JPH08290905A - Hexagonal boron nitride powder and its production - Google Patents

Abstract

PURPOSE: To obtain high-quality hexagonal boron nitride powder with even particle size. CONSTITUTION: The objective hexagonal boron nitride powder is characterized by being <=0.55 in coefficient of variation (σ/D50 ) (σ is the standard deviation of particle size distribution and Dig is the median diameter of the particles determined by laser diffractometry, respectively), being obtained by the following process: boric acid is mixed with melamine at such proportions as to be (1:1) to (1:6) in the atom ratio B/N, and the mixed gas is held in an atmosphere at 0-200 deg.C containing water vapor corresponding to a relative humidity of >=5% to effect formation of melamine borate, which is then held once at 500-<1400 deg.C in an ammonia-contg. atmosphere and then baked at 1400-2200 deg.C in a nonoxidative gas atmosphere except ammonia.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high quality hexagonal boron nitride powder having a uniform grain size and a method for producing the same.

[0002]

2. Description of the Related Art Hexagonal boron nitride (hBN) powder has a layered structure similar to graphite and has excellent properties such as thermal conductivity, insulation, chemical stability, solid lubricity, and thermal shock resistance. Utilizing these characteristics, it is applied to solid lubricants / release agents, fillers of resins and rubbers, heat resistant / insulating sintered bodies, etc.

The following is an example of a method for producing hBN powder. (1) A method of loading boron such as boric acid, boron oxide, or borax and a compound containing oxygen on a filler such as calcium phosphate, and then firing the mixture in an ammonia atmosphere. (2) A method of firing a mixture of the above boron compound and a compound containing nitrogen such as dicyandiamide, melamine and urea. (3) A method of firing a mixture of the above boron compound and a reducing substance such as carbon in a nitrogen gas atmosphere. (4) A method in which water is removed from a mixture containing boric acid or boron oxide, melamine and water by a method such as filtration, centrifugation or drying, and then the mixture is baked in a non-oxidizing gas atmosphere.

However, in (1), since the compound containing boron and oxygen melts during firing, the contact area with the ammonia atmosphere does not become sufficiently large, and in (2) the compound containing nitrogen is vaporized or burned during firing. Since it is easily decomposed, it is difficult to significantly increase the reaction rate by any method. In (3), a reducing substance that is difficult to remove by a simple post-treatment such as acid washing tends to remain as an impurity.

On the other hand, in (4), since boric acid or boron oxide and melamine form a salt (melamine borate) by the action of water to form a uniform raw material mixture, a homogeneous BN powder is produced at a high yield. It is known as a possible method (U.S. Pat. No. 3,241,918, JP-A-60-151202, JP-A-61-191505, JP-A-61-161).
However, unlike other methods, this method requires additional steps such as filtration, centrifugation, and drying for removing water from the raw material mixture, and thus has a problem of low productivity. .

Further, the hBN powder produced by any of the methods (1) to (4) has a remarkable variation in particle size. In recent years, various types of hBN powders have been increasingly required to have stable quality, and accordingly, it has become necessary to improve the homogeneity of the hBN powder itself. As a measure against this,
Although hBN powder with a uniform particle size is manufactured by classification, not only an extra post-process is required, but also the yield decreases due to classification, so the powder of the target particle size can be obtained with high yield. It could not be manufactured with high productivity.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems of the prior art and to provide hBN powder having a uniform particle size. Another object of the present invention is to produce hBN powder having a uniform particle size with high yield and high productivity without requiring an extra post-process such as classification.

[0008]

That is, according to the present invention, the coefficient of variation (σ / D ₅₀ ) consisting of the ratio of the standard deviation (σ) of the particle size distribution and the median diameter (D ₅₀ ) in the laser diffraction method is 0.55. Hexagonal boron nitride powder characterized by being the following, and B / N atomic ratio of oxalic acid and melamine of 1/1 to
Mix at a ratio of 1/6, and mix it at a temperature of 0-200 ° C.
A non-oxidizing gas that removes ammonia after holding it in an atmosphere containing water vapor with a relative humidity of 5% or more to form melamine borate and then holding it once in an atmosphere containing ammonia at a temperature of 500 to less than 1400 ° C. Temperature 1
It is a method for producing a hexagonal boron nitride powder characterized by firing at 400 to 2200 ° C.

The present invention will be described in more detail below.

