Crystallization Kinetics of Seeded SBT (Sr 0.7 Bi 2.4 Ta 2 O 9 ) Powders from Sol-Gel Derived Precursors
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CRYSTALLIZATION KINETICS OF SEEDED SBT ( Sr0.7Bi2.4Ta2O9 ) POWDERS FROM SOL-GEL DERIVED PRECURSORS Gopinathan M. A. Kumar, Woo-Chul Kwak, Se-Yon Jung, Seung-Joon Hwang and Yun-Mo Sung Department of Materials Science & Engineering, Daejin University Pochun-koon, Kyunggi-do 487-711 Korea (South) ABSTRACT Strontium bismuth tantalate (SBT) having composition of Sr0.7Bi2.4Ta2O9 has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt.% of nanometer sized SBT particles to the sol followed by pyrolysis. By applying non-isothermal kinetic analysis to the DTA results, activation energy values for the unseeded and seeded samples were determined. Enhanced crystallization kinetics was observed for the seeded one and the activation energy for the aurivillius phase formation was found to be 318 kJ/mol, while 375 kJ/mol for the unseeded. The Avrami exponent values for the seeded and unseeded were found to be 2.80 and 0.96 respectively. INTRODUCTION The bismuth layered-structure, perovskite compounds, belonging to the so-called Aurivillius class of layered perovskites, have attracted much attention due to the possibility of applications such as non-volatile memory devices. Among the various perovskite materials, strontium bismuth tantalate (SBT) has been identified for potential applications, especially in the thin film ferroelectric non-volatile random access memories [1]. This is basically because of its properties such as excellent retention characteristics, low leakage currents and almost polarization fatigue free operation for up to 1012 switching cycles [1]. Out of the various methods reported, sol-gel method is found to be a promising route for the synthesis of films and powders, since it offers a low crystallization temperature with enhanced homogeneity [2, 3]. Several attempts have been made to synthesize SBT at a low temperature and to improve the microstructure and hence ferroelectric properties [4, 5]. Even though the possible existence of many non-stoichiometric phases, which are stable within the (Sr-Bi-Ta-O) system, has been reported, SBT systems with a near stoichiometric composition (Sr1-x Bi2+yTa2O9) have been found to show significantly improved ferroelectic properties [6]. It has been reported that Bi substitution and cation vacancies at the strontium site enhance the structural distortion of the TaO6 octahedra, leading to a larger ferroelectric spontaneous polarization [6]. In order to reduce the transformation temperature and to refine the microstructure the effect of perovskite seeds has been studied in the fields of sol-gel derived Pb(Zr, Ti)O3 (PZT), PbTiO3 −Pb(Mg, Nb)O3 (PT-PMN) systems [7-9], since an addition of perovskite seed particles causes an increase in the number of nucleation sites for phase transformation. Recently, a non-isothermal kinetic analysis was carried out by Sung [10] to determine the activation energy and Avrami exponent for the phase formation reactions in the SBT (Sr0.7Bi2.4Ta2O9) system.
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