IMAGING OF TRANSPORT CURRENTS IN SUPERCONDUCTING (BI,PB)(2)SR2CA2CU3OX COMPOSITES

IMPROVEMENTS in the current carrying capacity of high-T-c superconduct ing composite conductors will come from a detailed understanding of th e connection between current how and microstructure. Slicing experimen ts(1-3) on silver-sheathed monofilamentary (Bi, Pb)(2)Sr2Ca2Cu3Ox (Bi- 2223), combined with microstructural studies(4,5), have suggested that current dow is enhanced in well textured Bi-2223 layers at the superc onductor/silver interface, but the spatial resolution of the slicing e xperiments is insufficient to identify the actual current path, More r ecently, magneto-optical imaging in an applied field(6-10) has been us ed to measure spatial variations in the shielding critical current den sity on a micrometre scale in Bi-2212 and Bi-2223 mono- and multifilam entary composites, Here we use an extension of this technique to measu re spatial variations in transport critical current density; J(c), in a Bi-2223 multifilamentary composite at close to real operating condit ions, Current densities of up to 8 x 10(4) a cm(-2) at 77 K in self-fi eld are observed in well aligned Bi-2223 grain colonies of 2-3 mu m wi dth. The superconductor/silver interface does not, in general, constit ute a continuous high-current path because of frequent interruptions b y second-phase particles; elimination of these particles in the interf ace layer could result in a threefold increase in J(c).