JPH0798683B2 - Manufacturing method of composite material mainly composed of carbon and boron - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a carbon-boron (hereinafter sometimes referred to as CB) composite material, and particularly when a boron (hereinafter sometimes referred to as B) component is carbon (hereinafter (Sometimes referred to as C) to a method for producing a composite material in which ultrafine particles are uniformly dispersed in a component.

[Conventional technology]

The C-B composite material has been widely studied and used as a neutron absorbing material in the nuclear industry, as a carbon material having oxidation resistance, and as a structural material for machinery.

The currently generally known method for producing this CB composite material is B ₄ C (boron carbide) produced separately.
It is well known that a carbon material or a carbonizable raw material is mixed and fired at a high temperature to form a solid solution of both. Examples thereof include JP-A-62-108767 and JP-A-1-138172 (Japanese Patent Application No. 62-297202).

[Problems to be Solved by the Invention]

The CB composite material obtained by the above conventional manufacturing method is
Coarse B ₄ C is crushed and mixed with carbon.
Since the pulverization is mechanical pulverization, there is a limit to the pulverization, and as the target C-B composite material, there are a B ₄ C portion, a portion in which B ₄ C and carbon are solid-solubilized, and a portion containing only carbon. When mixed and observed finely, it was hard to say that the whole was completely uniform.

Further, in the above-mentioned conventional method, since the powders are mixed, molded and sintered, there is an inconvenience in cutting and molding the material after sintering, and it is expensive due to waste of the cutting powder. There was a drawback that the raw material efficiency of B ₄ C decreased.

Further, the biggest problem in the conventional method is that it contains a large amount of inorganic impurities other than boron (usually about 5000 ppm).
These impurities are introduced by contact with steel machinery in each step of crushing, mixing, molding, firing of the blended raw materials, and it is extremely difficult to avoid them by the conventional method. Depending on the use of the BC composite material, the coexistence of a small amount of inorganic impurities may not be a big problem, but as a material for the nuclear industry, the mixed inorganic impurities may cause isotope conversion, fission, etc. due to radiation irradiation. When they are used in a nuclear fusion material, the reactions simultaneously occur, and the high Z element (high valence element) emitted from these impurities causes a definite adverse effect of significantly lowering the plasma temperature. Therefore, as a carbon material for the nuclear industry, a highly purified material, for example, an ultrahigh-purity carbon material having an inorganic impurity of 5 ppm or less, preferably 2 ppm or less, substantially 0 ppm (by atomic absorption analysis or emission line spectrum method) Is always used. As a method for producing such an ultra-high purity carbon material, for example, as shown in JP-A-63-79759 (Japanese Patent Application No. 61-224131), inorganic elements are removed by halogenation treatment with high volatility. but is at B ₄ a C-C in the composite material as a raw material, due to the presence of boron in the carbon material, it is impossible to remove impurities by using the this method was complexation.

The present invention solves the above-mentioned various drawbacks of using a conventional B ₄ C-C powder as a raw material and develops a more excellent B-C composite material.

[Means for Solving the Problems]

The present inventors have solved the drawbacks of the above-mentioned conventional method and proceeded with research to develop a more excellent material, first without using B ₄ C, which is a raw material of the conventional method, as a boron source, and newly used boron oxide or its The method of impregnating the carbon material with the hydrated compound in the form of melt or solution was adopted. By this method, the boron component permeates at the molecular level on the surface of fine carbon particles or in the fine pores in the carbon material, and is extremely fine compared to the case of using B ₄ C powder, and dispersed throughout the whole. Can be made.

Further, since boron is used in the form of an oxide as described above, a reaction method suitable for fixing boron in carbon by carbon reaction with an oxide to form a solid solution, that is, a boron compound in a liquid state in a carbon material is used. Following impregnation, we developed a manufacturing method in which the impregnated boron compound does not volatilize from the carbon material, and as a condition for the solid solution reaction of boron and carbon, a method of firing at high temperature and high pressure was developed, and the intended purpose was reached. .

[Operation and Configuration of Invention]

The action of the present invention will be described by exemplifying the reaction between boron oxide (B ₂ O ₃ ) and carbon.

