Isothermal melt processing of Bi-2212 tapes

Isothermal melt processing (IMP) has been used to produce phase-pure, high critical current density (J/sub c/) Bi-2212 tapes at temperatures as low as 780/spl deg/C, Bi-2212 tapes processed by IMP have yielded J/sub c/ values up to 250 kA/cm/sup 2/ (I/sub c/=345 A) at liquid helium temperatures. Small test coils have been produced with J/sub c/ values up to 150 kA/cm/sup 2/. The effects of the oxidation rate, processing temperature, and time spent in the partial melt on the superconducting properties and microstructure were investigated. Optimal superconducting properties result from the interplay of these parameters such that the grain size of the Bi-2212 phase in the polycrystalline core is allowed to coarsen and align itself with the silver sheath to form a well connected superconductor. Deviations from optimal conditions result in either too little coarsening to form a well aligned structure or instabilities in the coarsening process that lead to the formation of large secondary phases which disrupt connectivity.