JPH0818110A - Superconductor - Google Patents

Abstract

PURPOSE:To make a current path long and to made the amount of heat intrusion small by constituting a superconductor of an REBa2Cu3Ox-based bulk superconductor, wherein a meander structures are connected in three dimensions and three-dimensional current paths are provided, and conductors which are connected to its both ends. CONSTITUTION:A meander structure is the structure that cutting 2 is applied on an REBa2Cu3Ox-based superconductor 1 and a means for providing the three- dimensional zigzag current paths is provided. Here, the RE of the REBa2Cu3Ox- based bulk superconductor is the elements of one or more kinds selected from a group comprising Y, La, Ce. Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. When the three-dimensional meander structure such as this is adopted, the superconductor material having a long current path can be manufactured from the superconductor having a limited shape.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in current leads and current limiting devices used in superconducting equipment which is cooled by liquid helium or a refrigerator.

[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 heat ingress from room temperature. 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 such that its shape is optimized so that the latent heat of gas from liquid helium can be used while ensuring a 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 the 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 an attempt to utilize these in the space of liquid helium temperature. Replacing the current leads with oxide superconductors has two advantages. One is that Joule heat does not occur in the superconducting state because the electric resistance is zero, and the other is that the thermal conductivity is extremely low as compared with copper.

Another application of oxide superconductors is in fault current limiters. This is a switch utilizing the superconducting-normal conducting transition. This device is designed so that the current normally flows through the superconducting part with zero electric resistance, and when the abnormally large current flows, the superconductor changes to the normal conducting state, and it is connected to the superconductor in parallel. The current is diverted to the bypass circuit connected to and the energy is dissipated by the resistance of the bypass circuit to protect the terminal equipment. Current,
Studies using metal superconductors have been made, but in the future, studies are being made to replace them with oxide superconductors. The advantages of oxide superconductors are that the cryogen needs only liquid nitrogen and that the electrical resistance in the normal conducting state is higher than that of metal superconductors. The larger the resistance difference between the superconducting state and the normal conducting state, the better the characteristics of the switch.

As described above, oxide superconductors are extremely promising materials for current leads and current limiting devices, but in order to be used as these materials, a certain critical current density and length are required. Is. In particular, since the superconducting current lead has no Joule heat from the current lead itself, it is advantageous that the current path is long in the direction of the temperature gradient from the viewpoint of heat penetration. Bi ₂ Sr ₂ C produced by sintering method
The a ₂ Cu ₃ O ₁₀ material satisfies these conditions to some extent and is being used for a superconducting magnet that operates in a refrigerator. However, since it is a sintered body and a Bi-based material that does not have a large pinning force is used, the conductor cross-section area must be large, and the critical current density significantly deteriorates in a strong magnetic field. It is expected that the range of use will be limited.

Considering the critical current density and its characteristics in a magnetic field, the YBa ₂ Cu ₃ O _x superconducting material is a material having excellent characteristics. The position of Y is other La, Ce,
Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, H
1 selected from the group consisting of o, Er, Tm, Yb, and Lu
It may be substituted with one or more elements, and may be replaced by REBa ₂ Cu ₃
Notated as O _X. However, in the case of this material system, the crystal grains must be oriented because the crystal grain boundaries significantly reduce the critical current density. With current technology, oriented REB
As a method for producing a ₂ Cu ₃ O _{x, there} are a method of forming a film on a substrate having a close lattice constant and a melting method.

Of these, the QMG method (JP-A-63-261)
The melting method, which is represented by Japanese Patent Application No. 607204/1990 and Japanese Patent Application No. 2-402204), has a high critical current density and is a method for obtaining a relatively large material. The melting method is once RE ₂ BaC
heated to a temperature region where uO ₅ phase and Ba-Cu-O main component and liquid phase coexist, it was cooled to peritectic temperature just above the REBa ₂ Cu ₃ O _X is produced, slowly cooling from this temperature Is a method of growing crystals to obtain large crystal grains. By this method, at present, a bulk superconducting material having crystal grains of about 20 cm ³ or more can be produced. The critical current density of this material is 77K, 1000 at 1T
Since it is 0 A / cm ² and is bulk, a large critical current can be obtained.

[0008]

However, it is difficult to manufacture a conductor having a length long enough to be used as a current lead or a current limiter with the current technology. Especially when applied to a fault current limiter, the current value can be increased compared to a thin film, but 1
A conductor having a dimensional (linear) or two-dimensional current path cannot have a sufficient path for obtaining a large normal resistance. Therefore, an object of the present invention is to provide a superconducting conductor provided with a long current path by using a REBa ₂ Cu ₃ O _X type bulk superconductor having a limited critical length and a high critical current.

[0009]

DISCLOSURE OF THE INVENTION The present invention is to solve the above-mentioned problems, and REBA ₂ Cu ₃ O _X having a three-dimensional current path in which meander structures are three-dimensionally connected.
System bulk superconductor (RE is Y, La, Ce, Pr, N
d, Pm, Sm, Eu, Gd, Tb, Dy, Ho, E
A superconducting conductor comprising one or more elements selected from the group consisting of r, Tm, Yb, and Lu) and a lead wire connected to both ends thereof.

