WO2016207988A1 - Mgb2 superconducting wire - Google Patents

Abstract

The problem addressed by the disclosed invention is to provide an MgB₂ superconducting wire which achieves a high critical current density J_c by establishing a good balance between improvement of superconducting characteristics and increase of the cross-sectional area of an MgB₂ core of the MgB₂ superconducting wire by means of an internal diffusion method. A means for solving the above-described problem is the achievement of a superconducting wire comprising an MgB₂ core that has a larger cross-sectional area than conventional superconducting wires by configuring a filament region including the MgB₂ core such that the cross-sectional shape thereof has an inwardly projected region.

Description

MgB2 superconducting wire

The present invention relates to a MgB ₂ superconducting wire, and more particularly to a superconducting wire having a magnesium diboride (MgB ₂ ) core.

Magnesium diboride (MgB ₂ ) has a higher superconducting transition temperature (39K) than Nb-based metallic superconductors that have been put into practical use. Therefore, application is expected as a superconducting material that can be used at 10 to 20 K without using liquid He. In particular, application as an electromagnet wire for a nuclear magnetic resonance apparatus (NMR) or a magnetic resonance imaging apparatus (MRI) that generates a high magnetic field is expected. Superconducting wires applied to these high magnetic field generating magnets are required to have a high critical current density ( _Jc ) for flowing a large current in a magnetic field. _Jc is the maximum current density that can be passed while maintaining the superconducting state, which depends not only on the physical properties unique to the superconducting material but also on the method of manufacturing the wire.

In the development so far, the MgB ₂ wire is generally produced using a powder-in-tube method (PIT method). In this method, magnesium (Mg) and boron (B) are filled in a metal tube, and then drawing is performed, and heat treatment is performed to generate MgB ₂ inside the metal tube (sheath). Recently, based on PIT method, internal diffusion process in order to obtain a higher J _c (IMD) method has been developed (Patent Document 1). The manufacturing process of the wire by the IMD method is shown in FIG. An Mg rod 1 is inserted into the metal tube 10 (FIG. 1 (a)), and a gap between them is filled with a boron raw material 2 (FIG. 1 (b)). Thereafter, drawing is performed, and finally heat treatment is performed to generate MgB2 (FIG. 1 (c)).
FIG. 2 shows a schematic diagram of a cross section of a wire by a conventional IMD method. Mg in the center of the wire is diffused into the surrounding B by heat treatment, and the MgB ₂ core 20 is generated inside the sheath 10. Since Mg existing before the heat treatment is consumed by this diffusion reaction, the central portion of the cross section of the wire becomes a void 30. In the wire rod produced by this manufacturing method, the MgB ₂ core produced is higher in density than the PIT method, and as a result, _Jc is improved. In this figure, the cross-sectional shape is circular, and the constituent elements 10, 20, and 30 are present concentrically. In this figure, the cross-sectional shape is circular, but it may be elliptical. However, the display of the figure is omitted. In the case of concentric circles, the shape of each element is an ellipse concentrically.

In the wire by the IMD method, MgB ₂ is generated only on the inner side of the sheath as described above, and the central portion becomes a void, so that the sectional area of the MgB ₂ core tends to be essentially reduced. In order to increase the cross-sectional area of the MgB ₂ core, a method of reducing the volume of the Mg rod before heat treatment to the minimum amount necessary for MgB ₂ production and filling as much B raw material as possible is used. However, in this case, since the B layer inside the sheath becomes thick, there is a problem that the progress of the MgB ₂ generation reaction due to the diffusion of Mg becomes uneven in the thickness direction. In general, the microstructure of the MgB ₂ core depends on the reaction conditions, and the crystal grain size and the number of strains in the crystal vary. In insufficient reaction process is not MgB ₂ is produced, lowering the magnetic field J _c resulted in relief of coarsening and crystal distortion particle size in excess reaction process reversed. Therefore, in order to obtain a high magnetic field J _c it becomes necessary reaction process under optimum conditions. Problems That is, when the thick simply MgB ₂ core for the MgB ₂ core cross-sectional area increase in the IMD process, the reaction occurs unevenness in the thickness direction, can not be uniformly generate optimal MgB ₂ regions where high J _c is obtained was there.

