Mgb2 superconducting wire, method of manufacturing mgb2 superconducting wire, superconducting coil, and magnetism generation device

Abstract

To provide a MgB2 superconducting wire material in which the thickness of a barrier layer covering a MgB2 core is highly uniform, defects of MgB2 filaments are reduced, and which is suitable for thinning and lengthening, and to provide a manufacturing method of the MgB2 superconducting wire material for manufacturing the same, a superconducting coil using the same, and a magnetism generation device including the same.SOLUTION: The MgB2 superconducting wire 200 includes a MgB2 material 210, a MgB2 filament 220 in which a MgB2 core 221 is covered with a barrier layer 222, and a metallic sheath 230, and in the center filament 220, the thickness of the barrier layer 222 on the outer peripheral side of the wire is three times or less the thickness of the barrier layer 222 on the center side of the wire. The method of manufacturing the central superconducting wire includes a step of forming a single-core wire, a step of forming an embedded material, a step of performing area reduction processing, and a step of performing heat treatment, and the embedded material is formed by arranging the single-core wire in a recess formed on an outer surface of a MgB2 material or embedding a spacer in a gap.SELECTED DRAWING: Figure 2A

Description

[Technical Field]

[0001] The present invention relates to a magnesium diboride (MgB2) superconducting wire, a method for manufacturing the MgB2 superconducting wire, a superconducting coil, and a magnetic field generator. [Background technology]

[0002] The advantage of superconducting wire is that it allows current to flow with zero resistance. Traditionally, niobium titanium (NbTi) wire has been widely used for superconducting coils. Superconducting coils using NbTi wire have a low operating temperature of approximately 4K, so they are cooled with liquid helium. However, in recent years, there have been concerns about a tight supply and demand of helium. Therefore, efforts are being made to develop and produce wires of superconductors with high critical temperatures that do not require liquid helium.

[0003] Known superconductors with high critical temperatures include niobium tin (Nb3Sn), yttrium (Y)-based oxides, bismuth (Bi)-based oxides, and magnesium diboride (MgB2). MgB2 has the highest critical temperature of any metal-based superconductor, at approximately 39 K. In addition, the raw materials are relatively easy to obtain, and it has lightweight and excellent mechanical properties. For this reason, MgB2 superconducting wires using MgB2 are expected to be used in a variety of applications.

[0004] The Powder In Tube (PIT) method is a common method for manufacturing MgB2 superconducting wire. In the PIT method, raw material powder is filled into a metal tube, which is then subjected to wire drawing. There are two types of PIT methods: ex situ and in situ. In the ex situ method, pre-synthesized MgB2 is filled into a metal tube. In the in situ method, magnesium powder and boron powder are filled into a metal tube, which is then heat-treated to produce MgB2.

[0005] Most of the superconducting wires currently in practical use have a multi-core structure with many superconducting filaments to stabilize the magnetic flux, etc. Multi-core wires are also considered important for MgB2 superconducting wires due to magnetic and metallurgical factors. When manufacturing MgB2 superconducting wires, an embedded single-core wire is produced in which MgB2 raw material is filled in a metal tube that is less reactive with the raw material. Then, a multi-core embedded material is produced by embedding multiple embedded single-core wires in a metal sheath tube that forms a metal sheath. MgB2 superconducting wires are manufactured by subjecting the embedded material to area reduction processing and heat treatment.

[0006] Patent Document 1 discloses that there is a problem in that breakage occurs inside the multi-filament superconducting wire when the multi-filament superconducting wire is lengthened. Furthermore, a technology for providing a multi-filament superconducting wire that does not break, in response to this problem, is disclosed. In Patent Document 1, a hard metal portion is provided between adjacent single-filament superconducting wires. The hard metal portion is made of a metal that is harder than the metal that constitutes the stabilizing phase. By providing the hard metal portion, plastic deformation of the stabilizing phase due to wire drawing is suppressed, and damage to the barrier phase due to inhomogeneous deformation is prevented. [Prior art documents] [Non-patent literature]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-126950 Summary of the Invention [Problem to be solved by the invention]

[0008] There is a demand for further thinning and lengthening of MgB2 superconducting wires. To achieve this, it is desirable to more reliably prevent damage to the barrier layer covering the MgB2 core of each MgB2 filament forming a multi-core wire structure. In Patent Document 1, a plate-shaped hard metal member is placed between adjacent single-core superconducting wires. However, this technology leaves room for improvement in terms of the arrangement of the member, the degree of prevention of non-uniform deformation, and the uniformity of the thickness of the barrier layer.

[0009] Therefore, the present invention aims to provide an MgB2 superconducting wire that is suitable for thinning and elongating by increasing the uniformity of the thickness of the barrier layer that covers the MgB2 core in the MgB2 filament, thereby reducing defects in the MgB2 filament, a manufacturing method for the MgB2 superconducting wire, a superconducting coil using the same, and a magnetic generator equipped with the same. [Means for solving the problem]

[0010] In order to solve the above problems, the MgB2 superconducting wire of the present invention is an MgB2 superconducting wire with a multi-core structure including a central material arranged at the center of the wire, a plurality of MgB2 filaments arranged to surround the central material, each MgB2 filament having an MgB2 core covered with a barrier layer, and a metal sheath covering the plurality of MgB2 filaments, wherein the thickness of the barrier layer on the outer periphery of the MgB2 superconducting wire is three times or less the thickness of the barrier layer on the center side of the MgB2 superconducting wire.

[0011] Furthermore, a method for manufacturing an MgB2 superconducting wire according to the present invention is a method for manufacturing an MgB2 superconducting wire having a multi-core wire structure including a central material disposed at the center of the wire, multiple MgB2 filaments disposed so as to surround the central material, each having an MgB2 core covered with a barrier layer, and a metal sheath covering the multiple MgB2 filaments, and includes the steps of: forming multiple embedded single-core wires in which a metal barrier tube is filled with raw material MgB2; forming a recess on the outer surface of the central material; arranging the multiple embedded single-core wires in the recess, incorporating the central material and the multiple embedded single-core wires into a metal sheath tube that forms the metal sheath, and incorporating a spacer into a gap surrounded by the multiple embedded single-core wires and the inner surface of the metal sheath tube, reducing the area of ​​the embedded material; and heat-treating the area-reduced embedded material to generate MgB2. Alternatively, the method includes the steps of forming a plurality of embedded single-core wires in which raw material for the MgB2 core is filled in a metal barrier tube that forms the barrier layer, incorporating the central material and the plurality of embedded single-core wires into a metal sheath tube that forms the metal sheath, and incorporating an inner spacer into a gap surrounded by the plurality of embedded single-core wires and the outer surface of the central material, and incorporating an outer spacer into a gap surrounded by the plurality of embedded single-core wires and the inner surface of the metal sheath tube to form an embedded material, reducing the area of ​​the embedded material, and heat-treating the area-reduced embedded material to generate MgB2.

[0012] A superconducting coil according to the present invention is formed by winding the MgB2 superconducting wire. A magnetic field generator according to the present invention includes a superconducting coil wound with the MgB2 superconducting wire. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide an MgB2 superconducting wire having a highly uniform thickness of a barrier layer covering an MgB2 core, reduced defects in the MgB2 filaments, and suitable for thinning and elongating, a method for manufacturing the MgB2 superconducting wire, a superconducting coil using the same, and a magnetic generator equipped with the same. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view schematically showing an example of a precursor of an MgB2 superconducting wire according to an embodiment of the present invention. FIG. [Figure 2A] 1 is a cross-sectional view schematically showing an example of an MgB2 superconducting wire according to an embodiment of the present invention. [Figure 2B] FIG. 2B is an enlarged view of a part of FIG. 2A (the part enclosed by the thick dashed line). [Figure 3] 1 is a cross-sectional view schematically showing an example of a precursor of an MgB2 superconducting wire according to an embodiment of the present invention. FIG. [Figure 4] 1 is a cross-sectional view schematically showing an example of an MgB2 superconducting wire according to an embodiment of the present invention. [Figure 5] 1 is a cross-sectional view schematically showing an MgB2 superconducting wire according to a comparative example. [Figure 6] 1 is a diagram schematically illustrating an example of a superconducting coil using an MgB2 superconducting wire according to an embodiment of the present invention. [Figure 7] 1 is a cross-sectional view schematically illustrating an example of a magnetic field generating device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an MgB2 superconducting wire according to one embodiment of the present invention, a manufacturing method for the MgB2 superconducting wire, a superconducting coil using the same, and a magnetic field generator including the same will be described with reference to the drawings. In the following drawings, common components are designated by the same reference numerals, and duplicated explanations will be omitted.

[0016] The MgB2 superconducting wire according to this embodiment is manufactured by the powder-in-tube (PIT) method. The MgB2 superconducting wire according to this embodiment may be manufactured by either the ex situ method or the in situ method, but is preferably manufactured by the in situ method. The in situ method allows dense MgB2 to be produced by heat treatment at a relatively low temperature. In the following description, a configuration for manufacturing the MgB2 superconducting wire by the in situ method will be exemplified.

[0017] Fig. 1 is a cross-sectional view schematically showing an example of a precursor of an MgB2 superconducting wire according to an embodiment of the present invention. Fig. 1 shows the cross-sectional structure of an embedded material 100, which is an example of a precursor of the MgB2 superconducting wire according to this embodiment before heat treatment. The MgB2 superconducting wire according to this embodiment can be manufactured as a multi-filamentary wire structure composed of multiple MgB2 filaments by subjecting the embedded material 100, which is the precursor, to area reduction processing and heat treatment.

[0018] 1, the embedded material 100 is composed of a central material 110 arranged at the center of the embedded material 100, a plurality of embedded single-core wires 120 arranged so as to surround the central material 110, a metal sheath tube 130 covering the plurality of embedded single-core wires 120, and a spacer 140. The embedded material 100 shown in FIG. 1 is configured to use a central material 110 having recesses 110a formed on its outer surface and to incorporate spacers 140 into the gaps between the embedded single-core wires 120 in order to improve the uniformity of the thickness of the barrier layer covering the MgB2 core of the MgB2 superconducting wire.

[0019] The embedded material 100 is formed as a multi-billet by incorporating a central material 110, multiple embedded single-core wires 120, and a spacer 140 into a metal sheath tube 130. The central material 110 is incorporated at the center of the embedded material 100. The embedded single-core wires 120 are incorporated around the central material 110 so as to be positioned in recesses 110a formed in the outer surface of the central material 110. The spacer 140 is incorporated into gaps between the embedded single-core wires 120 that are located radially outward of the embedded material 100 and are surrounded by the multiple embedded single-core wires 120 and the inner surface of the metal sheath tube 130.

