Superconductive current lead

Abstract: A material representing a low thermal conductivity at a temperature lower than a room temperature is used for a basic member composing a superconductive current lead. The basic member is combined with an oxide system superconductive member. The basic member decreases the heat transfer from the superconductive current lead to an extreme low temperature side.

Description: This invention relates to a superconductive current lead, and more particularly to a superconductive current lead for connecting a superconductive machine placed in an atmosphere at an extreme low temperature to a power supply, etc. placed in an atmosphere at room temperature. A current lead which is used for a superconductive machine is designed to have a cross-section, such that the sum of heat generated by the current flow through the current lead and heat transferred from a high temperature portion to the current lead becomes minimum, and an evaporation amount of coolant such as helium, etc. becomes also minimum. In conventional cases, a current lead is composed of copper wires. Under this circumstance, a current lead using oxide system superconductive wires each covered with an Ag layer has been studied for the same purpose. A current lead composed of an Ag-layered oxide system superconductive wires, however, has the disadvantage in that the amount of heat transferred is large, because the thermal conductivity of Ag is large, at an extreme low temperature region, which is an operating region of a superconductive machine, as compared to other metals, although this results in an advantage in that the stability of the superconductive wires is increased. In this current lead, there is a further disadvantage in that Joule heat generated by eddy currents and coupling currents of the Ag layers is not negligible, because the electric resistance of the Ag layers becomes very small at an extreme low temperature, when AC current or transient current flows through the current lead. JP-A-4155715 and US-A-5975285 disclose a superconductive current lead comprising one or more superconductive wires, the or each wire including a first member of an oxide system superconductive material and a second member of an Ag-Au alloy. According to the present invention, such a lead is characterised in that the alloy is an Ag-Au alloy including Au of less than 15 atomic %, the alloy having a thermal conductivity at a temperature lower than room temperature which is lower than its thermal conductivity at room temperature. In the alloy used, lowering the thermal conductivity is associated with raising the electric resistance, so that a decrease in eddy current loss or coupling current loss is realized, when AC current or transient current flows through the superconductive current lead. In the invention, an oxide system superconductive material may be a material selected from, for instance, Y-Ba-Cu-O, Bi-Sr-Cu-O, Bi-Pb-Sr-Ca-Cu-O, Tl-Ba-Cu-O, Tl-Ba-Ca-Cu-O, Tl-Sr-Ca-Cu-O, La-Sr-Cu-O, La-Ba-Cu-O. A barrier layer of precious metal may be interposed between the oxide system superconductive material and the lower thermal conductivity alloy. The content of Au in the alloy is preferably 1 to 10%, by atomic ratio. A predetermined number of superconductive wires may be one or more superconductive wires. In the accompanying drawings: Fig.1 is an explanatory diagram showing a superconductive current lead connecting a superconductive machine to a power supply ; Fig.2 is a perspective view showing a superconductive current lead of a first preferred embodiment according to the invention ; and Fig.3 is a perspective view showing a superconductive current lead of a second preferred embodiment according to the invention. Fig. 1 shows a superconductive current lead 4 connected to a superconductive coil (machine) 3 and a current lead 5 by connectors 41 and 42, and a power supply 6 provided to apply a predetermined voltage across the superconductive coil 3 via the current lead 5 and the superconductive current lead 4, wherein the superconductive coil and current lead 3 and 4 are immersed in liquid helium 2 contained in a container 1. Fig.2 shows a superconductive current lead 4 of a preferred embodiment according to the invention which is used in the apparatus as shown in Fig.1. The superconductive current lead 4 comprises a plurality of tape-shaped wires 7 each comprising a core 8 of an oxide system superconductive material and an alloy covering layer 9. In an example of the first preferred embodiment, the core 8 is of an oxide system superconductive material (Bi-Sr-Ca-Cu-O) Bi-2212 system as a main component, and the alloy covering layer 9 is of Ag-3 atomic % Au alloy, wherein each superconducting tape is 2.4 mm in width, 1.4 mm in thickness, and 3.3 mm 2 in cross-sectional area. A critical current of this superconductive current lead 4 immersed in the liquid helium 2 at a temperature of 4.2K is 10 5 A/cm 2 which is the same value as a value obtained in case where pure Ag is used for a covering material. At the same time, a resistivity which is as high as 0.7 to 1.2 µ Ω ·cm is obtained to represent a significant change as compared to the case where pure Ag is used at an extreme low temperature region for a covering material. In relation to the resistivity of the alloy covering layer 9, a thermal conductivity of the superconductive current lead 4 is smaller than that of common current lead of phosphorus-deoxidized copper. Consequently, the superconductive current lead 4 can be used with an eddy current loss of one percent as compared to that in the case where pure Ag is used for a covering layer. In regard to electric resistance of the connectors 42 and 41, the resistance value is not only decreased, but also stabilized upon the elapse of time. The thermal conductivity (W/m · k) of the alloy covering layer 9 is shown below in a table relative to a temperature along with that of a pure Ag covering layer. TEMPERATURE (K) EXAMPLE PURE Ag 260 160 590 77 51 530 20 10 25,000 Fig. 3 shows a superconductive current lead of a second preferred embodiment according to the invention. The superconductive current lead comprises a basic member 10 of Ag-Au alloy, a thin layer 11 of Ag and an oxide system superconductive layer 12. As described above, a material representing a lower thermal conductivity at a temperature lower than a room temperature is used for a basic member in the invention, so that a heat transferring through the basic member to coolant is not only decreased, but a heat of the lead generated by the flow of current is also decreased, thereby decreasing the consumption of the coolant and a load of a refrigerator. A plurality of leads shown in Fig. 3 can be stacked in a similar way to the wires shown in Fig. 2.

Claims: A superconductive current lead comprising one or more superconductive wires (7), the or each wire including a first member (8) of an oxide system superconductive material and a second member of an Ag-Au alloy characterised in that the alloy is an Ag-Au alloy including less than 15 atomic % Au, the alloy having a thermal conductivity at a temperature lower than room temperature which is lower than its thermal conductivity at room temperature. A superconductive current lead according to claim 1, wherein the first member is covered with the second member to provide the superconductive wire; and the predetermined number of superconductive wires are stacked. A superconductive current lead according to claim 1, wherein the first member is layered on one surface of the second member to provide the superconductive wire; and the predetermined number of superconductive wires are stacked. A superconductive current lead according to any one of the preceding claims, wherein the superconductive wire is a tape-shaped wire. A superconductive current lead according to any one of the preceding claims, wherein the alloy includes 1 to 10 atomic % Au.

Inventors: SASAOKA TAKAAKI, SASAOKA, TAKAAKI
Applicants: HITACHI CABLE, HITACHI CABLE, LTD.
Application year: 1993
Main classification: H01B12
IPC codes: H01R4