JP2007008793A - Carbon-containing conductive ceramics mainly composed of Al-Si-C compounds - Google Patents

Abstract

Conductive ceramics such as a heating element that has excellent oxidation resistance and can be used at high temperatures in the atmosphere.
Sintering a conductive exothermic material such as carbon or silicon carbide comprising an aluminum-silicon-carbon compound compound by forming a mixed powder composed of aluminum, silicon and carbon powder and firing in a reducing atmosphere. It is present in the body. This compound sintered body is characterized in that an oxide film is formed on the surface when heated in the atmosphere, and this prevents oxidation of the conductive heat-generating substance existing inside the sintered body.
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Description

Detailed Description of the Invention

The present invention relates to a method for providing conductive ceramics, and more particularly, to a method for providing conductive ceramics such as a heating element that requires oxidation resistance in high-temperature air.

In electric heating such as an electric furnace, a silicon carbide (SiC) conductive ceramic sintered body is used as a heating element in an oxidizing and reducing atmosphere of 1600 ° C. or lower. In an oxidizing atmosphere, a molybdenum disilicide (MoSi 2) conductive ceramic sintered body can be used as a heating element to obtain a temperature of about 1900 ° C. In the reducing atmosphere, a temperature of about 2300 ° C. can be obtained by using a carbon (C) conductive ceramic sintered body as a heating element. Thus, appropriate heating is possible by selecting the material of the conductive ceramic sintered body according to the operating temperature and atmosphere. As described above, it is necessary to select a combination of materials for the conductive ceramic sintered body depending on the use atmosphere and temperature, and an inappropriate combination may damage the conductive ceramic sintered body.

Problems to be solved by the invention

The damage form of such a conductive ceramic sintered body varies depending on each type. In a silicon carbide based conductive ceramic sintered body, silicon oxide (SiO 2) generated as an oxide film functions as an appropriate protective film when used at about 1600 ° C. in the atmosphere. Silicon oxide has a melting point of 1723 ° C. and melts at a lower temperature when it contains impurities. As a result, due to the reaction between silicon carbide and silicon oxide, gas such as SiO (g) and CO (g) generated at the interface The protective layer is destroyed. For this reason, it is difficult to stably use silicon carbide as a heating element in the atmosphere at a temperature of about 1600 ° C. or higher.

In addition, in a carbonaceous conductive ceramic sintered body, wear can be minimized and used in a reducing atmosphere without oxygen. However, in an oxidizing atmosphere, carbon is oxidized and worn out violently.

Further, in the molybdenum disilicide conductive ceramic sintered body, a protective layer can be formed in an oxidizing atmosphere and wear of the heat generating material can be suppressed. However, in the reducing atmosphere, the protective layer is not formed, and the heat generating material itself is worn.

The present invention relates to the problem of wear when used in an oxidizing atmosphere in a graphite heating element and wear in a reducing atmosphere with molybdenum disilicide-based conductive ceramics, and 1600 ° C. in the atmosphere with silicon carbide-based conductive ceramics. An object of the present invention is to provide a conductive ceramic that is free from the problems of destruction of the antioxidation film when used at the above-mentioned high temperatures and that is dense and has excellent durability with as little restrictions on the atmosphere and temperature as possible.

Means for solving the problem

The method for producing a conductive ceramic according to the present invention comprises a mixed composition comprising aluminum, silicon and carbon, wherein the proportion of aluminum is 20 wt% or more and less than 66 wt%, the proportion of silicon is 8 wt% or more and less than 52 wt%, and the ratio of carbon Is carried out by reactive sintering of the raw material powder with a composition of 26 wt% or more and less than 63 wt%. By this sintering, a dense sintered body made of Al ₈ SiC ₇ , Al ₄ SiC ₄ , Al ₄ Si ₂ C ₅ , Al ₄ Si ₃ C _6, Al ₄ Si ₄ C ₇ or the like as an aluminum-silicon-carbon-based compound is obtained. Although formed, an oxidation-resistant protective layer made of aluminum oxide, silicon oxide, and silicon aluminum oxide is formed on the surface of the sintered body during oxidation, and the oxidation of the conductive heat-generating substance such as carbon or silicon carbide inside is suppressed.

Aluminum, silicon, and carbon powders are used as raw materials for the production. The aluminum powder may be large particles, but it is preferable that the average particle size is as small as possible, such as several μm. Further, the silicon powder may be large particles, but it is preferable that the average particle size is as small as possible, about several μm. The carbon powder may be a crystallized material such as graphite, but an amorphous material such as carbon black is highly reactive and good.

Firing and reaction firing are performed by mixing and forming aluminum powder at a ratio of 20 wt% or more and less than 66 wt%, a silicon powder ratio of 8 wt% or more and less than 52 wt%, and a carbon powder ratio of 26 wt% or more and less than 63 wt%. By bonding, an aluminum-silicon-carbon-based compound is brought into a state in which carbon or silicon carbide conductive ceramic exists in the compound, and the compound forms an oxide film during high-temperature oxidation, which is an oxidation-resistant protective substance for the sintered body. Become. This range is shown in FIG. More preferably, the proportion of aluminum powder is 35 wt% or more and less than 66 wt%, the silicon ratio is 8 wt% or more and less than 35 wt%, and the carbon ratio is 26 wt% or more and less than 50 wt%. . Carbon or silicon carbide present in a sintered body made of an aluminum-silicon-carbon compound can impart heat generated by energization. Aluminum-silicon-carbon-based compounds include Al ₄ SiC ₄ , Al ₄ Si ₂ C ₅ , Al ₄ Si ₃ C ₆ , Al ₄ Si ₄ C ₇ , and Al ₈ SiC ₇ . Carbon or silicon carbide is at the grain boundary of this compound, and can be stably heated without being affected by the atmosphere or a reducing atmosphere by the oxidizing protective material formed around the sintered body.

