JPH08217424A - Hexagonal boron nitride powder and its production - Google Patents

Abstract

PURPOSE: To obtain low-crystalline and highly purified hexagonal boron nitride (hBN) powder. CONSTITUTION: This hexagonal boron nitride powder has >=5 graphitized index (GI-value) by a powder X-ray diffraction method and contains <=1.0wt.% oxygen. In producing this powder by heating and holding a raw material mixture of a boron source compound containing boron and oxygen and a nitrogen source compound containing nitrogen and/or a reducing agent at >=1600 deg.C in a nitrogen gas atmosphere, the reaction is performed in such a state that a nitrogen gas in a supplying ratio of 1-200L/min in a normal state to 1kg of the raw material mixture and/or a gas generating a nitrogen gas at the above-mentioned temperature is circulated so as to be brought into contact with the raw material mixture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to hexagonal boron nitride (h
BN) powder, in particular hBN powder with low crystallinity and high purity, and a method for producing the same.

[0002]

2. Description of the Related Art hBN powder has a layered structure similar to that of graphite, and has thermal conductivity, insulation, chemical stability, solid lubricity,
Due to its excellent properties such as thermal shock resistance, it has been 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 in which a boron source compound containing boron and oxygen such as boric acid, boron oxide or borax is supported on a filler such as calcium phosphate and then baked in an ammonia atmosphere. (2) A method of firing a mixture of the above boron source compound and a nitrogen source compound containing nitrogen such as dicyandiamide, melamine or urea. (3) A method of firing a mixture of the boron source compound and a reducing agent such as carbon or magnesium in a nitrogen gas atmosphere.

In any of the above methods, the properties such as crystallinity and purity of the obtained hBN powder greatly depend on the firing conditions, especially the firing temperature. Particularly, at 1600 ° C. or higher, the graphitization index (GI value), which is an index of crystallinity, and the amount of oxygen, which is an index of purity, are decreased with an increase in firing temperature, and the crystallinity and purity are significantly improved.

The graphitization index means three diffraction peaks appearing in the X-ray diffraction pattern of hBN powder, (100), (101),
It is a value calculated from the peak area of (102) by the formula (1), and the value becomes smaller as the crystallinity of hBN is improved, and GI = 1.60 for completely crystallized hBN.
(J.Thomas et al., J.Chem.Soc., 84,24,4619-462
2 (1963)). GI = peak area [(100) + (101)] / peak area (102) (1)

On the other hand, the oxygen content of the hBN powder is usually larger in the low crystallinity (GI value = large) hBN powder, and smaller in the high crystallinity (GI value = small) hBN powder. It is considered that this is because the deoxygenation from the boron source compound and the bonding of nitrogen to the boron atom occur continuously in the production of the hBN powder using the boron source compound as a raw material. Therefore, in the case of a low crystalline hBN powder in which the crystal structure is incompletely formed, deoxidation from the boron source compound is not complete, so that a low purity hBN powder containing a large amount of oxygen is used.
It becomes BN powder. On the contrary, in the case of the highly crystalline hBN powder, deoxidation from the boron source compound is almost complete, so that the hBN powder of high purity has a small amount of oxygen.

[0007] Ordinary highly crystalline hBN powder has a GI value of 5
And the amount of oxygen is less than 1.0% by weight. Such highly crystalline and highly pure hBN powder is a solid lubricant / release agent,
It exhibits excellent properties in applications such as resin and rubber fillers, but as a raw material for hBN sintered compacts or a raw material for composite sintered compacts containing hBN, the strength of the sintered compact becomes insufficient, and low crystallinity The low-purity hBN powder has a problem that the density of the sintered body is not sufficient.

