The influence of the starting Cu-Sn phase on the superconducting properties of subsequently reacted internal-Sn Nb3Sn conductors

We have studied the reaction conditions and properties of a high-Sn, internal-Sn Nb₃Sn conductor whose overall Cu-Sn composition was Cu-29 at.%Sn. By adjusting the mixing heat treatment during which the Cu and Sn interdiffuse prior to A15 reaction, it is possible to substantially surround the Nb filaments with α and/or ε-phase Cu-Sn. It does not seem possible, however, to homogeneously mix the Cu and Sn prior to A15 formation. The T_c distributions, measured both by SQUID and heat capacity, showed no dependence on significantly different mixing heat treatments. Magnetization measurements at 4.2 K and 12 K also showed that there were no significant differences in the H^*, H_c2 and J _c (as indicated by Δm). This implies that the composition and reaction behavior of the A15 phase is independent of the Cu-Sn phase in contact with the Nb filament prior to the reaction heat treatment and that scoping experiments on high-Sn, high-J_c internal-Sn conductors can be considerably simplified. Our studies suggest a new Cu-Nb-Sn intermetallic phase with a composition of Cu-23 at.%Nb-62at.%Sn