Spark plug

Details Spark plug Spark plug

2002-06-25

2004-07-20

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

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

C313S118000, C313S130000, C252S519500, C501S014000

Reexamination Certificate

active

06765340

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a spark plug.
BACKGROUND OF THE INVENTION
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 projection 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 discharged 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 dilatometric 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 wasted 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 viscosity or an insufficient insulation resistance. In particular, in the case of the glaze for spark plugs, since being served together with engines, it more easily increases temperature than ordinary insulating porcelains (maximum: around 200° C.), and recently being accompanies with high performance of engines, voltage to be supplied to the spark plug has been high, and the glaze has been demanded to have the insulating performance durable against more severer. Actually, for restraining the flashover under a condition of increasing temperature, such a glaze is necessary which is more excellent in the insulating property under the condition of increasing temperature.
SUMMARY OF THE INVENTION
In the existing leadless glaze for spark plugs, for checking a melting point from going up effected by removing a lead component, an alkaline metal component has been mixed. The alkaline metal component is effective for securing fluidity when baking the glaze. However, the more the content of the alkaline metal component, the lower the insulating resistance of the glaze, and an anti-flashover property is easily spoiled. Therefore, the alkaline metal component in the glaze should be limited to a necessary minimum for increasing the insulating property.
So, the existing leadless glaze has inevitably wanted the content of the alkaline metal, a vitreous viscosity is easy to increase at high temperature (when melting the glaze) in comparison with a Pb-glaze, and after baking the glaze, there easily appear pinholes or glaze crimping.
It is an object of the invention to offer such a spark plug which contains a smaller Pb component, is excellent in the fluidity when baking the glaze, high in the insulating resistance, and good in the anti-flashover.
The spark plug according to the invention has a structure having an alumina ceramic insulator 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 of oxide being a main.
The glaze layer comprises
Pb component 1 mol % or less in terms of PbO;
Si component 40 to 60 mol % in terms of SiO
2
;
B component 20 to 40 mol % in terms of B
2
O
3
;
Zn component 0.5 to 25 mol % in terms of ZnO;
Ba and/or Sr components 0.5 to 15 mol % in terms of BaO or SrO in total;
the glaze layer comprises Zn component and Ba and/or Sr components 8 to 30 mol % in total in terms of ZnO, BaO or SrO, respectively,
alkaline metal components of 2 to 12 mol % in total of one kind or more of Na in terms Na
2
O, K in terms of K
2
O and Li in terms of Li
2
O, K being essential, respectively; and
one kind or more (hereinafter referred to as “necessary fluidity improving components) selected from Bi, Sb and rare earth elements RE (selected from a group of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu) of 0.1 to 5 mol % in total of Bi in terms of Bi
2
O
3
, Sb in terms of Sb
2
O
5
, as to RE, Ce in terms of CeO
2
, Pr in terms of Pr
7
O
11
, and others in terms of RE
2
O
3
.
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 layer 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 as mentioned above 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).
While lowering the Pb content as mentioned above, the invention selects the above mentioned particular compositions for providing the insulating performance, optimizing the glaze baking temperature and securing a good glaze-baked finish. In the existing glaze, the Pb component plays an important part as to adjustment of the dilatometric softening point (practically, appropriately lowering the dilatometric softening point of the glaze and securing the fluidity when baking the glaze) but in the leadless glaze, the B component (B
2
O
3
) and the alkaline metal have a deep relation with adjustment of the dilatometric softening point. Inventors found that the B component has a particularly convenient range for improving the glaze baking finish in relation with the content of the Si component, and if the necessary fluidity improving component is contained in the above mentioned range, the fluidity when baking the glaze may be secured, and in turn the baking of the glaze is possible at relatively low temperatures, the glaze layer having an excellent and smooth baked surface is available, and they completed this invention.
Each of these necessary fluidity improving components has effects of heightening the fluidity when baking the glaze, controlling the bubble forming in the glaze layer, or wrapping adhered substances to the glaze baked surface to prevent abnormal projections. Sb and Bi are especially remarkable in these effects (Bi has possibility to be designated as a limited substance in a future). The improvement of the fluidity when baking the glaze is more remarkable by combining two kinds or more of these fluidity improving components. Since the rare earth component comparatively takes cost for separation and refinement, use of non-separating rare earth elements (in this case, those are the composition particular to raw ores and a plurality of kinds of rare earth elements are mixed) is advantageous for saving cost. If

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