JPS6335564B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to a novel method for producing silicon nitride powder that is highly pure, fine, and has a uniform particle size distribution, and is suitable as a raw material powder for silicon nitride sintered bodies. (Background Art) Since silicon nitride sintered bodies have excellent heat resistance and high mechanical strength, attempts have been made in recent years to apply them to high-temperature, high-stress members. In order to apply silicon nitride sintered bodies to these parts, chemical stability and physical stability at high temperatures are important. In particular, heat resistance and mechanical strength, which are physical stability, largely depend on the physical properties of the silicon nitride powder that is the raw material, so it is desired that the powder be highly pure, fine, and have a uniform particle size distribution. Conventionally, the following method is generally used to produce silicon nitride powder. (1) Method of nitriding metal silicon powder 3Si+2N ₂ ―→Si ₃ N ₄ (2) Method of reacting silicon tetrachloride or silane with ammonia in the gas phase 3SiCl ₄ +4NH ₃ ―→Si ₃ N ₄ +12HCl (3) Silica 3SiO ₂ +6C+2N ₂ ―→Si ₃ N ₄ +6CO (problems with conventional technology) However, with method (1), the silicon nitride obtained in method (1) is coarse particles, so a pulverization process is required. and
There is a drawback that impurities cannot be avoided due to wear of the crusher itself during crushing. Furthermore, since this grinding is done in batches, there is a problem in that it requires manual labor.
Method (2) has a low yield and is not suitable for industrial production. Method (3) is a method in which silica and carbon are crushed and mixed in a batch manner using a ball mill, etc., and then nitrided. Therefore, it is difficult to avoid contamination of impurities during the crushing and mixing process, and there are dust particles in the crushing and mixing process. The production of silicon nitride powder causes serious problems in the working environment, and the resulting silicon nitride powder has various problems such as the unavoidable formation of coarse particles, which is presumed to be caused by the uneven mixing of the raw materials silica and carbon. be. In order to eliminate these coarse particles, the produced silicon nitride must be re-pulverized, but there are limits to pulverization using a ball mill, etc. Therefore, this method cannot produce a sintered body with excellent heat resistance and high purity. There is a fundamental problem in that it is actually extremely difficult to obtain silicon nitride powder suitable for use, that is, with high purity, fineness, and uniform particle size. (Summary of the Invention) As a result of various studies carried out by the present inventors in order to solve these problems, we first developed a composition consisting of a silicon oxide powder and an elemental carbon powder with sufficiently high uniformity and fine particle size. The present invention has been completed based on the discovery that the desired high purity and fine silicon nitride powder can be produced by producing and heating this in a nitrogen-containing compound gas atmosphere. That is, in the present invention, a decomposable silicon compound and a decomposable carbon compound are charged into hot gas containing water vapor to generate a mixed aerosol dispersoid containing silicon oxide and elemental carbon, and the dispersoid is collected. This invention relates to a method for producing silicon nitride, which is characterized in that the carbon-containing composition obtained is heated in a nitrogen-containing compound gas atmosphere in a state where the carbon-containing composition is compressed to a bulk density of 0.2 g/cc or more. (Detailed Disclosure of the Invention) The present invention will be described in detail below. The mixed aerosol dispersoid referred to in the present invention refers to one in which fine particles of silicon oxide and elemental carbon are mixed as a solid substance in a gas. Such silicon oxide and elemental carbon in the present invention are obtained by thermally decomposing, oxidizing, hydrolyzing, etc. a decomposable silicon compound and a decomposable carbon compound (hereinafter simply referred to as a silicon compound or carbon compound) in a hot gas containing water vapor. It can be obtained by tightening. To explain the present invention in more detail, first, an aerosol of simple carbon can be easily obtained by charging a carbon compound into hot gas. On the other hand, an aerosol of silicon oxide can be easily obtained by introducing a silicon compound such as silicon tetrachloride into a hot gas containing water vapor to cause thermal decomposition, oxidation, or hydrolysis. By simultaneously charging a carbon compound and a silicon compound into hot gas containing water vapor in this manner, a mixed aerosol containing silicon oxide and elemental carbon can be easily obtained. The silicon compound that can be used in the present invention is, for example, one represented by the general formula Si _o X _2o+2 (n is an integer from 1 to 4), where X is hydrogen or a halogen atom, and Compounds include SiCl ₄ ,
_HSiCl3 , _SiH4 , _Si2H6 , ( _CH3 ) _{4Si ,}
Examples include (CH ₃ ) ₂ SiCl ₂ , CH ₃ SiCl ₃ and SiF ₄ . Preferably, the carbon compound is a gaseous or liquid phase at room temperature, or a carbon hydrogen or a halogenated hydrocarbon that can easily become a liquid phase by increasing the temperature. Examples of hydrocarbons include petrochemical products such as methanol, ethanol, acetone, n-hexane, benzene, xylene, naphtha, propane, light oil, kerosene,
