Superconducting wires

Abstract

This provides a superconducting wire with high Tc and excellent flexibility. [Solution] A superconducting wire comprises a core material formed of a first material having superconducting properties, and a covering material formed of a second material different from the first material, which also has superconducting properties and covers the core material. In this superconducting wire, one of the first and second materials is MgB2, and the other of the first and second materials is a Cu oxide-based superconducting material.

Description

Technical Field

[0001] This specification discloses a technology related to superconducting wires.

Background Art

[0002] Various materials have been developed as materials for superconducting wires. Patent Document 1 lists MgB2 as a material for superconducting wires. MgB2 is relatively flexible and useful as a material for superconducting wires. Patent Document 1 discloses a technology for improving the manufacturing method when manufacturing MgB2 superconducting wires and realizing a superconducting wire with a high critical current density.

Prior Art Documents

Non-Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Superconducting wires are expected to be used, for example, as windings of superconducting motors. When using a superconducting wire as the winding of a superconducting motor, MgB2 with high flexibility is suitable as a material for the superconducting wire. In addition, liquid hydrogen is cited as a refrigerant for cooling the superconducting wire. However, the boiling point of liquid hydrogen is -252.6°C (approximately 20K), and the superconducting transition temperature (critical temperature) of MgB2 is 39K. Therefore, the difference between the temperature of liquid hydrogen and the critical temperature (Tc) of MgB2 is small, resulting in design constraints in the arrangement of the refrigerant and the superconducting motor. Considering the design constraints, it is preferable that the Tc of the superconducting wire is 40°C or higher than the temperature of liquid hydrogen. In addition, no material with high flexibility has been found among materials with a high Tc. Therefore, a superconducting wire with a high Tc and high flexibility is required. This specification aims to provide a superconducting wire with a high Tc and high flexibility.

Means for Solving the Problems

[0005] The first technology disclosed herein is a superconducting wire that comprises a core material formed of a first material having superconducting properties, and a covering material formed of a second material different from the first material, having superconducting properties and covering the periphery of the core material. In this superconducting wire, one of the first and second materials may be MgB2, and the other of the first and second materials may be a Cu oxide-based superconducting material.

[0006] The second technology disclosed herein is a superconducting wire of the first technology described above, wherein the first material is MgB2 and the second material is a Cu oxide-based superconducting material.

[0007] The third technology disclosed herein is a superconducting wire of the first or second technology described above, wherein the volume ratio of the Cu oxide-based superconducting material to the superconducting wire may be 30 vol% or more.

[0008] The fourth technology disclosed herein is a superconducting multi-core tube, wherein multiple superconducting wires from the first to third technologies described above may be arranged inside a hollow metal tube. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic diagram of a superconducting wire is shown. [Figure 2] A schematic diagram of a superconducting multi-core tube is shown. [Modes for carrying out the invention]

[0010] The superconducting wires disclosed herein are suitably used for windings (stator coils) of superconducting motors, windings of transformer cores, and the like. Furthermore, these superconducting motors and transformers can be used as components in fuel cell vehicles (FCEVs), hydrogen engine vehicles, hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs) that use hydrogen (liquid hydrogen) as fuel. When used as such components, the superconducting material can utilize liquid hydrogen as a refrigerant, either primarily or secondarily.

[0011] The superconducting wire disclosed herein is formed of a core material made of a first material having superconducting properties and a covering material that also has superconducting properties and covers the periphery of the core material. The first and second materials are different materials; one is MgB2 and the other is a Cu oxide-based superconducting material. The Tc of MgB2 is 39K, while the Tc of Cu oxide-based superconducting materials is generally 90-110K. Therefore, the superconducting wire disclosed herein exhibits a higher Tc overall than that of MgB2. As a result, when liquid hydrogen is used as a refrigerant, the superconducting wire disclosed herein can achieve a larger difference between the temperature of liquid hydrogen (refrigerant) and Tc (apparent critical temperature) compared to a superconducting wire made of MgB2. By increasing the difference between the temperature of liquid hydrogen and Tc, the distance between the liquid hydrogen and the superconducting wire (superconducting motor, transformer, etc.) can be increased. In other words, the design flexibility regarding the arrangement of liquid hydrogen and superconducting wires increases. Furthermore, a stabilizing layer made of Cu, Ag, or other material with good conductivity, or a protective layer made of Fe, resin, or other material to protect the superconducting wires, may be provided on the outer periphery of the coating layer.

