JP2006228797A - Permanent current switch using magnesium diboride and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a superconductivity laminate for a switch which is compact and excellent in thermal stability in a permanent current switch using a magnesium diboride and to provide a method of manufacturing it. <P>SOLUTION: The permanent current switch includes the superconductivity laminate including a magnesium diboride superconductor and a metal cover which covers it and laminated and integrated so that a plurality of layers of a flat angle or a tape shape superconductivity wire material in which the part of the superconductor is exposed are laminated in a circuit. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a permanent current switch, a superconducting laminate, and a superconducting laminate using magnesium diboride (MgB ₂ ), which can obtain high superconducting characteristics by using a superconductor exhibiting superconductivity in an environment below a critical temperature. It relates to a manufacturing method.

  Specifically, the present invention is applied to devices such as a nuclear magnetic resonance analyzer, a medical magnetic resonance diagnostic device, and a superconducting power storage device.

  For example, superconducting magnets used for levitation railway or nuclear magnetic resonance imaging (MRI) are used in a permanent current mode in which a constant current continues to flow for a long time. The permanent current mode is a state where the electric circuit of the superconducting magnet is closed and the current is confined.

  The permanent current switch (hereinafter, “Persistent Current Switch” is abbreviated as “PCS” as appropriate) is an important component having a function capable of opening and closing the superconducting magnet to or from the permanent current mode as described above. In the permanent current mode, the current in the closed loop electric circuit of the superconducting magnet is not attenuated and continues to flow semipermanently, so that the superconducting magnet can maintain a constant magnetic field.

  As such a PCS, a thermal type is widely adopted in which a superconducting wire is forced to be normally conducted by the heat generated by a built-in heater, and a switch is opened / closed so as to attenuate a current in a permanent current mode.

  There are two ways to operate the superconducting magnet: a method in which a current is always supplied from the power source to the coil, and a permanent current switch and a superconducting coil are connected in parallel to the power source, and after excitation, the coil is disconnected from the power source by this permanent current switch. There is a method of shifting to the permanent current mode. A general operation method in the permanent current mode using the permanent current switch will be described with reference to FIG. As shown in FIG. 6, a short-circuit switch 2 is provided between the terminals of the superconducting coil 1. This short-circuit switch 2 is called a permanent current switch, and a superconductor is usually used in order to reduce the resistance at the time of a short circuit. In a switch using a superconductor, for example, in a state where resistance is generated by setting the temperature of the superconductor to a critical temperature or higher by heating with a heater, current is passed from the power source 3 to the rated current value to the superconducting coil 1 to perform excitation.

  Next, if the heater heating is stopped and the switch 2 is moved to the superconducting state and then a short circuit is performed, even if the excitation power supply is removed, a constant current is continued. In this way, operation in the permanent current mode becomes possible.

The permanent current switch generally requires the following characteristics.
(1) The resistance when ON is 0 or very small value close to 0.
(2) The resistance at OFF is large.
(3) The necessary current can flow stably.
(4) Do not make a normal conducting transition except when necessary.

  For a superconducting magnet using a conventional alloy or compound superconducting wire, a permanent current switch using NbTi wire is generally used. In this case, the temperature for shifting to the permanent current mode is about 9 K, and it cannot be used at a higher temperature. The low critical temperature means that even if a small amount of disturbance energy enters the permanent current switch, the superconducting wire rises above the critical temperature, which may cause a rapid normal conduction transition, that is, quenching. When quenching, the current of the superconducting coil is lost, so there is a problem that the operation as a magnet has to be stopped once.

  In addition, a copper-coated wire is usually used for the NbTi wire, but a copper-nickel alloy is used as a stabilizing material because the permanent current switch needs to increase the electrical resistance when OFF. However, since the resistance of the copper-nickel alloy itself is not so large, a wire rod of about 30 to 60 m is used to produce a permanent current switch and is wound without induction. Although the resistance is increased by winding such a long wire, there is a problem that the permanent current switch is enlarged.

