JP2008140556A - MANUFACTURING METHOD OF MgB2 SUPERCONDUCTIVE WIRE ROD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of an MgB<SB>2</SB>superconductive wire material capable of simultaneously achieving lengthy wire material and high Jc necessary for making a practical superconductive wire material. <P>SOLUTION: In the manufacturing method of this MgB<SB>2</SB>superconductive wire material, first Mg primary particles and B primary particles are mixed, the B primary particles are made to be adhered/reacted on a surface of the first Mg primary particles, MgB<SB>4</SB>or MgB<SB>7</SB>is formed on a surface of the first Mg primary particles, the first Mg primary particles in which MgB<SB>4</SB>or MgB<SB>7</SB>is formed on the surface and the second Mg primary particles larger in a particle diameter than the first Mg primary particles in which MgB<SB>4</SB>or MgB<SB>7</SB>is formed on the surface are mixed, the first Mg primary particles in which MgB<SB>4</SB>or MgB<SB>7</SB>is formed on the surface are made to be adhered/reacted on the surface of the second Mg primary particles, and MgB<SB>2</SB>is made to be formd on the surface of the secondary Mg primary particles. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method for producing a MgB ₂ superconducting wire.

In the 21st century, as described in Non-Patent Document 1, it was discovered that magnesium diboride (MgB ₂ ) exhibits superconducting properties at 39K. This material is mainly known for the following characteristics.

  (1) The critical temperature (hereinafter Tc) is 39K, which is 20K or more higher than that of a conventional metal superconductor.

(2) The upper critical magnetic field (hereinafter referred to as Hc ₂ ) is about 20 T or more, which is larger than that of conventional metal superconductors.

(3) The transport critical current density (hereinafter, Jc) is on the order of 1000 A / mm ² at the maximum.

  (4) The magnetic anisotropy is small, and the same current can flow in any direction of the a-axis, b-axis, and c-axis of the crystal.

As described above, since the MgB ₂ superconductor has higher Tc and Hc ₂ than the conventional metal superconductor, when applied to a superconducting magnet, there is a merit that an extremely stable system without a quench accident can be constructed. .

As a general method for making MgB _{2 into} a wire, which is applied to wire production, a powder-in-tube (PIT) method suitable for industrialization is mainly used. This PIT method is
(I) Ex-situ method of filling MgB ₂ superconducting powder in a relatively strong stainless steel or other metal tube (ii) Superconductivity by filling Mg and B mixed powder in a metal tube of iron or the like and heat-treating It can be roughly divided into two types of in-situ methods.

Nature 410, 63-64 (2001)

In the case of the ex-situ method, MgB ₂ superconducting powders react with each other, and thus heat treatment at a high temperature for a long time is substantially inevitable. In heat treatment, increasing the temperature and lengthening the time lead to an increase in cost, which is undesirable in terms of application.

Further, in the ex-situ method, it largely depends on the characteristics of the MgB ₂ superconducting powder to be filled. Specifically, when an oxide film is formed on the surface of the MgB ₂ superconducting powder, the final heat treatment is performed. A heterogeneous phase is generated at the powder interface, blocking the current path. Therefore, at this stage, there is a problem with Jc particularly in a high magnetic field.

On the other hand, in the case of an in-situ method, a method of generating MgB ₂ by a diffusion reaction between Mg powder and B powder is common. Considering this reaction form (Mg + 2B → MgB ₂ ), each powder having a molar volume of Mg: 14 × 10 ⁻³ m ³ / mol and 2B: 9 × 10 ⁻³ m ³ / mol is subjected to heat treatment.
Since MgB _{2 is} 17 × 10 ⁻³ m ³ / mol, the sintered density is reduced by about 26%. This makes it difficult to increase the density in the wire.

  In the in-situ method, Fe is often used as a metal tube. Since Fe reacts thermally with B, a compound of Fe and B is used during heat treatment for superconductivity. Is formed. Thereby, there exists a subject that Jc falls.

Accordingly, the present invention is to provide a method for producing a MgB ₂ superconducting wire that can simultaneously achieve the lengthening of the long wire and the increase in Jc necessary for making a practical superconducting wire.

