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Synthesis of nanopowders of (Ba1−xSrx)TiO3
November 2002
Materials Letters 56 (2002) 1089 – 1092 www.elsevier.com/locate/matlet
Synthesis of nanopowders of (Ba1 xSrx)TiO3 I. Packia Selvam, V. Kumar* Centre for Materials for Electronics Technology, Ministry of Information Technology, Government of India, Thrissur 680 771, Kerala, India
Abstract A simple direct precipitation method has been developed for the synthesis of nanosized, homogeneous (Ba1 xSrx)TiO3 [BST] powders from stable precursor solutions. By appropriate control of pH and temperature among other processing variables, single-phase (Ba1 xSrx)TiO3 is obtained at temperatures less than 100 jC. The crystalline phase, particle size and morphology of the BST powders are examined by XRD and TEM. The stoichiometry of the (Ba1 xSrx)TiO3 powder is discussed in relation to the mechanism of formation of the perovskite phase. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanopowders; Synthesis; (Ba1 xSrx)TiO3
1. Introduction Barium titanate is used extensively in electronic components. Since the discovery of the applications of BaTiO3, several A-site isovalent substituted compositions have been investigated. One such material, (Ba1 xSrx)TiO3, finds extensive applications [1,2] in memory devices, capacitors, sensors, etc.: high chemical purity, precise composition, high density and uniform microstructure are among the most desirable features for the BST electroceramics in achieving the desired electrical characteristics. In order to achieve these characteristics, quality of the starting powder is a crucial factor. In the case of multicomponent systems, their synthesis through chemical solution methods offer several advantages such as high-purity, homogeneity and precise composition over the conventional solid state method. Several low-temperature *
Corresponding author. Tel.: +91-487-201156; fax: +91-487201347. E-mail address: [email protected] (V. Kumar).
chemical routes have been reported [3– 11] in the literature for the synthesis of BST. In the co-precipitation and hydrothermal methods, stoichometry deviations and formation of biphasic solid solutions have been reported. Also, the high temperatures (T >500 jC) required for achieving complete solid solution in some of the low-temperature methods such as sol – gel and metallo-organic decomposition (MOD) make them less favorable. Many of these problems can be overcome by using low-temperature chemical solution precipitation techniques. We have developed a simple chemical method for synthesizing nanosized powders of BST at low temperatures (T < 100 jC) without a calcination step. We had earlier reported [12] the usefulness of this novel chemical methodology for the synthesis of fine powders of BaTiO3. Although BST ceramics have been studied for many years, there is still no report of its synthesis by a single step. The purpose of the present investigation is to study the formation of the monophasic solid solution (Ba1 xSrx)TiO3 during its direct precipitation from stable precursor solutions derived from the system,
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 6 8 4 - 5
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I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
M I I ( O O C C H 3 ) 2 – CH 3 C OO H – (C H 3 ) 2 C H O H – Ti(OnBu)4, (MII = Ba, Sr) using sodium hydroxide as the precipitating agent.
2. Experimental For the preparation of the precursor solutions, barium acetate (Qualigens, India), strontium acetate and titanium tetrabutoxide monomer (Synthochem, India) were used as the starting materials. Appropriate amount of the alkaline-earth metal acetates dissolved in glacial acetic acid were mixed with a solution of titanium tetrabutoxide monomer in isopropyl alcohol at room temperature and constant stirring conditions. The resultant stable precursor solution was then added slowly to a hot stirred aqueous solution (T = 85– 90 jC) of sodium hydroxide. A white precipitate of the alkaline-earth metal titanate was observed. After the addition was complete, stirring and heating were continued for another hour. After cooling, the precipitate was filtered, washed first with warm dilute acetic acid
Fig. 1. XRD patterns of as-precipitated powders of (Ba1 1.5 and (d) 2.0, and (e) x = 1.00.
xSrx)TiO3
(0.1 N) and then with distilled water until the washings were alkali-free and dried at 110 jC for 4 h. The yield of the ceramic powder was almost quantitative. The XRD pattern of the powders were recorded on a Bruker, Model D5005 X-ray diffractometer. Particle size and morphology were determined using a Hitachi Model H-8100 transmission electron microscope.