[0020] 1, the central material 110 is formed by an inner central material 111 disposed at the center of the built-in material 100 and an outer central material 112 disposed toward the outside of the center of the built-in material 100. The inner central material 111 is provided as a cylindrical billet. The outer central material 112 is provided in a tubular shape. The outer central material 112 is disposed so as to accommodate the inner central material 111 therein and cover the outer surface of the inner central material 111.

[0021] The inner core material 111 can be made of copper, such as oxygen-free copper, phosphorus-deoxidized copper, or tough-pitch copper, or iron. The inner core material 111 is preferably made of oxygen-free copper. The inner core material 111 made of copper functions as a stabilizer that stabilizes superconductivity in the MgB2 superconducting wire after area reduction and heat treatment. The stabilizer is made of a good conductor with low electrical resistivity and high thermal conductivity. When the stabilizer is incorporated, the superconductivity is thermally and electromagnetically stabilized, suppressing quench propagation and thermal runaway when cooling is lost, etc. When the inner core material 111 is made of iron, it is preferable to arrange the stabilizer on the outside of the metal sheath tube 130.

[0022] The outer core material 112 can be made of, for example, iron. The outer core material 112 made of iron constitutes part of the base material supporting the MgB2 filaments and the like in the MgB2 superconducting wire after area reduction and heat treatment. The outer core material 112 is preferably made of a material harder than the material of the inner core material 111. The high hardness of the outer core material 112 improves the workability of the embedded single-core wire 120, particularly the side that contacts the core material 110, during area reduction of the embedded material 100. After heat treatment of the embedded material 100, dense MgB2 can be produced. As a result, an MgB2 superconducting wire with excellent superconducting properties such as critical current density can be obtained.

[0023] The embedded single-core wire 120 is composed of raw material powder 121 that constitutes the superconductor and a metal barrier tube 122 that accommodates the raw material powder 121. The embedded single-core wire 120 is formed by filling the raw material powder 121 into the metal barrier tube 122. The embedded single-core wire 120 can be formed as a single billet that is a round wire, and subjected to area reduction processing to adjust it to a predetermined wire diameter.

[0024] The embedded single-core wire 120 forms an MgB2 filament in an MgB2 superconducting wire, with an MgB2 core covered with a barrier layer. When producing the embedded material 100, a plurality of embedded single-core wires 120 are incorporated into a metal sheath tube 130. By incorporating a plurality of embedded single-core wires 120, the embedded material 100 is produced to form a multi-core wire structure composed of a plurality of filaments.

[0025] When manufacturing an MgB2 superconducting wire by the in situ method, the raw material powder 121 is prepared as a mixed powder of magnesium powder and boron powder. When the built-in material 100 is heat-treated at approximately 600°C or higher, the raw material powder 121 generates MgB2 through a reaction between magnesium and boron. The raw material powder 121 becomes an MgB2 core capable of superconducting transition in the MgB2 superconducting wire after area reduction processing and heat treatment.

[0026] The metal barrier tube 122 functions as a barrier material that prevents reactions between magnesium and components other than boron during heat treatment to produce MgB2. The use of a barrier material prevents the formation of heterogeneous phases due to reactions between magnesium and copper, etc., and also makes it difficult for the amount of MgB2 produced to decrease. The metal barrier tube 122 can be made of iron, niobium, tantalum, alloys of these, etc. In the MgB2 superconducting wire after area reduction processing and heat treatment, the metal barrier tube 122 serves as a barrier layer that covers the MgB2 core, and together with the MgB2 core, constitutes an MgB2 filament.

[0027] 1, the metal sheath tube 130 is formed by an inner sheath tube 131 arranged radially inside the embedded material 100, and an outer sheath tube 132 arranged radially outside the embedded material 100. The inner sheath tube 131 houses a plurality of embedded single-core wires 120 therein and is arranged so as to surround the plurality of embedded single-core wires 120. The outer sheath tube 132 houses the inner sheath tube 131 therein and is arranged so as to cover the outer surface of the inner sheath tube 131.

[0028] The inner sheath tube 131 can be formed of iron, niobium, tantalum, an alloy thereof, or the like. The inner sheath tube 131 is preferably formed of a material harder than the copper used for the inner core material 111. If the inner sheath tube 131 has high hardness, a large processing force can be applied to the embedded single-core wire 120 from the radial outside of the embedded material 100 during the area reduction process of the embedded material 100. Dense MgB2 can be generated after the embedded material 100 is heat treated. Therefore, an MgB2 superconducting wire with excellent superconducting properties such as critical current density can be manufactured. The inner sheath tube 131 is preferably formed of the same material as the metal barrier tube 122. If the inner sheath tube 131 is made of the same material as the metal barrier tube 122, variations in the direction and magnitude of the processing force acting on the metal barrier tube 122 during the area reduction process of the embedded material 100 are less likely to occur. Furthermore, thermal strain due to differences in thermal expansion coefficients is less likely to occur in the base material of the MgB2 superconducting wire.

[0029] The outer sheath tube 132 can be made of a nickel-copper alloy, stainless steel, low-carbon steel, or the like. Examples of nickel-copper alloys include Monel, which has a Cu content of 20% by mass or more and 35% by mass or less. Examples of low-carbon steel include carbon steel, which has a carbon content of 0.01% by mass or more and less than 0.25% by mass. The outer sheath tube 132 is preferably made of a material harder than the material of the inner sheath tube 131. High hardness of the outer sheath tube 132 allows a large processing force to be applied from the outside in the radial direction of the embedded material 100 during the surface reduction process of the embedded material 100. The surface reduction of each embedded single-core wire 120 can be achieved with high uniformity. This allows the production of an MgB2 superconducting wire with excellent superconducting properties, such as critical current density.

[0030] 1, a plurality of recesses 110a are formed on the outer surface of the central material 110. The recesses 110a are formed in a concave shape such that a portion of the outer surface of the central material 110 in the circumferential direction is recessed toward the center of the central material 110. The plurality of recesses 110a are formed at predetermined intervals around the entire circumference of the central material 110. The recesses 110a are formed in the shape of a groove in the longitudinal direction of the embedded material 100.

[0031] The recess 110a is a position where the embedded single-core wire 120 is placed. A plurality of recesses 110a are provided, the number corresponding to the number of embedded single-core wires 120 to be placed around the central material 110. The embedded single-core wires 120 are embedded along the recesses 110a when the embedded material 100 is produced. In a cross-sectional view, at least a portion of the embedded single-core wire 120 is housed in the recess 110a. By fitting along the recess 110a, the gap around the embedded single-core wire 120 is reduced.

[0032] The depth of the recess 110a is preferably 50% or less of the diameter of the embedded single-core wire 120, and more preferably 25% or less. Also, it is preferably 5% or more of the diameter of the embedded single-core wire 120. When the depth is 25% or less of the diameter of the embedded single-core wire 120, it is relatively easy to form the recess 110a by drawing. Also, when the depth is 5% or more of the diameter of the embedded single-core wire 120, the gap around the embedded single-core wire 120 can be significantly reduced.

[0033] The depth of the recess 110a can be calculated as the difference between the maximum and minimum radii of the central material 110. The maximum radius of the central material 110 can be measured as the radius of the central material 110 at the side edge of the recess 110a or the radius of the central material 110 at a portion where the recess 110a is not formed. The minimum radius of the central material 110 can be measured as the radius of the central material 110 at the bottom of the recess 110a, which is the shortest distance from the center of the central material 110.

[0034] 1, among the gaps around the embedded single-core wires 120, spacers 140 are embedded in gaps surrounded by multiple embedded single-core wires 120 and the inner surface of the metal sheath tube 130. The spacers 140 are provided as columnar billets so as to extend in the longitudinal direction of the embedded material 100. A plurality of spacers 140 are embedded in each embedded material 100 so as to fill each of the gaps aligned in the circumferential direction of the embedded material 100.

[0035] The spacer 140 can be formed of iron, niobium, tantalum, alloys thereof, or metals having a similar hardness. The spacer 140 is preferably formed of a material harder than the copper used for the inner core material 111. A high hardness of the spacer 140 allows a large processing force to be applied to the embedded single-core wire 120 from the radially outer side of the embedded material 100 during the area reduction process of the embedded material 100. After the embedded material 100 is heat-treated, dense MgB2 can be produced. Therefore, an MgB2 superconducting wire with excellent superconducting properties, such as critical current density, can be manufactured. The spacer 140 is preferably formed of the same material as the metal barrier tube 122. Using the same material as the metal barrier tube 122 reduces the likelihood of variations in the direction and magnitude of the processing force applied to the metal barrier tube 122 and the core material 110 during the area reduction process of the embedded material 100. Furthermore, thermal strain due to differences in thermal expansion coefficients is less likely to occur in the base material of the MgB2 superconducting wire.

[0036] When the embedded material 100 is produced, the spacer 140 is inserted into a gap surrounded by the multiple embedded single-core wires 120 and the inner surface of the metal sheath tube 130. The insertion of the spacer 140 reduces the gap around the embedded single-core wire 120. The spacer 140 constitutes a part of the base material that supports the MgB2 filaments and the like in the MgB2 superconducting wire after the area reduction processing and heat treatment.

[0037] In FIG. 1, the spacer 140 has a triangular cross-sectional shape. However, the cross-sectional shape of the spacer 140 may be other shapes such as a sector shape, a trapezoid shape, a rectangle shape, or a circle shape. The cross-sectional shape of the spacer 140 is preferably similar to the general shape of the gap. With such a shape, the gap around the embedded single-core wire 120 can be significantly reduced. The spacer 140 may be configured so that multiple pieces of the spacer 140 are inserted into one gap.

[0038] Generally, if the embedded material has many gaps inside, the processing force during area reduction tends to cause non-uniform deformation of the MgB2 filament. This can cause the embedded single-core wire embedded in the embedded material to shift position, or the embedded single-core wire to undergo non-uniform plastic deformation toward the gaps. As a result, the thickness of the metal barrier tube that forms the embedded single-core wire also becomes non-uniform, making the barrier tube more susceptible to tearing. If the metal barrier tube tears, it loses its function of isolating the raw material powder, which can lead to the problem of the formation of a heterogeneous phase due to a reaction between magnesium and components other than boron during heat treatment after area reduction. Furthermore, the formation of the heterogeneous phase consumes magnesium, resulting in a problem of a decrease in the amount of MgB2 produced.

[0039] One possible method for preventing the metal barrier tube from breaking is to increase its thickness. However, if the thickness of the metal barrier tube is increased, the core area ratio per cross section of the thinned MgB2 superconducting wire becomes smaller, which may make it difficult to obtain necessary superconducting properties such as critical current density. In addition, there is a risk that the symmetry of the MgB2 core portion may be reduced or breakage may occur, making it difficult to form a healthy MgB2 core, making it difficult to lengthen the MgB2 superconducting wire.