The reason for this is that when highly crystalline and highly pure hBN powder is used as the raw material for the sintered body, crystallization has already progressed sufficiently at the stage of the raw material and the atomic rearrangement during sintering is Since it hardly occurs, the mass transfer at the time of sintering becomes slow, and it is conceivable that a strong bond is less likely to occur between the hBN particles inside the sintered body and the strength does not increase. On the other hand, when hBN powder of low crystallinity and low purity is used, such a problem of strength development is reduced, but since the amount of oxygen is large, a liquid phase of boron oxide (B ₂ O ₃ ) is generated during sintering. The resulting hBN grains are likely to grow. Since hBN is a substance with a large anisotropy by nature, and since the grain growth direction is also anisotropic, when grain growth occurs, gaps are likely to occur in the sintered body and the density of the sintered body will decrease. Can be considered.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has a high density and high strength hB.
It is an object of the present invention to provide an hBN powder capable of producing an N sintered body or a composite sintered body containing hBN and a method for producing the same.

[0010]

That is, the present invention is characterized in that the graphitization index (GI value) in the powder X-ray diffraction method is 5 or more and the amount of oxygen is 1.0% by weight or less. A hexagonal boron nitride powder, a raw material mixture containing a boron source compound containing boron and oxygen, and a nitrogen source compound containing nitrogen and / or a reducing agent under a nitrogen gas atmosphere.
In the method for producing a hexagonal boron nitride powder by heating and holding at a temperature of 0 ° C. or higher, the nitrogen gas atmosphere is 1 to 200 liter / min of nitrogen gas in a standard state with respect to 1 kg of the raw material mixture and / or the above. The method for producing a hexagonal boron nitride powder is characterized in that the hexagonal boron nitride powder is formed by circulating a gas that produces nitrogen gas at a temperature so as to directly contact the raw material mixture.

The present invention will be described in more detail below.
The hBN powder of the present invention has a GI value of 5 or more and an oxygen content of 1.
The hBN powder has a low crystallinity and a high purity of 0% by weight or less, preferably 0.5% by weight or less. The hBN powder of the present invention is different from the conventional ones which are either highly crystalline and highly pure, low crystalline and low pure, or highly crystalline and low pure. The reason for this is that in the former two cases, the oxygen content of the hBN powder by the usual production method changes depending on the crystallinity as described above, and in the latter case, a highly crystalline hBN powder was produced. This is because the subsequent purification is insufficient or a substance containing oxygen is intentionally added.

In the present invention, when the GI value is less than 5 or the oxygen content exceeds 1.0% by weight, hB of high density and high strength is obtained.
It is not possible to manufacture an N sintered body or a composite sintered body containing hBN. Since the hBN powder of the present invention has low crystallinity, a strong bond is formed between particles inside the sintered body, and since it is highly pure, grain growth inside the sintered body can be suppressed.

Next, a method for producing the hBN powder of the present invention will be described. A major feature of the present invention is that when the raw material mixture is held in a nitrogen atmosphere at a temperature of 1600 ° C. or higher, it is 1 to 200 liters / liter in a standard state for 1 kg of the raw material mixture.
A minute nitrogen gas and / or a gas that produces nitrogen gas at a temperature of 1600 ° C. or higher is passed, and the nitrogen gas and / or a gas stream that produces nitrogen gas is brought into direct contact with the raw material mixture.

The effect of the nitrogen gas stream and / or the gas stream that produces nitrogen gas in the present invention is to remove oxygen contained in the hBN powder without improving the crystallinity of the hBN powder, and to decompose hBN. By this, it is possible to produce a fine hBN powder having a BET specific surface area of 10 m ² / g or more, which has low crystallinity, high purity, and suppressed grain growth. As a gas that produces nitrogen gas at a temperature of 1600 ° C. or higher, ammonia gas or the like can be used.

Although an inert gas such as argon or hydrogen gas can be used at a temperature lower than 1600 ° C., it is not suitable for the present invention at a temperature higher than that, since it has no effect of suppressing the decomposition of hBN.