3. Results and discussion BST powders with a ratio of {[Ba2 + ]/[Ba2 + + Sr ] = 0.5} in solution were prepared at a temperature of 90 jC that contained 2.5 M of NaOH, equal initial concentrations of Ba2 + and Sr2 + and the total concentration of the cations in solutions {[Ba2 + ]+ [Sr2 + ]+[Ti4 + ]} of 0.1 M. The initial ratio of barium and strontium in solution relative to titanium, [Ba2 + + Sr2 + ]/[Ti4 + ], was varied between 1.0 and 2.0. XRD patterns of the as-precipitated powders are shown in Fig. 1. XRD patterns confirmed the formation of BST along with small amounts of carbonate contamination 2+
with (a) x = 0 and with x = 0.5 for starting [Ba + Sr]/[Ti] ratios of (b) 1, (c)
I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
in samples with [Ba + Sr]/[Ti] ratio = 2.0. In order to understand the monophasic nature of the powders, slow XRD scan between 2h = 30 –34j for the most intense (111) peak was performed. It can be seen that monophasic solid solution was formed for a [Ba + Sr]/ [Ti] ratio of 1.0 itself as evidenced by the presence of sharp peak (Fig. 2b). The ratio of [Ba2 + + Sr2 + ]/
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[Ti4 + ] did not show any major influence on the formation of single-phase solid solution. The average particle size deduced from the peak broadening in XRD, as well as from the TEM micrograph (Figs. 3) for this powder, is 50 nm. The SAED pattern (Fig. 3) reveals their polycrystalline nature. The d values estimated from this pattern did not reveal the presence
Fig. 2. XRD patterns of as-precipitated powders of (a) BaTiO3, (b) (Ba0.5Sr0.5)TiO3 and (c) SrTiO3 between 2h = 30 – 34j showing the diffraction from (111) plane.
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I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
Fig. 3. TEM with SAED pattern of as-precipitated (Ba0.5Sr0.5)TiO3 powder.
of any other crystalline phases. In the BST specimens, a slight shift towards Sr-rich stoichiometry from the initial Ba/Sr ratio in solutions can be explained on the basis of the ionic radii of the two cations diffusing through the TiO2 gel network. The ionic radii of Sr2 + is lower than that of Ba2 +. Due to its smaller size, Sr2 + ions can diffuse more easily than Ba2 + through the network of TiO6 octahedra [12], causing a shift towards Sr-rich compositions. The formation of a monophasic solid solution, at such low temperatures, with minimal stoichiometry deviation, reflect the atomic level homogeneity of the cations achieved in solutions.
4. Conclusion Stoichiometric and monophasic (Ba1 xSrx)TiO3 powders were synthesized by a direct precipitation method at very low temperatures (T < 100 jC) from stable solutions. Formation of a monophasic solid solution was observed even for an even stoichiometry of the A2 + and B4 + cations in the solutions. Smaller ionic radius of Sr2 + relative to Ba2 + leads to enhanced
diffusion of them through the TiO2 gel network to yield slightly Sr-rich monophasic BST powders.
References [1] D. Tahan, A. Safari, L.C. Klein, in: R.K Pandey, M. Liu, A. Safari (Eds.), Proc. IEEE Int. Symp. Appl. Ferroelectr., vol. 9, IEEE, New York, 1994, p. 427. [2] X. Li, F. Qiu, K. Guo, B. Zou, J. Gu, J. Wang, B. Xu, Mater. Chem. Phys. 50 (1997) 227. [3] P.K. Gallagher, F. Schrey, F.V. DiMarcello, J. Am. Ceram. Soc. 46 (1963) 359. [4] F. Chaput, J.P. Boilot, J. Mater. Sci. Lett. 6 (1987) 1110. [5] F. Chaput, J.P. Boilot, A. Beauger, J. Am. Ceram. Soc. 73 (1990) 942. [6] D.M. Tahan, A. Safari, L.C. Klein, J. Am. Ceram. Soc. 79 (1996) 1593. [7] K. Kajiyoshi, M. Yoshimura, Y. Hamaji, K. Tomono, T. Kasanami, J. Mater. Res. 11 (1996) 169. [8] C.H. Lin, T.S. Yan, Mater. Res. Soc. Symp. Proc. 346 (1994) 231. [9] S. Komarneni, Q. Li, M. Stefansson, R. Roy, J. Mater. Res. 8 (1993) 3176. [10] R.W. Vest, Ferroelectrics 102 (1990) 53. [11] R.K. Roeder, E.B. Slamovich, J. Am. Ceram. Soc. 82 (1999) 1665. [12] V. Kumar, J. Am. Ceram. Soc. 82 (1999) 2580.
