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Microstructural and piezoelectric properties of low temperature sintering PMN-PZT ceramics with the amount of Li2CO3 addition
Materials Chemistry and Physics 90 (2005) 386–390
Review
Microstructural and piezoelectric properties of low temperature sintering PMN-PZT ceramics with the amount of Li2CO3 addition Juhyun Yooa,∗ , Changbae Leea , Yeongho Jeongb , Kwanghyun Chungc , Duckchool Leec , Dongsoo Paikd a
Department of Electrical Engineering, Semyung University, Jechon, Chungbuk 390-711, Korea b Korea Electric Power Research Institute, Yusung-Gu, Taejon 305-380, Korea c Department of Electrical Engineering, Inha University, Incheon 402-751, Korea d E2S Technologies Co. Ltd., KITI, Suwon University, Hwaseung, Gyeonggi 445-743, Korea Received 31 March 2004; accepted 23 September 2004
Abstract PMN-PZT ceramics doped with Li2 CO3 and Bi2 O3 as sintering aids were manufactured in order to develop the low temperature sintering ceramics for multilayer piezoelectric transformer, and their micro structural, dielectric and piezoelectric properties were investigated. The sintering aids were proved to lower the sintering temperature of doped PMN-PZT ceramics due to the effect of LiBiO2 liquid phase. Optimal values for multilayer piezoelectric transformer application, such as electromechanical coupling factor (kp ) of 0.50, mechanical quality factor (Qm ) of 2264, and dielectric constant (K) of 1216, and curie temperature (Tc ) of 317 ◦ C were found at 0.1 wt.% Li2 CO3 added ceramics sintered at 940 ◦ C. © 2004 Elsevier B.V. All rights reserved. Keywords: Low temperature sintering ceramics; Dielectric properties; Electromechanical coupling factor (kp ); Mechanical quality factor (Qm ); Sintering aids
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
386
2.
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.
Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Compared with conventional electromagnetic transformers, piezoelectric transformers are expected to be thin, minia∗
Corresponding author. Tel.: +82 43 649 1795; fax: +82 43 648 0868. E-mail address: [email protected] (J. Yoo).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.09.036
turized, non-flammable and highly efficient. Therefore, liquid crystal display (LCD) backlight inverter, DC-DC converter, AC-DC converter and ballast for fluorescent lamps have been widely developed using the piezoelectric transformers [1,2]. However, conventional Rosen-type piezoelectric transformers for step-up voltage operated at high voltage and low current could not be successfully used for high power devices,
J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
such as DC-DC converter, AC-DC converter and ballast [3,4]. It is, therefore, clear that the output structure of piezoelectric transformer for step-down voltage has to be newly designed with multilayer feature in order to increase output current and power. However, the multilayer piezoelectric transformer has several problems compared with single layer piezoelectric transformer [5,6]. When it is cofired with internal electrode (Ag/Pd), the piezoelectric properties, especially mechanical quality factor (Qm ), decrease because of the formation of interfacial micro defects and internal electrode loss. Since the sintering temperature of lead-based piezoelectric ceramics, such as Pb(Zr,Ti)O3 and Pb(Mg1/3 Nb2/3 )O3 is generally higher than 1200 ◦ C, the ratio of expensive Pd in Ag/Pd internal electrode must be increased. Therefore, the excellent piezoelectric ceramics, which have higher mechanical quality factor (Qm ) and can be sintered at the low temperature, are required in order to compensate for the deteriorated piezoelectric properties of the multilayer piezoelectric transformer and to decrease the amount of expensive Pd. In this study, in order to develop low temperature sintering piezoelectric ceramics for multilayer piezoelectric transformer for AC-DC converter, PMN-PZT ceramics were manufactured according to the amount of Li2 CO3 addition and their piezoelectric characteristics were investigated.
