JPH0779045B2 - Oxide superconducting current lead - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconducting current lead used for supplying a large current to a magnet for generating a strong magnetic field using a superconducting coil.

[0002]

2. Description of the Related Art Superconducting materials are materials that exhibit characteristics such as zero resistance, perfect diamagnetism, and Josephson effect at a critical temperature Tc or lower. Metal-based superconducting materials have a low critical temperature of less than 20K, but liquid helium temperature (4.2
By applying a large current to the superconducting coil in (K), it is possible to generate a high magnetic field without loss. These are used in magnetic levitation trains, nuclear magnetic resonance diagnostic equipment, and the like.

The current lead supplies a current of several hundred to several thousand amperes from a room temperature power source to a cryogenic superconducting magnet.
Conventionally, a copper wire was used. However, if normal conductive copper is used, Joule heat due to the electric resistance of the lead wire and heat inflow through the lead wire due to heat conduction are unavoidable. Since these lead to a loss of electric power and a loss of helium as a refrigerant, various studies have been conducted on the shape thereof so as to minimize the loss.

Y-Ba-Cu-discovered in 1987
O-based superconductors and Bi-Sr-C discovered in 1988
Oxide superconductors such as a-Cu-O-based superconductors have a critical temperature of liquid nitrogen temperature or higher and can achieve a superconducting state at a relatively high temperature of 77K, and are therefore promising as materials applied to the above applications. However, when using an oxide superconductor, connection with a copper wire poses a problem. Soldering cannot be applied to the oxide, and a method of applying a conductive paste can be considered as one means, but this method reduces the connection resistance to 1/10.
^It is as large as ² Ω · cm ² and cannot carry a large current.

An object of the present invention is to provide an oxide superconducting current lead capable of lowering connection resistance with a conductor such as copper when an oxide superconductor is used as a current lead, and thus allowing a large current to flow therethrough. .

[0006]

The oxide superconducting current lead of the present invention has a rod-shaped end portion of the oxide superconductor, which is provided with a silver foil portion, a silver paste portion or a silver coating portion obtained by sintering a silver sprayed layer. The object is achieved by an oxide superconducting current lead provided or an oxide superconducting current lead having a plurality of conductive wires dispersedly connected to the silver coated portion.

[0007]

In the oxide superconducting current lead according to the present invention as described above, the silver coat portion formed at the end portion of the oxide superconductor enables soldering, and enables connection with the lead wire with low connection resistance. . Furthermore, by connecting a plurality of conducting wires in a distributed manner to the silver-coated portion, the contraction stress generated in the fixing jig or the like during cooling is reduced, and the superconducting magnet or the like can be operated without damaging the oxide superconducting current lead. . Moreover, even if a large current is passed through the superconducting magnet, etc., there is no heat generation in the lead wire, and since the oxide superconductor has a lower heat transfer coefficient than metal, it is possible to reduce the heat inflow due to heat conduction. It is possible to reduce the consumption amount of. Specific examples of the present invention will be described below with reference to the drawings.

As the oxide superconductor which can be used in the present invention, Y-Ba-Cu-O system (critical temperature 90K),
Bi- (Pb) -Sr-Ca-Cu-O system (critical temperature 1
10K), Tl-Ba-Ca-Cu-O system (critical temperature 1
25K) or the like is applicable. The powder of these oxide superconductors is molded into a rod-shaped oxide superconductor 1 by cold isostatic pressing or the like. A silver foil was wrapped around the end of the rod-shaped oxide superconductor sample, and then 100 to 500 was obtained by cold isostatic treatment or the like.
After applying a step of pressurizing at kg / cm ² , or applying and drying a silver paste, spraying silver, and sintering the same, the silver-coated portion 2 is formed at a temperature of 800 to 950 ° C. The conductive wire 3 connected to the silver-coated portion 2 thus obtained is a single-core wire, a twisted wire, a mesh wire or the like of a low resistance normal conducting metal such as copper or aluminum, and a niobium-titanium alloy or a niobium-tin alloy. A multifilamentary wire obtained by adding a stabilizing material to a superconducting material such as a vanadium-gallium alloy is applicable. In particular, when one or both of the oxide superconducting current leads are used at the liquid helium temperature, it is useful to reduce the connection resistance by using the metal-based superconducting wire having zero resistance at the liquid helium temperature.

[0009]

According to the present invention as described above, the connection resistance between the oxide superconducting lead and the normal conducting lead is 1/10 ⁸ Ω.
-Because it can be reduced to cm ² or less, it is possible to suppress heat ingress when supplying current to a superconducting magnet, etc., reduce helium consumption, and downsize refrigeration equipment. In addition, the conductive wires are dispersed in the silver-coated part and connected to each other, so that the contraction stress generated during cooling is reduced as compared with the case where a copper block or the like is connected. Since the area is increased, the connection resistance is further reduced.

