FLUX MOTION, PROXIMITY EFFECT, AND CRITICAL CURRENT-DENSITY IN YBA2CU3O7-DELTA SILVER COMPOSITES

We report on studies of magnetic and transport properties, as well as on characterization of defects in the pure YBa2Cu3O7- and the YBa2Cu3O7-/Ag(10 and 30 wt. %) composites. The studies of magnetic properties include the diamagnetic shielding, the Meissner effect, the trapped field [for both zero-field-cooling (ZFC) and field-cooling (FC) cases], and their dependence on applied magnetic field, temperature, and time. High- and low-magnetic-field hysteresis loops were measured and the intragrain magnetic critical current density was calculated. The studies of transport properties include the resistivity and intergrain transport critical-current-density measurements. Distribution, spacing, and size of intragrain twin boundaries were investigated. The results show the degradation of superconducting properties if silver is added to YBa2Cu3O7- during the sintering process, except the enhancement of the intergrain critical current density JCT in YBa2Cu3O7-/Ag(10 wt. %) composite. The activation energy for intergranular flux creep of 1.6 and 0.3 eV was found for the pure YBa2Cu3O7- and the YBa2Cu3O7-/Ag composites, respectively. The results did not show any relationship between JCT, the activation energy, and the number of pinning centers (the trapped field) in these samples. It is suggested that the proximity junctions superconductor normal-metal superconductor built up by intergranular silver, and not flux pinning, are responsible for the increase of JCT. Defect characterization by transmission electron microscopy revealed that silver does not affect the structure of twin boundaries inside the grains of YBa2Cu3O7-. © 1990 The American Physical Society.