Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-04-29
2001-07-24
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S142000, C313S143000, C313S136000, C313S137000
Reexamination Certificate
active
06265816
ABSTRACT:
RELATED APPLICATION
This application claims the priority of Japanese Patent Application No. 10-121448 filed on Apr. 30, 1998, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug used for the ignition of an internal combustion engine, an insulator used in the spark plug as well as a process for fabricating the insulator.
In recent years, with the trend of higher output power of internal combustion engines used for automobiles and the like, the area occupied by suction and exhaust valves within the combustion chamber has been increasing. On account of this, the spark plug for igniting air-fuel mixture is required to be smaller in size, and besides the temperature in the combustion chamber has a tendency to increase due to turbochargers or other supercharging equipment and the like. Therefore, as insulators for spark plugs, those made of alumina base insulating materials, which are superior in thermal resistance, are widely used. Another reason that alumina base insulators for spark plugs are used is that alumina is superior in voltage endurance characteristics at high temperatures. However, in recent years, because the insulator tends to be thinner in thickness with the aforementioned miniaturization of spark plugs , and insulators more superior in voltage endurance characteristics are demanded.
For example, in recent years, insulators in which the alumina content is increased to 85 wt %, in some cases, 90 to 97 wt % for improvement in voltage endurance characteristics have been used (hereinafter, insulators having such high alumina contents will be referred to as high alumina insulators). However, in the present technical background, effects of the improvement in voltage endurance characteristics have not been achieved so remarkably for the increase in the alumina content. The reason of this could be that in conventional high alumina insulators, materials have not been sufficiently densified due to lack of sintering aid components, or even if densified, minute open voids are remaining in relatively large amounts so that effects of increasing in the alumina content on the voltage endurance characteristics are reduced.
Therefore, in Japanese Patent Laid-Open Publication SHO 63-190753, there has been disclosed an alumina insulator in which fine alumina powder having a mean particle size of approximately 0.1 to 0.5 &mgr;m is used as a raw material, to which at least one of Y
2
O
3
, MgO and La
2
O
3
is blended as a sintering aid, so that the alumina content is raised to approximately 95 wt % with the result that the voltage endurance characteristics can be improved correspondingly. As the reasons of the improvement in the voltage endurance characteristics, the publication describes that the insulator is less prone to initial deterioration by virtue of the formation of a high-melting-point grain boundary phase based on the aforementioned sintering aid components, and that the formation of the grain boundary phase suppresses the growth of alumina crystal grains, making the structure microfine, with the result that grain boundary portions serving as electrical conduction paths are elongated and bypassed.
However, in the insulator of this patent laid-open publication, because the mean particle size of alumina crystal grains is as microfine as 1 &mgr;m or less, there is a tendency that large amounts of residual voids that adversely affect the voltage endurance are involved in the insulator. Further, the publication also describes that the voltage endurance is improved notwithstanding high rates of voids by virtue of the formation of the high-melting-point grain boundary phase. However, it is essentially impossible to completely eliminate the effect of voids, and the upper limit of the content of alumina directly contributing to the improvement in voltage endurance could be around 95 wt % as shown in Examples of the patent laid-open publication. In conclusion, with alumina-richer compositions adopted for further improvement in voltage endurance, the rate of voids would increase more and more, while the high- melting-point grain boundary phase that suppresses the effects of the increased rate of voids, so that satisfactory voltage endurance characteristics could no longer be expected.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug having an insulator more superior in voltage endurance characteristics at high temperatures, as compared with the prior-art materials, as well as an insulator to be used in the spark plug.
In order to achieve the above object, in a first aspect, the present invention provides a spark plug comprising:
a center electrode;
a metallic shell placed outside the center electrode;
a ground electrode which has one end coupled to the metallic shell and which is placed opposite to the center electrode; and
an insulator placed between the center electrode and the metallic shell so as to cover exterior of the center electrode, wherein
the insulator is made of an insulating material which is composed mainly of alumina, and which contains Al component within a range of 95 to 99.7 wt % in Al
2
O
3
-converted weight, and in which an area ratio occupied by alumina base principal-phase particles with particle size not less than 20 &mgr;m is not less than 50% as a cross-sectional structure of the insulator is observed.
In a second aspect, the present invention provides a spark plug comprising:
A spark plug comprising:
a center electrode;
a metallic shell placed outside the center electrode;
a ground electrode which has one end coupled to the metallic shell and which is placed opposite to the center electrode; and
an insulator placed between the center electrode and the metallic shell so as to cover exterior of the center electrode, wherein
the insulator is made of an insulating material which is composed mainly of alumina, which contains Al component within a range of 95 to 99.7 wt % in Al
2
O
3
-converted weight, and in which a mean presence number per mm
2
in a cross section of voids having a size of not less than 10 &mgr;m observed in cross-sectional structure is less than 100.
The present invention also provides an insulator for spark plugs characterized by being formed from an insulating material which is composed mainly of alumina, and which contains Al component within a range of 95 to 99.7 wt % in weight converted to Al
2
O
3
, and in which an area ratio occupied by alumina base principal-phase particles with particle size not less than 20 &mgr;m is not less than 50% as a cross-sectional structure of the insulator is observed. It is noted that the “alumina base principal phase” refers to a phase containing 99.8 wt % or more of Al component in Al
2
O
3
-converted weight.
The other object of the present invention is to provide process for fabricating the insulator for spark plugs of this invention. In a first aspect, the present invention provides a process for fabricating the insulator for spark plugs comprising:
preparing a raw material base powder (PG) by blending alumina powder having a mean particle size of not more than 1 &mgr;m with 0.3 to 5 wt % of additional-element material serving as sintering aids in a ratio relative to a total of the alumina powder and the additional-element material;
molding the raw material base powder (PG) into a specified insulator configuration; and
firing the molded body at a temperature of 1450 to 1700° C., whereby an insulator in which an area ratio occupied by alumina base principal-phase particles with particle size not less than 20 pm is not less than 50% as a cross-sectional structure of the insulator is observed is obtained.
In a second aspect, the present invention provides a Process for fabricating the insulator for spark plugs comprising:
preparing a raw material base powder (PG) by blending alumina powder having a mean particle size of not more than 1 &mgr;m with 0.3 to 5 wt % of additional-element material serving as sintering aids in a ratio relative to a total of the alumina powder and the additional-element material;
Ito Hirohito
Tanabe Hiroyuki
NGK Spark Plug Co. Ltd.
Patel Vip
Quarterman Kevin
Sughrue Mion Zinn Macpeak & Seas, PLLC
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