Spark plug

Details Spark plug Spark plug

2001-06-29

2003-05-20

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

C313S130000, C313S143000, C501S014000, C501S151000, C501S153000

Reexamination Certificate

active

06566792

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass transition or an insufficient insulation resistance. To address this problem, JP-A-11-43351 proposes a leadless glaze composition having an adjusted Zn component to improve glass stability without increasing viscosity, and JP-A-11-106234 discloses a composition of leadless glaze for improving the insulation resistance by effects of joint addition of alkaline component.
The glaze layer for the spark plug not only prevents the insulator surface from adhering of dirt or stain, heightens withstand voltage of creeping discharge to prevent flashover, but also serves to bury defects in the insulator surface which are apt to cause a destruction starting point for increasing strength. However, in recent internal combustion engines remarkable in high output, vibration and impact received by the spark plug during working, so that problems often occur as breakage of the insulator though being formed with the glaze layer. In addition, when attaching the spark plug to a cylinder head (in particular when attaching with power tools such as impact wrench), if adding over tightening torque, the insulator will be broken. Further, since voltage applied to the spark plug is getting higher accompanied with high performance of engines, the glaze has been demanded to have an insulating performance durable against severe circumstances, but compositions of the glaze disclosed in JP-A-11-106234 or JP-A-11-43351 are involved with problems that the glaze compositions compatible in the insulation performance and mechanical properties are not always investigated.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide spark plugs having such glaze layers containing less Pb component, enabling to be baked at relatively low temperatures, having excellent insulating property, easily realizing smooth baked surfaces, and heightening the mechanical strength of the insulator with the glaze layer.
For solving the above problems, the spark plug of the invention has an insulator comprising alumina based ceramic disposed between a center electrode and a metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides, and is characterized
in that the glaze layer comprises
Pb component 1 mol % or less in terms of PbO;
Si component 15 to 60 mol % in terms of SiO
2
;
B component 22 to 50 mol % in terms of B
2
O
3
;
Zn component 10 to 30 mol % in terms of ZnO;
Ba and/or Sr components 0.5 to 35 mol % in terms of BaO or SrO;
F component 1 mol % or less,
Al component 0.1 to 5 mol % in terms of Al
2
O
3
; and
alkaline metal components of 5 to 10 mol % in total of one kind or more of Na, K and Li in terms of Na
2
O, K
2
O, and Li
2
, respectively, where Li is essential, and the amount of the Li component is 1.1 to 6 mol % in terms of Li
2
O.
In the spark plug according to the invention, for aiming at the adaptability to the environmental problems, it is a premise that the glaze to be used contains the Pb component 1.0 mol % or less in terms of PbO (hereafter called the glaze containing the Pb component reduced to this level as “leadless glaze”) When the Pb component is present in the glaze in the form of an ion of lower valency (e.g., Pb
2+
), it is oxidized to an ion of higher valency (e.g., Pb
3+
) by a corona discharge. If this happens, the insulating properties of the glaze layer are reduced, which probably spoils an anti-flashover. From this viewpoint, too, the limited Pb content is beneficial. A preferred Pb content is 0.1 mol % or less. It is most preferred for the glaze to contain substantially no Pb (except a trace amount of lead unavoidably incorporated from raw materials of the glaze).
However, according to an inventor's studies, it was proved that if the amount of Pb component was smaller, a mechanical strength of the glaze layer, in particular impact resistance was apt to relatively decrease. Therefore, it was found that if Si, B, Zn, Ea and/or Sr, and Al components, further alkaline metal component were contained in the above mentioned range, such glaze layers could be provided, enabling to be baked at relatively low temperatures, having excellent insulating property, easily realizing smooth baked surfaces, and heightening the mechanical strength, especially the impact resistance of the insulator formed with the glaze layer, and thus the present invention has been accomplished. Thereby, in case the spark plug is attached to the high output internal combustion engine, the insulator of the spark plug is unlikely to break by such as vibrations during working. Further, if tightening torque somewhat exceeds when attaching the spark plug to the cylinder head (especially when attaching with power tools such as an impact wrench), the insulator is unlikely to break.
In the following, reference will be made to critical meanings of ranges containing respective composing components of the glaze layer in the present spark plug. Si component is a skeleton forming component of the glaze layer of vitreous substance, and is indispensable for securing the insulating property. With respect to the Si component, being less than 15 mol %, it is often difficult to secure a sufficient insulating performance. Being more than 60 mol %, it is often difficult to bake the glaze. The Si containing amount should be more preferably 25 to 40 mol %.
B component is also a skeleton forming component of the glaze layer of vitreous substance, and if combined with Si a skeleton forming component of the glaze layer of vitreous substance, a softening point of the glaze is lowered and fluidity when baking the glaze is improved for easily obtaining smooth baked surfaces. If the B containing amount is less than 20 mol %, the softening point of the glaze goes up, and the baking of the glaze will be difficult. On the other hand, being more

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