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L. S. Chumbley Ames Laboratory, Iowa State University, 214 Wilhelm, Ames, Iowa 50011 (Received 2 November 1992; accepted 23 April 1993)

The solidification processes for the compositions Bi 2 Sr 2 CaCu 2 0 y (2212) and Bi2Sri.75Cao.25CuOy (2201) were determined as a function of oxygen partial pressure. During solidification in argon, the superconducting phases were generally not observed to form for either composition. In both cases, the solidus is lowered to approximately 750 °C. Solidification of Bi2Sri 75Ca0.25CuOy in Ar resulted in a divorced eutectic structure of Bi2Sr2_A:Cax0>, (22JC) and Cu2O while solidification of Bi 2 Sr 2 CaCu 2 0 > , in Ar resulted in a divorced eutectic structure of Bi2Sr3_xCa;c03, (23x) and Cu 2 O. Solidification of Bi2Sri.75Cao.25CuOy in O 2 resulted in large grains of 2201 interspersed with small regions containing the eutectic structure of 22* and CuO/Cu 2 O. Solidification of Bi2Sr2CaCu2Oy in partial pressures of 1%, 20%, and 100% oxygen resulted in multiphase samples consisting of 2212, 2201, some alkaline-earth cuprates, and both divorced eutectic structures found during solidification in Ar. For both compositions, these latter structures can be attributed to oxygen deficiencies present in the melt regardless of the overpressure of oxygen. These eutectic structures are unstable and convert into the superconducting phases during subsequent anneals in oxygen. The formation process of the 2212 phase during solidification from the melt was determined to proceed through an intermediate state involving the 2201 phase.

I. INTRODUCTION Heat treatments in the partially melted state are commonly employed during processing of the Bi 2Sr2CaCu2Oy (2212) superconductor in order to fabricate highly textured wires, tapes, and thick films capable of handling high critical current densities (Jc).ia These materials have been reported to have 7 c 's in excess of 1 X 104 A/cm 2 at 4.2 K in magnetic fields greater than 20 T.3"5 Hence, new possibilities may exist for transport or magnet applications if the production of these materials can be scaled up commercially. To accomplish this, a detailed understanding of the mechanisms involved in melt processing is required. Previous studies have shown that Sri_xCa^CuO2 and a Bi-rich liquid result from the incongruent melting of 2212 around 870 °C in air.6-9 With further heating, Srx^Ca^CuO 2 transforms into S ^ C a ^ C u ^ and finally (Sr, Ca)O.9 Excesses of Bi or the presence of Ag were also found to lower the melting point by as much as 30 °C.9 In contrast, less is understood about the sequence of crystallization events that occurs during solidification from the molten state. One study reported that the 2212 phase crystallizes from the melt around 850 °C and coexists with a liquid phase. With further cooling, the liquid phase crystallizes at 730 °C as the low-temperature superconductor Bi2Sr2_^Ca^Cu0J (2201).7 Another study reported that the 2212 phase J. Mater. Res., Vol. 8, No. 9, Sep 1993 http://journals.cambridge.org

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