2. Experimental The specimens were manufactured using a conventional mixed oxide process. The composition used in this study was as follows: [Pb0.97 Sr0.03 (Mn1/3 Nb2/3 )0.07 (Zr 0.48 Ti0.52 )0.93 O3 ] + 0.25 wt.% CeO2 + 0.3 wt.% Nb2 O5 + 0.3 wt.% CuO + 0.3 wt.% Bi2 O3 + x wt.% Li2 CO3 (x = 0, 0.05, 0.1, 0.15, 0.2and0.25) The raw materials, such as PbO, ZrO2 , TiO2 , MnO2 and Nb2 O5 among the given composition were weighted by mole ratio and the powders were ball-milled for 24 h. After drying, they were calcined at 850 ◦ C for 2 h. Thereafter, CuO, Bi2 O3 and Li2 CO3 were added, ball-milled, and dried again. A polyvinyl alcohol (PVA: 5%) was added to the dried powders. The powders were molded by the pressure of 1000 kg cm−2 in 21 mm Φ mold, burned out at 600 ◦ C for 3 h, and then sintered at 930–1030 ◦ C for 2 h. For measuring the piezoelectric characteristics, the specimens were polished to 1 mm thickness and then electrodeposited with Ag paste. Poling was carried out at 120 ◦ C in a silicon oil bath by applying fields of 30 kV cm−1 for 30 min. All samples were aged for 24 h prior to measuring the piezoelectric and dielectric properties. The microstructure and crystal structure of specimens were analyzed through SEM (Hitachi, S-2400) and XRD (Rigaku, D/MAX-2500H), respectively. For investigating the dielectric properties, capacitance was measured at 1 kHz using a
387
LCR meter (ANDO AG-4034) and dielectric constant was calculated. For investigating the piezoelectric properties, the resonant and antiresonant frequencies were measured by an Impedance Analyzer (Agilent 4294A) according to IRE standard and then the electromechanical coupling factor and mechanical quality factor were calculated.
3. Results and discussion Fig. 1 shows the variation of density with sintering temperature and the amount of Li2 CO3 addition. The density of specimens sintered at 930 ◦ C and 940 ◦ C showed the maximum value of 7.52 g cm−3 and 7.77 g cm−3 at 0.15 wt.% Li2 CO3 addition, respectively, and then was decreased after the maximum value. Specimens sintered at 970 ◦ C, 1000 ◦ C and 1030 ◦ C showed the maximum value of 7.78 g cm−3 , 7.77 g cm−3 and 7.77 g cm−3 at 0.1 wt.% Li2 CO3 addition, respectively. Therefore, dense ceramics were obtained at the sintering temperature more than 940 ◦ C and the Li2 CO3 addition of 0.1–0.2 wt.%. The doped Li2 CO3 apparently affects to lower the sintering temperature of PMN-PZT ceramics. Since the Bi2 O3 with melting point of 825 ◦ C forms a liquid phase at about 690 ◦ C by adding LiO2 , the liquid phase assists the lowering of sintering temperature [7]. Fig. 2 shows the SEM microstructure of specimens sintered at 940 ◦ C with various amount of Li2 CO3 addition. The grain size of specimen gradually increased up to 3 m with increase in Li2 CO3 addition and then decreased over 0.2 wt.% Li2 CO3 addition, since the sinterbility at low temperature was enhanced by the reaction between Bi2 O3 and Li2 CO3 , the grain size and density increased. However, the decrease of the grain size and density after the maximum value can be explained by the facts that a reactant is segregated at grain boundary by excessive addition, thus inhibiting the grain growth and densification by acting as impurities. As can see in Table 1, the grain size of specimens sintered at 970 ◦ C showed the maximum size of 3.64 m at 0.1 wt.% Li2 CO3 addition and then, decreased above the addition. This result can be explained by the fact that liquid phase formed by the reaction between Bi2 O3 and
Fig. 1. Density with Li2 CO3 addition.
