JPH0969427A - Oxide superconducting current lead - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To connect an oxide superconductor and a normal conductor in a current lead, which is cooled by liquid helium or a refrigerator and supplies current to equipment used at cryogenic temperature, (EN) Provided is a compact current lead having a large current capacity and capable of relieving thermal contraction stress applied to an oxide superconducting material itself. SOLUTION: At least one end of an oxide superconductor is connected to a good conductor which has a specific electric resistance at liquid helium temperature of 7 × 10 ^-11 Ωm or less and which can be deformed to expand or contract in length. An oxide superconducting current lead in which both ends of a superconductor and a good conductor are connected and fixed and supported. Also, these are oxide superconducting current leads that are housed in a rigid tube.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current lead that is cooled by liquid helium or a refrigerator and supplies a current to equipment used at cryogenic temperatures.

[0002]

2. Description of the Related Art At present, almost all superconducting devices are cooled to a liquid helium temperature (4.2K) and used. One of the major problems with these devices is the ingress of heat from the surroundings. Heat enters from various parts, but the heat is most intruded from the lead wire that supplies current to the superconducting device. The current lead is a conductor that has been devised so as to optimize the shape so that the latent heat of gas from liquid helium can be used for cooling while ensuring a current capacity (cross-sectional area) for supplying current.

The current lead used so far has mainly been made of copper, but recently, attempts have been made to replace it with an oxide superconducting material. Among oxide superconducting materials, YBa ₂ Cu ₃ O _x system, Bi ₂ Sr ₂ Ca ₂
It was discovered that the critical temperature exceeds the liquid nitrogen temperature (77K), such as Cu ₃ O ₁₀ series, Tl ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ or Hg ₂ Sr ₂ Ca ₂ Cu ₃ O ₈ series, and the liquid nitrogen temperature. It is intended to utilize these in the space of liquid helium temperature.

Replacing the current lead with an oxide superconductor has the following two advantages. One is that Joule heat does not occur because the electric resistance is zero in the superconducting state, and the other is that the thermal conductivity is lower than that of copper. Therefore, oxide superconductors are extremely promising materials for current leads.

In order for an oxide superconducting material to be used as a current lead, a certain level or more of critical current density and length are required. In particular, since the superconducting current lead has no Joule heat from the current lead itself, a longer current path is advantageous from the viewpoint of heat intrusion. Therefore, a typical current lead is a rod-shaped or tubular conductor.

However, oxide superconductors are generally fragile, and the longer the conductor is, the more easily it breaks when subjected to impact or stress. Therefore, some form of reinforcement is desirable. Although various methods are conceivable as the reinforcing method, the easiest and most effective method is to combine the reinforcing body with high rigidity, which is resistant to thermal shock during cooling. For the stress of the current lead, a metal material such as stainless steel or cupro-nickel having a low thermal conductivity as a reinforcement or G
Fiber reinforced plastics (FRP) such as -10 are possible. However, these reinforcing materials differ from the oxide superconductor in the heat shrinkage rate when cooled. Therefore, if both ends of the oxide superconductor are fixed with these reinforcing bodies, there is a risk that the superconducting properties of the superconductor may be deteriorated or destroyed due to stress due to thermal contraction during cooling.

In order to avoid this problem, it is desirable to flexibly connect the oxide superconductor and the surrounding structure, and a flat braided copper wire is generally used for the lead wire connected to the superconductor. However, it is common to pass a large current through the current leads, and if the capacitance of the current leads is large, a sufficiently low resistance cannot be obtained with the flat braided copper wire, which also causes heat generation. Therefore, as a special example, a bellows-shaped copper, a bellows of a copper alloy, a mesh-shaped flexible hose, and a porous metal layer are used (above JP-A-5-21228) and a tension spring is arranged (JP-A-5-212). 109531
JP-A-5-198432) and the like have been proposed.

Generally, a copper-based material having a low electric resistance is used as a conducting wire for conducting electricity. A typical copper wire has an electric resistance of 1 × 10 ⁻¹⁰ Ωm at a liquid helium temperature, as indicated by Cu-1 in Table 1 (which will be described later).
It is a degree. However, the amount of current passed through the superconducting wire is large, and the above-mentioned electrical resistance causes a large amount of heat generation, which may cause a problem. Further, generally used copper wires are work-hardened and hard, and the current flow becomes large, and if the cross-sectional area is increased in order to suppress Joule heat generation, sufficient elasticity cannot be obtained. This problem is magnified when trying to downsize the superconducting current flow lead.

