JPH0610081B2 - Method for producing hexagonal boron nitride - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a method for producing hexagonal boron nitride,
The present invention relates to a method for producing high-purity boron nitride by adding powder generated during low-temperature heat treatment to a raw material and performing heat treatment.

[Conventional technology]

Hexagonal boron nitride is a white powder, has a hexagonal layered structure similar to graphite, and has various characteristics. In particular, it is excellent in terms of thermal conductivity, electrical insulation, corrosion resistance, lubricity, heat resistance, machinability, etc., and these properties are utilized for a wide variety of applications. Applications as powders include additives to plastics, lubricants, etc., and molded articles and composites include applications such as jigs, electrical insulating materials, and mold materials.
Various methods have been developed for synthesizing hexagonal boron nitride (hereinafter referred to as BN), which has a wide range of uses, but the methods currently used industrially are: (1) A mixture of borax and urea in an ammonia atmosphere. 80
Method of synthesizing by heating at 0 ° C. or higher (Japanese Patent Publication No. 38-1610) (2) Method of mixing boric acid or boron oxide with calcium phosphate and heating in ammonia atmosphere (3) Boric acid and nitrogen-containing compound (urea, melamine, The main method is to heat dicyandiamide or the like) to 1600 ° C. or higher (Japanese Patent Publication No. 48-14559). There is also a vapor phase synthesis method using boron trichloride as a raw material, but the raw material cost is high, so it is limited to the case of producing a product for a special purpose.

In the method (1) above, since the borax contained in the raw material contains a sodium compound, if the temperature is set to 1000 ° C. or higher, the vapor of the sodium compound may adversely react with the material used in the reactor. Sodium compounds need to be removed. Therefore, a cleaning process is required and the process becomes complicated.

Also in the above method (2), it is necessary to remove the added calcium phosphate by pickling.

Further, the method (3) has a problem that a high-purity product cannot be produced unless the temperature is set to a high temperature of 1600 ° C. or higher. .

A method of using BN powder as a filler as a substitute for calcium phosphate in the above method (2) (JP-A-57 / 57).
22105, Japanese Patent Publication No. 45-19168) and the like are also disclosed. This method does not require a treatment step such as pickling as in the case of using calcium phosphate as a filler. However, when BN powder is used as a filler, the addition amount is about 50% in the embodiment for producing high-purity boron nitride, and it is uneconomical to return half the costly BN powder product. . In addition, raw material powder and B
An operation such as mixing and molding N powder becomes necessary, and the process becomes complicated.

[Problems to be solved by the invention]

As described above, the BN manufacturing method which is currently industrially adopted is desired to be improved so as to obtain a high quality product at a lower cost.

In order to improve the thermal and mechanical properties of the BN sintered body, it is preferable that the raw material BN has a high degree of purity and does not grow crystals and has fine particles. It is an object of the present invention to provide a method for producing BN powder which is optimal as a raw material for such a sintered body.

[Means for solving problems]

In the present invention, as a result of earnest efforts to make up for the drawbacks of the prior art, a BN powder having a high purity and no crystals developed, which is not obtained by the conventional method, and which is optimum as a raw material for a sintered body, is produced. On the other hand, by subjecting this BN powder to a hot press treatment, it has succeeded in producing a BN sintered body having a higher strength than the conventional method.

Hereinafter, the method for producing BN powder according to the method of the present invention will be described in detail.

In the present invention, boric acid or a dehydrated product thereof or ammonium borate and a nitrogen compound are mixed and the mixture is heated at 250 to 600 ° C.
5mm of the porous mass produced by low temperature heat treatment at
The following product powder is characterized by being mixed with boric acid or its dehydrated product or ammonium borate in an amount of 5 to 50% by weight and subjected to nitriding heat treatment.

Although there are various boron sources, it is preferable to use boric acid or its dehydrated product, ammonium borate, etc. in consideration of contamination with impurities, etc., and nitrogen sources that can be decomposed and removed at high temperatures such as urea, melamine, and dicyandiamide. Is preferred.

The boron source and the nitrogen source must be mixed such that the molar ratio (N / B) of the boron source and the nitrogen source is 1 or more.
However, when boric acid and melamine were mixed at a ratio of (N / B) = 2 and heated in an ammonia atmosphere at 900 ° C. for 2 hours, the BN content was about 70%, and at 1200 ° C. The BN content was about 80% even under the condition of holding for a time. Thus, it was revealed that the nitriding reaction did not proceed so much simply by mixing and heating the boron source and the nitrogen source.