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Fig. 2. Microstructure of specimen with Li2 CO3 addition (940 ◦ C). Table 1 Physical properties of specimen with Li2 CO3 addition Temperature (◦ C)
930
940
970
1000
1030
Li2 CO3 (wt.%)
Density (g cm−3 )
Dielectric constant
0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25
6.78 7.12 7.52 7.38 7.33 7.64 7.72 7.77 7.75 7.62 7.64 7.78 7.77 7.77 7.75 7.56 7.77 7.74 7.69 7.71 7.40 7.77 7.75 7.75 7.74
767 1074 1170 1167 1143 864 1216 1271 1297 1263 880 1246 1277 1313 1352 958 1248 1276 1307 1346 1063 1270 1270 1310 1336
Grain size (m) 1.83
1.07 2.34 2.78 3.04 2.57 1.30 3.64 3.29 3.07 2.97 2.39
2.68
kp
Qm
0.28 0.41 0.48 0.47 0.47 0.28 0.50 0.50 0.51 0.51 0.31 0.50 0.51 0.51 0.52 0.35 0.50 0.50 0.51 0.52 0.41 0.50 0.50 0.51 0.52
234 1053 1720 1793 1362 1038 2264 2014 1936 1813 1080 2414 2115 2079 2003 1273 2248 2020 1962 1920 1294 2012 2000 1753 1721
J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
Fig. 3. XRD patterns with Li2 CO3 addition (940 ◦ C).
Fig. 4. Temperature dependence (Tc ) of dielectric constant with Li2 CO3 addition (940 ◦ C).
Li2 CO3 is volatilized at high temperature. On the other hand, the effect of Bi2 O3 and Li2 CO3 is dominant at lower sintering temperature. Fig. 3 shows X-ray diffraction patterns of specimens sintered at 940 ◦ C with various amount of Li2 CO3 addition. All specimens showed the splitting of (0 0 2) and (2 0 0) peaks implying the tetragonal phase. Tetragonality was nearly constant for all the prepared powers. Fig. 4 shows the temperature dependence of dielectric constant of specimens sintered at 940 ◦ C with the amount
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Fig. 5. Electromechanical coupling factor (kp ) with Li2 CO3 addition.
of Li2 CO3 addition. Curie temperature (Tc ) showed the maximum value of 330 ◦ C at 0.15 wt.% Li2 CO3 and decreased after the maximum value. It means that ion substitution happens in the PMN-PZT system ceramics with perovskite structure of ABO3 . That is, the increase of Curie temperature appeared because the Li+ ion produced by Li2 CO3 addition was substituted for B-site ion of PMN-PZT ceramics with perovskite structure of ABO3 and reacted as hardner producing O-vacancy. Fig. 5 shows electromechanical coupling factor (kp ) according to the variation of the sintering temperature and the amount of Li2 CO3 addition. The kp of specimens sintered at 930 ◦ C and 940 ◦ C showed the maximum values of 0.48 and 0.51 at 0.15 wt.% and 0.2 wt.% Li2 CO3 additions, respectively. But the kp ’s of specimens sintered at 970 ◦ C, 1000 ◦ C and 1030 ◦ C were saturated over 0.1 wt.% Li2 CO3 addition and the highest kp was 0.52 at 0.3 wt.% Li2 CO3 addition. These results are coincident with the trends of density and grain size according to the amount of Li2 CO3 addition. Fig. 6(a) and (b) show mechanical quality factor (Qm ) according to the variation of the sintering temperature and the amount of Li2 CO3 addition. The behavior of Qm is similar to those of density, grain size, and kp . The maximum values of Qm were obtained at 0.1 wt.% Li2 CO3 addition regardless of the sintering temperature. To maximize the Qm , the sintering
Fig. 6. Mechanical quality factor (Qm ) with Li2 CO3 addition and sintering temperature.
