JP3367567B2 - Method for producing easily crushable and high α-type silicon nitride - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inexpensive method for producing easily crushable and high α-type silicon nitride. [0002] In recent years, from the viewpoint of energy saving and high energy efficiency, silicon nitride sintered bodies have been studied as engine parts of automobiles such as turbo rotors, valves and swirl chambers and various industrial machine parts. Since these are used under severe conditions, the conditions required for the silicon nitride powder are also strict as follows. (1) The α phase is mainly used. (2) Submicron fine particles. (3) High purity. (4) Inexpensive. Various methods for producing silicon nitride have been proposed so far, and these are roughly classified into the following four methods. (A) A direct nitriding method in which metallic silicon is nitrided using a reaction gas such as nitrogen or ammonia. (B) A reduction nitriding method in which silica is nitrided using a reducing agent such as carbon and a reaction gas. (C) An imide thermal decomposition method for thermally decomposing silicon diimide generated from silicon tetrachloride. (D) A gas phase method in which a gas such as monosilane or silicon tetrachloride is reacted with a gas such as ammonia by heating such as laser or plasma. [0004] Of these, the most popular method at present is the direct nitriding method. In the direct nitriding method, the produced ingot is pulverized into silicon nitride powder, and the powder characteristics largely depend on that of the ingot. An ingot is an aggregate of silicon nitride particles synthesized from metal silicon powder. The main raw material metal silicon powder is
Usually, it is formed into a compact or filled into a reactor as a powder to improve handling properties.Since the nitriding reaction of metal silicon is a large exothermic reaction, the silicon nitride particles generated are relatively strong aggregates. It becomes a body, that is, an ingot. [0005] There are wet methods and dry methods for grinding ingots, but they have advantages and disadvantages. In wet grinding, post-processes such as purification, filtration, drying, and disintegration of the pulverized material are required. In addition, grinding of the hard material to be pulverized, such as silicon nitride, is performed for a long time. As a result, a purification step for removing mixed media and increased surface oxygen becomes indispensable.
Furthermore, this purification step is expensive because it is an acid treatment.
In contrast, dry grinding does not have such a problem,
The specific surface area does not increase so much, and there are problems such as mixing of abrasion powder of the medium and a large increase in surface oxygen. Unless classified to reduce the yield, coarse particles of several tens μm or more remain. The coarse particles reduce the strength and toughness of the silicon nitride sintered body. To cope with such a problem, there have been many proposals for controlling a nitriding reaction to produce a silicon nitride powder having a high α ratio and a high specific surface area. For example, control of the reaction gas partial pressure and reaction temperature (for example, JP-A-48-102100, JP-A-49-94600, JP-A-52-117898, etc.), control of the rate of temperature rise (for example, JP-B-4-54607, JP-A-54-24300, JP-A-63-170203, etc.), addition of a nitriding catalyst (for example, JP-A-2-248309), Control of porosity (see, for example,
No. 8109), but it was not sufficient. As described above, according to the conventional method, a relatively inexpensive silicon nitride that satisfies the above conditions (1) to (4) and that can be used industrially can be obtained. It is difficult to manufacture and the emergence of new technologies has been awaited. The present inventors have conducted various studies to respond to such a demand, and as a result, using a mixture of a specific silicon nitride powder and a metal silicon powder as a nitriding raw material and nitriding while strictly controlling the reaction temperature, They have found good things and completed the present invention. [0008] That is, the present invention is based on 100 parts by weight of metal silicon powder and 30 to 100 parts by weight of silicon nitride powder having an α conversion of 85% or more and a specific surface area of 5 to ²⁰ m ² / g. In an atmosphere containing nitrogen and / or ammonia and having a total of an oxygen content and a water content of 0.2% by weight or less with respect to the metal silicon powder, the temperature of the internal It is characterized in that the temperature is raised to a temperature of 50 ° C. or less while the temperature is raised to nitridize, and that the metal silicon powder is nitrided by 30% or more by 1200 ° C. and 90% or more by 1350 ° C.
