JPH04255203A - Oxide superconducting current lead - Google Patents

Abstract

PURPOSE:To make possible to perform a soldering operation, and to reduce connection resistance by providing a silver-coated part, formed by sintering pressure-bonded silver foil, silver paste or a silver flame-sprayed layer, on the end part of a rod-like oxide superconducting material. CONSTITUTION:Y-Ba-Cu-O and the like can be used for oxide superconducting material, and a rod-like oxide superconducting body 1 formed by the power of the above-mentioned material. A sheet of silver foil is wound on the end part of the superconducting body 1 and pressure is applied thereto, or silver paste is applied and dried up. After silver frame-spraying process has been conducted, the material is sintered, and a silver-coated part 2 is formed. A stranded wire of the low-resistance normal conducting metal such as copper and the like, and a multi-core wire, which is formed by adding stabilized material to the superconducting substance such as a niobium-titanate alloy and the like can be used as a conductor to be connected to the above-mentioned silver-coated part 2. Especially, the use of a metal superconducting wire, having zero resistance at the temperature of liquid helium, for one or both of the oxide superconducting lead wires is good for reduction of connection resistance.

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

BACKGROUND OF THE INVENTION Superconducting materials are materials that exhibit properties such as zero resistance, perfect diamagnetism, and the Josephson effect below a critical temperature Tc. Metal-based superconducting materials have a low critical temperature of less than 20K, but the critical temperature is lower than the liquid helium temperature (4.2
K), it is possible to generate a high magnetic field without loss by passing a large current through the superconducting coil. These are used in magnetic levitation trains, nuclear magnetic resonance diagnostic equipment, etc.

[0003] The current lead supplies a current of several hundred to several thousand A from a power supply at room temperature to a superconducting magnet at an extremely low temperature.
Traditionally, copper wire was used. However, when normally conducting copper is used, 1) Joule heat due to the electrical resistance of the lead wire, and 2) heat inflow through the lead wire due to thermal conduction cannot be avoided. Since these lead to a loss of power and a loss of helium, which is a refrigerant, various studies are being carried out regarding the shape of the helium to minimize the loss.

[0004]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 higher than the liquid nitrogen temperature, and a superconducting state is achieved at a relatively high temperature of 77 K, so they are promising materials for the above applications. However, when using oxide superconductors, connection with copper wires becomes a problem. Soldering cannot be applied to oxides, and one possible method is to apply a conductive paste, but this method reduces the connection resistance by 1/10.
It has a large resistance of 2Ω/cm2, so large currents cannot be passed through it.

An object of the present invention is to provide an oxide superconducting current lead that can lower the connection resistance with a conductive wire such as copper when the oxide superconductor is used as a current lead, and therefore can flow a large current. .

[0006]

[Means for Solving the Problems] The oxide superconducting current lead of the present invention has a silver coated portion formed by sintering crimped silver foil, silver paste, or a silver sprayed layer on the end of a rod-shaped oxide superconductor. The above-mentioned object has been achieved by an oxide superconducting current lead formed by the present invention or an oxide superconducting current lead formed by distributing and connecting a plurality of conductive wires to the silver coated portion.

[0007]

[Function] In the oxide superconducting current lead according to the present invention, soldering is possible due to the silver coated portion formed at the end of the oxide superconductor, and connection with a conducting wire is possible with low connection resistance. . Furthermore, by distributing and connecting multiple conductive wires to this silver-coated part, the shrinkage stress that occurs in fixing jigs, etc. during cooling is reduced, and superconducting magnets, etc. can be operated without damaging the oxide superconducting current leads. . Moreover, even when a large current is passed through a superconducting magnet, etc., there is no heat generation in the lead wires, and since oxide superconductors have a lower heat transfer coefficient than metals, it is possible to reduce the heat inflow due to heat conduction, and the refrigerant consumption can be reduced. Specific examples of the present invention will be described below with reference to the drawings.

[0008] Oxide superconductors that can be used in the present invention include 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) etc. are applicable. These oxide superconductor powders are formed into a rod-shaped oxide superconductor 1 by cold isostatic pressure treatment or the like. After wrapping silver foil around the end of this rod-shaped oxide superconductor sample, a cold hydrostatic pressure treatment of 100 to 500
After applying a pressure at kg/cm2 or applying a silver paste and drying it, or thermally spraying silver, this is sintered at a temperature of 800 to 950°C to form the silver coated part 2. . The conductive wire 3 to be connected to the silver coated portion 2 thus obtained may be a single core wire, a stranded wire, a mesh wire, etc. made of a low-resistance normal conductive metal such as copper or aluminum, or a niobium-titanium alloy or a niobium-tin alloy. , multifilamentary wires made of superconducting materials such as vanadium-gallium alloys and a stabilizing material are applicable. In particular, when one or both of the oxide superconducting current leads are used at liquid helium temperatures, using a metal-based superconducting wire that has zero resistance at liquid helium temperatures is useful for reducing connection resistance.

