JPS6227002B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing hexagonal BN (hereinafter referred to as BN) powder, and particularly aims to obtain a BN powder with high filling properties and high crystallinity. In addition to being a lubricant, BN has excellent thermal conductivity and electrical insulation, and also has neutron absorption properties, so it is also used as a filler for heat dissipation materials and neutron shielding materials that use resin, rubber, ceramic materials, etc. as binding materials. There is. What is important for these fillers is that they have high filling properties and, furthermore, they have great crystallinity. BN is generally produced by heating boric acid, anhydrous boron oxide, or borax in ammonia gas, or by mixing and heating a nitrogen-containing organic compound that generates ammonia gas by thermal decomposition of urea, diamine, etc. Crude BN is obtained by heating it to a higher temperature to recrystallize it, and then it is made into a product. Crude BN is usually obtained by reaction at a temperature of 700 to 1400℃, and the crystals are underdeveloped, the size of the primary crystal is smaller than 0.1μ, and the C axis measured by X-ray diffraction method. The index Lc value indicating the degree of crystallinity in the direction is also 50 ~
It is as small as 300Å (JSPS Carbon Materials 117 Committee Act).
In addition, crude BN is used as a precursor for converting anhydrous boron oxide to BN in the form of a B-O-N intermediate compound.
The BN crystal contains ~12% oxygen, and unlike oxygen in the unnitrided free boron oxide state, it is thought that this oxygen cannot be removed by room-temperature treatments such as water washing. When this crude BN is reheated in an inert atmosphere at a temperature higher than the reaction formation temperature of the crude BN, the oxygen in the BN crystal is separated from the BN crystal as free anhydrous boron oxide, and is volatilized and removed. As the reheating temperature increases, more oxygen is liberated as boron oxide, and when the temperature exceeds 2000°C, almost the entire amount is liberated. In parallel with this, BN crystals also develop, and the size of the primary crystals becomes 1 to 10 μ, and the Lc value exceeds 10,000 Å. Usually commercially available BN is heated to about 1900℃ or above, and the primary crystal size is 1μ or more.
The Lc value is 1000 Å or more. In this way, in order to obtain BN powder with developed crystals, crude BN can be reheated in an inert atmosphere, but crude BN itself has poor sinterability, and the bulk density of the powder after reheating is The vibration bulk density of commercially available powder is only 0.1 to 0.3 g/cc, and the filling properties are poor when trying to obtain a composite of resin and BN powder.
It is difficult to obtain composites that exhibit the characteristics of BN. Conventionally, methods for obtaining BN powder with good filling properties include adding boron oxide or boron oxide-calcium oxide as a binder to commercially available BN powder and pulverizing a hot-pressed sintered body;
There is a method of adding calcium borate salt such as B ₂ O ₃ -CaO or borosilicate glass to the powder and pulverizing the sintered body, but in the former case, the productivity of the hot pressed sintered body itself is low; The powder obtained is therefore expensive. In addition, in the latter method, it is difficult to obtain commercially available calcium borate or borosilicate having a predetermined composition, and it is difficult to produce the calcium borate or borosilicate in-house by melting because the melt creeps up due to wetting with the container. The present inventor investigated a method for producing BN powder with easily developed crystals and good filling properties, and found that by utilizing the fact that oxygen present in crude BN becomes B ₂ O ₃ when reheated, an alkaline earth was added thereto. It was discovered that the presence of similar metal oxides or fluorides is effective in densification as a sintering binder, and is also effective in crystal growth, leading to the present invention. That is, the present invention uses crude BN powder containing 1 to 12%, preferably 2 to 10%, of oxygen in the BN crystal.
1 to 10%, preferably 2 to 7%, of one or more powders of alkaline earth metal oxides or fluorides is added and formed into a green compact;
This is carried out at a temperature of at least 1700℃ in a stream of inert gas, etc.
This method involves sintering at a temperature above, pulverizing, and classifying to obtain BN powder with developed crystals and good filling properties. If the oxygen content in the crude BN is less than 1%, the BN crystals will develop too much, making it difficult to form a green compact.
If it exceeds 12%, the amount of volatilization during heating will increase, the density of the sintered body will be low, and as a result, the bulk density of the powder obtained by crushing this will also decrease. As alkaline earth metal oxides and fluorides
Preferred are CaO, MgO, BaO, CaF 2 , MgF _{2 and BaF 2} which form a double salt with B ₂ O 3 and are effective for the growth of BN crystals. Further, instead of these oxides and fluorides, carbonates, hydroxides, basic carbonates, basic fluorides, etc. which become oxides and fluorides upon heating may be used. The amount may be adjusted to the above amount in terms of oxide or fluoride. If the amount of alkaline earth metal oxide or fluoride added is less than 1% or exceeds 10% (%), sinterability and crystallinity will be low, and in the case of fluoride,
If it exceeds 10%, the alumina protection tube and gas blowing tube used during sintering will be seriously damaged. Therefore, the amount of these added is suitably 1 to 10%, preferably 2 to 7%.
