Interfrnualr Phases n Cyclically Annealed YBa2Cu3O7-x and their Implications for Critical Current Densities

by Clarke, Andrew Peter

We report changes in the intergranular material and grain morphology of YBa2Cu3O7-x during cyclic anneals between 780 and 980 ºC in oxygen at atmospheric pressure. Two endothermic reactions were detected: (a) the eutectic reaction of YBa2Cu3O7-x with CuO and BaCuO2 at 900 ºC (enthalpy ?Ha) and (b) the peritectic reaction of YBa2Cu3O7-x with CuO at 950 ºC (?Hb). During the first anneal, only reaction (b) is detected, and although it should only occur if there is an excess of CuO, its signature is present in all published data. Cyclic annealing causes a monotonic decrease in ?Hb and an increase in ?Ha, larger average grain sizes, and greater volume fraction of the superconducting phase. A steady state is reached after 10 cycles at which point ?Hb = 0. We propose a model that explains the origin of the intergranular CuO and the changes in the intergranular material composition with cyclic annealing.