JPS62202811A - Production of high-purity boron carbide fine powder - Google Patents

Abstract

PURPOSE:To mass produce high-purity boron carbide fine powder suitable as a raw material for sintered material in high recovery ratio and at low cost, by heat-treating a mixture of a carbon material and a specific boron nitride under reduced pressure. CONSTITUTION:A boron compound [e.g. boric acid (anhydride)] and a nitrogenizing agent (e.g. urea or dicyandiamine) and heated in a nonoxidizing atmosphere such as N2, etc., at 600-1,300 deg.C to give a boron nitride of turbulent layer structure. Then, the boron nitride of turbulent layer structure is blended with a carbon material in a solid, liquid or gaseous state as a carbon source and heat- treated under reduced pressure <=20 Torr at 1,700-2,000 deg.C to give the titled fine powder having <=10mu particle size and >=99.5% purity.

Description

[Detailed Description of the Invention] [Industry, [Field of Application] The present invention relates to a method for producing high-purity boron carbide fine powder, and more specifically, to a method for producing a carbonized layer from boron nitride (BN) and a carbon material (C). The present invention improves the method for producing (B+C) and provides a technology for efficiently producing high-purity, fine boron carbide suitable as a raw material for sintered bodies.

[Conventional technology]

Boron carbide has the second highest hardness after diamond and cubic boron nitride, and is widely used as a material for nuclear reactors and wear-resistant materials due to its excellent corrosion resistance, chemical stability, lightweight, wear resistance, and neutron absorption ability. has been done.

Conventionally, various methods for manufacturing boron carbide have been disclosed, and typical manufacturing methods are shown below.

(1) A method of mixing and heating a carbon material with a first oxidation or boric acid (Japanese Patent Publication No. 53-10560).

This method is carried out on an industrial scale and is usually produced by the Acheson method. The produced boron carbide is collected in bulk, crushed and classified, and then used for various purposes. For this reason, the recovery rate of submicron particles is extremely low.

(2) A method of manufacturing by adding carbon powder and magnesium to boron oxide and boric acid for reduction and oxidation (Japanese Patent Application Laid-open No. 6O-46910
).

This method complicates the process because it is necessary to remove magnesium oxide by acid treatment after firing and there is a risk of magnesium igniting or exploding.

(3) A method of manufacturing using boron tri71 and methane.

This method has the disadvantage that the raw boron source is expensive.

(4) Method for producing boron carbide from a mixture of boron nitride and carbon (Japanese Patent Publication No. 45-19169, Japanese Patent Publication No. 40-2504
8).

Even with the method of Japanese Patent Publication No. 45-19169, it is difficult to obtain fine boron carbide.

The reason for this is that the synthesis furnace used is an arc furnace, the temperature distribution is non-uniform, and the resulting product is also produced in the form of molten lumps. Therefore, in order to obtain a fine powder of 104 m or less that is suitable as a raw material for a sintered body, it is necessary to manufacture it by pulverization. However, boron carbide has the disadvantages that it is hard and cannot be easily crushed, that it is difficult to obtain fine powder by crushing, and that it is often contaminated with impurities.

Furthermore, there is no control of the atmosphere, and nitrogen gas generated from boron nitride remains in the atmosphere, resulting in a high nitrogen partial pressure, making it difficult to effectively produce boron carbide.

For this reason, the actual synthesis temperature must be 2400° C. or higher, as seen in Japanese Patent Publication No. 40-25048. Since this temperature is close to the melting point of boron carbide, 2450° C., the boron carbide produced is sintered, causing grain growth, and is recovered in the form of a lump. Energy costs are also quite high due to high synthesis temperatures and temperature non-uniformity.

[Problem that the invention seeks to solve]

The present inventor has improved the drawbacks of the method (4) and achieved 99%
We have now found a method for producing ultrafine powder boron carbide of 10 ILm or less with high purity and high yield.

Mix boron nitride and carbon and use the following formula.

4BN+C+B4 G+2N2...°
- In the reaction of synthesizing boron carbide using (1), conventionally it was not possible to produce highly pure boron carbide unless the synthesis was carried out at a temperature of at least 2000°C or higher. On the other hand, the melting point of boron carbide is 2450°C, and grain growth inevitably occurs when synthesized at high temperatures. For this reason, the only way to produce boron carbide in the range of several gm to submicron, which is suitable as powder for sintered bodies, is to use pulverization.

In the pulverization method, the recovery rate of fine powder below loILm is poor, and there are many impurities mixed in during the pulverization process, so it is difficult to obtain boron carbide with high purity and fine particle size. As a result of research into a method for effectively producing fine powder boron carbide from raw materials, the present invention was completed.The present invention aims to provide such an improved method.

[Means for solving problems]

The present invention has focused on the fact that fine powder boron carbide cannot be obtained in the method for producing boron carbide from boron nitride and carbon material due to the high synthesis temperature, and has improved this point. The means for this are as follows: (1) Boron nitride with a turbostratic structure different from conventional ones is used as the raw material boron nitride. ′ ■Reduce the pressure of the atmosphere and immediately remove the generated nitrogen from the reaction system to lower the nitrogen partial pressure.

