JP2003146759A - METHOD OF MANUFACTURING MgB2 SUPERCONDUCTING MATERIAL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an MgB2 superconducting material which facilitates the manufacturing of the MgB2 superconducting material by suppressing the oxidation and evaporation of Mg as far as possible, can improve superconducting performance and reduce a manufacturing cost. SOLUTION: The sintered MgB2 compact having the superconducting performance is manufactured by embedding a molding made by mixing Mg power and B powder at an atomic ratio of 1:2 and compacting the mixture to form a molding body and embedding the molding body into a powder mixture composed of Ti and C, then heating strongly one end of the powder mixture composed of the Ti and C by ignition to induce the reaction expressed by Ti+ C→TiC, synthesizing the entire part of powder mixture composed of the Ti and C to TiC by the self-propagation high-temperature synthesis of the chain reaction resulting from the successive propagation of the heat of the formation generated at this time, inducing the reaction expressed by Mg+2B→MgB2 by the heat of the formation released during this time and leading this reaction also to the self-propagation high-temperature synthesis over the entire part of the molding body.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a MgB ₂ superconducting material. More specifically, the invention of this application facilitates the production of MgB ₂ superconducting material by suppressing the oxidation and evaporation of Mg as much as possible,
The present invention relates to a method for producing a MgB ₂ superconducting material which can be expected to have improved superconducting performance and can be produced at a reduced cost.

[0002]

_2. Description of the Related Art MgB ₂ superconducting materials are
It has been manufactured as follows until now.

First, magnesium (Mg) and boron (B) powders are mixed and mixed in an atomic ratio of 1: 2, and then pressed to prepare a compact. As is well known, Mg easily oxidizes into an oxide such as magnesium oxide (MgO). In order to prevent this oxidation, the shaped body is then made of, for example, iron (F
e), tantalum (Ta), tungsten (W), etc. are filled in a metal capsule. Then, after vacuum deaeration, argon gas is sealed at about 1/3 to 1/2 atm, and the mixture is held at high temperature for a long time and sintered to obtain a MgB ₂ sintered body having superconducting performance.

[0004]

However, the above manufacturing method has the following problems.

First, as the raw material Mg powder, a coarse powder is used in order to suppress oxidation during storage as much as possible. As a result, the heat treatment time until obtaining the MgB ₂ sintered body is increased. It's getting longer. This is reflected in the manufacturing process and manufacturing cost of the MgB ₂ superconducting material.

Secondly, since Mg easily evaporates at high temperature,
Therefore, the composition ratio of Mg and B in the sintered body is changed by heat treatment at too high temperature for a long time, which affects the superconducting performance.

Thirdly, if the atmosphere during the heat treatment is not a good non-oxidizing atmosphere, the oxidation of Mg is likely to occur. On the other hand, if a vacuum is applied to prevent this oxidation, evaporation of Mg is promoted. Become.

The invention of this application has been made in view of the above circumstances, and facilitates the production of MgB ₂ superconducting material while suppressing the oxidation and evaporation of Mg as much as possible.
It is an issue to be solved to provide a method for producing a MgB ₂ superconducting material which can be expected to improve the superconducting performance and can reduce the production cost.

[0009]

In order to solve the above problems, that is, to prevent the oxidation and evaporation of Mg as much as possible, the inventors of the present application
We have carried out research aiming at synthesizing MgB ₂ superconducting material by reacting B.

One of the inventors of the invention of this application has been involved in the development of self-propagating high temperature synthesis of various compounds including borides and has already been successful (eg, Patent No. 18).
No. 16876). Therefore, in order to solve the above-mentioned problems, MgB ₂
Various attempts have been made to apply the above-mentioned self-propagating high-temperature synthesis to superconducting materials, but have failed.

When the cause is examined, it is considered that the main reason is that the heat of formation of MgB ₂ is only 1/3 of the heat of formation of other borides. For example, the heat of formation of TiB ₂ is -27
The heat of formation of 9.9 kJ / mol, ZrB ₂ is -326.6 kJ / mol, the heat of formation of HfB ₂ is -328.9 kJ / mol, whereas the heat of formation of MgB ₂ is -92.
There is only 0 kJ / mol. Therefore, it is concluded that the reaction shown by Mg + 2B → MgB ₂ and its propagation are unlikely to occur.

Another factor is that Mg is likely to evaporate. That is, when one end of the raw material powder mixture is ignited by igniting it to cause the initial reaction, Mg evaporates, and the evaporation at this time is Mg + 2B.
→ It takes away the heat required for the reaction indicated by MgB ₂ . Moreover, the composition ratio of Mg and B changes due to evaporation of Mg, and as a result, self-propagating high temperature synthesis becomes difficult.

