JPH0791043B2 - Method for manufacturing silicon nitride - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing silicon nitride by a silica reduction method.

2. Description of the Related Art As a method for economically producing high-purity silicon nitride, a silica reduction method in which a mixture of silica and carbon is heated in a nitrogen atmosphere is known. For example, in Japanese Patent Publication No. 54-23917, a mixed powder of silica powder, carbon powder, silicon nitride powder, silicon carbide powder, and at least one of silicon oxynitride-based powder containing nitrogen is contained. A method for producing a silicon nitride powder has been proposed in which heat treatment is performed in an atmosphere to cause a reduction nitriding reaction, and then unreacted carbon is oxidized and removed by a decarburizing step.

Problems to be Solved by the Invention In the conventional method for producing a silicon nitride foundation by the silica reduction method, it is unavoidable that the produced silicon nitride powder contains a relatively large amount of carbon. However, as is well known, the carbon content in the silicon nitride powder hinders the densification of the sintered body when the powder is sintered, so the carbon content needs to be reduced as much as possible.

Further, the work of mixing carbon as one of the raw material powders is messy and is not preferable in terms of product yield and synthesis furnace efficiency, so a raw material system containing no carbon powder is desired. Furthermore, the conventional method requires decarburization of the synthetic powder, but if it can be eliminated, it is desirable from the viewpoint of production cost.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional state of the art, and an object of the present invention is to provide a method for significantly reducing the carbon content in silicon nitride and efficiently producing silicon nitride.

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention is characterized in that in a method for producing silicon nitride by a silica reduction method, silica is heated in a mixed gas of NH ₃ and a hydrocarbon gas (CmHn). The gist is the method of manufacturing silicon nitride.

Means for Solving the Problems In the method for producing silicon nitride according to the present invention, silica is heated in a mixed gas of NH ₃ and hydrocarbon gas (CmHn) (with or without addition of seed powder). . The hydrocarbon gas of the present invention includes a gas that becomes a hydrocarbon in the synthesis atmosphere. Include N ₂ or other inert gas in the gas mixture if necessary. Thereby, the amount of carbon contained in the silicon nitride powder is significantly reduced.

The inventors of the present invention have investigated the cause of carbon content in the silicon nitride powder in the silica reduction method, and have been able to clarify the cause different from what has been considered in the past. In the past, the raw material carbon was blended in excess of the amount required to reduce silica, and therefore, after synthesis, excess carbon was heated in the air to be oxidized and removed. It was thought that the cause of the inclusion was that the raw material carbon remained undecarburized. However, as a result of the research conducted by the present inventors, it was discovered that the contained carbon was generated during the synthesis and was present inside the silicon nitride powder. Therefore, it was clarified that it is difficult to effectively remove the contained carbon by externally oxidizing the silicon nitride powder.

According to the research results of the present inventors, the cause of carbon contamination in the silicon nitride powder is as follows.

The silica reduction reaction proceeds by the following reaction. In the formula, S represents a solid and G represents a gas.

SiO ₂ (S) + C (S) → SiO (G) + CO (G) SiO (G) + C (S) → Si (G) + CO (G) ₃ Si (G) + 2N ₂ (G) → Si ₃ N ₄ ( S) When the partial pressure of N ₂ in the reaction system is low, or when the CO concentration is locally high, the reverse reactions of the above-mentioned first and second equations occur and C (S) is Carbon precipitates and carbon is incorporated into the silicon nitride powder during molding.

Further, in the thermodynamic equilibrium relationship of the silica reduction reaction, the stable relationship of the condensed phase, which is related by the N ₂ partial pressure and the CO partial pressure, is shown in FIG. This shows an example at 1427 ° C. and shows the stability relationship of the condensed phase of the Si—C—N—O system. In such systems, Si ₃ N ₄ , SiC, Si
O ₂ and C is present but, C also in the generation region the Si ₃ N ₄ may be present in equilibrium with Si ₃ N _4. Thus, C in the synthesis process the Si ₃ N ₄ is taken into the Si ₃ N ₄ powder.

Due to the reasons described above, C generated in the reaction process is Si
_It is taken inside the ₃ N ₄ powder, and as a result, carbon is contained in the silicon nitride powder.

Therefore, in the present invention, silicon nitride is produced by the following reaction without using C.

3SiO ₂ (S) + 4NH ₃ (G) → Si ₃ N ₄ (S) + 6H ₂ O (G) Although the silicon nitride powder synthesized as described above does not contain carbon, the reaction of the above formula Si ₃ N ₄ is oxidized by the H ₂ O generated in. To prevent oxidation the Si ₃ N ₄ with in H ₂ O may be lowered partial pressure of H ₂ O atmospheric gas. H
As a method to lower the partial pressure of ₂ O, to increase the flow rate of the NH ₃ gas, increasing the flow rate of a mixture of inert gas such as N ₂ to NH ₃ gas, or mixed CmHn gas to NH ₃ gas It is possible to do it. When CmHn is mixed, H ₂ O can be removed by the reaction of the following formula.

mH ₂ O + CmHn → mCO + (m + n / 2) H _{2 When the} amount of hydrocarbon gas is large, the carbon produced when the hydrocarbon is thermally decomposed also acts as a reducing agent for silica.

