Critical current density in superconducting Bi-Pb-Sr-Ca-Cu-O wires and coils

The critical current density (J c ) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 11900 A cm -2 at 77 K in a zero field. A coil of diameter 35 mm made from the wire had a J c of 1913 A cm -2 at 77 K over the full length (one metre) of the coil. The high J c is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity through optimal heat treatment, crystal alignment through cold rolling, and enhancement of flux pinning through chemical doping and processing of high-purity powder by freeze drying. The critical current followed a power law of (1-T/T c ) 3/2 in the temperature range near T c , indicating that the weak link structure acts as a proximity effect junction.