JP3496795B2 - Method for producing silicon nitride powder - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride-based powder which is easily crushable and excellent in sinterability. More specifically, silicon nitride powder suitable for sintering structural parts such as automobile engine parts, cutting tools, molten aluminum jigs, and heat-resistant parts.
The final manufacturing method . [0002] Silicon nitride (Si ₃ N ₄ ) includes α type, β type
The existence of crystalline systems of the type is known. Conventionally, α-type silicon nitride powder has been mainly used as a raw material powder for producing a sintered body. This is because when β-type silicon nitride powder is used as a starting material, columnar particles do not develop and the resulting sintered body has low fracture toughness, so its application to structural materials is limited except for some refractories. I was [0003] In the method of synthesizing silicon nitride powder, the direct nitridation method of metallic silicon (Si) is the most general method currently industrialized. That is, when the nitriding furnace is filled with the raw material and undergoes a process of replacing with nitrogen gas, raising the temperature, nitriding, and cooling, it takes time to raise and lower the temperature, so that it is difficult to significantly reduce the cost. In addition, a method of synthesizing silicon nitride using a continuous furnace is known, but requires a post-process of pulverization and purification. In particular, in order to obtain a high α-type silicon nitride powder, it is necessary to control the use of a catalyst, a reaction rate, an atmosphere gas composition, and a nitriding temperature, and it takes a long time to complete the reaction. Further, in order to pulverize the obtained product to obtain a fine powder suitable for sintering, a long-term pulverization or a post-process such as classification is required. Furthermore, if a hard material to be ground such as silicon nitride is ground for a long period of time, the grinding media is severely worn, and the running cost is increased, and a purification process for removing mixed media and increased surface oxygen is indispensable. Processing is expensive. On the other hand, since β-type silicon nitride is a high-temperature stable type, it is synthesized by reacting at a high temperature or by increasing the reaction rate. For this reason, the reaction time is shorter than α to １ ／ of the α-type silicon nitride powder, and the reaction cost is slightly lower. However, the resulting silicon nitride particles undergo grain growth and sintering, and β-type silicon nitride is less liable to be crushed than α-silicon nitride product, so that obtaining fine powder for sintering is still expensive. I didn't get it. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to obtain an inexpensive β-type silicon nitride powder excellent in sinterability and easy to grind. I do.
Furthermore, it is an object of the present invention to provide a material which can synthesize a silicon nitride material in a short time by using an inexpensive raw material and can easily pulverize the obtained product. SUMMARY OF THE INVENTION The present inventors have made a study to properly mix silica (SiO ₂ ) powder and alkali metal halide or alkaline earth metal halide with metallic silicon in nitrogen. The inventors have found that a silicon nitride-based product with easy pulverizability can be obtained by rapid heating, and have completed the present invention. That is, according to the present invention, a metal silicon or a powder comprising metal silicon and silicon nitride
Quadruple silica content as the silica-containing material relative to 00 parts by weight
The amount unit above, 18 parts by weight or less and an alkali metal halide and / or alkaline earth metal halide of at least one or more of 0.1 or more parts by weight of the molded body of the powder composition was added 10 parts by weight or less of , Specified in nitrogen-containing atmosphere
The method for nitriding by raising the temperature to a temperature
The rate of temperature rise of the green body between 1000 ° C and 1300 ° C
Is rapidly heated to 100 ° C./min or more,
While expanding so that the bulk specific gravity ratio after the conversion becomes 1.5 or more .
