JP2003306383A - METHOD FOR PRODUCING MgB2 SUPERCONDUCTIVE MATERIAL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a MgB<SB>2</SB>superconductive material in which oxidation and evaporation of Mg are suppressed as much as possible; the production of the MgB<SB>2</SB>superconductive material is facilitated; the improvement of the superconductive properties is possible; and production cost is reduced. <P>SOLUTION: Mg and B powders are mixed in an atomic ratio of 1:2, and the mixture of the raw material powders is compressed to prepare a molding. The molding is buried in a mixture of Ti and B powders. Part of the mixture of Ti and B powders is intensely heated to be ignited to cause the reaction represented by Ti+2B→TiB<SB>2</SB>. The whole mixture of Ti and B powders is caused to react to synthesize TiB<SB>2</SB>by self-propagation high temperature synthesis in which the heat of formation generated by the reaction propagates in turn to cause chain reaction. The heat of formation released at the synthesis induces the reaction represented by Mg+2B→MgB<SB>2</SB>. This reaction also causes self- propagation high temperature synthesis to convert the whole molding to a MgB<SB>2</SB>sintered body with superconductive properties. <P>COPYRIGHT: (C)2004,JPO

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 suppresses the oxidation and evaporation of Mg as much as possible to facilitate the production of the MgB ₂ superconducting material, and
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 manufactured at a reduced cost.

[0002]

_2. Description of the Related Art A MgB ₂ superconducting material has been manufactured by powder metallurgy as follows.

That is, magnesium (Mg) and boron (B)
The powder of 1 is mixed and mixed so that the atomic ratio is 1: 2, and then the powder is pressed to prepare a molded body. As is well known, Mg easily oxidizes to an oxide such as magnesium oxide (MgO), and therefore, in order to prevent this oxidation, the molded body is made of iron (Fe), tantalum (Ta), tungsten ( It is packed in a metal capsule formed from W). Then, after vacuum deaeration, argon gas is sealed in at about 1/3 to 1/2 atm, and is held at a high temperature for a long time for sintering.

[0004]

However, the above manufacturing method has the following problems.

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

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

Thirdly, if the atmosphere during the heat treatment is not a good non-oxidizing atmosphere, the oxidation of Mg cannot be avoided, but if it is evacuated, the evaporation is promoted and the handling is difficult.

The invention of this application has been made in view of such circumstances, and suppresses the oxidation and evaporation of Mg as much as possible to facilitate the production of the MgB ₂ superconducting material, and
It is an issue to be solved to provide a manufacturing method of a MgB ₂ superconducting material which can be expected to improve the superconducting performance and can also reduce the manufacturing cost.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors of the present invention, namely, suppress the oxidation and evaporation of Mg as much as possible, and make Mg as short as possible.
Studies have been repeated to react B with B to synthesize a MgB ₂ superconducting material.

One of the inventors of the invention of this application has succeeded in self-propagating high-temperature synthesis of carbides, borides and the like (for example, Japanese Patent No. 1816876). Self-propagation high-temperature synthesis is that when a powder mixture is ignited by igniting a part thereof and an initial reaction occurs, the heat of formation generated at this time is successively propagated to cause a chain reaction, and the entire powder mixture is It is synthesized into compounds such as carbides and borides.

Therefore, various attempts were made to apply the self-propagating high-temperature synthesis to the MgB ₂ superconducting material, but the realization was not easy. When the cause was investigated from various directions, it was found that the heat of formation of MgB ₂ was only about 1/3 of the heat of formation of other borides. The heat of boride formation is, for example, -279.9 kJ / mol for TiB ₂ and -326.6 kJ / m for ZrB _2.
It is -328.9 kJ / mol for ol and HfB ₂ . On the other hand, the heat of formation of MgB ₂ is as small as -92.0 kJ / mol. From this, Mg + 2B →
It is concluded that the reaction shown by MgB ₂ and its propagation are unlikely to occur.

Another factor is that Mg is also likely to evaporate. When a portion of the powder mixture of the raw materials is ignited by heating to ignite the initial reaction, Mg evaporates, and this evaporation produces heat necessary for the reaction represented by Mg + 2B → MgB _2. It will rob you. Further, as described above, the composition ratio of Mg and B in the raw material fluctuates with the evaporation of Mg, which makes self-propagating high temperature synthesis even more 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, by utilizing the self-propagating high temperature synthesis of Ti and B, the above problems were solved and MgB ₂ It was found that self-propagating high temperature synthesis of superconducting materials can be realized.