JP 2004-47441 A

An object of the present invention, the MgB ₂ wire material by internal diffusion process (IMD) method, to achieve both an improvement in the cross-sectional area increases and the superconducting properties of MgB ₂ core, provides a MgB ₂ wire material to achieve a high critical current density J _c That is.

In the present invention, the cross-sectional shape of the filament region including the MgB ₂ core (region of the sheath inner wall) has a region that protrudes inward. Even if the thickness of the MgB ₂ core is the same as that of the prior art, the cross-sectional area of the core increases as the surface area of the sheath inner wall is larger. At the same time, the thickness of the core can be suppressed within a range in which uneven reaction of MgB ₂ generation does not occur, and MgB ₂ is generated under optimal reaction conditions over the entire area of the wire.

The present invention increases the quality MgB ₂ core, MgB ₂ superconducting wire exhibits higher _{J c} properties in a magnetic field is obtained.

It is a schematic view of a manufacturing process of the MgB ₂ single-core wire by conventional IMD technique. It is a schematic sectional view of MgB ₂ single-core wire by conventional IMD technique. 1 is a schematic cross-sectional view of an MgB ₂ single core wire of Example 1. FIG. 3 is a schematic diagram of a production process of the MgB ₂ single-core wire of Example 1. FIG. 4 is a schematic cross-sectional view of a MgB ₂ single core wire of Example 2. FIG. 4 is a schematic cross-sectional view of an MgB ₂ single-core wire of Example 3. FIG. 6 is a schematic cross-sectional view of an MgB ₂ multi-core wire of Example 4. FIG.

Embodiments of the present invention will be described in detail with reference to the drawings.

Example 1 proposes a single-core MgB ₂ wire. FIG. 3A shows a schematic cross-sectional view of the wire rod of Example 1. FIG. In the present embodiment, the shape of the region surrounded by the inner wall of the sheath 10 (the region where the MgB ₂ core 20 is generated) is not a simple ring shape but includes a region protruding inward (portion 40 in the figure). It is characterized by having the shape of Here, the “simple ring shape” means, for example, the one indicated by reference numeral 20 in FIG. This increases the surface area inside the sheath 10 and increases the volume of the MgB ₂ core 20 compared to the conventional IMD wire shown in FIG. In particular, when a wire rod having the same cross-sectional area as that of a conventional wire rod is manufactured based on this embodiment, the advantage that the ratio of the area of the MgB ₂ core 20 in the total single area can be made larger than that of the conventional wire rod. There is.

The relationship between the area (filament area) S of the closed curved surface surrounded by the boundary line between the MgB ₂ core 20 and the sheath 10 and the peripheral length L of the boundary line in the example having the wire cross section in the case of the present embodiment. explain.

Using the dimensional notation of each location in the cross section of Example 1 described in FIG. 3B, the filament area S and the circumferential length L are obtained by using some approximation S≈ (π−π (1−β) -Αβ) a ² , L≈2πa (2-β) + 2a (β-α). a is the radius of the filament region, and α and β are the relative ratio of the distances b and d to the radius a. In Example 1, α = 0.6 and β = 0.2, which is about L = 6√S. Based on this, L> 4√S is expressed in the claims.

Incidentally, in the case of a conventional configuration in which the filament region has a circular shape, assuming that the radius of the filament region is a, S ₀ = πa ² , circumferential length L ₀ = 2πa, and L = 3.5√S.

