Abstract
Effects of annealings under various atmospheres on the superconductivity of BiSrCaCuO materials are presented. Samples of nominal compositions 1112, 2212, and 2202 were annealed under vacuum or nitrogen gas, then reacted under oxygen or NF3 gas flows. Fully reversible and opposite effects on Tc are observed when the 110-K and 85-K phases are annealed under the same conditions. In particular in the 110-K phase, a decrease of Tc is obtained under reducing conditions, and then restored by short annealings under oxygen flow at low temperatures. Similar trends are reported by reaction under diluted NF3. Negligible weight variations were observed, suggesting that only a very small amount of oxygen (or fluorine) might be eventually inserted, but the main mechanism must be related to internal conversion between oxidation states of both bismuth and copper, controled by thermodynamical conditions.
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