Soon after entering the 21st century, it was discovered that MgB ₂ exhibits superconductivity, as reported in Nature 410, 63-64 (2001). The typical material properties of MgB ₂ are described below.

  First, the critical temperature is 39K. This is the highest value among conventional metallic superconducting materials. Therefore, when a permanent current switch is configured using this material, it can be turned on at 39K or lower and turned off at 39K or higher. Since the critical temperature is higher than other metal-based superconducting materials, the stability margin is increased, and therefore, a highly reliable permanent current switch that hardly causes quenching can be realized.

The critical magnetic field at 0T is about 20T. This belongs to a higher class than conventional metal superconducting materials. In the conventional permanent current switch, the critical magnetic field may be low, and there is a restriction that the permanent current switch is placed away from the superconducting coil in a place with a small magnetic field, or a magnetic shield is installed outside the permanent current switch. there were. However, when a permanent current switch is configured using MgB ₂ , the above-described restriction is eliminated, and the configuration of the permanent current switch is free.

Many studies have been made on the production of MgB _{2 as} a wire. Among these, it has been found that the MgB ₂ wire can obtain a practical critical current density only by machining. This property is completely different from conventional superconducting wires that do not exhibit superconducting phenomena without heat treatment. By using this property, (b) the manufacturing process can be shortened and (b) the range of metal coating materials can be expanded. (C) Improvement of coil winding and design flexibility can be realized. Therefore, it is considered that the cost can be greatly reduced. It is extremely important to increase the density of the superconducting powder for high performance. For this reason, a metal with high hardness is applied to the covering material.

Moreover, since it is also possible to manufacture a wire rod by the “powder in tube method” most suitable for industrialization, it is positioned as an extremely attractive material. However, the allowable bending strain of MgB ₂ wire is about 1.2%, which is inferior to NbTi wire. Therefore, making the permanent current switch using MgB ₂ wire material, applying the conventional winding method, there is a problem that increase in size.

Patent Document 1 describes that a metal tube having a Vickers hardness of 80 or more is filled with MgB ₂ powder and processed into a wire. In Patent Document 2, it is described that MgB ₂ powder or MgB ₂ compact is packed between CuNi alloys, surface-reduced, and then subjected to sintering heat treatment to obtain a filament. Furthermore, Patent Document 3 describes a superconductor in which MgB ₂ produced by heat treatment is enclosed in a metal tube such as Cu. Patent Document 4 describes that MgB ₂ powder is filled in an iron pipe, and the powder is subjected to heat treatment and drawing to reduce the surface.

JP 2003-31057 A JP 2002-352648 A JP 2003-89516 A JP 2002-151387 A

In order to solve the above-mentioned problems and realize a permanent current switch having high stability and high reliability, studies on improving the performance of NbTi and MgB ₂ and improving the bending characteristics of MgB ₂ are earnest. Has been done. However, at present, these technical problems cannot be cleared.

  An object of the present invention is to provide a permanent current switch, a superconducting laminate, and a method for manufacturing them, which are compact and excellent in thermal stability.

  In order to achieve the above-mentioned problems, the present inventors have found a novel permanent current switch and a manufacturing method thereof. The means will be described below.

The object can be achieved by a permanent current switch configured by stacking MgB ₂ superconducting wires coated with metal in one direction.

The MgB ₂ superconducting wire can be achieved by a permanent current switch having a flat or tape shape. Further, it can be achieved by a permanent current switch using a wire having a Vickers hardness of 50 or more at room temperature of the metal coating material of the MgB ₂ superconducting wire. Further, this can be achieved by a permanent current switch in which the length of the MgB ₂ superconducting wire is 10 m or less.

Furthermore, the step of removing the metal coating material end of the MgB ₂ superconducting wire to expose the superconducting core, the step of stacking so that the exposed portions of the superconducting wire are in contact with each other, and the gap between the stacked superconducting wires This can be achieved by applying a method for manufacturing a permanent current switch including a step of arranging an MgB ₂ superconductor.

