Superconducting properties of in situ--processed Nb3/Sn-Cu composites

Performance characteristics for a family of Nb3Sn-Cu superconducting composites have been studied to determine the optimum value of various fabrication variables such as the Cu-to-Nb ratio, Sn content, reaction temperature, reaction time, and dendrite size in the original casting. Basic superconducting properties such as the critical current and effective upper critical field are found to change in a very regular way as these parameters change and there are clear design tradeoffs which can be made depending on the wire specifications needed. A typical high-performance wire is made from a Cu70Nb30 casting with 2--8-μm dendrite size reacted with a stoichiometric amount of Sn at 550 0C for 6 days. In this study there has been a special emphasis on materials made from casting of a Cu--20 wt% Nb dendritic alloy. Critical currents of this alloy are smaller than for the 30% Nb samples, but the wire can withstand about 3% bending strain at the outer surface of the wire with only modest degradation of the J/sub c/ values