JPS63162516A - Production of silicon nitride - Google Patents

Abstract

PURPOSE:To remarkably reduce the carbon content in the title silicon nitride as well as to efficiently obtain silicon nitride, by heating silica (SiO2) in a gaseous mixture of NH3 and a gaseous hydrocarbon. CONSTITUTION:SiO2 with or without seed powder is heated in a gaseous mixture of NH3 and a gaseous hydrocarbon (e.g., C3H8). N2 or other inert gases can be incorporated, as required, into the gaseous mixture, and the carbon content in the silicon nitride powder to be formed can be remarkably reduced. Silicon nitride having extremely low contents of carbon and oxygen can be obtained in the production of silicon nitride (Si3N4) by this invention even if a silica reduction method is used. Besides, since raw material carbon is not used, the operation for mixing silica and carbon can be dispensed with.

Description

[Detailed Description of the Invention] Production Field 1 This invention relates to a method for producing silicon nitride by a silica reduction method.

[To L11] A silica reduction method in which a mixture of silica and carbon is heated in a nitrogen atmosphere is known as a method for economically producing high-purity silicon nitride. For example, Tokuko Sho 54-2
In Publication No. 3917, a mixed powder consisting of silica powder, carbon powder, silicon nitride powder, silicon carbide powder, and at least one of silicon oxynitride-based powders is heat-treated in an atmosphere containing nitrogen. A method for producing silicon nitride powder has been proposed in which a reductive nitriding reaction is performed, followed by a decarburization step to oxidize and remove unreacted carbon.

11 蒐゛し：'"1'-1 In the conventional method for producing silicon nitride using the silica reduction method, it is necessary to avoid containing a relatively large amount of carbon in the produced silicon nitride powder. However, as is well known, the carbon content in silicon nitride powder inhibits the densification of the sintered body when the powder is sintered, so it is necessary to reduce the amount of carbon content as much as possible. .

Further, since the work of mixing carbon as one of the raw material powders is dirty and undesirable from the viewpoint of product yield and synthesis furnace efficiency, a raw material system that does not contain carbon powder is desired. Furthermore, conventional methods require decarburization of synthetic powder, but it would be desirable from the viewpoint of production costs if this could be eliminated.

21 November 1 This invention aims to significantly reduce the amount of carbon contained in silicon nitride and to provide an efficient method for producing silicon nitride, in view of the current state of the prior art such as fleas. .

LL Minor 11 In order to achieve the above-mentioned object, the present invention provides a method for producing silicon nitride using a silica reduction method, in which silica is converted into NHa.
The gist of this article is a method for producing silicon nitride, which is characterized by heating in a mixed gas of CmHn and hydrocarbon gas (CmHn).

In the method for producing silicon nitride according to the present invention for preparing silicon nitride, silica (with or without addition of seed powder) is heated in a mixed gas of NH3 and hydrocarbon gas (Cm Hn ).

Note that the hydrocarbon gas of the present invention includes gases that become hydrocarbons in the synthesis atmosphere. Include N2 or other inert gas in the gas mixture as needed. This significantly reduces the amount of carbon contained in the silicon nitride powder.

The present inventors investigated the cause of carbon inclusion in silicon nitride powder in the silica reduction method, and were able to clarify a cause different from that previously thought. Conventionally, the raw material carbon was added in excess of the amount necessary to reduce silica, and the excess carbon was oxidized and removed by heating in the atmosphere after synthesis. It was thought that the reason for the inclusion was that raw carbon remained undecarburized. However, as a result of research conducted by the present inventors, it was discovered that the carbon contained is generated during synthesis and is present inside the silicon nitride powder. Therefore, it has been found that it is difficult to effectively remove the carbon contained in the silicon nitride powder by external oxidation.

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

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

Si 02 (S) + C (S) → Si O (G) + CO (G) Si O (G) + C (S) → Si (G) + CO (G) 3Si (G) + 2N2 (G) → Si 3 N4 ( S) When the N2 partial pressure in the reaction system is low or when the CO depth locally increases, the reverse reactions of the first and second equations above occur, and C(S) Carbon is precipitated and incorporated into the forming silicon nitride powder.

In addition, in the thermodynamic equilibrium relationship of the silica reduction reaction, N2
The stable relationship of the condensed phase, which is related to the partial pressure and the 00 partial pressure, is shown in Figure 1. This shows an example at 1427°C, and shows the stable relationship of the condensed phase of the sr -C-N-0 system. In such systems, S as the condensed phase
i 3 N4, Si C, Si 02 and C exist, but even in the Si 3 N4 generation region, C is Si
3 Can exist in equilibrium with N4.

Therefore, in the synthesis process of Si 3 N4, C becomes Si 3
It is taken inside the N4 powder.

Due to the causes mentioned above, C generated during the reaction process becomes S.
Carbon is incorporated into the i3N4 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.

