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Molten salt synthesis of lead lanthanum zirconate titanate stannate powders and ceramics
Materials Letters 60 (2006) 425 – 430 www.elsevier.com/locate/matlet
Molten salt synthesis of lead lanthanum zirconate titanate stannate powders and ceramics Shixi Zhao a,⁎, Qiang Li b,⁎, Lin Wang b , Yiling Zhang c a
c
Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, PR China b Department of Chemistry, Tsinghua University, Beijing, 100084, PR China Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR China Received 9 May 2005; accepted 3 September 2005 Available online 3 October 2005
Abstract Powders and ceramics with a composition (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 were prepared by the molten salt synthesis (MSS) method, using Li2SO4–Na2SO4 and NaCl–KCl eutectic mixtures as the flux. The influences of processing parameters, such as temperature, time, and type of molten salts, on the formation and sinterability of PLZST were investigated. XRD and SEM were used for characterization of the as-prepared powders and ceramics. It was found that the PLZST powders and ceramics obtained by the MSS method have a relatively uniform particle size and microstructure. With other conditions being kept the same, chloride flux is preferential to the formation of tetragonal perovskite structure PLZST. The compact PLZST ceramics can be obtained at 1150 °C by MSS method, and then their dielectric properties are excellent. © 2005 Elsevier B.V. All rights reserved. Keywords: PLZST; Perovskite phase; Molten salt synthesis
1. Introduction Lanthanum modified (Pb0.97La0.02) (Zr, Ti, Sn)O3 (PLZST) ceramics has been investigated for more than thirty years [1]. With composition near tetragonal antiferroelectric (AFET) and rhombohedral ferroelectric (FER) morphotropic phase boundary (MPB), the phase transition between AFET and FER can occur under application of a limited external field due to a little energy difference between them. Electric field induced AFE– FE transition produces an abrupt change of spontaneous polarization and a volume expansion, up to 0.8% of the longitudinal strain [2], which is the largest for the application of ceramics actuators. So this material has been investigated for various actuator applications [3,4]. It is well known that the macroscopic properties of ceramics are directly related to their phase composition, grain size, homogeneity and microstructure, while these compositional and structural parameters are substantially influenced by the method of their preparation and ⁎ Corresponding authors. Zhao is to be contacted at Tel.: +86 755 26036372; fax: +86 755 26036752. Li, Tel.: +86 10 62781694; fax: +86 10 62781694. E-mail addresses: [email protected] (S. Zhao), [email protected] (Q. Li). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.005
the starting materials. In previous studies [5–8], PLZST ceramics were mainly fabricated using a conventional mixed oxides (CMO) method at higher temperature (synthesized at around 1100 °C and sintered at around 1300 °C [9]). This method often led to volatilization of PbO, which not only caused environmental problems, but also rendered compositional fluctuation and structural inhomogeneities in the final products [10,11]. In recent years, the main attention was turned to the development of a simple and reproducible low temperature synthesis technique. Generally, wet chemical methods have more advantages in compositional and structural control as compared to CMO method. Two typical methods that have been attempted include sol-gel approach and coprecipitation approach. But the sol-gel method proved to be not suitable for the preparation of the bulk ceramics, since the cost of starting materials is high [12,13]. The coprecipitation approach is also inadequate, since lead chloride is quite insoluble, and stannic chloride is the only water-soluble stannic salt. Therefore, it is difficult to make solutions containing lead and stannic. In addition, the differences of optimal precipitating conditions between A site (Pb and La) and B site (Zr, Sn and Ti) hydroxides tend to result in stoichiometric problems in the final powders [14,15].
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of the components [16]. (2) Obtaining grain-oriented ceramic is possible by simple tape casting because control of particle morphology is possible by MSS method [17]. Several complex perovskite materials, like PFN, PMN, PMN-PT, BZN, BMN, BZT [18–22], etc., have been synthesized by MSS method. In the present study, MSS method, using Li2SO4–Na2SO4 and NaCl–KCl eutectic mixtures as the flux, was applied to prepare PLZST powders and ceramics. The influences of processing parameters, such as temperature, time, and type of salts, on the formation and sinterability of the perovskite PLZST were investigated. 2. Experimental
Fig. 1. Ternary phase diagram for the system (Pb0.97La0.02) (Zr, Sn, Ti)O3 (Ref. [1]).
