JPH0589919A - Low resistance current lead and low resistance current lead unit - Google Patents

Abstract

PURPOSE:To restrain the entry of heat into an extremely low temperature part by using silver or silver alloy for longitudinally bonding at least two current leads, which are made of oxide superconductors, and heating and melting it under a pressurized oxygen atmosphere so that the current lead is freely bendable. CONSTITUTION:By utilizing the property of silver that when heated under an oxygen pressure atmosphere it has a lower melting point than that of an oxide superconductor, many superconductors, if short, are longitudinally connected together with silver used as bond. In this way, of current leads of optional length can be obtained with reasonable curvature as a whole because silver is more flexible than the superconductor to allow external force in bending- direction to be absorbed at the bonded parts. Silver is used exactly only as bond and the main part of the lead is therefore made of a low heat conductivity conductor to block heat conduction at the conductor part and prevent the entry of heat. Concretely a lead 1 is made up with units 2a- and bonded parts 3a- provided with electrifying area enlarged portions 4a-.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current lead material and a current lead unit whose main constituent material is an oxide superconductor, and in particular, has a wide effective current-carrying area and is capable of carrying a large current and has a length. The present invention relates to a low resistance current lead unit that can be freely changed, and a low resistance current lead unit devised so that such a current lead can be easily manufactured.

[0002]

2. Description of the Related Art The development of superconducting technology has been remarkable recently, and the fields of application utilizing strong magnetic field magnets, large-current-capacity conductors, etc. have been remarkably expanding.

When using a superconducting material, in order to secure a superconducting transition temperature (Tc), a metallic superconducting material is brought to a liquid helium level, and an oxide superconducting material is brought to a cryogenic state of at least a liquid nitrogen level. It needs to be cooled, and for practical use, a current lead from the power supply unit at room temperature to the superconducting member kept at a cryogenic temperature is required.

Since a large current must be efficiently supplied to the superconducting member through the current lead, conventionally, a metal material such as a copper rod which is a good conductive material has been mainly used. However, even with good conductive materials such as copper rods, Joule heat is generated when a large current is applied. Therefore, copper rods and the like have to be made quite thick. For example, to supply a current of 1000 A, a diameter of about 6 mm is required. Requires copper rod lead. However, since copper rods and the like also have high thermal conductivity, the use of such a large diameter wire also increases the amount of heat invasion from the outside to the cryogenic part, increasing the evaporation amount of liquid helium and liquid nitrogen. That is a big drawback.

Therefore, as a means for alleviating these problems, a method of using an oxide superconductor as a current lead has been proposed. In other words, oxide superconductors have been developed that exhibit superconducting properties even in the temperature range of 100 to 130K due to the increase of Tc in recent years, and the thermal conductivity in the superconducting state is also small. The above-mentioned problems pointed out by the above are also solved.

[0006]

As an oxide superconductor that can be practically used as a current lead, a sintered oxide superconductor, a single crystalline oxide superconductor, and a tape-shaped oxide and silver are stacked. Combined complexes and the like are known. However, the sintered type has a drawback that it is difficult to obtain a long type because it is very fragile, and moreover, it is easily burned when it is transferred to the normal conducting state. It is difficult to obtain long ones due to crystal formation technology and equipment restrictions. Also, as in the case above, there is severe burnout at the time of transition to the normal conduction state, and mechanical strength and electrical conductivity decrease rapidly due to thermal cycles. There is a drawback that Further, the composite type has an advantage that brittleness is improved by combining with a silver tape and a long product can be easily molded, but since a large amount of heat penetration occurs through the composite silver sheath, The original purpose of utilizing oxide superconductors is significantly diminished.

The present invention has been made by paying attention to the problems as described above, and its purpose is to exhibit excellent conductivity in an extremely low temperature state, a small amount of heat penetration, and an arbitrary length. The present invention aims to provide a low resistance current lead that can be easily obtained, and can be easily repaired when it is damaged by fire, and a current lead unit having a structure suitable for manufacturing the current lead. Is.

[0008]

The structure of the low resistance current lead according to the present invention, which has been able to solve the above-mentioned problems, has two current lead units made of an oxide superconductor which are longitudinally arranged via silver or a silver alloy. The joint part made of silver or a silver alloy is formed by heating and melting the silver or silver alloy in a pressurized oxygen atmosphere to join the oxide superconductor. It is possible to receive a reliable joining state without deteriorating the characteristics. Further, if an energized area enlarged portion made of silver or a silver alloy is formed on the outer surface side of the joint portion, the energization property of the joint portion is enhanced and the energization loss can be reduced. As the oxide superconductor, all oxides exhibiting superconducting properties can be used. Among them, for example, Bi ₂ Sr ₂ CaCu ₂ O _x phase (Bi−
2212 phase) and other single crystalline materials are prized for showing excellent superconducting properties. Further, as a current lead unit, if silver or silver alloy joint portions having fitting surfaces are integrally formed in advance at both ends of a rod-shaped object made of an oxide superconductor, the unit can be optionally used. This is preferable because it is possible to easily manufacture a current lead having a length of, or to easily repair a current lead portion that has been damaged by burning or the like.