[0010]

In order to solve the above problem, the present invention provides a means for cutting the REBa ₂ Cu ₃ O _x superconductor 1 as shown in FIG. 1 to form a three-dimensional zigzag current path. It is provided. This is called a three-dimensional meander structure. 1A is a perspective view, and FIG. 1B is a perspective view.
(C) and (d) are the lower surface side which is not visible in (a),
The figures are seen from the back and left sides, respectively. The symbols P and Q in the drawings are added to show the mutual positional relationship between the drawings.

By taking a three-dimensional meander structure,
A superconducting material having a long current path can be manufactured from a superconductor having a limited shape. This is particularly useful as a method of using a bulk superconductor manufactured by the melting method. As a current limiting device, a structure having a long current path has a larger electric resistance in the normal conducting state by that amount, so that a current limiting device excellent in current limiting operation can be manufactured.

This structure is also useful as a conductor for current leads. The current idea of the current lead is to positively use the evaporated helium gas, and it was necessary to install a long current lead in the temperature gradient direction of the gas. But,
In the case of the current lead used for the superconducting magnet operating in the refrigerator as described above, it is not always necessary to lengthen it in one direction. In the case of such a superconducting device, the current lead is installed in a vacuum space of 10K-80K, and is cooled by conduction cooling from the refrigerator, so that a temperature gradient is formed along the current lead conductor. Even if it has such a structure, there is an effect due to the lengthening of the conduction path, and the three-dimensional structure is particularly advantageous for the current lead.

This structure has three advantages. The first is
Since the currents flowing through the adjacent current paths are in opposite directions, the inductance can be reduced. Secondly, it can be made compact. In particular, the REBa ₂ Cu ₃ O _x type bulk superconductor produced by the QMG method has a high critical current density and can have a small cross-sectional area, and can be made considerably compact. And, thirdly, it is easy to give a shape because all of the processing is linear.

The superconducting conductor of the present invention does not need to have a constant cross-sectional area at all times, and the width of the portion where the current is reversed and the portion where the metal conductor is connected is strict and the transition to normal conduction is likely to occur is wide. You may. In particular, since this material has a large anisotropy, when an electric current is to be applied three-dimensionally, an electric current having a component in the c-axis direction having a small critical current density must be applied. Therefore, the cross-sectional area of such a portion must be large. It may also be compounded with other materials in order to improve its mechanical properties. For example, when applied to a current limiting device, measures such as stabilization should be taken by coating, pasting, or filling the gap between current paths such as silver with good thermal conductivity, and applying to current leads. In that case, an epoxy resin or the like having poor heat conductivity should be used. In addition, packaging in a vacuum further enhances the effect because heat short circuit due to gas between current paths is eliminated.

[0015]

【Example】

(Example 1) A current lead as shown in FIGS. 2 and ₃ was produced from a YBa ₂ Cu ₃ O _x type bulk superconductor produced by the QMG method. The arrow 3 in the figure indicates the direction of current flow. These are designated as current lead A and current lead B, respectively. The material forming each current lead has no large tilt grain boundaries throughout,
Y ₂ BaCuO ₅ phase has an average of 2μ in the Ba ₂ Cu ₃ O _x phase.
It has a structure in which it is uniformly dispersed below m. Also, as shown in the figure, the widest surface of the sample is the ab surface,
The current was made to flow mainly parallel to the ab plane. YBa ₂ Cu ₃ for current lead A and current lead B at room temperature
The electrical resistance at both ends of O _X is 0.6Ω and 1.8, respectively.
Ω. The critical current density of this material is 77K,
It was 25000 A / cm ² at 1T.

These current leads were applied to a critical current density measuring holder for measuring the critical current density of a superconductor in liquid helium. The critical current density is measured by configuring a cryostat as shown in FIG. 4A and 4B are diagrams showing the state in which the current lead B is inserted. FIG. 4A is an overall view, and FIG. 4B is an enlarged view of an X portion. The sample holder 11 used is capable of measuring 6 samples, and a total of 7 current-carrying copper wires 23 having a diameter of 2 mm and having a common plus side are inserted along the mandrel 16 of the cupro-nickel pipe from the outside to the measurement part. (The copper wire is omitted in the figures other than the enlarged view). A copper wire of this diameter can normally be energized up to 200A.

The sample holder 11 is fixed to a stainless dewar 12 as shown by a flange 13, and the critical current density is measured while energizing the superconducting sample placed in the sample holder 11. The dewar 12 is thermally shielded by the vacuum 17 and the liquid nitrogen 14, and the level of the liquid helium 15 in the case where there is no copper wire for electricity is from 200 mm to 0.
The evaporation rate up to mm is 0.007 liters / minute (hereinafter, the evaporation rate means that the level of liquid helium is 2
Refers to the average evaporation rate from 00 mm to 0 mm). On the other hand, the evaporation amount of liquid helium was 0.08 liters / minute when seven current-carrying copper wires having a diameter of 2 mm entered from outside the dewar to the sample holder 11. Therefore, it was found that 90% or more of the heat entered due to the heat conduction through the current-carrying copper wire.

