JP2001261441A - Production process of electrically conductive silicon carbide sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production process of a conductive SiC sintered body which has a <=10 Ω.cm specific resistance value and a >=90% relative density value, that are suitable characteristic values as a heater element, electrode material, resistor, or the like, and also, the variation in specific resistance value of which is hardly caused in the production and the resistance of which can easily be controlled and further which has excellent durability. SOLUTION: This production process comprises: mixing powdery SiC having a <=2 μm average grain size with a sintering aid to obtain a mixture; forming the mixture into a green body; and heating and sintering the green body in an inert atmosphere containing 30-90 vol.% gaseous nitrogeg at 2,100-2,300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a conductive SiC sintered body, and more particularly, to various kinds of resistors such as a heating element requiring oxidation resistance and corrosion resistance, and a high temperature such as a spark rod or a frame rod. The present invention relates to a method for producing a conductive SiC sintered body used as an electrode material for a vehicle.

[0002]

2. Description of the Related Art A SiC sintered body has heat resistance, thermal shock resistance,
Since it has excellent corrosion resistance and high-temperature strength characteristics, it is used in many fields.SiC is also known as an impurity semiconductor, and exhibits various conductive characteristics depending on the manufacturing method. Also widely used.

[0003] The SiC sintered body is generally produced by a sublimation recrystallization method,
It is manufactured by a reaction sintering method, a normal pressure sintering method, or the like. In the sublimation recrystallization method, generally, SiC powder having a two-stage particle distribution is heated and sintered by a vaporization / condensation process, thereby obtaining
Although a sintered body with open pores of about 0% is obtained, its oxidation resistance at high temperatures is inferior to that of reaction sintered materials and normal pressure sintered materials due to its low density. There is a problem in using it stably. In the reaction sintering method, a mixed powder of SiC and carbon is impregnated with molten Si at a high temperature to obtain a SiC sintered body by a reaction between C and Si. However, the resistance value tends to fluctuate and control is difficult.

As a method for obtaining a dense SiC sintered body, S
Pressure sintering methods such as a normal pressure sintering method in which B, Al, C, Be or a compound thereof are added to iC powder as a sintering aid, and sintering is performed in an argon gas atmosphere, and a hot pressing method, The SiC sintered body obtained by these methods has a high specific resistance at room temperature, and has a very large resistance change due to temperature, which is difficult to control, and thus has a problem in use as a resistor or an electrode material. For example, when used as a heating element, there is a problem that it is difficult to conduct electricity from room temperature and generate a thermal runaway due to a sudden increase in current due to a sudden decrease in resistance due to a temperature change.

As a method for improving the electric characteristics of a SiC sintered body in which B and C are added as a sintering aid, for example, 0.3% by weight of B is added to SiC powder having an average particle size of 1.0 μm or less. Amount of B or B compound and 0.1-0.6 wt%
The raw material system to which an amount of C or a C compound corresponding to the amount of C is added is formed, and a sintered body having a density of 70 to 95% of the theoretical density is obtained by primary sintering. A method of obtaining a low-resistance SiC heating element through secondary sintering has been proposed (Japanese Patent Publication No. 61-56187), but a sintered body in which the ultimate density during primary sintering is obtained after secondary sintering Has a problem that the density of the sintered body after the primary sintering obtained by stopping the sintering when the density reaches a predetermined range is large. Further, dividing the sintering process into two steps is disadvantageous in terms of manufacturing cost.

In order to solve the above problems, the inventor obtains the composition of the SiC sintered body based on the normal pressure sintering method, the relation between the texture and the specific resistance, and the desired composition and the texture. As a result of conducting tests and examinations from various angles on manufacturing conditions for the purpose, it is necessary to obtain 0.1% by weight or more of nitrogen and 0.1 to 0.5% of boron to obtain improved electrical characteristics.
t%, specific resistance of 0.05-5 Ω · cm, 15%
A heating element made of a SiC sintered body having an open porosity of less than 10 mm is preferable. As a method for obtaining this SiC sintered body, a mixed powder of a SiC powder having an average particle diameter of 5 μm or less and a sintering aid is molded. The molded body is placed in a nitrogen gas atmosphere or an inert gas atmosphere containing nitrogen gas in an atmosphere of 2100-2.
A method of manufacturing a sintered body in a single sintering step by heating to a temperature of 300 ° C. has been proposed (Japanese Patent Application No. 11-291970).
issue).

