Through-process study of factors controlling the critical currentdensity of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract

The connectivity and flux pinning components of the critical currentdensity Jc in a high-Jc Ag-clad, 19-filament (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) composite throughout importantsteps in its thermomechanical treatment were investigated by mass densitymeasurement, microstructural observation and extensive superconductingproperty characterization. The mechanical integrity of the filaments wasgreatest after the first heat treatment, because damage produced byintermediate rolling was not completely erased by subsequent heat treatment.The final relative filament mass density was less than 75%, even though Jcmore than tripled to reach ~50 kA cm-2 (77 K, 0 T) afterfinal heat treatment. Filament resistance and Jc data showed that theelectrical connectivity improved significantly throughout the process,although ultrasonic fracturing, magneto-optical images and densitymeasurements all showed that the mechanical and physical connectivitydegraded. Our study suggests that the attainable Jc of Bi-2223 isdetermined by the electrical connectivity of each filament which is itself anuncertain compromise between minimizing the significant porosity produced bythe Bi-2223 formation reaction and the extensive crack network that porosityreduction by intermediate rolling produces.