Petroleum such as heavy oil can be used, as well as petroleum pit, methyl oil, anthracene oil, refining residues such as creosote, _C9 distillate mixtures, and petrochemical residues such as ethylene bottoms. Examples of halogenated hydrocarbons include chloroform, vinyl chloride, and chlorobenzene. It is preferable to use a furnace to obtain the carbon-containing composition of the present invention. The heating device for the furnace is preferably a combustion burner, an energized heating element, etc., but is not particularly limited. Further, the furnace is equipped with a charging nozzle for silicon compounds and carbon compounds, a hot air supply duct, and a mixed aerosol discharge duct, and preferably has a structure surrounded by refractory material. FIG. 1 shows one example. There must be a space inside the furnace with a temperature of at least 600°C. At temperatures above this temperature, elemental carbon is obtained from carbon compounds, and silicon oxide is obtained from silicon compounds through thermal decomposition, oxidation, and hydrolysis reactions in an atmosphere containing water vapor, and the gas and these solids are combined. Generates a mixed aerosol, which is a mixture. The proportions of silicon and carbon in the resulting carbon-containing composition can be adjusted simply by adjusting the amount of silicon compound and carbon compound charged into the hot gas from the nozzle. As a method for obtaining hot gas containing water vapor, water vapor may be injected into hot gas obtained by an electric heating method, a high frequency heating method, or a discharge method.
The method of burning combustible substances such as methane, propane, and butane that produce water vapor by combustion with air is simple in terms of equipment and economical in terms of thermal efficiency. The mixed aerosol thus obtained is guided out of the furnace, and then the solids contained are removed through a bag filter.
Although it is collected using a collection device such as a cyclone, it is desirable to cool it beforehand in order to reduce the heat load on the collection device. Cooling may be achieved by cooling the zone after the reaction or by injecting water into the aerosol. The carbon-containing composition obtained as described above is heated at about 1300°C to 1500°C in an atmosphere of a nitrogen-containing compound gas such as nitrogen gas or ammonia using a high frequency heating furnace, an electric current resistance furnace, etc. The silicon nitride powder that is the object of the present invention can be obtained. In the present invention, in the step of heating in the nitrogen-containing compound atmosphere, it is desirable to heat the carbon-containing composition after the carbon-containing composition has been compressed to a bulk density of 0.2 g/cc or more. If the bulk density of the carbon-containing composition is less than this, whisker-like substances tend to be mixed in the silicon nitride powder produced, and if the bulk density is tightened to 0.2 g/cc or more in advance, fine and grainy This is based on the inventors' experimental findings that silicon nitride powder with a uniform diameter can be obtained. In the present invention, austerity refers to an operation to increase the bulk density of powder, and can be easily carried out by compression, stirring granulation, or the like. The ratio of silicon and carbon in the carbon-containing composition is a formula ratio (gram atom or gram mole ratio).
same as below. )(C/Si) is preferably 2.5 or more. This is because if the formula weight ratio C/Si is less than 2.5, the rate of conversion of silicon oxide to silicon nitride in the carbon-containing composition will sharply decrease. However, if this ratio is made too large beyond this value, there is nothing to be gained and the result is only a mere loss of the carbon compound. If elemental carbon remains in the silicon nitride powder obtained as a result of implementing the present invention, the remaining carbon can be easily combusted by heating the silicon nitride powder to about 500 to 800°C in the presence of oxygen. Can be removed. For example, heating silicon nitride powder in air,
Residual carbon can be easily removed by burning the fuel with excess air and placing it in a hot gas atmosphere containing oxygen. (Operations and Effects of the Invention) As explained in detail above, the present invention first charges a silicon compound and a carbon compound into a hot gas containing water vapor to generate a mixed aerosol dispersoid containing silicon oxide and elemental carbon, This dispersoid is collected to continuously produce a carbon-containing composition, and then this carbon-containing composition is compressed to a bulk density of 0.2 g/cc or more and heated in a nitrogen-containing compound atmosphere. This is a method to obtain the desired nitrogen powder. Therefore, the mixed state of silicon oxide and elemental carbon in the carbon-containing composition obtained by the method of the present invention is much more uniform and fine than that obtained by conventional mechanical mixing, and is also continuous. Therefore, there are no problems such as generation of dust or contamination with impurities. Furthermore, since the silicon nitride powder of the present invention is obtained by heating such a fine carbon-containing composition as described above, the silicon nitride obtained as a result of implementing the present invention does not contain coarse particles and is already fine. Since the powder is uniform in particle size, there are no problems such as contamination with impurities, which has been a problem with conventional methods of pulverizing powders containing coarse particles. Although the details of why fine powder of silicon nitride can be easily obtained in the present invention cannot be clarified exactly, it is likely that the mixed form of silicon oxide and elemental carbon in the obtained carbon-containing composition itself is different from the conventional one. This is presumed to be because it is extremely uniform and fine. Since the silicon nitride powder obtained in the present invention is a powder of high purity, fineness, and uniform particle size, the sintered body obtained by sintering it has chemical and mechanical stability at high temperatures. An extremely excellent product can be obtained. (Examples and Comparative Examples) The present invention will be specifically explained below using Examples and Comparative Examples. It should be noted that all percentages shown in Examples and Comparative Examples are percentages by weight. Example 1 Using the furnace shown in Fig. 1, air was introduced from duct 2.