The inventor has further studied for the purpose of obtaining a conductive SiC sintered body which can be used not only as a heating element for high temperature, but also as an electrode material, various resistors, and the like. As a characteristic of the sintered body, it has been found that a combination of a specific resistance of 10 Ωcm or less, preferably 0.1 Ωcm or less and a relative density of 90% or more is preferable.

[0008]

SUMMARY OF THE INVENTION The present invention has been made as a result of further testing and study based on the one-step manufacturing method in order to obtain a SiC sintered body having the above-described combination. Its purpose is to provide conductivity suitable for applications such as heating elements, electrode materials, resistors, etc., excellent heat resistance and corrosion resistance, easy resistance control during manufacturing, and relatively inexpensive manufacturing costs. An object of the present invention is to provide a method for producing a SiC sintered body.

[0009]

According to a first aspect of the present invention, there is provided a method for producing a conductive SiC sintered body having a specific resistance of 10 Ωcm or less and a relative density of 90% or more. A mixture of SiC powder having a particle size of 2 μm or less and a sintering aid is formed into a molded body, and the molded body is formed into a mixture of 30 to 30 μm.
2100 in an inert atmosphere containing 90% by volume of nitrogen gas
It is characterized by heating and sintering to a temperature of ２2300 ° C.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, SiC powder having an average particle size of 2 μm or less is used as a SiC raw material. When the average particle size exceeds 2 μm, a sufficient driving force for sintering cannot be obtained, and it becomes difficult to obtain a sintered body having a relative density of 90% or more. Preferably, SiC powder having an average particle size of 1 μm or less is used as a raw material. Further, the N content in the SiC powder is preferably 0.05% by weight or more, and if it is less than 0.05% by weight, it is difficult to obtain a SiC sintered body having a specific resistance of 10 Ωcm or less.

The above-mentioned SiC powder is mixed with a sintering aid powder, and a binder, a plasticizer, a dispersant,
A solvent or the like is added and molded by a method such as extrusion molding, sheet molding by a doctor blade method, cast molding, press molding, etc. to obtain a molded body. As the sintering aid, for example, B, B
B compounds such as ₄ C and BN, and carbon sources such as carbon black and phenolic resin can be used.
Al, Al ₄ C ₃ , Al ₂ O to improve sinterability
_A small amount of an Al compound such as ₃ may be added.

The preferable content of B added as a sintering aid in the SiC sintered body is 0.1 to 0.5% by weight.
By containing B in this range,
An SiC sintered body having high density and low specific resistance can be obtained. B
If the content is less than 0.1% by weight, sufficient sinterability cannot be obtained, and it becomes difficult to obtain a sintered body having a relative density of 90% or more.
If the content exceeds 10 wt%, it becomes difficult to obtain a low-resistance sintered body.

Next, the compact is placed in an inert gas atmosphere containing 30 to 90% by volume of nitrogen gas in an atmosphere of 2100 to 2300%.
Heat to ℃ and sinter. By containing 30 to 90% by volume of nitrogen gas in the sintering atmosphere, sintering of SiC and solid solution of nitrogen in SiC proceed simultaneously, and a dense, low-resistance SiC sintered body can be obtained. As the inert gas, an argon gas, a helium gas, or the like is used.

In the present invention, the nitrogen is brought into contact with the SiC crystal grains from the initial stage of sintering before the SiC crystal grains grow large, so that the nitrogen can be efficiently dissolved by the room temperature sintering method. Thus, the specific resistance of the SiC sintered body can be controlled to be low.

An inert gas atmosphere containing nitrogen gas, for example, a mixed gas atmosphere of argon gas and nitrogen gas, a nitrogen gas in a mixed gas atmosphere of helium gas and nitrogen gas, and an inert gas such as argon gas and helium gas. By adjusting the mixing ratio, the specific resistance value and the sintered density of the SiC sintered body can be controlled. The mixing ratio of the inert gas does not need to be always constant during sintering, but can be adjusted according to the sintering temperature range. It is possible to control the specific resistance value and the sintered density of the sintered body.

If the mixing ratio of the nitrogen gas in the inert gas atmosphere is less than 30% by volume, the nitrogen is not sufficiently dissolved in the SiC sintered body, and it is difficult to obtain a specific resistance of 10 Ωcm or less. If the ratio exceeds 90% by volume, the effect of nitrogen to inhibit the densification of the SiC sintered body increases,
It becomes difficult to obtain a sintered body having a relative density of 90% or more. In order to obtain a sintered body having a specific resistance of 0.1 Ωcm or less, the mixing ratio of the nitrogen gas is preferably set to 50% or more.

