EFFECT OF INTERGRAIN JUNCTIONS AND FLUX-PINNING ON TRANSPORT CRITICALCURRENTS IN YBA2CU3O7-DELTA GRANULAR SUPERCONDUCTORS

Studies of the dependence of the intergrain critical currents on tempe rature (60-90 K) and magnetic field (0-100 G) were performed for sampl es of YBa2Cu3O7-delta ceramics (YBCO) and of a 2-wt.% Ag-doped YBa2Cu3 O7-delta composite (YBCO-Ag). The measurements were done using ceramic rings in a persistent mode and a scanning cryogenic Hall probe. This technique allowed us to satisfy conditions not available in the studie s of critical currents before. A self-sustaining supercurrent (sensiti ve to any resistive dissipation) was used to detect the intergrain cri tical current I(c). Its magnitude and relaxation were measured using a contactless method (a Hall probe). The results of the measurements of I(c) on temperature close to the intergrain T(c) confirmed the prese nce of the superconductor-insulator-superconductor tunnel intergrain j unctions in YBCO and the superconductor-normal-metal-superconductor pr oximity junctions in YBCO-Ag. For YBCO, the I(c)(T)=const(1 - T/T(c) dependence is not affected by the applied magnetic field, however, for YBCO-Ag the I(c)(T)=const(1 - T/T(c))2 dependence is not preserved, suggesting a strong effect of magnetic flux creep on critical currents . The intergrain T(c) is about 5 K lower than the intragrain T(c) and about 4 K lower than the ''zero-resistance T(c)'' measured using a co nventional I-V technique. Dissipation of a persistent current measured in the YBCO-Ag ring provided strong evidence that the transport curre nt is controlled by an intergrain flux creep with the energy barrier p roportional to I(c). The results revealed that the intergrain critical current density J(cT) in YBCO-Ag has lower values than J(cT) of YBCO, and the observed higher total critical currents are due to a larger s urface area of grain-boundary conduction in this composite.