New high-field (Nb, Ta)3Sn superconductors prepared From Sn-Ta matrix

Structure and high-field performance of (Nb, Ta)₃Sn conductors fabricated by using ductile Sn-Ta matrix are reported. A mixed powder of Sn and Ta with or without a small amount of Cu addition was melted at 800°C in vacuum. The resulting Sn-Ta alloy was rolled into a sheet 100 μm in thickness. The Sn-Ta sheet was laminated with a Nb foil of the same thickness, and wound around a Nb-4at%Ta rod forming a jelly roll composite. The composite was encased in a Nb-4at%Ta tube, and then fabricated into a wire without intermediate annealing. The wire with no Cu addition to the Sn-Ta sheet and reacted at 900°C showed a non-Cu J_c of 100 A/mm² at 23 T and 4.2 K. The non-Cu J_c of the wire at 2.1 K and 25 T reached 100 A/mm², the upper critical field of the wire being 28.4 T. A small amount of Cu addition to the Sn-Ta sheet appreciably reduced the reaction temperature, and produced a non-Cu J_c of 100 A/mm₂ at 22 T and 4.2 K after the reaction at 800°C. Another process for fabricating multifilamentary-type (Nb, Ta)₃Sn conductors using ductile Sn-Ta/Nb composite is also being studied.