Molten salt synthesis (abbreviated as MSS) is one of the simplest methods for obtaining complex compound, in which the molten salt is used as a reaction aid or medium. Advantages of MSS are as fellows: (1) Preparation temperature can be lower and reaction time can be shorter because of the high diffusivities
A composition of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 was chosen near the boundary between the antiferroelectric tetragonal phase (AFET) and low temperature ferroelectric rhombohedral phase (FER) and it is shown in Fig. 1 [1]. PLZST powders were prepared by MSS method. Li2SO4–Na2SO4 salt with a 0.635 : 0.365 eutectic composition was used, its melting point is about 594 °C; NaCl–KCl salt with a 1 : 1 eutectic composition was used, its melting point is about 650 °C. The weight ratio (W) of salts to oxides was 1 : 1. 3 mol% excess PbO was used to avoid forming pyrochlore phases, owing to lead
Fig. 2. XRD patterns of PLZST powders prepared by MSS at different temperatures for 1 h (a) and (b) using sulfates, (c) and (d) using chlorides.
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insufficiency. Appropriate amounts of Reagent-grade oxides (PbO, La2O3, ZrO2, SnO2, TiO2) were mixed with Li2SO4– Na2SO4 or NaCl–KCl salts by planetary ball milling in ethanol with agate balls for 4–6 h to obtain uniformly distributed mixtures. The dried powders were fired at 800 to 900 °C for different time in covered alumina crucibles. After being cooled to room temperature, the powders were washed with deionized water until no sulfate or chloride was detected using Ba(NO3)2 and AgNO3, respectively. The dried resultant powder was pressed to pellets 10 mm in diameter and 2 mm in thickness. After binder burnout, the green plates were sintered in a leadrich atmosphere at different temperature for 2 h. Silver paste was painted onto the surfaces of the samples, which were then baked at 600 °C to form electrodes. The crystal structure of the resulting PLZST powders were determined using X-ray diffraction with CuKα radiation (λ = 0.15405 nm) at room temperature over an angular range of 10° ⩽ 2θ ⩽ 70° by a step of 0.02° (Model: Brüker D8). The morphology of the powders was observed by a field emission scanning electron microscopy (Model: JSM-6301F). The microstructure of ceramics was observed by scanning electron microscopy (JSM-6460LV). Relative density of the samples sintered at various temperatures was determined by the water immersion method. Dielectric properties at 1 kHz to 100 kHz
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were measured with a HP4192 A impedance analyzer (HP, USA) in a temperature-controlled chamber from − 50 to 200 °C. 3. Results and discussion 3.1. Formation of PLZST perovskite phases Fig. 2 shows the X-ray diffraction patterns of PLZST powders synthesized by MSS from two molten salt systems. It can be seen that the products synthesized by using sulfate salts at 800 °C for 1 h contain impurities (Fig. 2a), which may be pyrochlore phases [10] or unreactive SnO2. At temperatures above 850 °C, no pyrochlore phase was detected. This indicates that the product phase of PLZST synthesized by using sulfate salts at 850 °C for 1 h is as pure as that synthesized by CMO at 1100 °C for 2 h (Fig. 2a). The chosen PLZST composition point is located in the tetragonal AFE region, the (200)-diffraction peak of the XRD spectra represent tetragonal antiferroelectric phases. Fig. 2 (b) shows the XRD patterns of the (200)-diffraction peak for perovskite PLZST 66/27/7 powder prepared by using sulfate salts at different temperatures. This figure exhibits that the (200)-diffraction peak changes from broad singlet to duplet (T(200) and T(002)) with the synthesis temperature increasing, which indicated that the perovskite structure of PLZST could be formed below 850 °C, but there were the coexistence of rhombohedral and tetragonal phases. The peak R(200) and T(200) represent the rhombohedral and tetragonal phases, respectively. Experimentally, the peak T(002) of tetragonal phases was
Fig. 3. XRD patterns of PLZST powders synthesized by MSS at 900 °C for different soaking time (a) and (b) using sulfate flux; (c) and (d) using chloride flux.
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Fig. 4. SEM photographs of PLZST powders synthesized by MSS method. (a) 850 °C/1 h, (c) 900 °C/1 h, (e) 900 °C/2 h (using sulfate flux); (b) 850 °C/1 h, (d) 900 °C/ 1 h, (f) 900 °C/2 h (using chloride flux).
