Nb3Sn wires synthesized by rapid-heating/quenching process of rod-in-tube wire precursors

A rod-in-tube technique (RIT) for the fabrication of Nb₃Sn precursor wire has been developed. Through this process we expect to increase the volume fraction of A15 phase in the wire, since no bronze remains unlike commercial Nb₃Sn wires. However, it is very difficult to cold-work a Nb/Sn composite, because pure Sn is too soft compared with Nb, as well known. We found that the hardness of Sn increases with the addition of small amount of Bi, and resulting Nb/Sn-Bi composite wires with 50 m length could be fabricated by the RIT technique. The RHQ (rapid-heating/quenching) process was applied to these strands as the heat treatment technique. The Nb₃Sn precursor wire has a Ta sheath in order to bear the high temperature RHQ treatment. Although the Nb-Sn supersaturated bcc solid solution was not synthesized, unlike the case of Nb/Al micro composites, the A15 phases formed directly by the RHQ treatment. The maximum T_c of 18.3 K was obtained, which is slightly higher than that of the bronze-processed Nb₃Sn. T_c, J_c (4.2 K) and B_c2 were increased with decreasing peak heating temperature. The addition of Bi seems not to have negative influences on the superconducting properties of Nb₃Sn.