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Low temperature synthesis of Ba0.70Sr0.30TiO3 powders by the molten-salt method
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Materials Chemistry and Physics 106 (2007) 164–167
Materials science communication
Low temperature synthesis of Ba0.70Sr0.30TiO3 powders by the molten-salt method Chaoliang Mao, Genshui Wang, Xianlin Dong ∗ , Zhiyong Zhou, Yuanyuan Zhang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 31 January 2007; received in revised form 18 June 2007; accepted 23 June 2007
Abstract Pure perovskite phase Ba0.70 Sr0.30 TiO3 (BST) powders were successfully synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 850 ◦ C for 2 h, which is 300 ◦ C lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcining of the mixtures led to BST powders at 800–900 ◦ C. XRD and SEM techniques are used to characterize the phase and morphology of the fabricated BST powders, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Molten salts; BST; Powder; Perovskite
1. Introduction Barium strontium titanate (BST) [Ba1−x Srx TiO3 , 0 < x < 1] is a kind of favorable electronic materials for its high dielectric constant and composition-dependent Curie temperature [1,2]. It has been found that BST have applications in piezoelectric sensors, dynamic random access memories (DRAM), microwave phase shifters and uncooled infrared detectors for its high dielectric and pyroelectric properties [3–5]. A complete solid solution possessing lots of phases and structures exists throughout most of the BaTiO3 –SrTiO3 system and the low temperature phase equilibria study is still in argument [6]. This is because the studies of low temperature phase equilibria in this system are often hampered by sluggish kinetics arising from extremely low cation and/or anion diffusion coefficients in the conventional solid-state (CS) method of which the synthesis temperature is too high. The molten-salt (MS) method provides a way to circumvent this problem [7]. Furthermore, in recent years, low-temperature synthesis of BST powders has attracted considerable interest due to the technological importance of these powders in the fabrication of fine-grained and homogenized ceramics which improves the performance and reliability of physical properties
to a large extent. Besides lots of chemical methods [8,9], the molten-salt process can also be used to synthesize perovskite fine-grained ceramics [10]. Molten-salt synthesis method is one of the simplest methods for obtaining highly reactive powders of a single phase at low temperatures, in which the molten salt is used as a reaction aid. Materials such as PZT (lead zirconate titanate) [11], Pb(Fe0.5 Nb0.5 )O3 [12] and Bi3 NbTiO9 [13] have been synthesized using this method. However, to our best of the knowledge, there is little report about the molten-salt synthesis of BST powders. In this work, we report the synthesis of Ba0.70 Sr0.30 TiO3 powders at low temperatures by molten-salt method using NaCl–KCl eutectic mixtures. The effect of synthesizing temperature on the formation and morphology of BST powders in this chloride flux is investigated. Meanwhile, the comparison of molten-salt method with conventional solid-state process is also discussed. The goals of this paper are to develop the fabrication technology of materials and facilitate the study of low temperature phase equilibria in BaTiO3 –SrTiO3 system. 2. Experimental 2.1. Synthesis of Ba0.70 Sr0.30 TiO3 powders
∗
Corresponding author. Tel.: +86 21 52411412; fax: +86 21 52411104. E-mail address: [email protected] (X. Dong).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.052
Barium carbonate, (BaCO3 , 99.0%), strontium carbonate (SrCO3 , 99.0%) and titanium oxide (TiO2 , 99.44%) were used as starting materials to synthe-
C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167 size Ba0.70 Sr0.30 TiO3 powders. They were mixed with an equimolar mixture of sodium chloride (NaCl, 99.5%) and potassium chloride (KCl, 99.5%) in a polyethylene pot for 24 h using alcohol as the grinding medium. Eutectic temperature for the kind of chloride flux is 650 ◦ C. Subsequently, the mixture was heated in a sealed alumina crucible at a selected temperature between 800 and 900 ◦ C for 2 h. The resulting powders were crushed and washed with hot distilled water for several times to remove the chloride salts. For comparison, conventional solid-state reaction process was also used to prepare the Ba0.70 Sr0.30 TiO3 powders. In order to obtain the pure phase BST, the calcining temperature should be increased to 1150 ◦ C. The powder obtained by the molten-salt method and conventional solid-state reaction process will be hereinafter referred to as the MS and CS powder, respectively.