<First Step> In a pressure-resistant container, a carbon material, for example, an isotropic high-density carbon material (Toyo Tanso Co., Ltd. “IG-11”) cut molded body, molten boron oxide is 600 to 1400 ° C., preferably Impregnate under pressure at 800-1200 ℃. At this time, it is desirable to depressurize the pressure vessel once to remove the air contained in the pores of the carbon material before impregnation, but it is not always necessary to degas beforehand.

The carbon material may be impregnated with B ₂ O ₃ under a pressure of several kg / cm ² , but 50 to 100 kg / can be used to completely infiltrate deep into the material.
It is desirable to have cm ² . This pressurization is appropriately determined depending on the porosity, particle size, pore distribution, temperature, etc. of the carbon material.

<Second Step> The carbon material impregnated with B ₂ O ₃ is subjected to heat treatment (hereinafter sometimes referred to as HIP) at high temperature and high pressure using an inert gas as a pressure medium. By heat treatment, for example, by using an inert gas such as Ar as a medium, the carbon material and the B ₂ O ₃ liquid are evenly pressed from each direction so as to be pressed by water pressure, while preventing evaporation of B ₂ O _3. The carbon is confined in the carbon material, and the chemical reaction between carbon and boron proceeds due to the effect of temperature.

The pressure and temperature inside the heat treatment device is 100 kg / cm ² or more, 150
Temperature of 0 ℃ or higher, desirably 2000 ℃ or higher, 1500-2000kg /
cm ² is good. In this case, if the temperature exceeds 2300 ° C, the decomposition reaction of the solid solution of carbon and boron occurs simultaneously, which is not preferable.

The above first and second steps are essential, but depending on the purpose of use of this composite material, a small amount of B ₂ O ₃ may remain in the carbon material. Can also be added.

<Third step> The composite material subjected to the HIP treatment in the second step is subjected to a reduced pressure of 10 Torr or less, preferably 5 Torr or less, 1000 ° C or more, preferably 1
B ₂ O ₃ in the composite material is treated at high temperature of 500 ℃ or more under reduced pressure.
The amount can be reduced to less than 0.1%. In the B-C composite material thus obtained, boron is finely dispersed and uniformly dispersed throughout the composite material as compared with the composite material using B ₄ C powder as in the conventional method.

The carbon materials used in the present invention include isotropic carbon materials as exemplified above, general carbon materials, anisotropic carbon materials (for example, pyrocarbon, pyrographite, etc.), carbon-carbon composite materials (hereinafter It is applicable regardless of the type of carbon material such as C / C material). Since the present invention does not use carbon powder, the greatest feature is that carbon can be borated while the shape, texture, and skeleton of the molded carbon material remain unchanged.

For example, when boron is used by using ultra-high purity isotropic high-density graphite material as a base material, a composite material obtained by using a material having a high purity boro compound is an impurity of elements other than carbon and boron. Is very small, 5 ppm or less, which is almost the same as the purity of the base material. It is considered that this is because in the present invention, there is no contamination during mechanical processing steps such as crushing, mixing, compression molding of raw materials.

A simple example showing the characteristics of the method of the present invention is the case of boration of a carbon / carbon composite material. B ₄ as in conventional method
When using C powder, it requires special equipment and technology to grind it to a particle size of 1 μm or less, even if it is ground very finely. This particle is mixed with a resin component and applied to carbon fiber to make a prepreg. It is conceivable to make a boronized C / C material product by further molding, heat curing, carbonizing, and then cutting.
The biggest drawback of this method is that the carbon material cannot be completely graphitized. The reason for this is that although the graphitization of carbon requires high temperature firing at 2500-3000 ° C, the B ₄ C content is 2300
This is because decomposition begins at around ℃. Therefore, 3000 ° C in advance
It is almost impossible to push the B4C fine powder into the fine pores of the C / C material graphitized by high-temperature firing at ₄₀ ° C, let alone evenly distribute the boron component to the deep part of the C / C material. I can't. This can be said for the general carbon material block as well, but for the C / C material, it was particularly difficult to measure the boronation while maintaining the strength of the carbon fiber.

In this respect, according to the method of the present invention, the C / C material can be boronized very easily. As described above, the boron component can be forcibly pressed into the pores of the carbon material by a pressure in a molten or solution form at a molecular level, and can be uniformly dispersed to a deep portion. In addition, there is no change in the structure of the C / C material due to the forced press-in work of this boron component and the subsequent firing work, and since the carbon material has been graphitized at 3000 ° C in advance, in order to promote the boration reaction. Even if it is fired at 2000 ° C., the obtained borated compact has sufficient physical properties as a C / C material.

