Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-03-08
2003-12-02
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A, C501S097200
Reexamination Certificate
active
06657166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon nitride sintered material and to a production process thereof, and more particularly to a silicon nitride sintered material which exhibits excellent mechanical characteristics and anti-corrosion property, which has a high thermal expansion coefficient, and which has high heat resistance so as to make the sintered material suitable for use as an insulating material such as a base material used in a ceramic glow plugs, as well as to a production process thereof.
2. Description of the Related Art
Silicon nitride sintered material, having excellent mechanical characteristics and heat resistance, has been employed as an insulating material for use in ceramic heaters in which a resistance heater is embedded or for use in similar products. In this case, increase in the weight of silicon nitride sintered material through oxidation is desired as low as possible, so that the sintered material attains excellent anti-corrosion property. When silicon nitride sintered material is used as an insulating material, a problem arises in that cracks are possibly generated in the insulating material during application or generation of heat, because silicon nitride has a thermal expansion coefficient lower than that of tungsten, tungsten carbide, molybdenum silicide, or a similar substance that is generally employed as a resistance heater (i.e. a resistance heating element) embedded in the insulating material. Therefore, in order to prevent generation of cracks, the thermal expansion coefficient of the insulating material must be substantially as large as that of the resistance heater. Thus, when the silicon nitride sintered material is used as an insulating material for ceramic heaters including a glow plug, the sintered material must have both excellent anti-corrosion property and a high thermal expansion coefficient.
In order to increase the thermal expansion coefficient of the insulating material, particles of high thermal expansion coefficient compounds such as rare earth element compounds, metallic carbides, metallic nitrides, metallic silicides, etc., having a thermal expansion coefficient higher than that of the silicon nitride have been conventionally incorporated into a raw material powder of silicon nitride, and have been dispersed therein. Typically, such a high thermal expansion coefficient compound incorporated into the silicon nitride sintered material is in an amount of a few % to about 30% by volume.
However, incorporation of a rare earth element compound that has a higher thermal expansion coefficient than a silicon nitride deteriorates the anti-corrosion property of the sintered material, particularly anti-corrosion property at about 1,000° C., because formation of a crystalline phase having oxy-nitride, such as an H (RE
20
Si
12
N
4
O
48
) phase, a J (RE
4
Si
2
N
2
O
7
) phase, or an M (RE
2
Si
3
N
4
O
3
) phase (RE: rare earth element) is formed during firing. Therefore, use of the sintered material as an insulating material for ceramic heaters is problematic. Thus, in order to prevent deterioration of the anti-corrosion property of the sintered material, conventionally, studies have been carried out on the compositions and particle sizes of silicon nitride raw material and sintering aids. However, obtaining silicon nitride sintered material having a high thermal expansion coefficient while maintaining excellent anti-corrosion property has remained difficult so far.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a silicon nitride sintered material which exhibits excellent mechanical characteristics and anti-corrosion property, which has a high thermal expansion coefficient and which has high heat resistance so as to make the silicon nitride sintered material suitable for use as an insulating material such as a base material for ceramic glow plugs, as well as a production process thereof.
The present inventors have performed studies on the relation of components of silicon nitride sintered material and amounts thereof vs. insulating property and thermal expansion coefficient thereof; and have found that when the amount of a rare earth element as reduced to a certain oxide thereof, the element being contained in the sintered material, is determined so as to fall within a specific range, and when the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is determined so as to fall within a specific range, the silicon nitride sintered material has a high thermal expansion coefficient, and exhibits excellent anti-corrosion property and mechanical characteristics. The present invention has been accomplished on the basis of this finding.
The present invention provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 5.7-10.3 mol %, and the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is at least 0.50 and less than 0.70.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %, and the amount of the silicon carbide is 0.8-3 mass %.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 5.7-10.3 mol %, and a crystalline phase of the sintered material contains no J phase.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is at least 0.50 and less than 0.70, and a crystalline phase of the sintered material contains no J phase.
The present invention also provides a silicon nitride sintered material produced by firing a raw material powder mixture containing silicon nitride powder, powder of a rare earth element compound, powder of a compound of any of Group 4a through 6a elements, and silicon carbide powder, wherein the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, and the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %.
The present invention also provides a process for producing a silicon nitride sintered material, characterized by preparing a raw material powder mixture by mixing silicon nitride powder, powder of a rare earth element compound, powder of a compound of any of Group 4a through 6a elements, and silicon carbide powder, such that the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, and the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %; and firing the raw material powder mixture.
In the silicon nitride sintered material of the present invention, examples of the a
Funaki Kouji
Ito Masaya
Matsubara Katsura
Watanabe Hiroki
Jeffery John A.
NGK Spark Plug Co. Ltd.
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