In the present invention, the nitrogen gas and / or the gas that produces the nitrogen gas must be circulated. This is because when these gases are allowed to stand, the gas containing oxygen volatilized from the hBN powder is saturated in the gas phase and the volatilization thereafter becomes difficult to proceed.

Further, in the present invention, it is necessary that the nitrogen gas stream and / or the gas stream for producing nitrogen gas be brought into direct contact with the raw material mixture. If this is neglected, the deoxidation from the hBN powder in the parts that do not come into contact with these gases does not proceed smoothly.

As a method for directly contacting the nitrogen gas stream and / or the gas stream for producing nitrogen gas with the raw material mixture, for example, after filling the raw material mixture into a basket-like crucible having a large number of holes, these crucibles are placed under the crucible. A method of circulating gas,
For example, a method may be used in which a pipe is inserted into the filling portion of the raw material mixture and these gases are caused to flow out through a hole provided at the tip of the pipe or the side surface of the pipe.

The flow rate of the nitrogen gas and / or the gas for producing the nitrogen gas is 1 to 1 kg in a standard state for 1 kg of the raw material mixture.
200 liters / minute, preferably 5 to 100 liters / minute. If it is less than 1 liter / min, deoxidation from the hBN powder is not sufficient, and if it exceeds 200 liter / min, the heat carried away by these gases becomes large, which is not preferable. The flow of the nitrogen gas and / or the gas that generates the nitrogen gas is performed at a temperature of 1600 ° C. or higher. When the temperature is lower than this, deoxygenation from the hBN powder does not sufficiently occur regardless of whether or not the nitrogen gas and / or the gas that generates the nitrogen gas is passed.

The raw material mixture used in the present invention is a mixture of a boron source compound containing boron and oxygen, and a nitrogen source compound containing nitrogen and / or a reducing agent. If these formulation examples are shown, the nitrogen source compound and / or the reducing agent is 10 to 200 parts by weight with respect to 100 parts by weight of the boron source compound.

Examples of the boron source compound include boric acid,
Boron oxide, borax, a compound containing boron and oxygen such as anhydrous borax, a nitrogen source compound, melamine, dicyandiamide, a nitrogen-containing organic compound such as urea, further reducing agents, carbon and magnesium, A reducing metal such as calcium is used.

[0022]

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

Example 1 990 grams of boric acid (H ₃ BO ₃ ) and melamine (C ₃ N
_After 1010 grams of ₆ H ₆ ) was mixed in a ball mill for 1 hour, a large number of raw material mixtures consisting of cylindrical molded bodies having a diameter of 20 mm and a height of 20 mm were molded using a mold. These have a diameter of 30 mm with a large number of holes of 10 mm in diameter on the bottom.
A crucible made of 0 mm and a height of 300 mm, which had no lid and was made of pyrolytic boron nitride, was filled and placed in a graphite reaction tube of a high-frequency induction heating furnace.

Next, 1 liter / minute (0.5 liter / minute per 1 kg of the raw material mixture) of nitrogen gas was supplied in a standard state from the nitrogen gas introduction hole provided at the bottom of the graphite reaction tube. The crucible is arranged so that the opening of the nitrogen gas introduction hole is in contact with the bottom of the crucible so that the nitrogen gas flows through the crucible.

The raw material mixture is heated and heated at a rate of 15 ° C./minute, and when the temperature reaches 1600 ° C., the nitrogen gas flow rate is 2 liters / minute (1 liter / kg of the raw material mixture).
Minutes). Then, again at a rate of 15 ° C / min for 20
The temperature was raised to 00 ° C., the temperature was maintained for 2 hours, the heating was stopped, and the temperature was naturally cooled to room temperature.

The product obtained was a white mass which could be easily ground by applying a light impact. This was ground in a ball mill for about 1 hour to obtain about 400 grams of white powder. When this was subjected to X-ray diffraction analysis, it showed a diffraction pattern of hBN, and its graphitization index (GI
The value) was 7.42. Moreover, when the oxygen amount was measured by an O / N simultaneous analysis apparatus (EMGA-2800) manufactured by Horiba, Ltd., it was 0.23% by weight.