2.2. Characterization of BST powder X-ray diffraction (XRD) with Cu K␣ radiation (Model Rax-10, Rigaku, Tokyo, Japan) was carried out to examine the phase of the BST powders. The microstructure of BST powders was investigated by a scanning electron microscopy (Model JSM-6700F, JEOL, Tokyo, Japan).
3. Results and discussion 3.1. Powder phase characterization Fig. 1 shows the X-ray diffraction patterns of Ba0.70 Sr0.30 TiO3 powders synthesized under different conditions. It is obvious that single BST perovskite phase is formed by molten-salt synthesis at 850 ◦ C for 2 h, whereas, the carbonates and oxide without the mixture of NaCl–KCl cannot fully crystallize to pure phase until 1150 ◦ C. It should be emphasized that conventional solid-state synthesis of this solid solution is virtually impossible at a temperature <1000 ◦ C [14] due to very low solid-state diffusion. In the present study, the fact that the pure BST perovskite phase forms at a temperature only 850 ◦ C demonstrates that the molten salt eutectic indeed accelerates the kinetics at this low temperature by orders of magnitude and facilitates the formation of BST solid solution, similar to the results found in the solutions of BaZrO3 –SrZrO3 [7]. This could be attributed to the enhanced diffusion coefficients in the molten chloride liquid phase compared to that in the solid
Fig. 1. XRD patterns of the BST powder obtained at 800 ◦ C (a), 850 ◦ C (b), 900 ◦ C (c) for 2 h by MS process; and 1150 ◦ C (d) for 2 h by CS method.
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state. Thus, perovskite phase can be synthesized at a relative low temperature using molten-salt method, which can make the low temperature phase equilibria study more feasible. In Fig. 1, another significant feature should also be noted: besides the characteristic peaks of BST in Fig. 1(d), there are only two weak peaks corresponding to the carbonates (Ba,Sr)CO3 including BaCO3 and SrCO3 [15]. As the temperature increased, the two peaks disappeared and a pure phase is obtained at 850 ◦ C. During this process, there could not be found any other compounds or intermediate phases. It is known that usually, there are two reaction mechanisms in the MS process. In some cases, the molten salt participates in the chemical reaction. It reacts with the oxide or carbonate precursor, subsequently decomposing to the product, and returns the original salt in its uncombined state. In the case of Bi4 Ti3 O12 , for instance, the component oxides react with LiCl to form an intermediate compound which then decomposes into Bi4 Ti3 O12 and LiCl [16]. In other cases, the molten salt acts as a mere solvent which accelerates the rate of formation of the desired compound. The latter reaction mechanism seems to be closer to the process investigated in this study. Nevertheless, the exact formation mechanism is very complex and further work of this is in progress. 3.2. Powder morphology analysis Fig. 2 shows the SEM images of BST powders fabricated under different conditions. It can be seen that most of the CS powders calcined at 1150 ◦ C are badly agglomerated and random conglomerated. Compared to CS powders, the MS powders show quadrate-like morphology. Furthermore, the MS powders are more homogenized and fine. This can be attributed to the high reaction activity and low synthesis temperature introduced by the molten salts. Comparing Fig. 2(b) with (c), it should be noted that the powders synthesized at 850 ◦ C are fine but aggregated. When the temperature increased to 900 ◦ C, the powders dispersed well and most of them are quadrate accompanied by a few of coarse particles. It is obvious that calcining temperature plays an important role in the development of BST particle morphology. It is generally believed that the particle morphology is initially dominated by the nucleation process and later growth process during the MS synthesis. The morphology change of the BST powder with the different temperatures is controlled by the mechanism of the crystal growth in the chloride flux. Fig. 3 shows the main processing stages of the MS method with increasing temperature [10]. First, when the mixtures including carbonates, oxides and salts are fired at a temperature above the melting point of the salts and form a flux, the raw materials rearrange and diffuse quickly in a liquid state of the salts. Small and aggregated BST particles are rapidly formed in the initial period of molten-salt reaction: nucleation process. Then, as the calcining temperature increases, the BST nucleus grow to relatively big ones by consuming the small and agglomerated particles in accordance with the Ostwald ripening mechanism in the growth process [13]. Finally, discrete BST powders are formed at 900 ◦ C in the molten NaCl–KCl flux. However, as we can see in Fig. 2(c), there are some abnormal coarse particles.
