SURFACE STABILITY OF SUPERCONDUCTING OXIDES

One factor which may limit the scale of application of oxide superconductors is their intrinsic instability when in contact with air containing water vapour. All families of these materials are to some extent subject to surface degradation reactions to form insulating products. These reactions may be monitored conveniently by surface sensitive techniques such as photoemission. Here we show examples of the application of this technique to degradation reactions occurring in ceramic and single crystal material, and initial results from superconducting tapes. Systems studied include YBa2Cu3O7-x, Bi2Sr2CaCu2O8+x and BaPb1-xBixO3. For the latter, we show that the extent of degradation is sensitively dependent on the chemical composition, with metallic compositions being dramatically more unstable than non-metallic compositions. Amongst the degradation products formed from materials containing Ba, Sr or Ca is the alkaline earth carbonate. This is formed by reaction with CO2 in a reaction catalysed by water vapour. The carbonate is particularly stable in the case of Ba, and this may go some way towards explaining the rapid rates of degradation of YBa2Cu3O7-x and BaPb1-xBixO3, when compared with Bi2Sr2CaCu2O8+x.