THE DEGRADATION OF YBA2CU3O7 RESULTING FROM EXPOSURE TO WET AND DRY STEAM

The severity of degradation of YBa2Cu3O7-x from exposure to an aqueous phase has the following dependence: wet steam (condensed thin film) > bulk fluid phase >> dry steam, other variables being constant. Also, the surface reactivity increases with oxygen deficiency. It is found that the early stages of degradation are associated with surface hydroxylation and preferential extraction of Ba. Subsequently, dissolution takes place, followed by solution saturation and sequential precipitation, nucleation, and growth of secondary phases. These observations form the basis of a model which incorporates surface hydroxylation and OH- penetration of the bulk, formation of planar defects and extraction of Ba, lattice dissolution, and mobility and solubility constraints.