ELECTROMAGNETIC GRANULARITY, CRITICAL CURRENT-DENSITY, AND LOW-TC PHASE FORMATION AT THE GRAIN-BOUNDARIES IN (BI,PB)2SR2CA2CU3OX SILVER-SHEATHED TAPES

Transport critical currents are correlated with DC magnetization, AC susceptibility and high resolution electron microscopy in progressively heat treated Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes. Both the zero field cooled magnetization shielding curves and the real component of the AC susceptibilities show two diamagnetic transitions. One transition is found to occur at 109 K and is the signature of the 2223 phase. A second, much larger transition occurs over a range of temperatures between 75 and 105 K. The transport critical current densities correlate directly with the lower magnetic transition temperature. High resolution electron microscopy reveals a few layers of Bi2Sr2CaCu2Ox (2212) phase clustered at the numerous (001) twist grain boundaries. These residual layers require a long reaction for their elimination and directly control the critical current while they are present.