JPS63162512A - Production of silicon nitride - Google Patents

Abstract

PURPOSE:To efficiently produce the title Si3N4 with a remarkably reduced content of carbon, by heating silica in NH3 or in a gaseous mixture of NH3 and N2 at the time of producing Si3N4 by a silica reduction method. CONSTITUTION:Silica (single or added with seed powder) is heated in NH3 or in a gaseous mixture of NH3 and N2. Consequently, the carbon content in the Si3N4 powder can be remarkably reduced. Since the Si3N4 powder is synthesized in this method without using C as the reducing agent for SiO2, C is not contained in the Si3N4 powder, however the Si3N4 is oxidized by the H2O generated in the reaction of SiO2 and NH3, hence the O content in the powder tends to increase. The reaction can be controlled by decreasing the partial pressure of H2O in the atmosphere, and the flow rate of NH3 is increased or gaseous N2 is mixed to increase the gas flow rate for the purpose.

Description

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

Row 11 L 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, in Japanese Patent Publication No. 54-23917, a mixed powder consisting of silica powder, carbon powder, and at least one of silicon nitride powder, silicon carbide powder, and silicon oxynitride-based powder is used as a nitrogen-containing powder. A method for producing silicon nitride powder has been proposed in which the silicon nitride powder is heat-treated in an atmosphere to cause a reduction-nitridation reaction, and then unreacted C@oxidation is removed.

Let's do it.

In the conventional method for producing silicon nitride using a silica reduction method, it was inevitable that the produced silicon nitride powder would contain a relatively large amount of carbon. However, as is well known, carbon content in silicon nitride powder inhibits densification of a sintered body when the powder is sintered, so the amount of carbon content needs to be reduced as much as possible.

Furthermore, when carbon is used as one of the raw material powders, the working environment tends to become dirty, which is unfavorable from the viewpoint of product performance and synthesis furnace efficiency.

Furthermore, although the conventional method requires decarburization treatment after synthesis, it would be desirable from the viewpoint of production costs if this could be omitted.

In view of the current state of the prior art as described above, it is an object of the present invention to provide a method for producing silicon nitride that significantly reduces the amount of carbon contained in silicon nitride and has high production efficiency.

In order to achieve the above-mentioned object, the present invention provides a method for producing silicon nitride by a silica reduction method, in which silica is reduced to NH3.
Alternatively, the gist is a method for producing silicon nitride, which is characterized by heating in a mixed gas of NH3 and N2.

In the method for producing silicon nitride according to the present invention for making silica, silica (alone or with the addition of seed powder) is added to NH3 or N
Heating in a mixed gas of H3 and N2. 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, raw carbon was added in excess of the age required 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) 3 Si (G) + 2 N 2 (G
)→Si 3 N4 (S) When the N2 partial pressure in the reaction system is low or when the COx degree locally increases, the reverse reactions of the first and second equations above occur, and C (S) precipitates and carbon is incorporated into the silicon nitride powder being produced.

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 stability relationship of the condensed phase of the st -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 3i 3 N4 production 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 i 3 N4 powder, and as a result, carbon is contained in the silicon nitride powder.

Therefore, in order to prevent C from being included in the Si 3 N, i powder, it is only necessary to synthesize the Si 3 N4 powder without using C as a reducing agent for SiO2.

As a result of various experimental studies from this point of view, we found that SiO2 as a powder raw material or 3i as a seed in addition to SiO2 was used as a powder raw material.
3 It was discovered that the best method was to use NH3 as the reaction gas with addition of N4, etc., and this invention was completed. The nitriding reaction in this case is expressed by the following equation.

3Si 02 (S) + 4NH3 (G) - + Si 3
N4 (S) + 6H20 (G) Although the silicon nitride powder synthesized as above contains no carbon at all, the H2O produced in the above reaction contains J: 3i 3
N4 is oxidized and the oxygen content of the powder tends to increase.

As is clear from the above reaction equation, 3 moles of SiO2 and 4
1 mole of NH3 reacts with 1 mole of Si 3 N4 and 6
Produces moles of H2O.

The H2O generated at this time reacts with Si3N4 according to the following equation, reducing Oλ by one.

Si 3 N4 +6H20→3Si 02 +2N2
+6 days 2 This is the reason for the increase in oxygen in the powder.

This reaction can be suppressed by reducing the 1-120 partial pressure in the atmosphere. -120 partial pressure in the atmosphere can be reduced by increasing the flow rate of NH3 or adding N2 to NH3.
Possible methods include mixing gas and increasing the gas flow rate.