Silicon nitride characterized by pulverizing after nitriding
Powder composed of silicon and silicon oxynitride having an average particle size of 5.1 μm or less
Wherein the β ratio of silicon nitride is 50% or more and silicon oxynitride is
8% or more and 37% or less of silicon nitride-based powder
It is a manufacturing method. Hereinafter, the present invention will be described in detail. The room temperature bending strength of the sintered body using the silicon nitride powder containing silicon oxynitride according to the present invention as a raw material is higher than that of the sintered body using the conventional β silicon nitride powder containing no silicon oxynitride. was gotten. Although the details of the mechanism of the increase in bending strength are unknown, as described in JP-A-8-2908, the hardness of the glass phase increases due to the solid solution of silicon oxynitride in the grain boundary glass phase. It is presumed that high strength was developed because of the role of the binder. Next, the production method of the present invention will be described. By adding oxygen to the raw silicon powder and rapidly heating it, the obtained reaction product is in an expanded state as compared with the state of the temporary formed body of the raw powder, and has a low bulk specific gravity. As a result, β-type silicon nitride containing easily crushable silicon oxynitride is generated. It is presumed that this reaction / expansion process is performed in the following order. Si (g) + SiO ₂ (s) → 2SiO (g) Formula 1 At a high temperature, the silica present simultaneously with the metallic silicon in the raw material powder preform is subjected to a reaction shown in formula 1 to form the inside of the preform. By generating a SiO gas and expanding the particles, the constituent particles are ruptured into fine particles, and by changing the position, the entire molded body expands. At the same time, the surface area of the raw material powder exposed to high-temperature nitrogen increases, so that the nitriding reaction is promoted. Further, since the contact area between the particles is reduced by expanding the molded body, sintering of the generated particles is less likely to occur, and the particles can be collected in a pulverizable state. A portion of the silica is consumed by the reaction of Formula 1 and at the same time produces silicon oxynitride by the reaction of Formula 2. Si ₃ N ₄ (S) + SiO ₂ (S) → Si ₂ ON ₂ (S) Formula 2 BEST MODE FOR CARRYING OUT THE INVENTION Silicon nitride has a β ratio of less than 50%, ie, α silicon nitride If the average particle size increases, sintering will not proceed unless the average particle size is reduced to ultrafine powder of 1 μm or less, which is inappropriate. It is necessary that silicon oxynitride be contained at 7% or more and 37% or less. If less than 7%, the sintering density is sufficient,
The strength after sintering is low. On the other hand, if it exceeds 37%, only a sintered material having a low sintering density is obtained, resulting in insufficient strength. If the average particle size exceeds 5.1 μm,
The strength is low. Here, the amount of silicon oxynitride is a value calculated from a ratio of specific peaks obtained by a powder X-ray diffraction method.
That is, the (210) plane of α silicon nitride and the (2) plane of β silicon nitride
10) The peak heights caused by the reflection of the plane and the (200) plane of silicon oxynitride were determined. It refers to the ratio of the peak height due to silicon oxynitride to the total amount. It is not preferable to estimate the amount of silicon oxynitride from the amount of oxygen obtained by oxygen analysis. This is because oxygen in the powder includes silica, surface oxygen and internal oxygen of silicon nitride, oxides of impurities, and attached moisture in addition to silicon oxynitride, and it is difficult to distinguish these. Next, the form of the manufacturing method will be described. The purity and particle size of the metal silicon powder are not particularly limited, but the specific surface area is BET from the viewpoint of reactivity.
It is preferably adjusted to 0.5 to ⁵ m ² / g by the method. Further, the amount of silica contained in the raw material must be 4 or more and 18 parts by weight or less based on 100 parts by weight of metallic silicon. More preferably, it is in the range of 6 to 16 parts by weight. If the amount of silica exceeds 18 parts by weight, the proportion of the fibrous product in the reaction product increases, and part of the product remains even after pulverization, resulting in a reduction in the strength of the sintered body. On the other hand, if the amount is less than 4 parts by weight, the change in the bulk specific gravity ratio during the reaction is poor, the reaction product is obtained in a tightly bound state, and the pulverizability is poor. Regarding the method of incorporating silica in the raw material powder, a method of heating silicon in the air to oxidize the surface is preferable. However, quartz glass powder, aerosil or quartzite, porcelain stone, rockite, , Etc., may be added and mixed. Further, even if the above-mentioned methods are used alone or in combination, the proportion of the total silica content may be within the range of 4 to 18 parts by weight with respect to 100 parts by weight of metallic silicon. The siliceous mineral is pulverized and used until the specific surface area becomes 4 m ² / g or more. When the specific surface area is 4 m ² / g or less, the reaction represented by the formula 1 proceeds only slowly, and the expansion of the molded body is small. Further, the purity of the siliceous mineral is 80% for the silica component.
