Effects of silver additions on resistance to thermal shock and delayed failure of YBa₂Cu₃O_7−δ superconductors

The effect of silver (Ag) addition on the resistance to thermal shock and delayed failure of YBa₂Cu₃O_7−δ superconducting wires has been evaluated. Resistance to thermal shock was evaluated by measuring the critical-current density (J_c) as a function of the number of thermal cycles for YBCO and its composites with 15 vol. % Ag additions. Composite YBCO-Ag wires show a slower decrease in J_c than does monolithic YBCO as the number of thermal cycles increases. The nature of delayed failure was studied by measuring strength (σ_f) as a function of loading rate (σ) and evaluating the value of the subcritical crack growth parameter N[σ_f = Aσ^1/(N+1)]. The value of the N parameter was observed to increase slightly from 35 for YBCO to 41 for YBCO-Ag composites. The improved resistance to thermal shock and subcritical crack growth is believed to be due to improvements in mechanical properties (strength and fracture toughness) and thermal conductivity of YBCO as a result of Ag addition.