JPH0611643B2 - Boron Nitride Manufacturing Method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing boron nitride, and more specifically,
The present invention relates to an improved method for producing boron nitride, which shortens the heating time, lowers the maximum temperature, increases the throughput, obtains large crystals, and is easy to operate.

[Conventional technology]

Conventionally, as an industrial production method of boron nitride, boric acid, boric anhydride or borax, and a mixture of dicyandiamide, melamine, a mixture of organic compounds that generate ammonia by thermal decomposition of urea, or boric acid There is a method in which is granulated with a filler having a large specific surface area such as calcium phosphate and heated in an ammonia stream. With this method, it is difficult to retain the reaction surface of boric acid or borax that is nitrided at a temperature above its melting point, and the reaction rate is low, so the production batch becomes larger and the reaction time due to rapid heating is shortened. Is hindering Further, this method has a problem that carbonization of melamine or the like causes blackening of the obtained boron nitride and non-uniform nitriding.

On the other hand, boric acid or boric anhydride and melamine are mixed and water is further added to form a boron nitride precursor, which is heated in an inert atmosphere. Since it reacts completely, there is no melamine carbonization due to subsequent heating, and the heating residue of the precursor remains only boron nitride, and all other components are vaporized,
It has been found that high yield and high purity boron nitride can be easily obtained.

[Problems to be solved by the invention]

Therefore, when various studies were conducted on the above method,
The boron nitride precursor, in the presence of excess water,
In addition to nitrogen and boron, which remain as boron nitride after heating, a large amount of heated volatile components are contained, and these volatile components including excess moisture evaporate during heating.Therefore, when the precursor is heated, the precursor powder is scattered. Yes, and it is difficult for heat to reach the inside,
It was difficult to heat a large amount of precursors uniformly and rapidly. Furthermore, after the volatile components are removed, the residual boric anhydride also volatilizes rapidly at a low temperature, so there is no crystal growth of boron nitride at 700 ° C. or higher, and it is difficult to obtain boron nitride with good crystallinity. .

Therefore, the present inventors previously made the precursor a molded body, increased the bulk density to improve the thermal conductivity, made it possible to rapidly raise the temperature, and caused the scattering of the powder along with the evaporation of volatile components. Proposed a method to prevent. This method improves thermal conductivity,
Although a great improvement was recognized, for rapid uniform heating,
Furthermore, it was necessary to improve the thermal conductivity of the molded body.

In view of the above circumstances, the present invention provides a method for obtaining a boron nitride having good crystallinity in a short time by significantly increasing the thermal conductivity of a molded precursor, allowing it to be uniformly and rapidly heated. With the goal.

[Means for solving problems]

The present invention has been made to achieve the above object,
The gist thereof is a method for producing boron nitride in which boron nitride and water are added to a mixture of boric acid or boric anhydride and an organic cyclic compound having an NH ₂ group, and the mixture is then molded to increase the bulk density and then heated.

[Specific Structure and Action of Invention]

Boric acid or boric anhydride (B ₂ O used in the present invention
₃ ) is orthoboric acid (H ₃ B)
O ₃ ), metaboric acid (HBO ₂ ), tetraboric acid (H ₂ B ₄
Any of O ₇ ) can be used.

Moreover, the organic cyclic compound having an NH ₂ group is preferably one that does not melt during the heating reaction of boron nitride.
For example, melamine, ammeline, ammelide, melam, melem, melon, cyanomelamine, guanylmelamine and the like. The reason why it is desirable not to melt is that foaming occurs when heated, the decomposed gas does not easily escape, carbonization easily occurs due to insufficient decomposition, and the purity of boron nitride decreases.
Urea (H ₂ NCONH ₂ ) easily dissolves in water and decomposes to generate NH ₃ , which is not preferable because it is difficult to form a stable precursor with boric acid.

Now, a case will be described in which boron nitride and water are further added to boric acid or boric anhydride and melamine to produce boron nitride (hereinafter referred to as BN). First, when boric acid or boric anhydride is mixed at a B / N atomic ratio of 1/3 to 2/1, and 40% by weight or less of BN is added to the total amount of boric acid. Alternatively, boric anhydride, melamine, and water react to give a precursor represented by the molecular formula of C ₃ N ₃ (NH ₂ · H ₃ BO ₃ ) ₃ , and BN retains its state as it is without undergoing a chemical change. . This precursor is molded with a granulating machine such as a tablet machine, and the molded body is dried and further calcined, or by firing in a non-oxidizing atmosphere after drying or calcining, the precursor is a crystal with high purity. A hexagonal BN with good properties and formed with the precursor
Is contained in the BN produced from the precursor as it is.

By molding and using the above mixture, the precursor has a high bulk density, the thermal conductivity of the precursor is good, and further BN having good thermal conductivity is dispersed and contained, which is promoted. Therefore, when a large amount of molded bodies are heated, heat is easily transmitted to the center, rapid temperature rise is possible, and uniform heating is performed in a short time. In addition, since a gap is formed between the individual molded particles, the gas generated by thermal decomposition can easily escape, and blow-up and scattering that occur when the powder is heated without being molded can be prevented. Further, since the bulk density is high, boric anhydride is heated to a high temperature and remains for a long time even after volatilization of a volatile component having a high initial decomposition pressure, so that boron nitride crystal growth is extremely well performed.