It is preferable that it is above. If the silica component is 80% or less, the impurity component may form a liquid phase at the time of nitriding, liquify silicon and hinder the nitriding reaction, and may cause a reduction in the strength of the sintered body. . By adding one or more kinds of alkali metal and alkaline earth metal halides to the raw material during nitriding, the generation of SiO gas and the nitriding reaction according to the present invention can be promoted. Addition amount of metal silicon 1
It is necessary that the content be 0.1 to 10 parts by weight based on 00 parts by weight. If the added amount is more than 10 parts by weight, the reaction product after nitriding remains in a lump or powder, and the strength of the sintered body using the same is reduced. 0.
If the amount is less than 1 part by weight, the effect of promoting the reaction is poor. In the calculation of the silica content of the metal silicon or the alkali metal and the alkali metal halide, when the metal silicon surface is oxidized and used, the silica content formed on the surface by oxygen analysis or the like is used. Is calculated, and the net amount of metallic silicon needs to be grasped. The silicon nitride added together with the metallic silicon is
This is for diluting the heat of reaction during the reaction, and the particle size is not particularly limited. The crystal system may be either α-type or β-type. The addition amount is 0 to 60 wt. % May be appropriately determined. Regarding the density of the raw material powder after the preliminary molding, the bulk density of the molded body is preferably 1.3 g / cm ³ or less. When the bulk density of the molded body exceeds 1.3 g / cm ³ ,
Sintering of the metallic silicon during nitriding occurs, and there is a possibility that unnitrided metallic silicon remains. If it is difficult to mold only with the raw material powder, a molding binder may be added. Next, the raw material powder is thrown into a reaction furnace for synthesis. The reaction atmosphere is a nitrogen gas atmosphere or a non-oxidizing nitrogen-containing gas obtained by mixing a gas such as argon and helium with nitrogen and / or ammonia gas. The atmosphere needs to be. The reactor is not particularly limited as long as it can rapidly heat the raw materials, but a batch-type or continuous-type reactor can be used. What is important here is that the raw material powder is preheated from room temperature or a predetermined temperature lower than 1000 ° C.
After that, it is moved to a high-temperature atmosphere of 1300 ° C. or more and rapidly heated. The rapid heating causes the reaction while the raw material temporary molded body expands, and at this time, the bulk specific gravity of the raw material temporary molded body is 1.
It is necessary to heat so that it becomes 8 times or more, preferably 2 times or more. The heating rate is 100
If the temperature between 0 ° C. and 1300 ° C. is 100 ° C./min or more, the expansion is performed smoothly. Therefore, the set temperature of the furnace on the high temperature side is 1300 ° C. or higher, preferably 1400 to 1600 ° C. If the rate of temperature rise is lower than 100 ° C./min, the generation of SiO gas is performed gradually, and the expansion becomes insufficient. There is no particular upper limit on the heating rate, but in order to perform ultra-rapid heating such as 1000 ° C./min, the electric energy to be charged into the furnace is increased, or the amount of raw material charged is extremely reduced. There are financial problems. Although a substantial portion of the raw material powder after the rapid heating has already reacted, the raw material powder is kept at a temperature of 1400 ° C. to 1600 ° C. for several hours to complete the reaction more completely. EXAMPLE A commercially available high-purity metallic silicon powder (average particle diameter: 25 μm) was pulverized by a ball mill using silicon nitride balls to an average particle diameter of 21 μm, sieved with a 210 μm sieve, and then dried in air. Oxidation was performed by heating at 1000 ° C. for 2 hours to produce oxidized silica. The silica was adjusted so as to be 10.4 parts by weight with respect to 100 parts by weight of metallic silicon in the mixed powder. Next, silicon nitride powder (SN-
7) Magnesium fluoride was added in an amount of 1.0 part by weight based on 50 parts by weight and further 100 parts by weight of the metal silicon powder, and dry-mixed with a ball mill to obtain a mixed powder. After adding and kneading 30 g of a 4% PVA binder aqueous solution to 100 g of the mixed powder, a mold is formed.
