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

IMPROVEMENTS in the current carrying capacity of high-T-c superconducting composite conductors will come from a detailed understanding of the connection between current how and microstructure. Slicing experiments(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 superconductor/silver interface, but the spatial resolution of the slicing experiments is insufficient to identify the actual current path, More recently, magneto-optical imaging in an applied field(6-10) has been used to measure spatial variations in the shielding critical current density on a micrometre scale in Bi-2212 and Bi-2223 mono- and multifilamentary composites, Here we use an extension of this technique to measure spatial variations in transport critical current density; J(c), in a Bi-2223 multifilamentary composite at close to real operating conditions, Current densities of up to 8 x 10(4) a cm(-2) at 77 K in self-field are observed in well aligned Bi-2223 grain colonies of 2-3 mu m width. The superconductor/silver interface does not, in general, constitute a continuous high-current path because of frequent interruptions by second-phase particles; elimination of these particles in the interface layer could result in a threefold increase in J(c).