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J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
4. Conclusions In this study, in order to develop the low temperature sintering piezoelectric ceramics for multilayer piezoelectric transformer, which has a high electromechanical coupling factor (kp ) and mechanical quality factor (Qm ), PMN-PZT ceramics were manufactured with various amount of Li2 CO3 addition. Their dielectric and piezoelectric characteristics were investigated. The results obtained from the experiment are as follows:
Fig. 7. Dielectric constant (K) with Li2 CO3 addition.
temperature of 970 ◦ C seems to be adequate for the purpose of multilayer piezoelectric transformer application. And also, as can see in Fig. 6(b), the excessive Li2 CO3 addition over 0.1 wt.% did not contribute the increase of Qm regardless of the sintering temperature. And the decrease of Qm after the maximum value can be illustrated by over addition and the increase of porosity due to over firing. The PMN-PZT ceramics doped with low temperature sintering aids exhibit optimal behavior at the sintering temperature of less than 970 ◦ C. Fig. 7 shows dielectric constant according to the variation of the sintering temperature and the amount of Li2 CO3 addition. Dielectric constant of specimens sintered at 930 ◦ C showed the maximum value of 1170 at 0.15 wt.% Li2 CO3 addition. But, those sintered at more than 930 ◦ C increased with the amount of Li2 CO3 addition. These results can be illustrated by the facts that the dielectric constants of specimen sintered below 940 ◦ C increase with increasing density according to the amount of Li2 CO3 addition, but decrease after the maximum value because of segregation of the sintering aids with a low dielectric constant to grain boundary by amorphous forms. However, those sintered above 940 ◦ C increase continuously by volatilization of liquid phase with a low dielectric constant. From these results, the simultaneous increase of kp and Qm values according to Li2 CO3 addition can be partially illustrated by the facts that Li2 CO3 and Bi2 O3 react as hardner and softener producing Pb-vacancy and O-vacancy in the ˚ and PZT composition ceramics by substituting Bi3+ (0.96 A) ˚ for Pb2+ (1.18 A) ˚ and Ti4+ (0.68 A), ˚ respectively Li+ (0.74 A) [8,9], as well as the formation of liquid phase which increase the density and grain size. Consequently, Li2 CO3 addition to the PZT composition ceramics could lead to low temperature sintering, and at the same time improve piezoelectric properties. Table 1 shows the piezoelectric and dielectric characteristics of specimens manufactured according to the amount of Li2 CO3 addition.
(1) Li2 CO3 doped PMN-PZT ceramics showed the splitting of (0 0 2) and (2 0 0) peaks implying the tetragonal phase without phase transition. (2) Dense ceramics were obtained at the sintering temperature more than 940 ◦ C and the Li2 CO3 addition of 0.1–0.2 wt.%. (3) Optimal values for multilayer piezoelectric transformer application, such as electromechanical coupling factor (kp ) of 0.50, mechanical quality factor (Qm ) of 2264, and dielectric constant (K) of 1216, and curie temperature (Tc ) of 317 ◦ C were found at 0.1 wt.% Li2 CO3 added ceramics sintered at the low sintering temperature of 940 ◦ C. (4) Li2 CO3 addition was proved to lower the sintering temperature of piezoelectric ceramics due to the effect of LiBiO2 liquid phase and to improve the piezoelectric properties by acting as the hardner and softner.
Acknowledgements This study was supported by Korea Electric Power Research Institute (Grant No. R-2004-0-114).