[0012] In the superconducting wires disclosed herein, it is preferable that the volume ratio of Cu oxide-based superconducting material to the superconducting wire is 30 vol% or more. For example, when using a Cu oxide-based superconducting material with a Tc of 110 K, by setting the volume ratio of the Cu oxide-based superconducting material to 30 vol% or more, the Tc of the superconducting wire can be made 60 K or more. This makes it possible to ensure a temperature difference of 40°C or more between the liquid hydrogen temperature and the Tc, thereby improving the flexibility of the arrangement between the refrigerant and the superconducting wire (equipment utilizing the superconducting wire). The volume ratio of Cu oxide-based superconducting material to the superconducting wire may be 40 vol% or more, 50 vol% or more, 60 vol% or more, or 70 vol% or more. Note that if the superconducting wire is directly cooled with liquid hydrogen, the superconducting wire will become brittle due to hydrogen embrittlement. Furthermore, when superconducting wires are used as windings in superconducting motors, direct cooling with liquid hydrogen can cause the motor's permanent magnets to become brittle. Therefore, when cooling superconducting wires with liquid hydrogen as a coolant, indirect cooling (heat transfer cooling) is preferable to direct cooling. From this perspective, it is preferable that the temperature difference between the coolant (liquid hydrogen) and the Tc of the superconducting wire is large.

[0013] From the perspective of increasing the Tc of a superconducting wire, a higher volume ratio of Cu oxide-based superconducting material to the superconducting wire is preferable. However, Cu oxide-based superconducting material is less flexible than MgB2. Therefore, if the volume ratio of Cu oxide-based superconducting material becomes too high, cracks may occur in the portion of the superconducting wire formed by the Cu oxide-based superconducting material when the superconducting wire is bent. Even if cracks occur, the Tc of the Cu oxide-based superconducting material itself does not change. However, if cracks occur in the portion formed by the Cu oxide-based superconducting material, the conductive resistance of the superconducting wire increases, and the effect of using the Cu oxide-based superconducting material (increase in Tc) decreases. Simulation results show that when the volume ratio of Cu oxide-based superconducting material to the superconducting wire exceeds 85 vol%, the apparent effect of increasing Tc decreases compared to when the volume ratio of Cu oxide-based superconducting material is 30 vol%. Therefore, in the superconducting wire disclosed herein, it is preferable that the volume ratio of Cu oxide-based superconducting material to the superconducting wire is 85 vol% or less. However, the volume ratio of Cu oxide-based superconducting material to the superconducting wire may be 80 vol% or less, 70 vol% or less, 60 vol% or less, or 50 vol% or more.

[0014] The superconducting wire disclosed herein has a core material and a coating material, one of which is MgB2 and the other is a Cu oxide-based superconducting material. That is, the core material (first material) may be MgB2 and the coating material (second material) may be a Cu oxide-based superconducting material, or the core material may be a Cu oxide-based superconducting material and the coating material may be MgB2. Preferably, the core material (first material) is MgB2 and the coating material (second material) is a Cu oxide-based superconducting material. As described above, MgB2 is highly flexible, while Cu oxide-based superconducting materials are less flexible than MgB2. By using highly flexible MgB2 as the core material (first material), it is possible to suppress the occurrence of cracks in the center of the superconducting wire (core material) when the superconducting wire is bent, and to secure a conductive path in the center of the superconducting wire. Examples of Cu oxide-based superconducting materials include Bi-based Cu oxides (BiBa2Cu3Ox, (Bi,Pb)2Sr2Ca2Cu3Ox) y Bi2Sr2CaCu2O y Bi2Sr2Ca2Cu3O y ), Tl2Ba2Ca2Cu3Ox, YBa2Cu3O7, HgBa2Ca2Cu3O3, etc. can be used.

[0015] Furthermore, the superconducting multi-core tube disclosed herein has multiple superconducting wires arranged inside a metal tube. Copper, iron, silver, and the like can be used as the material for the metal tube. [Examples]

[0016] (Superconducting wire) Referring to Figure 1, the superconducting wire 10 will be described. The superconducting wire 10 comprises a core material 2 made of MgB2 and a coating material 4 made of a Bi-based Cu oxide superconducting material. MgB2 is an example of the first material, and the Bi-based Cu oxide superconducting material is an example of the second material. The diameter of the superconducting wire 10 is adjusted to 6 to 129 μm. The diameter of the core material 2 is adjusted to 5 to 50 μm, and the volume ratio of the core material 2 to the coating material 4 is adjusted to core material 2:coating material 4 = 0.15:0.85 to 0.7:0.3.

