FURTHER EVIDENCE THAT THE CRITICAL-CURRENT DENSITY OF (BI, PB)(2)SR2CA2CU3OX SILVER-SHEATHED TAPES IS CONTROLLED BY RESIDUAL LAYERS OF (BI, PB)(2)SR2CACU2OY AT (001) TWIST BOUNDARIES

Field-dependent AC susceptibility measurements are correlated with the transport critical current density (J(ct)) and microstructure for some 20 Ag-clad (Bi, Pb)(2)Sr(2)Ca(2)CU(3)Ox (2223) tapes having J(ct)(0 T, 77 K) values ranging from 0-20 000 A/cm(2). Fields of 0.1-1 mT induced a double transition in the real component of the susceptibility of all samples. The field dependence of the lower temperature transition (T'(H)) exhibited a marked kink at 80 K and was always very sensitive to the applied field above 80 K, indicating an electromagnetically granular behavior. High-resolution transmission electron microscopy often revealed (Bi, Pb)(2)Sr2CaCu2Ox (2212 phase) intergrowths at the numerous (001) twist grain boundaries. Samples with a high J(ct)(0 T, 77 K) had only occasional half layers of 2212 at the (001) boundaries and required correspondingly higher fields to induce a kink in the T'(H) characteristic. The universal appearance of a kink at 80 K in all samples and the strong correlation of the field required to produce the kink to the zero-field, 77 K transport critical current density strongly suggests that c-axis transport across (001) twist boundaries within grain colonies controls the critical current density, even in relatively high J(c)BSCCO-2223 tapes.