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C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167
Fig. 2. SEM micrographs of BST powders synthesized at 850 ◦ C (a), 900 ◦ C (b) for 2 h by MS process; and 1150 ◦ C (c) for 2 h by CS method.
This is because the raw powders used in this experiment are not well homogenized. 4. Conclusions The molten-salt method was successfully used to synthesize BST powders at a low temperature. The pure perovskite phase BST could be obtained at 850 ◦ C for 2 h. XRD results revealed that the molten salts acted as a mere solvent which accelerated the rate of formation of the desired BST compound but not participated in the chemical reaction. SEM analysis indicated that the calcining temperature played an important role in the development of BST particle morphology. This simple and novel fabrication method makes the fine-grained ceramics synthesis and low temperature phase equilibria study of BST more feasible. Acknowledgment The authors gratefully acknowledge the financial support granted by the QiMingXing Project of Shanghai City (No. 06QA14055). References
Fig. 3. Schematic illustration of the main processing stages in the MS method.
[1] J.L. Davis, L.D. Rubin, J. Appl. Phys. 24 (1953) 1194. [2] Y.B. Khollam, H.S. Potdar, S.B. Deshpande, A.B. Gaikwad, Mater. Chem. Phys. 97 (2006) 295. [3] G.W. Dietz, W. Antpohler, M. Klee, R. Waser, J. Appl. Phys. 78 (1995) 6113.
C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167 [4] T. Karaki, K. Yan, T. Miyamoto, M. Adachi, Jpn. J. Appl. Phys. 46 (2007) L97. [5] R.W. Whatmore, P.C. Osbond, N.M. Shorrocks, Ferroelectrics 76 (1987) 351. [6] B.A. Wechsler, K.W. Kirby, J. Am. Ceram. Soc. 75 (1992) 981. [7] S. Gopalan, K. Mehta, A.V. Virkar, J. Mater. Res. 11 (1996) 1863. [8] C. Mao, X. Dong, T. Zeng, Mater. Lett. 61 (2007) 1633. [9] C. Mao, X. Dong, T. Zeng, H. Chen, F. Cao, Ceram. Int., in press. [10] K.H. Yoon, Y.S. Cho, D.H. Kang, J. Mater. Sci. 33 (1998) 2977.
167
[11] R.J. Arendt, H.H. Rosolowski, J.W. Szymaszek, Mater. Res. Bull. 14 (1979) 703. [12] C.C. Chiu, C.C. Li, S.B. Desu, J. Am. Ceram. Soc. 74 (1991) 38. [13] P.H. Xiang, Y. Kinemuchi, K. Watari, Mater. Lett. 59 (2005) 1876. [14] S. Kumar, G.L. Messing, W.B. White, J. Am. Ceram. Soc. 76 (1993) 617. [15] C. Mao, X. Dong, T. Zeng, G. Wang, S. Chen, Mater. Res. Bull. 42 (2007) 1602. [16] Y. Hayashi, T. Kimura, T. Yamaguchi, J. Mater. Sci. 21 (1986) 2876.
