Further investigations of the solid-liquid reaction and high-field critical current density in liquid-infiltrated Nb-Sn superconductors

Abstract

Superior superconducting properties, such as high J c 's and T c 's, have been obtained from reacted liquid-infiltrated Nb-Sn composite wires. These excellent properties are attributed to the chemistry and structure of the material, which is prepared by a unique solid (Nb) - liquid (Sn) reaction. From heat capacity measurements, sharp bulk superconducting transitions of the Al5 phase occur at 17.2-18 K and the weight fraction of Al5 in the composite wire is ~23%. Analytical electron microscopy techniques have shown that: the microstructure of these conductors consists of alternating large-grain and small-grain filaments; these two types of filaments correspond to BCC Nb(Sn) and cubic Al5 Nb_{75\pm{x}}Sn_{25\mp{x}} phases, respectively; the Al5 filaments ( \leq0.5 \mu m) are chemically homogeneous in terms of measured X-ray intensity ratios to within ±7%, which implies that x\sim1.5 ; and the Al5 grains are essentially free of extended lattice disorder down to a resolution of ∼0.34nm. Recent work in which Nb is alloyed with Ta has shown that these superconducting properties can be improved upon; e.g. high overall J c 's of \sim1.8 \times 10^{4} A/cm2at 20 T and 4.2 K have been measured. Also, the liquid-infiltrated Nb(Ta)-Sn composites have a damage strain tolerance nearly double that of commercial bronze-processed Nb-Sn conductors.