Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

The dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties (Tsub(c2) = 12.0 K and Jsub(c) = 130 A cm-2) in a Cu-0.30 at % Nb-0.15 at % Sn alloy were obtained when the sample was aged at 5500C for 384h. This sample exhibited a structure of fine Nb3Sn precipitates of 200 to 500 A diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb3Sn particles. In spite of the similar structure obtained by ageing at 8000C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloys and this would be explained qualitatively by the difference in the mean free path in the two alloys. (author)