Based on the investigation of these causes, the inventors of the invention of the present application have made earnest studies, and as a result, the above problems have been solved by utilizing the self-propagating high temperature synthesis of Ti and C, and the MgB ₂ superconductivity has been solved. It was found that self-propagating high temperature synthesis of materials was realized.

The heat of formation of TiC is -184.1 kJ / mol, and self-propagating high temperature synthesis easily occurs. The adiabatic temperature at this time is 2937
The temperature is ℃, and the reaction shown by Ti + C → TiC and the heat of reaction during its self-propagating high-temperature synthesis induce self-propagating high-temperature synthesis of Mg and B. That is, the heat of formation of MgB ₂ is small as described above, but the heat required for self-propagating high temperature synthesis of Mg and B is TiC.
It is compensated by the heat of formation of.

The time required for self-propagating high-temperature synthesis of Mg and B is as follows: Mg powder and B powder are mixed at an atomic ratio of 1: 2,
When the compact produced by compaction is about several to several tens of grams, 1
It is a short time of less than a second, and therefore, the oxidation and evaporation of Mg can be minimized.

That is, the invention of this application is based on Mg powder and B
The powder was mixed in an atomic ratio of 1: 2, and the compacted body made by compaction was embedded in the powder mixture of Ti and C, and then one end of the powder mixture of Ti and C was ignited by heating. , Ti + C → TiC occurs, and the heat of formation generated at that time propagates one after another to cause a chain reaction. The entire powder mixture of Ti and C is synthesized into TiC by self-propagating high temperature synthesis, and then released. The heat of formation induces a reaction represented by Mg + 2B → MgB _2, which is also characterized by self-propagating high-temperature synthesis over the whole of the compact to produce a MgB ₂ sintered body having superconducting performance. A method for manufacturing a MgB ₂ superconducting material (claim 1) is provided.

In the invention of this application, the self-propagating high-temperature synthesis of MgB ₂ is carried out in vacuum at a temperature of room temperature or higher and 300 ° C. or lower (claim 2), and the degree of vacuum at that time is 5 × 10 ^-1. Tor
r or less (Claim 3), and heating the powder mixture of Ti and C in a vacuum in advance to remove water and volatile impurities contained in the powder mixture (Claim 4), Further, the powder mixture of Ti and C has a mixing ratio of Ti and C of 1: 1 in atomic ratio and contains 0 to 20 mass% of TiC (claim 5) as one embodiment.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, Mg of the invention of this application
In the method for producing a B ₂ superconducting material, Mg powder and B powder are mixed at an atomic ratio of 1: 2, and a compact produced by compaction is embedded in a powder mixture of Ti and C, and then Ti One end of the powder mixture of C and C is ignited by igniting to cause a reaction represented by Ti + C → TiC, and the heat of formation generated at that time is propagated one after another to cause a chain reaction. The entire powder mixture of was synthesized into TiC and the heat of formation released at that time caused M
A reaction represented by g + 2B → MgB ₂ is induced, and this reaction is also self-propagating at high temperature over the entire compact to produce a MgB ₂ sintered body having superconducting performance. In carrying out this, for example, the self-propagating high temperature synthesis apparatus shown in FIG. 1 can be used.

FIG. 1 is a cross-sectional view showing an outline of a self-propagating high-temperature synthesis apparatus applicable to the method for producing an MgB ₂ superconducting material of the invention of this application and an outline of producing an MgB ₂ superconducting material using this apparatus. It is a figure.

The self-propagating high temperature synthesis apparatus is provided with a vacuum container (1), which can be sealed by a sealing mechanism (2) and is connected to an air supply / exhaust system (3). Supply and exhaust is possible. Inside the vacuum vessel (1), an electric furnace (5) equipped with a heater (4) is arranged. The electric furnace (5) can be loaded with a refractory crucible (6) in which powder for self-propagating high temperature synthesis is put. This electric furnace (5) is located near the upper end opening of the refractory crucible (6) and is capable of igniting and igniting by contact with one end of the powder for self-propagating high temperature synthesis. A coil (7) formed of wires or the like is arranged. A thermocouple (8) for temperature control is also arranged in the electric furnace (5). The above heater (4), coil (7), and thermocouple (8) are pulled out from the vacuum container (1) so that the airtight state is maintained, and electrically connected to a power supply, a controller, etc. And can be operated from the outside.