The hBN powder of the present invention is characterized in that the particle size is uniform, and the laser diffraction method is adopted for its measurement in terms of simplicity and high reliability. The evaluation of the uniform particle size in the present invention is made by a coefficient of variation consisting of the ratio of the standard deviation (σ) of the particle size distribution measured by the laser diffraction method to the average particle size [median size (D ₅₀ )]. (Σ / D ₅₀ ) can be used. The coefficient of variation corresponds to the width of the particle size distribution, and if it is large, the width of the particle size distribution, that is, the variation becomes large, and conversely, if it is small, the powder has a uniform particle size.

The coefficient of variation of the conventional hBN powder is 0.7-
While it was 1.0 and the variation was large,
The coefficient of variation of the hBN powder of the present invention is 0.55 or less, which is different from the conventional hBN powder.

Next, a method for producing the hBN powder of the present invention will be described. The method for producing the hBN powder of the present invention is a production method applicable to the production of the hBN powder of the present invention.

The boric acid used in the present invention comprises melamine and
It reacts to form melamine borate, an example of which
Is orthoboric acid (H₃BO₃), Metaboric acid (HB
O₂), Tetraboric acid (H₂B_FourO₇), Boric anhydride
(B₂O₃) And the general formula (B₂O ₃) ・ (H₂O) x
[However, 1 type or 2 types or more of the compound shown by x = 0 to 3]
As mentioned above, orthoboric acid is easy to obtain and
It is suitable for the present invention because it has good miscibility with min.
It

Boric acid and melamine are mixed by a ball mill,
It can be performed using a general mixer such as a ribbon blender or a Henschel mixer. The mixing ratio is B / of boron atom (B) of boric acid and nitrogen atom (N) of melamine.
Ratio of N atomic ratio of 1/1 to 1/6, preferably 1 /
It is a ratio of 2 to 1/4. The B / N atomic ratio is 1/1
If it exceeds, the residual unreacted boric acid becomes remarkable after firing,
On the other hand, if it is less than 1/6, the sublimation of unreacted melamine becomes remarkable during firing, and the yield decreases in any case.

Boric acid and a polymer satisfying the B / N atomic ratio of the present invention are
Ramin (C₃N₆H₆), The specific mixing ratio is
Is orthoboric acid (H₃BO₃), H₃BO₃
/ C ₃N₆H₆Is a molar ratio of 6/1 to 1/1 and a weight ratio of
Is 2.94 / 1 to 0.49 / 1.

In the present invention, the mixture of boric acid and melamine is kept in an atmosphere containing water vapor having a relative humidity of 5% or more at a temperature of 0 to 200 ° C., preferably 40 to 100 ° C. to form melamine borate, which is then calcined. To do. If the formation temperature of melamine borate is less than 0 ° C, the reaction rate becomes remarkably slow, and if it exceeds 200 ° C, boric acid melts into a glass and the surface area becomes small, so that the reaction rate does not increase.
Furthermore, the formation atmosphere of melamine borate is set to a relative humidity of 5%.
The above-mentioned atmosphere containing water vapor was set to have a relative humidity of 5%.
This is because the reaction rate becomes remarkably slow in an atmosphere of less than.

The holding time in the atmosphere of the above temperature and relative humidity is about 1 to 100 hours. Among them, the temperature T (° C.), the relative humidity Ψ (%) and the holding time t (hr) satisfy the following relational expression to hold a mixture of boric acid and melamine to form melamine borate. Such an atmosphere can be easily formed by using a thermo-hygrostat or a steam heating furnace. The gas other than water vapor forming the atmosphere is not particularly limited, and examples thereof include atmospheric gas, nitrogen gas, and inert gas. T ≧ −20 · log ₁₀ (t / 4) + {(Ψ−100) ²
/ 20} +60

The formation of melamine borate can be easily confirmed by thermogravimetry / differential thermal analysis (TG / DTA), X-ray diffraction measurement and the like. For example, in the TG / DTA curve of a mixture of boric acid and melamine, a rapid weight loss and endotherm due to sublimation of melamine appear near 350 ° C.
Almost never appears with melamine borate. Also, in X-ray diffraction measurement, melamine borate formed at a temperature of 120 ° C. or lower exhibits a unique diffraction pattern different from both boric acid and melamine, and thus can be easily confirmed (Take Hagio, et al .; Journal of the Ceramic Society of Japan; Volume 102, No. 11, 1051-1052 [199
4 years]). However, it should be noted that melamine borate formed at a temperature over 120 ° C. is amorphous and does not show its own diffraction pattern.