In the method of manufacturing the MgB ₂ superconducting wire of the present invention, the first Mg primary particles and the B primary particles are mixed, the B primary particles are adhered to the surface of the first Mg primary particles, and the first heat treatment is performed. A step of causing the first Mg primary particles to react with the B primary particles attached to the surface of the first Mg primary particles to generate MgB ₄ or MgB ₇ on the surface of the first Mg primary particles; a first Mg primary particles MgB ₄ or MgB ₇ is formed on the surface, and MgB ₄ or the first particle diameter than Mg primary particles is larger second Mg primary particles MgB ₇ is formed on the surface were mixed, The first Mg primary particles produced with MgB ₄ or MgB ₇ are attached to the surface of the second Mg primary particles, and the first Mg primary particles produced with MgB ₄ or MgB ₇ are attached to the surface. The second Mg primary particles made into a tube-shaped metal tube Filling the sheath), subjected to wire drawing, by a second heat treatment, is reacted with a second Mg primary particles, and MgB ₄ or MgB _7, MgB ₂ on the surface of the second Mg primary particles And a step of generating.

  In the present invention, the primary particles are single crystal particles.

Also, when the first Mg primary particles react with the B primary particles attached to the surface of the first Mg primary particles to produce MgB ₄ or MgB ₇ on the surface of the first Mg primary particles, and When the second Mg primary particles are reacted with MgB ₄ or MgB ₇ to produce MgB ₂ on the surface of the second Mg primary particles, the first Mg primary particles The particle diameter and the particle diameter of the second Mg primary particles are respectively smaller than before the reaction.

The generated MgB ₂ is continuously generated inside the metal tube, that is, electrically continuously. The produced MgB ₂ is produced in the metal tube at a sintering density of 85% or more with respect to the theoretical density (2.63 g / cm ³ ) of MgB ₂ .

  The first heat treatment is preferably performed at a temperature of 800 to 1200 ° C., and the second heat treatment is preferably performed at a temperature of 600 to 750 ° C.

Further, the reaction between the first Mg primary particles and the B primary particles attached to the surface of the first Mg primary particles and the reaction between the second Mg primary particles and MgB ₄ or MgB ₇ are diffusion reactions.

  In addition, the particle diameter of the 1st Mg primary particle is 10-30 micrometers, the particle diameter of B primary particle is 1/10-1/100 with respect to the particle diameter of 1st Mg primary particle, 2nd It is preferable that the particle diameter of Mg primary particle is 40-60 micrometers.

Moreover, MgB ₂ superconducting wire of the present invention, is continuously MgB ₂ is formed in the longitudinal direction of the wire, an arbitrary position of the wire that MgB ₂ is formed in (cross), MgB ₂ is around the Mg particles The average radius of the Mg particles is 5 μm or less, and the average thickness of MgB ₂ is equal to or greater than (1 ×) the average radius of the Mg particles.

Then, MgB ₂ superconducting wire of the present invention, is continuously MgB ₂ is formed in the longitudinal direction of the wire, an arbitrary position of the wire that MgB ₂ is formed in (cross-section), a first Mg primary particles, has a larger second Mg primary particles of particle diameter than the first Mg primary particles, MgB ₂ around each particle, characterized in that it is produced.

  It is also effective to use a manufacturing method including a step of forming a film of B or a compound composed of Mg and B on the surface of Mg particles.

Moreover, since the wire rod produced by the production method of the present invention uses Mg powder and B powder independently and causes a diffusion reaction to produce MgB ₂ , the volume shrinkage rate is small. The sintered density in the core of the MgB ₂ superconducting wire can be improved, and a high Jc can be achieved.

  The method for producing a superconducting wire according to the present invention can simultaneously achieve a long wire and a high Jc necessary for making a practical superconducting wire.

The present inventors have advanced research and development of superconducting wires and their magnets, and have clarified that the following four items are particularly important as items indispensable for producing high-performance superconducting wires. That is,
(1) Selection of metal tube material that does not thermally react with the superconductor (2) Improvement of packing density of the superconductor when processed into the final shape (3) Improvement of bondability between crystal grains (4) Quantization This is the introduction of a pinning center that traps the magnetic flux lines that are trapped and prevents the magnetic flux lines that have entered from moving.