Since MgB ₂ has a higher critical temperature than other metal-based superconducting materials, the stability margin is increased, so that it is possible to realize a reliable permanent current switch that is less likely to cause a rapid normal conduction transition, that is, a quench phenomenon. .

  In addition, there is an extremely great effect that the manufacturing cost can be greatly reduced by not performing the heat treatment process for the superconducting wire during the manufacturing process for producing the superconducting wire.

Furthermore, ₂ to 30% by volume of copper, indium, tin, lead, iron, aluminum, magnesium, titanium, silicon oxide, silicon carbide, and silicon nitride are mixed alone or in combination with MgB _{2 in the} filling powder. When added, the critical current density is improved. In particular, it is more effective to reduce the particle size to the nano order.

  Hereinafter, representative embodiments of the present invention will be described in an organized manner.

  The first embodiment of the present invention includes a magnesium diboride superconductor and a metal coating covering the magnesium diboride, and a plurality of long superconducting wires such as a rectangular or tape shape in which a part of the superconductor is exposed. A permanent current switch having a superconducting laminate laminated and integrated in a circuit so that the superconductors exposed by adjacent wires are stacked and connected. As the permanent current switch, in addition to the superconducting laminate, a thermal or magnetic means for transferring the superconducting state of the laminate is required.

  In this permanent current switch, it is desirable that a gap between adjacent wires of the laminate is filled with a magnesium diboride superconductor to ensure connection between the superconductors. In addition, it is desirable to use a metal coating material having a Vickers hardness of 50 or more at room temperature, such as SUS.

  Furthermore, it is desirable to provide means for pressurizing and holding the laminate of the magnesium diboride superconducting wire. By this means, the connection between the exposed superconductors can be secured, and the alignment and parallelism between the superconducting wires can be contributed. Further, the permanent current switch can be reduced in size by setting the length of the magnesium diboride superconducting wire to 10 m or less, particularly 3 m or less, and further 2 m or less.

  The present invention includes a magnesium diboride superconductor and a metal coating covering the magnesium superboride, and a plurality of long superconducting wires such as a rectangular shape or a tape shape in which a part of the superconductor is exposed are stacked and adjacent to each other. The present invention provides a superconducting laminate that is laminated and integrated so that the superconductor with exposed wire is connected.

  Filling the gap between adjacent wires of the laminate with a magnesium diboride superconductor, and using a metal coating material of the magnesium diboride superconducting wire having a Vickers hardness of 50 or more at room temperature. As described above, a means for holding the laminate under pressure is provided, and the length of the magnesium diboride superconducting wire is 10 m or less.

  The present invention also includes a step of removing a part of the metal coating material of the superconducting wire including the magnesium diboride superconductor and the metal coating material covering the superconductor core to expose the superconducting core, and an exposed portion of the superconducting wire. And a step of laminating a plurality of superconducting wires so that they are in contact with each other. In this method, it is desirable to perform a step of filling the gap between the superconducting wires stacked with the magnesium diboride superconductor to ensure the connection between the superconductors. Further, it is desirable to maintain the connection between the superconductors of the superconducting wire or the alignment between the wires by means of holding the superconducting laminate under pressure.

  The permanent current switch according to the present invention is effective when applied to devices such as a nuclear magnetic resonance analyzer, a medical magnetic resonance diagnostic device, and a superconducting power storage device.

  As described above, according to the present invention, a permanent current switch having the following characteristics can be provided.

The characteristics of the class current switch are summarized in the picture using the superconducting laminate according to the present invention as follows.
(1) The electrical resistance is remarkably small when the permanent current switch is ON.
(2) The electrical resistance is sufficiently large when the permanent current switch is OFF.
(3) A sufficient current can be stably supplied to the permanent current switch.
(4) In the permanent current switch, there is no normal conduction transition except when necessary.