JP-B-57-44602 discloses that silica is a hydrocarbon gas,
A method of reducing and nitriding in a mixed gas of ammonia gas and hydrogen gas is disclosed. In this method, hydrogen and nitrogen in the active phase generated during the thermal decomposition process of NH ₃ are used to cause a reduction nitriding action. According to the experimental results of the present inventors, adding hydrogen to the original gas as in JP-B-57-44602 only suppresses the decomposition of NH ₃ and delays the Si ₃ N ₄ synthesis reaction. However, the effect of adding hydrogen could not be found.

Example SiO ₂ (silica) powder having an average particle size of 20 μm and Si ₃ N ₄ (silicon nitride) powder having an average particle size of 0.1 μm were mixed in a ratio shown in Table 1, and some of them were mixed with a catalyst. Add
The reduction nitriding treatment was performed under the conditions shown in Table 1.

Examination of the silicon nitride powder thus obtained revealed that the content of carbon was extremely low.

In addition, experiments were performed on four comparative examples shown in Table 1. In these comparative examples, reductive nitriding and decarburization were performed only in N ₂ (nitrogen) not containing NH ₃ or in an atmosphere of a mixed gas of H ₂ , NH ₃ , and C ₃ H ₈ .

In addition, the characteristics of the sintered bodies were compared using the generated powders of Example 1 and Comparative Example 7. 5 parts by weight of Y ₂ O _{3 and} 5 parts by weight of Al ₂ O ₃ were added to each powder and mixed in n-butanol for 40 hours. After that, the mixed powder obtained by evaporating the solvent is molded with a mold so that the length is 50 mm, the width is 50 mm, and the thickness is 40 mm, and then the mixed powder is pressure-molded by a rubber press at a pressure of 1 ton / cm ^2. A molded body was obtained. When this compact was fired for 3 hours in a nitrogen atmosphere at 1760 ° C., the bulk density of the sintered body using the powder of Example 1 was 3.19 g / cm ³ , whereas that of Comparative Example 7 was 2.91. It was g / cm ³ , which was an extremely low value in the comparative example.

The optimum conditions in the method for producing silicon nitride according to the present invention will be described. The mixing ratio of the mixed gas is NH ₃ / CH ₄ = 0.5 to 2000 (volume ratio) by converting CmHn into CH ₄ of carbon standard.
Is appropriate. If necessary, N ₂ or other inert gas may be contained. If the amount of NH ₃ is less than the NH ₃ / CH ₄ = 2000, becomes weak reducing action of silica by NH _3,
The reaction does not proceed and it becomes difficult to generate silicon nitride. . Conversely, NH
_When the amount of ₃ is more than NH ₃ / CH ₄ = 2000, it is formed during the reaction.
The effect of the hydrocarbon gas for removing H ₂ O is reduced, and the total oxygen content of the synthetic powder is increased. . Also, the heating temperature is 800-1600
The temperature is preferably set to ° C. If the heating temperature is lower than 800 ° C, the reaction may not proceed substantially. Also, 1600 ℃
If it is higher than this, the thermal decomposition rate of NH ₃ itself may be too fast to obtain the desired effect.

EFFECTS OF THE INVENTION In the method for producing silicon nitride according to the present invention, it is possible to obtain silicon nitride having an extremely low carbon content and an extremely low oxygen content, despite the silica reduction method.

Furthermore, since the raw material carbon is not used, the work of mixing silica and carbon is eliminated. Product yield is improved. Also, it is not necessary to decarburize with NH ₃ following the synthesis. Therefore, it is easy to improve the work environment.

As seed powder in the raw material, Si ₃ N ₄ , SiC, Si ₂ N ₂ O, Si
Etc. can be added. Further, Mg, Ca, Zr, Be, Sr, Sn, Ge, Ti, Hf, or compounds thereof can be added to the raw material as a catalyst.

[Brief description of drawings]

FIG. 1 is a diagram showing the stability relationship of the condensed phase of the Si—C—N—O system.

Claims (6)

[Claims] 1. A method for producing silicon nitride by a method for producing silicon nitride by a silica reduction method, which comprises heating silica in a mixed gas of NH ₃ and a hydrocarbon gas (CmHn). 2. The mixing ratio of the mixed gas is CmHn to CH based on carbon.
_The method for producing silicon nitride according to claim 1, wherein NH ₃ / CH ₄ = 0.5 to 2000 (volume ratio) when converted to ₄ . 3. The method for producing silicon nitride according to claim 1 or 2, wherein the heating temperature is 800 to 1600 ° C. 4. The method according to claim 1, wherein the total carbon content of the silicon nitride is 0.3% by weight or less and the total oxygen content is 3.5% by weight or less. Of manufacturing a processed silicon nitride. 5. The method for producing silicon nitride according to any one of claims 1 to 4, wherein the mixed gas further contains N ₂ or another inert gas. 6. The silica further comprises 0.005 to 1 part by weight of at least one of silicon nitride powder, silicon carbide powder, silicon oxynitride powder, and metal silicon powder as seed powder with respect to 1 part by weight of silica. Or, as a catalyst, Mg, Cg, Zr, Be, Sr, Sn, Ge, Ti, Hf or at least one of these compounds is converted into each element weight by 0.001 to 0.1.
The method for producing silicon nitride according to any one of claims 1 to 5, which includes parts by weight.