As described above, the heat of formation of TiB ₂ is -279.9 kJ / mol.
And self-propagating high temperature synthesis easily occurs. The adiabatic temperature during self-propagating high-temperature synthesis is 2917 ℃, and Ti + 2B → TiB
_The reaction represented by ₂ and the heat of reaction during the self-propagating high temperature synthesis induce the reaction represented by Mg + 2B → MgB ₂ and the self-propagating high temperature synthesis. In other words, the heat of formation of MgB ₂ is
The heat required for self-propagating high-temperature synthesis of Mg and B, which is the smallest among borides, is supplemented by the heat of formation of TiB ₂ .

As described above, the self-propagating high-temperature synthesis of Mg and B induced by the self-propagating high-temperature synthesis of TiB ₂ is prepared by mixing Mg powder and B powder in an atomic ratio of 1: 2 and compacting. When the formed compact has a weight of several grams to several tens of grams, the time required is a short time of 1 second or less, and therefore, the oxidation and evaporation of Mg can be minimized.

The invention of this application has been completed based on the technical knowledge as described above.

That is, the invention of this application is based on Mg powder and B
The powder was mixed at an atomic ratio of 1: 2, and the molded body produced by compacting this raw material mixed powder was embedded in the powder mixture of Ti and B, and then the powder mixture of Ti and B was partially used. At the same time, it ignites and ignites to cause the reaction shown by Ti + 2B → TiB ₂ , and the heat of formation generated at that time is propagated one after another to cause a chain reaction.
While synthesizing into ₂ , the heat of formation released at this time causes Mg + 2B
→ Manufacture of MgB ₂ superconducting material characterized by inducing a reaction represented by MgB ₂ and also self-propagating high temperature synthesis of this reaction to make the whole of the compact into a MgB ₂ sintered body having superconducting performance. A method (claim 1) is provided.

In the invention of this application, the self-propagating high-temperature synthesis of MgB ₂ is performed in vacuum at room temperature or higher and 300 ° C. or lower (claim 2), and the degree of vacuum is 5 × 10 ^-1 Torr or lower. It is provided as an aspect to do so (claim 3) and to preheat the powder mixture of Ti and B in a vacuum to remove water and volatile impurities contained in the powder mixture (claim 4).

Hereinafter, the method for producing the MgB ₂ superconducting material of the invention of this application will be described in more detail with reference to Examples.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a MgB ₂ superconducting material according to the invention of the present application, as described above, Mg powder and B powder are mixed at an atomic ratio of 1: 2, and this raw material mixed powder is compacted. The compact thus produced is embedded in a powder mixture of Ti and B. Next, the powder mixture of Ti and B is ignited by igniting a part of it, causing a reaction represented by Ti + 2B → TiB ₂ , and the heat of formation generated at that time is propagated one after another and a chain reaction occurs. The entire powder mixture of Ti and B is synthesized into TiB ₂ by synthesis. On the other hand, in the method for producing the MgB ₂ superconducting material of the invention of this application, the reaction shown by Mg + 2B → MgB ₂ is induced by the heat of formation released during the self-propagating high-temperature synthesis of TiB ₂ , and this reaction is also self-sustaining. Propagation at high temperature is performed, and the entire compact is made into a MgB ₂ sintered compact having superconducting performance.

Specifically, when carrying out the method for producing the MgB ₂ superconducting material of the invention of this application, the self-propagating high temperature synthesis apparatus outlined in FIG. 1 can be used.

As shown in FIG. 1, the self-propagating high temperature synthesis apparatus comprises a vacuum container (1). Vacuum container (1)
Is sealed by a sealing mechanism (2) and is connected to an air supply / exhaust system (3) to enable the internal air supply / exhaust. Inside the vacuum container (1), an electric furnace (5) equipped with a heater (4) and a thermocouple (8) is arranged. The electric furnace (5) has a refractory crucible (6) inside.
Are placed. The refractory crucible (6) is filled with a powder mixture of Ti and B (10).

In addition, the self-propagating high-temperature synthesizer includes Ti and B.
An electric heating coil (7) that can be formed from a tungsten wire, a nichrome wire, or the like for igniting and igniting a part of the powder mixture (10) is provided, and the electric heating coil (7) is usually a refractory crucible. It is arranged so as to contact the upper end of the powder mixture of Ti and B (10) filled in (6).