References [1] J.H. Yoo, K.H. Yoon, Y.W. Lee, S.S. Suh, J.S. Kim, C.S. Yoo, Jpn. J. Appl. Phys. 39 (2000) 2680. [2] J. Hu, Y. Fuda, M. Katsuno, T. Yoshida, Jpn. J. Appl. Phys. 38 (1999) 3208. [3] Y. Sasaki, M. Yamamoto, A. Ochi, T. Inoue, S. Takahshi, Jpn. J. Appl. Phys. 38 (1999) 5598. [4] N.Y. Wong, Y. Zhang, H.L.W. Chan, C.L. Choy, Mater. Sci. Eng. B99 (2003) 164. [5] R. Zuo, L. Li, Z. Gui, Mater. Sci. Eng. A326 (2002) 202. [6] T. Hayashi, T. Hasegawa, J. Tomizawa, Y. Akiyama, Jpn. J. Appl. 42 (2003) 6074. [7] E.M. Levin, C.R. Robbins, H.F. Mcmurdie, in: M.K. Reser (Ed.), Phase Diagrams for Ceramists, vol. 1, fourth ed., The American Ceramic Society, Ohio, 1979, p. 126. [8] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971, p. 154. [9] Y. Xu, Ferroelectric Materials and their Application, North-Holland, Amsterdam, 1991, p. 130.
Review
Microstructural and piezoelectric properties of low temperature sintering PMN-PZT ceramics with the amount of Li2CO3 addition Juhyun Yooa,∗ , Changbae Leea , Yeongho Jeongb , Kwanghyun Chungc , Duckchool Leec , Dongsoo Paikd a
Department of Electrical Engineering, Semyung University, Jechon, Chungbuk 390-711, Korea b Korea Electric Power Research Institute, Yusung-Gu, Taejon 305-380, Korea c Department of Electrical Engineering, Inha University, Incheon 402-751, Korea d E2S Technologies Co. Ltd., KITI, Suwon University, Hwaseung, Gyeonggi 445-743, Korea Received 31 March 2004; accepted 23 September 2004
Abstract PMN-PZT ceramics doped with Li2 CO3 and Bi2 O3 as sintering aids were manufactured in order to develop the low temperature sintering ceramics for multilayer piezoelectric transformer, and their micro structural, dielectric and piezoelectric properties were investigated. The sintering aids were proved to lower the sintering temperature of doped PMN-PZT ceramics due to the effect of LiBiO2 liquid phase. Optimal values for multilayer piezoelectric transformer application, such as electromechanical coupling factor (kp ) of 0.50, mechanical quality factor (Qm ) of 2264, and dielectric constant (K) of 1216, and curie temperature (Tc ) of 317 ◦ C were found at 0.1 wt.% Li2 CO3 added ceramics sintered at 940 ◦ C. © 2004 Elsevier B.V. All rights reserved. Keywords: Low temperature sintering ceramics; Dielectric properties; Electromechanical coupling factor (kp ); Mechanical quality factor (Qm ); Sintering aids
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
386
2.
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.
Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
390
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Compared with conventional electromagnetic transformers, piezoelectric transformers are expected to be thin, minia∗
Corresponding author. Tel.: +82 43 649 1795; fax: +82 43 648 0868. E-mail address: [email protected] (J. Yoo).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.09.036
turized, non-flammable and highly efficient. Therefore, liquid crystal display (LCD) backlight inverter, DC-DC converter, AC-DC converter and ballast for fluorescent lamps have been widely developed using the piezoelectric transformers [1,2]. However, conventional Rosen-type piezoelectric transformers for step-up voltage operated at high voltage and low current could not be successfully used for high power devices,
J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
such as DC-DC converter, AC-DC converter and ballast [3,4]. It is, therefore, clear that the output structure of piezoelectric transformer for step-down voltage has to be newly designed with multilayer feature in order to increase output current and power. However, the multilayer piezoelectric transformer has several problems compared with single layer piezoelectric transformer [5,6]. When it is cofired with internal electrode (Ag/Pd), the piezoelectric properties, especially mechanical quality factor (Qm ), decrease because of the formation of interfacial micro defects and internal electrode loss. Since the sintering temperature of lead-based piezoelectric ceramics, such as Pb(Zr,Ti)O3 and Pb(Mg1/3 Nb2/3 )O3 is generally higher than 1200 ◦ C, the ratio of expensive Pd in Ag/Pd internal electrode must be increased. Therefore, the excellent piezoelectric ceramics, which have higher mechanical quality factor (Qm ) and can be sintered at the low temperature, are required in order to compensate for the deteriorated piezoelectric properties of the multilayer piezoelectric transformer and to decrease the amount of expensive Pd. In this study, in order to develop low temperature sintering piezoelectric ceramics for multilayer piezoelectric transformer for AC-DC converter, PMN-PZT ceramics were manufactured according to the amount of Li2 CO3 addition and their piezoelectric characteristics were investigated.