Materials Chemistry and Physics 106 (2007) 164–167
Materials science communication
Low temperature synthesis of Ba0.70Sr0.30TiO3 powders by the molten-salt method Chaoliang Mao, Genshui Wang, Xianlin Dong ∗ , Zhiyong Zhou, Yuanyuan Zhang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 31 January 2007; received in revised form 18 June 2007; accepted 23 June 2007
Abstract Pure perovskite phase Ba0.70 Sr0.30 TiO3 (BST) powders were successfully synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 850 ◦ C for 2 h, which is 300 ◦ C lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcining of the mixtures led to BST powders at 800–900 ◦ C. XRD and SEM techniques are used to characterize the phase and morphology of the fabricated BST powders, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Molten salts; BST; Powder; Perovskite
1. Introduction Barium strontium titanate (BST) [Ba1−x Srx TiO3 , 0 < x < 1] is a kind of favorable electronic materials for its high dielectric constant and composition-dependent Curie temperature [1,2]. It has been found that BST have applications in piezoelectric sensors, dynamic random access memories (DRAM), microwave phase shifters and uncooled infrared detectors for its high dielectric and pyroelectric properties [3–5]. A complete solid solution possessing lots of phases and structures exists throughout most of the BaTiO3 –SrTiO3 system and the low temperature phase equilibria study is still in argument [6]. This is because the studies of low temperature phase equilibria in this system are often hampered by sluggish kinetics arising from extremely low cation and/or anion diffusion coefficients in the conventional solid-state (CS) method of which the synthesis temperature is too high. The molten-salt (MS) method provides a way to circumvent this problem [7]. Furthermore, in recent years, low-temperature synthesis of BST powders has attracted considerable interest due to the technological importance of these powders in the fabrication of fine-grained and homogenized ceramics which improves the performance and reliability of physical properties
to a large extent. Besides lots of chemical methods [8,9], the molten-salt process can also be used to synthesize perovskite fine-grained ceramics [10]. Molten-salt synthesis method is one of the simplest methods for obtaining highly reactive powders of a single phase at low temperatures, in which the molten salt is used as a reaction aid. Materials such as PZT (lead zirconate titanate) [11], Pb(Fe0.5 Nb0.5 )O3 [12] and Bi3 NbTiO9 [13] have been synthesized using this method. However, to our best of the knowledge, there is little report about the molten-salt synthesis of BST powders. In this work, we report the synthesis of Ba0.70 Sr0.30 TiO3 powders at low temperatures by molten-salt method using NaCl–KCl eutectic mixtures. The effect of synthesizing temperature on the formation and morphology of BST powders in this chloride flux is investigated. Meanwhile, the comparison of molten-salt method with conventional solid-state process is also discussed. The goals of this paper are to develop the fabrication technology of materials and facilitate the study of low temperature phase equilibria in BaTiO3 –SrTiO3 system. 2. Experimental 2.1. Synthesis of Ba0.70 Sr0.30 TiO3 powders
∗
Corresponding author. Tel.: +86 21 52411412; fax: +86 21 52411104. E-mail address: [email protected] (X. Dong).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.052
Barium carbonate, (BaCO3 , 99.0%), strontium carbonate (SrCO3 , 99.0%) and titanium oxide (TiO2 , 99.44%) were used as starting materials to synthe-
C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167 size Ba0.70 Sr0.30 TiO3 powders. They were mixed with an equimolar mixture of sodium chloride (NaCl, 99.5%) and potassium chloride (KCl, 99.5%) in a polyethylene pot for 24 h using alcohol as the grinding medium. Eutectic temperature for the kind of chloride flux is 650 ◦ C. Subsequently, the mixture was heated in a sealed alumina crucible at a selected temperature between 800 and 900 ◦ C for 2 h. The resulting powders were crushed and washed with hot distilled water for several times to remove the chloride salts. For comparison, conventional solid-state reaction process was also used to prepare the Ba0.70 Sr0.30 TiO3 powders. In order to obtain the pure phase BST, the calcining temperature should be increased to 1150 ◦ C. The powder obtained by the molten-salt method and conventional solid-state reaction process will be hereinafter referred to as the MS and CS powder, respectively.