On the other hand, the boron component impregnated in the carbon material can be used for this purpose as a boron compound which can be melted by heating or made into a solution by a solvent in principle. It is not preferable because it causes contamination of the material and limits the use. Therefore, a compound that decomposes and volatilizes leaving only boron by thermal decomposition or reaction with carbon by firing is desirable. From this point, boron-containing organic compounds, boron halides, etc. may be mentioned, but in the present invention, boron oxide (B ₂ O ₃ ) and its hydrate are included from the viewpoints of economy and ease of handling. Compound, eg
H ₃ BO ₃ orthoboric acid can be exemplified as the optimum one.
For example, as a chemical reaction between B ₂ O ₃ and carbon, 2B ₂ O ₃ + 7C → B ₄ C + 6CO is known as a calcination reaction of B ₄ C. It is not clear whether the (carbon-boron) composite material produced by dispersing B ₂ O ₃ having a molecular level is proceeding according to the above reaction formula. Example 1
As a result of various analyzes of the composite material obtained by the method shown in Fig. 4, 4% by weight of boron component was measured by chemical analysis (free B ₂ O ₃ 0.02%), and it was also clear from the result of neutron irradiation. Despite the fact that the neutron absorption by the boron component is observed, the observation by the X-ray diffractometer shows that
There are few peaks indicating the presence of B ₄ C. Since there are few peaks showing other specific crystal systems and there are many broad portions, it is considered to be in an amorphous or solid solution state. Therefore, it is speculated that the final product may be in the form of a solid solution as the form of (BxCy + C), rather than the definite form indicating the specific compound of B ₄ C, but the present invention is restricted to such a form of solid solution. Not something.

As the boron component, besides boron oxide (B ₂ O ₃ ), a hydrated compound thereof can be used as well. Examples of the hydrated compound include boric acid (H ₃ BO ₃ , B (OH) ₃ ).

These boric acids have a relatively low melting point (185 ° C) compared to boron oxide (B ₂ O ₃ ), and decompose at higher temperatures (B ₂ O ₃ · ⁿ H ₃ BO) while releasing water. ₃ ) It becomes a solid solution form and remains liquid. Therefore, when boric acid is used as a raw material, boric acid is melted while maintaining a suitable viscosity in the container, that is, 300 to 500 ° C., a carbon material is dipped in this, and carbon is impregnated by pressure. Forcibly press into the material pores. The second step (HIP) that follows the above first step (impregnation)
The treatment step) can be performed in the same manner as in the case of B ₂ O ₃ described above.

Next, a mode for carrying out the method of the present invention using these boron compound and carbon material as raw materials will be described.

The boron compound is heat-melted and pressure-impregnated in a liquid state or a solution state dissolved in an appropriate solvent. For example
B ₂ O _{3 has} a melting point of 450 ° C. at normal pressure and a boiling point of 1500 ° C., and becomes liquid in this temperature range, but impregnation operation is 600 to 1400 ° C.,
A temperature range of 800 to 1200 ° C. is suitable.

First, as the first step, B ₂ O ₃ and a carbon material are put in a pressure vessel, and B ₂ O ₃ is press-fitted into the pore voids of the carbon forming body by a heating (vacuum) / pressurizing method. At this time, if the air inside the pores of the carbon material is removed by temporarily reducing the pressure inside the container prior to the B ₂ O ₃ press-fitting, the B ₂ O ₃ press-fitting is complete and easy, but Since the pressure is high, this temporary depressurization operation is not essential.
The press-fitting pressure may be several kg / cm ² , but is preferably 50-100 k
It is g / cm ² .

Next, HIP processing is performed as a second step. Even if the carbon material impregnated with the boron compound in the first step is heated at 2000 ° C. under normal pressure, the carbon material is hardly borated. This is probably because the boron component evaporates by heating at high temperature and does not form a solid solution due to the reaction with the carbon material. The heating in the second step needs to be performed under high pressure. The process performed at high temperature and high pressure is 100 kg / cm with an inert gas such as Ar as a medium.
² or more, temperature of 1500 ℃ or more, desirably 100 to 2000 kg / c
m ^2, carried out in 2000 ℃ or more of the conditions. By this HIP treatment, the boron compound can be solid-solution diffused and fixed in the carbon material.

