Isothermal melt processing of Bi-2212 tapes

Isothermal melt processing (IMP) has been used to produce phase-pure, high critical current density (J(c)) Bi-2212 tapes at temperatures as low as 780 degrees C. Bi-2212 tapes processed by IMP have yielded J(c) values up to 2 50 kA/cm(2) (I-c = 345 A) at liquid helium temperatures. Small test coils h ave been produced with J(c) values up to 150 kA/cm(2). The effects of the o xidation rate, processing temperature, and time spent in the partial melt o n the superconducting properties and microstructure were investigated. Opti mal superconducting properties result from the interplay of these parameter s such that the grain size of the Bi-2212 phase in the polycrytalline core is allowed to coarsen and align itself with the silver sheath to form a wel l-connected superconductor, Deviations from optimal conditions result in ei ther too little coarsening to form a well-aligned structure or instabilitie s in the coarsening process that lead to the formation of large secondary p hases which disrupt connectivity.