too weak to be discerned below 850 °C. The typical tetragonal AFE phase was formed at 900 °C. Fig. 2c shows the XDR patterns of PLZST powders synthesized using NaCl–KCl molten salts. This figure exhibits that the single perovskite phase can be obtained at 800 °C for 1 h in chloride molten salt. Fig. 2d shows that, in the case of chloride flux, the typical tetragonal AFE phases can be formed at 800 °C. With increasing synthesis temperature, the tetragonal AFE phases become more full-grown. This indicates that the chloride molten salt is preferential to the formation of tetragonal perovskite phase when other conditions, such as salts / oxides ratio, synthesis temperature and soaking times, were the same. It is known that, usually, the reaction mechanism of a given phase in MSS is somewhat different from that of the solid-state reaction, the molten salt provides the channels of fast diffusion for the constituents, so the reaction rates are faster in the MSS than that in the CMO. In this study, we examine the influence of time on the formation of PLZST perovskite phases. The X-ray diffraction patterns of PLZST powder synthesized by MSS for both molten salt systems at 900 °C for different soaking times are shown in Fig. 3. The results indicated that, in molten sulfates, the tetragonal characteristics of PLZST perovskite phase obtained at 900 °C for less 30 min were unapparent, but become apparent at 900 °C for more than 1 h. In molten chlorides, however, the tetragonal perovskite phase can be synthesized at 900 °C for only 15 min. The results confirmed that molten salt synthesis was a fast reaction process, and can greatly shorten the reactive times, which usually needs
2 h in CMO method. They also revealed that the chloride flux is preferential to the formation of tetragonal perovskite phase. The differences of phase structure are explained by considering the different size anions between two types of salts. Because Cl− ion size is smaller
Fig. 5. Relative density of PLZST ceramics sintered at different temperature for 2 h.
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than that of SO42− ions, the viscosity of NaCl–KCl flux is also smaller than that of Li2SO4–Na2SO4 flux, at same temperature, the mobility of species is larger in chloride flux than that in sulfate flux, which accelerate the formation of tetragonal perovskite phase. 3.2. Morphological development and characteristics in the MSS Fig. 4 shows the micrographs of the PLZST powders prepared by using sulfate and chloride salts respectively. An average particle size around 0.2∼0.3 μm with a relatively uniform size distribution can be obtained at 850 °C for 1 h (Fig. 4a and b). No large agglomerates were found in these powders. With an increase of synthesis temperature and time, the particle sizes became larger. The average particle size of around 0.5 μm can be obtained at 900 °C for 1 h (Fig. 4c and d). When the soaking time was prolonged to 2 h, the particles grew up ≈1 μm in average (Fig. 4e and f). Under same conditions, the morphological development of the grains was more full-grown and the dispersity of particles was apparently improved as the salts changed from sulfates to
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chlorides. Because the viscosity of chloride flux was smaller than that of sulfate flux, so it was easy that the chloride salt infiltrated into interspace of oxide particles or perovskite PLZST particles. This not only promoted morphological development of the grains, but also was in favor of dispersing of product particles when they were washed with deionized water. The SEM observations are consistent with the XRD results. 3.3. Microstructure and dielectric properties of PLZST ceramics To examine the sinterability of the PLZST powders synthesized by MSS, we used the powders synthesized at 900 °C for 1 h by using sulfate salt and chloride salt respectively as the raw materials to fabricated ceramics. The dry-pressed pellets were sintered at different temperatures. Fig. 5 shows the effect of sintering temperature on the relative density of PLZST ceramics. As shown in Fig. 5, in the case of sulfate flux, the relative density of PLZST ceramics changed little in the range of 1100 to 1280 °C, all most are above 97%; in the case of chloride flux,
Fig. 6. SEM micrographs and dielectric properties of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 ceramics sintered at 1150 °C for 2 h by using (a) sulfate flux, (b) chloride flux.
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however, the relative density of specimens has a great difference, a maximum relative density of 98% was achieved from the specimens sintered at 1150 °C for 2 h. The optimal sintering temperature of PLZST ceramics made from the powders synthesized by using sulfate flux is 1100–1150 °C, that by using chloride flux is 1150–1180 °C. Compared with CMO method, the powders synthesized by MSS can lower the sintering temperature of PLZST ceramics up to 150 to 200 °C [9]. Fig. 6 provides SEM micrographs and dielectric properties of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 ceramics sintered at 1150 °C for 2 h. It can be seen that PLZST ceramics obtained by using MSS method have a very uniform microstructure, their fracture surface are intergranular fracture. The average grain size of ceramics obtained by using sulfate flux is about 2 μm and smaller than that by using chloride flux, which is about 4 μm. As illustrated in Fig. 6, the dielectric constant and dielectric loss of the two specimens have almost same variational character with temperatures. The crystal structure of antiferroelectric PLZST was relaxed which resulted dielectric constant dispersed with frequency at low temperature side of dielectric peak. In the case of sulfate flux, Tm is about 168 °C, the max dielectric constant is about 1430, in the case of chloride flux, Tm is about 172 °C, the max dielectric constant is about 1390. Although the frequency dispersive phenomena also existed in PLZST antiferroelectric materials, but the Tm is independent of frequency. This is different from the relaxor ferroelectric materials [23]. The dielectric loss of specimens was decreased with temperature increasing. At the temperature Tm, the dielectric loss of PLZST ceramics obtained by MSS were below 0.5%. This indicates the MSS method can't result in the dielectric loss increasing.
