Optimization of Cu addition and RHQ-treatment time for Cu-added Nb3Al wire

In a Nb/Al-Cu precursor wire, Nb₃Al filaments are formed directly through a proper rapid heating and quenching (RHQ) treatment, and show the excellent superconducting properties at two different regions in the RHQ-treatment conditions. The region at heating energy density, W, of 4.4-4.6 J/mm³ is one of the best RHQ treatments to obtain the excellent superconducting properties, when the Cu content in Al-Cu alloy is 1.2 to 5.2at%. The other optimized region was observed at W of 5.2-5.6 J/mm³, which exists clearly for the 2.5 and the 5.2at%Cu added wires, but unclearly for the 1.2at%Cu added wire. The best improvement in J_c is obtained for 2.5at%Cu addition, such as J_c(4.2 K) values of 800 A/mm² in 20 T and 1200 A/mm² in 18 T, which are the highest values among the metallic multifilamentary superconductors. The best T_c of 18.34 K is obtained for the 2.5at%Cu added wire. The wires with 1.2at%Cu addition show intermediate J_c vs. B curves between those of the 2.5at%Cu added and non-Cu added wires.