[0010]

Example 1 Bi (Bi-Pb-Sr-Ca-Cu-
O) -based oxide superconductor (Bi: Pb: Sr: Ca: Cu)
= 0.8: 0.2: 0.8: 1.0: 1.4) powder by cold hydrostatic pressure treatment (1 ton / cm ² ) with a diameter of 12 mm,
It was formed into a rod shape having a length of 200 mm. 2 at 845 ℃
After firing for 4 hours, a thickness of 20 μm was applied to the end of the rod-shaped sample,
A silver foil is wound so as to have a width of 20 mm, subjected to cold hydrostatic pressure treatment (1000 kg / cm ² ), coated with silver paste, or sprayed with silver, and then cold hydrostatic pressure treatment (1 t.
on / cm ² ). It was fired again at 845 ° C. for 24 hours to obtain an oxide superconducting current lead.

The critical current characteristics and connection resistance of the present oxide superconducting current lead were evaluated by the DC four-terminal method shown in FIG. Since almost no current flows between the current lead and the voltage lead among the four terminals of the measurement system, it is considered that the potential difference between both leads is almost due to the connection resistance, and the connection resistance is calculated from the potential difference. did. If the measurement result is,
The critical current was 200 A, and the potential difference between the current and voltage leads of the four terminals was very small at 1.5 μV, and the connection resistance was 5.6 × (1/10) ⁸ (Ω · cm ² ). there were. In the case of, the critical current was 200 A, the potential difference between the current lead and the voltage lead was 1.2 μV, which was very small, and the connection resistance value was 4.5 × (1/10 ⁸ ) (Ω · cm ² ).
Met. In the case of, the critical current is 200 A, and the potential difference between the current lead and the voltage lead is 2.0 μV.
And the connection resistance is 7.5 x (1/10 ⁸ )
(Ω · cm ² ).

[0012]

Example 2 A silver-coated portion was formed in the same manner as in Example 1 except that a silver foil having a width of 50 mm was wrapped around the end of the rod-shaped sample obtained in the same manner as in Example 1. A plurality of (1, 2, 4, 24, 120) twisted copper wires having an outer diameter of 3 mm and a length of 50 mm were connected to the silver-coated portion at the end of the oxide superconducting current lead to prepare an oxide superconducting current lead.

The connection resistance of this lead was evaluated by the DC four-terminal method shown in FIG. The result is shown in FIG. In FIG. 3, the horizontal axis represents the energization current and the vertical axis represents the potential difference between the copper wire and the superconductor. Therefore, the inclination of FIG. 3 becomes the connection resistance.
The numbers in FIG. 3 indicate the number of conductors connected. This Figure 3
From this, it is understood that the inclination becomes smaller and the connection resistance is reduced as the number of solder connections of the copper wire is increased. 1
The data when 20 wires are connected overlaps with the horizontal axis in FIG. 3 and therefore is omitted, but the connection resistance value is 1 μΩ. With this connection resistance value, the amount of heat generated is 1 W even if 1000 A is applied.
It is a small value and can be processed sufficiently even with an existing refrigerator.

[0014]

[Embodiment 3] The conductor wire in Embodiment 2 has a length of 50 m.
24 niobium-titanium alloy superconducting wires of m were connected, and the connection resistance was measured at the liquid helium temperature in the same manner as in Example 2. As a result, the connection resistance value was 200 nΩ, and a low connection resistance that was not a practical problem was obtained.

[0015]

COMPARATIVE EXAMPLE Ten superconductors obtained in the same manner as in Example 2 were soldered to the silver-coated portions at both ends with round rod-shaped copper having an outer diameter of 30 mm. The copper rods at both ends were fixed and the thermal cycle between room temperature and liquid helium temperature was repeated 10 times. As a result, generation of cracks was observed in 3 out of 10 pieces.

[0016]

[Example 4] Oxide superconducting current leads were prepared by soldering 120 twisted copper wires having an outer diameter of 3 mm and a length of 50 mm to the silver-coated portions on both ends of 10 superconductors obtained in the same manner as in Example 2. Got Fix the copper wire to the same round bar copper as a comparative example,
Further, after fixing copper, heat cycle treatment was performed in the same manner as in the comparative example. As a result, no cracks were found in all 10 pieces, and it became clear that the stress was reduced by using the copper wire as compared with the case where the copper wire was fixed to the rod-shaped copper.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a measurement system of a DC four-terminal method used in Example 1.

FIG. 2 is an explanatory diagram showing a measuring system of a DC four-terminal method used in Example 2.

3 is a measurement result diagram showing a connection resistance value in Example 2. FIG.

[Explanation of symbols]

 1 Oxide superconductor 2 Silver coat part 3 Conductor wire

Claims (2)

[Claims] 1. An oxide superconducting current lead having a bar-shaped end portion of an oxide superconductor and a silver coating portion obtained by sintering a pressure-bonded silver foil, silver paste or silver sprayed layer. 2. An oxide superconducting current lead in which a plurality of conducting wires are dispersed and connected to the silver coated portion.