Effect of carbon dioxide on the reaction of YBa₂Cu₂O_6.5+x with water vapour

The influence of carbon dioxide on the reaction between YBa₂Cu₃O_6.5+x powder and air saturated with water vapour was studied. Superconductor powder samples were exposed to air that was saturated with water vapour which (a) was free of CO₂, (b) was saturated with CO₂, and (c) contained ambient levels of CO₂. In another experiment, the effect of pure, dry CO₂ (1 atmosphere partial pressure) was studied. The time variations of sample weight, oxygen content and X-ray diffraction (XRD) spectrum were determined. In water-saturated air, the rate of decrease of the oxygen content in YBa₂Cu₃O_6.5+x decreased with increasing partial pressure of CO₂. XRD was used to identify the reaction products. Higher partial pressures of CO₂ favour the formation of a barrier layer (BaCO₃) which inhibits the reaction of the underlying YBa₂Cu₃O_6.5+x with water vapour. Very little decrease in oxygen content occurred on exposure of YBa₂Cu₃O_6.5+x to pure, dry CO₂.