This is a method for producing high α-type silicon nitride. Now, the present invention will be described in further detail. There are two main features of the present invention. One is the condition of the aggregate to be mixed with the metallic silicon powder, and the other is the reaction condition. The conditions of the aggregate are the α ratio, the specific surface area and the compounding amount of the silicon nitride powder, and the reaction conditions are the oxygen amount and the water amount of the nitriding atmosphere, the temperature difference between the internal temperature of the nitriding raw material and the ambient temperature, and the temperature 1200. Up to ℃ and temperature 1350 ℃
Is the nitriding rate up to. In the present invention, these technical items interact with each other to achieve the object of the present invention. It has been difficult to produce a silicon nitride powder having a sufficiently high α ratio and a high specific surface area by a conventionally proposed method of controlling a nitridation reaction because each control is performed under other effective conditions. Due to the lack of rigor. [0010] Since the nitridation of metallic silicon is a reaction that generates heat of as much as 170 kcal per mole of silicon nitride, unless the reaction is controlled, this reaction heat leads to a runaway reaction in which nitridation proceeds at an accelerated rate. In this case, the molten metal silicon remains unnitrided. In the present invention, an aggregate is used to easily control the reaction. The aggregate is a silicon nitride powder having an α conversion of 85% or more and a specific surface area of 5 to ²⁰ m ² / g. Since the aggregate hardly changes while the metallic silicon undergoes the nitriding reaction, the aggregate must also have a high α and a high specific surface area in order to obtain a silicon nitride having a high α ratio and a high specific surface area. The preferred alpha ratio of the aggregate is 90% or more, and the specific surface area is 6 to 18 m ² / g. Regarding the amount of aggregate used, the larger the amount, the easier the control of the reaction and the better the crushability of the synthesized ingot, but if the amount is too large, the productivity is reduced. 30 to 100 parts by weight
Parts by weight, preferably 40 to 90 parts by weight. The metal silicon powder may be an ordinary industrial metal silicon powder, but a high-purity product is used to produce a high-purity silicon nitride powder. If the particle size is too fine, the pulverizing process will be burdened, and if it is too coarse, the nitriding reaction will be difficult, so 0.2 to 5 m ² / g, especially 0.5 to 4
m ² / g is preferred. The method of mixing the metal silicon powder and the aggregate is not particularly limited as long as both can be uniformly mixed, and a method in which pulverization and mixing can be performed simultaneously can also be adopted. In addition, it is necessary to give due consideration to the contamination of impurities, especially impurities due to abrasion of the media, and the oxidation of the metal silicon powder. In the case of manufacturing high-purity products in particular, use media made of silicon nitride in a non-oxidizing atmosphere. Do it below. The nitriding raw material composed of a mixture of the metal silicon powder and the aggregate may be a compact or a powder. Next, in the present invention, the control of the amount of oxygen and the amount of water in the nitriding reaction atmosphere and the temperature control are strictly performed. The temperatures controlled by the present invention are three points: the difference between the internal temperature of the nitriding raw material and the ambient temperature, and the reaction temperature for achieving a nitriding rate of 30% and a nitriding rate of 90%. As described above, the nitridation of metallic silicon is a highly exothermic reaction. Therefore, when the reaction occurs, the inside of the nitriding raw material is stored and the temperature rises. This increase in internal temperature depends on many factors such as the particle size and purity of the metallic silicon powder, the filling bulk density, the filling thickness, the filling amount, the amount of aggregate, the reaction rate, and the atmosphere gas composition. Since only some were controlled, the internal temperature rise could not be controlled sufficiently, and the reaction proceeded rapidly and the internal temperature further increased. As a result, the specific surface area, α-rate, and grindability of the inside and the surface of the produced ingot differ greatly, and even if such an ingot is pulverized, silicon nitride powder with high α and high specific surface area is produced. It could not be produced and coarse particles remained. In the present invention, during the temperature rise, the difference between the internal temperature of the nitriding raw material and the ambient temperature must be kept at 50 ° C. or less. The reaction is controlled as the temperature difference is reduced, and an ingot with less variation can be manufactured. In order to reduce this temperature difference, it is necessary to take measures such as a very slow reaction rate, a very thin nitriding raw material, and a very small packing density. The temperature difference is preferably 4 to 50 ° C., particularly preferably 6 to 40 ° C., because productivity is significantly reduced. The internal temperature of the nitriding material can be easily measured by inserting a thermocouple, which can be represented by the temperature near the center of the hottest maximum thickness. Since the above-mentioned temperature difference is caused by the heat generated by the nitriding reaction, the control can be performed by controlling the nitriding reaction. That is, in order to reduce the temperature difference, the heating rate is decreased or the heating is stopped, or an inert gas such as hydrogen gas, argon, helium or the like is added to the atmosphere, and a reaction gas containing nitrogen and / or ammonia is added. Or by lowering the partial pressure. Conversely, to increase the temperature difference, the reverse operation may be performed. In the present invention, in order to produce silicon nitride having a high α-conversion rate, it is necessary to further increase the temperature while maintaining the above temperature difference. It has a nitridation rate of 30% and a nitridation rate of 90.