4. Conclusions PLZST powders and ceramics were prepared by MSS method using Li2SO4–Na2SO4 and NaCl–KCl fluxes respectively. In the case of sulfate flux, the PLZST powder with single perovskite structure can be obtained above 850 °C, but there are more distorted in perovskite lattice, which shows the coexistence of rhombohedral and tetragonal phases below 850 °C. The typical tetragonal AFE phase was formed at 900 °C. The tetragonal AFE phase obtained at 900 °C for less 30 min is unapparent, it became apparent when the dwell time was more than 1 h. In the case of chloride flux, the single perovskite phase can be synthesized at 800 °C, which has the typical characteristics of tetragonal structure. The tetragonal perovskite phase can be synthesized at 900 °C for only 15 min. This indicates that the chloride flux is preferential to the formation of tetragonal perovskite structure. Average practical size PLZST powders have a uniform size distribution with no large agglomerates. As synthesis temperature and time increase, the particle sizes became larger. Under same conditions, the grain size prepared in chlorides is larger and the size distribution is
more uniform than that in sulfates. PLZST ceramics prepared by MSS method at 1150 °C for 2 h had better microstructure and dielectric properties, their relative densities are above 97%, the dielectric loss are below 0.5% at Tm. The data indicates that MSS may be better than CMO. MSS method is a promising method for antiferroelectric phase PLZST powders and ceramics preparation. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No. 50272030, and by National High-tech R and D Program (863 Program) of China, No. 2002AA325060. References [1] D. Berlincourt, IEEE Trans. Sonics Ultrason. su-3 (1968) 116. [2] W.Y. Pan, Q.M. Zhang, A. Balla, L.E. Cross, J. Appl. Phys. 66 (1989) 6014. [3] W.Y. Pan, Q.M. Zhang, A. Bhalla, L.E. Cross, J. Am. Ceram. Soc. 72 (1989) 571–578. [4] K. Uchino, Mater. Lett. 22 (1995) 1–4. [5] K. Markowski, S.E. Park, S. Yoshikawa, L.E. Cross, J. Am. Ceram. Soc. 79 (1996) 3297–3304. [6] L.H. Xue, Y.L. Zhang, Q. Li, J. Inorg. Mater. 19 (2004) 566–570 (Chinese). [7] Y. Akiyama, E. Fujisawa, J. Appl. Phys. 36 (1997) 5997–6000. [8] Y.J. Feng, Z. Xu, X. Yao, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 99 (2003) 499–501. [9] Peng Liu, Xi Yao, Solid State Commun. 132 (2004) 809–813. [10] S.E. Park, M.J. Pan, K. Markowski, S. Yoshikawa, L.E. Cross, J. Appl. Phys. 82 (1997) 1798–1803. [11] K. Markowski, S.E. Park, S. Yoshikawa, L.E. Cross, J. Am. Ceram. Soc. 79 (1996) 3297–3304. [12] B.M. Xu, N.G. Pai, L.E. Cross, Mater. Lett. 34 (1998) 157–160. [13] B.M. Xu, L.E. Cross, D. Ravichandran, J. Am. Ceram. Soc. 82 (1999) 306–312. [14] J.H. Lee, Y.M. Chiang, J. Mater. Chem. 9 (1999) 3107–3111. [15] M. Chen, X. Yao, L.Y. Zhang, J. Eur. Ceram. Soc. 21 (2001) 1159–1164. [16] R.H. Arent, Z.H. Rosolowski, J.W. Szymaszek, Mater. Res. Bull. 14 (1979) 703–709. [17] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84–87. [18] C.C. Chiu, C.C. Li, S.B. Desu, J. Am. Ceram. Soc. 74 (1991) 38–41. [19] K.H. Yoon, Y.S. Cho, D.H. Lee, D.H. Kang, J. Am. Ceram. Soc. 76 (1993) 1373–1376. [20] K.H. Yoon, Y.S. Cho, D.H. Kang, J. Mater. Sci. 33 (1998) 2977–2984. [21] S.X. Zhao, Q. Li, F.B. Song, Mat. Sci. Forum 475–479 (2005) 1153–1156. [22] M. Thirumal, P. Jain, A.K. Ganguli, Mater. Chem. Phys. 70 (2001) 7–11. [23] X. Duan, W. Luo, W. Wu, J.S. Yuan, Solid State Commun. 114 (2000) 597–600.
