PROCESS OPTIMIZATION FOR 123-BASED AND BI-BASED SUPERCONDUCTORS

The development of pure phase 123 and Bi-based 2223 superconductors ha s been optimized. The pre-heat processing appears to be a very importa nt parameter in achieving this objective. In addition, the synthesis o f pure phases in the Bi-based system involves effects due to oxygen pa rtial pressure, time and temperature. The optimization of this process ing is a key step towards the successful continuation of the supercond ucting materials development, i.e. in substitutions, c-axis orientatio n, melt-sintering processes, and in synthesizing single crystals. An e xample of optimized bulk substitution is the 70% limit of Sr for Ba su bstitution in the yttrium-based 123 superconductor. In a nominal compo sition containing no barium, a majority 123 phase has also been observ ed, though this phase is apparently not superconducting. In the Bi-bas ed system, substitution of Sb and Pb for Bi has been obtained in the 2 223 phase. The Sb substitution into the pure phase has no apparent adv antage over the parent Bi compound or the Pb-substituted phase. Orient ation/melt-sintering effects include the extreme c-axis orientation of yttrium 123 and of the bisumuth 2223, 2212 and 2201 phases. Surface o rientation of all three Bi-based phases can be obtained from one sampl e of a specific nominal bulk composition. The surface orientation for both series of samples is dependent upon a number of factors including pre- heat processing, exact composition and/or extraneous phases pres ent, heat rates, temperatures, and times. A procedure for establishing Sr substitution for Ba into 123 single crystals has been determined. Systematic substitution of up to 30% Sr for Ba in the structure has be en successful. This procedure as well as those above will be described and discussed.