MICROSTRUCTURAL DEVELOPMENT AND CRITICAL CURRENTS IN LONG-LENGTH (TL, BI)-(SR, BA)-CA-CU-O WIRES

Synthesis of long Tl-Ba-Ca-Cu-O wires has been achieved by in situ synthesis techniques. Substitutions of Bi for Tl and of Sr for Ba were found to greatly enhance formation of the superconducting phase. Detailed microstructural characterization was performed on wires processed under various conditions in order to elucidate the reaction mechanisms. In addition, microstructural studies were carried out to investigate the beneficial effects of uniaxial compression on critical current density. It was found that the in situ reaction follows a ''natural'' two-powder process with early formation of Tl-1212. This reaction pathway provides a mechanism for sintering and crack healing and protects existing Tl-1223 for regrowth. Intermediate heat treatment allows more complete advantage to be taken of the above processes, while subsequent pressing increases the density and promotes grain alignment with minimum damage. Further heat treatment sinters grains and allows completion of the transformation to Tl-1223. Elements of this process have been incorporated in a sandwich-type geometry using plate-like grains to promote additional grain alignment.