Propane as a hot air fuel is fed from the combustion burner 3 at a flow rate of 100Nm ³ /h and 3Nm ³ /h, respectively, and heavy oil A as the carbon compound and SiCl ₄ as the silicon compound are mixed in advance at a weight ratio of 1.5:1.0. The mixture was charged into the furnace through nozzle 4 at a flow rate of 25 kg/h. The combustion zone was kept at a temperature of approximately 1200°C. The aerosol produced in the furnace was collected by a bag filter after cooling, yielding 8.9 kg/h of carbon-containing compounds. Carbon-containing composition contains 39.1% silicon dioxide and 60.9% elemental carbon.
was contained, and the bulk density was 0.09 g/cc. To tighten this carbon-containing composition, add 30g of this %
After putting it in a cylindrical container and uniaxially compressing it to a bulk density of 0.38 g/cc, it was heated at 1400℃8 in an _N2 atmosphere using a high frequency furnace.
The mixture was heated for several hours, and further heated to 700°C in air to burn off the remaining elemental carbon, yielding 9.0 g of powder.
The obtained powder was confirmed to be α-Si ₃ N ₄ as a result of X-ray diffraction. As a result of electron microscopic analysis, the average particle size was 0.15 μm, and the particle shape was spherical with a uniform particle size distribution. Examples 2 to 4 The compositions shown in Table 2 were prepared in the same manner as in Example 1, using methane and hydrogen in addition to propane as hot air fuels, and using the silicon compounds and carbon compounds shown in Table 1, respectively. A carbon-containing composition was obtained. After compressing and tightening the obtained carbon-containing composition in exactly the same manner as in Example 1, in addition to N ₂ as a nitrogen-containing compound,

【table】

[Table] The temperatures shown in Table 1 using NH ₃ gas,
A powder was obtained by heating for several hours. The obtained powder is
As a result of line diffraction, it was confirmed that all of them were α-Si ₃ N ₄ . As a result of electron microscopic analysis, the average particle size was as shown in Table 2, and the particle shape was all spherical with a uniform particle size distribution. Comparative Example 1 Example 1 except that 6 g of the carbon-containing composition obtained in Example 1 was heated while the bulk density remained 0.09 g/cc.
In exactly the same manner as above, the mixture was heated at 1400°C for 8 hours in a N ₂ atmosphere, and further heated at 700°C in air to burn off the remaining elemental carbon, yielding 1.7 g of powder. The obtained powder was confirmed to be α-Si ₃ N ₄ as a result of X-ray diffraction. As a result of electron microscopy analysis, the particle shape was whisker-like. Comparative Example 2 39.1 g of Erosil with a specific surface area of 200 m ² /g and 60.9 g of carbon black with a specific surface area of 120 m ² /g were mixed in a ball mill for 24 hours so as to have the same composition as the carbon-containing composition obtained in Example 1. Grind and mix. This 30
g was placed in a cylindrical container and uniaxially compressed to give a bulk density of 0.38 g/cc, and then heated and elemental carbon was removed under exactly the same conditions as in Example 1 to obtain 8.8 g of powder. The obtained powder was confirmed to be α-Si ₃ N ₄ as a result of X-ray diffraction. As a result of electron microscopy analysis, the average particle size was 0.77 μm, and many coarse particles of 1 μm or more were observed, and the particle size had a wide distribution.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing one example of a furnace used in carrying out the present invention. In the drawings, 1...Furnace material, 2...Duct, 3
... combustion burner, 4 ... nozzle, 5 ... duct.

Claims (1)

[Claims] 1 Charge a decomposable silicon compound and a decomposable carbon compound into hot gas containing water vapor to generate a mixed aerosol dispersoid containing silicon oxide and elemental carbon,
The carbon-containing composition obtained by collecting the dispersoids has a bulk density of
1. A method for producing silicon nitride powder, which comprises heating in a nitrogen-containing compound gas atmosphere under tight conditions of 0.2 g/cc or more.