About the substitutions for CuO in YBCO high temperature superconductor

Abstract

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 Ag 2S 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 T c of 92 K and minimum Δ T c 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.