[0040] In contrast to this, when a recess 110a is formed on the outer surface of the central material 110, the gap around the embedded single-core wire 120 is reduced on the radially inner side of the embedded material 100. Furthermore, when a spacer 140 is incorporated into the gap surrounded by the multiple embedded single-core wires 120 and the inner surface of the metal sheath tube 130, the gap around the embedded single-core wire 120 on the radially outer side of the embedded material 100 is reduced.

[0041] As a result, in the initial stage of the area reduction process, the adhesion between the components that make up the embedded material 100 is increased, reducing variations in the direction and magnitude of the processing force acting on the embedded single-core wire 120. This suppresses misalignment of the embedded single-core wire 120 and uneven plastic deformation of the embedded single-core wire 120 toward the gaps. Therefore, during the area reduction process of the embedded material 100, the thickness of the metal barrier tube 122 of the embedded single-core wire 120 becomes more uniform, making the metal barrier tube 122 less likely to break.

[0042] Fig. 2A is a cross-sectional view schematically showing an example of an MgB2 superconducting wire according to an embodiment of the present invention. Fig. 2B is an enlarged view of a portion of Fig. 2A (the portion within the thick dashed frame indicated by symbol A). Figs. 2A and 2B show the cross-sectional structure of an MgB2 superconducting wire 200 obtained by subjecting the embedded material 100 having the structure shown in Fig. 1 to area reduction processing and heat treatment.

[0043] 2A, the MgB2 superconducting wire 200 includes a core material 210 disposed at the center of the wire, a plurality of MgB2 filaments 220 disposed so as to surround the core material 210, a metal sheath 230 covering the plurality of MgB2 filaments 220, and a filling layer 240. The MgB2 superconducting wire 200 shown in FIG. 2 includes the core material 210 derived from the core material 110 having recesses 110a formed on the outer surface thereof, and the filling layer 240 derived from the spacer 140.

[0044] 2A , the core material 210 is formed by an inner core material 211 disposed at the center of the wire and an outer core material 212 disposed toward the outside of the center of the wire. The outer core material 212 is provided so as to cover the inner core material 211. The inner core material 211 is formed by subjecting the inner core material 111 constituting the embedded material 100 to area-reducing processing. The outer core material 212 is formed by subjecting the outer core material 112 constituting the embedded material 100 to area-reducing processing. The inner core material 211, which is made of copper, functions as a stabilizer that stabilizes superconductivity. The outer core material 212 constitutes a part of the base material of the MgB2 superconducting wire 200.

[0045] 2A, the MgB2 filament 220 is formed by an MgB2 core 221 capable of superconducting transition and a barrier layer 222 covering the MgB2 core 221. The MgB2 filament 220 has a structure in which the MgB2 core 221 is covered with the barrier layer 222. The MgB2 core 221 is formed by subjecting the raw material powder 121 constituting the built-in material 100 to area reduction processing and heat treatment. The barrier layer 222 is formed by subjecting the metal barrier tube 122 constituting the built-in material 100 to area reduction processing. The barrier layer 222 constitutes a part of the base material of the MgB2 superconducting wire 200.

[0046] 2A, the metal sheath 230 is formed by an inner sheath 231 arranged radially inside the wire and an outer sheath 232 arranged radially outside the wire. The inner sheath 231 is provided so as to cover the multiple MgB2 filaments 220. The inner sheath 231 is formed by subjecting the inner sheath tube 131 that constitutes the embedded material 100 to area reduction processing. The outer sheath 232 is provided so as to cover the inner sheath 231. The outer sheath 232 is formed by subjecting the outer sheath tube 132 that constitutes the embedded material 100 to area reduction processing. The metal sheath 230 contributes to protecting the MgB2 core 221, etc. and dispersing current.

[0047] As shown in Fig. 1, the embedded material 100 has a plurality of recesses 110a formed on the outer surface of the core material 110, and a spacer 140 is embedded in a gap surrounded by a plurality of embedded single-core wires 120 and the inner surface of the metal sheath tube 130. Therefore, as shown in Fig. 2, the MgB2 superconducting wire 200 has recess traces 210a formed on the outer surface of the core material 210 by subjecting the recesses 110a to area-reducing processing. Also, a filling layer 240 formed by subjecting the spacer 140 to area-reducing processing is included in the space surrounded by a plurality of MgB2 filaments 220 and the inner surface of the metal sheath 230. The filling layer 240 constitutes a part of the base material of the MgB2 superconducting wire 200.

[0048] When manufacturing the MgB₂ superconducting wire 200 using such an embedded material 100 as a precursor, the thickness of the barrier layer 222 on the outer peripheral side of the wire can be limited to 3 times or less the thickness of the barrier layer 222 on the central side of the wire over the longitudinal direction of the wire. That is, the thickness of the barrier layer 222 on the central side of the wire can be ensured to be 1 / 3 or more the thickness of the barrier layer 222 on the outer peripheral side of the wire over the longitudinal direction of the wire.

[0049] Figure 2B is an enlarged view of a part of Figure 2A, that is, part A surrounded by the thick dashed line frame in Figure 2A. In this Figure 2B, when the thickness of the barrier layer 222 on the outer peripheral side of the wire is Tout and the thickness of the barrier layer 222 on the central side of the wire is Tin, the thickness Tout is thicker than the thickness Tin, and the ratio of the thickness is 3 times or less. That is, 1 < Tout / Tin ≤ 3. In Figure 2B, the dashed-dotted line indicates the diameter line C passing through the center of the wire and the center of the MgB₂ filament.

[0050] It is preferable that the thickness of the barrier layer 222 on the outer peripheral side of the MgB₂ filament 220 in the MgB₂ superconducting wire 200 is 1 time or more and 3 times or less (1 ≤ Tout / Tin ≤ 3) the thickness of the barrier layer 222 on the central side of the MgB₂ superconducting wire 200, and more preferably 1.5 times or more and 3 times or less (1.5 ≤ Tout / Tin ≤ 3). That is, it is preferable that the thickness of the barrier layer 222 on the central side of the wire is 1 / 3 or more and 1 time or less (1 / 3 ≤ Tout / Tin ≤ 1) the thickness of the barrier layer 222 on the outer peripheral side of the wire, and more preferably 1 / 3 or more and 2 / 3 or less (1 / 3 ≤ Tout / Tin ≤ 2 / 3). With such a thickness, the thickness of the barrier layer 222 becomes more uniform, and the area ratio of the MgB₂ core 221 per cross-section of the wire increases. On the other hand, when the thickness on the outer peripheral side exceeds 3 times the thickness on the central side, the thickness on the central side becomes less than 1 / 3 of the thickness on the outer peripheral side, and breakage of the metal barrier tube 122 or the barrier layer 222, asymmetry or disconnection of the MgB₂ filament 220 are likely to occur, and it is difficult to obtain good superconducting characteristics.

[0051] The ratio between the thickness of the barrier layer 222 on the outer periphery of the MgB2 superconducting wire 200 and the thickness of the barrier layer 222 on the center side of the MgB2 superconducting wire 200 can be adjusted more precisely by changing the shape and size of the recess 110a, the shape, size and number of the spacers 140, the shape and size of the embedded single-core wire 120, the area reduction rate of the area reduction process applied to the embedded material 100, the thickness of the metal barrier tube 122 that constitutes the embedded single-core wire 120, etc. in the embedded material 100 having the structure shown in Figure 1.

[0052] The thickness of the barrier layer can be measured by cutting out any cross section of the MgB2 superconducting wire and targeting any MgB2 filament. For any MgB2 filament observed in any cross section, the thickness of the barrier layer at the center of the wire can be compared with the thickness of the barrier layer at the outer periphery of the wire on a diameter line (see the dashed line in Figure 2B) passing through the center of the wire and the center of the MgB2 filament. The ratio of the barrier layer thickness at the outer periphery to the barrier layer thickness at the center can be calculated as the average value of the measurement results for any MgB2 filament. The number of samples measured is, for example, 5 or more.

[0053] The thickness of the barrier layer is measured after cutting out an arbitrary cross section and polishing it. If the interface of the barrier layer is unclear, the thickness can be measured after performing a process to clarify the interface. An example of a process to clarify the interface is chemical etching. It is also effective to distinguish the interface based on component analysis. An example of component analysis is energy dispersive X-ray spectroscopy (EDX).

[0054] With such an MgB2 superconducting wire 200, the thickness of the metal barrier tube 122 and the barrier layer 222 becomes more uniform, and the metal barrier tube 122 and the barrier layer 222 are less likely to break, making it possible to use a thinner metal barrier tube 122 when producing the embedded material 100. Using a thinner metal barrier tube 122 can reduce the material costs and processing costs of the embedded single-core wire 120 and the embedded material 100. Furthermore, the area ratio of the MgB2 core per cross section of the wire increases, making it possible to improve superconducting properties such as critical current density.

[0055] Furthermore, the uniformity of the thickness of the metal barrier tube 122 and the barrier layer 222 is increased, making the metal barrier tube 122 and the barrier layer 222 less likely to break, which reduces the likelihood of heterophase formation and inhibition of MgB2 production during heat treatment of the embedded material 100. Furthermore, geometrically asymmetric MgB2 filaments and breakage of MgB2 filaments are less likely to occur. This prevents a decrease in critical current (Ic), making it easier to lengthen the wire.

[0056] Therefore, the structure of the MgB2 superconducting wire 200 is a multi-core wire structure composed of multiple MgB2 filaments each having an MgB2 core, and the thickness of the barrier layer in the circumferential direction of the MgB2 filaments is highly uniform, reducing defects in the MgB2 core and resulting in an MgB2 superconducting wire suitable for thinning and elongating. Because the barrier layer can be made thinner, the area occupancy rate of MgB2 on the cross section of the wire and the critical current density can be improved. A structure in which the recess 110a is formed on the outer surface of the central material 110 makes it easy to arrange the embedded single-core wire 120 relative to the central material 110, facilitating the production of elongated wire.

[0057] Fig. 3 is a cross-sectional view schematically showing an example of a precursor of an MgB2 superconducting wire according to an embodiment of the present invention. Fig. 3 shows the cross-sectional structure of an embedded material 300, which is an example of a precursor of the MgB2 superconducting wire according to this embodiment before heat treatment. The MgB2 superconducting wire according to this embodiment can be manufactured as a multi-filamentary wire structure composed of multiple MgB2 filaments by subjecting the embedded material 300, which is the precursor, to area reduction processing and heat treatment.