As a result of studying to overcome these drawbacks and further improve the nitriding rate, it was considered that the poor contact between the raw material and the atmospheric gas was the cause of not improving the nitriding reaction. Therefore, the same raw material as above was first preheated at low temperature at 400 ° C,
The generated porous lumps were subjected to various heat treatment furnaces (rotary furnace, pusher furnace, rigid filling furnace) in an ammonia atmosphere for 9 minutes.
When subjected to a high temperature heat treatment of holding at 00 ° C for 2 hours,
Improved contact with atmospheric gas and BN content of 85-90
% Product was obtained.

Further, the synthesis method was further investigated for the purpose of improving the nitriding rate and improving the recovery of raw materials.

First, when a lump sample is taken out from the product in the low temperature heat treatment step, the recovery rate of the lump is only about 50% because the product is soft due to degassing reaction and the like. Furthermore, when this agglomerated product was put into the reaction furnace, some of the samples had weak compressive strength, so when it was put in the lower part of the reaction layer, it was pulverized and pores were blocked, gas flow became poor, and the nitriding rate was also reduced. It does not improve, and the precipitation of carbon contained in melamine increases. For this reason, the reactor used was also restricted. In order to solve this point, research was conducted to see if it would be possible to improve the strength of the lump product and improve the recovery rate of the porous body by using additives.

As a result, we came to the following conclusions.

When selecting additives, the following two points are important.

(1) High recovery rate of agglomerated products (2) B without the step of removing additives by washing treatment, etc.
Being able to manufacture N.

As an additive satisfying these conditions, it was examined to use a powder having a size of 5 mm or less, which is generated when recovering a lump product. It was confirmed that even if the generated powder at this time was heated again at low temperature, boric acid, boron oxide, etc. (low melting point compound) were present in a small amount due to the nitriding reaction, and thus did not sinter and remained porous. . By adding this powder to the raw material and subjecting it to low-temperature heat treatment, a lump product was obtained in which the additive served as a filler and had porous strength.
When the addition amount of the powder added to the raw material exceeded 50% by weight, the collection rate of the lumps was worse than that when the powder was not added. Therefore, the addition amount is preferably 50% or less. The amount of powder added is about 5% or more, which is sufficient to achieve the object of the present invention. The particle size of the powder to be added may be 5 mm or less.

Dehydration of boric acid and ammonium borate requires 250 ° C. or higher, and heating to 600 ° C. is sufficient to decompose and remove organic nitrides such as melamine and dicyandiamide. Therefore, the low temperature heat treatment temperature is 250 to 600
C is optimal.

The agglomerated product obtained by mixing the powders and heat-treating at a low temperature is then adjusted to a particle size of 5 to 30 mm in order to ensure sufficient gas flow, and in a non-oxidizing atmosphere (preferably, in order to prevent oxidation). High-purity BN was obtained by performing a nitriding reaction treatment by heating at a high temperature of 800 to 1200 ° C. in an ammonia atmosphere).

The above research was conducted in a moving bed heating furnace such as a rotary furnace and a filling furnace.
In the case of this moving bed, in order to avoid troubles such as blockage of the heating furnace due to the melting of boric acid, a low-temperature heat treatment was performed to form a lump product, and then a high-temperature nitriding treatment was performed. A two-step treatment process is required. However, in the case of a fixed bed heating furnace such as a pot furnace, troubles such as blockage do not occur. So when I studied in fixed bed,
Without heating the mixture in which the powder is added to the raw material at low temperature,
Even when heat-treated directly to 1000 ° C., high-purity BN similar to the two-step treatment was obtained.

Therefore, the feature of the present invention resides in that the nitriding reaction treatment is performed by adding the product powder obtained by heat-treating a mixture of a boron compound such as boric acid and a nitrogen compound to a boron compound such as boric acid.

As the nitrogen source in this nitriding reaction treatment, there is ammonia or a nitrogen compound as a solid nitrogen source. In the case of ammonia, if the produced powder is mixed with a boron compound and heat-treated in an ammonia atmosphere, nitriding proceeds and the desired high-purity BN is obtained. In the case of solid nitrogen compounds such as urea, melamine, and dicyandiamide, a boron compound, a nitrogen compound, and the product powder are mixed, and heat treated in a non-oxidizing atmosphere such as an inert gas or ammonia to obtain high purity. BN is obtained.