2.2. Characterization of BST powder X-ray diffraction (XRD) with Cu K␣ radiation (Model Rax-10, Rigaku, Tokyo, Japan) was carried out to examine the phase of the BST powders. The microstructure of BST powders was investigated by a scanning electron microscopy (Model JSM-6700F, JEOL, Tokyo, Japan).
3. Results and discussion 3.1. Powder phase characterization Fig. 1 shows the X-ray diffraction patterns of Ba0.70 Sr0.30 TiO3 powders synthesized under different conditions. It is obvious that single BST perovskite phase is formed by molten-salt synthesis at 850 ◦ C for 2 h, whereas, the carbonates and oxide without the mixture of NaCl–KCl cannot fully crystallize to pure phase until 1150 ◦ C. It should be emphasized that conventional solid-state synthesis of this solid solution is virtually impossible at a temperature <1000 ◦ C [14] due to very low solid-state diffusion. In the present study, the fact that the pure BST perovskite phase forms at a temperature only 850 ◦ C demonstrates that the molten salt eutectic indeed accelerates the kinetics at this low temperature by orders of magnitude and facilitates the formation of BST solid solution, similar to the results found in the solutions of BaZrO3 –SrZrO3 [7]. This could be attributed to the enhanced diffusion coefficients in the molten chloride liquid phase compared to that in the solid
Fig. 1. XRD patterns of the BST powder obtained at 800 ◦ C (a), 850 ◦ C (b), 900 ◦ C (c) for 2 h by MS process; and 1150 ◦ C (d) for 2 h by CS method.
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state. Thus, perovskite phase can be synthesized at a relative low temperature using molten-salt method, which can make the low temperature phase equilibria study more feasible. In Fig. 1, another significant feature should also be noted: besides the characteristic peaks of BST in Fig. 1(d), there are only two weak peaks corresponding to the carbonates (Ba,Sr)CO3 including BaCO3 and SrCO3 [15]. As the temperature increased, the two peaks disappeared and a pure phase is obtained at 850 ◦ C. During this process, there could not be found any other compounds or intermediate phases. It is known that usually, there are two reaction mechanisms in the MS process. In some cases, the molten salt participates in the chemical reaction. It reacts with the oxide or carbonate precursor, subsequently decomposing to the product, and returns the original salt in its uncombined state. In the case of Bi4 Ti3 O12 , for instance, the component oxides react with LiCl to form an intermediate compound which then decomposes into Bi4 Ti3 O12 and LiCl [16]. In other cases, the molten salt acts as a mere solvent which accelerates the rate of formation of the desired compound. The latter reaction mechanism seems to be closer to the process investigated in this study. Nevertheless, the exact formation mechanism is very complex and further work of this is in progress. 3.2. Powder morphology analysis Fig. 2 shows the SEM images of BST powders fabricated under different conditions. It can be seen that most of the CS powders calcined at 1150 ◦ C are badly agglomerated and random conglomerated. Compared to CS powders, the MS powders show quadrate-like morphology. Furthermore, the MS powders are more homogenized and fine. This can be attributed to the high reaction activity and low synthesis temperature introduced by the molten salts. Comparing Fig. 2(b) with (c), it should be noted that the powders synthesized at 850 ◦ C are fine but aggregated. When the temperature increased to 900 ◦ C, the powders dispersed well and most of them are quadrate accompanied by a few of coarse particles. It is obvious that calcining temperature plays an important role in the development of BST particle morphology. It is generally believed that the particle morphology is initially dominated by the nucleation process and later growth process during the MS synthesis. The morphology change of the BST powder with the different temperatures is controlled by the mechanism of the crystal growth in the chloride flux. Fig. 3 shows the main processing stages of the MS method with increasing temperature [10]. First, when the mixtures including carbonates, oxides and salts are fired at a temperature above the melting point of the salts and form a flux, the raw materials rearrange and diffuse quickly in a liquid state of the salts. Small and aggregated BST particles are rapidly formed in the initial period of molten-salt reaction: nucleation process. Then, as the calcining temperature increases, the BST nucleus grow to relatively big ones by consuming the small and agglomerated particles in accordance with the Ostwald ripening mechanism in the growth process [13]. Finally, discrete BST powders are formed at 900 ◦ C in the molten NaCl–KCl flux. However, as we can see in Fig. 2(c), there are some abnormal coarse particles.
