Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_<i>x</i>tapes

Abstract

The connectivity and flux pinning components of the critical current density Jc in a high-Jc Ag-clad, 19-filament (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though Jc more than tripled to reach ~50 kA cm-2 (77 K, 0 T) after final heat treatment. Filament resistance and Jc data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable Jc of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.