Method of making Nb.sub.3 Sn composite wires and cables

#The present invention relates generally to superconducting wires and cables and is more particularly concerned with a novel and economical method for making Nb.sub.3 Sn composite wires and cables.

#The combination in Nb.sub.3 Sn of brittleness and superior superconducting properties has stimulated a number of prior art attempts to devise methods enabling fabrication of various forms, particularly wires and cables, useful in the construction of devices such as high-field magnets, motors and generators. One such method is an external diffusion process which involves drawing niobium in a pure copper matrix to provide a multifilament niobium wire, plating tin on the copper surface of the wire and diffusing the tin into the copper matrix for reaction with the niobium. After coiling or otherwise forming the wire to preferred shape, the wire is heated to react the tin with the niobium and produce the superconducting Nb.sub.3 Sn phase in the wire. While the brittleness problem is thereby avoided during the forming or shaping stage, superconducting properties are degraded to the extent that the tin tends to form droplets (tin spots) on the wire surface during the diffusion-heating step. These tin spots remain as flaws and mechanically weak spots in the ultimate superconducting product.

#Tin spots can be avoided in this process by applying the tin in incremental amounts in a number of separate plating steps with intermediate anneals to homogenize the materials. This procedure, however, is time consuming and expensive to carry out.

#Niobium-copper composite filaments of about 120-mil diameter and about 90-mil niobium core diameter were prepared by co-drawing niobium rods in oxygen-free, high-conductivity, commercial grade (OFHC) copper tubes. Three hundred of these filaments were loaded into a OFHC copper extrusion can of 21/8 inch outside diameter and the assembly was then extruded to produce a 1/2-inch rod which was then cold drawn to about 10-mil diameter wire size. The wire was cleaned in commercial copper bright dip solution and water-rinsed and then provided with a five-micron coating of tin which was electroplated from an aqueous acid-tin bath at a current density of five amperes per square decimeter. The bath was maintained at about 25.degree. C during the plating operation and the bath contained 100 grams per liter stannous sulfate, 100 grams per liter sulfuric acid, two grams per liter gelatin and one gram per liter .beta.-naphthol.

#A 1/2-micron nickel overcoat was electroplated on the tin-coated copper-niobium composite wire from a Watts nickel bath containing 300 grams per liter nickel sulfate (NiSO.sub.4 .7H.sub.2 O), 50 grams per liter nickel chloride (NiCl.sub.2.6H.sub.2 O) and 35 grams per liter boric acid (H.sub.3 BO.sub.3). The bath was maintained at about 50.degree. C during the plating operation, which was carried out at a current density of 5 amps/dm.sup.2.

#The resulting wire was then assembled with five other composite wires made as just described plus a 10-mil diameter stainless steel core wire to provide a seven-strand cable of about 0.015 inch outside diameter. This cable proved to be bendable around a 1/2-inch diameter without degradation. The cable was heat treated in two stages to diffuse the tin into the copper matrix of each of the wires and then to react the substantially homogenized tin with the niobium of the filaments within the wires. The homogenizing heating step was conducted at about 580.degree. C in four hours and the Nb.sub.3 Sn reaction step was accomplished in 50 hours at about 750.degree. C. These heat treatments were carried out in an argon atmosphere (-70.degree. F dewpoint).