[0009]

FUNCTION AND EXAMPLES Oxide superconductors have excellent superconducting properties, and as described above, they are excellent as current lead materials. However oxide superconductor hardly be obtained as long for a very fragile, especially B ₁ -2,212 for single crystalline oxide superconductor of equality is difficult to mold its own long product , Lacks suitability as a current lead material.

Therefore, the present inventors proceeded with the research in anticipation of the development of a low resistance current lead which is mainly composed of an oxide superconductor and whose length can be arbitrarily adjusted. As a result, even if a short length oxide superconductor is joined through silver or a silver alloy (hereinafter represented by silver) that is a good conductor, the length can be changed by changing the number of combinations. In addition, the silver constituting the joint is flexible, and strain is absorbed by the joint when an external force in the bending direction is applied. However, it was confirmed that the flexibility was remarkably improved, and that flexibility could be imparted.

Further, according to another confirmation by the present inventors, the melting point of silver significantly changes depending on the oxygen partial pressure in the heating atmosphere, and by increasing the oxygen partial pressure, the melting point is considerably lower than that of the oxide superconductor. Can be made That is, FIG. 1 is a graph showing the relationship between the partial pressure of oxygen (PO ₂ ) in the heating atmosphere and the melting point of silver. The melting point of silver under the partial pressure of oxygen in a normal atmosphere is about 950 ° C. The temperature is higher than the melting point of most oxide superconductors (about 900 ° C or higher), and when trying to obtain a completely joined state by melting silver, the oxide superconductor also melts, especially single-crystal oxides. In the case of a superconductor, the junction becomes amorphous and the superconducting properties are significantly impaired.

However, the oxygen partial pressure (PO
The melting point of silver as enhancing ₂₎ is rapidly lowered, PO ₂
At 10 atm, the melting point of silver drops to about 880 ℃,
When O ₂ reaches 100 atm, the melting point of silver drops to about 760 ° C. On the other hand, the melting point of an ordinary oxide superconductor is
For example, "JJAP.26 (1990), 2729-2.
731 ", it increases as the oxygen partial pressure increases. Therefore, when the oxide superconductor and silver are heated in a pressurized oxygen atmosphere, only the silver can be melted without melting the oxide superconductor, and as a result,
Oxide superconductors can be reliably bonded to each other with a large contact area by using silver as a bonding material, whereby the current-carrying resistance at the bonding portion can be reduced to a minimum and a large current can be carried.

As described above, the present invention utilizes the characteristic of silver that the melting point becomes lower than the melting point of the oxide superconductor when heated in a pressurized oxygen atmosphere, and is a short oxide superconductor. However, by connecting a large number of these in the longitudinal direction using silver as a bonding material, it is possible to obtain a current lead having an arbitrary length as necessary. Moreover, silver is more flexible than oxide superconductors, and even when subjected to an external force in the bending direction, the external force is absorbed at the joint of the silver, so the current lead as a whole exhibits appropriate curvature. .. Furthermore, the silver itself forming the joint has a high thermal conductivity, which may cause heat intrusion. However, silver is only used as a joining material, and the main component of the current lead is oxidation with low thermal conductivity. Since it is composed of a physical superconductor and heat transfer is blocked at the oxide superconductor portion, there is no risk of heat intrusion.

By the way, the silver constituting the joint is one of the highest in conductivity among metals, but it has a lower conductivity under cryogenic conditions than an oxide superconductor, and therefore silver in the silver joint. The energization resistance may increase slightly and energization loss may occur. However, such energization loss is caused by the current lead unit 2a made of an oxide superconductor, as shown in the current lead of FIG.
.. and silver joints 3a, 3b, .. .. are formed on the outer surface side of silver (or silver alloy) energized area enlarged portions 4a, 4b ,. It is possible to suppress the energization loss generated at the joint.

Since the silver energization area enlarging portion is provided only to increase the energization amount of the joint, it does not have to be provided over the entire circumference of the joint, and the essential point is mutual. It may be formed in two or three or more strips as long as it is connected between the current lead units joined to each other. Also, when joining the silver energized area enlarging portion, it is desired to heat and join the energized area enlarging silver member in a pressurized oxygen atmosphere for the same purpose as above.