[0058] As shown in Fig. 3, the embedded material 300 is composed of a central material 310 arranged at the center of the embedded material 300, a plurality of embedded single-core wires 320 arranged so as to surround the central material 310, a metal sheath tube 330 covering the plurality of embedded single-core wires 320, and a spacer 340. The embedded material 300 shown in Fig. 3 is configured such that the spacers 340 are incorporated into the gaps between the embedded single-core wires 320 in order to improve the uniformity of the thickness of the barrier layer covering the MgB2 core of the MgB2 superconducting wire. The spacers 340 are incorporated into the gaps on both the outer periphery and the center of the embedded material 300.

[0059] The embedded material 300 is formed as a multi-billet by incorporating a central material 310, multiple embedded single-core wires 320, and a spacer 340 into a metal sheath tube 330. The central material 310 is incorporated at the center of the embedded material 300. The embedded single-core wires 320 are incorporated around the central material 310. The spacers 340 are incorporated into gaps between the embedded single-core wires 320 that are located on the radially outer side of the embedded material 300 and are surrounded by the multiple embedded single-core wires 320 and the inner surface of the metal sheath tube 330, and into gaps that are located on the radially central side of the embedded material 300 and are surrounded by the multiple embedded single-core wires 320 and the outer surface of the central material 310.

[0060] In FIG. 3 , the central material 310 is formed by an inner central material 311 disposed at the center of the built-in material 300 and an outer central material 312 disposed toward the outside of the center of the built-in material 300. The inner central material 311 is provided as a cylindrical billet. The outer central material 312 is provided in a tubular shape. The outer central material 312 accommodates the inner central material 311 and is disposed so as to cover the outer surface of the inner central material 311. The inner central material 311 and the outer central material 312 can be formed using the same materials and methods as the inner central material 111 and the outer central material 112 described above. The central material 310 can be one without recesses formed on its outer surface.

[0061] The embedded single-core wire 320 is formed by raw material powder 321 that constitutes the superconductor and a metal barrier tube 322 that accommodates the raw material powder 321. The embedded single-core wire 320 is formed by filling the raw material powder 321 into the metal barrier tube 322. The embedded single-core wire 320 can be formed as a single billet that is a round wire, and subjected to area reduction processing to adjust it to a predetermined wire diameter.

[0062] The embedded single-core wire 320 forms an MgB2 filament in an MgB2 superconducting wire, with the MgB2 core covered with a barrier layer. When producing the embedded material 300, a plurality of embedded single-core wires 320 are incorporated inside a metal sheath tube 330. By incorporating a plurality of embedded single-core wires 320, the embedded material 300 is produced to form a multi-core wire structure made up of a plurality of MgB2 filaments. The raw material powder 321 and the metal barrier tube 322 can be formed using the same materials and methods as those for the raw material powder 121 and the metal barrier tube 122 described above.

[0063] 3, the metal sheath tube 330 is formed by an inner sheath tube 331 arranged radially inside the embedded material 300, and an outer sheath tube 332 arranged radially outside the embedded material 300. The inner sheath tube 331 houses a plurality of embedded single-core wires 320 therein and is arranged so as to surround the plurality of embedded single-core wires 320. The outer sheath tube 332 houses the inner sheath tube 331 therein and is arranged so as to cover the outer surface of the inner sheath tube 331. The inner sheath tube 331 and the outer sheath tube 332 can be formed using the same materials and methods as the inner sheath tube 131 and the outer sheath tube 132 described above.

[0064] 3, an inner spacer 341 is incorporated into a gap surrounded by the plurality of embedded single-core wires 320 and the outer surface of the central material 310. An outer spacer 342 is incorporated into a gap surrounded by the plurality of embedded single-core wires 320 and the inner surface of the metal sheath tube 330. The inner spacer 341 and the outer spacer 342 are provided as columnar billets so as to extend in the longitudinal direction of the embedded material 300. A plurality of inner spacers 341 and outer spacers 342 are incorporated into one embedded material 300 so as to fill each gap lined up in the circumferential direction of the embedded material 300.

[0065] The inner spacer 341 and the outer spacer 342 can be formed of iron, niobium, tantalum, alloys thereof, or metals with equivalent hardness. The inner spacer 341 and the outer spacer 342 are preferably formed of a material harder than the copper used for the inner core material 311. High hardness allows a processing force to be applied to the embedded single-core wire 320 from the radial outside toward the center of the embedded material 300 during the area reduction process of the embedded material 300. After the embedded material 300 is heat-treated, dense MgB2 can be produced. Therefore, an MgB2 superconducting wire with excellent superconducting properties, such as critical current density, can be manufactured. The inner spacer 341 and the outer spacer 342 are preferably formed of the same material as the metal barrier tube 322. Using the same material as the metal barrier tube 322 reduces the variation in the direction and magnitude of the processing force acting on the metal barrier tube 322 during the area reduction process of the embedded material 300. Furthermore, thermal strain due to differences in thermal expansion coefficients is less likely to occur in the base material of the MgB2 superconducting wire.

[0066] The inner spacer 341 is inserted into a gap surrounded by the multiple embedded single-core wires 320 and the outer surface of the central material 310 when the embedded material 300 is produced. The outer spacer 342 is inserted into a gap surrounded by the multiple embedded single-core wires 320 and the inner surface of the metal sheath tube 330 when the embedded material 300 is produced. The insertion of the inner spacer 341 and the outer spacer 342 reduces the gap around the embedded single-core wire 320. The inner spacer 341 and the outer spacer 342 form part of the base material that supports the MgB2 filaments and the like in the MgB2 superconducting wire after area reduction processing and heat treatment.

[0067] In FIG. 3, the inner spacer 341 and the outer spacer 342 have a circular cross-sectional shape. However, the cross-sectional shape of the inner spacer 341 and the outer spacer 342 can also be other shapes, such as a triangular shape, a sector shape, a trapezoidal shape, or a rectangular shape. The cross-sectional shape of the inner spacer 341 and the outer spacer 342 is preferably a shape similar to the general shape of the gap or a shape with an outline parallel to the wall surface of the gap. Such a shape can significantly reduce the gap around the embedded single-core wire 320. The inner spacer 341 and the outer spacer 342 may be configured so that multiple pieces are inserted into one gap.

[0068] When an inner spacer 341 is incorporated into the gap surrounded by the multiple embedded single-core wires 320 and the outer surface of the central material 310, the gap around the embedded single-core wires 320 is reduced on the radially inner side of the embedded material 300. Furthermore, when an outer spacer 342 is incorporated into the gap surrounded by the multiple embedded single-core wires 320 and the inner surface of the metal sheath tube 330, the gap around the embedded single-core wires 320 is reduced on the radially outer side of the embedded material 300.

[0069] As a result, in the initial stage of the area reduction process, the adhesion between the components that make up the embedded material 300 is increased, reducing variations in the direction and magnitude of the processing force acting on the embedded single-core wire 320. This suppresses misalignment of the embedded single-core wire 320 and uneven plastic deformation of the embedded single-core wire 320 toward the gaps. Therefore, during the area reduction process of the embedded material 300, the thickness of the metal barrier tube 322 of the embedded single-core wire 320 becomes more uniform, making the metal barrier tube 322 less likely to break.

[0070] Fig. 4 is a cross-sectional view schematically illustrating an example of an MgB2 superconducting wire according to an embodiment of the present invention. Fig. 4 shows the cross-sectional structure of an MgB2 superconducting wire 400 obtained by subjecting the embedded material 300 having the structure shown in Fig. 3 to area reduction processing and heat treatment.

[0071] 4, MgB2 superconducting wire 400 includes a core material 410 disposed at the center of the wire, a plurality of MgB2 filaments 420 arranged to surround core material 410, a metal sheath 430 covering the plurality of MgB2 filaments 420, and a filling layer 440. MgB2 superconducting wire 400 shown in FIG. 4 includes, as filling layer 440 derived from spacer 340, inner filling layer 441 derived from inner spacer 341 and outer filling layer 442 derived from outer spacer 342.

[0072] In FIG. 4 , the core material 410 is formed by an inner core material 411 disposed at the center of the wire and an outer core material 412 disposed toward the outside of the center of the wire. The outer core material 412 is provided so as to cover the inner core material 411. The inner core material 411 is formed by subjecting the inner core material 311 constituting the embedded material 300 to area-reducing processing. The outer core material 412 is formed by subjecting the outer core material 312 constituting the embedded material 300 to area-reducing processing. The inner core material 411, which is made of copper, functions as a stabilizer that stabilizes superconductivity. The outer core material 412 constitutes a part of the base material of the MgB2 superconducting wire 400.

[0073] 4, the MgB2 filament 420 is formed by an MgB2 core 421 capable of superconducting transition and a barrier layer 422 covering the MgB2 core 421. The MgB2 filament 420 has a structure in which the MgB2 core 421 is covered with the barrier layer 422. The MgB2 core 421 is formed by subjecting the raw material powder 321 constituting the built-in material 300 to area reduction processing and heat treatment. The barrier layer 422 is formed by subjecting the metal barrier tube 322 constituting the built-in material 300 to area reduction processing. The barrier layer 422 constitutes a part of the base material of the MgB2 superconducting wire 400.

[0074] 4, the metal sheath 430 is formed by an inner sheath 431 arranged radially inside the wire and an outer sheath 432 arranged radially outside the wire. The inner sheath 431 is provided so as to cover the multiple MgB2 filaments 420. The inner sheath 431 is formed by subjecting the inner sheath tube 331 that constitutes the embedded material 300 to area reduction processing. The outer sheath 432 is provided so as to cover the inner sheath 431. The outer sheath 432 is formed by subjecting the outer sheath tube 332 that constitutes the embedded material 300 to area reduction processing. The metal sheath 430 contributes to protecting the MgB2 core 421, etc. and dispersing current.

[0075] As shown in Fig. 3, the embedded material 300 has an inner spacer 341 embedded in a gap surrounded by the plurality of embedded single-core wires 320 and the outer surface of the core material 310, and an outer spacer 342 embedded in a gap surrounded by the plurality of embedded single-core wires 320 and the inner surface of the metal sheath tube 330. Therefore, as shown in Fig. 4, the MgB2 superconducting wire 400 has an inner filling layer 441 formed by subjecting the inner spacer 341 to area reduction processing in a space surrounded by the plurality of MgB2 filaments 420 and the outer surface of the core material 410. Also, the MgB2 superconducting wire 400 has an outer filling layer 442 formed by subjecting the outer spacer 342 to area reduction processing in a space surrounded by the plurality of MgB2 filaments 420 and the inner surface of the metal sheath 430. The inner filling layer 441 and the outer filling layer 442 constitute part of the base material of the MgB2 superconducting wire 400.