  According to the present invention, a permanent current switch is manufactured using a magnesium diboride superconducting wire having a high critical temperature, a critical magnetic field, and a high critical current, and the superconducting coil is short-circuited with this, thereby providing a compact and highly reliable permanent current. A switch can be provided.

In the present invention, MgB ₂ superconducting wire is used for the permanent current switch. The MgB ₂ superconductor can relatively easily obtain a wire having a high critical temperature, a critical magnetic field and a critical current. Therefore, in particular, if the MgB ₂ superconducting wire is used for the permanent current switch, a necessary current can be flowed stably, and a permanent current switch with high reliability and operability can be provided.

Examples of the powder filled when producing the MgB ₂ superconducting wire include superconducting powder and precursor powder. Further, as described above, it is effective to add a third element typified by 2 to 30% by volume of metal fine powder.

  The heat treatment temperature for superconducting powder synthesis in the present invention is in the range of 200 to 1200 ° C. Further, if necessary, heat treatment is performed with nitrogen gas, argon gas, hydrogen gas, oxygen gas or the like alone or in combination. Further, if necessary, heat treatment is performed while pressurizing at a pressure higher than atmospheric pressure.

The MgB ₂ superconducting wire used for the permanent current switch in the present invention is manufactured by a method in which a raw material powder of MgB ₂ superconductor is filled in a sheath made of a stabilizing material and plastic processing is performed, so-called powder-in-tube method. Can be used. In such a method, it is effective to use a metal having a Vickers hardness of 50 or more at room temperature as the metal coating material. Examples of high-strength metals having a Vickers hardness of 50 or more at room temperature include SUS304, SUS316, SUS310, SUS430, Hastelloy B, Hastelloy C, carbon steel, inconel, cobalt, tungsten, nickel, molybdenum, titanium, monel, aluminum There are base alloys, titanium base alloys, nickel base alloys, copper base alloys, niobium base alloys, magnesium base alloys and the like.

  The wire length necessary for manufacturing the permanent current switch is expressed by the equation (1).

Resistance at OFF x Wire cross-sectional area / Wire specific resistance --------- (1)
That is, if the specific resistance of the wire is high, the wire length of the permanent current switch can be shortened, and if the resistance is increased 10 times, the wire length can be reduced to 1/10. In general, the specific electrical resistance of metal and the Vickers hardness are in a proportional relationship. For this reason, it can be said that the above-mentioned metal having a Vickers hardness of 50 or more has a relatively high specific electric resistance.

  In the method of manufacturing a wire rod, plastic processing includes wire drawing, groove roll processing, roller die processing, isostatic pressing, rolling, and the like. In particular, a round cross-section wire, a square cross-section wire, a tape wire or the like can be obtained with high accuracy by a combination with wire drawing or rolling. However, in addition to these methods, it is possible to obtain equivalent superconducting characteristics even when using a wire produced by, for example, a thermal spraying method, a doctor blade method, a dip coating method, a spray pyrolysis method, or a jelly roll method. It is. The superconducting wire used may be either a single core wire or a multi-core wire.

  In addition to superconducting magnets, the above superconducting wires include transmission cables, current leads, MRI equipment, NMR equipment, SMES equipment, superconducting generators, superconducting motors, magnetic levitation trains, superconducting magnetic propulsion ships, superconducting transformers, superconducting fault current limiters Etc. can be used.

Any superconducting material may be used for the superconducting coil connected to the permanent current switch. For example, a metallic superconductor such as NbTi, Nb ₃ Sn, Nb ₃ Al, or V ₃ Ga, an oxide superconductor such as Bi-2212, Bi-2223, or Y-123, or MgB ₂ may be used. As a winding method, non-inductive winding is applied in order to make the generated magnetic field of the coil itself zero.