As a result of various experiments, it was found that the flow rate of NH3 (NH3 flowed from the start to the completion of the reaction) is required to obtain powder of practically acceptable quality.
81 calculation amount) is three times the theoretical amount obtained from the nitriding reaction equation mentioned above, that is, 4 for 1 mole of SiO2.
It turns out that it has to be more than a mole.

In addition, the maximum flow rate of NH3 that can be flowed during synthesis varies depending on the type of synthesis furnace, the form of the raw material powder, etc., but in practice, the limit is about 3000 times the theoretical amount (4000 mol vs. 1 mol of 5iOz), and the most preferable NH3 flow rate is The flow rate is the theoretical amount of 40
It is 0 to 2000 times.

Furthermore, from an economical point of view, it is also effective to mix N2 gas, which is available at a lower price than NH3, with NH3. In this case, if the flow rate of NH3 is constant, the rate of nitriding reaction will slow down as the amount of N2 gas mixed in increases, that is, as the concentration of NH3 decreases, so the practically acceptable mixing ratio of N2 is 0 to 60% (volume percentage). Therefore, the proportion of NH3 in the mixed gas is 40~
100% by volume is appropriate. If necessary, N2 may contain an inert gas or a non-oxidizing gas.

Moreover, it is preferable that the heating temperature is 800 to 1600°C. If the heating temperature is lower than 800°C, the reaction may not substantially proceed.

Furthermore, if the temperature is higher than 1600°C, the rate of thermal decomposition of NH3 itself may be too fast, making it difficult to obtain the desired effect.

5i02 (silica) powder with an average particle size of 20 μm and Si3N4 (silicon nitride) powder with an average particle size of 0.1 μm were blended in the proportions shown in Table 1, and in some cases a catalyst was added. Then, a reduction nitriding treatment was performed under the conditions shown in Table 1.

When the powder thus obtained was examined, it was found that the amount of carbon contained was extremely low and that the powder consisted only of silicon nitride.

In addition, experiments were also conducted on three comparative examples shown in Table 1. In these comparative examples, cases were shown in which the atmosphere contained NH3 and its flow rate was small, and in an atmosphere containing only N2 (nitrogen) without NH3.

Further, the properties of the sintered bodies were compared using the powders produced in Example 1 and Comparative Example 7. Y2035 in each powder
A portion of heavy water and 35 parts by weight of ΔQ were added and mixed in n-butanol for 40 hours. After that, the mixed powder obtained by evaporating the solvent is 5Qmm vertically, 5Qmm horizontally, and 4Qm thick.
After molding with a mold to a size of m, 1 ton/c
The mixed powder was pressure-molded using a rubber press at a pressure of m2 to obtain a molded body. When this compact was fired in a nitrogen atmosphere at 1760°C for 3 hours, the bulk density of the sintered compact using the powder of Example 1 was 3.20 g/cm3, whereas
The value of Comparative Example 7 was 2.91 g/cm3, which was an extremely low value.

11 Kotatsul In the method for producing silicon nitride according to the present invention, silicon nitride containing no carbon at all can be obtained despite the silica reduction method.

Furthermore, since NH3 does not easily react with 5iOz, a great practical effect can be obtained in that the synthesis temperature can be set low.

Furthermore, by using a mixture of N1-13 and N2, it is possible to reduce manufacturing costs.

Furthermore, since carbon, which has conventionally been used as a reducing agent, is not used at all, it is easy to improve the conventional environment and reduce manufacturing costs. No decarburization step is required after synthesis.

In addition, Si 3 N 4, Si
C, Si 2 N20. Si or the like can be added. Furthermore, Mg, Ca, Z as catalysts are contained in the raw materials.
r, 3e, Sr, Sn, Ge.

'li, )lf, 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 s+ -C-N-0 system.

Claims (6)

[Claims] (1) A method for producing silicon nitride by a silica reduction method, which comprises heating silica in NH_3 or a mixed gas of NH_3 and N_2. (2) The method for producing silicon nitride according to claim 1, wherein the mixing ratio of the mixed gas is 40 to 100% by volume of NH_3. (3) The cumulative flow rate of NH_3 is 4 to 1 mole of silica.
4000 mol of silicon nitride as claimed in claim 1. (4) The method for producing silicon nitride according to claim 1, 2, or 3, wherein the heating temperature is 800 to 1600°C. (5) Any one of claims 1 to 4, wherein the total carbon content of silicon nitride is 0.2% by weight or less and the total oxygen amount is 4.0% by weight or less. The method for producing silicon nitride described in Section. (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 a claim containing 0.001 to 0.1 parts by weight of Mg, Ca, Zr, Be, Sr, Ge, Ti, Hf, or at least one of these compounds as a catalyst in terms of the weight of each element. 1st of
The method for producing silicon nitride as described in any one of Items 1 to 5.