QMG method (Japanese Patent Publication No. 4-40289)
Among the oxide superconducting materials, the YBa ₂ Cu ₃ O _x type bulk material produced by the melting method as described above has a particularly high critical current density, and it is possible to construct a small current lead having a large current capacity. However, if the oxide superconductor is made smaller, the area of the electrode portion is also limited, so that the electrical resistance of the metal to be connected is required to be lower. In particular, in order to protect the current leads in a protective tube and to provide an elastic part in the space inside, it is necessary to reduce the cross-sectional area of the metal wire. It is difficult to reduce the size while ensuring the capacity (low resistance).

[0010]

Therefore, the present invention is to connect an oxide superconductor and a normal conductor and has a large current capacity (low resistance of the normal conductor portion), and the oxide superconductor material itself. It is an object of the present invention to provide a compact current lead having a large current capacity that can relieve the heat shrinkage stress applied to the.

The present invention is to solve the above-mentioned problems, in which at least one end of the oxide superconductor has a specific electrical resistance at liquid helium temperature of 7 × 10 ^-11 Ωm or less and is deformable in length. A good conductor is connected, and both ends of the above-mentioned oxide superconductor and a good conductor are fixed and supported, which is an oxide superconducting current lead. At least one end of the oxide superconductor has an electrical resistivity of 7 × 10 ^-11 Ω at liquid helium temperature.
A good conductor having a length of less than or equal to m and capable of expanding and contracting is connected, and the one in which the oxide superconductor and the good conductor are connected are housed in a rigid pipe and fixed to both ends of the rigid pipe. An oxide superconducting current lead, which is supported by a good conductor for an electrode. Here, the inside of the rigid tube is also characterized by being sealed by enclosing a dry gas or by making a vacuum. In addition, in these oxide superconducting current leads, the good conductor is obtained by combining a plurality of strip-shaped thin plates curved in the length direction in an axially symmetrical manner.
It is possible to make the length expand and contract, and the good conductor is 99.
It is also characterized in that it is silver having a purity of 9% or more, and that the oxide superconductor is an oriented REBa ₂ Cu ₃ O _x system superconductor.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The current lead of the present invention has a structure in which a metal which is a good conductor is connected to at least one end of an oxide superconductor. FIG. 1 shows an example of an oxide superconducting current lead of the present invention, in which 1 is an oxide superconductor and 2 is a metal plate as a good conductor. In this example, four curved metal plates are combined, but this allows the length to expand and contract when a force is applied in the length direction. By axially symmetrically combining a plurality of strip-shaped thin plates curved in the lengthwise direction as described above, it is possible to expand and contract without applying bending force to the connected oxide superconductors. In addition, 3 is a rigid pipe and is shown by a cross section cut along the center line, and 4 is a good conductor for electrodes, which will be described later.

The above metal has an electric resistivity of 7 × 10 ^-11 Ωm or less at the temperature of liquid helium. The electrical resistance at liquid helium temperature depends on the type, purity and structure of the metal. When the purity of good conductors such as silver, copper, aluminum and gold is increased, the electric resistance in the low temperature region is remarkably reduced. Further, even with metals having the same purity, if the number of dislocations, vacancies, and grain boundaries is reduced, the electric resistance in the low temperature region is reduced. Further, in the state where the above-mentioned defects are few, the metal is soft, and by connecting as shown in FIG. 1, a large current can be passed, and the strain due to thermal contraction or the like can be alleviated.

The soft metal having a low electric resistance in the low temperature region can be obtained by using high purity silver, copper, aluminum or gold as described above and sufficiently annealing it at a high temperature. The annealing temperature is preferably 60% or more of the melting point. Further, in order to prevent oxidation, it is desirable to carry out in an inert gas, a reducing gas or in a vacuum.

Silver having a purity of 99.9% or more has an electric resistance comparable to that of a commonly used copper wire, and is superior to the above material due to its softness. Further annealing makes it possible to dramatically reduce the electric resistance at extremely low temperatures and soften it. Also, the solderability is good, and silver having a purity of 99.9% or more is most suitable as the metal used in the present invention.

Although there is a temperature gradient across the current leads, it is desirable to use the metal portion as described above on the cold side. This is because the low resistance generally depends on the RRR but is large in the low temperature region of 20 K or less, and the purity and annealing effects almost disappear at the liquid nitrogen temperature (77 K). If the low temperature side is made flexible, it is not necessary to make the high temperature side flexible. For example, as shown in FIG. 1, a metal block (good conductor for electrode 4)
By directly connecting to, the problem of connection resistance can be greatly reduced.