  A mechanism such as a thermal type or a magnetic type can be used as the permanent current switch mechanism. In general, a thermal switch mechanism is often used. For the heating element used for switching, a material that generates heat by electrical resistance such as manganin wire can be preferably used. When the superconducting switch is used, the heating element is connected to a power supply circuit and a means for controlling heat generation. The heating element may be provided so as to be in contact with the superconducting wire, or may be provided on the superconducting wire via an electrically insulating material.

When the permanent current switch in the present invention is brought into contact with a refrigerant such as liquid helium or gaseous helium, the heating element is covered with a heat insulating material. The thermal insulating material forms an appropriate temperature gradient between the environment for cooling, the superconducting wire and the heating element, and serves as a thermal buffer so that the heating element and the wire are not rapidly cooled. Further, the heat insulating material protects the heating element so that the superconducting wire can be sufficiently heated when the switch is turned off. For example, a resin material such as an epoxy resin, a resin compound, a silicon compound, or the like can be used as the heat insulating material. As the thermal insulating material, a material having a thermal conductivity of preferably 10 ⁻² W / cm · K to 10 ⁻⁶ W / cm · K can be used at the temperature to be used.

  Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to these.

Example 1
Using magnesium powder (Mg; purity 99%) and amorphous boron powder (B; purity 99%) as starting materials, the magnesium and boron were weighed so as to have an atomic molar ratio of 1: 2, and 10-60 Mix for minutes. Next, this mixture is heat-treated at a temperature of 700 to 1000 ° C. for 2 to 20 hours to produce a MgB ₂ superconductor. At this time, it may heat-process by applying the pressure more than atmospheric pressure. When X-ray diffraction of the obtained powder was performed, it was found that 95% or more of MgB ₂ superconductor was contained in terms of intensity ratio. Besides MgB ₂ was also included some MgO and MgB _4.

  The obtained mixed powder is filled into a stainless steel pipe having an outer diameter of 5 mm and an inner diameter of 4 mm and a circular cross-sectional shape. At this time, in addition to filling in powder form, a cylindrical or square rod produced by pressing or the like may be filled. This is drawn at a cross-sectional area reduction rate of 3 to 20% and reduced in diameter to a predetermined shape. If necessary, the cross-sectional shape of the wire is reduced to an elliptical, hexagonal, flat or round cross-sectional shape.

In this example, the diameter was reduced to a tape-shaped wire having a thickness of 0.2 to 0.5 mm and a width of 2 to 5 mm. FIG. 1 shows an example of a schematic cross-sectional view of the MgB ₂ superconducting wire 4 produced. In the MgB ₂ superconducting wire 4, an MgB ₂ superconductor 6 is filled or included in a metal coating material 5. The wire size used in the present embodiment is 0.4 mm thick and 3.7 mm wide. Here, no heat treatment such as heat treatment or annealing was performed during wire processing. Note that the thickness and width of the tape-shaped wire and the aspect ratio that is the ratio of the thickness to the width are not limited. For example, the wire shape varies from a round shape or extremely close to a round shape to a wide ultra-thin tape. There is no problem. However, a rectangular shape or a tape shape is a preferable shape because the contact area of the wire can be increased and lamination is easy.

The obtained MgB ₂ superconducting wire 4 was cut to a length of 30 mm, and only the metal coating material portion was removed from the end portions. 2A and 2B show the removal process of the metal coating 5. Removal of the metal coating 5 was 4 mm in the length direction and 3 mm in the width direction from the end of the wire. At this time, the upper surface was removed at one end and the lower surface was removed at the other end. In FIG. 3, a black portion 9 indicates that the metal coating on the upper surface has been removed, and a portion 10 indicated by a dotted line indicates the lower surface removal. In this way, the metal coating removal portion is formed on both surfaces of the end portion of the wire to expose the superconductor.