[0009]

According to the present invention as described above, the connection resistance between the oxide superconducting lead and the normal conducting lead can be reduced to 1/108Ω.
- Since it can be reduced to less than cm2, it is possible to suppress heat intrusion when supplying current to superconducting magnets, etc., making it possible to reduce helium consumption and downsize refrigeration equipment. Furthermore, by dispersing the conductor wires in the silver coated part and connecting multiple conductors, the shrinkage stress generated during cooling is reduced compared to when connecting copper blocks, etc., thereby preventing disconnection of the connecting wires and preventing contact. Since the area is increased, connection resistance is further reduced.

0010

[Example 1] Bi(Bi-Pb-Sr-Ca-Cu-O
) system oxide superconductor (Bi:Pb:Sr:Ca:Cu=
0.8:0.2:0.8:1.0:1.4) was molded into a rod shape with a diameter of 12 mm and a length of 200 mm by cold isostatic pressure treatment (1 ton/cm2). This was heated to 845℃ for 24 hours.
After baking for a period of time, wrap silver foil around the end of the rod-shaped sample to a thickness of 20 μm and width of 20 mm, and apply cold isostatic pressure treatment (
1000 kg/cm2), apply silver paste or spray silver, and then cold isostatically process (1 to
n/cm2) was applied. 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 using the DC four-terminal method shown in FIG. Of the four terminals of the measurement system, almost no current flows between the current lead and the voltage lead, so the potential difference between the two leads is thought to be mostly due to the connection resistance, and the connection resistance can be calculated from that potential difference. did. As a result of the measurement, in the case of ■, the critical current is 200A, and the potential difference between the current lead and the voltage lead of the four terminals is very small at 1.5 μV, and the connection resistance value is 5.6 × (1/10). 8 (Ω·cm2). In the case of (2), the critical current is 200A, the potential difference between the current lead and the voltage lead is very small at 1.2μV, and the connection resistance value is 4.5×(1/108)(Ω・cm2)
Met. In addition, in the case of ■, the critical current is 200A, and the potential difference between the current lead and the voltage lead is 2.0μV.
The connection resistance value is 7.5 x (1/108) (
Ω·cm2).

0012

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

The connection resistance of this lead was evaluated using the DC four terminal method shown in FIG. The results are shown in FIG. In FIG. 3, the horizontal axis represents the current flowing, and the vertical axis represents the potential difference between the copper wire and the superconductor. Therefore, the slope in FIG. 3 becomes the connection resistance. Note that the numbers in FIG. 3 indicate the number of connected conductive wires. This figure 3
It can be seen that as the number of soldered copper wires is increased, the slope becomes smaller and the connection resistance is reduced. 1
The data when 20 wires are connected is omitted in FIG. 3 because it overlaps with the horizontal axis, but the connection resistance value is 1 μΩ. With this connection resistance value, even if 1000A is applied, the amount of heat generated is 1W.
This is a small value that can be easily processed by existing refrigerators.

[0014]

[Example 3] As the conductor in Example 2, the length is 50 m.
24 superconducting wires made of niobium-titanium alloy with a diameter of 1.5 m were connected, and the connection resistance was measured at liquid helium temperature using the same method as in Example 2. As a result, the connection resistance value was 200 nΩ, which was a low connection resistance that was not a problem in practical use.

[0015]

[Comparative Example] A round copper rod having an outer diameter of 30 mm was soldered to the silver coated portions of both ends of 10 superconductors obtained in the same manner as in Example 2. The copper rods at both ends were kept fixed and thermal cycles between room temperature and liquid helium temperature were repeated 10 times. As a result, cracks were observed in 3 out of 10 pieces.

[0016]

[Example 4] Oxide superconducting current leads were obtained by soldering 120 twisted copper wires with an outer diameter of 3 mm and a length of 50 mm to the silver-coated parts of both ends of 10 superconductors obtained in the same manner as in Example 2. I got it. A copper wire was fixed to a round bar-shaped copper similar to the comparative example,
Furthermore, after fixing the copper, a heat cycle treatment was performed in the same manner as in the comparative example. As a result, no cracks were observed in any of the 10 wires, and it became clear that by using copper wires, stress was reduced more than when the wires were fixed to a rod-shaped copper wire.

[Brief explanation of the drawing]

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

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

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

[Explanation of symbols]

1 Oxide superconductor 2　Silver coat section 3 Conductor wire

Claims (2)

[Claims] 1. An oxide superconducting current lead comprising a rod-shaped oxide superconductor end having a silver coated portion formed by sintering a crimped silver foil, silver paste, or a silver sprayed layer. 2. An oxide superconducting current lead comprising a plurality of conducting wires connected in a distributed manner to the silver coated portion.