Molten salt synthesis of lead lanthanum zirconate titanate stannate powders and ceramics Shixi Zhao a,⁎, Qiang Li b,⁎, Lin Wang b , Yiling Zhang c a
c
Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, PR China b Department of Chemistry, Tsinghua University, Beijing, 100084, PR China Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR China Received 9 May 2005; accepted 3 September 2005 Available online 3 October 2005
Abstract Powders and ceramics with a composition (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 were prepared by the molten salt synthesis (MSS) method, using Li2SO4–Na2SO4 and NaCl–KCl eutectic mixtures as the flux. The influences of processing parameters, such as temperature, time, and type of molten salts, on the formation and sinterability of PLZST were investigated. XRD and SEM were used for characterization of the as-prepared powders and ceramics. It was found that the PLZST powders and ceramics obtained by the MSS method have a relatively uniform particle size and microstructure. With other conditions being kept the same, chloride flux is preferential to the formation of tetragonal perovskite structure PLZST. The compact PLZST ceramics can be obtained at 1150 °C by MSS method, and then their dielectric properties are excellent. © 2005 Elsevier B.V. All rights reserved. Keywords: PLZST; Perovskite phase; Molten salt synthesis
1. Introduction Lanthanum modified (Pb0.97La0.02) (Zr, Ti, Sn)O3 (PLZST) ceramics has been investigated for more than thirty years [1]. With composition near tetragonal antiferroelectric (AFET) and rhombohedral ferroelectric (FER) morphotropic phase boundary (MPB), the phase transition between AFET and FER can occur under application of a limited external field due to a little energy difference between them. Electric field induced AFE– FE transition produces an abrupt change of spontaneous polarization and a volume expansion, up to 0.8% of the longitudinal strain [2], which is the largest for the application of ceramics actuators. So this material has been investigated for various actuator applications [3,4]. It is well known that the macroscopic properties of ceramics are directly related to their phase composition, grain size, homogeneity and microstructure, while these compositional and structural parameters are substantially influenced by the method of their preparation and ⁎ Corresponding authors. Zhao is to be contacted at Tel.: +86 755 26036372; fax: +86 755 26036752. Li, Tel.: +86 10 62781694; fax: +86 10 62781694. E-mail addresses: [email protected] (S. Zhao), [email protected] (Q. Li). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.005
the starting materials. In previous studies [5–8], PLZST ceramics were mainly fabricated using a conventional mixed oxides (CMO) method at higher temperature (synthesized at around 1100 °C and sintered at around 1300 °C [9]). This method often led to volatilization of PbO, which not only caused environmental problems, but also rendered compositional fluctuation and structural inhomogeneities in the final products [10,11]. In recent years, the main attention was turned to the development of a simple and reproducible low temperature synthesis technique. Generally, wet chemical methods have more advantages in compositional and structural control as compared to CMO method. Two typical methods that have been attempted include sol-gel approach and coprecipitation approach. But the sol-gel method proved to be not suitable for the preparation of the bulk ceramics, since the cost of starting materials is high [12,13]. The coprecipitation approach is also inadequate, since lead chloride is quite insoluble, and stannic chloride is the only water-soluble stannic salt. Therefore, it is difficult to make solutions containing lead and stannic. In addition, the differences of optimal precipitating conditions between A site (Pb and La) and B site (Zr, Sn and Ti) hydroxides tend to result in stoichiometric problems in the final powders [14,15].
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of the components [16]. (2) Obtaining grain-oriented ceramic is possible by simple tape casting because control of particle morphology is possible by MSS method [17]. Several complex perovskite materials, like PFN, PMN, PMN-PT, BZN, BMN, BZT [18–22], etc., have been synthesized by MSS method. In the present study, MSS method, using Li2SO4–Na2SO4 and NaCl–KCl eutectic mixtures as the flux, was applied to prepare PLZST powders and ceramics. The influences of processing parameters, such as temperature, time, and type of salts, on the formation and sinterability of the perovskite PLZST were investigated. 2. Experimental
Fig. 1. Ternary phase diagram for the system (Pb0.97La0.02) (Zr, Sn, Ti)O3 (Ref. [1]).
