JP2012046380A - Method for producing silicon carbide porous body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To materialize a method for producing a silicon carbide porous body, with which electroconductive characteristics of the silicon carbide porous body can be controlled as desired.SOLUTION: In the method for producing the silicon carbide porous body, predetermined amounts of silicon nitride powder and carbonaceous solid powder having a molar ratio (Si/C) of a silicon component in silicon nitride to carbon of 0.5-1.5, is mixed with silicon carbide powder as aggregate and molded, and the obtained molded body is fired in nitrogen atmosphere, and thereby silicon nitride as a silicon source and a carbonaceous solid as a carbon source react with each other so as to generate silicon carbide and form a neck to connect the aggregate. The electroconductive characteristics are controlled according to the mixing ratio of silicon nitride powder and carbonaceous solid powder to silicon carbide powder.

Description

The present invention relates to a method for producing a silicon carbide porous body having conductive properties.

Conventionally, a silicon carbide sintered body has been widely used as a ceramic resistance heating element used in an electric furnace or the like (for example, Patent Document 1). Silicon carbide is a semiconductor and has a temperature dependency in which the resistance value rapidly decreases as the temperature rises. Since the resistance value is related to the energization characteristics and the heat generation amount, it is an important technical problem to keep the resistance value within a desired range in heating control. Since temperature control becomes difficult when the temperature dependence of the resistance value is large, it is desired that both the resistance value and the temperature dependence thereof can be controlled.

JP-A-6-506530

In general, doping of impurities is performed in order to control the resistance value of a material having such semiconductor characteristics. However, with this method, it is difficult to control the amount of a small amount of doping, and when a resistance value is adjusted by adding a small amount of impurities when blending raw materials, it is finally obtained due to a minute error in weighing, volatilization during firing, etc. There is a problem that the resistance value of the sintered body to be obtained may be greatly deviated from the target value.

Then, an object of this invention is to implement | achieve the manufacturing method of the silicon carbide porous body which can be controlled to a desired electroconductivity characteristic.

In order to achieve the above object, according to the present invention, in the invention described in claim 1, the silicon carbide powder as an aggregate is 50 to 95% by weight, and the molar ratio of silicon component to carbon in silicon nitride (Si / C) Silicon carbide powder in which 50 to 5 wt% mixed powder of silicon nitride powder and carbonaceous solid powder with 0.5 to 1.5 is molded, and the resulting molded body is fired in a nitrogen atmosphere A method for producing a porous body, which uses a technical means for controlling the conductive characteristics of the silicon carbide porous body by the mixing ratio of the silicon nitride powder and the carbonaceous solid powder to the silicon carbide powder.

According to the first aspect of the present invention, the silicon carbide powder as the aggregate is nitrided with a silicon component / carbon molar ratio (Si / C) in the silicon nitride of 0.5 to 1.5. A predetermined amount of silicon powder and carbonaceous solid powder are mixed and molded, and the resulting molded body is fired in a nitrogen atmosphere to react silicon source silicon nitride and carbon source carbonaceous solid. It is possible to produce a silicon carbide porous body in which a neck for generating aggregate and connecting aggregates is formed.
Here, by firing in a nitrogen atmosphere, nitrogen generated by decomposition of silicon nitride is contained as impurities in the silicon carbide neck and the silicon carbide as the aggregate. Thereby, impurities can be introduced by easily controlling the content in silicon carbide, and the silicon carbide porous body becomes an n-type semiconductor, so that the conductivity is improved. And, the mixing ratio of the silicon nitride powder and the carbonaceous solid powder to the silicon carbide powder can control the porous structure such as the nitrogen content, neck shape, porosity, etc., so that the conductive characteristics are controlled. be able to.

According to a second aspect of the invention, in the method for producing a silicon carbide porous body according to the first aspect, the specific resistance is reduced by increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder. Use technical means.

As in the second aspect of the invention, the specific resistance can be reduced by increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder.

According to a third aspect of the present invention, in the method for producing a silicon carbide porous body according to the first or second aspect, the resistivity is increased by increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder. Use technical means to reduce temperature dependence.

As in the third aspect of the invention, the temperature dependency of the specific resistance can be reduced by increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder.