FIG. 3 shows another embodiment of the current lead according to the present invention. The current lead is composed of a plurality of current lead units 2a, 2b joined through silver joints 3a, 3b, .... The outer peripheral surface of No. 1 is covered with an organic film 5 to suppress deterioration of the lead body when subjected to a thermal cycle. The organic film used here is, for example, low temperature epoxy resin (product name "Stycast")
Examples include polyester resins and the like.

FIG. 4 shows still another embodiment of the present invention, in which a plurality of current lead units 2a, 2b, ... Having different cross-sectional areas are joined via silver joints 3a, 3b ,. The structure is shown. That is, the current lead of the present invention is provided mainly for electrically connecting the extremely low temperature and the low temperature or normal temperature power source portion as described above, and the conductivity of the oxide superconductor increases as the temperature increases. It tends to decrease. Therefore, if the cross-sectional area of the lead unit is increased toward the high temperature side as shown in FIG. 4, the energization amount on the high temperature side can be increased, and a large current can be more smoothly supplied to the cryogenic portion.

Further, FIG. 5 is a cross-sectional explanatory view illustrating a preferred structure of the current lead unit, in which both ends of the lead unit 2 made of an oxide superconductor are integrally formed with silver coupling portions 6a and 6b having a structure of fitting with each other. Have been formed in the same way. With such a configuration, as shown by the broken line in FIG. 5, a plurality of lead units 2a, 2 are formed by the connecting portions 6a, 6b.
Since b, ... Are fitted and connected in a pressurized oxygen atmosphere and heated to melt the joint portion, a reliable melted joint state can be obtained, and the jointing work is remarkably simplified. It is advantageous. The shapes and structures of the connecting portions 6a and 6b are not limited to those shown in the figures, and any shape
It can be a structure.

[0019]

The present invention is configured as described above, and the effects thereof are summarized as follows. (1) Oxide superconductors are melt-bonded to each other using silver as a bonding material, and they have excellent conductivity and can carry a large current. For example, when using a lead with a diameter of 5 mm, It is possible to energize more than 500A at 77K. (2) Since an oxide superconductor having a low thermal conductivity is the main constituent material, heat intrusion to extremely low temperatures can be suppressed to a minimum. (3) A current lead of arbitrary length can be obtained by changing the number of combinations of lead units. (4) Since the joint is made of soft silver, it exhibits excellent flexibility even for long products, can be bent freely, and has stress due to thermal expansion and contraction in the longitudinal direction. Is also absorbed at the joint. (5) Even if partial burnout occurs, only that part can be cut out and replaced with another lead unit for easy repair. (6) By coating the surface of the lead with an organic film, the resistance to heat cycles can be increased. (7) If a lead unit having silver joints formed at both ends is used, a fusion-bonding operation combining these can be performed further easily.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the oxygen partial pressure (PO ₂ ) in a heating atmosphere and the melting point of silver.

FIG. 2 is a cross-sectional explanatory view of main parts illustrating a current lead according to the present invention.

FIG. 3 is a cross-sectional explanatory view of a main part illustrating another current lead according to the present invention.

FIG. 4 is a cross-sectional explanatory view of main parts illustrating still another current lead according to the present invention.

FIG. 5 is a cross-sectional explanatory view illustrating a current lead unit according to the present invention.

[Explanation of symbols]

 1 Current Leads 2, 2a, 2b, 2c Current Lead Units 3a, 3b, 3c Silver Joints 4a, 4b, 4c Current Enlarged Area 5 Organic Films 6a, 6b Silver Joints

Claims (7)

[Claims] 1. A low resistance current lead characterized in that at least two current lead units made of an oxide superconductor are joined in the longitudinal direction via silver or a silver alloy. 2. The low resistance current lead according to claim 1, wherein the current lead units are joined by heating and melting silver or a silver alloy in a pressurized oxygen atmosphere. 3. The low resistance current lead according to claim 1, wherein the oxide superconductor is single crystalline. 4. The low resistance current lead according to claim 1, wherein an energized area enlarged portion made of silver or a silver alloy is formed on the outer surface side of the joint portion made of silver or a silver alloy. 5. The energized area expanding portion made of silver or a silver alloy,
5. The low resistance current lead according to claim 4, which is formed on the outer surface side of the joint by heating and melting silver or a silver alloy in a pressurized oxygen atmosphere. 6. The low resistance current lead according to claim 1, wherein the outer peripheral surface is coated with an organic film. 7. A low resistance current lead unit comprising a current lead unit made of an oxide superconductor, and silver or silver alloy joint portions having mating surfaces formed on both ends thereof.