When carrying out the method for producing a MgB ₂ superconducting material according to the invention of this application, a compact (9) prepared by mixing Mg powder and B powder in an atomic ratio of 1: 2 and compacting the mixture. ) Is embedded in a powder mixture of Ti and C (10) placed in a refractory crucible (6). It is preferable to heat the powder mixture (10) of Ti and C in a vacuum in advance to remove water and volatile impurities contained in the powder mixture (10). This is because it is possible to manufacture a higher quality MgB ₂ superconducting material.

Then, the refractory crucible (6) is loaded into the electric furnace (5), and the vacuum container (1) is sealed by the sealing mechanism (2). Then, the inside of the vacuum container (1) is evacuated by the operation of the supply / exhaust system (3). MgB ₂
The vacuum degree suitable for self-propagating high temperature synthesis of is 5 × 10 ^-1 Tor
It is r or less, and the higher the degree of vacuum, the more the generation of oxides such as MgO can be suppressed.

After that, the coil (7) is brought into contact with the upper end which is one end of the powder mixture (10) of Ti and C, and is energized to ignite to heat the upper end of the powder mixture (10) of Ti and C. Ignite. After ignition, the reaction represented by Ti + C → TiC occurs, and the heat of formation that is generated at that time propagates one after another, causing self-propagating high temperature synthesis in which a chain reaction occurs, and the entire powder mixture of Ti and C (10) becomes TiC. Is synthesized. Also, due to the heat of formation at this time, Mg +
The reaction represented by 2B → MgB ₂ is induced, and the molded body (9) is
Self-propagating high-temperature synthesis occurs, and the whole becomes a MgB ₂ sintered body having superconducting performance. The time required for the self-propagating high temperature synthesis of MgB ₂ is 1 second or less if the molded body (9) is about several to several tens of grams.

The powder mixture of Ti and C (10) is Ti
It is preferable that the mixing ratio of C and C is 1: 1 by atomic ratio,
TiC may be contained in an amount of 0 to 20% by mass in order to adjust the calorific value.

During the self-propagating high-temperature synthesis of MgB ₂ , it is preferable to make the atmosphere in the vacuum container (1) a vacuum as described above, and the temperature in the electric furnace (5) is set to the heater (4). Therefore, it is also preferable to maintain the temperature at room temperature or higher and 300 ° C. or lower. The yield of the MgB ₂ superconducting material obtained under such conditions is about 50 to 100%.

For example, as shown above, the MgB ₂ superconducting material of the present invention can be produced by self-propagating high temperature synthesis by the method for producing an MgB ₂ superconducting material, which requires a short time. , Long-time heat treatment such as the conventional manufacturing by powder metallurgy is eliminated. Therefore, the oxidation and evaporation of Mg can be suppressed as much as possible, the production of the MgB ₂ superconducting material is facilitated, and Mg and
The composition ratio of B is almost stabilized, and it can be expected that the superconducting performance is improved. Moreover, as well as facilitating production, Ti powder and C powder used for causing the self-propagating high temperature synthesis of MgB ₂ are
Since neither is particularly expensive, the manufacturing cost of the MgB ₂ superconducting material can be reduced.

Examples will be shown below.

[0028]

[Examples] Angular Mg powder having an average particle size of about 400 μm, amorphous boron powder having an average particle size of about 1 μm, and average particle size of 30 μ
Square m-shaped Ti powder and graphite powder having an average particle size of about 15 μm were used.

The above Mg powder and B powder were mixed at an atomic ratio of 1: 2, the Mg and B powder mixture was packed into a polyurethane rubber mold, and cold isostatic pressing (CIP) was performed to obtain 1 MPa of 300 MPa. It was kept for a minute and pressed to prepare a molded body. The powder mixture of Ti and C was kept at 200 ° C. for 12 hours to be dried.

Then, a powder mixture of Ti and C (10) was placed in a refractory crucible (6) of a self-propagating high temperature synthesizer shown schematically in FIG. 1, and a compact of Mg and B (9) was placed therein. ) Was buried. Then, the refractory crucible (6) is placed in an electric furnace (7).
A coil (7) made of a tungsten wire having a wire diameter of 0.6 mm was placed in contact with the upper end of the powder mixture of Ti and C (10). In this state, after sealing the vacuum container (1) with the sealing mechanism (2), the inside of the vacuum container (1) is evacuated by the air supply / exhaust system (3) to reduce the degree of vacuum to 1 × 10 ⁻³ Pa or less. I kept it all the time. Then, a current of about 20 A was applied to the coil (7) to ignite the upper end of the powder mixture (10) of Ti and C by igniting it.

