EFFECT OF A CONTROLLED MELT PROCESS ON PHASE-TRANSFORMATION AND ELECTROMAGNETIC PROPERTIES OF BIPBSRCACUO/AG SUPERCONDUCTING WIRES

The melt process has been successfully used in fabricating Ag-sheathed YBa2Cu3Oy wires and (Bi,Pb)2Sr2CaCu2Oy (2212) wires. However, difficulties were encountered in applying the melt process to the (Bi,Pb)2Sr2Ca2Cu3Oy (2223) system, because the high T(c) (2223) phase decomposed during the melting and was difficult to recover by further heat treatment. In this paper, we present a high T(c) phase formation-decomposition-recovery process (PFDR) through the use of a short-period partial melt, with which the decomposed (2223) phase in the core of the wires can be recovered with subsequent annealing. The resultant tapes show a significant enhancement in critical current density (J(c)) and improved J(c)-magnetic field (H) dependence; in particular, a much slower drop of J(c) in the low magnetic field regime, indicating the weak links between grains has been greatly improved. A J(c) of 40,000 A/cm2 at 77 K and zero field, and 9000 A/cm2 at 77 K and 1 T has been achieved in the PFDR processed tapes. a.c. susceptibility measurements under a d.c. magnetic field reveal that the irreversibility lines shift to a higher temperature, suggesting an improvement of flux pinning in the PFDR processed tapes compared with the solid-state reaction (SSR) treated tapes. The high mass density, excellent grain alignment, large grain size and uniform distribution of fine impurity particles are considered to be responsible for the improvement of tapes' electromagnetic properties.