[0076] When an MgB2 superconducting wire 400 is manufactured using such an embedded material 300 as a precursor, the thickness of the barrier layer 422 on the outer periphery of the wire can be limited to no more than three times the thickness of the barrier layer 422 on the center side of the wire along the longitudinal direction of the wire. In other words, the thickness of the barrier layer 422 on the center side of the wire can be ensured to be at least one-third the thickness of the barrier layer 422 on the outer periphery of the wire along the longitudinal direction of the wire.

[0077] In the MgB2 filament 420, the thickness of the barrier layer 422 on the outer periphery of the MgB2 superconducting wire 400 is preferably 1.5 to 3 times the thickness of the barrier layer 422 on the center side of the MgB2 superconducting wire 400. That is, the thickness of the barrier layer 422 on the center side of the wire is preferably 1 / 3 to 2 / 3 the thickness of the barrier layer 422 on the outer periphery of the wire. With such a thickness, the thickness of the barrier layer 422 becomes more uniform, and the area ratio of the MgB2 core 421 per cross section of the wire becomes high. On the other hand, if the thickness on the outer periphery exceeds three times the thickness on the center side, the thickness on the center side becomes less than 1 / 3 of the thickness on the outer periphery, which may easily cause breakage of the metal barrier tube 322 or the barrier layer 422, or asymmetry or breakage of the MgB2 filament 420, making it difficult to obtain good superconducting properties.

[0078] The ratio between the thickness of the barrier layer 422 on the outer periphery of the MgB2 superconducting wire 400 and the thickness of the barrier layer 422 on the center side of the MgB2 superconducting wire 400 can be adjusted more precisely by changing the shape, size and number of the spacers 340, the shape and size of the embedded single-core wire 320, the area reduction rate of the area reduction processing applied to the embedded material 300, the thickness of the metal barrier tube 322 that constitutes the embedded single-core wire 320, etc. in the embedded material 300 having the structure shown in Figure 3.

[0079] With such an MgB2 superconducting wire 400, the thickness of the metal barrier tube 322 and the barrier layer 422 becomes more uniform, and the metal barrier tube 322 and the barrier layer 422 are less likely to break, which makes it possible to use a thinner metal barrier tube 322 when producing the embedded material 300. Using a thinner metal barrier tube 322 makes it possible to reduce the material costs and processing costs of the embedded single-core wire 320 and the embedded material 300. Furthermore, the area ratio of the MgB2 core per cross section of the wire increases, which makes it possible to improve superconducting properties such as critical current density.

[0080] Furthermore, the thickness uniformity of the metal barrier tube 322 and the barrier layer 422 is improved, making them less likely to break, and therefore the formation of different phases and the inhibition of MgB2 production are less likely to occur during heat treatment of the embedded material 300. Furthermore, geometrically asymmetric MgB2 filaments and breakage of MgB2 filaments are less likely to occur. As a result, the critical current (Ic) is less likely to decrease, making it easier to lengthen the wire.

[0081] Therefore, the structure of the MgB2 superconducting wire 400 is a multi-core wire structure composed of multiple MgB2 filaments each having an MgB2 core. The thickness of the barrier layer in the circumferential direction of the MgB2 filaments is highly uniform, reducing defects in the MgB2 core and resulting in an MgB2 superconducting wire suitable for thinning and elongating. Because the barrier layer can be made thinner, the area occupancy rate of MgB2 on the cross section of the wire and the critical current density can be improved. Compared to a case where the single-core wire 120 is placed in a recess 110a formed on the outer surface of the central material 110, the manufacturing process can be simplified because there is no need to form the recess 110a.

[0082] Fig. 5 is a cross-sectional view schematically showing an MgB2 superconducting wire according to a comparative example. Fig. 5 shows the cross-sectional structure of an MgB2 superconducting wire 500 obtained when the thickness of the barrier tube constituting the embedded material is increased. The MgB2 superconducting wire 500 is obtained by increasing the thickness of the metal barrier tube in the embedded material 100 having the structure shown in Fig. 1 to 1.5 times that of the metal barrier tube 122 shown in Fig. 1.

[0083] 5, MgB2 superconducting wire 500 includes a core material 510 disposed at the center of the wire, a plurality of MgB2 filaments 520 disposed so as to surround core material 510, a metal sheath 530 covering the plurality of MgB2 filaments 520, and a filling layer 540. MgB2 superconducting wire 500 shown in FIG. 5 includes core material 510 derived from a core material having recesses formed on the outer surface, and filling layer 540 derived from a spacer.

[0084] 5, the core material 510 is formed of an inner core material 511 disposed at the center of the wire and an outer core material 512 disposed toward the outside of the center of the wire. The MgB2 filament 520 is formed of an MgB2 core 521 capable of superconducting transition and a barrier layer 522 covering the MgB2 core 521. The metal sheath 530 is formed of an inner sheath 531 disposed radially inside the wire and an outer sheath 532 disposed radially outside the wire.

[0085] As shown in Fig. 1, the embedded material 100 has a plurality of recesses 110a formed on the outer surface of the core material 110, and a spacer 140 is embedded in a gap surrounded by a plurality of embedded single-core wires 120 and the inner surface of the metal sheath tube 130. Therefore, as shown in Fig. 5, the MgB2 superconducting wire 500 has recess traces 510a formed on the outer surface of the core material 510 by subjecting the recesses 110a to area-reducing processing. Also, a filling layer 540 formed by subjecting the spacer 140 to area-reducing processing is included in the space surrounded by a plurality of MgB2 filaments 520 and the inner surface of the metal sheath 530. The filling layer 540 constitutes a part of the base material of the MgB2 superconducting wire 500.

[0086] When an MgB2 superconducting wire 500 is manufactured using such an embedded material 100 as a precursor, the thickness of the barrier layer 522 on the outer periphery of the wire is not limited to three times the thickness of the barrier layer 522 on the center side of the wire along the longitudinal direction of the wire because a thick metal barrier tube is used. In other words, the thickness of the barrier layer 522 on the center side of the wire is not ensured to be one-third or more of the thickness of the barrier layer 522 on the outer periphery of the wire along the longitudinal direction of the wire.

[0087] If the ratio of the thickness of the barrier layer on the outer periphery of the wire to the thickness of the barrier layer on the center side of the wire is too large, the area ratio of the MgB2 core per cross section of the wire will be small, resulting in deterioration of superconducting properties such as critical current density. Therefore, in order to improve the uniformity of the thickness of the barrier layer, it is preferable to adjust the thickness of the metal barrier tube constituting the embedded single-core wire so that the thickness of the barrier layer on the outer periphery of the wire is no more than three times the thickness of the barrier layer on the center side of the wire along the longitudinal direction of the wire.

[0088] Next, a method for manufacturing the MgB2 superconducting wire will be described. In the following description, a method for manufacturing the MgB2 superconducting wire shown in Fig. 2A and Fig. 4 by an in situ method will be exemplified.

[0089] The manufacturing method of the MgB2 superconducting wire according to this embodiment includes a preparation step of forming a built-in single-core wire, an assembly step of forming a precursor built-in material, a surface-reducing process of reducing the surface area of ​​the built-in material, and a heat treatment step of heat-treating the surface-reduced built-in material to generate MgB2. When manufacturing the MgB2 superconducting wire 200 shown in FIG. 2A, a recess forming process of forming a recess on the outer surface of the core material is performed before the assembly step, and a spacer is prepared. When manufacturing the MgB2 superconducting wire 400 shown in FIG. 4, an inner spacer and an outer spacer are prepared before the assembly step.

[0090] In the preparation process, a plurality of embedded single-core wires are formed by filling the MgB2 core raw material into a metal barrier tube that forms the barrier layer. The MgB2 core raw material is prepared by weighing out magnesium powder and boron powder, which are the raw materials for MgB2, so that the molar ratio of Mg to B is approximately 1:2, and then grinding and mixing them.

[0091] If necessary, a carbon source can be added to the raw material for the MgB2 core to perform element substitution in MgB2. Adding a carbon source allows some of the boron atoms in MgB2 to be replaced with carbon atoms during the heat treatment that produces MgB2. The introduction of carbon atoms as impurities into the superconductor improves the critical current and critical magnetic field of the MgB2 superconducting wire. Examples of carbon sources that can be used include inorganic carbon compounds such as B4C and SiC, hydrocarbons such as benzene, naphthalene, coronene, and anthracene, organic acids such as stearic acid, and magnesium salts of organic acids.

[0092] The raw material for the MgB2 core is preferably handled in a non-oxidizing atmosphere such as an inert gas atmosphere such as nitrogen or argon, or a vacuum atmosphere. The oxygen content in the atmosphere is preferably 10 ppm or less. The moisture content in the atmosphere is preferably 10 ppm or less. The raw material powder for the MgB2 core can be pulverized and mixed using a ball mill, planetary mixer, V-type mixer, mortar, etc.

[0093] The raw materials for the MgB2 core can also be pulverized and mixed by mechanical milling. In mechanical milling, the particles constituting the powder are vigorously collided with media such as zirconia balls or the inner wall of a pot, and pulverized and mixed while being subjected to intense processing. In mechanical milling, it is preferable to apply a collision energy that is not sufficient to clearly produce MgB2. The production of MgB2 can be confirmed by the substantial presence or absence of an MgB2 peak in powder X-ray diffraction.

[0094] Mechanical milling allows boron particles to penetrate magnesium particles, resulting in a powder structure with a high degree of mixing, in which boron is finely dispersed and encapsulated in a magnesium matrix. Heat treatment of this powder structure results in the formation of a core with many bonds between MgB2 particles and few voids. This results in a high critical current density.

[0095] As the embedded single-core wire, a plurality of wires are produced to form a multi-core wire structure. The embedded single-core wire, in which the raw material for the MgB2 core is filled in a metal barrier tube, can be adjusted to a predetermined wire diameter by performing area reduction processing. The area reduction processing of the embedded single-core wire can be performed with an appropriate number of passes. The area reduction processing is preferably performed with an area reduction rate of 8 to 12% per pass.

[0096] The area reduction process for the embedded single-core wire can be performed by drawing, extrusion, swaging, cassette roll processing, groove roll processing, etc. As the processing device, a draw bench, a hydrostatic extruder, a wire drawing machine, a swager, a cassette roller die, a groove roll, etc. can be used.