1 cube of 50mm in width, 50mm in width and 50mm in thickness
Five were molded. The bulk density of the dried molded article is 1.0 g / cm ³
Met. The molded body was put in a nitriding furnace adjusted to a temperature of 1500 ° C. in a nitrogen atmosphere from a room temperature, and kept for 4 hours to be nitrided. The rate of temperature rise of the compact at this time was measured and found to be 280 ° C./min. The compact after the nitriding reaction was recovered, cooled to 900 ° C. in a nitrogen atmosphere, and then allowed to cool in air. The cooled molded body was expanded as compared to before the injection, and was weak enough to collapse when strongly picked with a finger. This molded body is crushed and sieved in a silicon nitride mortar,
The specific surface area was measured using a sample having a particle size range of 0.1 to 1 mm, and the oxygen content and the nitrogen content were measured using a sample having a particle size range of 0.1 mm or less. Further, 100 g of the powder after the coarse pulverization was pulverized by a vibration mill filled with 15φ silicon nitride balls, and then the specific surface area and the particle size distribution were measured. The specific surface area was measured by a flow-through one-point method using a helium-nitrogen mixed gas as a standard gas with a cantasorb manufactured by Yuasa Ionics. The particle size distribution was measured by a laser diffraction light scattering method using a micro track manufactured by Nikkiso Co., Ltd. The amount of silicon oxynitride is a value determined by the X-ray diffraction method. The sinterability of the silicon nitride powder obtained by the vibration mill pulverization was evaluated as follows. 5 parts by weight of Y ₂ O ₃ powder and 3 parts by weight of Al ₂ O ₃ powder are mixed with 100 parts by weight of the silicon nitride powder, and 5% of the total amount of the powder is mixed with an organic binder and water. Then, a slurry having a powder content of 30% by weight was prepared. It was granulated and dried with a spray drier, press-molded in a mold, and then CIP-molded at 3 t / cm ² . This was sintered at 1750 ° C. for 10 hours, and the obtained sintered body was measured for the four-point bending strength at room temperature in accordance with JIS-R1601. In Examples 2 to 6 and Comparative Examples 1 to 9,
The procedure was performed in the same manner as in Example 1 except that the reaction was performed under the starting materials and reaction conditions shown in Table 1. Table 2 shows the contents of the siliceous raw materials shown in Table 1. [Table 1] [Table 2] In Reference Example 1, β-type silicon nitride powder (SN-B manufactured by Denki Kagaku Kogyo Co., Ltd.) was pulverized with a vibration mill to obtain a specific surface area of 4%.
^2. Adjusted to m ² / g, purity 99%, specific surface area 3.
4 m ² / g of silicon oxynitride powder was mixed to obtain a silicon nitride powder. This mixed silicon nitride powder was mixed with an auxiliary in the same manner as in Example 1 and sintered. Comparative Example 10 was conducted in the same manner as in Example 1 except that commercially available β-type silicon nitride powder (SN-B manufactured by Denki Kagaku Kogyo Co., Ltd.) was pulverized with a vibration mill and the specific surface area was adjusted to 4 m ² / g.
A sintered body was fabricated and evaluated under the same conditions. Table 3 shows the properties of the powders obtained in each of the examples and comparative examples and the physical properties of the sintered bodies using the powders. [Table 3] As is apparent from Table 3, the β silicon nitride-based powder containing silicon oxynitride according to the present invention has excellent sinterability, and the silicon nitride-based powder obtained by the production method of the present invention has a short duration. It can be understood that the composition can be synthesized, is easily crushable, and has excellent sinterability. According to the present invention, excellent sinterability and easy pulverization are obtained.
Inexpensive β-type silicon nitride powder
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Continuation of the front page (56) References JP-A-62-5599 (JP, A) JP-A-2-248309 (JP, A) JP-A-63-69759 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) C01B 21/068 C04B 35/591

Claims (1)

(57) [Claim 1] In a powder comprising metallic silicon or metallic silicon and silicon nitride, a silica content is used as a siliceous raw material in an amount of 4 to 18 parts by weight based on 100 parts by weight of metallic silicon. below and an alkali metal halide and / or alkaline earth metal halide, at least one or more of 0.1 or more parts by weight of the powder composition was added 10 parts by weight or less
The molded body of
In the method, the compact of the powder composition is heated from 1000 ° C. to 1300 ° C.
The temperature is rapidly increased to 100 ° C / min or more during
As the bulk density ratio after nitriding before / reaction nitride is one. 5 or more
Pulverizing after nitriding while expanding to an average particle size of 5.1 μm or less, comprising silicon nitride and silicon oxynitride.
The powder below, wherein the β ratio of silicon nitride is 50% or more,
Silicon nitride containing 8% or more and 37% or less of silicon nitride
Method for producing porous powder.