Kinetics of solid state reaction between barium carbonate and cupric oxide
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INTRODUCTION
S T U D I E S in solid state reaction have for some time been stimulated by their applications in metallurgy, ceramics processing, laser chemistry, chemistry of polymers and propellants, manufacture of artificial gems, semiconductors, ferrites, and catalysts. More recently, interest in this area has been further catalyzed by the development of the new high-temperature superconductors, many of which are currently synthesized by routes involving solid state reactions, ti} A compound that has received much attention among these is the yttriumbarium-copper superconductor, Yna2CU3OT_x, which has a relatively high critical temperature of over 90 K. The most common method of preparation of this superconductor is that of calcination of a stoichiometric mixture of BaCO3, C u O , and Y203 at temperatures of about 1173 K, but this is a slow process because of the low reactivity of BaCO3 and produces large particles of wide size distribution which, in turn, adversely affect the product density. Ruckenstein et al. ~21 have suggested a modification of this technique, whereby the slow reaction between BaCO3 and CuO is isolated in an initial step so that shorter calcination times for the subsequent reaction with Y203 and CuO are required and a higher density product can be obtained. The initial step follows the stoichiometry. BaCO3 + CuO ,~ BaCuO2 + CO2
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and has a high degree of reversibility, because BaCuO2 adapts itself to a structure similar to that of BaO, the decomposition product of BaCO3. As a consequence, it may be anticipated that the rate of mass transfer of CO2 out of the reaction zone will have a significant role in determining the reaction rate. The work reported in this S.G. ACHARYA, Graduate Student, and S.K. BHATIA and H.S. SHANKAR, Professors, are with the Department of Chemical Engineering, Indian Institute of Technology, Powai, Bombay 400 076, India. Manuscript submitted August 2, 1991. METALLURGICALIRANSACTIONSB
article concerns the kinetics of this initial step in the twostage process for the synthesis o f the yttrium-bariumcopper superconductors. While extensive studies have been done with the single step ternary reaction system, little has been reported regarding the binary BaCO3-CuO system. Some investigators I3:.51 have reported the formation of BaCuO2 as the double metal oxide following X-ray diffraction studies. Recently, Halasz et al. t6J have reported the existence of a new phase Ba2Cu305 in addition to BaCuO2 when investigating the consequences of preparative chemistry on Y-Ba-Cu-O superconductors. Subsequently, they proposed the two-stage preparative method, in the first stage of which BaCO3 and CuO are reacted. More recently, Ruckestein et al. t2j also proposed a similar procedure, since the rate-limiting step is isolated in this reaction and consequently rapid formation of the superconducting phase is achieved in the second step involving subsequent reaction with Y203. These authors followed the progress of the BaCO3-CuO reaction by X-ray diffraction and speculated that tran
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