Molten salt synthesis (abbreviated as MSS) is one of the simplest methods for obtaining complex compound, in which the molten salt is used as a reaction aid or medium. Advantages of MSS are as fellows: (1) Preparation temperature can be lower and reaction time can be shorter because of the high diffusivities
A composition of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 was chosen near the boundary between the antiferroelectric tetragonal phase (AFET) and low temperature ferroelectric rhombohedral phase (FER) and it is shown in Fig. 1 [1]. PLZST powders were prepared by MSS method. Li2SO4–Na2SO4 salt with a 0.635 : 0.365 eutectic composition was used, its melting point is about 594 °C; NaCl–KCl salt with a 1 : 1 eutectic composition was used, its melting point is about 650 °C. The weight ratio (W) of salts to oxides was 1 : 1. 3 mol% excess PbO was used to avoid forming pyrochlore phases, owing to lead
Fig. 2. XRD patterns of PLZST powders prepared by MSS at different temperatures for 1 h (a) and (b) using sulfates, (c) and (d) using chlorides.
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insufficiency. Appropriate amounts of Reagent-grade oxides (PbO, La2O3, ZrO2, SnO2, TiO2) were mixed with Li2SO4– Na2SO4 or NaCl–KCl salts by planetary ball milling in ethanol with agate balls for 4–6 h to obtain uniformly distributed mixtures. The dried powders were fired at 800 to 900 °C for different time in covered alumina crucibles. After being cooled to room temperature, the powders were washed with deionized water until no sulfate or chloride was detected using Ba(NO3)2 and AgNO3, respectively. The dried resultant powder was pressed to pellets 10 mm in diameter and 2 mm in thickness. After binder burnout, the green plates were sintered in a leadrich atmosphere at different temperature for 2 h. Silver paste was painted onto the surfaces of the samples, which were then baked at 600 °C to form electrodes. The crystal structure of the resulting PLZST powders were determined using X-ray diffraction with CuKα radiation (λ = 0.15405 nm) at room temperature over an angular range of 10° ⩽ 2θ ⩽ 70° by a step of 0.02° (Model: Brüker D8). The morphology of the powders was observed by a field emission scanning electron microscopy (Model: JSM-6301F). The microstructure of ceramics was observed by scanning electron microscopy (JSM-6460LV). Relative density of the samples sintered at various temperatures was determined by the water immersion method. Dielectric properties at 1 kHz to 100 kHz
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were measured with a HP4192 A impedance analyzer (HP, USA) in a temperature-controlled chamber from − 50 to 200 °C. 3. Results and discussion 3.1. Formation of PLZST perovskite phases Fig. 2 shows the X-ray diffraction patterns of PLZST powders synthesized by MSS from two molten salt systems. It can be seen that the products synthesized by using sulfate salts at 800 °C for 1 h contain impurities (Fig. 2a), which may be pyrochlore phases [10] or unreactive SnO2. At temperatures above 850 °C, no pyrochlore phase was detected. This indicates that the product phase of PLZST synthesized by using sulfate salts at 850 °C for 1 h is as pure as that synthesized by CMO at 1100 °C for 2 h (Fig. 2a). The chosen PLZST composition point is located in the tetragonal AFE region, the (200)-diffraction peak of the XRD spectra represent tetragonal antiferroelectric phases. Fig. 2 (b) shows the XRD patterns of the (200)-diffraction peak for perovskite PLZST 66/27/7 powder prepared by using sulfate salts at different temperatures. This figure exhibits that the (200)-diffraction peak changes from broad singlet to duplet (T(200) and T(002)) with the synthesis temperature increasing, which indicated that the perovskite structure of PLZST could be formed below 850 °C, but there were the coexistence of rhombohedral and tetragonal phases. The peak R(200) and T(200) represent the rhombohedral and tetragonal phases, respectively. Experimentally, the peak T(002) of tetragonal phases was
Fig. 3. XRD patterns of PLZST powders synthesized by MSS at 900 °C for different soaking time (a) and (b) using sulfate flux; (c) and (d) using chloride flux.
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Fig. 4. SEM photographs of PLZST powders synthesized by MSS method. (a) 850 °C/1 h, (c) 900 °C/1 h, (e) 900 °C/2 h (using sulfate flux); (b) 850 °C/1 h, (d) 900 °C/ 1 h, (f) 900 °C/2 h (using chloride flux).
