Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-01-16
2003-05-13
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A, C501S097400
Reexamination Certificate
active
06563089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon nitride-tungsten carbide composite sintered material and to a process for producing the sintered material; and more particularly to a silicon nitride-tungsten carbide composite sintered material exhibiting well-balanced properties in terms of specific resistance and thermal expansion coefficient while maintaining specific resistance at a low level, and to a process for producing the sintered material. The silicon nitride-tungsten carbide composite sintered material of the present invention is used as, for example, a material for a heater of a glow plug.
2. Description of the Related Art
Conventionally, silicon nitride-tungsten carbide composite sintered material is used as, for example, a material for a heater of a glow plug, since electrical resistance of the sintered material can be controlled easily. In order to attain intended characteristics through control of electrical resistance, for example, as disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-64346, the amount of added tungsten carbide, which serves as a conductive substance, in a sintered material, and the amount of added silicon nitride, which serves as an insulating substance, in the sintered material are regulated.
A heating element of a glow plug has a structure including a heater which is formed from silicon nitride-tungsten carbide composite sintered material, a silicon nitride ceramic support section for embedding the heater therein, and a lead wire section formed from a high-melting-point metal. An important factor for the formation of this element structure is balancing in thermal expansion coefficient between the materials of the heater, the support, and the lead wire. Great mismatch between the thermal expansion coefficients of the materials raises problems, including generation of cracking during firing, lowering of strength of the resultant sintered element, and deterioration of durability of the element under application of electricity. Therefore, regulation of the thermal expansion coefficient of the material of the heater is also important.
The thermal expansion coefficient of the material of the heater is greatly affected by the incorporation amount of conductive tungsten carbide. Therefore, depending on the incorporation amount of tungsten carbide, the thermal expansion coefficient and specific resistance of the material are determined substantially uniquely. However, in order to reduce power consumption of the heater, the specific resistance must be reduced for a given thermal expansion coefficient.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a silicon nitride-tungsten carbide composite sintered material exhibiting well-balanced properties in terms of specific resistance and thermal expansion coefficient while maintaining specific resistance at a low level; and a process for producing the sintered material.
In view of the foregoing, the present inventors have performed extensive studies on developing a silicon nitride-tungsten carbide composite sintered material exhibiting well-balanced properties in terms of specific resistance and thermal expansion coefficient while maintaining specific resistance at a low level, as well as on a process for producing the sintered material; and have accomplished the present invention on the basis of the studies.
The present invention provides a silicon nitride-tungsten carbide composite sintered material comprising silicon nitride and tungsten carbide, characterized in that, in an arbitrary cross-section of the sintered material, the ratio of the area of a tungsten carbide portion to that of the entirety of the cross-section is 20-30%; and a tungsten carbide aggregation portion having a longitudinal length of at least 5 &mgr;m is present in the arbitrary cross-section.
No particular limitation is imposed on the silicon nitride incorporated into the silicon nitride-tungsten carbide composite sintered material of the present invention, and the silicon nitride may be of two or more grain types of different sizes. When the total amount of silicon nitride and tungsten carbide is 100 mass %, the incorporation amount of silicon nitride is preferably 33-38 mass %. When the amount of silicon nitride is less than 33 mass %, the amount of tungsten carbide becomes large, resulting in an increase in thermal expansion coefficient, whereas when the amount of silicon nitride exceeds 38 mass %, the specific resistance of the composite sintered material increases, which is not preferable.
The silicon nitride-tungsten carbide composite sintered material of the present invention may contain an oxide of a rare earth element. The “rare earth element” which constitutes the oxide is one or more elements selected from among, for example, Y, Sc, La, Ce, Pr, Nd, Gd, Tb, Dy, Er, and Yb.
Intergrain regions of the silicon nitride-tungsten carbide composite sintered material of the present invention may contain a crystalline phase in addition to an amorphous phase of, for example, rare earth elements, silicon, oxygen, and/or nitrogen. When such a crystalline phase is present in the intergrain regions, softening of grain boundary phase at high temperature is prevented, and mechanical characteristics of the sintered material at high temperature can be enhanced. Examples of the crystalline phase include RE
2
Si
2
O
7
and RE
2
SiO
5
(RE=rare earth element). The crystalline phase may contain one or more of such species.
In an arbitrary cross-section of the aforementioned composite sintered material, the ratio of the area of a tungsten carbide portion to that of the entirety of the cross-section is obtained as described below. Specifically, the composite sintered material is cut along a hot pressing direction so as to obtain an arbitrary cross-section (i.e., a cross-section to be observed), and the cross-section is subjected to, for example, surface grinding and mirror polishing. A certain region of the resultant cross-section is observed at a magnification of 2,000, and the ratio of the area of a region at which W detection sensitivity is high to the area of the entirety of the observed region is calculated by use of an electron probe micro-analyzer.
The ratio of the area of the tungsten carbide portion is preferably 25-28%. When the ratio of the area is less than 20%, specific resistance increases, whereas when the ratio of the area exceeds 30%, specific resistance decreases, which is not preferable.
The longitudinal length of a tungsten carbide aggregation portion in the aforementioned arbitrary cross-section is obtained by measuring the longitudinal length of a portion in which tungsten carbide grains are linked together, on the basis of the reflection electron image of the cross-section as taken by use of a scanning electron microscope.
The aforementioned longitudinal length is preferably 8 &mgr;m or more. When the length is less than 5 &mgr;m, specific resistance increases, which is not preferable.
The silicon nitride-tungsten carbide composite sintered material of the present invention has a specific resistance of 10,000 &mgr;&OHgr;·cm or less, preferably 8,000 &mgr;&OHgr;·cm or less, more preferably 6,000 &mgr;&OHgr;·cm or less, and has a thermal expansion coefficient of 3.6-4.2 ppm/° C., preferably 3.7-4.2 ppm/° C., more preferably 3.7-4.1 ppm/° C., between room temperature and 1,000° C.
When the aforementioned specific resistance exceeds 10,000 &mgr;&OHgr;·cm, conductivity is lowered, and the voltage required for heat generation increases, which is not preferable. When the aforementioned thermal expansion coefficient is less than 3.6 ppm/° C., the difference between the thermal expansion coefficient of the sintered material and that of a lead wire formed from a high-melting-point metal (4.8 ppm/° C.) becomes large, whereas when the thermal expansion coefficient exceeds 4.2 ppm/° C., the difference between the thermal expansion coefficient of the sintered material and that of insulating ceramic (3.6-3.8 ppm/° C.) becomes large, resulting in
Ito Masaya
Matsubara Katsura
Watanabe Hiroki
Jeffery John A.
NGK Spark Plug Co. Ltd.
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