JPS6337062B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a silicon carbide sintered body having high density and low thermal conductivity. Silicon carbide is chemically stable at room and high temperatures, and has excellent mechanical properties, so
It is beginning to be used as a heat-resistant structural material and sliding material, including ceramic engine parts. Conventionally, when silicon carbide is used as such a material, a sintered body is manufactured by molding fine silicon carbide powder and firing it at a high temperature. However, since silicon carbide is inherently difficult to sinter, a sintering aid is mixed with it during firing. For example, (1) β-SiC
A method of mixing 0.3 to 3.0% by weight of boron and 0.1 to 1.0% by weight of carbon in powder and firing it (Japanese Patent Application Laid-Open No. 50-78609
issue). (2) Hot pressing method by adding BeO to α-SiC powder (Ceramics, 18, P217-223,
1983) is known. However, although these methods have good sinterability and produce a high-density sintered body, the thermal conductivity of the sintered body is high, and the silicon carbide sintered body requires insulation such as a ceramic insulation engine. When used as a high-temperature structural material, the heat insulation properties and thermal efficiency are lowered, which is not preferable. For example, the thermal conductivity of the sintered body (2) is as high as 270W/mK. The present invention was made to improve these conventional drawbacks, and its purpose is to provide a method for producing a silicon carbide sintered body with high density and low thermal conductivity (excellent heat insulation properties). . In order to achieve the above object, the present inventors have
Barium and carbon were used as sintering aids in silicon carbide powder of 0.5 μm or less, and the amounts used were varied and sintered under vacuum or in an inert atmosphere. 20% by weight,
It was discovered that a sintered body with high density and low thermal conductivity could be obtained by using both sintering aids containing 0.1 to 5% by weight of carbon, and based on this knowledge, the present invention was completed. It is something. The particle size of silicon carbide used in the present invention is
It needs to be 5.0 μm or less, preferably 1 μm or less. If the particle size is larger than this, the density of the sintered body decreases and the strength also decreases. The crystal forms of silicon carbide contained in the raw material powder are α form,
Either β-type may be used. Alternatively, a mixture of these may be used. Barium and carbon are used as sintering aids. Barium sources include barium or barium-containing compounds such as barium oxide, barium carbide,
Barium carbonate etc. are used. The amount of barium (converted to barium for barium-containing compounds) added is 0.5 to 20 to silicon carbide.
% by weight, preferably from 1 to 10% by weight. If it is less than 0.5% by weight, densification progresses slowly and a high-density sintered body cannot be obtained. Also, 20
If it exceeds % by weight, a large amount of barium remains in the sintered body, resulting in a drawback of lowering the mechanical strength of the silicon carbide sintered body. As the carbon source, carbon such as carbon black or an organic compound such as phenol resin, which produces carbon during firing, is used. The amount of carbon (in the case of an organic compound, in terms of carbon) added must be 0.1 to 5% by weight based on silicon carbide. If it is less than 0.1% by weight, sintering is difficult;
If it exceeds 5% by weight, carbon remains in the sintered body, resulting in a disadvantage that the mechanical strength of the silicon carbide sintered body is reduced. The function of barium and carbon as sintering aids is not clear, but barium and carbon react to form barium carbide, which reacts with silicon carbide to form a liquid phase and promote sintering. It is considered that Furthermore, it is thought that the thermal conductivity decreases due to the effect of this generated compound. The mixture of silicon carbide powder, barium and carbon is
Wet mixing using an organic solvent such as ethanol or acetone or water is suitable. Particularly when using an organic compound as a carbon source, it is desirable to use an organic solvent that dissolves the organic compound. Molding of these mixtures is performed by die molding, rubber press, injection molding, or the like. Alternatively, a method of simultaneously performing molding and sintering such as hot pressing or hot isostatic pressing may be used. Firing is carried out in vacuum or in an inert atmosphere. Examples of the inert atmosphere include helium, argon, and the like. It is necessary to reduce oxygen in this atmosphere as much as possible, and it is desirable that the degree of vacuum be 10 ^-4 atmospheres or less, and the oxygen concentration contained in the inert atmosphere be 10 ^-6 or less. This is because if a large amount of oxygen is contained in the atmosphere, the surface of the silicon carbide powder will be oxidized and the sinterability will be reduced. Firing temperature is 1800~2400℃, preferably 2000~
Perform at 2100℃. If the temperature is lower than 1800°C, sintering progresses slowly and a dense sintered body cannot be obtained, and if the temperature exceeds 2400°C, crystal grain growth becomes significant and silicon carbide decomposes, which is not preferable. The sintering method may be any method such as normal pressure sintering, hot pressing, hot isostatic pressing, etc., but when the amount added is small, it is better to use hot pressing or hot isostatic pressing. Example 1 Average particle size 0.3μm made by reacting SiO ₂ and carbon
β-SiC powder, 5% by weight of barium oxide (4.5% by weight in terms of barium), 1% by weight of resol type phenolic resin (in terms of carbon)
mixed in the ratio of Ethanol was added to this and mixed in a ball mill for 24 hours, then dried and pulverized to obtain a raw material mixed powder. Next, the mixed powder was placed in a graphite die, heated at a rate of 50°C/min to 2050°C while applying a pressure of 200Kg/cm ² in a vacuum of 10 ^-4 Torr, and hot pressed for 30 minutes. A sintered body having a diameter of 10 mm and a thickness of 2 mm was obtained. The density of the obtained sintered body was 3.05 g/cm ³ and the thermal conductivity was 6.4 W/mK. Note that the thermal conductivity is a value measured by a laser flash method. Example 2 Example 1 except that 10% by weight of barium carbonate (7% by weight in terms of barium) was added as a barium source and 2% by weight of resol type phenolic resin (in terms of carbon) was added as a carbon source. A sintered body was obtained by hot pressing under the same conditions as above. The density of the obtained sintered body was 3.06 g/cm ³ and the thermal conductivity was 4.4 W/mK. Comparative Example A sintered body was obtained by hot pressing in the same manner as in Example 1 using additives having the composition shown in Table 1. All of the obtained sintered bodies had high densities, but all had high thermal conductivity and were unsuitable as heat insulating materials.

[Table] As described above, according to the method of the present invention, density 3.0
g/cm ³ or more and a thermal conductivity of 10 W/mK or less, resulting in an excellent silicon carbide sintered body with high density and low thermal conductivity.

Claims (1)

[Claims] 1 Silicon carbide powder with an average particle size of 5.0 μm or less, 0.5 to 20% by weight of barium of the silicon carbide powder, or a barium compound containing the same amount of barium, and 0.1 to 5% of the silicon carbide powder by weight. After mixing and shaping carbon or an organic compound that produces the same amount of carbon, in vacuum or in an inert atmosphere, 1800 to 2400
A method for producing a high-density silicon carbide sintered body characterized by firing at ℃.