It is explanatory drawing which shows the relationship between the specific resistance of the silicon carbide porous body manufactured by the manufacturing method of this invention, its temperature dependence, and the mixing ratio of a silicon nitride powder and a carbonaceous solid powder.

The method for producing a silicon carbide porous body according to the present invention will be described.

Silicon nitride having 50 to 95% by weight of silicon carbide powder as aggregate as a starting material and a molar ratio (Si / C) of silicon component to carbon in silicon nitride of 0.5 to 1.5 A mixture of powder and carbonaceous solid powder (hereinafter referred to as neck material) is blended in an amount of 50 to 5% by weight.

The silicon carbide powder preferably has an average particle size of 50 μm or less, and a plurality of powders having different particle size distributions can be mixed.
The silicon nitride powder preferably has an average particle size of 100 μm or less in order to promote the reaction with the carbonaceous solid powder forming the neck.
Carbon black, acetylene black, etc. can be used as the carbonaceous solid powder. In order to promote the reaction with the silicon nitride powder, the average particle size is preferably 10 μm or less. As the carbonaceous solid powder, an organic resin such as phenol, furan or polyimide which is thermally decomposed to become a carbon source can be used.

The above raw material powder is blended at a predetermined ratio, an organic binder such as methylcellulose and additives such as moisture are added, and the mixed and kneaded kneaded product is extruded, cast, press molded, injection molded, etc. Mold into a desired shape.

The produced molded body is fired at 1600 to 2300 ° C. in a nitrogen atmosphere. In the firing process, silicon nitride reacts with carbonaceous solids to form silicon carbide, which forms a neck that connects the silicon carbide as an aggregate and reacts and sinters, and pores are formed and silicon carbide is formed. A porous material is formed.

Here, by firing in a nitrogen atmosphere, nitrogen generated by decomposition of silicon nitride is contained as impurities in the silicon carbide neck and the silicon carbide as the aggregate. Thereby, impurities can be introduced by easily controlling the content in silicon carbide, and the silicon carbide porous body becomes an n-type semiconductor, so that the conductivity is improved. Nitrogen not only dissolves in silicon carbide, but also may exist as a compound by reaction with silicon carbide.

When the firing temperature is lower than 1600 ° C., the carbonization reaction of silicon nitride is not sufficient, and the silicon nitride remains in the neck connecting the silicon carbide of the aggregate, so that the specific resistance increases. Further, when the firing temperature exceeds 2300 ° C., sublimation starts, the neck portion becomes thin, and the specific resistance similarly increases.

When the silicon carbide powder as the aggregate is less than 50% by weight, the mechanical strength of the silicon carbide porous body is lowered. When the amount is more than 95% by weight, the neck raw material is small, so that the amount of neck is insufficient and sintering becomes insufficient.

When the neck raw material Si / C is smaller than 0.5, the carbon content remaining without contributing to the carbonization reaction of silicon nitride increases, which causes rough atmospheric pores and silicon carbide produced as a neck. Grain growth is inhibited. If Si / C is greater than 1.5, the amount of neck silicon carbide produced is reduced, resulting in insufficient sintering.

According to this production method, the content of nitrogen, the neck shape of the silicon carbide porous body, the porous structure such as the porosity, and the like can be controlled by the mixing ratio of the neck raw material.
It is considered that when the mixing ratio of the neck raw material increases, the amount of nitrogen contained in silicon carbide increases, so that the specific resistance decreases. Moreover, the porosity of the silicon carbide porous body is increased, and the neck shape tends to be thickened.
As shown in the following examples, as the mixing ratio of the neck raw material increases, the specific resistance decreases and the temperature dependency thereof decreases. Thereby, it can control to a desired specific resistance and its temperature dependence by the mixing ratio of a neck raw material.

In addition, the specific resistance can be controlled in a wider range by using silicon carbide powders having different conductive characteristics, for example, by appropriately selecting the grain size and the crystal phase.

Since the silicon carbide porous body manufactured by this manufacturing method can control the specific resistance and its temperature dependence to desired characteristics, heating control is facilitated and particulates discharged from the diesel engine are collected. It can be suitably used as an electric heating type diesel particulate filter having a heater performance for combustion and incineration or a heated gas decomposition application (exhaust gas for automobile, VOC). Moreover, since the heat generation area is large and the thermal efficiency can be increased, it can be suitably used as a heater for hot air generators used as a heat source for duct heaters and large dryers. Furthermore, it can also be suitably used as a heater used in heating equipment, cooking equipment, drying equipment, firing furnaces, and the like.

