About the substitutions for CuO in YBCO high temperature superconductor

Copper oxide is one of the critical ingredients for superconductivity in high temperature superconductors like YBCO. The bulk YBCO is a porous material with loosely packed grains separated by non-superconducting grain boundaries. Ag addition to YBCO is found to overcome this problem partially. Keeping this fact in view, samples with CuO substituted by Ag2S up to 50% were synthesised by solid state route to understand the role of CuO in this material. Ag is found to substitute at the Cu chain site, the maximum substitution level being 3%, however S is found to enter the YBCO matrix for all substitutions tried. Ag segregates near grain boundaries for doping between 3 and 30% and then starts forming globules. The sintered samples have less porous and more uniform microstructures with well developed grains. The superconducting and normal state properties exhibit a characteristic variation. The sample with 10% doping has a maximum Tc of 92 K and minimum ΔTc of 0.77 K. The temperature derivative of resistivity indicates the transition to the superconducting state to be a two step process. Ag segregated near grain extremities possibly form an Ag-O eutectic which enhances densification. The formation of impurity is explained in terms of the wetting reaction between segregated pure silver and bulk YBCO grains. (orig.)