A method for manufacturing the wire will be described with reference to FIGS. A perspective view of each process is shown in the upper stage, and a sectional view in the lower stage. First, B powder and Mg raw material rod are prepared. The amount of the B powder to be prepared and used is weighed so that the molar ratio to Mg is 2 or less. The Mg raw material rod 1 is processed into a shape like the gap 30 in the sectional view of FIG. 3, that is, a shape substantially similar to the sectional shape of the gap 30 in FIG. The Mg rod 1 is inserted into the Fe tube 10 (which is a kind of the above-described metal tube) (FIG. 4 (a)), and further, a region corresponding to the constricted portion 40 in FIG. A metal (Fe) member 11 processed into a shape (keyhole shape) that can fill the gap and the center of the mold is inserted (FIG. 4B). B powder 2 is filled into the space between the assembled Fe tube 10 / Fe member 11 and the Mg raw material rod 1 (FIG. 4 (c)). When the filling of the B powder 2 is completed, the shape of the cross section is similar to the shape of the finished product shown in FIG. 2 (FIG. 4 (d)). As shown in the lower part of FIG. 4, the composition at the time of filling is B powder filled with the MgB ₂ core 20 region in FIG. 2, the void 30 region is Mg, and the sheath 10 region is an Fe tube and an Fe member. It becomes the composition which becomes. The end portion of the material thus produced is sealed. Subsequently, the surface is reduced to a diameter of 0.5 mm by drawing to form a wire. Finally, heat treatment is performed at 600 to 800 ° C. to generate MgB ₂ in the sheath.

In the case of the present embodiment, the cross-sectional area of the core increases as the surface area is large even if the thickness of the MgB ₂ core is the same as that of the conventional structure. At the same time, the thickness of the core can be suppressed within a range in which uneven reaction of MgB ₂ generation does not occur, and MgB ₂ is generated under optimal reaction conditions over the entire area of the wire. The MgB ₂ wire prototype manufactured with the configuration of Example 1 has an MgB ₂ core cross-sectional area of about 1.3 times that of the wire having the conventional structure shown in FIG. 2, thereby improving J _c by 1.3 times. .

In this embodiment, an Fe tube is used as the sheath material, but a composite material of Cu and Fe may be used in order to improve heat transfer. At that time, it is desirable to arrange the Fe so as to be in contact with MgB ₂ . Further, in this embodiment using the B powder in the raw material, it may be added materials such as C and B _{4 C} in order to further improve the J _c. In the present embodiment, as shown in the cross-sectional view of FIG. 2, the MgB ₂ core 20 is continuously present in the cross section inside the sheath. However, the MgB ₂ core 20 does not necessarily have to be continuous in the entire arbitrary cross section with respect to the longitudinal direction of the wire. Since it has a complicated core shape instead of a simple ring shape, the core may be discontinuous due to shape disturbances in some places during manufacturing. However, the direction in which the current flows is the longitudinal direction of the wire, and there is no adverse effect that significantly cuts the current path.

Example 2 proposes a single-core MgB ₂ wire. FIG. 5A shows a schematic cross-sectional view of the wire rod of Example 2. FIG. The wire rod of the present embodiment is a flat wire, and the outer shape of the cross section is a rectangle. Similar to the first embodiment, the shape of the region surrounded by the inner wall of the sheath 10 (the region where the MgB ₂ core 20 is generated) is not a simple square, but U having a convex region (part 40 in the figure) inside. It is a letter shape. Compared with the wire material by the conventional IMD method, the surface area inside the sheath 10 increases, and the volume of the MgB ₂ core 20 increases. The relationship between the area (filament area) S of the closed curved surface surrounded by the boundary line between the MgB ₂ core 20 and the sheath 10 and the peripheral length L of the boundary line in the cross section of the wire rod of the present embodiment will be described. Using the dimension notation of each location in the cross section of Example 2 described in FIG. 5B, the filament area S and the circumferential length L are approximately equal to S≈ ( 2αβ + 2β ² −β) a ² , L≈2 (1 + 2α−β) a. a is the short side of the filament region, and α and β are the relative ratio of the long side b to the short side a and the distance d. In the second embodiment, α = 2 and β = 0.3, which is about L = 9√S. By the way, in the case of the conventional configuration in which the filament region is a simple rectangle, S ₀ ≈αa ² and circumferential length L ₀ ≈2 (1 + α) due to a slight approximation in which the short side of the filament region is a and the corner curved portion is a right-angled shape. ) A, and when α = 2, L = 4.2√S. Based on this, L> 4√S is expressed in the claims.

