Superconducting Properties of the <I>in situ</I> Cu-22.5 mass%Nb-Sn Composites

Superconducting properties, Jc and Tc of in situ Cu-22.5 mass%Nb-7∼8 mass%Sn composite wires (φ0.24 mm) were measured to make clear the influence of the heat treatment condition and the Sn content on them. Structures were also observed by an optical microscopy, SEM and EPMA from a metallurgical point of view. The results obtained can be summerized as follows.(1) The overall Jc increased with increasing heat treatment time at 823 and 873 K, while it scarcely depended on time when heat treated at 923 K. The highest overall Jc at 9 T was 5×108 A/m2, and was obtained after the heat treatment at 873 K for 346 ks.(2) Tc of about 17 K was attained after heat treatments, but the value was mostly unaltered even if the heat treatment condition varied in the range of 823∼923 K for 86.4∼691 ks.(3) The EPMA observation showed that the electroplated Sn diffused homogenously to the center of wires after the heat treatment at 873 K for 346 ks. This was in accord with the results of Jc measurement. With the prolonged diffusion process most Sn reacted with Nb fibers to form the Nb3Sn compound and, consequently, there remained little Sn in the Cu matrix. The structural changes of fibers due to the formation of Nb3Sn after heat treatment were also detected by the SEM observation.