Effects of heat treatment and doping with Zr on the superconducting critical current densities of multifilamentary Nb3Sn wires

Abstract

Various wire samples consisting of 19-Nb3Sn filaments in a normal metal matrix were made by heat treating composite wires of 19-Nb, Nb-0.5 wt% Zr, Nb-0.75 wt% Zr, and Nb-2.5 wt% Zr filaments embedded in a Cu-10 wt% Sn matrix. The Nb-0.75 wt% Zr rod contained the precipitate ZrO2, while the Zr in the remaining Nb rods was in solid solution. The over-all size of the composite was 2.5×10−2 cm and each filament was approximately 2.5×10−3 cm in diameter. The critical current density Jc at 4.2 °K was measured at 40 and 100 kG for a variety of heat treatments. It was found that the critical current density was a strong function of the thickness of the Nb3Sn layers formed during heat treatment. This dependence of Jc on the thickness d could be approximately represented by the expression Jc(d)≃J0 exp (−nd), where J0 is the extrapolated zero thickness critical current density. J0 was found to be approximately 2.4×106 A/cm2 at 40 kG for the Nb3Sn formed from pure Nb, 1.2×106 A/cm2 for Nb doped with solid-solution additions of Zr, and 9.5×105 A/cm2 for that formed from Nb doped with ZrO2. The exponent n falls in the range 0.52–0.67 (μm)−1 for pure Nb3Sn depending on heat treatment temperature, 0.154–0.287 (μm)−1 for Nb doped with solid-solution additions of Zr, and only 0.11 (μm)−1 for the Nb3Sn composite wire containing ZrO2. The magnetic field dependence of Jc for some of the composites was also measured to 150 kG.