Critical current densities of the multifilamentary Nb3Sn superconductors produced by the solid-liquid diffusion method

The critical current and critical temperature of the multifilamentary Nb3Sn superconductors produced by the solid-liquid diffusion method were measured for various heat treatments, and were discussed in relation to the process of Nb3Sn formation. The critical current density Jc for the fine-grained Nb3Sn layer, which was formed at the interface of the intermediately formed Nb6Sn5 layer and the Nb matrix, was calculated to be 5--6 x 105 A/cm2 at 9.5 T and 4.2 K, which is a little higher than the best Jc value for the Nb3Sn layers of the bronze-processed conductors. The Jc value increased during the formation of the fine-grained Nb3Sn layers and reached its maximum value when the reaction process was nearly completed and thick Nb3Sn layers were formed, though Jc decreases during the growth of Nb3Sn layers in the bronze process. By this solid-liquid diffusion method, thick (approx.5--20 μm) and high Jc Nb3Sn layers are obtained, so that the overall Jc of the conductor (without a Cu stabilizer area) exceeds 1.5 x 105 A/cm2 at 9.5 T, and is twice as high as that of the bronze-processed conductors