  By realizing the above items simultaneously, a superconducting wire having high characteristics can be obtained.

However, Jc is not a value specific to the substance, but greatly depends on the configuration of the wire and the manufacturing method of the wire. For this reason, it has been found that the Jc of the MgB ₂ superconducting wire is not significantly improved by the manufacturing method applied to the conventional metal-based superconducting wire and oxide-based superconducting wire.

Therefore, it is necessary to optimize each superconducting material, and the MgB ₂ superconductor has to be independently studied.

Thus, as a result of intensive studies on a method for producing an MgB ₂ superconducting wire, the present inventors have found means as in the present invention. By applying this means, it is possible to manufacture an MgB ₂ superconducting wire that has a high Jc and can be easily formed into a long wire regardless of the shape of the wire.

  Furthermore, this embodiment will be described in more detail with reference to the drawings. However, the present invention is not limited to the examples described below.

  FIG. 1 shows a production flow diagram of a method for manufacturing a superconducting wire in this embodiment.

  The first Mg primary particles having a particle size of 10 μm and B primary particles having a particle size of 0.3 μm were weighed so that the atomic molar ratio of Mg to B was 1: 4. Mixed with a planetary ball mill.

  The atmosphere during mixing was in Ar, and the time was 2 hours. The material of the pot and ball was made of alumina.

  When the mixed powder is observed with a scanning electron microscope, it can be seen that B primary particles having a particle size of 0.3 μm are almost uniformly adhered to the surface of the first Mg primary particles having a particle size of 10 μm. It was.

  Next, first heat treatment was performed at a temperature of 800 ° C. using the obtained powder.

When the heat-treated powder was observed with a scanning electron microscope, it was found that MgB ₄ was formed on the surface of the first Mg primary particles. Hereinafter, this is abbreviated as first Mg primary particles + MgB ₄ .

Each value in the first Mg primary particles + MgB ₄ is slightly different depending on the position (cross section) of the wire, but the first Mg primary particles are 3 μm, MgB ₄ is 3.5 μm, and the particle diameter is approximately 10 μm. .

  The reaction between the first Mg primary particles and the B primary particles attached to the surface of the first Mg primary particles is a diffusion reaction.

Next, the first Mg primary particles + MgB ₄ powder and the second Mg primary particles having a particle size of 45 μm were weighed so that the atomic molar ratio of Mg to B was 1: 2, and then both Were mixed with a planetary ball mill.

  The atmosphere during mixing was in Ar, and the time was 2 hours. The material of the pot and ball was made of alumina.

When the mixed powder was observed with a scanning electron microscope, it was found that the first Mg primary particles + MgB ₄ adhered almost uniformly to the surface of the second Mg primary particles.

The respective particle sizes were 45 μm for the second Mg primary particles and 10 μm for the first Mg primary particles + MgB ₄ .

  The obtained powder was filled into a Cu / Nb composite tube having an outer periphery of Cu and an inner periphery of Nb. The outer diameter of the tube was 12 mm, the inner diameter was 7 mm, and the length was 300 mm.

  Thereafter, the wire was subjected to wire drawing to reduce the wire diameter to 1.0 mm.

  A superconducting wire was obtained by subjecting the obtained wire to a second heat treatment at 640 ° C. for 1 hour in an Ar atmosphere.

As a result of observing the cross section of the wire with a scanning electron microscope, it was confirmed that MgB ₂ was formed on the surface of the second Mg primary particles. Unreacted Mg was observed, and both the first Mg primary particles and the second Mg primary particles were confirmed as unreacted Mg.

The particle diameter of each particle varies slightly depending on the location, but the second Mg primary particle is 33 μm,
MgB ₂ was 10 μm, and the first Mg primary particles were 3 μm. FIG. 3 schematically shows such a state.

The reaction between the second Mg primary particles and MgB ₄ or MgB ₇ is a diffusion reaction.

Next, the sintered density of the cross-section core part in the produced MgB ₂ superconducting wire was evaluated. Here, the sintered density was calculated by the following method.

  Specific examples are described below.

