The influence of intermediate roll characteristics on the residual crack density and critical current density in multifilamentary (Bi, Pb)2Sr2Ca2Cu3Ox tapes

Abstract

We studied the influence of the intermediate rolling deformation on the residual crack network and critical current density in multifilamentary (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes. Although an intermediate rolling step between heat treatment is generally beneficial in raising the critical current density, it also has the negative effect of causing cracking. Cracking was evaluated both by magneto optical imaging and by ultrasonic fracture of extracted filaments. We found that the characteristic fragment size was sensitive to both roll diameter and reduction percentage, decreasing from ~210 µm for a 10% reduction to ~115 µm for a ~20% reduction with 152 mm diameter rolls. At a constant 10% reduction, the fragment size decreased from ~210 µm to ~140 µm when changing from 152 mm to 38 mm diameter rolls. We conclude that intermediate deformation is positive in so far as it decreases the filament porosity, but that it cannot reduce this too much without introducing more cracking than can be healed in subsequent heat treatment. The critical current density thus maximizes at an intermediate deformation that counterbalances these two oppositely directed influences on the filament connectivity.