The above is the second step, which is an essential operation together with the first step,
For ordinary use and purpose as a [carbon-boron] solid solution, further finishing such as cutting and forming treatment is carried out at this stage and the product is put on the market.

However, for special applications such as neutron absorbers for nuclear reactors, the amount of residual B ₂ O ₃ should be as small as possible. When such B ₂ O ₃ was used in a nuclear reactor and used under high temperature conditions, it vaporized.
This is because B ₂ O ₃ precipitates and solidifies in a relatively low temperature part, causing troubles that hinder the operation, corroding metal parts, and solidifying during storage.

Therefore, when it is necessary to remove the remaining B ₂ O ₃ , a third step described below can be added as necessary.

<Third Step> The solid solution obtained in the second step is placed in a pressure vessel, and under reduced pressure, preferably 10 Torr or less, particularly preferably 5 Torr or less under strong reduced pressure, and subjected to heat treatment at 1500 ° C. or higher, B ₂ O ₃ Is added to the step of removing by evaporation.

By performing such treatment, the residual amount of B ₂ O ₃ is 0.01
It can be as low as wt%.

Due to its diversity, the material obtained by the method of the present invention is more excellent as an anti-oxidation consumable material, as a reactor material such as a neutron absorber, and also as a plasma facing wall for a fusion reactor, an aerospace material, a metallurgy, a machine. It exerts its power in all fields such as parts.

〔The invention's effect〕

In the composite material of the present invention thus obtained, boron is uniformly dispersed in carbon. In addition, carbon moldings can be selected from all kinds of carbon materials and shapes such as high density graphite including C / C materials, and carbon-boron composite materials that cannot be obtained by conventional methods can be produced. There is a big advantage of being able to do it.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to examples.

Example 1 <First Step> A fine-grained isotropic graphite material (“IG-11” manufactured by Toyo Tanso Co., Ltd.) was immersed in B ₂ O ₃ melted at 1200 ° C. using an autoclave, and N _Two gases were applied at a pressure of 150 kg / cm ² for 1 hour to impregnate B ₂ O ₃ into the pores of the graphite material.

<Second step> After the impregnation is completed, a HIP treatment device is used and the temperature is 2000 ° C and 2000k.
A pressure of g / cm ² was maintained for 1 hour (pressure medium Ar), and boron was diffused into the graphite material to form a solid solution. During the HIP treatment, the article to be treated was put in a graphite cylindrical sheath and the lid was closed.

<Third Step> After that, a vacuum treatment was performed in a vacuum container at 1 Torr and 2000 ° C. for 1 hour. The boron concentration of the obtained composite material was measured by the Mannitol method and was 4.0% by weight (as boron element).
Met. Among them, B ₂ O ₃ was 0.02% by weight, and almost all unreacted B ₂ O ₃ was evaporated and removed.

Example 2 The carbon-boron composite material obtained in Example 1 was repeatedly treated in the same manner as in Example 1. The boron concentration of the boron composite material thus obtained was 7% by weight. Of that
B ₂ O ₃ was 0.03% by weight.

As is clear from the above, it was found that the boron content in the composite material can be increased by repeating the treatment shown in Example 1.

Example 3 PAN-based high strength carbon fiber (3000 filament, fiber diameter 7μ
m, a tensile strength of 300 kg / mm ² ) of a plain weave cloth with 5-20% by weight of boron carbide (diameter 7 μm or less) uniformly dispersed (a phenol resin solution diluted 2 to 3 times with methanol) ) Was applied by impregnation and air-dried for 24 hours to obtain a prepreg sheet.

This prepreg sheet is heat treated in a dryer (100 ° C x
0.5 hours), then packed in a die, hydraulic press at 140 ℃, 50
It was kept under the condition of kg / cm ² for 1 hour to obtain a 2D molded body as two laminated bodies.

The obtained compact was packed in coke powder and treated in a non-oxidizing atmosphere up to 1000 ° C at a heating rate of 10 ° C / hour, then in a vacuum furnace under reduced pressure of 5 Torr to 2000 ° C up to 100 ° C / hour. High temperature treatment was performed at a speed of. A crack-free 2D C / C composite was obtained.