% Reaction temperature. If the metal silicon is nitrided at a low temperature, an α phase is generated, and if it is nitrided at a high temperature, a β phase is easily generated. In the present invention, the atmospheric temperature is 1200 ° C.
The nitriding rate must be 30% or more by 1350 ° C., and the nitriding rate must be 90% or more by 1350 ° C. If such conditions have not been reached, it is necessary to allow the nitridation reaction to proceed without raising the temperature to reach the nitriding rate. Since the product up to a temperature of 1350 ° C. mainly comprises an α phase, even if the temperature is raised to 1400 to 1450 ° C. to terminate nitriding, the product hardly changes to a β phase. Therefore, while maintaining the difference between the internal temperature of the nitriding raw material and the ambient temperature at 50 ° C. or less as in the present invention, 90 ° to 1350 ° C.
% Or more, it is possible to produce silicon nitride having a pregelatinization ratio of 90% or more. The ratio of α-formation in the present invention refers to the volume ratio of α-phase in the produced silicon nitride, and since the true specific gravity of α-phase and β-phase has almost no difference, No problem. In the present invention, the measurement of alpha ratio, the powder X-ray method is adopted by the diffraction ray intensity ratio in the diffraction, alpha phase (102) plane diffraction intensity Ia ₁₀₂ and the (21
0) plane is represented by the sum of the diffraction intensity Ia ₂₁₀ of, while the β-phase (101) plane and (210) diffraction line of surface intensity Ib ₁₀₁ and I
b Represented by the sum of ₂₁₀ and calculated by the following equation. α-conversion rate _{(%) = (Ia 102 +} Ia 210) / (Ia 102 + Ia 210 +
Ib ₁₀₁ + Ib ₂₁₀ ) × 100 The nitridation rate in the present invention is a rate at which metal silicon in a nitriding raw material is consumed by nitriding.
It measures the amount of nitrogen and / or ammonia reactant gas consumed by nitriding, which is 3Si + 2N ₂ → S
Assuming that 1.5 times as much metal silicon has been consumed according to i ₃ N ₄ , the method of converting it to weight is simple and accurate. Specifically, if the volume of the reaction system is known, the reaction system can be obtained by sealing the reaction system and calculating the change in the furnace pressure and the change in the reaction gas concentration in the furnace. The difference between the amount and the amount of reaction gas discharged out of the system can be calculated as the amount of consumed reaction gas. In the present invention, the nitriding atmosphere is nitrogen and / or
Or an atmosphere containing a reaction gas composed of ammonia, but in order to make the above temperature control effective for the reaction control, the total amount of oxygen and water in the atmosphere is determined in the nitriding raw material before the nitriding reaction. Must be 0.2% by weight or less based on the metallic silicon content. In the prior art, there have been proposals to reduce the amount of oxygen and the amount of moisture in the atmosphere (for example, see Japanese Patent Publication No. 2-18).
No. 284, Japanese Patent Publication No. 5-221617, etc.) These are intended to produce low-oxygen silicon nitride, and produce silicon nitride having high α, high specific surface area and easy pulverization as in the present invention. The purpose was different from the above, and the mixing of the aggregate and the control of the reaction temperature were not performed. Oxidation with oxygen or water vapor is a thermodynamic reaction that occurs preferentially to the nitridation reaction of metallic silicon. Therefore, in the related art, it was necessary to suppress the amount of oxygen and the amount of moisture in the atmosphere. In the present invention, in order to suppress the formation of an oxide layer on the surface of the ingot, which causes the reaction inside the ingot to be extremely uneven, the amount of oxygen and the amount of water in the atmosphere are compared with the amount of metallic silicon before the nitriding reaction. Need to control in relation. The sum of the amount of oxygen and the amount of moisture in the atmosphere in the present invention is the total of those existing in the atmosphere in the reactor. For example, in a reactor having a volume of 1 m ³ , 100 ppm of oxygen and 1000 ppm of moisture are reduced. If measured, oxygen
14 g and a water content of 0.80 g.
g will be present. In this case, assuming that the metal silicon content in the nitriding raw material was 1 kg, 0.94 g was 0.1 kg.