[0097] Furthermore, when manufacturing an MgB2 superconducting wire having the structure shown in FIG. 2A, a recess formation step is performed before the assembly step, in which a recess is formed on the outer surface of the core material used to manufacture the assembly material. The method for forming the recess is not particularly limited. For example, a method of drawing the core material using a predetermined die can be used to form the recess. The die can be one having a die hole with a convex portion formed in a shape that is the inverse of the concave portion. Furthermore, when the length of the assembly material is short, cutting, grinding, beam machining, electric discharge machining, etching, etc. can also be used.

[0098] Next, in the assembly process, the assembled single-core wire, the core material, etc. are assembled into a metal sheath tube to form an assembled material, which is a precursor of the MgB2 superconducting wire. The core material can be a cylindrical inner core material covered by a tubular outer core material. The metal sheath tube can be a double tube in which an outer sheath tube for forming the outer sheath is placed over an inner sheath tube for forming the inner sheath.

[0099] When manufacturing the MgB2 superconducting wire 200 shown in Figure 2A, in the assembly process, multiple embedded single-core wires 120 are placed in a recess 110a formed on the outer surface of the central material 110, and the central material 110 and the multiple embedded single-core wires 120 are assembled into a metal sheath tube 130, and a spacer 140 is assembled into the gap surrounded by the multiple embedded single-core wires 120 and the inner surface of the metal sheath tube 130.

[0100] When manufacturing the MgB2 superconducting wire 400 shown in Figure 4, in the assembly process, the central material 310 and multiple embedded single-core wires 320 are assembled into a metal sheath tube 330, an inner spacer 341 is assembled in the gap surrounded by the multiple embedded single-core wires 320 and the outer surface of the central material 310, and an outer spacer 342 is assembled in the gap surrounded by the multiple embedded single-core wires 320 and the inner surface of the metal sheath tube 330.

[0101] Next, in the area reduction process, the built-in material with the built-in single-core wire etc. is subjected to area reduction. By performing area reduction on the built-in material at a predetermined area reduction rate, the built-in material is drawn to be elongated and thinned to a predetermined wire diameter. The area reduction of the built-in material can be performed with an appropriate number of passes. The area reduction of the built-in material can be performed, for example, so that the wire diameter is 0.3 to 2.0 mm.

[0102] The area reduction of the embedded material can be performed by drawing, extrusion, swaging, cassette roll processing, groove roll processing, etc. Processing equipment that can be used includes a draw bench, hydrostatic extruder, wire drawing machine, swager, cassette roller die, groove roll, etc.

[0103] During the area reduction process, the built-in material may be annealed to remove residual stress and soften the processed structure. Furthermore, the built-in material processed to a predetermined wire diameter may be twisted into a spiral. Twisting into a spiral reduces the coupling current between the cores. The twist pitch may be, for example, 10 to 100 mm.

[0104] In the heat treatment process, the thinned embedded material is heat treated to generate MgB2. When the embedded material is heat treated at a specified temperature, the magnesium and boron filled in the barrier tube react to form an MgB2 core.

[0105] The heat treatment atmosphere is preferably a non-oxidizing atmosphere such as an inert gas atmosphere of nitrogen, argon, or the like, or a vacuum atmosphere. The oxygen content in the atmosphere is preferably 10 ppm or less. The moisture content in the atmosphere is preferably 10 ppm or less. The heat treatment may be carried out after or before the thinned embedded material is wound into a coil or the like. If the material is wound before the heat treatment, an insulating coating can be applied using a heat-resistant insulating material such as glass fiber.

[0106] The heat treatment temperature is, for example, 550 to 800°C, preferably 560 to 700°C, and more preferably 580 to 620°C. The higher the heat treatment temperature is above 550°C, the more easily the reaction of generating MgB2 by magnesium diffusion progresses. Also, the lower the heat treatment temperature is below 800°C, the more difficult it is for MgB2 to grow into grains, increasing the density of grain boundaries that act as pinning centers and resulting in a high critical current density.

[0107] The heat treatment time is, for example, several tens of minutes to several tens of hours, preferably 2 to 16 hours, and more preferably 3 to 12 hours. When the heat treatment time is 3 hours or more, it is usually possible to generate a sufficient amount of MgB2. When the heat treatment time is 12 hours or less, the grain growth of MgB2 is difficult, so the density of grain boundaries that become pinning centers increases, and a high critical current density can be obtained.

[0108] The above-described manufacturing method for MgB2 superconducting wire includes a process for forming a recess on the outer surface of the core material and a process for incorporating a spacer into the gap around the embedded single-core wire, thereby reducing the gap around the embedded single-core wire. This reduces variation in the direction and magnitude of the processing force acting on the metal barrier tube, suppressing misalignment of the embedded single-core wire and uneven plastic deformation of the metal barrier tube toward the gap. This increases the uniformity of the thickness of the metal barrier tube during area reduction, making the metal barrier tube less likely to break. This enables the use of a thinner metal barrier tube, reducing the material and processing costs of the embedded single-core wire. Furthermore, the formation of heterophases and the inhibition of MgB2 production are less likely to occur, reducing the likelihood of geometrically asymmetric MgB2 filaments and MgB2 filament breakage. This also reduces the likelihood of a decrease in critical current (Ic), making it easier to produce long wires. Therefore, it is possible to efficiently manufacture MgB2 superconducting wire that has a multi-core wire structure composed of multiple MgB2 filaments with an MgB2 core, has a high uniformity in the thickness of the metal barrier layer in the circumferential direction of the MgB2 filaments, has reduced defects in the MgB2 filaments, and is suitable for thinning and lengthening.

[0109] Next, a superconducting coil using the above-mentioned MgB2 superconducting wire and a magnetic field generator equipped with a superconducting coil using the above-mentioned MgB2 superconducting wire will be described with reference to the drawings.

[0110] Fig. 6 is a diagram schematically illustrating an example of a superconducting coil using an MgB2 superconducting wire according to an embodiment of the present invention. Fig. 6 shows a cross-sectional view of a superconducting coil 600 cut parallel to the coil axis. As shown in Fig. 6, the MgB2 superconducting wire in which the ratio of the thickness of the barrier layer on the outer periphery to the thickness of the barrier layer on the central side is adjusted can be used as a winding for the superconducting coil 600. The superconducting coil 600 includes a bobbin 601, a winding section 602, and a cooling vessel 603.

[0111] The superconducting coil 600 is a coil formed of a superconductor capable of superconducting transition, and constitutes a superconducting magnet device that generates a magnetic force. The superconducting coil 600 can be provided in, for example, an MRI (Magnetic Resonance Imaging) device, an NMR (Nuclear Magnetic Resonance) device, etc.

[0112] The superconducting coil 600 may be formed by either the wind-and-react method or the react-and-wind method. The wind-and-react method is a method in which a precursor of a superconducting wire is wound into a coil and then heat-treated. The react-and-wind method is a method in which a heat-treated superconducting wire is wound into a coil.

[0113] The bobbin 601 can be made of a metal with high thermal conductivity. The bobbin 601 is preferably made of copper, and particularly preferably made of oxygen-free copper. The bobbin 601 is covered with an insulating material (not shown). When the wind and react method is used, a heat-resistant material that can withstand the heat treatment temperature is used as the insulating material. An example of a heat-resistant insulating material is glass braid using glass fiber.

[0114] The winding portion 602 is formed by the MgB2 superconducting wire wound into a coil. When the wind-and-react method is used, the winding portion 602 can be insulated with a heat-resistant insulating material before the heat treatment that produces MgB2. Alternatively, the winding portion 602 may be impregnated with an insulating resin after the heat treatment that produces MgB2. When the react-and-wind method is used, the winding portion 602 may be impregnated with an insulating resin after the heat treatment that produces MgB2 and after winding into a coil.

[0115] The cooling vessel 603 is a vessel with a sealed structure, which houses the MgB2 superconducting wire 602 wound in a coil shape around the bobbin 601, and cools the MgB2 superconducting wire 602. The cooling vessel 603 is provided in a vacuum insulation structure or a structure insulated from the inside and outside by a heat insulating material, a heat shield, or the like. The cooling vessel 603 may be filled with a cooling medium such as liquid helium, or may be cooled by conduction using a refrigerator.

[0116] In the superconducting coil 600, the ratio of the barrier layer thickness on the outer periphery of the MgB2 filaments constituting the multi-core structure of the MgB2 superconducting wire used in the winding section 602 to the barrier layer thickness on the central side of the wire is limited, and the thickness of the barrier layer is highly uniform and less likely to break, so that the MgB2 superconducting wire used in the winding section 602 can be made thinner and longer than before. This makes it possible to provide a superconducting magnet device that is suitable for miniaturization and high magnetic force.

[0117] Fig. 7 is a cross-sectional view schematically showing an example of a magnetic field generator according to an embodiment of the present invention. Fig. 7 shows an MRI apparatus 700 as an example of the magnetic field generator. As shown in Fig. 7, a superconducting coil 600 wound with the above-mentioned MgB2 superconducting wire can be provided in the MRI apparatus 700.

[0118] The MRI apparatus 700 includes a pair of static magnetic field generating units 701, an imaging region 702, and a gradient magnetic field generating unit 703. The static magnetic field generating units 701 are arranged above and below each other so as to face each other with the imaging region 702 in between. The static magnetic field generating units 701 are connected to each other via connecting members (not shown). The gradient magnetic field generating units 703 are arranged between the static magnetic field generating units 701 and the imaging region 702, respectively.

[0119] The MRI apparatus 700 also includes a bed 705 on which a subject 704 rests, and a transport mechanism 706 that transports the bed 705. The bed 705 is provided so as to be movable toward and away from the imaging region 702. When the bed 705 is transported by the transport mechanism 706, the subject 704 placed on the bed 705 moves toward and away from the imaging region 702.

[0120] The static magnetic field generating unit 701 generates a static magnetic field that is constant over time in the imaging region 702. The static magnetic field generating unit 701 includes a coil unit and a persistent current switch. The coil unit can be formed by the superconducting coil 600 described above. The persistent current switch can be formed by the MgB2 superconducting wire described above, in which the ratio between the thickness of the barrier layer on the outer periphery and the thickness of the barrier layer on the center side is adjusted. The circuit of the static magnetic field generating unit 701 is electrically connected to a power supply (not shown) via a normal conductor.

[0121] An excitation current flows through the coil section of the static magnetic field generating unit 701 when the persistent current switch is in the OFF state. When the persistent current switch is switched to the ON state, a persistent current flows. The persistent current flowing through the coil section generates a static magnetic field with high temporal stability in the imaging region 702. The stronger the static magnetic field, the higher the nuclear magnetic resonance frequency, and therefore the frequency resolution of the MRI apparatus 700 can be improved.