As described above, by connecting a well-annealed metal of high purity, the oxide superconducting material can be expanded and contracted in the length direction in a narrow space, and the electric resistance can be lowered (the current capacity can be increased). It will be possible. Therefore, as shown in FIG. 1, the structure is accommodated in a rigid pipe 3 made of stainless steel or the like and supported by good conductors 4 for electrodes such as copper fixed to both ends of the rigid pipe 3, thereby It can withstand current power and realizes a compact current lead compared to the current capacity.

This type of oxide superconducting conductor may be used as a gas-cooled type having a structure in which vaporized helium gas passes through the rigid body tube 3, but in the case of a use environment where there is a lot of contact with the atmosphere, the rigid body is made rigid. It is desirable to have a sealed structure inside the tube. This is highly necessary when an YBa ₂ Cu ₃ O _x oxide superconductor, which is highly deteriorated by water, is used as the oxide superconducting material. Therefore, it is desirable that the inside of the rigid tube is filled with vacuum or dry gas.

[0019]

【Example】

Example 1 For copper, silver, and aluminum, room temperature, liquid nitrogen temperature,
The electrical resistance at liquid helium temperature was investigated. The tested materials are a commonly used copper wire (Cu-1) and a commercially available plate material with high purity. The results are shown in Table 1. The electrical resistances at room temperature and liquid nitrogen temperature were almost the same regardless of the material and heat treatment, but the electrical resistance at liquid helium temperature was significantly different depending on the material and heat treatment conditions. Approximately 1 x 10 for coated copper wire (Cu-1) that is commercially available
It was ^-11 Ωm.

[0020]

[Table 1]

When annealed at a high temperature, the electrical conductivity was dramatically improved even with the same purity. The sample used this time has a melting point of 80.
It was annealed in an argon stream at a temperature of 2% for 2 days, but the electric resistance decreased from about 1/2 to 1/13 or less. It can be said that such a material is excellent as a low temperature side lead wire of a current lead which is cooled to a temperature close to liquid helium, that is, 20 K or less.

Further, among these, the material which was annealed and whose electric resistance was reduced became dramatically softer than that before the annealing. Especially silver and aluminum had a great effect. This is because defects such as vacancies, dislocations, and crystal grain boundaries are reduced, and it can be said that such a material is excellent as a material for relaxing the above-mentioned heat shrinkage strain.

Example 2 An oxide superconducting current lead as shown in FIG. 1 was produced. Oxide superconductor used for this current lead 1
Is a YBa ₂ Cu ₃ O _x type bulk superconducting material produced by the QMG method. The characteristic of this material is that there is no large tilt grain boundary throughout the material and that the Y ₂ BaCuO ₅ phase is uniformly dispersed in the YBa ₂ Cu ₃ O _x phase of the matrix with an average of 2 μm or less. The critical current density is extremely high. The length of the oxide superconductor used is 43m.
m, the cross-sectional area is 3 × 4 mm ² .

This oxide superconductor 1 is electrically connected to a copper electrode as a good conductor 4 for an electrode. In order to reduce the contact resistance, silver is deposited on the electrode part, and the high temperature side is directly soldered to the copper electrode, but the low temperature side is bonded to the copper electrode.
The rigid pipe 3 of S304 stainless steel (outer diameter 10 mm, thickness 0.3 mm, length 80 mm) is connected via a soft metal as a good conductor 2 for absorbing heat shrinkage strain. Soft metal plates for stress relaxation are two plates with a width of 3 mm x 35 mm and a width of 4
Two plates each having a size of mm × 35 mm and a total of four plates were connected. The rigid tube was sealed with helium gas.

The low temperature part of the current lead is cooled to 10 K or less, but the high temperature part becomes about 60 K. The required characteristics of this current lead were 200 A at an external magnetic field of 5000 Gauss. The critical current density of a current lead of a typical Bi-based sintered body that has been recently studied is about 30 at 62K and 5000 gauss.
0 A / cm ² (Advances in Sup
erconductivity V, Springer
-Verlag, p. 643), assuming that the shape is rod-like, a cross-sectional area of about 10 mm diameter is required only with the superconducting material. Therefore, the prototyped current lead is extremely compact for this class of current lead.
This is because an oxide superconducting material having a large critical current was used. The critical current of the material used this time is 77K, 5
It is 1430 A at 000 gauss, and it is possible to pass a larger current when the temperature becomes lower.