166
C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167
Fig. 2. SEM micrographs of BST powders synthesized at 850 ◦ C (a), 900 ◦ C (b) for 2 h by MS process; and 1150 ◦ C (c) for 2 h by CS method.
This is because the raw powders used in this experiment are not well homogenized. 4. Conclusions The molten-salt method was successfully used to synthesize BST powders at a low temperature. The pure perovskite phase BST could be obtained at 850 ◦ C for 2 h. XRD results revealed that the molten salts acted as a mere solvent which accelerated the rate of formation of the desired BST compound but not participated in the chemical reaction. SEM analysis indicated that the calcining temperature played an important role in the development of BST particle morphology. This simple and novel fabrication method makes the fine-grained ceramics synthesis and low temperature phase equilibria study of BST more feasible. Acknowledgment The authors gratefully acknowledge the financial support granted by the QiMingXing Project of Shanghai City (No. 06QA14055). References
Fig. 3. Schematic illustration of the main processing stages in the MS method.
[1] J.L. Davis, L.D. Rubin, J. Appl. Phys. 24 (1953) 1194. [2] Y.B. Khollam, H.S. Potdar, S.B. Deshpande, A.B. Gaikwad, Mater. Chem. Phys. 97 (2006) 295. [3] G.W. Dietz, W. Antpohler, M. Klee, R. Waser, J. Appl. Phys. 78 (1995) 6113.
C. Mao et al. / Materials Chemistry and Physics 106 (2007) 164–167 [4] T. Karaki, K. Yan, T. Miyamoto, M. Adachi, Jpn. J. Appl. Phys. 46 (2007) L97. [5] R.W. Whatmore, P.C. Osbond, N.M. Shorrocks, Ferroelectrics 76 (1987) 351. [6] B.A. Wechsler, K.W. Kirby, J. Am. Ceram. Soc. 75 (1992) 981. [7] S. Gopalan, K. Mehta, A.V. Virkar, J. Mater. Res. 11 (1996) 1863. [8] C. Mao, X. Dong, T. Zeng, Mater. Lett. 61 (2007) 1633. [9] C. Mao, X. Dong, T. Zeng, H. Chen, F. Cao, Ceram. Int., in press. [10] K.H. Yoon, Y.S. Cho, D.H. Kang, J. Mater. Sci. 33 (1998) 2977.
167
[11] R.J. Arendt, H.H. Rosolowski, J.W. Szymaszek, Mater. Res. Bull. 14 (1979) 703. [12] C.C. Chiu, C.C. Li, S.B. Desu, J. Am. Ceram. Soc. 74 (1991) 38. [13] P.H. Xiang, Y. Kinemuchi, K. Watari, Mater. Lett. 59 (2005) 1876. [14] S. Kumar, G.L. Messing, W.B. White, J. Am. Ceram. Soc. 76 (1993) 617. [15] C. Mao, X. Dong, T. Zeng, G. Wang, S. Chen, Mater. Res. Bull. 42 (2007) 1602. [16] Y. Hayashi, T. Kimura, T. Yamaguchi, J. Mater. Sci. 21 (1986) 2876.