The basic production method of the wire of Example 2 is the same as that of Example 1. However, in the case of Example 2, the Fe member 11 to be inserted is not a keyhole-shaped cross section as in Example 1, but a simple square, and a die having a square hole shape is used in the middle of the drawing process. While forming the shape into a quadrangle, it is made into a wire while performing surface-reducing processing.

In the case of the present embodiment, the cross-sectional area of the core increases as the surface area is large even if the thickness of the MgB ₂ core is the same as that of the conventional structure. At the same time, the thickness of the core can be suppressed within a range in which uneven reaction of MgB ₂ generation does not occur, and MgB ₂ is generated under optimal reaction conditions over the entire area of the wire. The MgB ₂ wire prototype manufactured with the configuration of Example 2 has an MgB ₂ core cross-sectional area of about 1.5 times that of the wire material obtained by the conventional IMD method, thereby improving J _c by 1.5 times.

In this embodiment, an Fe tube is used as the sheath material, but a composite material of Cu and Fe may be used in order to improve heat transfer. At that time, it is desirable to arrange the Fe so as to be in contact with MgB ₂ . Further, in this embodiment using the B powder in the raw material, it may be added materials such as C and B _{4 C} in order to further improve the J _c. In the present embodiment, the MgB ₂ core 20 is continuous inside the sheath as shown in the cross-sectional view of FIG. 5, but not all of them are necessarily continuous. Due to the complex core shape, the core may become discontinuous in some places during manufacturing, and the core may be discontinuous, but the direction of current flow is the longitudinal direction of the line, and the adverse effect of significantly cutting the current path There is no.

Example 3 proposes a single-core MgB ₂ wire. FIG. 6A shows a schematic cross-sectional view of the wire rod of Example 3. FIG. Similar to the first embodiment, the shape of the region surrounded by the inner wall of the sheath 10 (the region where the MgB ₂ core 20 is generated) is not a simple circle, but has a convex region (portion 40 in the figure) on the inside. Compared with the wire material by the conventional IMD method, the surface area inside the sheath 10 increases, and the volume of the MgB ₂ core 20 increases.

The relationship between the area (filament area) S of the closed curved surface surrounded by the boundary line between the MgB ₂ core 20 and the sheath 10 and the peripheral length L of the boundary line in the cross section of the wire rod of the present embodiment will be described. Using the dimensional notation of each location in the cross section of Example 3 shown in FIG. 6B, the filament area S and the circumferential length L can be obtained by using some approximation S≈πa ² −6bd / 2 = (π ^-3Arufabeta) is expressed as ^{a 2, L ≒ 2πa-6βd} + 12c = 2πa-6βa + 12a√ (α 2 + β 2/4). a is the radius of the filament region, and α and β are the relative ratio of the distances b and d to the radius a. In Example 3, α = 0.6 and β = 0.25, which is about L = 7.4√S. Incidentally, in the case of a conventional configuration in which the filament region has a circular shape, assuming that the radius of the filament region is a, S ₀ = πa ² , circumferential length L ₀ = 2πa, and L = 3.5√S. Based on this, L> 4√S is expressed in the claims.

The basic production method of the wire of Example 3 is the same as that of Example 1. However, in the case of Example 3, the Mg rod 1 shown in FIG. 4 uses a raw material having a cross-sectional shape similar to the gap 30 shown in FIG. Further, in order to form a region corresponding to the constricted portion 40 in FIG. 6, the Fe member 11 shown in FIG. 4 uses a member having a triangular cross section, and is arranged on the inner wall of the Fe tube 10 serving as a sheath. In the composite body incorporating the metal member as described above, B is filled between the Mg rod and the Fe tube, and then drawing and heat treatment are performed to produce a wire.