Since melamine borate produced according to the present invention does not contain water as in the conventional case, it can be immediately used without any steps such as filtration, centrifugation, and drying as it is or after a light crushing step. It can be baked. Further, in the conventional method, the bulk density of melamine borate is remarkably reduced, so that a recovery treatment thereof is necessary (Japanese Patent Laid-Open Publication No. Sho 6-62).
However, in the present invention, such a process is not necessary, so that the productivity is improved. Further, since melamine borate does not melt during firing, it is possible to suppress the wear of the furnace material, which facilitates firing in a continuous furnace.

In the present invention, a temperature of 50 is used before firing.
It is once held in an atmosphere containing ammonia of 0 or more and less than 1400 ° C. By this operation, melamine borate is changed to amorphous boron nitride (a-BN) powder, but the action of ammonia on melamine borate forms a homogeneous a-BN powder with a low oxygen and carbon content. . 5
If the temperature is lower than 00 ° C, the production rate of a-BN becomes remarkably slow,
At 1400 ° C or higher, decomposition of ammonia becomes remarkable. The holding time is preferably 0.5 to 12 hours, particularly 1 to 2 hours.

The firing is carried out at a temperature of 1400 to 2200 ° C. in an atmosphere of non-oxidizing gas excluding ammonia. 140
If the temperature is lower than 0 ° C, the production rate of hBN becomes remarkably slow, and if it exceeds 2200 ° C, the decomposition of hBN becomes remarkable. Examples of the non-oxidizing gas include nitrogen gas, hydrogen gas, hydrocarbon gas such as methane and propane, rare gas such as helium and argon, and carbon monoxide gas. Of these, nitrogen gas is most suitable because it is easily available, inexpensive, and highly effective in suppressing the decomposition of hBN at high temperatures of 2000 to 2200 ° C. The firing time is preferably 0.5 to 24 hours, particularly 2 to 10 hours.

As a furnace used for holding or firing in an atmosphere containing ammonia, batch type furnaces such as muffle furnace, tubular furnace, atmosphere furnace, rotary kiln, screw conveyor furnace, tunnel furnace, belt furnace, pusher furnace, vertical furnace A continuous furnace such as a continuous mold furnace can be used. These are used properly according to the purpose, for example, hBN of many varieties
A batch-type furnace is used when producing powder in small quantities, and a continuous-type furnace is used when producing large quantities of a certain variety.

Further, a catalyst may be added to control the crystallinity and particle size of hBN before or after holding in an atmosphere containing ammonia. As a catalyst,
Compounds such as alkali metal or alkaline earth metal borate, boric acid and / or alkali metal or alkaline earth metal carbonate which reacts with boric acid at high temperature to form a salt are used.

The hBN powder produced as described above is subjected to post-treatment steps such as crushing, classification, acid treatment to remove (purify) residual catalyst, washing, and drying, if necessary.
It is put to practical use.

[0025]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

Example 1 200 g of orthoboric acid and 200 g of melamine (B / N atomic ratio: 1/3) were mixed in a ball mill, and the mixture was heated at a temperature of 80 ° C. and a relative humidity of 80% in the atmosphere using a thermo-hygrostat. It was kept for 10 hours in an atmosphere containing water vapor. The obtained sample was coagulated to such an extent that it could be easily crushed when lightly pressed with a finger. After crushing, it was confirmed by X-ray diffraction measurement that melamine borate was produced.

This sample was held at a temperature of 800 ° C. for 2 hours in an ammonia atmosphere using a tubular furnace. The retained sample contained 3.2% by weight of oxygen and 0.5% by weight of carbon. Calcium borate (2CaO · B ₂ O ₃ ) 3
g was added and mixed by a ball mill, and baked at a temperature of 2000 ° C. for 2 hours in a nitrogen atmosphere using a high frequency heating furnace.

The obtained fired product was washed with dilute nitric acid, filtered, dried and then subjected to X-ray diffraction analysis. As a result, it was confirmed to be hexagonal boron nitride (hBN). When the particle size distribution was measured by a laser diffraction method, the median diameter (D ₅₀ ) was 16.8 μm, the standard deviation (σ) was 8.2 μm, and the variation coefficient (σ / D ₅₀ ) was 0.49.

