Synthesis processes and sintering behavior of layered-perovskite barium bismuth tantalate ceramics
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Synthesis processes and sintering behavior of layered-perovskite barium bismuth tantalate ceramics Chung-Hsin Lua) and Buh-Kuan Fang Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China (Received 15 July 1996; accepted 20 October 1997)
Ferroelectric layered-perovskite BaBi2 Ta2 O9 has been successfully prepared through a novel process using BiTaO4 as a precursor. Heating the mixtures of BiTaO4 and BaCO3 at 900 ±C without soaking results in the complete formation of the monophasic powder. In contrast, the conventional solid-state reaction requires soaking at 900 ±C for 2 h to obtain the pure compound. Such prolonged heat treatment causes unfavorable growth of particles. In the new process, the formation of BaBi2 Ta2 O9 is markedly accelerated due to the suppression of the formation of a stable intermediate Ba5 Ta4 O15 . In addition, this process yields submicron BaBi2 Ta2 O9 powder with significantly improved sinterability. Sintering at 1000 ±C affords well-densified ceramics. On the other hand, heating at temperatures greater than 1100 ±C causes BaBi2 Ta2 O9 to thermally decompose and form Bi2 O3 and rod-like BaTa2 O6 . The formation of these rod-like grains results in the expansion of the matrix, thereby reducing the density.
I. INTRODUCTION
The layered-structure perovskites have recently received considerable interest in view of their potential application as low-voltage, high-speed nonvolatile random access memory (NvRAM).1,2 These materials were first synthesized by Aurivillius,3–5 and were named Aurivillius compounds. The chemical formula of these compounds is expressed as (Bi2 O2 )21 (Ax21 Bx O3x11 )22 , where x indicates the number of perovskite building blocks between two (Bi2 O2 )21 layers, and A and B represent the different cations with low and high valences in the structure.6 This group of compounds exhibits a pervoskite-like structure in a way that B cations form chemical bonding with the oxygen ions to construct BO6 octahedrons, whereas A cations are located in the space between BO6 octahedrons. The BO6 octahedrons constitute the continuous layers perpendicular to the c-axis direction, but are interrupted by (Bi2 O2 )21 layers along the c-axis direction. The BO6 octahedrons exhibit spontaneous polarization, thereby resulting in ferroelectric properties. Several layered perovskites with x 2, e.g., BaBi2 Ta2 O9 , SrBi2 Ta2 O9 , and SrBi2 Nb2 O9 , have been deposited in the form of thin film on silicon wafers by using various processes. Desu7 and Dat8 have employed the pulse laser deposition technique to prepare SrBi2 Tax Nb12x O9 and SrBi2 Ta2 O9 thin films. Araujo,1 Chu,9 and Amanuma10 have used solution-deposition methods to prepare similar ferroelectric thin films. These thin films have been confirmed to possess excellent fatigue a)
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J. Mater. Res., Vol. 13, No. 8, Aug 1998
resistance and exhibit no degradation of polarization up to 109 –1012 switching cycles.7
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