094% by weight. This ratio is preferably as small as possible,
In the present invention, 0.2% by weight or less, preferably 0.1% by weight.
If it is less than the weight%, the aggregate condition and the reaction condition work effectively. In the present invention, the composition ratio of the amount of oxygen and the amount of moisture in the atmosphere does not matter so much, and the total amount thereof is important. The method of controlling the amount of oxygen and the amount of water in the atmosphere is as follows.
For example, the reaction can be performed by supplying a low-oxygen, low-moisture reaction gas into the furnace.However, since furnace materials used in the reaction furnace often contain a large amount of oxygen and water, In some cases, the nitriding reaction must be performed while deoxidizing and / or dehydrating the furnace atmosphere. In such a case, measures such as taking out the reaction gas in the furnace once with a pump or a compressor outside the system, treating it with a deoxidizer or a dehumidifier, and returning it to the furnace again are taken. The pulverization of the silicon nitride produced according to the present invention can be carried out by either a wet method or a dry method, but the dry method is well suited for achieving the object of the present invention. In this case, from the viewpoint of reducing oxygen and increasing the specific surface area, under a non-oxidizing atmosphere such as nitrogen, ammonia, and argon, a non-oxidizing pulverization auxiliary agent that can prevent aggregation of the silicon nitride powder, for example, triethylamine , N-butylamine, acetonitrile, methyl ethyl ketone and the like are preferably added and pulverized, and the amount used is about 0.2 to 3% by weight. As the pulverizing medium, those mainly composed of silicon nitride are preferable. EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. Examples 1 to 5 Comparative Examples 1 to 7 100 parts by weight of a commercially available high-purity metal silicon powder were mixed with an amount of silicon nitride powder as an aggregate as shown in Table 1, and mixed with a ball mill to obtain a nitriding raw material. 1.5kg of this is 16x
After being formed into a molded body of 16 × 6 cm, the molded body was placed in a reaction furnace, and a thermocouple placed in a protective tube made of alumina was attached so that a temperature measuring part was located at a thickness of 3 cm at the center. After evacuation, the temperature was increased after replacing with nitrogen gas. The temperature rising rate at 1100 ° C. or higher was 5 ° C./hr, and the reaction rate was controlled by controlling the difference between the central temperature of the nitrided raw material compact and the ambient temperature. The temperature difference occurred between 1130 and 1150 ° C in each case.
In this case, the reaction control is such that when the temperature difference reaches 10 ° C., argon gas is supplied, and when the temperature difference reaches 5 ° C., the supply is stopped and nitrogen gas is supplied at a rate of 2 liter / minute. Was done by doing. The nitriding ratio was calculated in accordance with the above, with an integrating flow meter installed at the inlet and outlet of the reaction gas and the difference between the two as the amount of consumed reaction gas. The nitriding rates at the temperatures of 1200 ° C. and 1350 ° C. were adjusted as shown in Table 1 by maintaining the ambient temperature at 1190 ° C. and 1330 ° C., respectively, and proceeding with nitriding. The amount of oxygen and the amount of moisture in the atmosphere relative to the metal silicon powder in the nitriding raw material before the nitriding reaction were calculated according to the following formula by installing an oxygen meter and a moisture meter at the outlet of the reaction gas, measuring the oxygen concentration and the moisture concentration. . Table 1 shows the results.
