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 som e 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 indu ced 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 gr anular behavior. High-resolution transmission electron microscopy ofte n revealed (Bi, Pb)(2)Sr2CaCu2Ox (2212 phase) intergrowths at the nume rous (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) c haracteristic. The universal appearance of a kink at 80 K in all sampl es and the strong correlation of the field required to produce the kin k to the zero-field, 77 K transport critical current density strongly suggests that c-axis transport across (001) twist boundaries within gr ain colonies controls the critical current density, even in relatively high J(c)BSCCO-2223 tapes.