A MgB ₂ superconducting wire having a constant weight (M _t (g)) is cut. Next, the cut superconducting wire is immersed in alcohol, the weight (W (g)) of the wire in alcohol is measured, and the buoyancy acting on the superconducting wire is calculated.

Then, the volume (V _t (cm ³ )) of the superconducting wire is calculated using the alcohol density (ρ = 0.789 (g / cm ³ ). Specifically, when the buoyancy is F _t , the following ( 1), the equation (2), V _t is calculated.

F _t = M _t −W (1)
V _t = F _t / ρ (2)
Subsequently, the superconducting wire is dissolved in nitric acid, and the solution is subjected to ICP (Inductive Coupled Plasma) emission analysis, whereby the sheath portion of the metal tube composed of Cu and Nb is quantified, and the sheath occupies the weight of the superconducting wire. The ratio (Y) of parts is calculated.

Then, from the weight of the superconducting wire, the weight of the cross-section core part (M _f (g)) and the weight of the sheath part (M _s (g)) are calculated by the following expressions (3) and (4). The

M _s = M _t × Y (3)
M _f = M _t −M _s (4)
Next, the volume of the sheath part (V _s (cm ³ )) is known specific gravity (for example, 8.82 (g / cm ³ ) when the sheath part is Cu, and 8.57 (g / cm ³ ) when Nb is used. ³ )), and the volume of the core section (V _f (cm ³ )) is calculated from the volume of the sheath part.

Then, the density ρ _{f of the} cross-sectional core part is calculated from the volume of the cross-sectional core part. Specifically, ρ _f is calculated by the following equations (5) to (7).

V _s = M _s / specific gravity of sheath material (5)
V _f = V _t −V _s (6)
ρ _f = M _f / V _f (7)
On the other hand, when the cross-sectional core portion is made of MgB ₂ , the theoretical density is 2.63 g / cm ³ . From the ratio of the theoretical density and the density ρ of the cross-sectional core part, the sintered density of the cross-sectional core part is calculated.

  Specifically, it is calculated by equation (8).

Sintering density (%) = (ρ _f /2.63)×100 (8)
As a result of evaluating the sintered density after the second heat treatment in the superconducting wire produced in this example, the density of the cross-sectional core portion is 2.32 g / cm ³ and the sintered density is 88%. all right.

  Next, Jc measurement was performed at a temperature of 4.2K. Here, a magnetic field of 4 to 7 T was applied to the superconducting wire.

The result is shown in FIG. Jc of 1050 A / mm ² was obtained at a temperature of 4.2 K, an applied magnetic field of 4 T, and 4100 A / mm ² and an applied magnetic field of 6 T.

[Comparative Example 1]
As a comparative example, a superconducting wire as a comparative example with respect to Example 1 was manufactured by the steps described below.

  The first Mg primary particles having a particle size of 10 μm and the B primary particles having a particle size of 0.3 μm were weighed so that the atomic molar ratio of Mg to B was 1: 2, and then both were connected to a planetary ball mill. Mixed. The atmosphere during mixing was in Ar, and the time was 2 hours. The material of the pot and ball was made of alumina.

  When the mixed powder was observed with a scanning electron microscope, it was found that B primary particles were evenly adhered to the surface of the first Mg primary particles.

  The obtained powder was filled into a Cu / Nb composite tube having an outer periphery of Cu and an inner periphery of Nb. The outer diameter of the tube was 12 mm, the inner diameter was 7 mm, and the length was 150 mm.

  Thereafter, the wire was subjected to wire drawing to reduce the wire diameter to 1.0 mm.

  The obtained wire was heat-treated at 640 ° C. for 1 hour in an Ar atmosphere to obtain a superconducting wire.

As a result of observing the cross section of the wire with a scanning electron microscope, it was confirmed that MgB ₂ produced by the diffusion reaction was present on the surface of the first Mg primary particles.

In the superconducting wire produced in this comparative example, the sintered density after heat treatment at 640 ° C. for 1 hour was evaluated. As a result, the density of the cross-sectional core portion was 1.88 g / cm ³ , and the sintered density was 71%. I found out.