[0017] A method for manufacturing a superconducting wire 10 will be described. In the following description, a method for manufacturing a superconducting wire 10 provided with a stabilizing Cu layer (not shown) on the outer periphery will be described. First, a Bi-based Cu oxide superconducting material is formed on the surface of a stabilizing Cu foil to produce a Cu foil on which a Bi-based Cu oxide superconducting thin film is formed. The Bi-based Cu oxide superconducting thin film can be produced using a sputtering method, MOD (Metal Organic Decomposition), PLD (Pulsed Laser Deposition), or the like.

[0018] Next, the Cu foil with the Bi-based Cu oxide superconducting thin film is wound around an iron dummy core. Then, after inserting the dummy core into a stabilizing Cu tube, the dummy core is removed from the stabilizing Cu tube. As a result, a space is formed inside the Cu foil with the Bi-based Cu oxide superconducting thin film. The Cu foil with the Bi-based Cu oxide superconducting thin film corresponds to a coating material 4 provided with a Cu foil on the outer periphery.

[0019] Next, the inner space of the Cu foil with the Bi-based Cu oxide superconducting thin film is filled with magnesium powder and boron powder at a molar ratio of Mg:B approximately 1:2. Then, wire drawing is performed using a die to adjust the wire to a predetermined diameter. After adjusting the diameter of the wire, the wire is heat-treated at 600 to 700 °C to form a core material 2 of MgB2 inside the Cu foil with the Bi-based Cu oxide superconducting thin film. Thus, the superconducting wire 10 provided with a stabilizing Cu layer on the outer periphery is formed. Note that instead of the stabilizing Cu, an iron tube or a silver tube may be used. In this case, a superconducting wire 10 provided with a protective layer of an iron tube or a stabilizing Ag layer on the outer periphery can be formed.

[0020] Note that the above manufacturing method is a method for generating MgB2 (core material 2) inside the coating material 4 (Bi-based Cu oxide superconducting material), and can be evaluated as an in-situ method. The superconducting wire 10 can also be manufactured using an ex-situ method described below.

[0021] In the ex-situ method, a dummy core material wrapped with a Cu foil with a Bi-based Cu oxide superconducting thin film is inserted into a stabilizing Cu tube. After removing the dummy core material from the stabilizing Cu tube, the inner space of the Cu foil with the Bi-based Cu oxide superconducting thin film is filled with pre-generated MgB₂ powder. Then, after wire drawing to adjust the diameter of the wire, the wire is heat-treated at 800 to 900 °C to sinter the MgB₂ powder (particles). Also in this case, a superconducting wire 10 provided with a stabilizing Cu layer on the outer periphery is formed. The MgB₂ powder can be manufactured using a diffusion method, a CVD method (Chemical Vapor Deposition), or the like.

[0022] Also, when using the ex-situ method, other manufacturing methods can be further used. Specifically, after inserting a hollow (cylindrical) dummy core material into a stabilizing Cu tube, the inside of the dummy core material is filled with MgB₂ powder, and the outside of the dummy core material is filled with Bi-based Cu oxide superconducting material powder. After removing the dummy core material, wire drawing is performed to adjust the diameter of the wire, and the wire is heat-treated at 800 to 900 °C to sinter the MgB₂ powder (particles) and the Bi-based Cu oxide superconducting material powder (particles). This manufacturing method is useful when the volume ratio of the Bi-based Cu oxide superconducting material (coating material 4) in the superconducting wire 10 is high.

[0023] Next, we will explain the simulation results regarding the preferred volume ratio of the core material 2 and the covering material 4 in the superconducting wire 10. In the superconducting wire 10, since the material of the covering material 4 is a Bi-based Cu oxide superconducting material, cracks may occur in the covering material 4 when the superconducting wire 10 is bent. When cracks occur in the covering material 4, current does not flow in the cracked area, and the amount of current per unit cross-sectional area of ​​the superconducting wire 10 decreases. In other words, when cracks occur in the covering material 4, the effective Tc (apparent critical temperature) of the superconducting wire 10 decreases. When the superconducting wire 10 is used as a winding for a transformer core, the superconducting wire 10 may be bent by 180°. In this case, there is a high possibility that cracks will occur in the covering material 4. Therefore, considering the effects of cracks etc. that occur in the covering material 4 when the superconducting wire 10 is bent 180° (a decrease in the apparent critical temperature Tc'), we simulated the volume ratio of the covering material 4 to the superconducting wire 10 and Tc'.