The reason that the B / N atomic ratio is in the range of 1/3 to 2/1 is that when the atomic ratio is less than 1/3, melamine that does not become a precursor in the presence of water remains and carbonizes and is nitrided during firing. Makes boron black or brown. To obtain BN with good crystallinity, B / N = 1 /
It is preferably 1 or more, and the crystallinity is improved up to about 2/1 depending on the amount of boric acid. Even if the amount of boric acid or boric anhydride exceeds the B / N atomic ratio of 2/1, the crystallinity is increased. There is no improvement in sex and it is ineffective in the product. That is, in the above range, if the proportion of B is large, it is easy to obtain BN having good crystallinity.

On the other hand, in order to obtain amorphous boron nitride, melamine excess, usually B / N, is preferably 1/2 or less. Therefore, the preferable range of B / N is 1/3 to 2/1.

Further, the addition of BN is to increase the thermal conductivity of the molded product by adding BN having good thermal conductivity. Naturally, the larger the amount of addition, the more effective. However, since this BN is included in the product as it is, if too much,
Although the temperature rise time is shortened, the amount of the precursor is reduced, which is economically disadvantageous. Therefore, the amount contained in the molded body is preferably 40% by weight or less, particularly preferably 10 to 30% by weight. Further, in order to obtain a uniform temperature by the rapid temperature rise, it is necessary to uniformly disperse BN in the molded body, which is added as a powder. In consideration of dispersion, the powder has a finer one, but in handling, it is 30 to 325 mesh, particularly 80 to 20 mesh.
0 mesh is preferable.

The amount of water mixed is H ₃ BO ₃ , HBO ₂ and H ₂ as raw materials.
It is slightly different depending on which one of B ₄ O ₇ and B ₂ O ₃ is used. HBO ₂ , H ₂ B ₄ O ₇ , and B ₂ O ₃ react with water to form H ₃ BO ₃ , so it is necessary to increase the amount of water consumed by it.

When H ₃ BO ₃ is used, powdered melamine or boric acid can be mixed and molded by adding 0.2 to 0.3 g of water per 1 g of melamine and stirring with a screw. Also, in the case of mixing powder tube of granular melamine and boric acid,
If 1.5 to 2.0 g of water is added per 1 g of melamine, it becomes lake-like and difficult to stir, but if it exceeds 2 g, fluidity gradually occurs and stirring can be performed with a ball mill. However, if the amount of water is too large, it cannot be molded unless it is dried. Therefore, on the assumption of molding, water is 0.
About 2 to 2 g is preferable. Excess water may be dehydrated to this range prior to molding. The addition of BN may be appropriately performed when the boric acid and melamine are mixed, at the time when they can be uniformly dispersed.

The molding can be performed by press molding, a pan pelletizer, etc., and is made into a lump, a granule, or the like.

The heating temperature for BN generation is preferably 300 to 2300 ° C.
The atmosphere at that time is N ₂ , Ar, and NH ₃ .

Further, in order to enhance the crystallinity of the generated BN, it can be heated to a higher temperature. The molded product may be dried before heating.

Next, the method of the present invention will be described with reference to Examples and Comparative Examples.

[Example 1] Boric anhydride powder: 30 kg, Melamine powder: 40 kg were put in a Shinagawa stirrer, water: 30 kg was gradually added with stirring, and BN powder under 100 mesh was further added.
Add (a) 15kg, (b) 25kg, (c) 40kg, (d) 55kg,
Powder mixture of precursor and BN, 115kg, 125kg, 14
0 kg and 155 kg were obtained. This mixed powder was granulated and formed by a mechanical press into tablets of 20 mmφ × 10 mmt.
The molded body was dried at 80 ° C., then charged into a stainless steel container, and heated to 1000 ° C. in a nitrogen stream for each optimum time. None of the obtained products had the same external shape as before charging, and neither disintegration nor pulverization was observed. Further, each of these products was placed in a graphite container lined with BN and heated to 1400 ° C. in a nitrogen gas stream to obtain a final product.

The final product was confirmed to be BN by X-ray diffraction.

[Comparative Example 1] BN was not added, and a tablet containing only the precursor was prepared.
BN was prepared in the same manner as in Example except that the temperature was raised to 0 ° C. for 4 hours.

The final product was confirmed to be BN by X-ray diffraction.

The results of Example 1 and Comparative Example 1 are shown in Table 1.

As is clear from the table, the more the amount of BN added, the shorter the optimal treatment time and the better the crystallinity and yield of the product. However, the apparatus for treating the same amount of precursor becomes large, and it is estimated economically that the amount of BN added is up to about 40%.

[Example 2, Comparative Example 2] The molded body 10 of Example 1, Comparative Example 1 was dried at 80 ° C,
Put each in a stainless steel container, 100 in a nitrogen stream
The temperature was raised to 0 ° C. over 150 minutes, and the product was placed in a graphite container lined with titanium and heated to 1400 ° C. in a nitrogen stream to obtain a product, and the product physical properties were compared. The results are shown in Table 2.

[Effects] As described above, in the method of the present invention, BN having good thermal conductivity is dispersed in the precursor and molded. Therefore, the increase in the bulk density of the precursor and the action of BN cause It has extremely good thermal conductivity, enables rapid and uniform heating, improves both productivity and quality, and the generated gas escapes between the granules of the compact, so that powder blowing up occurs. There are many advantages such as no.

Claims (1)

[Claims] 1. Boron nitride or a mixture of boric anhydride and an organic cyclic compound having an NH ₂ group is added with boron nitride and water, which is then molded and heated to increase the bulk density. Manufacturing method.