  Next, Jc measurement was performed at a temperature of 4.2K. Here, a magnetic field of 4 to 7 T was applied to the superconducting wire.

  The result is shown in FIG.

  Jc accumulated at a low value as compared with the superconducting wire produced in Example 1. This is presumably because the decrease in the sintered density became voids and the current path was interrupted.

An MgB ₂ superconducting wire was produced using the same manufacturing method as in Example 1 except that the first heat treatment temperature in Example 1 was changed from 600 ° C. to 1300 ° C. in increments of 100 ° C.

  Table 1 shows the crystal phase observed with a scanning microscope after the first heat treatment temperature and the first heat treatment.

  Moreover, the sintered density of the cross-sectional core part of the superconducting wire after the final heat treatment, and Jc in 4.2K and 4T are also shown.

In the heat treatment at 600 ° C. and 700 ° C., an MgB ₂ phase was observed in addition to Mg and B.
It was found that the MgB ₄ phase was formed in the heat treatment at 800 ° C. to 1000 ° C. In addition, MgB ₇ was generated in the heat treatment at 1100 ° C. and 1200 ° C. However, it was found that when the temperature was raised to 1300 ° C, most of the phases could not be identified, and superconducting properties were not exhibited.

On the other hand, by performing heat treatment at 800 ° C. to 1200 ° C., the sintered density of the cross-sectional core portion of the superconducting wire can be 85% or more of the MgB ₂ theoretical density, and Jc is clearly increased reflecting this. became.

  From this, it was found that the first heat treatment temperature is effectively in the range of 800 ° C to 1200 ° C.

An MgB ₂ superconducting wire was produced using the same manufacturing method as in Example 1 except that the second heat treatment temperature in Example 1 was changed from 500 ° C. to 850 ° C. in increments of 50 ° C.

  Table 2 shows the crystal phase observed with a scanning microscope after the second heat treatment temperature and the second heat treatment.

  Also. The Jc in 4.2K and 4T is also shown.

At 500 ° C., MgB ₂ was not generated, so Jc was zero. When the temperature is raised to 550 ° C., MgB ₂ starts to be produced, but the temperature is insufficient, so that MgB ₂ and MgB ₄ are mixed.

In addition, at a temperature of 800 ° C. or higher, once generated MgB ₂ changes to MgB ₄ ,
Jc decreased. In the heat treatment at 550 ° C., 800 ° C., and 850 ° C., Jc accumulated at ²⁰⁰ to 720 A / mm ² in 4.2 K and 4 T.

When the heat treatment temperature is set to 600 ° C. to 750 ° C., MgB ₂ is generated, and in 4.2K and 4T,
Jc exceeding 3900 A / mm ² was obtained. From this, the second heat treatment is from 600 ° C to
It has been found that it is effective to set the temperature within the range of 750 ° C.

  In this embodiment, a superconducting wire was produced using a PIT method in which a pipe-shaped metal tube is filled with powder and subjected to plastic working. However, a compacted body formed by molding the powder is filled into the pipe-shaped metal tube. In addition, a rod-in-tube method or the like that performs plastic working may be employed. Since the superconductor and the metal tube may react thermally and Jc may decrease, it is preferable to select a material that does not react with the superconductor as the metal sheath material as the metal tube that is in direct contact with the superconductor. .

  A drawing bench, hydrostatic extrusion, swager, cassette roller die, or groove roll can be used for wire drawing to reduce the diameter of the superconducting wire, and the cross-section reduction rate per pass is about 8% to 12%. Repeat the wire drawing process.

  Further, in order to improve the bending characteristics and increase the density of the superconducting core part, the number of cores is increased as necessary. In order to increase the number of cores, in general, a wire rod drawn into a round or hexagonal cross section is incorporated into a multi-core metal pipe, and the cross-sectional reduction rate per pass is set to about 5% to 30%. The method of drawing to a wire diameter is taken.

  In addition to these methods, equivalent superconducting characteristics can be obtained using superconducting wires produced by, for example, thermal spraying, doctor blade method, dip coating method, spray pyrolysis method or jelly roll method. It is.

  Depending on the purpose, two or more superconducting wires produced can be combined and wound in a spiral shape, or formed into a lead wire shape or a cable wire shape.

