Metallurgical Route to High‐Tc Superconducting Ceramics

Abstract

We have prepared various high‐Tc superconductors, of the LnBa2Cu3O7‐x (Ln = Y, Eu, Yb) and Bi‐Sr‐Ca‐Cu‐O families, by oxidation of rapidly quenched precursor alloys. In this metallurgical approach, appropriate metallic constituents are first combined into homogeneous alloys by rapid solidification processing. Subsequent oxidation of the as‐obtained precursors leads to formation of the desired superconducting phases. For EuBa2Cu3O7‐x, a notable increase in critical current density is observed, and we attribute this to both the uniform grain size and a more homogeneous composition. In the case of bismuth compounds, the Bi4Sr3Ca3Cu4O16+x (4334) and Bi2Sr2Ca2Cu3O10+x (2223) phases obtained have properties similar to those of their ceramic‐processed counterparts, and there is evidence for accelerated kinetics for the 2223 phase formation. We have also studied effects of additions and substitutions in the Bi‐Sr‐Ca‐Cu‐O system prepared by this method, where passage through the liquid state can ensure good chemical homogeneity for multicomponent alloys. By this means, the substitution limit of bismuth by lead in the 2223 phase is estimated to be about 30 at.%, and there is evidence that lead substitution both accelerates the formation of the 2223 phase and sharpens the superconducting transition. Finally, silver addition to Bi4Sr3Ca3Cu4 alloy leads to the production of flexible metallic ribbons, which, after oxidation, form Ag/4334 microcomposites. These superconducting ribbons remain relatively ductile because of the formation of a layered microstructure in which superconducting regions alternate with silver bands aligned along the ribbon direction.