LACK OF INFLUENCE OF THE Cu-Sn MIXING HEAT TREATMENTS ON THE SUPERCONDUCTING PROPERTIES OF TWO HIGH-Nb, INTERNAL-Sn Nb_3Sn CONDUCTORS

摘要

The heat treatment of internal-Sn, Nb_3Sn superconductors has two primary purposes - (1) to mix the Cu and Sn, and (2) to form the A15 phase. We have studied the effect of the Cu-Sn mixing heat treatments on the superconducting properties of two high-Nb internal-Sn conductors. The samples underwent continuous ramps of 6, 30 or 60℃/h from room temperature to the A15 reaction temperature, or used isothermal hold treatments to mix the Cu and Sn prior to A15 formation. The strands were then reacted for 180 hours at 650℃ to form the A15 phase. Inductive T_c measurements showed no significant difference arising from vastly different mixing steps. However, the T_c values were ～2 K below that of stoichiometric Nb_3Sn (18.3 K) due to a high Nb:Sn ratio and Sn leakage from the filament bundle. H~*, H_(c2) and J_c (as inferred from magnetization measurements) were also unaffected by the Cu-Sn mixing heat treatments used. Unlike lower Nb content, lower J_c internal-Sn conductors, we conclude that the superconducting properties of these internal-Sn composites are essentially independent of how the Cu and Sn are mixed, greatly simplifying heat treatment optimization.