  Further, in the present embodiment, high Jc can be obtained by using a superconductor that exhibits superconducting characteristics in an environment below a critical temperature.

  And not only the cooling by liquid helium but also the cooling by liquid hydrogen, refrigerator conduction cooling, etc., the device can be operated, and a high Jc can be obtained even in a magnetic field.

  Specifically, current leads, power transmission cables, large magnets, nuclear magnetic resonance analyzers, medical magnetic resonance diagnostic devices, superconducting power storage devices, magnetic separation devices, single crystal pulling devices in magnetic fields, refrigerator-cooled superconducting magnet devices, It is applied to equipment (superconducting equipment) such as superconducting energy storage, superconducting generator, and fusion reactor magnet.

The present invention can be used for a superconducting device using an MgB ₂ superconducting wire.

The flowchart which shows the manufacturing method of the superconducting wire in this embodiment. The figure which shows the critical current density regarding the applied magnetic field of the superconducting wire manufactured in Example 1 and Comparative Example 1. FIG. The cross-sectional schematic diagram of the superconducting wire in this embodiment.

Explanation of symbols

1 Mg (secondary)
2 Mg (primary)
3 MgB ₂

Claims (9)

First Mg primary particles and B primary particles are mixed, and the B primary particles are adhered to the surface of the first Mg primary particles,
By performing the first heat treatment, the first Mg primary particles react with the B primary particles attached to the surface of the first Mg primary particles, and MgB is formed on the surface of the first Mg primary particles. ₄ or MgB ₇
Mixing and MgB ₄ or the first Mg primary particles MgB ₇ is generated, and MgB ₄ or larger particle size than the MgB ₇ has generated the first Mg primary particles a second Mg primary particles on the surface to the surface Then, the first Mg primary particles having MgB ₄ or MgB ₇ formed on the surface are attached to the surface of the second Mg primary particles,
Filling the tube-shaped metal tube with the second Mg primary particles having the first Mg primary particles produced on the surface with the MgB ₄ or MgB ₇ adhered to the surface, and drawing the wire,
By performing a second heat treatment, the second Mg primary particles react with the MgB ₄ or MgB ₇ to generate MgB ₂ on the surface of the second Mg primary particles. Manufacturing method of MgB ₂ superconducting wire. A method of manufacturing a MgB ₂ superconducting wire according to claim 1, MgB ₂ the generated method for manufacturing a MgB ₂ superconducting wire, characterized in that the interior of the metal tube is continuously produced . _{2. The} method of manufacturing a MgB ₂ superconducting wire according to claim 1, wherein the generated MgB ₂ is generated at a sintering density of 85% or more of the theoretical density of MgB ₂ inside the metal tube. A method for producing a MgB ₂ superconducting wire, characterized in that: The method for producing an MgB ₂ superconducting wire according to claim 1, wherein the first heat treatment is performed at a temperature of 800 to 1200 ° C. The method for producing a MgB ₂ superconducting wire according to claim 1, wherein the second heat treatment is performed at a temperature of 600 to 750 ° C. _2. The MgB ₂ superconducting wire according to claim 1, wherein a reaction between the first Mg primary particles and the B primary particles attached to the surface of the first Mg primary particles is a diffusion reaction. Production method. The method for producing a MgB ₂ superconducting wire according to claim 1, wherein a reaction between the second Mg primary particles and the MgB ₄ or MgB ₇ is a diffusion reaction. MgB ₂ superconducting wire,
Said wire longitudinally continuous MgB ₂ is formed of, in any cross section of the wire that the MgB ₂ is formed, MgB ₂ is formed around the Mg particles, the average radius of the Mg particles with 5μm or less A MgB ₂ superconducting wire characterized in that the average thickness of MgB ₂ is equal to or greater than the average radius of the Mg particles. MgB ₂ superconducting wire,
MgB ₂ is continuously formed in the longitudinal direction of the wire, and the first Mg primary particles and the particle diameter larger than the first Mg primary particles in an arbitrary cross section of the wire formed with the MgB ₂ A MgB ₂ superconducting wire, characterized in that it has second Mg primary particles, and MgB ₂ is generated around each particle.

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