Critical current and upper critical field of multifilament Nb3−xTaxSn superconductor

Superconductive wires containing 78 filaments of Nb3−xTaxSn for 4–16 at. % Ta were produced using the external bronze process to yield a composite with an overall composition 21.4 wt. % Nb–Ta, 64.9 wt. % Cu, and 13.6 wt. % Sn. Reaction periods of 20 to 260 h at temperatures from 650 to 735 °C were used. Critical-current densities over the entire wire cross section were 5×104 A/cm2 at 14 T and 1×104 at 19.7 T, the measured Hc2 was 27 T at 4.2 K for wires with 8 μm filaments containing approximately 4 at. % Ta. Single filament wires using a 13 wt. % bronze matrix were fabricated to determine the effect of Ta on reaction kinetics. The A15 layer thickness was approximately proportional to the square root of the reaction time and independent of tantalum concentration. The critical-current densities are discussed in terms of A15 grain structure, stoichiometry, filament uniformity, and Hc2 enhancement.