2. Experimental The specimens were manufactured using a conventional mixed oxide process. The composition used in this study was as follows: [Pb0.97 Sr0.03 (Mn1/3 Nb2/3 )0.07 (Zr 0.48 Ti0.52 )0.93 O3 ] + 0.25 wt.% CeO2 + 0.3 wt.% Nb2 O5 + 0.3 wt.% CuO + 0.3 wt.% Bi2 O3 + x wt.% Li2 CO3 (x = 0, 0.05, 0.1, 0.15, 0.2and0.25) The raw materials, such as PbO, ZrO2 , TiO2 , MnO2 and Nb2 O5 among the given composition were weighted by mole ratio and the powders were ball-milled for 24 h. After drying, they were calcined at 850 ◦ C for 2 h. Thereafter, CuO, Bi2 O3 and Li2 CO3 were added, ball-milled, and dried again. A polyvinyl alcohol (PVA: 5%) was added to the dried powders. The powders were molded by the pressure of 1000 kg cm−2 in 21 mm Φ mold, burned out at 600 ◦ C for 3 h, and then sintered at 930–1030 ◦ C for 2 h. For measuring the piezoelectric characteristics, the specimens were polished to 1 mm thickness and then electrodeposited with Ag paste. Poling was carried out at 120 ◦ C in a silicon oil bath by applying fields of 30 kV cm−1 for 30 min. All samples were aged for 24 h prior to measuring the piezoelectric and dielectric properties. The microstructure and crystal structure of specimens were analyzed through SEM (Hitachi, S-2400) and XRD (Rigaku, D/MAX-2500H), respectively. For investigating the dielectric properties, capacitance was measured at 1 kHz using a
387
LCR meter (ANDO AG-4034) and dielectric constant was calculated. For investigating the piezoelectric properties, the resonant and antiresonant frequencies were measured by an Impedance Analyzer (Agilent 4294A) according to IRE standard and then the electromechanical coupling factor and mechanical quality factor were calculated.
3. Results and discussion Fig. 1 shows the variation of density with sintering temperature and the amount of Li2 CO3 addition. The density of specimens sintered at 930 ◦ C and 940 ◦ C showed the maximum value of 7.52 g cm−3 and 7.77 g cm−3 at 0.15 wt.% Li2 CO3 addition, respectively, and then was decreased after the maximum value. Specimens sintered at 970 ◦ C, 1000 ◦ C and 1030 ◦ C showed the maximum value of 7.78 g cm−3 , 7.77 g cm−3 and 7.77 g cm−3 at 0.1 wt.% Li2 CO3 addition, respectively. Therefore, dense ceramics were obtained at the sintering temperature more than 940 ◦ C and the Li2 CO3 addition of 0.1–0.2 wt.%. The doped Li2 CO3 apparently affects to lower the sintering temperature of PMN-PZT ceramics. Since the Bi2 O3 with melting point of 825 ◦ C forms a liquid phase at about 690 ◦ C by adding LiO2 , the liquid phase assists the lowering of sintering temperature [7]. Fig. 2 shows the SEM microstructure of specimens sintered at 940 ◦ C with various amount of Li2 CO3 addition. The grain size of specimen gradually increased up to 3 m with increase in Li2 CO3 addition and then decreased over 0.2 wt.% Li2 CO3 addition, since the sinterbility at low temperature was enhanced by the reaction between Bi2 O3 and Li2 CO3 , the grain size and density increased. However, the decrease of the grain size and density after the maximum value can be explained by the facts that a reactant is segregated at grain boundary by excessive addition, thus inhibiting the grain growth and densification by acting as impurities. As can see in Table 1, the grain size of specimens sintered at 970 ◦ C showed the maximum size of 3.64 m at 0.1 wt.% Li2 CO3 addition and then, decreased above the addition. This result can be explained by the fact that liquid phase formed by the reaction between Bi2 O3 and
Fig. 1. Density with Li2 CO3 addition.