Orthoboric acid (H ₃ BO ₃ ) 1 for the above 2D C / C composite
A solution obtained by adding 1% by weight of water to the% by weight was added, dipped and impregnated. The water was evaporated in a dryer maintained at 120 ° C. After that, 1 more impregnation treatment
Conducted once. It was confirmed that the aqueous solution had a relatively low viscosity and was easily impregnated into the voids and pores of the C / C composite material to a deep portion.

Using the above as the first step (impregnation treatment), two steps were performed under the same conditions as shown in Example 1 to obtain a carbon-boron composite material using the C / C composite material as a base material. . The boron concentration in the obtained product was 3.7% by weight (boron element conversion value).

Example 4 As a carbon substrate, a high-purity ultrafine isotropic graphite material (hereinafter referred to as ISO
(Abbreviated as -880) was used to carry out the boration reaction in the same manner as in Example 1.

This carbon substrate is a carbon material that has dense and high strength characteristics.
Although it is a material having a small pore volume, when boronation was performed by the method shown in Example 1, the boron concentration in the obtained C-B composite material was 2.6% (by weight), and The amount of residual B ₂ O ₃ after the treatment was measured to be 0.01% or less.

Table 1 shows the analysis values of elements other than boron before and after the boration treatment, that is, the ISO-880 raw material and after the boration reaction according to this example. The unit is ppm.

In addition, the general carbon material usually contains about 400ppm of impurities,
This is subjected to high temperature halogenation treatment (see, for example, JP-A-63-79759
No.) 10ppm or less, depending on the purpose the total ash content is 1-2ppm
It can be: The analysis method is a combination of atomic absorption spectrometry and emission line spectrum method. Further, (−) indicates that no detection was performed.

As is clear from the analysis results of the amount of impurities before and after the boration treatment in Table 1, it is understood that the elements other than boron have not increased.

Examples 5, 6 and Comparative Example 1 A neutron irradiation test was conducted using the sample prepared by the method of the present invention and the sample prepared by the conventional method.

<Test sample> sample -A (Comparative Example 1): pulverizing a commercially available B ₄ C powder, carries the particles having the size of 3~7μm diameter, were prepared. Separately, coal coke powder (average particle size 15μ
50 parts by weight, artificial graphite powder (average particle size 40 μm or less)
Mix 10 parts by weight and 40 parts by weight of the pitch, and heat (230
After kneading, the mixture is molded and ground. 7.7 parts by weight of the above B ₄ C particles were added to this pulverized product 100, and the mixture was heated and kneaded with a small amount of a binder. This kneaded product is pressure-molded and 2000 ° C
The material was obtained by firing. As a result of chemical analysis, the boron content was 4.2% by weight (pure boron conversion value).

Sample-B (Example 5): Raw material obtained by the method described in Example 1 above.

Sample-C (Example 6): Raw material obtained by the method described in Example 3 above.

2mm thickness of 3 kinds of raw materials obtained by the above method
Was cut into a thin plate and subjected to a neutron irradiation test.

Neutron irradiation test equipment: Sumiju Test Co., Ltd. Neutron radiography beam irradiation amount: 34.4 μA, 4653 sec (160.0 m Cb) Neutron irradiation method: A sample was placed on a dry plate and irradiated with neutrons. The areas where neutrons are absorbed are exposed in white, and the areas where they are not absorbed are exposed in black.

Test results: The test results are shown in FIGS.

In the case of Sample-A, as shown in FIG. 2, the boron component exists as B ₄ C particles, and the neutron-absorbed portion remains as white spots as an unexposed state. The part without boron, that is, the part irradiated with neutrons, is exposed in black. In addition, this figure shows a magnified 10 times in order to clearly show the spots.

In the case of Sample-B, the boron component is extremely finely and uniformly dispersed. No white spots are observed even when enlarged.
Therefore, the entire surface of the obtained photograph is exposed as a uniform intermediate color of white and black, and white spots are not observed as shown in FIG.

As described in the section of Example 1, the boron component is 4%.
The absence of absorption points as white spots, regardless of the presence, indicates that boron is in a state of being dispersed in a very fine state.

In the case of Sample-C, the (carbon-carbon) composite material was impregnated with boron, and although there was no analysis result by photography, it was uniformly distributed in the entire sample in an ultrafine dispersion state.

