Patent GB-1470733-A (GB)
Title: Manufacture of superconductors
IPC: C22F1/00
Filed: 1975-05-16
Abstract: 1470733 Making superconductors SIEMENS AG 16 May 1975 [16 May 1974] 21021/75 Heading B3A [Also in Division H1] In making a superconductor conductor, a conductor structure comprising a first conductor component containing one element but not all elements of a superconductive compound, bound to a second conductor component containing a ductile carrier metal, e.g. copper or silver or an alloy thereof, is subjected to a process including a chemical transport reaction (i.e. a solid or liquid is transported by conversion to a gaseous compound by reaction with a gas, the gaseous compound being transported) whereby the second component is supplied with the or each remaining element of the superconductive compound, which element diffuses through the carrier metal to form the superconductive compound in the first component. In a first example, niobium rod is pushed into a copper tube, and drawn to a long wire without intermediate annealing. Nineteen pieces of this wire are formed into a bundle, pushed into a copper tube, and cold-drawn. A piece 11 of this wire is wound on to a ceramic tube 12, and placed with a supply of tin into a quartz ampoule 14. The ampoule is then evacuated, filled with HCl at 0À05 Torr at room temperature, sealed by fusion, and heated for 16 hours at 750 C., whereby tin is transported as chloride to form a copper-tin alloy in the conductor sheath, the tin diffusing through the copper to form Nb 3 Sn at the Nb core faces. Further Nb 3 Sn is formed upon heating for several hours at 750 C. in argon. The quantity of HCl is preferably such as not to transport tin faster than the diffusion rate, to avoid the formation of brittle tin-rich alloy. Even distribution of tin deposition on the conductor may be achieved by a fan in the ampoule, or by having a tubular tin source round the tube 12. The source may be a tin-coated tube, e.g. tin vapour-coated on to porous ceramic, or a vacuum-tinned steel wire mesh tube, or a tin-rich alloy tube. Alternatively, the tube 12 may be rotated over a quartz boat of tin. Excesses of the transported element in the conductor matrix may be removed by cooling part of the ampoule to set up a chemical transport reaction along the temperature gradient. In a second example, steam under a pressure of 20 Torr at room temperature was used to transport tin. In a third example, a 61-core conductor of vanadium in a copper-gallium alloy matrix (18 atom per cent Ga) was made by drawing with intermediate annealing, and placed below a compartmented boat containing Ga in a quartz ampoule together with some CuI. The ampoule is filled with argon at 1 Torr, sealed, and heated for 49 hours at 660 C., to form V 3 Ga at the core faces. In a modification, enrichment of the Cu-Ga matrix may take place at 600 C., followed by formation of V 3 Ga at 660 C. The source may be an alloy richer in Ga than the matrix. In a fourth example, a Nb core in Cu matrix strip is passed through a tubular oven containing a tin supply, and HCl in an argon carrier is fed into the oven to effect the chemical transport reaction, the gas escaping through an extraction passage or through the strip entry and exit passages. Generally, the source and matrix may be at the same or different temperatures. A superconductive plate or cylinder may be made, e.g. by coating a V plate or cylinder with Cu on one face, and transporting Ga by I, V 3 Ga forming at the interface, but not at the copper-free face. The element in the first component should have a higher melting point than the transported element. The first component may contain additives, e.g. to Nb or V, there may be added Ti, Zr, or Ta up to 30% by weight, or Hf. An Nb-V alloy may be used.