Phase Formation Kinetics in Sol-Gel Derived Strontium Bismith Tantalate
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PHASE FORMATION KINETICS IN SOL-GEL DERIVED STRONTIUM BISMITH TANTALATE Yun-Mo Sung and Woo-Chul Kwack Department of Materials Science & Engineering, Daejin University Pochun-koon, Kyunggi-do 487-711 Korea (South) ABSTRACT Phase formation characteristics of Sr0.7Bi2.4Ta2O9 (SBT) powder, synthesized via sol-gel and pyrolysis process, was investigated by using thermal analysis. Each of the two exotherms, appearing in differential thermal analysis (DTA) scan curves, was identified as crystallization of fluorite phase and transformation of fluorite to aurivillius phase, respectively by using x-ray diffraction (XRD). By applying non-isothermal kinetic analyses to the DTA results, activation energy values for the formation of fluorite and aurivillius phases were determined as 192 and 375 kJ/mol, respectively and Avrami exponent values for each reaction were determined as 0.91 and 0.96, respectively. These activation energy and Avrami exponent values were discussed in detail to understand phase formation mechanism in SBT system.
INTRODUCTION Oxides with alternating perovskite and bismuth oxide layers, called aurivillius structure, have been studied to replace PZT, since they show much less fatigue problem even up to ~1012 switching cycles. SrBi2Ta2O9 (SBT) is the most famous material among the aurivillius oxides [14]. SBT forms the ferroeelctric aurivillius phase having orthorhombic structure at ~750°C. Recently, by using x-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, Koiwa et al [5,6] and Lee et al. [7] have revealed that SBT forms a metastable phase with fluorite (CaF2) structure at a low temperature range. However, any thermal analysis to identify characteristics of fluorite phase formation in SBT, has not been performed. Moreover, kinetic analysis results on both fluorite formation and fluorite-to-aurivillius phase transformation have not been reported so far. For the present study, Sr0.7Bi2.4Ta2O9 composition was selected for SBT powder synthesis since it has been known that partial substitution of Sr2+ ions in the stochiometric SrBi2Ta2O9 by Bi3+ ions can improve ferroelectric properties of SBT [8,9]. SBT powder, fabricated by sol-gel and pyrolysis, was analyzed for phase formation characteristics using differential thermal analysis (DTA) and XRD. Also, non-isothermal kinetic analyses were applied to the DTA results to determine activation energy and Avrami exponent for each phase formation reaction.
EXPERIMENTAL PROCEDURE Sr-isopropoxide, Bi-triamyloxide, and Ta-ethoxide were used as received from High-Purity Chemical Co. in Japan and these metal alkoxides were handled in a glove box at dried Ar atmosphere. Sr-isopropoxide was dissolved in 2-methoxyethanol by refluxing at 130°C and distilled via thermal and vacuum processes. Bi-amyloxide was added into the SrF7.4.1
methoxyethoxide solution by refluxing at 60°C and Sr-Bi double methoyethoxide solution was distilled again. Ta-ethoxide was mixed with 2-methoxyethanol by refluxing at 130°C and the solution was distilled. This Ta-metho
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