Nonisothermal phase formation kinetics in sol-gel-derived strontium bismuth tantalate
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Phase formation characteristics of Sr0.7Bi2.4Ta2O9 (SBT) powder, synthesized via the sol-gel and pyrolysis processes, was investigated using the thermal analysis technique. The two exotherms, appearing in the differential thermal analysis (DTA) curve, were identified as crystallization of fluorite phase and transformation of fluorite to Aurivillius phase, respectively, using x-ray diffraction. Nonisothermal kinetic analysis of the DTA results shows activation energy values for the formation of fluorite and Aurivillius phases as 192 and 375 kJ/mol, respectively, and Avrami exponent values for each reaction as 0.91 and 0.96, respectively. The results of this investigation are presented and discussed in detail to understand the phase formation mechanism in the SBT system.
I. INTRODUCTION
As a next-generation memory, ferroelectric randomaccess memory possesses a variety of excellent properties, including a large capacity of dynamic random-access memory, high speed of static random-access memory, and data retention ability of flash memory. Oxides with alternating perovskite and bismuth oxide layers, called the Aurivillius structure, have been studied to replace Pb(Zr, Ti)O3 solid solutions, since they show much less fatigue problems even up to approximately 1012 switching cycles. SrBi2Ta2O9 (SBT) is one of the most important materials among the aurivillius oxides.1– 4 It has been suggested that in the case of SBT, the oxygen vacancies are fast enough not to be trapped −at the interfaces or bismuth oxide layers can supply O2 ions to oxygen vacancy sites. It is comprised of alternating Bi2O2 layers and SrBi2O7 perovskite layers. SBT forms the ferroelectric Aurivillius phase having an orthorhombic structure at approximately 750 °C. Recently, by using x-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, Koiwa et al.,5,6 Lee et al.,7 and Boyle et al.8 have revealed that SBT forms a metastable phase with fluorite (CaF2) structure at a low temperature range. However, no thermal analysis study has been reported for the characterization of the fluorite phase formation in SBT. Moreover, kinetic analysis results on both fluorite formation and fluorite-to-Aurivillius phase transformation
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e-mail: [email protected] J. Mater. Res., Vol. 16, No. 7, Jul 2001
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have not been reported so far. Among the various methods reported, the sol-gel method is found to be a promising route for the synthesis of oxide powders. In 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. 9 , 1 0 Also, Bi excess composition is widely used for most of the SBT synthesis in order to compensate for the Bi loss during the processing. The phase formation characteristics of the solgel-derived and pyrolyzed SBT powders were analyzed using differential thermal analysis (DTA) and XRD. Also, no
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