Enhancement of critical currents in bulk and Ag-sheathed Bi(Pb)-2223 superconductors

The superconducting properties of the Bi(Pb)-2223 phase have been optimized. Prolonged annealing of coarse powder raised the j/sub c/ in bulk samples to as high as 1100 A-cm/sup -2/ at 77 K. The temperature dependence between 96 and 4.2 K was studied, where j/sub c/ rose gradually from 120 to 2100 A-cm/sup -2/. T/sub c/ dropped from 107 K to 64 K in magnetic fields up to 4 T, but decreased by only 1 K/T in higher fields. A tape prepared by rolling and subsequent annealing of Ag tubes containing Bi(Pb)2223 powder exhibited a j/sub c/ of 1700 A-cm/sup -2/ at 77 K and 11000 A-cm/sup -2/ at 4.2 K. The latter j/sub c/ dropped to 3000 A-cm/sup -2/ in a magnetic field of 0.25 T, but it was still 1200 A-cm/sup -2/ in a 10-T field. A tape fabricated by pressing a drawn and annealed wire proved to be less sensitive to low magnetic fields.