We report on studies of magnetic and transport properties, as well as on characterization of defects in the pure ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/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 ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ during the sintering process, except the enhancement of the intergrain critical current density ${\mathit{J}}_{\mathrm{CT}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/Ag(10 wt. %) composite. The activation energy for intergranular flux creep of 1.6 and ∼0.3 eV was found for the pure ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/Ag composites, respectively. The results did not show any relationship between ${\mathit{J}}_{\mathrm{CT}}$, 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 ${\mathit{J}}_{\mathrm{CT}}$. Defect characterization by transmission electron microscopy revealed that silver does not affect the structure of twin boundaries inside the grains of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$.

• Received 18 June 1990

DOI:https://doi.org/10.1103/PhysRevB.42.6181