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-29at.%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 alpha and/or epsilon -phase Cu-Sn, It does not seem possible, however, to homogeneously mix the Cu and Sn prior to A15 formation. The T,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 Deltam). 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-gn conductors can be considerably simplified, Our studies suggest a new Cu-Nb-Sn intermetallic phase with a composition of Cu-23at, %Nb-62at. %Sn.