It is. A green compact obtained by mixing these additives with BN powder is formed using a forming device such as a mold press or a rubber press, and the green bulk density of the green compact is preferably 1.3 g/cc or more. If this bulk density is low, the vibration-packed bulk density of the powder obtained by pulverization after sintering will not increase. If the density of the green compact is 1.3g/cc or more, the vibration filling bulk density is preferably 0.6g/cc.
The above product powder is obtained. To heat the compact, use an inert gas such as Ar or _N2 gas,
The test is carried out in an atmosphere of NH ₃ gas or a mixture of these gases.
Heating temperature is 1700℃ or higher, preferably 2000-2200℃
It is. At temperatures below 1,700℃, crystal development is insufficient, and at temperatures above 2,200℃, a reaction between graphite used as a heating container and BN begins, resulting in the formation of B ₄ C.
Undesirable. After heating, the green compact is cooled, pulverized by a pulverizer such as a roll mill or a ball mill, and classified. usually,
When used as a filler for synthetic resins, etc., it is preferable to classify with a 40-mesh sieve and select crushing conditions so that the average particle diameter under the sieve is 45 to 80 μm. If there are particles coarser than 40 mesh, the surface will be rough when combined with resin etc. If the average particle size exceeds 80 μm, the bulk density will be low, and if it is smaller than 45 μm, the hygroscopicity of the fine powder will affect the fluidity. It's going to decline. Since the BN powder of the present invention has good filling properties and crystallinity, it can be used in composites with resins and silicone rubber.
If a large amount of BN can be contained, for example in a heat dissipation sheet, the heat dissipation properties will be good. Examples 1-4 Mixing boric acid with calcium phosphate (filler),
After crushing, granulating and drying in ammonia gas 850
The mixture was heated to 0.degree. C. and nitrided, and the filler and unreacted substances were removed by a conventional method to obtain crude BN. The amount of oxygen in this crude BN was 10%. Calcium carbonate, magnesium carbonate, barium carbonate, and calcium fluoride were added and mixed to this crude BN, respectively. The amount added is 5% to crude BN in the form of oxides and fluorides.
%. This was molded into a 60 mm cylindrical shape under a pressure of 1500 Kg/cm ² to obtain a compact having a bulk density of 1.37 to 1.47 g/cc. This green compact is placed in a graphite crucible, inserted into a high-frequency heating furnace, and nitrogen gas is introduced into the crucible at 1.
The temperature was raised to 2,000 to 2,100°C for 2 hours while flowing water for 2 hours, and this temperature was maintained for 2 hours. After cooling while flowing nitrogen gas, the BN compact was taken out from the graphite crucible, pulverized with a small alumina pot mill, and sieved with a 40-mesh sieve. The obtained BN powder had a high bulk density regardless of the additives used. Comparative Example 1 Crude BN obtained in the same manner as in the above example was made into a 60 mmφ green compact under the same conditions as in the example, and heated, cooled, crushed, and classified while flowing nitrogen gas. obtained
The vibrational bulk density of BN powder is 0.55 as shown in Table 1.
g/cc. Examples 5 to 8 Dry dehydrated borax and urea were mixed, heated to 1000°C in ammonia gas and nitrided, and unreacted substances were removed by a conventional method to obtain crude BN powder. The amount of oxygen in this crude BN powder was 3%. This crude BN powder contains CaO, MgO,
MgF ₂ and BaF ₂ were added and mixed, heated in the form of a green compact in the same manner as in Examples 1 to 4, and then crushed and classified to obtain BN powder having a bulk density as shown in Table 1. Comparative Example 2 Crude BN obtained in the same manner as in Examples 5 to 8 above was treated under the same conditions as above without adding any additives,
BN powder was obtained. The BN powders obtained in Examples 5 to 8 have a sintering effect and a higher vibration bulk density than those obtained in Comparative Example 2. Comparative Example 3 Crude BN powder obtained in the same manner as Examples 5 to 8 was
By reheating to 1400°C, washing with water, and drying, a crude BN powder with an oxygen content of 0.5% was obtained. This low oxygen crude
Calcium carbonate equivalent to 2% CaO was added to the BN powder, and sintering, pulverization, and classification were carried out in the same manner as in the above examples. The bulk density of the powder after setting was also lower than that of the example. 【table】

Claims (1)

[Claims] 1 Add at least one powder selected from alkaline earth metal oxides, fluorides, or compounds that become oxides or fluorides upon heating to undeveloped BN powder containing 1 to 12% by weight of oxygen. A method for producing BN powder, which comprises mixing 1 to 10% by weight in terms of fluoride, molding, sintering at 1700°C or higher in an inert gas or NH ₃ stream, and crushing and classifying the sintered body. .