(■ Synthesis is carried out at a temperature below 2000°C by these operations.

In other words, the gist of the present invention is a method for producing high-purity boron carbide fine powder, which is characterized in that a mixture of boron nitride and a carbon material having a turbostratic structure is heat-treated in a temperature range from 1700°C to less than 2000°C under reduced pressure. It is.

Further, preferred embodiments of the present invention will be described.

a) A boron compound and a nitriding agent were mixed as raw materials and synthesized at 600 to 1300°C in a non-oxidizing atmosphere.

Boron nitride having a turbostratic structure that has not completely grown into a hexagonal crystal structure is used, a carbon material is mixed therein to obtain the mixture, and the mixture is heat-treated at 1700° C. or more and less than 2000° C. under reduced pressure.

b) A boron compound, a nitriding agent, and a carbon material are mixed, and a mixture of boron nitride and carbon material with a turbostratic structure is obtained by heat treatment at 600 to 1300°C in a non-oxidizing atmosphere, and this is heated to 1700 °C under reduced pressure.
Below ℃ J: Treat at a temperature below ~2000℃.

[Effect]

The method for producing fine boron carbide with high purity according to the present invention will be described in detail below along with its operation.

The raw material boron nitride is not completely crystallized.

The turbostratic structure has higher reactivity and is more likely to form boron carbide at low temperatures.

The boron nitride powder with a turbostratic structure used as a raw material in the method of the present invention does not have a graphite structure completely, but has a layered structure in which adjacent layers are randomly positioned with respect to each other, and it is clearly different from the normal graphite structure. It is different from crystalline boron nitride.

The crystal structure of boron nitride differs depending on the synthesis conditions. For example, when boron nitride is synthesized in a nitrogen atmosphere using boric acid and urea as raw materials, boron nitride is formed at 600°C, but in this case it is completely However, it does not have a hexagonal crystal structure, but instead has a structure in which adjacent layers are randomly positioned relative to each other, which is called a crystallographic turbostratic structure.
Ordinary crystalline boron nitride has a hexagonal layered structure similar to graphite, and each layer is completely parallel, which is different from boron nitride, which has a turbostratic structure. As the temperature is further raised, the turbostratic structure gradually changes to a hexagonal structure, and at the same time grain growth and impurities such as boron oxide, ammonium borate, ugliness in the crystal, and carbon are removed, improving purity. I will do it.

When the temperature exceeds 1600℃, the secondary particle size also becomes 11Lm or more, and as the heating continues, the hexagonal crystal structure becomes completely 2.
The resulting boron nitride has a primary particle diameter of pm to 6 pm and a purity (calculated from the analytical value of N) of 99% or more.

There is a method of measuring the size of crystallites (7-117 committee method) to determine the crystal structure.The size of crystallites is the average thickness in the C-axis direction L c) It is expressed by the average diameter (La) in the axial direction, but since the (002) peak is the strongest, it is more accurate to display it by Lc.

When the crystal structure was evaluated using Lc, it was found to be a turbostratic structure below 100 Å, a quasi-graphitic structure between 100 and 400 Å, and a completely hexagonal structure north of 400 Å. Therefore, if the turbostratic structure in the present invention is expressed by Lc, it means boron nitride with a thickness of about 100 Å or less.

As an efficient method for producing 4a carbide, a boron compound (
(boric acid or its dehydrate, etc.), a nitriding agent (urea, dicyandiamide melamine, ammonium chloride, cyanuric acid, etc.), and a carbon material.

It can be efficiently produced by heating to 600° C. to 1300° C. in a non-oxidizing atmosphere to generate boron nitride with a turbostratic structure, followed by heat treatment under reduced pressure. According to such a method, boron carbide can be produced in one heat treatment, which is advantageous in terms of energy.

Another method is to mix boron compounds such as borax and urea or boric acid and ammonia with a nitriding agent and heat the mixture to 600 to 1300°C in a non-oxidizing atmosphere. Boron carbide can be produced by adding raw materials and heating under reduced pressure.

The finer the carbon material as the carbon source, the more preferable it is, and the method of adding it may be by kneading it in solid or liquid form.
When using boric acid and a nitriding agent as raw materials, add carbon material in advance, or insert or add iron plates or iron powder into the raw materials to allow carbon-containing organic decomposition gas to remain in the sample. It can also be used as a carbon source by precipitating carbon in the agent. According to this method, the carbon source is uniformly precipitated and remains highly reactive.

In this method, iron remains, so it is necessary to heat it to a temperature of around 1000° C. and then cool it, and to remove it by pickling in the case of iron powder and by sampling in the case of iron plates.

A gaseous carbon source (methane, ethane,
Injecting a liquid carbon source (such as propane, etc.) or vaporizing a liquid carbon source (methanol, ethanol, tar, etc.) is also effective in uniformly supplying the carbon source.