[0024] The simulation results showed that when the volume ratio of the covering material 4 to the superconducting wire 10 was 10 Vol%, Tc' was 46 K; when it was 20 Vol%, Tc' was 53 K; when it was 30 Vol%, Tc' was 60 K; when it was 40 Vol%, Tc' was 64 K; when it was 50 Vol%, Tc' was 70 K; when it was 60 Vol%, Tc' was 76 K; when it was 70 Vol%, Tc' was 79 K; when it was 80 Vol%, Tc' was 72 K; when it was 85 Vol%, Tc' was 60 K; and when it was 90 Vol%, Tc' was 45 K. Furthermore, when the volume ratio of coating material 4 is 0 Vol% (no coating material 4, core material 2 only), Tc is 39K, and when the volume ratio of coating material 4 is 100 Vol% (no core material 2, coating material 4 only), Tc is 110K.

[0025] When liquid hydrogen (boiling point: 20K) is used as the refrigerant for the superconducting wire 10, considering the degree of freedom in the arrangement of the refrigerant and the superconducting wire 10, it is preferable that the Tc of the superconducting wire 10 be 40°C or higher than the temperature of the liquid hydrogen (i.e., Tc: 60K or higher). Therefore, as a result of the above simulation, it is preferable that the volume ratio of the coating material 4 to the superconducting wire 10 be 30 vol% to 85 vol%. In particular, it was confirmed that when the volume ratio of the coating material 4 to the superconducting wire 10 is 50 vol% to 80 vol%, the Tc is 70K or higher. For this reason, it is particularly preferable that the volume ratio of the coating material 4 to the superconducting wire 10 be 50 vol% to 80 vol%.

[0026] (Superconducting multicore tube) Referring to Figure 2, the superconducting multi-core tube 14 will be described. The superconducting multi-core tube 14 has multiple superconducting wires 10 arranged inside a hollow iron tube 12. The iron tube 12 is an example of a metal tube. When manufacturing the superconducting multi-core tube 14, the superconducting wires 10 (intermediate products) are inserted into the iron tube 12 during the manufacturing process of the superconducting wires 10. Specifically, after inserting the superconducting wires 10 (intermediate products) into the iron tube 12, the diameter of the wires is adjusted by drawing, and then the wires are heat-treated at a predetermined temperature. Instead of the iron tube 12, stabilized Cu tubes, iron tubes, etc., may be used. Also, when inserting the superconducting wires 10 (intermediate products) into the iron tube 12, stabilized Cu powder, resin, etc., may be filled into the gaps between the superconducting wires 10 (intermediate products).

[0027] In the above embodiment, a superconducting wire 10 in which the core material 2 is made of MgB2 and the covering material 4 is made of a Bi-based Cu oxide superconducting material, and a superconducting multi-core tube 14 having a plurality of superconducting wires 10 were described. However, the technology disclosed herein can also be applied to a superconducting wire in which the core material 2 is made of a Bi-based Cu oxide superconducting material and the covering material 4 is made of MgB2. In addition, other Cu oxide superconducting materials can be used instead of the Bi-based Cu oxide superconducting material. The important point is that one of the core material 2 and the covering material 4 is made of MgB2 and the other is made of a Cu oxide superconducting material.

[0028] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. Furthermore, the technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of Symbols]

[0029] 2: Core material 4: Covering material 10: Superconducting wires 14: Superconducting multicore tube

Claims

1. A core material formed from a first material having superconducting properties, It comprises a covering material which is formed of a second material different from the first material, has superconducting properties, and covers the periphery of the core material, One of the first and second materials is MgB ２ And, A superconducting wire in which the other of the first and second materials is a Cu oxide-based superconducting material.

2. The first material is MgB ２ The superconducting wire according to claim 1, wherein the second material is a Cu oxide-based superconducting material.

3. The superconducting wire according to claim 1 or 2, wherein the volume ratio of Cu oxide-based superconducting material to the superconducting wire is 30 vol% or more.

4. A superconducting multi-core tube in which a plurality of superconducting wires according to claim 1 or 2 are arranged inside a hollow metal tube.

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