The above-mentioned heater (4), electrothermal coil (7) and thermocouple (8) are all pulled out from the vacuum container (1) so as to maintain an airtight state, and a power source, It is electrically connected to a controller and can be operated from the outside.

In the production of the MgB ₂ superconducting material, Mg powder and B powder are mixed at an atomic ratio of 1: 2, and the raw material mixed powder is compacted to prepare a compact (9). This compact (9) is embedded in a powder mixture (10) of Ti and B filled in a refractory crucible (6). Incidentally, the powder mixture (10) of Ti and B is heated in vacuum in advance before embedding the molded body (9) to remove water and volatile impurities contained in the powder mixture (10). , Better quality MgB ₂
A superconducting material is obtained.

After that, the refractory crucible (6) is placed inside 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). The vacuum degree at this time is preferably set to be suitable for causing self-propagating high-temperature synthesis of the MgB ₂ superconducting material, and is, for example, 5 × 10 ^-1 Torr or less. The higher the degree of vacuum, the more effective it is to suppress the formation of oxides such as MgO.

Next, an electric heating coil (7) is placed in contact with a part of the powder mixture (10) of Ti and B, specifically, the upper end portion as shown in FIG. , Ti and B
The upper end, that is, a part of the powder mixture (10) is ignited. After ignition, in the powder mixture of Ti and B (10), Ti + 2B
→ The reaction indicated by TiB ₂ occurs, the heat of formation generated at that time propagates one after another, causing a chain reaction, and self-propagating high temperature synthesis occurs. Finally, a powder mixture of Ti and B (1
The whole of 0) becomes TiB ₂ . On the other hand, the heat of formation released during the self-propagating high-temperature synthesis of the powder mixture of Ti and B (10) induces the reaction represented by Mg + 2B → MgB ₂ , and the compact (9) also has a small heat of formation. Irrespective of the fact that Mg that has self-propagating high temperature synthesis and has superconducting performance
It becomes a B ₂ sintered body. Generally, when the molded body (9) is about several grams to several tens of grams, the time required for self-propagating high-temperature synthesis of MgB ₂ is as short as 1 second or less.

The self-propagating high-temperature synthesis of MgB ₂ is preferably carried out in vacuum as described above. In addition to this, the temperature in the electric furnace (5) is kept at room temperature or higher by the heater (4).
When kept below 0 ℃, the yield of MgB ₂ superconducting material is almost 90%.
~ 100%.

As described above, the MgB ₂ superconducting material, which was not capable of self-propagating high-temperature synthesis due to the small heat of formation, can be manufactured by self-propagating high-temperature synthesis, and the required time becomes short. Long-term heat treatment in powder metallurgy is eliminated. Therefore, it becomes possible to suppress the oxidation and evaporation of Mg as much as possible, and the manufacture of the MgB ₂ superconducting material is facilitated. Further, the composition ratio of Mg and B is almost stabilized, and improvement of superconducting performance can be expected. Moreover, since the Ti powder and the B powder used for the self-propagating high-temperature synthesis of MgB ₂ are not particularly expensive, the manufacturing cost can be reduced as well as facilitating the manufacturing.

[0030]

Example: Angular Mg powder having an average particle size of about 400 μm, amorphous boron (B) powder having an average particle size of about 1 μm, and an average particle size of 3
0 μm square Ti powder was used. The above Mg powder and B powder are mixed at an atomic ratio of 1: 2, the raw material mixed powder is packed in a polyurethane rubber mold, and 300% is obtained by cold isostatic pressing (CIP).
It was held at MPa for 1 minute and pressed to prepare a molded body. Also, Ti powder and B powder are mixed, and this powder mixture is heated to 200 ° C.
Hold for 12 hours to dry.

Then, a powder mixture (10) of Ti and B was put into a refractory crucible (6) of a self-propagating high temperature synthesis apparatus as shown in FIG. 1, and a compact (9) of Mg and B was put therein. Buried. After this, a refractory crucible (6) is placed inside the electric furnace (5), at the upper end of the powder mixture of Ti and B (10),
An electrothermal coil (7) formed of a tungsten wire having a wire diameter of 0.6 mm was placed in contact with each other. In this state, the vacuum container (1) is sealed by the sealing mechanism (2), and the inside of the vacuum container (1) is evacuated by the air supply / exhaust system (3) to constantly maintain a vacuum degree of 1 × 10 ⁻³ Pa or less. I kept it. Then, a current of about 20 A was applied to the electric heating coil (7) to ignite the upper end of the powder mixture (10) of Ti and B by igniting it.

