Dependence of critical current density on microstructures in superconducting Bi(Pb)-Sr-Ca-Cu-O wires

The dependence of the critical current density J c on microstructures in superconducting Ag-sheathed Bi(Pb)-SrCa-Cu-O wires has been investigated. A mixture of the oxide powders with a composition of Bi 1.84 Pb 0.34 Sr 1.9 Ca 2.0 Cu 3.1 O x was calcined and the resulting powder was placed in silver tubes. The tubes were processed by drawing and rolling to give 0.3 nm thick tapes. The tapes were annealed at 825-843 degrees C for 300 h with intermediate pressing. Different annealing temperatures gave three types of sample with different microstructures. The critical current density of the samples depended on the kind of foreign phases, such as the Cu-free (Bi,Pb)-(Sr,Ca)-O phase or 2201 phase, as well as the orientation of the a-b plane. The weak link structure of the samples was an S-N-S junction regardless of the annealing conditions. The flux pinning force density F p followed the power law (1-(T/T c ) 2 ) m ; the exponent m was much larger than that for metal superconductors and was different in the foreign phases.