In addition, while conducting experiments to obtain high-purity BN,
It was revealed that the increase in the BN content at a higher synthesis temperature is largely due to the evaporation of boron oxide present in the synthetic powder, rather than the nitridation of boron. Therefore, when heat treatment was performed under reduced pressure, the evaporation amount of boron oxide was increased and the BN content was improved.

The product powder of 5 mm or less used in the method of the present invention is
It may be a return in the same production line, or may be obtained by pretreatment with separate equipment.

[Action]

By recycling the powder having a size of 5 mm or less generated from the product obtained by the low temperature heat treatment at 250 to 600 ° C. as a raw material, a porous massive product having high strength can be obtained.

This massive product has high strength, and during the nitriding reaction treatment,
Since air permeability is maintained, reactivity is remarkably improved, and BN having a high purity and no crystal growth is produced.

〔The invention's effect〕

By the method of the present invention, it has become possible to easily obtain a high-purity hexagonal boron nitride in which crystals are not developed at low cost.

〔Example〕

Example 1 Boric acid and melamine were mixed together at 400 ° C. in an N ₂ atmosphere to give 1
After processing for 400 hours, 400 g of powder of 5 mm or less generated from the agglomerated product was added to 2 kg of boric acid and 2 kg of melamine, and after mixing with a mixer, the raw material was placed in an N ₂ atmosphere in a pot furnace with an inner diameter of 400 mmφ and a height of 400 mmH. A low temperature heat treatment was carried out under the condition of medium 400 ° C. × 1 hour. The bulk product obtained is removed 5
After adjusting the grain size to ~ 30 mm, insert it into a rotary furnace (diameter 80 mmφ, soaking length 400 mm) and 1000 ° C in N ₂ atmosphere.
When the high temperature heat treatment was performed for 2 hours, the processed product was B
The N content was 94.8%. The pressure of this product is 200 Kg
When a molded product was manufactured by hot pressing at 1900 ° C./cm ² for 1 hour, the bending strength of the hot-pressed BN obtained by the conventional high temperature processing was 8.
It was ² kg / mm ² , but 11.2 was produced by this method.
It had a strength of Kg / mm ² .

It is considered that the reason for this is that the BN produced by this method was a BN powder having a submicron particle size and had good sinterability.

Thus, by obtaining a product of high purity at a temperature lower than that of the conventional method, the properties of the molded product can be greatly improved and the product can be manufactured at low cost.

Example 2 2 kg of boric acid and 900 g of powder of 5 mm or less generated from a low-temperature heat treatment product of boric acid-melamine were mixed and subjected to a nitriding treatment under the same two-step heating condition as in Example 1 in an ammonia atmosphere. It was possible to produce boron nitride having a content of 92.3%.

Subsequently, when a molded product was manufactured by hot pressing in the same manner as in Example 1, the bending strength was 10.8 Kg / mm ² .

Example 3 1 kg of the low temperature heat treatment product powder of 5 mm or less in Example 1 added to 40% boric acid was heat-treated in an ammonia atmosphere at 1000 ° C. for 2 hours in a pot furnace to give 94% of the product.
It was BN of.

A molded body was produced from this product by hot pressing in the same manner as in Example 1, and the strength was measured. The bending strength was 11.3.
It was Kg / mm ² .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Wangaki Sasaki 17 Kawasaki Furnace Co., Ltd. Technical Research Institute, 1576, 1576, East Offshore, Nakahiro, Ako City, Hyogo Prefecture

Claims (2)

[Claims] 1. Boric acid or its dehydrated product or ammonium borate is mixed with a nitrogen compound, and the mixture is added to 250 to 6 parts.
Hexagonal nitriding, characterized by mixing 5 to 50% by weight of boric acid or its dehydrated product or ammonium borate with 5 to 50% by weight of a powder of a lump product obtained by treating at 00 ° C. Method for producing boron. 2. Boric acid or its dehydrated product or ammonium borate and a nitrogen compound are mixed together, and the mixture is added to 250 to 6 parts.
A powder of 5 mm or less of a lump product obtained by treating at 00 ° C. is mixed with boric acid or its dehydrated product or ammonium borate in an amount of 5 to 50% by weight, and a nitrogen compound is further added to this to add it in a non-oxidizing atmosphere. A method for producing hexagonal boron nitride, which comprises heating with hexagonal boron nitride.