In order to evaluate the sinterability of the obtained hBN powder, a green body having a diameter of 40 mm and a height of 60 mm was die-molded and pressure-sintered under the conditions shown in Table 1. The bulk density and bending strength shown in (JIS R 160
A hBN sintered body having a three-point bending test in 1) was produced.

Example 2 The hBN powder produced in Example 1 was internally compounded with the additives shown in Table 1 and molded by a mold in the same manner as in Example 1 and then baked under normal pressure under the conditions shown in Table 1. Upon binding, an hBN sintered body having the density and bending strength shown in Table 1 was produced.

Example 3 560 grams of boron oxide, 250 grams of carbon powder (acetylene black) and 200 grams of water were mixed for 20 minutes in a Henschel mixer, and then molded in the same manner as in Example 1 to prepare a large number of raw material mixtures. did. These are 300m in diameter
It was filled in a crucible made of an hBN sintered body having a height of m and a height of 200 mm. At this time, a pipe made of hBN sintered body having a diameter of 20 mm and a length of 300 mm was inserted so that the opening at the lower end was located at a height of about 15 mm from the bottom surface of the inner wall of the crucible for nitrogen gas flow. After that, the crucible was placed in a graphite reaction tube of a high-frequency induction heating furnace, and the upper end of the hBN sintered pipe for gas distribution was connected to an existing nitrogen gas pipe, and 10 l / min of nitrogen was supplied under standard conditions. Gas was supplied.

The raw material mixture was heated and heated at a rate of 20 ° C./min. When the temperature reached 1800 ° C., the heating was stopped and the temperature was maintained for 4 hours and then naturally cooled to room temperature. The obtained product was a white lump similar to that in Example 1, and this was crushed in the same manner as in Example 1 to obtain about 300 g of a white powder. The X-ray diffraction analysis result of this white powder shows the diffraction pattern of hBN, and its graphitization index (G
The I value) was 9.12. The amount of oxygen was 0.42% by weight.

The hBN powder produced as described above was die-molded in the same manner as in Example 1 and hot-press sintered under the conditions shown in Table 1. As a result, hBN having the density and bending strength shown in Table 1 was obtained. A sintered body was produced.

Example 4 The hBN powder produced in Example 3 was internally compounded with the additives shown in Table 1, molded in the same manner as in Example 1, and then baked under normal pressure under the conditions shown in Table 1. Upon binding, a composite sintered body containing hBN having the density and bending strength shown in Table 1 was produced.

Example 5 Anhydrous borax (Na₂B_FourO₇) 800 grams, dicyan
Diamide (C₂N_FourH _Four) 1000 grams of mixed powder
The raw material mixture consisting of
After filling in the crucible, place it in the heating furnace and
To supply 2 liter / min of ammonia gas.

Then, the mixture was heated and heated at a rate of 25 ° C./minute, kept at a temperature of 1900 ° C. for 3 hours, naturally cooled to room temperature, and pulverized in the same manner as in Example 1 to obtain about 400 g of a white powder. Obtained. The X-ray diffraction analysis result of this white powder is h
The diffraction pattern of BN was shown, and the GI value was 6.14. The amount of oxygen was 0.66% by weight.

The hBN powder produced above was internally blended with the additives shown in Table 1, mold-molded in the same manner as in Example 1, and then pressure-sintered under the conditions shown in Table 1. An hBN sintered body having the density and bending strength shown in Table 1 was produced.

Comparative Example 1 When operated in the same manner as in Example 1 except that the nitrogen gas flow rate was not increased at 1600 ° C., about 5
00 grams of white powder was obtained. The X-ray diffraction analysis result of this powder showed the diffraction pattern of hBN, and the GI value was 2.60. The amount of oxygen was 0.15% by weight.