Materials Letters 56 (2002) 1089 – 1092 www.elsevier.com/locate/matlet
Synthesis of nanopowders of (Ba1 xSrx)TiO3 I. Packia Selvam, V. Kumar* Centre for Materials for Electronics Technology, Ministry of Information Technology, Government of India, Thrissur 680 771, Kerala, India
Abstract A simple direct precipitation method has been developed for the synthesis of nanosized, homogeneous (Ba1 xSrx)TiO3 [BST] powders from stable precursor solutions. By appropriate control of pH and temperature among other processing variables, single-phase (Ba1 xSrx)TiO3 is obtained at temperatures less than 100 jC. The crystalline phase, particle size and morphology of the BST powders are examined by XRD and TEM. The stoichiometry of the (Ba1 xSrx)TiO3 powder is discussed in relation to the mechanism of formation of the perovskite phase. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanopowders; Synthesis; (Ba1 xSrx)TiO3
1. Introduction Barium titanate is used extensively in electronic components. Since the discovery of the applications of BaTiO3, several A-site isovalent substituted compositions have been investigated. One such material, (Ba1 xSrx)TiO3, finds extensive applications [1,2] in memory devices, capacitors, sensors, etc.: high chemical purity, precise composition, high density and uniform microstructure are among the most desirable features for the BST electroceramics in achieving the desired electrical characteristics. In order to achieve these characteristics, quality of the starting powder is a crucial factor. In the case of multicomponent systems, their synthesis through chemical solution methods offer several advantages such as high-purity, homogeneity and precise composition over the conventional solid state method. Several low-temperature *
Corresponding author. Tel.: +91-487-201156; fax: +91-487201347. E-mail address: [email protected] (V. Kumar).
chemical routes have been reported [3– 11] in the literature for the synthesis of BST. In the co-precipitation and hydrothermal methods, stoichometry deviations and formation of biphasic solid solutions have been reported. Also, the high temperatures (T >500 jC) required for achieving complete solid solution in some of the low-temperature methods such as sol – gel and metallo-organic decomposition (MOD) make them less favorable. Many of these problems can be overcome by using low-temperature chemical solution precipitation techniques. We have developed a simple chemical method for synthesizing nanosized powders of BST at low temperatures (T < 100 jC) without a calcination step. We had earlier reported [12] the usefulness of this novel chemical methodology for the synthesis of fine powders of BaTiO3. Although BST ceramics have been studied for many years, there is still no report of its synthesis by a single step. The purpose of the present investigation is to study the formation of the monophasic solid solution (Ba1 xSrx)TiO3 during its direct precipitation from stable precursor solutions derived from the system,
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 6 8 4 - 5
1090
I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
M I I ( O O C C H 3 ) 2 – CH 3 C OO H – (C H 3 ) 2 C H O H – Ti(OnBu)4, (MII = Ba, Sr) using sodium hydroxide as the precipitating agent.
2. Experimental For the preparation of the precursor solutions, barium acetate (Qualigens, India), strontium acetate and titanium tetrabutoxide monomer (Synthochem, India) were used as the starting materials. Appropriate amount of the alkaline-earth metal acetates dissolved in glacial acetic acid were mixed with a solution of titanium tetrabutoxide monomer in isopropyl alcohol at room temperature and constant stirring conditions. The resultant stable precursor solution was then added slowly to a hot stirred aqueous solution (T = 85– 90 jC) of sodium hydroxide. A white precipitate of the alkaline-earth metal titanate was observed. After the addition was complete, stirring and heating were continued for another hour. After cooling, the precipitate was filtered, washed first with warm dilute acetic acid
Fig. 1. XRD patterns of as-precipitated powders of (Ba1 1.5 and (d) 2.0, and (e) x = 1.00.
xSrx)TiO3
(0.1 N) and then with distilled water until the washings were alkali-free and dried at 110 jC for 4 h. The yield of the ceramic powder was almost quantitative. The XRD pattern of the powders were recorded on a Bruker, Model D5005 X-ray diffractometer. Particle size and morphology were determined using a Hitachi Model H-8100 transmission electron microscope.