After ignition, the reaction represented by Ti + C → TiC occurred, and the heat of formation generated at that time was propagated one after another to cause self-propagating high temperature synthesis in which chain reaction occurred. The entire powder mixture of Ti and C (10) was synthesized into TiC within a short period of time, and Mg + 2B was added in the Mg and B compact (9) that had been charged by the heat of formation released at this time. The reaction represented by MgB ₂ was induced, and self-propagating high temperature synthesis took place, and the entire compact (9) became a MgB ₂ sintered compact.

FIG. 2 is a diagram in which the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the inside of the electric furnace at room temperature is plotted.

It is confirmed that the superconducting transition temperature is 39 K and the change in the magnetic susceptibility is large, and therefore the superconducting performance is excellent.

FIG. 3 is a diagram in which the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the inside of the electric furnace at 150 ° C. is plotted.

It is confirmed that the superconducting transition temperature is 39 K, and the change in the magnetic susceptibility is large, and therefore the superconducting performance is excellent.

FIG. 4 is a diagram in which the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the inside of the electric furnace at 200 ° C. is plotted.

It is confirmed that the superconducting transition temperature is 39 K and the change in the magnetic susceptibility is large, and therefore the superconducting performance is excellent. The MgB ₂ yield measured by SQUID (superconducting quantum interferometer) was 100%.

Of course, the invention of this application is not limited to the above-described embodiments and examples. It goes without saying that various configurations are possible for the detailed configuration and structure of the self-propagating high-temperature synthesis apparatus, and the particle size and shape of the powder used.

[0039]

As described in detail above, according to the invention of this application, oxidation and evaporation of Mg are suppressed as much as possible,
In addition to facilitating the manufacture of MgB ₂ superconducting materials, it is expected that the superconducting performance will be improved and the manufacturing cost will be reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an outline of a self-propagating high-temperature synthesis apparatus applicable to a method for producing an MgB ₂ superconducting material of the invention of this application and an outline when producing an MgB ₂ superconducting material using this apparatus. .

FIG. 2 is a diagram in which a change in magnetic susceptibility of a MgB ₂ sintered body produced by keeping the inside of an electric furnace at room temperature with temperature is plotted.

FIG. 3 is a diagram plotting temperature changes of magnetic susceptibility of a MgB ₂ sintered body produced by keeping the inside of an electric furnace at 150 ° C.

FIG. 4 is a diagram plotting temperature changes of magnetic susceptibility of a MgB ₂ sintered body produced by keeping the inside of an electric furnace at 200 ° C.

[Explanation of symbols]

1 vacuum container 2 sealing mechanism 3 air supply / exhaust system 4 heater 5 electric furnace 6 Fireproof crucible 7 coils 8 thermocouple Molded body of 9 Mg and B Powder mixture of 10 Ti and C

Claims (5)

[Claims] 1. A powder mixture of Ti and C, which is obtained by mixing Mg powder and B powder at an atomic ratio of 1: 2 and squeezing the compacted body to be embedded in the powder mixture of Ti and C. One end of the powder is ignited and ignited to cause a reaction represented by Ti + C → TiC, and the generated heat generated at that time is propagated one after another, and the entire powder mixture of Ti and C is subjected to chain reaction by self-propagating high temperature synthesis. Synthesized to TiC,
The heat of formation released at that time induces a reaction represented by Mg + 2B → MgB ₂ , and this reaction is also self-propagating at high temperature over the entire body of the compact, and MgB ₂ having superconducting performance is obtained.
A method for producing a MgB ₂ superconducting material, which comprises producing a sintered body. 2. The MgB _{2 according to} claim 1, wherein self-propagating high-temperature synthesis of MgB ₂ is carried out in a vacuum at a temperature of room temperature or higher and 300 ° C. or lower.
₂ Manufacturing method of superconducting material. 3. The method for producing a MgB ₂ superconducting material according to claim 2, wherein the degree of vacuum is 5 × 10 ⁻¹ Torr or less. 4. The MgB _{2 according} to any one of claims 1 to 3, wherein the powder mixture of Ti and C is previously heated in vacuum to remove water and volatile impurities contained in the powder mixture.
Manufacturing method of superconducting material. 5. The MgB according to claim 1, wherein the powder mixture of Ti and C has a mixing ratio of Ti and C of 1: 1 in atomic ratio and contains 0 to 20 mass% of TiC. ₂ Manufacturing method of superconducting material.