Actual fluorination of Y-Ba-Cu-O high-Tc superconductors by a solid-gas reaction under NF3 flow

Abstract

Sintered samples of YBa2Cu3Ox with x approximately=6, 6.7 and 7 have been reacted with diluted NF3 gas at various temperatures around 300 degrees C. Evidence for fluorine insertion in the material is given by weight increase and by concomitant evolution of the X-ray patterns and superconducting behaviour. The kinetics of the fluorine insertion clearly gives evidence for a direct correlation between the amount of inserted fluorine and the oxygen stoichiometry in the starting samples, namely the vacancy concentration. The fluorination effect on the superconducting properties has been studied by AC and DC susceptibility; after fluorination, for the compound in which x approximately=7, no modification of the Tc onset is observed, but the Meissner effect amplitude decreases in relation to the granularity effect. For the compound in which x approximately=6.7, fluorination increases the Tc onset up to 91 K. Further annealings of the fluorinated samples have been performed under O2, N2 and dynamic vacuum, and their influence on the superconducting behaviour is discussed.