Influence of additions and radiation damage on the superconducting properties of sintered YBa₂Cu₃O_{7 - gd}

Using the high-resolution Faraday (HRF) technique, domain patterns of sintered YBa₂Cu₃O_{7 - δ} samples containing additions of Ag₂O and Y₂BaCuO₅ are obtained. Similar observations are carried out on sintered YBa₂Cu₃O_7-δ samples with reduced oxygen content and on electron-irradiated samples. The penetration of flux into sintered high-T_c superconductors is found to be totally different from single crystalline materials (single crystals and epitaxial thin films) as the Abrikosov vortices penetrate the sample not as a whole thus forming a so-called flux front. Only a flux penetration into single superconducting grains is observed. From the measured flux density profiles, the intragrain critical current densities are determined locally. It is found that the chemical impurity phases do not affect the intragranular flux density distribution and the intragranular critical current density, but the decomposition of the Ag₂O leads to an improved oxygen stoichiometry in these samples. Electron irradiation is found to enhance the intragrain critical current densities, whereas the oxygen reduction leads to reduced intragrain currents. To determine the influence of the additions also on the intergrain critical current densities, standard four-point transport current measurements are carried out on the same samples. It is found that a maximum enhancement of the critical current density is obtained by the addition of 10 wt% silver oxide; however, this critical current density is drastically reduced by applying small external magnetic fields. The lower critical field of the matrix and the corresponding Josephson penetration depth are determined from transport current measurements. The Ag₂O additions and the green phase particles are found to separate the superconducting YBa₂Cu₃O_7-δ grains. The resulting larger widths of the barriers between the superconducting grains are found to cause a stronger influence of external magnetic fields on the intergrain critical current densities.