Process optimization for 123 and Bi-based superconductors

Abstract

Abstract The development of pure phase 123 and Bi-based 2223 superconductors has been optimized. The pre-heat processing appears to be a very important parameter in achieving this objective. In addition, the synthesis of pure phases in the Bi-based system involves effects due to oxygen partial pressure, time and temperature. The optimization of this processing is a key step towards the successful continuation of the superconducting materials development, i.e. in substitutions, c-axis orientation, melt-sintering processes, and in synthesizing single crystals. An example of optimized bulk substitution is the 70% limit of Sr for Ba substitution in the yttrium-based 123 superconductor. In a nominal composition containing no barium, a majority 123 phase has also been observed, though this phase is apparently not superconducting. In the Bi-based system, substitution of Sb and Pb for Bi has been obtained in the 2223 phase. The Sb substitution into the pure phase has no apparent advantage over the parent Bi compound or the Pb-substituted phase. Orientation/melt-sintering effects include the extreme c-axis orientation of yttrium 123 and of the bisumuth 2223, 2212 and 2201 phases. Surface orientation of all three Bi-based phases can be obtained from one sample 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 present, 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 been successful. This procedure as well as those above will be described and discussed.