3. Results and discussion BST powders with a ratio of {[Ba2 + ]/[Ba2 + + Sr ] = 0.5} in solution were prepared at a temperature of 90 jC that contained 2.5 M of NaOH, equal initial concentrations of Ba2 + and Sr2 + and the total concentration of the cations in solutions {[Ba2 + ]+ [Sr2 + ]+[Ti4 + ]} of 0.1 M. The initial ratio of barium and strontium in solution relative to titanium, [Ba2 + + Sr2 + ]/[Ti4 + ], was varied between 1.0 and 2.0. XRD patterns of the as-precipitated powders are shown in Fig. 1. XRD patterns confirmed the formation of BST along with small amounts of carbonate contamination 2+
with (a) x = 0 and with x = 0.5 for starting [Ba + Sr]/[Ti] ratios of (b) 1, (c)
I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
in samples with [Ba + Sr]/[Ti] ratio = 2.0. In order to understand the monophasic nature of the powders, slow XRD scan between 2h = 30 –34j for the most intense (111) peak was performed. It can be seen that monophasic solid solution was formed for a [Ba + Sr]/ [Ti] ratio of 1.0 itself as evidenced by the presence of sharp peak (Fig. 2b). The ratio of [Ba2 + + Sr2 + ]/
1091
[Ti4 + ] did not show any major influence on the formation of single-phase solid solution. The average particle size deduced from the peak broadening in XRD, as well as from the TEM micrograph (Figs. 3) for this powder, is 50 nm. The SAED pattern (Fig. 3) reveals their polycrystalline nature. The d values estimated from this pattern did not reveal the presence
Fig. 2. XRD patterns of as-precipitated powders of (a) BaTiO3, (b) (Ba0.5Sr0.5)TiO3 and (c) SrTiO3 between 2h = 30 – 34j showing the diffraction from (111) plane.
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I. Packia Selvam, V. Kumar / Materials Letters 56 (2002) 1089–1092
Fig. 3. TEM with SAED pattern of as-precipitated (Ba0.5Sr0.5)TiO3 powder.
of any other crystalline phases. In the BST specimens, a slight shift towards Sr-rich stoichiometry from the initial Ba/Sr ratio in solutions can be explained on the basis of the ionic radii of the two cations diffusing through the TiO2 gel network. The ionic radii of Sr2 + is lower than that of Ba2 +. Due to its smaller size, Sr2 + ions can diffuse more easily than Ba2 + through the network of TiO6 octahedra [12], causing a shift towards Sr-rich compositions. The formation of a monophasic solid solution, at such low temperatures, with minimal stoichiometry deviation, reflect the atomic level homogeneity of the cations achieved in solutions.
4. Conclusion Stoichiometric and monophasic (Ba1 xSrx)TiO3 powders were synthesized by a direct precipitation method at very low temperatures (T < 100 jC) from stable solutions. Formation of a monophasic solid solution was observed even for an even stoichiometry of the A2 + and B4 + cations in the solutions. Smaller ionic radius of Sr2 + relative to Ba2 + leads to enhanced
diffusion of them through the TiO2 gel network to yield slightly Sr-rich monophasic BST powders.
References [1] D. Tahan, A. Safari, L.C. Klein, in: R.K Pandey, M. Liu, A. Safari (Eds.), Proc. IEEE Int. Symp. Appl. Ferroelectr., vol. 9, IEEE, New York, 1994, p. 427. [2] X. Li, F. Qiu, K. Guo, B. Zou, J. Gu, J. Wang, B. Xu, Mater. Chem. Phys. 50 (1997) 227. [3] P.K. Gallagher, F. Schrey, F.V. DiMarcello, J. Am. Ceram. Soc. 46 (1963) 359. [4] F. Chaput, J.P. Boilot, J. Mater. Sci. Lett. 6 (1987) 1110. [5] F. Chaput, J.P. Boilot, A. Beauger, J. Am. Ceram. Soc. 73 (1990) 942. [6] D.M. Tahan, A. Safari, L.C. Klein, J. Am. Ceram. Soc. 79 (1996) 1593. [7] K. Kajiyoshi, M. Yoshimura, Y. Hamaji, K. Tomono, T. Kasanami, J. Mater. Res. 11 (1996) 169. [8] C.H. Lin, T.S. Yan, Mater. Res. Soc. Symp. Proc. 346 (1994) 231. [9] S. Komarneni, Q. Li, M. Stefansson, R. Roy, J. Mater. Res. 8 (1993) 3176. [10] R.W. Vest, Ferroelectrics 102 (1990) 53. [11] R.K. Roeder, E.B. Slamovich, J. Am. Ceram. Soc. 82 (1999) 1665. [12] V. Kumar, J. Am. Ceram. Soc. 82 (1999) 2580.