After ignition, the reaction represented by Ti + 2B → TiB ₂ occurs, and the heat of formation generated at that time is propagated one after another, causing a chain reaction to cause the entire powder mixture (10) to fall within a short time.
Self-propagating high temperature synthesis was performed on TiB ₂ . Furthermore, the heat of formation released during the self-propagating high-temperature synthesis of TiB ₂ induces a reaction represented by Mg + 2B → MgB ₂ in the compact (9), causing self-propagating high-temperature synthesis, and thus the compact (9) The whole of
It became a MgB ₂ sintered body.

FIG. 2 is a graph plotting the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the electric furnace of the self-propagating high temperature synthesis apparatus shown in FIG. 1 at room temperature.

As can be seen from FIG. 2, it is understood that the MgB ₂ sintered body has a superconducting transition temperature of 39 K, a large change in magnetic susceptibility, and good superconducting performance.

FIG. 3 is a diagram in which the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by holding the electric furnace of the self-propagating high temperature synthesis apparatus shown in FIG. 1 at 150 ° C. is plotted.

As can be seen from FIG. 3, it is understood that the MgB ₂ sintered body has a superconducting transition temperature of 39 K, has a large change in magnetic susceptibility, and has excellent superconducting performance.

FIG. 4 is a graph plotting the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the electric furnace of the self-propagating high temperature synthesis apparatus shown in FIG. 1 at 200 ° C.

As can be seen from FIG. 4, it is understood that the MgB ₂ sintered body has a superconducting transition temperature of 39 K, a large change in magnetic susceptibility, and good superconducting performance. SQ
MgB ₂ yield measured by UID (superconducting quantum interferometer) is 100%
Met.

Of course, the invention of this application is not limited to the above-described embodiments and examples. It goes without saying that various aspects are possible in terms of details such as the configuration and structure of the self-propagating high-temperature synthesis apparatus, the conditions at the time of self-propagating high-temperature synthesis, the shape and particle size of the powder used.

[0040]

As described in detail above, according to the invention of this application, the oxidation and evaporation of Mg are suppressed as much as possible.
B ₂ superconducting materials can be easily manufactured, and superconducting performance can be expected to be improved, so that manufacturing costs can 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 manufacturing a MgB ₂ superconducting material according to the invention of this application.

FIG. 2 is a diagram in which a change in magnetic susceptibility of an MgB ₂ sintered body produced by keeping the electric furnace of the self-propagating high temperature synthesis apparatus shown in FIG. 1 at room temperature is plotted.

FIG. 3 shows an electric furnace of the self-propagating high-temperature synthesis apparatus shown in FIG.
FIG. 3 is a diagram in which a temperature change of magnetic susceptibility of an MgB ₂ sintered body produced by holding at 150 ° C. is plotted.

FIG. 4 is a graph plotting the temperature change of the magnetic susceptibility of the MgB ₂ sintered body produced by keeping the electric furnace of the self-propagating high temperature synthesis apparatus shown in FIG. 1 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 Electric heating coil 8 thermocouple 9 molded products Powder mixture of 10 Ti and B

Claims (4)

[Claims] 1. A Mg powder and a B powder are mixed at an atomic ratio of 1: 2, and a green body prepared by compacting this raw material mixed powder is embedded in a powder mixture of Ti and B, and then Ti is mixed. Part of the powder mixture of B and B is ignited by igniting, causing a reaction represented by Ti + 2B → TiB ₂ , and the heat of formation generated at that time is propagated one after another and chain reaction occurs. And B
While the entire powder mixture of No. ₁ is synthesized into TiB ₂ , the heat of formation released at this time induces a reaction represented by Mg + 2B → MgB ₂ , and this reaction is also self-propagating at high temperature to synthesize the entire compact body. MgB ₂ superconductor material production method which is characterized in that the MgB ₂ sintered body having a superconducting performance. Wherein the MgB ₂ a combustion synthesis, in a vacuum, MgB of claim 1, wherein to perform the following conditions 300 ° C. above room temperature 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 claim 1, 2 or 3, wherein a powder mixture of Ti and B is previously heated in vacuum to remove water and volatile impurities contained in the powder mixture.
Manufacturing method of superconducting material.