When a ceramic made of an aluminum-silicon-carbon compound is exposed to a high temperature in the atmosphere, the surface is oxidized to form a layer made of aluminum oxide, silicon oxide, and silicon aluminum oxide. When the ratio of the raw material aluminum powder is 13 wt% or more and less than 50 wt% and the silicon ratio is 3 wt% or more and less than 38 wt%, the eutectic point of the oxide film to be formed is 1850 ° C., and up to this temperature, A film is stably maintained even at a high temperature of 1800 ° C. or higher. When the mixing ratio of the raw materials exceeds the above range and the aluminum becomes excessive, the oxide film cracks and the oxidation reaction proceeds. Further, when the mixing ratio of the raw materials exceeds the above range and the silicon powder is excessive, at 1600 ° C. or higher, a part of the film melts and the structure deteriorates in order.

When the ratio of the raw material aluminum powder is 20 wt% or more and less than 66 wt% and the silicon ratio is 8 wt% or more and less than 52 wt%, the carbon ratio is 26 wt% or more and less than 63 wt%. When the ratio exceeds this range, the range surrounded by the oxidation-resistant protective substance becomes narrow, and the exposed carbon is oxidized and released into the gas as a gas when heated in the atmosphere, and the tissue deteriorates. .

A mixed powder consisting of aluminum powder, silicon powder and carbon powder composed in these ranges is mixed with a dry ball mill or the like, and this powder is loaded into a graphite crucible, and 1150 to 1700 in a non-oxidizing atmosphere such as nitrogen and argon. Calcination (calcination) at 0 ° C. is performed and further pulverized with a ball mill or the like. A molding binder is added to the pulverized product, and pressure molding is performed at about 50 MPa or more with a mold and a press. By firing the compact in a non-oxidizing atmosphere such as nitrogen and argon at 1700 ° C. to 1850 ° C. for about 3 hours, a conductive ceramic can be obtained. When a sintering method such as hot pressing or discharge plasma is used for sintering, conductive ceramics can be obtained even from a mixed powder that is not calcined. Sintering by hot pressing or discharge plasma may be performed with the calcined powder. When sintering by hot pressing using calcined powder, holding at 1750 ° C. for 1 h, and sintering by discharge plasma at 1750 ° C. with a temperature increase of about 20 minutes and a holding time of about 20 minutes 98% or more of conductive ceramics can be obtained.

Examples of the present invention will be specifically described below in comparison with comparative examples, but these examples show one embodiment of the present invention, and the present invention is not limited thereto.

In Examples 1 to 4, a metal Al powder having an average particle diameter of 15 μm and a purity of 99.99%, an Si powder having an average particle diameter of 15 μm and a purity of 99.99%, and a carbon black powder having an average particle diameter of 28 nm are combined in Table 1. A total amount of 100 g was dry-mixed, and the obtained mixed powder was filled in a graphite crucible and pre-baked at 1300 ° C. for 3 h in an argon atmosphere. The pre-fired powder was filled into a graphite mold having an inner diameter of Φ50 mm, and pulsed current pressure sintering was carried out under the conditions of heating at 1750 ° C. for 20 minutes and pressing at 50 MPa to produce a ceramic sintered body having Φ50 × thickness 6 mm.

A rod-shaped body having a width of 10 mm, a thickness of 5 mm, a length of 40 mm, a width of 4 mm, a thickness of 3 mm, a length of 40 mm, and a width of 4 mm, a thickness of 3 mm, and a length of 15 mm was cut out from the sintered body.

A rod-shaped body having a width of 10 mm, a thickness of 5 mm, and a length of 40 mm was sandwiched between pressurizing electrodes that can be attached with a constant load of 40 kgf, and a voltage of about 10 V was applied to conduct current heating to confirm the state of heat generation. Further, the rod-shaped body having a width of 4 mm, a thickness of 3 mm, and a length of 15 mm was fired at 1750 ° C. for 1 h once and five times in an air atmosphere, and a change in weight was confirmed. The results are shown in Table 1.

The invention's effect

As described above, when a mixed powder molded body having an aluminum-silicon-carbon based chemical composition is fired, the sintered body having a microstructure in which the conductive materials of carbon and silicon carbide exist at the grain boundaries of the aluminum-silicon-carbon based compound. A knot is produced. This sintered body is stable in a reducing atmosphere, and on the other hand, in an oxidizing atmosphere, oxidation is prevented by forming an oxide film on the surface. That is, it is possible to obtain a conductive ceramic sintered body having a property excellent in durability, in which the internal conductive material is stably held in any atmosphere.

The composition range of aluminum, silicon, and carbon and the composition range of an aluminum-silicon-carbon compound according to the present invention.

Claims (1)

In a mixed composition composed of aluminum, silicon and carbon, the proportion of aluminum is 20 wt% or more and less than 66 wt%, the proportion of silicon is 8 wt% or more and less than 52 wt%, and the ratio of carbon is 26 wt% or more and less than 63 wt%. In the structure, after firing, one or more of Al ₈ SiC ₇ , Al ₄ SiC ₄ , Al ₄ Si ₂ C ₅ , Al ₄ Si ₃ C ₆ or Al ₄ Si ₄ C ₇ as an aluminum-silicon-carbon compound Compound ceramics.