From the above comparison between the conventional method (Sample-A) and the method of the present invention (Samples-B and C), there is a significant difference in the dispersion state of the boron component between them, and in the case of the method of the present invention, the boron is uniformly distributed over the entire surface. And, it is clear that they are finely dispersed so as not to be compared with the B ₄ C powder.

Example 7 and Comparative Examples 2 and 3 The oxidation resistance of the (boron-carbon) composite material produced by the method of the present invention was investigated.

Samples -D (Comparative Example 2): a conventional method using in Comparative Example 1 sample prepared in (B ₄ C powder used) (boron content 4.2%).

Sample-E (Example 7): Sample prepared by the method of the present invention used in Example 4 (boron content 4.0%).

Sample-F (Comparative Example 3): Carbon raw material (boron content 0.0%) used in the preparation of the sample used in Example 6.

The above three samples (DF) were cut into (32 × 20 × 12.5 mm) and left in an air bath heater maintained at 700 ° C., weight loss was measured at appropriate time intervals, and oxidation loss rate was measured. Was measured. The measurement results are shown in FIG.

As is clear from FIG. 3, the sample-E prepared by the method of the present invention is the raw material (IG-1
When compared with the sample F used as 1) (see Example 1), it is clear that the impregnation with the boron component significantly improves the oxidation resistance. And, surprisingly, it was found that when the boron content was set to the same level, the oxidation resistance was remarkably high as compared with the conventional method of Sample-D to which B ₄ C powder was added.

The reason for this is that the conventional method has the effect of suppressing oxidation.
Boron component as B ₄ C powder is locally unevenly distributed as particles, and there are some microscopically no boron component, and oxidation starts from the vicinity thereof. It can be understood that the result is that the oxidation reaction is suppressed as a whole because the fine particles are uniformly dispersed.

Examples 8 and 9 and Comparative Examples 4 and 5 are characterized by the boration reaction of the carbon material according to the method of the present invention, in which any kind of carbon material and carbon material having any shape can be borated and the raw material The feature is that the properties and physical properties of are almost not impaired.

Table 1 shows the physical properties of the carbon material before and after the boration reaction of the carbon material according to the method of the present invention, that is, the physical properties of the raw materials (Comparative Examples 4 and 5) used in the experiments of Examples 1 and 4. And the physical properties of products subjected to the boration reaction (Examples 1 and 4).

As is clear from Table 2 above, it is shown that the structure and skeleton of the raw carbon material are not changed and the physical properties are not changed by the boriding reaction.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic explanatory views in which exposure photographs by neutron irradiation are drawn. FIG. 3 is a graph showing the oxidation consumption rate of various carbon materials.

Claims (10)

[Claims] 1. Main components of carbon and boron, characterized in that a carbon material is impregnated with boron oxide or (and) a hydrated compound thereof and is fired under a pressure of an inert gas at a temperature of 1500 ° C. or higher. A method of manufacturing a composite material. 2. The method for producing a composite material according to claim 1, wherein the carbon material is a high-density isotropic graphite material. 3. The method for producing a composite material according to claim 1, wherein the carbon material is a high-purity isotropic carbon material in which the total amount of inorganic impurities other than the boron component is 20 ppm or less. 4. A carbon material reinforced by carbon fibers,
The method for producing a composite material according to claim 1, which is a carbon-carbon composite material. 5. The method for producing a composite material according to claim 4, wherein the carbon-carbon composite material is a high-purity material in which the total amount of inorganic impurities other than the boron component is 20 ppm or less. 6. The method for producing a composite material according to claim 1, wherein the remaining amount of liberated B ₂ O ₃ is 0.1% by weight or less. 7. A carbon material is impregnated with molten boron oxide or (and) a hydrated compound thereof under pressure, and then calcined under a pressure of an inert gas at 1500 ° C. or higher. The method for producing the composite material according to claim (1). 8. The method for producing a composite material according to claim 1, wherein the hydrated boron oxide compound is a melt of boric acid (B (OH) ₃ or H ₃ BO ₃ ) or a solution thereof. 9. The method according to claim (7), wherein when the carbon material impregnated with boron oxide is fired under the pressure of an inert gas, it is heated at a high temperature of 100 kg / cm ² or more and 1500 ° C. or more at a high pressure. Process for producing the described composite material. 10. The method for producing a composite material according to claim 9, wherein the reaction system is maintained at 1500 ° C. or higher and a reduced pressure of 10 Torr after the heat treatment at high temperature and high pressure.