In the case of the present embodiment, the cross-sectional area of the core increases as the surface area is large even if the thickness of the MgB ₂ core is the same as that of the conventional structure. At the same time, the thickness of the core can be suppressed within a range in which uneven reaction of MgB ₂ generation does not occur, and MgB ₂ is generated under optimal reaction conditions over the entire area of the wire. The MgB ₂ wire prototype manufactured with the configuration of Example 2 has a MgB ₂ core cross-sectional area of about 1.8 times that of the wire material obtained by the conventional IMD method, and thereby _Jc is also improved by 1.8 times.

In this embodiment, an Fe tube is used as the sheath material, but a composite material of Cu and Fe may be used in order to improve heat transfer. At that time, it is desirable to arrange the Fe so as to be in contact with MgB ₂ . Further, in this embodiment using the B powder in the raw material, it may be added materials such as C and B _{4 C} in order to further improve the J _c. In this embodiment, the MgB ₂ core 20 is continuous inside the sheath as shown in the cross-sectional view of FIG. 6, but not all may be necessarily continuous. Due to the complex core shape, the core may become discontinuous in some places during manufacturing, and the core may be discontinuous, but the direction of current flow is the longitudinal direction of the line, and the adverse effect of significantly cutting the current path There is no.

Example 4 proposes a multi-core MgB ₂ wire. FIG. 7A shows a schematic cross-sectional view of the fourth embodiment. Seven filament regions in which the MgB ₂ core 20 is formed are arranged. As shown in the drawing, each of the six filament regions arranged on the outer side has a side close to the outer periphery of the wire cross section narrowed inward (portion 40 in the figure), and is generated inside the sheath due to its shape. The cross-sectional area of the MgB ₂ core 20 increases compared to the shape without the constricted portion 40. For each of the six filament regions arranged outside, the relationship between the area (filament area) S of the closed curved surface surrounded by the boundary line between the MgB ₂ core 20 and the sheath 10 and the peripheral length L of the boundary line is as follows. explain. When the dimension notation of each place is used for each of the six filament regions arranged outside in the cross section of Example 4 shown in FIG. 7B, the filament area S and the circumferential length L are slightly approximated. S≈πa ² / 6−γ ² a ² √3 / 4−bd / 2 = (π / 6−γ ² √3 / 4αβ / 2) a ² , L≈2 (1−γ) a + 2πa / 6-d + 2c = a (2 (1-γ) + π / 3-β + 2a√ (α 2 + β 2/4)) is expressed as. a, b, c, d, and e are line segments as shown in FIG. 7B, and α, β, and γ are ratios of b, d, and e with reference to a. In Example 4, α = 0.5, β = 0.2, γ = 0.2, and approximately L = 5.1√S. Incidentally, in the case of the configuration in which the six filament regions arranged outside are not constricted, the line segment c shown in FIG. 7B is eliminated, so that S ₀ ≈ (π / 6−γ ² √3 / 4) a ^2. Circumference L ₀ ≈ (2 + π / 3−γ) a, and approximately L = 4√S. Based on this, L> 4√S is expressed in the claims.

The manufacturing process of the wire of Example 4 will be described. First, six composites composed of Fe tubes, Mg bars, and B powder are prepared in the same procedure as in Example 1. In addition, based on the normal IMD method, one composite composed of an Fe tube, a simple cylindrical Mg rod, and B powder is prepared. After hexagonal molding of these external shapes, a cylindrical metal tube is filled in a hexagonal arrangement. In that case, it arrange | positions so that the constriction part 40 may finally become the outer side of a wire. The metal composite is subjected to surface reduction to a diameter of 1.5 mm by drawing, and then heat-treated at 600 to 800 ° C. to obtain a multi-core wire having a cross-sectional shape as shown in FIG.