too weak to be discerned below 850 °C. The typical tetragonal AFE phase was formed at 900 °C. Fig. 2c shows the XDR patterns of PLZST powders synthesized using NaCl–KCl molten salts. This figure exhibits that the single perovskite phase can be obtained at 800 °C for 1 h in chloride molten salt. Fig. 2d shows that, in the case of chloride flux, the typical tetragonal AFE phases can be formed at 800 °C. With increasing synthesis temperature, the tetragonal AFE phases become more full-grown. This indicates that the chloride molten salt is preferential to the formation of tetragonal perovskite phase when other conditions, such as salts / oxides ratio, synthesis temperature and soaking times, were the same. It is known that, usually, the reaction mechanism of a given phase in MSS is somewhat different from that of the solid-state reaction, the molten salt provides the channels of fast diffusion for the constituents, so the reaction rates are faster in the MSS than that in the CMO. In this study, we examine the influence of time on the formation of PLZST perovskite phases. The X-ray diffraction patterns of PLZST powder synthesized by MSS for both molten salt systems at 900 °C for different soaking times are shown in Fig. 3. The results indicated that, in molten sulfates, the tetragonal characteristics of PLZST perovskite phase obtained at 900 °C for less 30 min were unapparent, but become apparent at 900 °C for more than 1 h. In molten chlorides, however, the tetragonal perovskite phase can be synthesized at 900 °C for only 15 min. The results confirmed that molten salt synthesis was a fast reaction process, and can greatly shorten the reactive times, which usually needs
2 h in CMO method. They also revealed that the chloride flux is preferential to the formation of tetragonal perovskite phase. The differences of phase structure are explained by considering the different size anions between two types of salts. Because Cl− ion size is smaller
Fig. 5. Relative density of PLZST ceramics sintered at different temperature for 2 h.
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than that of SO42− ions, the viscosity of NaCl–KCl flux is also smaller than that of Li2SO4–Na2SO4 flux, at same temperature, the mobility of species is larger in chloride flux than that in sulfate flux, which accelerate the formation of tetragonal perovskite phase. 3.2. Morphological development and characteristics in the MSS Fig. 4 shows the micrographs of the PLZST powders prepared by using sulfate and chloride salts respectively. An average particle size around 0.2∼0.3 μm with a relatively uniform size distribution can be obtained at 850 °C for 1 h (Fig. 4a and b). No large agglomerates were found in these powders. With an increase of synthesis temperature and time, the particle sizes became larger. The average particle size of around 0.5 μm can be obtained at 900 °C for 1 h (Fig. 4c and d). When the soaking time was prolonged to 2 h, the particles grew up ≈1 μm in average (Fig. 4e and f). Under same conditions, the morphological development of the grains was more full-grown and the dispersity of particles was apparently improved as the salts changed from sulfates to
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chlorides. Because the viscosity of chloride flux was smaller than that of sulfate flux, so it was easy that the chloride salt infiltrated into interspace of oxide particles or perovskite PLZST particles. This not only promoted morphological development of the grains, but also was in favor of dispersing of product particles when they were washed with deionized water. The SEM observations are consistent with the XRD results. 3.3. Microstructure and dielectric properties of PLZST ceramics To examine the sinterability of the PLZST powders synthesized by MSS, we used the powders synthesized at 900 °C for 1 h by using sulfate salt and chloride salt respectively as the raw materials to fabricated ceramics. The dry-pressed pellets were sintered at different temperatures. Fig. 5 shows the effect of sintering temperature on the relative density of PLZST ceramics. As shown in Fig. 5, in the case of sulfate flux, the relative density of PLZST ceramics changed little in the range of 1100 to 1280 °C, all most are above 97%; in the case of chloride flux,
Fig. 6. SEM micrographs and dielectric properties of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 ceramics sintered at 1150 °C for 2 h by using (a) sulfate flux, (b) chloride flux.
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however, the relative density of specimens has a great difference, a maximum relative density of 98% was achieved from the specimens sintered at 1150 °C for 2 h. The optimal sintering temperature of PLZST ceramics made from the powders synthesized by using sulfate flux is 1100–1150 °C, that by using chloride flux is 1150–1180 °C. Compared with CMO method, the powders synthesized by MSS can lower the sintering temperature of PLZST ceramics up to 150 to 200 °C [9]. Fig. 6 provides SEM micrographs and dielectric properties of (Pb0.97La0.02) (Zr0.66Sn0.27Ti0.07)O3 ceramics sintered at 1150 °C for 2 h. It can be seen that PLZST ceramics obtained by using MSS method have a very uniform microstructure, their fracture surface are intergranular fracture. The average grain size of ceramics obtained by using sulfate flux is about 2 μm and smaller than that by using chloride flux, which is about 4 μm. As illustrated in Fig. 6, the dielectric constant and dielectric loss of the two specimens have almost same variational character with temperatures. The crystal structure of antiferroelectric PLZST was relaxed which resulted dielectric constant dispersed with frequency at low temperature side of dielectric peak. In the case of sulfate flux, Tm is about 168 °C, the max dielectric constant is about 1430, in the case of chloride flux, Tm is about 172 °C, the max dielectric constant is about 1390. Although the frequency dispersive phenomena also existed in PLZST antiferroelectric materials, but the Tm is independent of frequency. This is different from the relaxor ferroelectric materials [23]. The dielectric loss of specimens was decreased with temperature increasing. At the temperature Tm, the dielectric loss of PLZST ceramics obtained by MSS were below 0.5%. This indicates the MSS method can't result in the dielectric loss increasing.