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J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
Fig. 2. Microstructure of specimen with Li2 CO3 addition (940 ◦ C). Table 1 Physical properties of specimen with Li2 CO3 addition Temperature (◦ C)
930
940
970
1000
1030
Li2 CO3 (wt.%)
Density (g cm−3 )
Dielectric constant
0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25
6.78 7.12 7.52 7.38 7.33 7.64 7.72 7.77 7.75 7.62 7.64 7.78 7.77 7.77 7.75 7.56 7.77 7.74 7.69 7.71 7.40 7.77 7.75 7.75 7.74
767 1074 1170 1167 1143 864 1216 1271 1297 1263 880 1246 1277 1313 1352 958 1248 1276 1307 1346 1063 1270 1270 1310 1336
Grain size (m) 1.83
1.07 2.34 2.78 3.04 2.57 1.30 3.64 3.29 3.07 2.97 2.39
2.68
kp
Qm
0.28 0.41 0.48 0.47 0.47 0.28 0.50 0.50 0.51 0.51 0.31 0.50 0.51 0.51 0.52 0.35 0.50 0.50 0.51 0.52 0.41 0.50 0.50 0.51 0.52
234 1053 1720 1793 1362 1038 2264 2014 1936 1813 1080 2414 2115 2079 2003 1273 2248 2020 1962 1920 1294 2012 2000 1753 1721
J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
Fig. 3. XRD patterns with Li2 CO3 addition (940 ◦ C).
Fig. 4. Temperature dependence (Tc ) of dielectric constant with Li2 CO3 addition (940 ◦ C).
Li2 CO3 is volatilized at high temperature. On the other hand, the effect of Bi2 O3 and Li2 CO3 is dominant at lower sintering temperature. Fig. 3 shows X-ray diffraction patterns of specimens sintered at 940 ◦ C with various amount of Li2 CO3 addition. All specimens showed the splitting of (0 0 2) and (2 0 0) peaks implying the tetragonal phase. Tetragonality was nearly constant for all the prepared powers. Fig. 4 shows the temperature dependence of dielectric constant of specimens sintered at 940 ◦ C with the amount
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Fig. 5. Electromechanical coupling factor (kp ) with Li2 CO3 addition.
of Li2 CO3 addition. Curie temperature (Tc ) showed the maximum value of 330 ◦ C at 0.15 wt.% Li2 CO3 and decreased after the maximum value. It means that ion substitution happens in the PMN-PZT system ceramics with perovskite structure of ABO3 . That is, the increase of Curie temperature appeared because the Li+ ion produced by Li2 CO3 addition was substituted for B-site ion of PMN-PZT ceramics with perovskite structure of ABO3 and reacted as hardner producing O-vacancy. Fig. 5 shows electromechanical coupling factor (kp ) according to the variation of the sintering temperature and the amount of Li2 CO3 addition. The kp of specimens sintered at 930 ◦ C and 940 ◦ C showed the maximum values of 0.48 and 0.51 at 0.15 wt.% and 0.2 wt.% Li2 CO3 additions, respectively. But the kp ’s of specimens sintered at 970 ◦ C, 1000 ◦ C and 1030 ◦ C were saturated over 0.1 wt.% Li2 CO3 addition and the highest kp was 0.52 at 0.3 wt.% Li2 CO3 addition. These results are coincident with the trends of density and grain size according to the amount of Li2 CO3 addition. Fig. 6(a) and (b) show mechanical quality factor (Qm ) according to the variation of the sintering temperature and the amount of Li2 CO3 addition. The behavior of Qm is similar to those of density, grain size, and kp . The maximum values of Qm were obtained at 0.1 wt.% Li2 CO3 addition regardless of the sintering temperature. To maximize the Qm , the sintering
Fig. 6. Mechanical quality factor (Qm ) with Li2 CO3 addition and sintering temperature.
