Effect of silver solubility on microstructure and superconducting properties of YBa₂Cu_3-xAg_xO_7-δ superconductors

The effect of silver (Ag) solubility on microstructure and superconducting properties of YBa₂Cu_3-xAg_xO_7-δ (0≤x≤0.5) superconductors has been evaluated. The solubility limit of Ag for copper sites in YBCO at ambient conditions (room temperature, atmospheric pressure) was evaluated to be x≅0.06. Analysis revealed the Ag content (x) of YBCO grains to be lower than the nominal Ag content (x_n) used for fabrication, probably due to processing-related problems. Part of the Ag used for fabrication segregates on the YBCO grain boundaries. The segregated Ag particles are believed to pin YBCO grains, resulting in fine-grain microstructures. For low nominal Ag content, x_n<0.2, critical current density (J_c) increased from 98 to 160 A/cm² as a function of increasing Ag content. These improvements are believed to be due to the increased oxygen content and associated microstructural changes in the YBCO. For higher nominal Ag content, x_n≳0.2, however, J_c decreased rapidly with the increasing Ag content due to the precipitation of nonsuperconducting phases.