Imaging of transport currents in superconducting (Bi, Pb)₂Sr₂Ca₂Cu₃O_x composites

Abstract

IMPROVEMENTS in the current carrying capacity of high- T_c superconducting composite conductors will come from a detailed understanding of the connection between current flow and microstructure. Slicing experiments^1–3 on silver-sheathed mono-filamentary (Bi, Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223), combined with mic-rostructural studies^4,5, have suggested that current flow 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 multifllamentary composite at close to real operating conditions. Current densities of up to 8 x 10⁴ A cm^-2 at 77 K in self-field are observed in well aligned Bi-2223 grain colonies of 2–3 μm 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.