GB1285442A - Improvements in or relating to electrical conductors using superconducting material - Google Patents

Abstract

1285442 Superconductors; cables CENTRAL ELECTRICITY GENERATING BOARD 11 Aug 1969 6539/72 Divided out of 1285441 Headings H1A and H1K A superconductor comprises a laminar structure of thin continuous layers of a first type II superconductor having a transition temperature T CA separated by layers of a second superconductor having a lower transition temperature T CB so that its rate of change of critical current with temperature is positive or for the same critical current density less negative than that of the first superconductor at the operating temperature. The layers have a thickness between 0-05 and 1-0 microns. As shown, Fig. 1, an A.C. power cable comprises two coaxial tubes 21, 25 of Cu or Al coated with laminated layers 22, 26 of superconductors having different transition temperatures, on top of which are provided laminated layers 23, 27 of a type II superconductor and a dielectric or resistive material. In layers 23, 27 the superconductor may be Nb or a Nb alloy (e.g. Nb-Zr) and the spacing material may be stainless steel, a Cu-Ni alloy, a brass, silicon monoxide, or niobium oxide. The gap between the tubes may be evacuated or filled with high pressure helium, spacers are provided to support the tubes and the cable is surrounded with thermal insulation and cooled to a cryogenic temperature. Under fault conditions the laminated superconductor layers 22, 26 exhibit a rate of change of critical current with temperature which is positive or less negative than that of the higher critical temperature material due to flux pinning at the interfaces. This improves the magnetic field stability under fault conditions. The operating load current is carried by the laminated layers 23, 27 which exhibit a higher critical current density than the layers 22, 26. Layers 22 and 26 may be provided by alternately sputtering Nb in an atmosphere of N to produce a niobium nitride coating and an inert atmosphere to produce a Nb coating. Layers 23 and 27 may be produced by depositing alternately by sputtering coatings of niobium nitride and silicon monoxide. In a modification, Fig. 2, each of the laminated . load conductor layers is replaced by a single layer of load superconductor. The high stability layers (32, 36) are of laminated coatings of Nb and Nb-Ta and the load current layers (33, 37) are of Nb. The arrangement may be produced by stacking appropriate foils, wrapping round a mandrel, placing in a Cu can and warm extruding followed by cold drawing to the required size. A plurality of materials having different transition temperatures may be used for the lower transition temperature coatings. A multiple phase cable can be produced by mounting several coaxial conductor pairs in a common thermal enclosure or by forming a multiple coaxial structure in which the superconductor layers are formed on both the insides and the outsides of some of the support tubes.