The turbostratic boron nitride used gradually changes into crystalline boron nitride through heat treatment.For this purpose, the highly reactive turbostratic boron nitride has a boron carbide synthesis temperature of 1700℃.
In the process of heat treatment to ℃, it changes to crystalline boron nitride and its reactivity decreases. After examining various ways to avoid this, it became clear that boron nitride having a turbostratic structure will not crystallize if heated at a reduced pressure f of 20 torr or less.

Therefore, the heat treatment conditions for boron carbide synthesis are preferably 20 I·-L or less.

As for the heat treatment temperature, if it is less than 1700℃, boron carbide will not be sufficiently generated, and if it exceeds 2000℃, boron carbide will be generated by 10J.
Grain growth exceeds Lm and Wk particles cannot be recovered. In this case, the higher the degree of vacuum, the faster the boron carbide production rate and the lower the synthesis temperature. 19
When synthesizing at 90°C, the reaction can be completed in several hours if the temperature is 0.1 Torr or less. Furthermore, the degree of vacuum that can be achieved with a rotary pump or a diffusion pump is 10-5 Torr.

When heat-treated with 1O-5)-ol, high-purity boron carbide fine powder could be synthesized at 1700°C.

From the above, in order to obtain high purity boron carbide fine powder, 1700
The temperature shall be at least ℃ but less than 2000℃.

By the above method, the purity is 99% or more and the average particle size is 10μ.
It was possible to produce high-purity boron carbide fine powder of less than m. From the viewpoint of suppressing grain growth and energy cost, it is preferable to end the holding time when the reaction is completed.

〔Example〕

Example 1 2.4 g of carbon black was added to 100 g of a raw material prepared by mixing boric acid and dicyandiamide at a weight ratio of 1:1, and the mixture was heated at 900° C. for 2 hours in a nitrogen atmosphere. Subsequently, the atmosphere was reduced to 0.1 Torr and maintained at 1950° C. for 10 hours.

After cooling, the product was identified by X-ray diffraction, and only a boron carbide peak was detected. Furthermore, when B and C were analyzed by chemical analysis (B + C), it was 99.7%.

The average particle size by Microtrack was 2.6 p, m.

Examples 2 to 8 Anhydrous borax and urea were mixed at a weight ratio of 1:2, held at 900"O for 2 hours in an ammonia atmosphere, and then washed with water to remove the sodium content, resulting in a product with a turbostratic structure. Boron nitride (boron nitride purity 91.3%, 97 primary particles) was obtained. 10 g of this turbostratic boron nitride and 1.2 g of carbon black were mixed and placed in a graphite crucible at a packing density of 0.25 g.
/crn'. This boron nitride was heated under various vacuum and temperature conditions. The results are shown in Table 1.

Comparative Example 1 Anhydrous borax and urea were mixed at a weight ratio of 1:2 and held at 1800"C for 2 hours in an ammonia atmosphere.
After washing with water, crystalline hexagonal boron nitride (h-BN) was obtained. 10 g of this boron nitride and 1.2 g of carbon black were mixed, filled into a crucible at a packing density of 0.25 g/c rn', and heated at a vacuum level of 10-5 torr or less at 1600°C for 10 min.
When the product obtained by heat treatment for several hours was examined by X-ray diffraction, boron carbide was not identified.

Example 9 1k of raw material prepared by mixing boric acid and urea at a weight ratio of 1:2
g was heat-treated at 900° C. for 2 hours in a nitrogen atmosphere, the pressure was reduced to 1 Torr, the temperature was raised to 1900° C., and the mixture was held for 5 hours while blowing methane gas at a rate of 100 cc/min.

When the resulting product was examined by X-ray diffraction, only a peak of boron carbide was detected. In chemical analysis (B + C
) was 99.6%. Further, the f average particle size measured by Microtrack was 1.7 gm.

Example 1O The same heat treatment as in Example 9 was carried out, but methyl alcohol was blown into the sample packed bed instead of methane. When the product was identified by X-ray diffraction, only a peak of boron carbide was obtained. Chemical analysis showed that (B + C) was 99.2%. Furthermore, the average particle size measured by Microtrack was 2.1 gm.

[Effects of the Invention] According to the present invention, boron carbide in the form of a fine powder of 10 gm or less with a high purity of 99.5% or more and suitable as a raw material for a sintered body can be produced with a high recovery rate. In particular, since it can be manufactured with a simple operation of just heat treatment, it has become possible to produce large gutters at low cost.

Claims (1)

[Claims] 1. A method for producing high-purity boron carbide fine powder, which comprises heat-treating a mixture of boron nitride and a carbon material having a turbostratic structure at a temperature of 1700°C or more and less than 2000°C under reduced pressure. 2. Claim 1, characterized in that boron nitride with a turbostratic structure obtained by mixing a boron compound and a nitriding agent and subjecting the mixture to heat treatment at 600 to 1300°C in a non-oxidizing atmosphere is used.
The method described in section. 3. Claims characterized in that a boron compound, a nitriding agent, and a carbon material are mixed and heat treated at 600 to 1300°C in a non-oxidizing atmosphere to form a mixture of boron nitride and carbon material with a turbostratic structure. The method described in paragraph 1.