Alumina-based sintered body insulator for spark plugs

Details Alumina-based sintered body insulator for spark plugs Alumina-based sintered body insulator for spark plugs

2000-11-29

2003-05-06

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Shaped electrode chamber, insulator end, shell skirt, baffle...

C313S118000, C313S141000, C313S142000, C313S130000, C313S137000

Reexamination Certificate

active

06559579

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug to be used as a source for igniting a mixed gas in an internal combustion engine and an insulator to be incorporated in such a spark plug.
2. Description of the Related Art
The insulator for spark plug (hereinafter referred as “insulator”) constituting the spark plug for use in internal combustion engines such as automobile engine is normally formed by an alumina-based sintered body obtained by sintering an alumina (Al
2
O
3
)-based insulation material. This is because alumina ceramics are excellent in heat resistance, mechanical strength, dielectric strength, etc. In particular, the insulator for spark plug is liable to exposure to a heat of from about 500° C. to 700° C. developed by the combustion (about 2,000° C. to 3,000° C.) of a gas ignited by spark discharge in the combustion chamber of internal combustion engine. Thus, it is important that the insulator for spark plug is excellent in dielectric strength over a temperature range of from room temperature to the foregoing high temperature. Such an insulator (alumina-based sintered body) has heretofore been formed by, e.g., a three-component system comprising silicon oxide (SiO
2
) calcium oxide (CaO) and magnesium oxide (MgO) as a sintering aid for the purpose of lowering the required sintering temperature and improving the sinterability.
However, the insulator formed merely by the foregoing three-component system sintering aid is disadvantageous in that the three-component system sintering aid (mainly composed of Si component) is present as a low melting glass phase on boundaries of alumina crystal particles after sintering. Thus, when the insulator is exposed to a heat of around 700° C., the heat effect causes the low-melting glass phase to soften, possibly resulting in the deterioration of dielectric strength of the insulation material. It can be therefore proposed to merely reduce the amount of such a sintering aid to be added during the formation of the insulator for the purpose of reducing the occurrence of low-melting glass phase. However, this approach is disadvantageous in that the densification of insulator cannot proceed. Even if the densification of insulator proceeds apparently, numeral pores remain in boundaries of alumina crystal particles, possibly causing the deterioration of dielectric strength of insulator.
For the purpose of densifying the insulator, JP-A-62-100474 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes that a raw material composition obtained by granulating a raw material powder comprising alumina powder and the foregoing three-component system sintering aid to a predetermined particle diameter be blended with the same raw material composition which has not been granulated to reduce the amount of residual pores present on boundaries of alumina-based sintered body. JP-A-62-143866 proposes that a raw material powder comprising two alumina powders having different particle diameters and the foregoing three-component system sintering aid be sintered to reduce the amount of residual pores present on boundaries of alumina-based sintered body.
For the purpose of improving the dielectric strength of glass phase present on boundaries of alumina crystal particles, JP-B-7-17436 (The term “JP-B” as used herein means an “examined Japanese patent application”), for example, proposes that an alumina-based sintered body be formed by a sintering aid such as Y
2
O
3
, La
2
O
3
and ZrO
2
to reduce the amount of residual pores and raise the melting point of glass phase present on boundaries of alumina crystal particles. Further, Japanese Patent 2564842 proposes that an alumina powder as a main component be blended with an organic metal compound and an aluminum compound to prepare a raw material powder having Y
4
Al
2
O
9
phase uniformly dispersed in uniform alumina crystal particles at triple point so that the dielectric strength of the resulting alumina-based sintered body can be improved.
In recent years, with the enhancement of output of internal combustion engines and the reduction of the size of engines, the inlet valve and exhaust value have occupied more in the combustion chamber and the size of the spark plug has been reduced. Thus, the insulator constituting the spark plug has been required to be thinner and hence have a higher dielectric strength. Under these circumstances, however, even an insulator formed by the alumina-based sintered body according to the foregoing various patents can hardly meet the requirements for dielectric strength at a temperature as high as around 700° C. sufficiently. Accordingly, such an insulator can undergo dielectric breakdown.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug comprising an insulator containing alumina as a main component, which is less liable to occurrence of dielectric breakdown due to the effect of residual pores or low-melting glass phase present on boundaries of alumina-based sintered body constituting the insulation material and exhibits a higher dielectric strength at a temperature as high as around 700° C. than the conventional materials and an insulator for use in such a spark plug.
The insulator for spark plug according to the invention which has been worked out to solve the foregoing problems comprises an alumina-based sintered body comprising Al
2
O
3
(alumina) as a main component and at least one component (hereinafter referred to as “&bgr; component”) selected from the group consisting of Ca (calcium) component, Sr (strontium) component and Ba (barium) component, the alumina-based sintered body having at least partly particles including a compound comprising the &bgr; component and Al (aluminum) component at an Al to &bgr; molar ratio of from 4.5 to 6.7 as calculated in terms of oxides thereof and having a relative density of 90% or more.
It is most noteworthy in the invention that the alumina-based sintered body comprising alumina as a main component comprises at least partly particles of a compound comprising &bgr; component and Al component at a molar ratio (Al
2
O
3
/&bgr;O) of from 4.5 to 6.7 as calculated in terms of oxides thereof.
Since it can be presumed that the foregoing compound comprising specific components at a specific molar ratio is a compound having a high melting point, an insulator for spark plug formed by an alumina-based sintered body with particles made of such a compound present thereon can be provided with an extremely excellent dielectric strength at a temperature as high as around 700° C. as compared with conventional insulators comprising alumina as a main component. Examples of the foregoing compound having a molar ratio (Al
2
O
3
/&bgr;O) of from 4.5 to 6.7 include BaAl
9.2
O
14.8
, (molar ratio: 4.6; &bgr; component: Ba component), and BaAl
13.2
O
20.8
, (molar ratio: 6.6; &bgr; component: Ba component). Alternatively, compounds other than hexaaluminate and analogy thereof may be used.
The term “particles” as used herein is meant to indicate particles other than alumina particles observed on cut area obtained by cutting the insulator. The presence of these particles can be easily confirmed by mirror-polishing the cut surface of the insulator, and then observing the cut surface under SEM. If necessary, the presence of these particles may be confirmed by observing under TEM. Subsequently, these particles can be subjected to EDS analysis to confirm that &bgr; component and Al component are present therein.
Subsequently, the presence of the “compound” contained in the foregoing particles can be confirmed by various measuring methods. By way of example, an insulator which has been confirmed for the presence of particles comprising &bgr; component and Al component by observation under SEM and EDS analysis can be crushed to give a powder which is then subjected to X-ray diffractometry to see if there occurs a spectrum corresponding to the compound having a molar ratio (Al
2
O
3
/&bgr;O) of from 4.5 to 6.7. If there is a spec

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