Analysis for formation of current path in the superconducting joint between Nb-Ti wires with the solder matrix replacement method

Abstract

We investigated a mechanism for formation of a current path in a superconducting joint between Nb-Ti wires with the solder matrix replacement method, where Pb-Bi connects Nb-Ti filaments. Superconducting joints were formed in the following steps. Firstly, the ends of wires were soaked in molten tin. Secondly, the ends of wires were soaked in molten Pb-Bi. Thirdly, the two ends of wires were set into the container filled with molten Pb-Bi and the Pb-Bi was solidified. One factor in the current path is the interface between Nb-Ti and Pb-Bi, where there was an intermediate layer composed of Nb3Sn and Ti-S. The intermediate layer appeared to be formed in the first step; niobium in Nb-Ti reacted with tin, and leftover titanium reacted with sulfur which may be contained as an impurity in the copper matrix of the wires. Another factor is the microstructure of Pb-Bi, which consisted of a superconducting phase and a bismuth phase. The specific surface area of the bismuth particles, which was affected by the composition and solidification rate of Pb-Bi in the third step, was positively correlated with the critical current density of Pb-Bi. This is because the bismuth particles are dominant flux pinning centers, as shown by Campbell. The present superconducting joints had excellent properties; the resistances were lower than 10−13 Ω.