These wires are regularly laminated in one direction so that the superconductors are connected as shown in FIG. The wire contact portion from which the metal coating material has been removed has a step as shown in FIG. 2 (b), so that a gap 11 is formed at the end of the laminate, and an MgB ₂ powder or MgB ₂ compact is inserted there. To do. After insertion, the permanent current switch 7 using a wire can be produced by pressurizing the whole with a uniaxial press. Moreover, a laminated body can be pressurized and hold | maintained using a holding means from the upper and lower sides. The number of stacked layers is preferably 5 or more, particularly 7 or more and 20 or less. As the resistance value of the metal coating is higher, the number of layers can be increased.

  Next, characteristics evaluation as a permanent current switch was performed. FIG. 5 shows a conceptual diagram of the superconducting device. In the figure, the superconducting wire used for the superconducting coil is NbTi superconducting wire 8 having a diameter of 1 mm using NbTi as the superconducting material and copper as the stabilizing material. This was wound around an aluminum alloy bobbin 9 to obtain a solenoid coil having an inner diameter of 50 mmφ, an outer diameter of 76 mmφ, and a height of 79.5 mm. Insulation between the windings was performed by enamel insulation. The number of turns of the solenoid coil was 800 turns, and the central magnetic field when a current of 20 A was passed was 0.1 T.

The permanent current switch 72 was manufactured using the MgB ₂ wire as described above. Since 100 wires cut to a length of 30 mm are stacked, the total length of the wire is 3 m. The permanent current switch has a very small size. An insulating material 10 using an epoxy resin is applied on the laminated MgB ₂ superconducting wire, and a manganin wire 11 is wound thereon. Copper wires 12 and 13 are connected to the manganin wire, and an external heater power source can be connected to the copper wire.

The MgB ₂ superconducting wire 4, the NbTi superconducting wire 8, and the current lead wires 14 and 15 are connected at the superconducting wire connecting portions 16 and 17. Further, an excitation power source can be connected to the other ends of the current lead wires 14 and 15.

The structure configured as described above is immersed in liquid helium, and a current is applied to the manganin wire 11 through the copper wires 12 and 13 to heat the same, so that the portion of the MgB ₂ superconducting wire acting as a permanent current switch is normally conducted. Transition to a state (temperature of 40K or higher). In this state, a current was supplied from the power source to the solenoid coil via the current lead wires 14 and 15. When the solenoid coil was excited, heating by the manganin wire 11 was stopped, and the MgB ₂ superconducting wire 4 in the switch portion was returned to the superconducting state.

  As a result of removing the excitation power source in this state, a state of the permanent current mode was obtained. The electrical resistance of the junction at this time was 0.05 nΩ or less. On the other hand, when lead-tin solder was used for connection between the superconducting cores in the superconducting wire connecting portions 16 and 17, the measured value of the electrical resistance was about 20 nΩ. As described above, according to the present invention, the electrical resistance due to bonding can be significantly reduced.

This example was the result of study using stainless steel as the coating material for the MgB ₂ superconducting wire 4. However, for example, when a metal having a Vickers hardness of 50 or more is used as the sheath material, It was confirmed that a result was obtained.

As described above, the end of the MgB ₂ superconducting wire 4 coated with metal was removed to expose the superconducting core, and they were stacked in one direction, thereby producing a permanent current switch having excellent characteristics.

(Example 2)
A similar permanent current switch 7 was produced by changing the wire shape of the MgB ₂ superconducting wire 4 described in Example 1, and the yield as the permanent current switch was examined. In this embodiment, the wire has three shapes, round, flat, and tape. The round wire has a diameter of 1 mm, the flat wire has a thickness of 1.1 mm and a width of 2.2 mm, and the tape-like wire has a thickness of 0.45 mm and a width of 3.6 mm.

  For the yield calculation method as the permanent current switch, a permanent current switch similar to that shown in FIG. 4 was produced, and then the wiring shown in FIG. 5 was performed to measure the electrical resistance of the junction. At this time, an electrical resistance of 0.1 nΩ or less was accepted. In FIG. 5, 1 is an NbTi superconducting wire, 9 is an aluminum alloy bobbin, 10 is an insulating material, 11 is a manganin wire, 12 is a copper wire, 13 is a copper wire, 14 is a current lead wire, 15 is a current lead wire, 16 Is a superconducting wire connection part, and 17 is a superconducting wire connection part.