3Si 02 (S) + 4NH3 (G) → Si 3
N4 (S) + 6820 (G) Although the silicon nitride powder synthesized as above does not contain carbon, 3i 3
N4 oxidizes. In order to prevent the oxidation of Si 3 N4 by N20, the partial pressure of 820 in the atmospheric gas may be lowered. As a method to lower the partial pressure of N20, NH3
Increase the gas flow rate. Mix NH3 gas with an inert gas such as N2 to increase the flow rate. Or C in NH3 gas
Possible solutions include mixing l1l 1-1n gas. Cm
When Hn is mixed, 1-120
can be removed. .

m t-120+Cm Hn −+m CO+ (m
Hn/2) Furthermore, when a large amount of hydrocarbon gas is mixed in, the carbon produced when the hydrocarbon gas is thermally decomposed also acts as a reducing agent for silica.

Japanese Patent Publication No. 57-44602 discloses a method of reducing and nitriding silica in a mixed gas of hydrocarbon gas, ammonia gas and hydrogen gas. In water droplets, NH3
This method uses hydrogen and nitrogen in the active phase generated during the thermal decomposition process to produce a reductive nitriding effect. According to the experimental results of the present inventors, adding hydrogen to the original gas as in Japanese Patent Publication No. 57-44602 only suppresses the decomposition of NH3 and delays the Si3N4 synthesis reaction. No additional curing could be found.

Tue 11 SiO2 (silica) powder having an average particle size of 20μI,
Si3N4 (silicon nitride) powder having an average particle size of 0.1 μm was blended in the proportions shown in Table 1, and some of the powders were added with a catalyst and subjected to reductive nitriding treatment under the conditions shown in Table 1.

When the silicon nitride powder thus obtained was examined, it was found that the amount of carbon contained was extremely small.

In addition, experiments were also conducted on four comparative examples shown in Table 1. In these comparative examples, N2 which does not contain N1-13
Reductive nitriding and decarburization were performed in an atmosphere of (nitrogen) alone or a mixed gas of H2, NH3, and CaHa.

Further, the properties of the sintered bodies were compared using the powders produced in Example 1 and Comparative Example 7. Yz 03 in each powder
5 parts by weight and Δ112035 weight part were added and mixed in n-tsukunol for 40 hours. After that, the mixed powder obtained by evaporating the solvent was placed in a size of 50 mm vertically and 50 mm horizontally.
After molding with a metal mold to a thickness of 40 raIIl, the mixed powder was press-molded with a rubber press at a pressure of 1 ton/cm2 to obtain a molded body. This molded body is 176
When baked for 3 hours in a nitrogen atmosphere at 0°C, Example 1
The bulk density of the sintered body using this powder is 3.19! J/
Cm3, whereas in Comparative Example 7 it was 2.91g/C
m3, and the comparative example had an extremely low value.

Silicon nitride according to the present invention! ! To explain the optimal conditions for the 4-method, the mixing ratio of the mixed gas is (:
, m)in converted to carbon-based CH4, NH3/C
H4=0.5 to 2000 (Yoyo Ratio) is appropriate. Furthermore, N2 or other inert gas may be contained as necessary. When the amount of NH3 is less than NH3/CH4 = 2000, the reducing effect of silica by NH3 becomes weak,
The reaction does not proceed and it becomes difficult to produce silicon nitride. . On the contrary, N
If the H3 cloudiness is NH3/CH4=2000J:
The effectiveness of the hydrocarbon gas in removing l-120 generated during the reaction becomes lower, and the total amount of oxygen in the synthetic powder increases. . Moreover, it is preferable that the heating temperature is 800 to 160O<0>C.

If the heating temperature is lower than 800°C, the reaction may not substantially proceed. Also, if the temperature is higher than 1600℃, N
The rate of thermal decomposition of H3 itself may be too fast, making it difficult to obtain the desired effect.

11 Starvation In the method for producing silicon nitride according to the present invention, silicon nitride with extremely low carbon content and oxygen content can be obtained despite using the silica reduction method.

Furthermore, since raw carbon is not used, there is no need to mix silica and carbon.

Product yield is improved. Further, there is no need for decarburization with NH3 subsequent to the synthesis. Therefore, it is easy to improve the working environment.

In addition, Si 3 H4, Si
C, Si 2 N20. Si or the like can be added. Furthermore, MQ, Ca 1Zr as catalysts are included in the raw materials.
, [3e 1sr, 3n, Qe 1Ti,
Hf, these compounds, etc. can be added.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the stability relationship of the condensed phase of the 5i-C-N-0 system.

Claims (6)

[Claims] (1) A method for producing silicon nitride by a silica reduction method, which is characterized in that silica is heated in a mixed gas of NH_3 and hydrocarbon gas (CmHn). (2) The mixing ratio of the mixed gas is CmHn to CH based on carbon.
When converted to _4, NH_3/CH_4=0.5~200
0 (capacity ratio). (3) The method for producing silicon nitride according to claim 1 or 2, wherein the heating temperature is 800 to 1600°C. (4) Any one of claims 1 to 3, wherein the total carbon content of silicon nitride is 0.3% by weight or less and the total oxygen amount is 3.5% by weight or less. The method for producing silicon nitride described in . (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 other inert gas. (6) The silica further contains 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 a seed powder per 1 part by weight of silica; , or as a catalyst Mg, Ca, Zr, Be, Sr, Sn, Ge, Ti, H
The nitriding agent according to any one of claims 1 to 5, which contains at least one of f and these compounds in an amount of 0.001 to 0.1 part by weight in terms of the weight of each element. Method of manufacturing silicon.