Each of the copper wires is provided with a current lead A,
The voltage across the current lead and the evaporation amount of liquid helium when the current lead B was inserted and connected were measured. The connection method is as follows: YBa ₂ Cu ₃ O _x current lead 18 1 cm on both ends of the electrode portion 19 (ab surface) is deposited with a RF sputtering device to form a silver film of 1 μm, and this is energized through a 5 cm long copper braided wire 20. It was performed by soldering to the copper wire 23. The reason for interposing the copper braided wire 20 between the copper wire and the YBa ₂ Cu ₃ O _x current lead 18 is to release the heat shrinkage strain due to cooling by inserting the flexible copper braided wire therebetween.

Even when the level of liquid helium was zero, the current lead was cooled to less than 50K by heat conduction from the copper wire and vaporized gas from liquid helium. In FIG. 4, 21 is the position where the level of liquid helium is zero. The sample mounting portion of the sample holder 11 is replaced with a copper block 22.
, And a direct current of 200 A was passed through the current lead.
No voltage was detected across all current leads.

Table 1 shows the evaporation amount of liquid helium when each current lead is installed. As shown in the table, in the case where the current lead of YBa ₂ Cu ₃ O _x is inserted, the heat penetration is suppressed as compared with the case where the current lead is not inserted, and the effect becomes larger as the current path becomes longer. .

[0021]

[Table 1]

(Embodiment 2) Next, the inside of the dewar is evacuated and the current lead B is set to 1 when the YBa ₂ Cu ₃ O _x current lead is not used and when one current lead A is inserted.
The evaporation amount of liquid helium when the main insertion was performed was measured. When exhausting, the temperature of liquid helium drops, so
It is often used to measure critical current density at very low temperatures. YB
When the a ₂ Cu ₃ O _x current lead was not used, the evaporation amount of liquid helium was large and the liquid level of liquid helium became 0 mm when the temperature dropped to 2K. On the other hand, when the YBa ₂ Cu ₃ O _x current lead was used, the temperature of liquid helium was able to reach 1.8 K or less as a result of decompression. Table 2 shows the consumption of liquid helium when the temperature reaches a steady state. A large difference was observed between the case where the current lead A was used and the case where the current lead B was used. It was found that the three-dimensional meander structure with a long current path reduced the consumption of liquid helium and significantly reduced the heat intrusion from the current lead.

[0023]

[Table 2]

[0024]

As described above, REBa ₂ Cu ₃
O _X system cuts giving the process the bulk superconductor, zigzag current path as shown in FIG. 1 (three-dimensional meandering structure)
By taking advantage of the above, it is possible to manufacture a superconducting conductor having a long current path and a small amount of heat penetration. It is useful as a current lead or current limiter used in vacuum or under reduced pressure.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a three-dimensional meander structure, in which (a) is a perspective view, (b) is a bottom view, (c) is a back side, and (d) is a left side view.

FIG. 2 is a perspective view showing a current lead A according to an embodiment.

FIG. 3 is a perspective view showing a current lead B in the example.

5A and 5B are diagrams showing an experimental method in Examples, in which FIG. 5A is an overall view and FIG. 5B is an enlarged view of a portion X.

[Explanation of symbols]

1 REBa ₂ Cu ₃ O _X type bulk superconductor ₂ Cut ₃ Direction of current flow 11 Sample holder 12 Cryostat 13 Flange 14 Liquid nitrogen 15 Liquid helium 16 Mandrel 17 Vacuum 18 Current lead 19 Electrode part 20 Copper braid 21 Level of liquid helium Zero position 22 Copper block 23 Conductive copper wire

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[Procedure amendment]

[Submission date] June 22, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a diagram illustrating a three-dimensional meander structure, in which (a) is a perspective view, (b) is a bottom view, (c) is a back side, and (d) is a left side view.

FIG. 2 is a perspective view showing a current lead A according to an embodiment.

FIG. 3 is a perspective view showing a current lead B in the example.

[Figure 4] a view showing an experimental method in Example, (a) shows the overall view, (b) is an enlarged view of a portion of the X

[Explanation of Codes] 1 REBa ₂ Cu ₃ O _x type bulk superconductor 2 Cut ₃ Direction of current flow 11 Sample holder 12 Cryostat 13 Flange 14 Liquid nitrogen 15 Liquid helium 16 Mandrel 17 Vacuum 18 Current lead 19 Electrode part 20 Copper braided wire 21 Position where liquid helium level is zero 22 Copper block 23 Conductive copper wire

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H01B 12/16 ZAA H01F 6/00 ZAA H01L 39/16 ZAA

Claims (1)

[Claims] 1. The meander structure is three-dimensionally connected and 3
REBa ₂ Cu ₃ O _x type bulk superconductor having a dimensional current path (RE is Y, La, Ce, Pr, Nd, P
m, Sm, Eu, Gd, Tb, Dy, Ho, Er, T
A superconducting conductor comprising one or more elements selected from the group consisting of m, Yb, and Lu) and conductors connected to both ends thereof.