Example 2 185 g of orthoboric acid and 250 g of melamine (B / N ratio =
1/4) and 10 g of calcium carbonate (CaCO ₃ ) were stirred with a Henschel mixer for 20 minutes and then held in a steam heating furnace under a nitrogen atmosphere at a temperature of 100 ° C. and a relative humidity of 80% for 5 hours. X-ray diffraction analysis of the product confirmed that it was a mixture of melamine borate and CaCO ₃ .
Further, using a tubular furnace, in an ammonia atmosphere, at a temperature of 10
After holding at 00 ° C. for 1 hour, it was fired at a temperature of 1800 ° C. for 1 hour in an argon atmosphere using a graphite resistance heating furnace.

The calcined product was washed with dilute nitric acid, filtered, dried and subjected to X-ray diffraction analysis. As a result, it was confirmed to be hBN powder. Further, the particle size distribution was measured by a laser diffraction method. As a result, D ₅₀ = 8.9 μm, σ = 4.1 μm
m, σ / D ₅₀ = 0.46.

Example 3 500 kg of boric anhydride and 600 kg of melamine (B / N ratio = 1/2) were heated in the atmosphere at a temperature of 1 using a steam heating furnace.
It was kept for 5 hours in an atmosphere of 00 ° C. and 90% relative humidity.
The product was confirmed to be melamine borate by X-ray diffraction analysis. It was maintained at a temperature of 700 ° C. for 10 hours in an ammonia atmosphere using a screw conveyor furnace.
Oxygen 1.5% by weight, carbon 0.3% by weight after holding
Was included. Then, it was fired at a temperature of 1900 ° C. for 12 hours in a nitrogen atmosphere using a vertical continuous furnace.

The calcined product was washed with dilute nitric acid, filtered, dried and subjected to X-ray diffraction analysis. As a result, it was confirmed to be hBN powder. Further, the particle size distribution was measured by a laser diffraction method. As a result, D ₅₀ = 5.8 μm, σ = 3.0 μ
m, σ / D ₅₀ = 0.52.

Comparative Example 1 Orthoboric acid and melamine were mixed in the same manner as in Example 1 and then immediately held in a tubular furnace in an ammonia atmosphere at 800 ° C. without holding in an atmosphere containing water vapor having a relative humidity of 80%.
Held for 2 hours. The oxygen and carbon contents of the sample after holding were measured and found to be 10.2% by weight and 3.5% by weight, respectively. Then, the product obtained by firing in exactly the same manner as in Example 1 was confirmed to be hBN powder.
It was confirmed by line diffraction measurement. Further, when the particle size distribution was measured by a laser diffraction method, D ₅₀ = 15.6 μm, σ
= 10.8 μm, and the coefficient of variation (σ / D ₅₀ ) was 0.69.

Comparative Example 2 Instead of holding the sample in which melamine borate was formed in the ammonia atmosphere at 800 ° C. for 2 hours in Example 1, after holding at 800 ° C. for 2 hours in a nitrogen atmosphere, oxygen and oxygen were added. The carbon contents were measured to be 10.5% by weight and 12.5% by weight, respectively. Then, it was confirmed by X-ray diffraction measurement that the product obtained by firing in exactly the same manner as in Example 1 was hBN powder.
Furthermore, when the particle size distribution was measured by a laser diffraction method, the powder was D ₅₀ = 18.5 μm, σ = 16.9 μm, and the variation coefficient (σ / D ₅₀ ) was 0.91.

[0035]

The hBN powder of the present invention is of high quality with uniform particle size. Further, according to the method for producing hBN powder of the present invention, high-quality hBN powder having a uniform particle size can be obtained in high yield,
It can be manufactured with high productivity.

Claims (2)

[Claims] 1. A hexagonal crystal characterized by a coefficient of variation (σ / D ₅₀ ) consisting of a ratio of a standard deviation (σ) of a particle size distribution and a median diameter (D ₅₀ ) in a laser diffraction method of 0.55 or less. Boron nitride powder. 2. Boric acid and melamine having a B / N atomic ratio of 1
Mix at a ratio of 1/1 to 1/6, and mix it at a temperature of 0 to 20.
After forming melamine borate by holding it in an atmosphere containing water vapor at 0 ° C. and a relative humidity of 5% or more, hold it once in an atmosphere containing ammonia at a temperature of 500 to less than 1400 ° C. and then remove ammonia. Under an oxidizing gas atmosphere,
A method for producing a hexagonal boron nitride powder, which comprises firing at a temperature of 1400 to 2200 ° C.