[0026] In general the copper wire (electric resistance at liquid helium temperature, 1 × 10 ^-10 about [Omega] m), in order to reduce the influence of heat generated in the liquid helium is, 1 mm ² per 45
A is the limit. Therefore, the electric resistance of the metal plate at the liquid helium temperature shown in Example 1 was 1.48 × 10 ^−10.
If Cu-2 of Ωm is used, the thickness is 0.5 m.
About m is necessary. In particular, in the case of a conduction cooling type current lead that does not come into direct contact with liquid helium as shown in FIG. 1 and the temperature is higher than the liquid helium temperature, this condition becomes more severe. However, it was not possible to obtain sufficient repellency with this thickness.

On the other hand, for example, when a material having an electric resistance of 1 × 10 ^-11 Ωm or less similar to Ag-la is used, the thickness is 1 /
10 will suffice. In fact, Cu-2 and Ag-
The current lead shown in FIG. 1 was produced using the same material and heat treatment condition as la, and a thickness of 0.3 mm, and an external magnetic field of 5000
The Gaussian critical current characteristics were compared.

In the case of using copper, the electric resistance of the current lead sharply increased at a current of 150 A, making it impossible to carry a current. This is because the copper plate has a large resistance and thus generates heat, and the heat propagates to the oxide superconductor to quench the superconductor. Further, when cooling and heating were repeated with room temperature, the critical current gradually deteriorated, and the critical current became 100 A or less after 5 thermal cycles. It is considered that this is because a contact strain between the oxide superconducting material and the copper plate was increased due to thermal strain and a crack was generated, resulting in an increase in contact resistance. On the other hand, in the case of using a silver plate, it was possible to stably energize up to 200 A, and deterioration due to thermal cycle and deterioration with time in half a year were not observed.

Example 3 A current lead similar to that of Example 2 was used, and an Al-layer having a thickness of 0.3 mm was used.
A prototype was made using the same material as la. The procedure of Example 2 was repeated except that the solder for aluminum was used for the connection between the oxide superconductor and aluminum. When the critical current density was measured under the same conditions, it was quenched at 200A. However, since the aluminum used was soft and had a margin for expansion and contraction, it had a thickness of 0.
Further experiments were conducted using a 4 mm one. As a result, it was confirmed that stable energization was possible up to 200 A, no deterioration due to heat cycle was observed, and the above effects obtained by using a general copper material were obtained.

[0030]

As described above, by connecting a highly pure and well-annealed metal material having an extremely small electric resistance to a superconducting material, a large current can be passed in a smaller space, and a compact size is achieved. Thus, it is possible to manufacture an oxide superconducting current lead which is resistant to thermal cycles.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an oxide superconducting current lead of the present invention.

[Explanation of symbols]

 1 oxide superconductor 2 good conductor 3 rigid tube 4 good conductor for electrodes

Claims (6)

[Claims] 1. An oxide superconductor is connected to at least one end thereof with a good deformable conductor whose electric resistivity at liquid helium temperature is 7 × 10 ⁻¹¹ Ωm or less and which is capable of expanding and contracting in length. An oxide superconducting current lead characterized in that both ends of a superconductor and a good conductor are fixed and supported. 2. An oxide superconductor is connected to at least one end thereof with a deformable good conductor having an electric resistivity at liquid helium temperature of 7 × 10 ⁻¹¹ Ωm or less and capable of expanding and contracting in length. An oxide superconducting current lead, characterized in that an object superconductor and a good conductor are connected to each other and housed in a rigid tube and supported by good conductors for electrodes fixed to both ends of the rigid tube. 3. The oxide superconducting current lead according to claim 2, wherein the rigid tube is hermetically sealed by filling a dry gas or by applying a vacuum. 4. The good conductor is capable of being deformed so that its length expands and contracts by combining a plurality of strip-shaped thin plates curved in the length direction in an axially symmetrical manner.
5. The oxide superconducting current lead according to any one of 1. 5. The oxide superconducting current lead according to claim 1, wherein the good conductor is silver having a purity of 99.9% or more. 6. The oxide superconductor is oriented REBA ₂ C.
u ₃ O _x based oxide superconducting current lead according to any one of claims 1 to 5, characterized in that a superconductor.