As a result, the yield was 75% for the round shape, but improved to 97% for the flat shape and 99% for the tape shape. As described above, it has been clarified that it is effective that the shape of the MgB ₂ superconducting wire 4 is a flat or tape shape.

The cross-sectional schematic diagram of the superconducting wire of this invention. The external appearance schematic diagram explaining the removal process of the metal coating | covering material of the superconducting wire in this invention. The expansion | deployment perspective view of the laminated body in this invention. The side perspective view which shows the superconducting laminated body for permanent current switches of this invention. The conceptual diagram of the superconducting apparatus provided with the permanent current switch using the superconducting laminated body of this invention. The circuit diagram explaining the superconducting coil with a permanent current switch.

Explanation of symbols

1 ... superconducting magnet, 2 ... permanent current switch, 3 ... Power, 4 ... superconducting wire, 5 ... metal covering material, 6 ... MgB ₂ superconductors, 8 ... NbTi superconducting wire, 9 ... aluminum alloy bobbin, 10 ... insulating material , 11 ... Manganin wire, 12 ... Copper wire, 13 ... Copper wire, 14 ... Current lead wire, 15 ... Current lead wire, 16 ... Superconducting wire connecting portion, 17 ... Superconducting wire connecting portion.

Claims (13)

  A magnesium diboride superconducting wire and a metal coating covering the same are laminated, and a plurality of long superconducting wires exposed from a part of the superconducting wire are stacked, and the above-mentioned superconducting wires exposed from adjacent wires are connected. A permanent current switch comprising a superconducting laminate laminated and integrated in a circuit.   2. The permanent current switch according to claim 1, wherein a magnesium diboride superconductor is filled in a gap between adjacent wires of the laminate.   3. The permanent current switch according to claim 1, wherein the metal coating material of the magnesium diboride superconducting wire has a Vickers hardness of 50 or more at room temperature. 4.   3. The permanent current switch according to claim 1, further comprising means for pressurizing and holding the laminate.   4. The permanent current switch according to claim 1, wherein the magnesium diboride superconducting wire has a length of 10 m or less.   A magnesium diboride superconducting wire and a metal coating for covering the same, and a plurality of long superconducting wires with a part of the magnesium diboride superconducting wire exposed, are laminated, and the superconducting wire is exposed on an adjacent wire. Superconducting laminate that is laminated and integrated so that wires are connected.   7. The superconducting laminate according to claim 6, wherein a gap between adjacent magnesium diboride superconducting wires of the laminate is filled with a magnesium diboride superconductor.   8. The superconducting laminate according to claim 6, wherein the metal coating material of the magnesium diboride superconducting wire has a Vickers hardness of 50 or more at room temperature.   The superconducting laminate according to any one of claims 6 to 8, further comprising means for pressurizing and holding the laminate.   The superconducting laminate according to any one of claims 6 to 9, wherein the magnesium diboride superconducting wire has a length of 10 m or less.   The step of removing the metal coating material end of the magnesium diboride superconducting wire to expose the superconducting core, the step of stacking so that the exposed portions of the superconducting wire are in contact with each other, and the gap between the stacked superconducting wires A method for manufacturing a permanent current switch comprising the step of arranging a magnesium boride superconductor.   A step of removing a part of the metal covering material of the superconducting wire including a magnesium diboride superconductor and a metal covering material covering the superconductor core to expose the superconducting core part, and an exposed part of the superconducting wire contacting each other And a step of laminating a plurality of superconducting wires. A method for producing a superconducting laminate, comprising:   The method of manufacturing a superconducting laminate according to claim 12, further comprising a step of filling a gap between the laminated superconducting wires with a magnesium diboride superconductor.

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