Effect of Nb₃Sn Coarse Grains on Critical Current Densities of Internal Tin Nb₃Sn Strand

Nb₃Sn strands have significant applications in constructing high-field (>10 T) magnets and have great potential value in the next 20 years. Internal-tin (IT) Nb₃Sn strand has been developed by many methods for the future fusion reactor after ITER with low hysteresis loss. However, how to improve the current densities of Nb₃Sn strand has become very important with the increasing demand for high J_c Nb₃Sn strand. In our study, we found that more than double Nb content should be added within Internal Tin Nb₃Sn strand and 2 times critical current densities can be enhanced compared with ITER Nb₃Sn strand. With this Nb₃Sn strand, the phase transition during heat treatment was investigated and new phase (Cu-Nb-Sn) can be formed at 400 °C due to higher ratio of Sn/Cu and more Cu₆Sn₅ phase decomposition. The coarse Nb₃Sn grains were analyzed and a composite structure (Cu and Nb₃Sn) can be formed as Nb element diffusion during Cu-Nb-Sn phase formation. We also found that phase-balance (less Cu-Nb-Sn produced), which can control the formation of Nb₃Sn coarse during filaments, can be obtained through element ratio optimization. With this method, coarse grains among filaments can be refined from micron size into nano-size. Effect of varisized coarse grains on the critical current densities was also studied and we found that micron sized Nb₃Sn (1∼2 μm) grains reduced the critical current densities severely.