In the case of the present embodiment, the cross-sectional area of the core increases as the surface area is large even if the thickness of the MgB ₂ core is the same as that of the conventional structure. At the same time, the thickness of the core can be suppressed within a range in which uneven reaction of MgB ₂ generation does not occur, and MgB ₂ is generated under optimal reaction conditions over the entire area of the wire. MgB ₂ wire prototyped in the configuration of Example 3, MgB ₂ core area is approximately 1.2 times that of the multi-core wire by conventional IMD process, whereby _{J c} was also improved 1.2 times.

Further, in this embodiment, the filaments are arranged in a hexagonal shape, but other shapes may be arranged, for example, a quadrangular shape or an octagonal shape. In this embodiment, an Fe tube is used as the sheath material, but a composite material of Cu and Fe may be used in order to improve heat transfer. At that time, it is desirable to arrange the Fe so as to be in contact with MgB ₂ . Further, in this embodiment using the B powder in the raw material, it may be added materials such as C and B _{4 C} in order to further improve the J _c. In the present embodiment, as shown in the cross-sectional view of FIG. 7, the MgB ₂ core 20 is continuous inside each filament. However, not all of them are necessarily continuous. Due to the complex core shape, the core may become discontinuous in some places during manufacturing, and the core may be discontinuous, but the direction of current flow is the longitudinal direction of the line, and the adverse effect of significantly cutting the current path There is no.

1 ... metal tube, 2 ... boron raw material, 10 ... sheath, 11 ... metal member, 20 ... MgB ₂ core, 30 ... gap, 40 ... constricted site

Claims (10)

1. A metal sheath whose outer shape is cylindrical,
   When viewed in an arbitrary cross section in the extending direction of the metal sheath, the inside of the metal sheath is hollow, and at least one filament having an MgB ₂ core and a void in the inner periphery of the metal sheath It is provided so that the side of the area touches,
   The MgB ₂ superconducting wire, wherein when the distance between the outer portion of the filament region and the inner portion of the filament region is the thickness of the filament region, the thickness varies depending on the location.
2. The outer peripheral shape of the cross section of the metal sheath is circular, and the thickness of a part of the filament region measured in the circumferential direction from the center point of the circle is larger than the thickness of other parts of the other filament region. The MgB ₂ superconducting wire according to claim 1, wherein the thickness varies depending on the location.
3. The MgB ₂ superconducting wire according to claim 2, wherein the number of the voids is one or more.
4. The outer peripheral shape of the cross section of the metal sheath is substantially a quadrangle, and the thickness of a part of the filament region measured from the centroid of the quadrangle in the outer peripheral direction of the quadrangle is larger than the thickness of other parts of the other filament region. The MgB ₂ superconducting wire according to claim 1, wherein the thickness differs depending on the location due to its large size.
5. The outer peripheral shape of the cross section of the metal sheath is substantially square, and the thickness varies depending on the location because the thickness of a part of the sheath when viewed in the cross section of the sheath is thicker than the thickness of the sheath of the other part. The MgB ₂ superconducting wire according to claim 1, wherein:
6. An MgB ₂ superconducting wire having at least one filament region including an MgB ₂ core and voids inside the metal sheath,
   The outside of the filament region is covered with a metal sheath material, and the MgB ₂ core in the filament region is disposed outside the gap,
   The filament region has one or more inwardly convex shapes, and the relationship between the outer peripheral length L and the cross-sectional area S of the filament region is L> 4√S, MgB ₂ superconducting wire,
7. In MgB ₂ superconducting wire according to claim 6, the outer shape of the metal sheath MgB ₂ superconducting wire, which is a circular shape.
8. In MgB ₂ superconducting wire according to claim 6, MgB ₂ superconducting wire, characterized in that the outer shape of the metal sheath is a polygonal shape.
9. The single core MgB ₂ superconducting wire according to claim 6, wherein the MgB ₂ superconducting wire has one filament region.
10. The multifilamentary MgB ₂ superconducting wire according to claim 6, wherein there are two or more filament regions.

Images