundaries, resulting in domains bound within grains and decrease of dielectric constant. In addition, Fig. 6 also shows that the difference of dielectric constants before and after poling vary with Pb(Mn1/3 Nb2/3 )O3 content. K = (Kpole − Kunpole ) is positive for the amount of Pb(Mn1/3 Nb2/3 )O3 below 10 mol% and is negative for the amount of Pb(Mn1/3 Nb2/3 )O3 above 15 mol%. This transition from positive K for tetragonal phase to negative K for cubic phase has also been previously observed in Pb(Mg,Nb)O3 –PbZrO3 –PbTiO3 system [14]. Fig. 7 shows the variation of dielectric constant with temperature for the composition containing 10 and 20 mol% Pb(Mn1/3 Nb2/3 )O3 . Compared with the high Curie temperature (Tc = 140 ◦ C) of Pb(Zn1/3 Nb2/3 )O3 , Pb(Mn1/3 Nb2/3 )O3 has a low Curie temperature (Tc = 20 ◦ C) [11]. So with increasing Pb(Mn1/3 Nb2/3 )O3 addition, the Curie temperature of PMZN–PZT becomes lower and consequently the peak of dielectric spectrum corre-
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Fig. 5. SEM micrographs for the composition containing (a) 5, (b) 10, (c) 15 and (d) 20 mol% Pb(Mn1/3 Nb2/3 )O3 .
sponding to the Curie temperature moves toward room temperature as shown in Fig. 7. The piezoelectric modulus (d33 ), planar coupling factor (kp ), dielectric loss (tan δ) and mechanical quality
factor (Qm ) versus the content of Pb(Mn1/3 Nb2/3 )O3 in PMZN–PZT are shown in Fig. 8. It is observed in Fig. 8 that as the increase of Pb(Mn1/3 Nb2/3 )O3 , Qm present a peak of 2528 at 10% Pb(Mn1/3 Nb2/3 )O3 ; meanwhile, the increasing
Fig. 6. Dielectric constant of poled and unpoled PMZN–PZT as a function of Pb(Mn1/3 Nb2/3 )O3 content.
Fig. 7. Temperature dependence of dielectric constants for PMZN–PZT containing 10 and 20 mol% Pb(Mn1/3 Nb2/3 )O3 .
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Fig. 8. Qm , d33 , kp and tan δ as a function of Pb(Mn1/3 Nb2/3 )O3 content.
of Pb(Mn1/3 Nb2/3 )O3 leads to the decrease of d33 , kp and tan δ. These variation in electrical properties can be contributed to both effects of compensation of oxygen vacancies for the charge deficits at B sites [11] and segregation at grain boundaries of excess Pb(Mn1/3 Nb2/3 )O3 which inhibit the movement of domain walls [4]. In particular, it should be noted that although the composition containing 10 mol% Pb(Mn1/3 Nb2/3 )O3 shows d33 and kp slightly less than that containing 5 mol% Pb(Mn1/3 Nb2/3 )O3 , the former has much larger Qm and lower tan δ, which is promising as an optimal material for high-power piezoelectric transformer.
4. Conclusions PMZN–PZT ceramics of pure perovskite structure were prepared via the two-stage processing route. The appropriate range of sintering temperature was from 1250 to 1300 ◦ C and lattice structure of system was transformed from the tetragonal to the cubic with the increasing amount of Pb(Mn1/3 Nb2/3 )O3 . The composition containing 10 mol% Pb(Mn1/3 Nb2/3 )O3 gave the desirable piezoelectric properties, i.e. Qm (2528), kp (0.55), and tan δ (0.003), which is a good candidate for piezoelectric transformer application.
Acknowledgements The authors are grateful to Science & Technology Development Project of Beijing Education Committee for financial support. References [1] Y. Fuda, K. Kumasaka, M. Katsuno, H. Sato, Y. Ino, Piezoelectric transformer for cold cathode fluorescent lamp inverter, Jpn. J. Appl. Phys. 36 (1997) 3050–3052. [2] J.H. Hu, H.L. Li, H.L.W. Chan, C.L. Choy, A ring-shaped piezoelectric transformer operating in the third symmetric extensional vibration mode, Sens. Actuators A 88 (2001) 79–86.
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Biographies Yu-Dong Hou was born in China in November 1974. He received BS and MS degrees in inorganic chemistry from Northwest University, and PhD
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degree in materials science from Northwestern Polytechnical University, China, in 1997, 1999, and 2002, respectively. He has been a postdoctoral research fellow at the Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing. His main research interest is the preparation, structure and properties of ferroelectric, piezoelectric and dielectric ceramic materials and their applications for transformers, resonators and actuators. Chang-Sheng Tian was born in China in 1938. He received BS and MS degrees in materials science from Northwestern Polytechnical University, China, in 1961 and 1965, respectively. He is currently a professor of functional materials in Northwestern Polytechnical University and led a group to make creative studies on multilayer ceramic capacitor, piezoelectric transformer, relaxor ferroelectric ceramics and ferroelectric oxide thin films. He has published over 100 papers and has co-authored three books and dictionaries.
Man-Kang Zhu was born in China in 1963. He received BS degree from Zhejiang University, China, in 1983, and MS degree from Wuhan University of Technology, China in 1986, both in materials science and engineering. He has devoted to ferroelectric, piezoelectric and dielectric ceramic materials and their applications. Now, he is an associate professor in materials institute, Beijing University of Technology, and a PhD candidate in subject of physics and chemistry of materials. Hui Yan was born in China in 1963. He received BS degree in semiconductor physics from Lanzhou University of China and PhD degree in solid electronics from Kanazawa University of Japan in 1984 and 1993, respectively. Dr. Yan worked as a postdoctoral at the electronic department of the Chinese university of Hong Kong from 1993 to 1996. He is currently a professor materials science and condensed state physics at Beijing University of Technology.