[0122] The gradient magnetic field generating unit 703 is supplied with a time-varying current to generate a gradient magnetic field having a spatial distribution in the imaging region 702. When an oscillating magnetic field of a nuclear magnetic resonance frequency is applied to the imaging region 702, a resonance signal is emitted from the subject 704. The resonance signal is received by a receiving coil (not shown). The received resonance signal is converted into a spectrum by Fourier transform. The spectral information is visualized as a magnetic resonance tomographic image of the subject 704. The subject 704 can be examined by imaging, such as a two-dimensional contrast image.

[0123] According to the MRI device 700, for the MgB2 filaments that make up the multi-core structure of the MgB2 superconducting wire used in the coil section and persistent current switch, the ratio of the barrier layer thickness on the outer periphery of the wire to the barrier layer thickness on the center side of the wire is limited, and the barrier layer thickness is highly uniform and less likely to break, so the MgB2 superconducting wire used in the coil section and persistent current switch can be made thinner and longer than before.As a result, a static magnetic field generator suitable for miniaturization and high magnetic force can be formed, allowing the magnetic generator to be made smaller and more space-saving than before.

[0124] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention. For example, the present invention is not necessarily limited to those having all of the configurations of the above-described embodiments. Part of the configuration of an embodiment can be replaced with another configuration, part of the configuration of an embodiment can be added to another form, or part of the configuration of an embodiment can be omitted.

[0125] For example, the MgB2 superconducting wire described above has a configuration in which 10 MgB2 filaments are arranged in one layer in the radial direction of the wire as shown in Figures 1 to 5, but the MgB2 superconducting wire can be multi-layered in two or more layers in the radial direction of the wire, or can be multi-cored with an appropriate number of MgB2 filaments of four or more. When multi-layered in two or more layers, the ratio of the thickness of the barrier layer on the outer periphery of the wire to the thickness of the barrier layer on the center side of the wire is limited for the MgB2 filaments in the nearest layer arranged around the central wire. [Example]

[0126] The present invention will be specifically described below with reference to examples, but the technical scope of the present invention is not limited to these examples.

[0127] Example 1 As Example 1, an assembled material 100 having the structure shown in FIG. 1 was produced, and the assembled material 100 was subjected to area reduction processing and heat treatment to produce an MgB2 superconducting wire 200 having the structure shown in FIG. 2A.

[0128] First, a precursor for the embedded single-core wire was prepared using the following procedure. The magnesium powder and boron powder, which are the raw materials for the MgB2 core, were weighed out so that the molar ratio of Mg to B was approximately 1:2. The magnesium powder and boron powder were then pulverized and mixed in a ball mill to prepare a mixed powder. The resulting mixed powder was then filled into a metal barrier tube made of a barrier material to form a precursor for the embedded single-core wire. The precursor for the embedded single-core wire was then subjected to area reduction processing to produce the embedded single-core wire. The precursor for the embedded single-core wire was reduced in area to the specified wire diameter by drawing using a die, and was then drawn.

[0129] Next, a double-structure core material with a recess formed on its outer surface was produced. An outer core material was placed over the outside of an inner core material that had been processed to a specified wire diameter. Then, a recess was formed on the outer surface of the outer core material that covered the inner core material by drawing using a specified die. The die used had a die hole with a projection shaped like the inverse of the recess, and the same number of die holes as the number of single-core wires. Copper was used for the inner core material. A metal tube made of the same material as the metal barrier tube was used for the outer core material.

[0130] Next, an embedded material, which is a precursor to the MgB2 superconducting wire, was prepared. Ten embedded single-core wires were arranged around the central material, each fitted into a recess in the central material. The central material and embedded single-core wires were then inserted into an inner sheath tube, which was then inserted into an outer sheath tube. A billet-shaped spacer was inserted into each gap surrounded by the embedded single-core wires and the inner surface of the inner sheath tube.

[0131] Next, the assembled material was subjected to area reduction processing. This was repeated using a drawing die until the assembled material reached the specified wire diameter. The thinned assembled material was then subjected to heat treatment to produce MgB2 superconducting wire.

[0132] Next, the cross section of the obtained MgB2 superconducting wire was observed. A cross section perpendicular to the longitudinal direction of the MgB2 superconducting wire was cut out and polished. Furthermore, chemical etching was performed to make the boundaries between the constituent elements visible, and then the cross section was observed using an optical microscope. As a result, as shown in Figure 2A, 10 MgB2 filaments were confirmed around the core material. An inner sheath and an outer sheath were arranged in that order outside the MgB2 filaments. A filling layer originating from the spacer was confirmed in the space surrounded by the multiple MgB2 filaments and the inner surface of the metal sheath.

[0133] Next, the thickness of the barrier layer of the MgB2 filaments constituting the multi-filamentary wire structure was measured in the cross section of the obtained MgB2 superconducting wire. As a result, it was confirmed that the thickness of the barrier layer on the outer periphery of the MgB2 superconducting wire was limited to 2.2 times the thickness of the barrier layer on the center side of the MgB2 superconducting wire. No heterogeneous phases due to reactions between magnesium and copper, etc. were observed, and it was confirmed that no breakage of the barrier layer had occurred.

[0134] <Example 2> As Example 2, an embedded material 300 having the structure shown in FIG. 3 was fabricated, and the embedded material 300 was subjected to area reduction processing and heat treatment to fabricate an MgB2 superconducting wire 400 having the structure shown in FIG.

[0135] First, a precursor for the embedded single-core wire was prepared using the following procedure. The magnesium powder and boron powder, which are the raw materials for the MgB2 core, were weighed out so that the molar ratio of Mg to B was approximately 1:2. The magnesium powder and boron powder were then pulverized and mixed in a ball mill to prepare a mixed powder. The resulting mixed powder was then filled into a metal barrier tube made of a barrier material to form a precursor for the embedded single-core wire. The precursor for the embedded single-core wire was then subjected to area reduction processing to produce the embedded single-core wire. The precursor for the embedded single-core wire was reduced in area to the specified wire diameter by drawing using a die, and was then drawn.

[0136] Next, a double-layered core was fabricated. The inner core, which had been processed to a specified wire diameter, was covered with an outer core. The inner core was made of copper. The outer core was made of a metal tube made of the same material as the metal barrier tube.

[0137] Next, an embedded material, which is a precursor of the MgB2 superconducting wire, was prepared. Ten embedded single-core wires were arranged around the central material. The central material and the embedded single-core wires were then inserted into an inner sheath tube, and the inner sheath tube was then inserted into an outer sheath tube. An inner spacer was inserted into each gap surrounded by the multiple embedded single-core wires and the outer surface of the central material. An outer spacer was also inserted into each gap surrounded by the multiple embedded single-core wires and the inner surface of the inner sheath tube.

[0138] Next, the assembled material was subjected to area reduction processing. This was repeated using a drawing die until the assembled material reached the specified wire diameter. The thinned assembled material was then subjected to heat treatment to produce MgB2 superconducting wire.

[0139] Next, the cross section of the obtained MgB2 superconducting wire was observed. A cross section perpendicular to the longitudinal direction of the MgB2 superconducting wire was cut and polished. Furthermore, chemical etching was performed to make the boundaries between the constituent elements visible, and then the cross section was observed using an optical microscope. As a result, as shown in Figure 4, 10 MgB2 filaments were confirmed around the core material. An inner sheath and an outer sheath were arranged in that order outside the MgB2 filaments. An inner filling layer originating from the inner spacer was confirmed in the space surrounded by the multiple MgB2 filaments and the outer surface of the core material. An outer filling layer originating from the outer spacer was confirmed in the space surrounded by the multiple MgB2 filaments and the inner surface of the inner sheath.

[0140] Next, the thickness of the barrier layer of the MgB2 filaments constituting the multifilamentary wire structure was measured in the cross section of the obtained MgB2 superconducting wire. The results confirmed that the thickness of the barrier layer on the outer periphery of the MgB2 superconducting wire was limited to three times the thickness of the barrier layer on the center side of the MgB2 superconducting wire. No heterogeneous phases due to reactions between magnesium and copper or the like were observed, confirming that no breaks in the barrier layer had occurred. It was also confirmed that placing an inner spacer in the gap surrounded by the MgB2 filament and the outer surface of the core material produced the same effect as forming a recess on the outer surface of the core material.

[0141] <Comparative Example 1> As Comparative Example 1, an MgB2 superconducting wire having the structure shown in Fig. 5 was produced. The MgB2 superconducting wire of Comparative Example 1 was produced by increasing the thickness of the metal barrier tube compared to the embedded material having the structure shown in Fig. 1.

[0142] First, an embedded material, which is a precursor of the MgB2 superconducting wire, was prepared in the same manner as in Example 1. Next, the material was drawn to a predetermined wire diameter, and finally, a heat treatment was performed to prepare the MgB2 superconducting wire. However, the metal barrier tube used was about 1.5 times thicker than the metal barrier tube used in Examples 1 and 2.

[0143] Next, the cross section of the obtained MgB2 superconducting wire was observed. A cross section perpendicular to the longitudinal direction of the MgB2 superconducting wire was cut out and polished. Furthermore, chemical etching was performed to distinguish the boundaries between the constituent elements, and then the cross section was observed using an optical microscope. As a result, as shown in Figure 5, 10 MgB2 filaments were confirmed around the core material. An inner sheath and an outer sheath were arranged in that order outside the MgB2 filaments. A filling layer originating from the spacer was confirmed in the space surrounded by the multiple MgB2 filaments and the inner surface of the metal sheath. It was confirmed that the degree of deformation of the MgB2 core shape along the radial direction of the wire was greater than in Examples 1 and 2.

[0144] Next, the thickness of the barrier layer of the MgB2 filaments that make up the multifilamentary wire structure was measured in the cross section of the obtained MgB2 superconducting wire. As a result, it was confirmed that the thickness of the barrier layer on the outer periphery of the wire was four times that of the barrier layer on the center side of the wire. It is thought that if the thickness of the metal barrier tube is increased, the proportion of metal that plastically deforms toward the outside in the radial direction of the wire during area reduction processing increases.

[0145] Next, the critical current value of the obtained MgB2 superconducting wire was measured. The MgB2 superconducting wire was cooled to 4.2 K and current was passed through it. As a result, the critical current value of the MgB2 superconducting wire was about 20% lower than that of Example 1. It is effective to limit the thickness of the barrier layer on the outer periphery of the wire to at least four times the thickness of the barrier layer on the center side of the wire.

[0146] Example 3 2A was produced as Example 3. The MgB2 superconducting wire of Example 3 was produced by making the thickness of the metal barrier tube thinner than that of the embedded material having the structure shown in FIG.