The amount of oxygen and the amount of moisture in the atmosphere were adjusted by the rate of temperature rise and the supply amount of the reaction gas during the temperature rise. Oxygen content (% by weight) = a × V _t ÷ 22.4 × 32 ÷ Me ×
100 Water content (% by weight) = b × V _t ÷ 22.4 × 18 ÷ Me ×
100 a: Oxygen concentration (vol%) in the atmosphere b: water concentration in the atmosphere (vol%) V _t: the reaction gas quantity in the furnace at a temperature (l) Me: metal silicon in before nitriding reaction of the starting materials nitride Amount (g) After the temperature was raised to 1420 ° C. to complete nitriding, the mixture was allowed to cool to room temperature while flowing nitrogen gas, and the synthesized ingot was taken out. Then, using a silicon nitride ball, 1.5% by weight of triethylamine was added as a grinding aid, and ball milling was performed in a nitrogen atmosphere for 6 hours to produce a silicon nitride powder. The α-formation ratio, specific surface area and residual amount of coarse particles of the obtained silicon nitride powder were measured as follows. Table 2 shows the results. (1) α conversion: powder X using CuKα ray
It was measured by the line diffraction method and calculated by the above equation. (2) Specific surface area: It was measured with a cantasorb manufactured by Yuasa Ionics Co., using a helium-nitrogen mixed gas as a standard gas by a flow-type one-point method. (3) Residue of coarse particles: 200 g of silicon nitride powder was mixed with 500 ml of water, ultrasonically dispersed for 30 minutes, and the operation of sieving water with an aperture of 25 μm was repeated three times. The dry weight was calculated as a ratio to the original weight. Further, the sinterability of the obtained silicon nitride powder was evaluated as follows. 6% by weight in silicon nitride powder
Of Y ₂ O ₃ powder and 3% by weight of Al ₂ O ₃ powder,
A slurry aqueous solution of 50% by weight of the mixed powder was prepared using an organic binder, granulated and dried by a spray drier, and then press-molded into a 2.7 ton / cm ² CIP.
Molded. This was sintered at a temperature of 1850 ° C. for 6 hours, and the obtained sintered body was measured for a four-point bending strength at room temperature in accordance with JIS R1601. Table 2 shows the results. [Table 1] (Note) The amount of aggregate is a ratio to 100 parts by weight of metallic silicon. (Note) The maximum temperature difference is the maximum value of (nitriding raw material internal temperature−ambient temperature) at a temperature of 1100 to 1420 ° C. [Table 2] Tables 1 and 2 show the following. The silicon nitride produced according to the present invention (Examples 1 to 5)
A silicon nitride powder having a sufficiently high specific surface area and a high α ratio can be produced by ordinary dry pulverization, almost no residual coarse particles are observed, and the strength of the sintered body is 100%.
It is beyond the kgf / mm ^2. On the other hand, in Comparative Example 1 using an aggregate having an inappropriate α-rate, the α-rate was small, and in Comparative Example 2 using an aggregate having an inappropriate specific surface area, the specific surface area was small. On the other hand, in Comparative Example 3, in which the amount of aggregate was small, the temperature was 120.
Comparative Example 4, in which the nitriding ratio at 0 ° C. is not appropriate, temperature 135
In Comparative Example 5 in which the nitridation rate at 0 ° C. is not appropriate, Comparative Example 6 in which the difference between the internal temperature of the nitriding raw material and the ambient temperature is large, and Comparative Example 7 in which the total amount of oxygen and moisture in the atmosphere is not appropriate, Both the ratio and the specific surface area were reduced, and coarse particles were also observed to remain. The strength of the silicon nitride sintered body manufactured using such a silicon nitride powder was significantly lower than that of the example. According to the present invention, a relatively inexpensive metal silicon raw material is used, and a complicated post-treatment such as a special pulverizing step or a wet refining treatment is not required.・ Easily pulverizable silicon nitride having a high specific surface area can be easily produced.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C01B 21/068 C04B 35/591 C04B 35/64

Claims (1)

(1) A nitriding raw material comprising 100 parts by weight of metal silicon powder and 30 to 100 parts by weight of silicon nitride powder having an α conversion of 85% or more and a specific surface area of 5 to ²⁰ m ² / g. In an atmosphere containing nitrogen and / or ammonia and the total amount of oxygen and water is 0.2% by weight or less with respect to the metal silicon powder, the difference between the internal temperature of the nitriding raw material and the ambient temperature is 50 ° C. The temperature was raised while the temperature was controlled as follows, and nitriding was performed.
A method for producing easily pulverizable and high α-type silicon nitride, comprising nitriding at least 30% of the metal silicon powder by 200 ° C. and at least 90% by 1350 ° C.