When nitrogen (N) forms a solid solution in SiC, N acts as a donor and generates electrons by thermal excitation.
Many of the electrons generated by the solid solution of N recombine with holes generated by the solid solution of B and Al added as sintering aids to cancel out the electric charge, and the remaining excess free electrons cause normal pressure sintering. It is considered that N-type conductivity is given to the obtained SiC.

In the normal-pressure sintering method in which B and C are added as sintering aids, there is generally a peak of sintering shrinkage at around 1950 ° C., but when nitrogen gas is present in the heating atmosphere during sintering, Since the sinterability of SiC decreases and the peak of sintering shrinkage shifts to a higher temperature side, it is necessary to perform sintering at a temperature of 2100 ° C. or higher to obtain a SiC sintered body having a relative density of 90% or higher. is there. When the heating temperature exceeds 2300 ° C., sublimation of SiC starts, and an extreme increase in resistance tends to occur. A more preferred sintering temperature is 2200 ° C. ± 50 ° C. By performing sintering at a temperature in this range, a specific resistance of 10 Ωcm
Hereinafter, a SiC sintered body having a relative density of 90% or more can be obtained most reliably.

In order to improve the sinterability, it is necessary to increase the temperature from room temperature to 150.
It is preferable to raise the temperature to about 0 ° C. in a vacuum atmosphere. By heating in a vacuum, SiO _{2 on the} surface of the SiC particles is effectively removed by reaction with C, and sinterability is improved.

[0020]

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

EXAMPLE 1 0.25% by weight as a sintering aid for 100% by weight of β-SiC powder having a specific surface area of 18 m ² / g, an average particle size of 0.55 μm and an N content of 0.12% by weight. B (boron) powder and 2.0% by weight of a mixed powder of carbon black are added and mixed, and an organic binder, a plasticizer, and a solvent are further added.
After sufficiently kneading with a kneader, it was formed into a rod shape using an extruder equipped with a mold having an outer diameter of 10 mm.

The obtained molded body was subjected to a binder removal treatment by heating at a temperature of 600 ° C. for 3 hours in a nitrogen gas atmosphere, and then sintered in a graphite heater furnace.
The sintering conditions were as follows: after heating in a vacuum at a temperature of 1500 ° C. for 1 hour, a mixed gas of argon gas and nitrogen gas was introduced into the furnace, and the temperature was raised to the temperature shown in Table 1 and sintered. The flow rates of the argon gas and the nitrogen gas were controlled at a predetermined ratio by a mass flow controller, mixed through a static mixer, and then introduced into a sintering furnace.

Table 1 shows the characteristic values of the SiC sintered body obtained by firing. As can be seen in Table 1, S according to the present invention
Each of the iC sintered bodies (Examples 1 to 6) has a specific resistance of 10
Ωcm or less, and the relative density is 90% or more.

[0024]

[Table 1]

COMPARATIVE EXAMPLE 1 Moldings were prepared in the same manner as in Example 1, and these molded bodies were heated and sintered in the same manner as in Example 1 except that the atmosphere of the inert gas was changed. Table 2 shows the characteristic values of the obtained SiC sintered body. As shown in Table 2, the mixture ratio of nitrogen gas was 30%
When the value is less than 1, the specific resistance sharply increased, and a remarkable resistance change depending on the temperature was exhibited. When the mixture ratio of nitrogen gas exceeded 90%, a sintered body having a relative density of 90% or more was not obtained.

[0026]

[Table 2]

[0027]

According to the present invention, suitable specific resistance of a heating element, an electrode material, a resistor, etc. is 10 Ωcm or less, and a relative density is 90%.
A method for producing a conductive SiC sintered body that has the above characteristics, has little variation in specific resistance value during production, easily controls resistance, and has a dense and superior durability.

Claims (1)

[Claims] 1. A molded product obtained by molding a mixture of a SiC powder having an average particle size of 2 μm or less and a sintering aid.
21 in an inert atmosphere containing 0 to 90% by volume of nitrogen gas.
A method for producing a conductive SiC sintered body having a specific resistance of 10 Ωcm or less and a relative density of 90% or more, characterized by heating and sintering to a temperature of 00 to 2300 ° C.