[0147] First, an embedded material, which is a precursor of the MgB2 superconducting wire, was prepared in the same manner as in Example 1. Next, the material was drawn to a predetermined wire diameter, and finally, a heat treatment was performed to prepare the MgB2 superconducting wire. However, the metal barrier tube used was approximately 0.6 times thicker than the metal barrier tube used in Examples 1 and 2.

[0148] Next, the cross section of the obtained MgB2 superconducting wire was observed. A cross section perpendicular to the longitudinal direction of the MgB2 superconducting wire was cut out and polished. Furthermore, chemical etching was performed to make it possible to distinguish the boundaries between the constituent elements, and then the cross section was observed using an optical microscope. As a result, 10 MgB2 filaments were confirmed around the central material. An inner sheath and an outer sheath were arranged in that order outside the MgB2 filaments. A filling layer originating from the spacer was confirmed in the space surrounded by multiple MgB2 filaments and the inner surface of the metal sheath.

[0149] Next, the thickness of the barrier layer of the MgB2 filaments that make up the multifilamentary wire structure was measured in the cross section of the obtained MgB2 superconducting wire. As a result, it was confirmed that the thickness of the barrier layer on the outer periphery of the wire was limited to no more than three times the thickness of the barrier layer on the central side of the wire. It is thought that by reducing the thickness of the metal barrier tube, the proportion of metal that plastically deforms toward the outside in the radial direction of the wire during the area reduction process is reduced. It can be said that the thickness of the metal barrier tube can be reduced by approximately 40%.

[0150] <Comparative Example 2> As Comparative Example 2, an MgB2 superconducting wire was fabricated having a structure in which an outer spacer was embedded in a gap surrounded by a plurality of embedded single-core wires and the inner surface of a metal sheath tube. The MgB2 superconducting wire of Comparative Example 2 was fabricated without forming a recess on the outer surface of the core material and without embedding an inner spacer in the gap surrounded by a plurality of embedded single-core wires and the outer surface of the core material.

[0151] First, an embedded material, which is a precursor of the MgB2 superconducting wire, was produced in the same manner as in Example 1. However, the metal barrier tube used was approximately 0.6 times thicker than the metal barrier tube used in Examples 1 and 2. The embedded material, which is a precursor of the MgB2 superconducting wire, was produced by arranging 10 embedded single-core wires around the central material without forming any recesses on the outer surface of the central material. Next, the wire was drawn to a predetermined wire diameter, and finally, heat treatment was performed to produce the MgB2 superconducting wire.

[0152] Next, the cross section of the obtained MgB2 superconducting wire was observed. A cross section perpendicular to the longitudinal direction of the MgB2 superconducting wire was cut and polished. Furthermore, chemical etching was performed to distinguish the boundaries between the components, and then the cross section was observed using an optical microscope. As a result, 10 MgB2 filaments were confirmed around the core material. An inner sheath and an outer sheath were arranged in that order around the MgB2 filaments. A filling layer originating from the spacer was confirmed in the space surrounded by the multiple MgB2 filaments and the inner surface of the metal sheath. It was confirmed that the degree of deformation along the radial direction of the wire was greater than that of Example 3. Some of the barrier layers had tears near the center of the wire. A heterogeneous phase due to a reaction between magnesium and copper was formed around the tears.

[0153] Next, the thickness of the barrier layer of the MgB2 filaments that make up the multi-core wire structure was measured in the cross section of the obtained MgB2 superconducting wire. As a result, it was confirmed that the thickness of the barrier layer on the outer periphery of the wire was more than three times that of the barrier layer on the center side of the wire. It is thought that the presence of gaps surrounded by multiple embedded single-core wires and the outer surface of the core wire reduced the proportion of metal on the radially inner side of the wire during the area reduction process.

[0154] Next, the critical current value of the obtained MgB2 superconducting wire was measured. The MgB2 superconducting wire was cooled to 4.2 K and current was passed through it. As a result, the critical current value of the MgB2 superconducting wire was about 40% lower than that of Example 3. The metal barrier tube used was about 0.6 times thicker than the metal barrier tubes used in Examples 1 and 2, but the presence of gaps surrounded by the multiple embedded single-core wires and the outer surface of the core material made it more susceptible to breakage during the wiredrawing process, which is thought to have led to the formation of heterophases, etc.

[0155] Example 4 As Example 4, a superconducting coil having the structure shown in FIG. 6 was fabricated using an MgB2 superconducting wire having the structure shown in FIG. 2A.

[0156] First, a built-in material was prepared using the same procedure as in Example 1, and the built-in material was subjected to area reduction processing. Then, the built-in material, which had been thinned by area reduction processing, was covered with glass fiber, an insulating material. A metal bobbin was also covered with glass fiber, an insulating material. The insulated built-in material was then wound around the bobbin and subjected to heat treatment at a predetermined temperature to generate MgB2. After that, insulating resin was impregnated into the gaps in the coil-wound MgB2 superconducting wire, and a superconducting coil in which the MgB2 superconducting wire was fixed was obtained.

[0157] The resulting superconducting coil was then placed in a cryostat and electrically connected to a power supply. The superconducting coil was then cooled below its critical temperature and excited by passing current from the power supply to confirm the magnetic field stability of the superconducting coil.

[0158] <Example 5> As Example 5, an MRI apparatus was fabricated that was equipped with a superconducting coil using an MgB2 superconducting wire having the structure shown in FIG. 2A.

[0159] The MRI apparatus includes a pair of static magnetic field generators and a gradient magnetic field generator. The static magnetic field generators are connected via a connecting member and arranged above and below each other so as to face each other. The gradient magnetic field generators are arranged between the static magnetic field generators so as to sandwich the imaging region. A transport mechanism is also provided for transporting the bed so that it can be moved toward and away from the imaging region.

[0160] A superconducting coil using MgB2 superconducting wire was housed in a cryocontainer. The cryocontainer housing the superconducting coil was placed in a static magnetic field generator. The superconducting coil was electrically connected to a power source. The superconducting coil can generate a stable static magnetic field by being excited by passing current from the power source. [Explanation of symbols]

[0161] 100,300 embedded materials 110,310 Core material 110a recess 111,311 Inner center material 112,312 Outer center material 120,320 Embedded single core wire 121,321 Raw material powder 122,322 Metal Barrier Pipe 130,330 Metal sheath tube 131,331 Inner sheath tube 132,332 Outer sheath tube 140,340 Spacer 341 Inner spacer 342 Outer spacer 200, 400, 500 MgB2 superconducting wire 210,410,510 Core material 210a, 510a Remains of depressions 211,411,511 Inner core material 212,412,512 Outer center material 220, 420, 520 MgB2 filament 221,421,521 MgB2 cores 222,422,522 Barrier layer 230,430,530 Metal sheath 231,431,531 Inner sheath 232,432,532 Outer sheath 240,440,540 packed bed 441 Inner packed layer 442 Outer packing layer Tin: Thickness of the barrier layer at the center Tout: Thickness of the barrier layer on the outer periphery

Claims

1. A core material is disposed at the center of the wire, and MgB is disposed to surround the core material. ２ A plurality of MgB cores formed by the method described above are covered with a barrier layer. ２ a filament and a plurality of the MgB ２ A multi-filamentary MgB wire having a metal sheath covering the filaments. ２ A superconducting wire, The MgB ２ The filament is the MgB ２ The thickness of the barrier layer on the outer periphery of the superconducting wire is ２ The thickness of the barrier layer at the center of the superconducting wire is three times or less than that of the MgB ２ Superconducting wire.

2. The MgB of claim 1 ２ A superconducting wire, The MgB ２ The filament is the MgB ２ The thickness of the barrier layer on the outer periphery of the superconducting wire is ２ The thickness of the MgB barrier layer at the center of the superconducting wire is 1.5 times or more and 3 times or less. ２ Superconducting wire.

3. The MgB of claim 1 ２ A superconducting wire, The metal sheath is made of the MgB ２ An MgB filament having an inner sheath covering the filament and an outer sheath covering the inner sheath. ２ Superconducting wire.

4. The MgB according to claim 3 ２ A superconducting wire, the inner sheath is made of iron, niobium, or tantalum; The outer sheath is made of a nickel-copper alloy, stainless steel, or low carbon steel. ２ Superconducting wire.

5. The MgB of claim 1 ２ A superconducting wire, The core material is the MgB ２ An MgB superconductor having an inner core disposed at the center of a superconducting wire and an outer core covering the inner core. ２ Superconducting wire.

6. The MgB according to claim 5 ２ A superconducting wire, the barrier layer is formed of iron, niobium, or tantalum; the inner core is formed of copper or iron; The outer core material is made of the same material as the barrier layer, MgB ２ Superconducting wire.

7. A core material is disposed at the center of the wire, and MgB is disposed to surround the core material. ２ A plurality of MgB cores formed by the method described above are covered with a barrier layer. ２ a filament and a plurality of the MgB ２ A multi-filamentary MgB wire having a metal sheath covering the filaments. ２ A method for manufacturing a superconducting wire, comprising: The MgB ２ forming a plurality of single-core wires in which the raw material is filled in a metal barrier tube that forms the barrier layer; forming a recess on the outer surface of the core; a step of arranging a plurality of the single-core wires in the recess, incorporating the central member and the plurality of the single-core wires into a metal sheath tube that forms the metal sheath, and incorporating a spacer into a gap surrounded by the plurality of the single-core wires and the inner surface of the metal sheath tube to form an incorporated member; a step of subjecting the built-in material to surface reduction processing; The reduced-area embedded material is heat-treated to form MgB ２ and generating MgB ２ A method for manufacturing superconducting wire.

8. A core material is disposed at the center of the wire, and MgB is disposed to surround the core material. ２ A plurality of MgB cores formed by the method described above are covered with a barrier layer. ２ a filament and a plurality of the MgB ２ A multi-filamentary MgB wire having a metal sheath covering the filaments. ２ A method for manufacturing a superconducting wire, comprising: The MgB ２ forming a plurality of single-core wires in which the raw material is filled in a metal barrier tube that forms the barrier layer; a step of incorporating the central material and the plurality of single-core wires into a metal sheath tube that forms the metal sheath, incorporating an inner spacer into a gap surrounded by the plurality of single-core wires and an outer surface of the central material, and incorporating an outer spacer into a gap surrounded by the plurality of single-core wires and an inner surface of the metal sheath tube to form an incorporated material; a step of subjecting the built-in material to surface reduction processing; The reduced-area embedded material is heat-treated to form MgB ２ and generating MgB ２ A method for manufacturing superconducting wire.

9. The MgB according to any one of claims 1 to 6. ２ A superconducting coil made of wound superconducting wire.

10. The MgB according to any one of claims 1 to 6. ２ A magnetic generator equipped with a superconducting coil wound with superconducting wire.

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