A silicon carbide powder obtained by mixing a coarse raw material with an average particle diameter of 11 μm as an aggregate raw material and a fine raw material with an average particle diameter of 1 μm in a ratio of 65:35, and a molar ratio of metal silicon to carbon as a neck raw material. 10 parts by weight of organic binder and 20 parts by weight of water are added to 100 parts by weight of raw materials in which a mixed powder of silicon nitride powder and carbon black adjusted to 1.0: 1.1 is blended in the ratio shown in Table 1. Then, after mixing and kneading, it was formed into a circular tube having an outer diameter of 6 mm and an inner diameter of 4 mm by extrusion molding, and fired in a nitrogen atmosphere at 2200 ° C. for 5 hours.

About the obtained sintered compact, the specific resistance and the porosity were measured. The specific resistance was measured by a four-terminal method using alternating current at a measurement temperature in the range of 50 ° C. to 350 ° C. or less. The porosity was measured by Archimedes method.

FIG. 1 shows the specific resistance and its temperature dependence for each mixing ratio of the neck material. As shown in FIG. 1, the specific resistance decreased as the mixing ratio of the neck material increased. In particular, the tendency is remarkable on the low temperature side. For example, the specific resistance at 50 ° C. is 68 Ω · cm at a mixing ratio of 5% of the neck material, and is 28 Ω · cm at a mixing ratio of 50%, which is less than half. Met.
Further, when the mixing ratio of the neck raw material was increased, the decrease rate of the specific resistance accompanying the temperature rise was reduced, and the temperature dependency was reduced.
Thereby, it was confirmed that the specific resistance of the silicon carbide porous body and its temperature dependency can be controlled by the mixing ratio of the neck raw material.

The porosity of the silicon carbide porous material is 47% when the mixing ratio of the neck raw material is 15% and 58% when the mixing ratio is 50%. The higher the mixing ratio of the neck raw material, the higher the porosity tends to be. It was.

[Effect of the embodiment]
According to the method for producing a silicon carbide porous body of the present invention, the silicon carbide powder as an aggregate has a silicon component to carbon molar ratio (Si / C) of 0.5 to 1. 5 by mixing a predetermined amount of silicon nitride powder and carbonaceous solid powder 5 and firing the resulting molded body in a nitrogen atmosphere, so that the silicon source silicon nitride and the carbon source carbonaceous solid Can produce a silicon carbide porous body in which a silicon carbide is formed by the reaction to form a neck for connecting the aggregates.
Here, by firing in a nitrogen atmosphere, nitrogen generated by decomposition of silicon nitride is contained as impurities in the silicon carbide neck and the silicon carbide as the aggregate. Thereby, impurities can be introduced by easily controlling the content in silicon carbide, and the silicon carbide porous body becomes an n-type semiconductor, so that the conductivity is improved. And, the mixing ratio of the silicon nitride powder and the carbonaceous solid powder to the silicon carbide powder can control the porous structure such as the nitrogen content, neck shape, porosity, etc., so that the conductive characteristics are controlled. be able to.
By increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder, the specific resistance can be reduced and the temperature dependence can be reduced.

Claims (3)

Silicon nitride powder and carbonaceous material in which the silicon carbide powder as an aggregate is 50 to 95% by weight and the molar ratio (Si / C) of silicon component to carbon in silicon nitride is 0.5 to 1.5 A method for producing a silicon carbide porous body, comprising molding a mixed powder in which 50 to 5% by weight of a solid powder is mixed, and firing the obtained molded body in a nitrogen atmosphere,
A method for producing a silicon carbide porous material, wherein the conductive characteristics of the silicon carbide porous material are controlled by a mixing ratio of the silicon nitride powder and the carbonaceous solid powder to the silicon carbide powder. 2. The method for producing a silicon carbide porous body according to claim 1, wherein a specific resistance is reduced by increasing a mixing ratio of the silicon nitride powder and the carbonaceous solid powder. The method for producing a silicon carbide porous body according to claim 1 or 2, wherein the temperature dependence of the specific resistance is reduced by increasing the mixing ratio of the silicon nitride powder and the carbonaceous solid powder. .

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