The hBN powder produced as described above was die-molded in the same manner as in Example 1 and hot-press sintered under the conditions shown in Table 1. As a result, hBN having the density and bending strength shown in Table 1 was obtained. A sintered body was produced.

COMPARATIVE EXAMPLE 2 The hBN powder produced in Comparative Example 1 was internally compounded with the additives shown in Table 1, molded in the same manner as in Example 1, and then baked under normal pressure under the conditions shown in Table 1. Upon binding, an hBN sintered body having the density and bending strength shown in Table 1 was produced.

Comparative Example 3 The same raw material mixture as in Example 1 was filled in a graphite crucible having a diameter of 300 mm and a height of 300 mm with no holes on the bottom and made of a lid, and placed in a graphite reaction tube of a high-frequency induction heating furnace. After arranging, 3 liter / min (1.5 liter / min per 1 kg of raw material mixture) nitrogen gas in a standard state so as to directly hit the surface layer of the raw material mixture in the crucible through the nitrogen gas introduction hole in the upper part of the reaction tube. Supplied.

After that, the raw material mixture was heated and heated at a rate of 15 ° C./min and kept at a temperature of 1500 ° C. for 2 hours,
It was naturally cooled to room temperature.

In the obtained product, transparent glass-like precipitates were found on the surface and bottom, and elemental analysis revealed that it was boron oxide (B ₂ O ₃ ). The other parts are white lumps similar to those in Example 1.
Trituration in the same manner as above gave 420 grams of white powder. This was treated with an aqueous nitric acid solution, washed with water and dried to remove B ₂ O ₃ to obtain about 400 g of a white powder. The X-ray diffraction analysis result of this white powder showed the diffraction pattern of hBN, and the GI value was 8.08. The amount of oxygen was 1.16% by weight.

The hBN powder produced above was molded in the same manner as in Example 1 and thereafter pressure-sintered under the conditions shown in Table 1. As a result, hBN having the density and bending strength shown in Table 1 was obtained. A sintered body was produced.

Comparative Example 4 The additives shown in Table 1 were added to the hBN powder produced in Comparative Example 3.
After compounding the inner ratio (confirmation ), molding the mixture in the same manner as in Example 1, and performing hot press sintering under the conditions shown in Table 1, a composite containing hBN having the density and bending strength shown in Table 1 was obtained. A sintered body was produced.

Reference Example 1 Commercially available hBN powder having a GI value and an oxygen content shown in Table 1 was mold-molded in the same manner as in Example 1 and hot-press sintered under the conditions shown in Table 1. An hBN sintered body having the density and bending strength shown in Table 1 was produced.

Reference Example 2 A commercially available hBN powder having a GI value and an oxygen content shown in Table 1 was internally compounded with the additive shown in Table 1 and molded in the same manner as in Example 1, and then shown in Table 1. When hot press sintering under the conditions, hB having the density and bending strength shown in Table 1 is obtained.
A composite sintered body containing N was produced.

[0046]

[Table 1]

[0047]

According to the present invention, an unprecedented low crystallinity and high purity hBN powder is provided, and a high strength and high density hBN sintered body or a composite sintered body containing hBN can be manufactured. it can.

Claims (2)

[Claims] 1. A graphitization index (G) in a powder X-ray diffraction method.
A hexagonal boron nitride powder having an I value of 5 or more and an oxygen content of 1.0% by weight or less. 2. A boron source compound containing boron and oxygen,
In the method for producing a hexagonal boron nitride powder by heating and holding a raw material mixture comprising a nitrogen source compound containing nitrogen and / or a reducing agent at a temperature of 1600 ° C. or higher in a nitrogen gas atmosphere, the nitrogen gas atmosphere is a raw material. 1 to 200 liters / min of nitrogen gas in a standard state for 1 kg of a mixture and / or a gas that produces nitrogen gas at the above temperature is circulated so as to be in direct contact with the raw material mixture. And a method for producing a hexagonal boron nitride powder.