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J. Yoo et al. / Materials Chemistry and Physics 90 (2005) 386–390
4. Conclusions In this study, in order to develop the low temperature sintering piezoelectric ceramics for multilayer piezoelectric transformer, which has a high electromechanical coupling factor (kp ) and mechanical quality factor (Qm ), PMN-PZT ceramics were manufactured with various amount of Li2 CO3 addition. Their dielectric and piezoelectric characteristics were investigated. The results obtained from the experiment are as follows:
Fig. 7. Dielectric constant (K) with Li2 CO3 addition.
temperature of 970 ◦ C seems to be adequate for the purpose of multilayer piezoelectric transformer application. And also, as can see in Fig. 6(b), the excessive Li2 CO3 addition over 0.1 wt.% did not contribute the increase of Qm regardless of the sintering temperature. And the decrease of Qm after the maximum value can be illustrated by over addition and the increase of porosity due to over firing. The PMN-PZT ceramics doped with low temperature sintering aids exhibit optimal behavior at the sintering temperature of less than 970 ◦ C. Fig. 7 shows dielectric constant according to the variation of the sintering temperature and the amount of Li2 CO3 addition. Dielectric constant of specimens sintered at 930 ◦ C showed the maximum value of 1170 at 0.15 wt.% Li2 CO3 addition. But, those sintered at more than 930 ◦ C increased with the amount of Li2 CO3 addition. These results can be illustrated by the facts that the dielectric constants of specimen sintered below 940 ◦ C increase with increasing density according to the amount of Li2 CO3 addition, but decrease after the maximum value because of segregation of the sintering aids with a low dielectric constant to grain boundary by amorphous forms. However, those sintered above 940 ◦ C increase continuously by volatilization of liquid phase with a low dielectric constant. From these results, the simultaneous increase of kp and Qm values according to Li2 CO3 addition can be partially illustrated by the facts that Li2 CO3 and Bi2 O3 react as hardner and softener producing Pb-vacancy and O-vacancy in the ˚ and PZT composition ceramics by substituting Bi3+ (0.96 A) ˚ for Pb2+ (1.18 A) ˚ and Ti4+ (0.68 A), ˚ respectively Li+ (0.74 A) [8,9], as well as the formation of liquid phase which increase the density and grain size. Consequently, Li2 CO3 addition to the PZT composition ceramics could lead to low temperature sintering, and at the same time improve piezoelectric properties. Table 1 shows the piezoelectric and dielectric characteristics of specimens manufactured according to the amount of Li2 CO3 addition.
(1) Li2 CO3 doped PMN-PZT ceramics showed the splitting of (0 0 2) and (2 0 0) peaks implying the tetragonal phase without phase transition. (2) Dense ceramics were obtained at the sintering temperature more than 940 ◦ C and the Li2 CO3 addition of 0.1–0.2 wt.%. (3) Optimal values for multilayer piezoelectric transformer application, such as electromechanical coupling factor (kp ) of 0.50, mechanical quality factor (Qm ) of 2264, and dielectric constant (K) of 1216, and curie temperature (Tc ) of 317 ◦ C were found at 0.1 wt.% Li2 CO3 added ceramics sintered at the low sintering temperature of 940 ◦ C. (4) Li2 CO3 addition was proved to lower the sintering temperature of piezoelectric ceramics due to the effect of LiBiO2 liquid phase and to improve the piezoelectric properties by acting as the hardner and softner.
Acknowledgements This study was supported by Korea Electric Power Research Institute (Grant No. R-2004-0-114).
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