4. Conclusions PLZST powders and ceramics were prepared by MSS method using Li2SO4–Na2SO4 and NaCl–KCl fluxes respectively. In the case of sulfate flux, the PLZST powder with single perovskite structure can be obtained above 850 °C, but there are more distorted in perovskite lattice, which shows the coexistence of rhombohedral and tetragonal phases below 850 °C. The typical tetragonal AFE phase was formed at 900 °C. The tetragonal AFE phase obtained at 900 °C for less 30 min is unapparent, it became apparent when the dwell time was more than 1 h. In the case of chloride flux, the single perovskite phase can be synthesized at 800 °C, which has the typical characteristics of tetragonal structure. The tetragonal perovskite phase can be synthesized at 900 °C for only 15 min. This indicates that the chloride flux is preferential to the formation of tetragonal perovskite structure. Average practical size PLZST powders have a uniform size distribution with no large agglomerates. As synthesis temperature and time increase, the particle sizes became larger. Under same conditions, the grain size prepared in chlorides is larger and the size distribution is
more uniform than that in sulfates. PLZST ceramics prepared by MSS method at 1150 °C for 2 h had better microstructure and dielectric properties, their relative densities are above 97%, the dielectric loss are below 0.5% at Tm. The data indicates that MSS may be better than CMO. MSS method is a promising method for antiferroelectric phase PLZST powders and ceramics preparation. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No. 50272030, and by National High-tech R and D Program (863 Program) of China, No. 2002AA325060. References [1] D. Berlincourt, IEEE Trans. Sonics Ultrason. su-3 (1968) 116. [2] W.Y. Pan, Q.M. Zhang, A. Balla, L.E. Cross, J. Appl. Phys. 66 (1989) 6014. [3] W.Y. Pan, Q.M. Zhang, A. Bhalla, L.E. Cross, J. Am. Ceram. Soc. 72 (1989) 571–578. [4] K. Uchino, Mater. Lett. 22 (1995) 1–4. [5] K. Markowski, S.E. Park, S. Yoshikawa, L.E. Cross, J. Am. Ceram. Soc. 79 (1996) 3297–3304. [6] L.H. Xue, Y.L. Zhang, Q. Li, J. Inorg. Mater. 19 (2004) 566–570 (Chinese). [7] Y. Akiyama, E. Fujisawa, J. Appl. Phys. 36 (1997) 5997–6000. [8] Y.J. Feng, Z. Xu, X. Yao, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 99 (2003) 499–501. [9] Peng Liu, Xi Yao, Solid State Commun. 132 (2004) 809–813. [10] S.E. Park, M.J. Pan, K. Markowski, S. Yoshikawa, L.E. Cross, J. Appl. Phys. 82 (1997) 1798–1803. [11] K. Markowski, S.E. Park, S. Yoshikawa, L.E. Cross, J. Am. Ceram. Soc. 79 (1996) 3297–3304. [12] B.M. Xu, N.G. Pai, L.E. Cross, Mater. Lett. 34 (1998) 157–160. [13] B.M. Xu, L.E. Cross, D. Ravichandran, J. Am. Ceram. Soc. 82 (1999) 306–312. [14] J.H. Lee, Y.M. Chiang, J. Mater. Chem. 9 (1999) 3107–3111. [15] M. Chen, X. Yao, L.Y. Zhang, J. Eur. Ceram. Soc. 21 (2001) 1159–1164. [16] R.H. Arent, Z.H. Rosolowski, J.W. Szymaszek, Mater. Res. Bull. 14 (1979) 703–709. [17] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84–87. [18] C.C. Chiu, C.C. Li, S.B. Desu, J. Am. Ceram. Soc. 74 (1991) 38–41. [19] K.H. Yoon, Y.S. Cho, D.H. Lee, D.H. Kang, J. Am. Ceram. Soc. 76 (1993) 1373–1376. [20] K.H. Yoon, Y.S. Cho, D.H. Kang, J. Mater. Sci. 33 (1998) 2977–2984. [21] S.X. Zhao, Q. Li, F.B. Song, Mat. Sci. Forum 475–479 (2005) 1153–1156. [22] M. Thirumal, P. Jain, A.K. Ganguli, Mater. Chem. Phys. 70 (2001) 7–11. [23] X. Duan, W. Luo, W. Wu, J.S. Yuan, Solid State Commun. 114 (2000) 597–600.