Electric lamp and discharge devices – Spark plugs – Shaped electrode chamber – insulator end – shell skirt – baffle...
Reexamination Certificate
2001-02-28
2003-07-08
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Spark plugs
Shaped electrode chamber, insulator end, shell skirt, baffle...
Reexamination Certificate
active
06590318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a spark plug and production thereof.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of automobiles, etc. generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, etc. which is disposed inside the metal shell, 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 spark plug terminal (hereinafter “terminal”) is inserted into the projecting part of the insulator and connected to the center electrode via a conductive glass seal layer, which is formed by a glass sealing technique, a resistor, and the like. A high voltage is applied to the terminal 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 a flashover occurs between the terminal and the metal shell, going round the projecting insulator. Primarily for the purpose of avoiding flashovers, most of commonly used spark plugs have a glaze layer on the surface of the insulator. A glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
Lead glazes have been applied to alumina ceramics as an insulator. A lead glaze is silicate glass compounded with a relatively large amount of PbO to have a lowered softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing lead 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 lead grazes in the future, taking into consideration the adverse influencs of waste spark plugs on the environment.
Borosilicate glass- or alkali borosilicate glass-based glazes have been studied as a substitute for the conventional lead glazes, but they have their own disadvantages, such as a high glass transition point or an insufficient insulation resistance. To address this problem, JP-A-11-43351 proposes a leadless glaze composition having an adjusted Zn component content, etc. to improve glass stability without increasing the viscosity (i.e., without reducing flowability) and JP-A-11-106234 discloses a leadless glaze composition which contains a combination of two or more alkali components to improve insulation resistance.
A glaze formed on an insulator of a spark plug is more apt to rise in temperature than on general insulating porcelain because, for one thing, the spark plug is used as fitted into an engine block of a vehicle. Further, in recent years the voltage applied to a spark plug has been increasing with advancing performance of engines. For these, a glaze for this use has been required to have insulation performance withstanding severer conditions of use.
In the light of these circumstances, the glaze composition disclosed in JP-A-11-106234 supra is not always satisfactory in high-temperature insulating performance, particularly the performance as evaluated as a glaze layer formed on an insulator in a spark plug (e.g., anti-flashover properties).
Both the compositions reported in JP-A-11-43351 and JP-A-11-106234, especially the former, have a relatively high Zn component content (10 to 30 mol %). According to the present inventors' researches, it has been revealed that too high a Zn component content tends to make it difficult to obtain a smooth glazed surface. This tendency is conspicuous when firing is carried out in an atmosphere containing much steam as in a gas-firing furnace.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug having a glaze layer on the insulator thereof, wherein the glaze layer has a reduced Pb content, is capable of being fired at a relatively low temperature, exhibits excellent insulation properties, and can have a smooth surface.
The above object is accomplished by the present invention.
The present invention provides, in its first aspect, a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze of the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
25 to 60 mol %, in terms of SiO
2
, of an Si component,
10 to 40 mol %, in terms of B
2
O
3
, of a B component,
0.5 to 9.5 mol %, in terms of ZnO, of a Zn component,
5 to 25 mol %, in terms of BaO, of a Ba component,
the total content of the Si component, the B component, the Zn component, and the Ba component being 60 to 98 mol % in terms of the respective oxides,
the total content of the Zn component and the Ba compound being 9 to 30 mol % in terms of the respective oxides, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na
2
O, K
2
O or Li
2
O.
From the environmental consideration it is a premise of the present invention that the Pb component content in the glaze be 1.0 mol % or less in term of PbO. This premise applies to not only the above-described first aspect but the second and the third aspects of the present invention hereinafter described. A glaze with its Pb component content reduced to this level will hereinafter be referred to as a leadless glaze. When a Pb component is present in a glaze in the form of an ion of lower valency (e.g., Pb
2+
), it can be oxidized to an ion of higher valency (e.g., Pb
3+
) by a corona discharge from the glaze layer surface, etc. If this happens, the insulating properties of the glaze layer are reduced, which can result in a flashover phenomenon. From this viewpoint, too, the limited Pb content is beneficial. A preferred Pb content is 0.1 mol % or less. It is the most preferred for the glaze to contain substantially no Pb, except a trace amount of lead unavoiodably incorporated together with raw materials.
The glaze used in the first aspect of the present invention has a specifically designed composition for securing insulating properties, optimizing the firing temperature, and improving the firing finish while reducing the Pb content. A Pb component in a conventional glaze has played an important role in softening point adjustment. That is, a Pb component serves to lower the softening point of a glaze moderately to secure flowability in application. In a leadless glaze, it is a B component (B
2
O
3
) and an alkali metal component that take part in softening point adjustment. The present inventors have proved that there is a specific range for a B component content that is suited to improve firing finish (specifically a range of from 10 to 40 mol % in terms of B
2
O
3
). Particularly when firing is carried out in an atmosphere containing relatively much steam as in a gas-firing furnace, it is very advantageous to limit the B component content within this range.
To limit the total content of the alkali metal component as well as the B component content is effective in facilitating formation of a glaze layer with a uniform thickness and few defects such as seeds or bubbles. For example, in preparing a glaze slurry of a mixed powder, the alkali metal component and the B component, if present in excessive amounts, dissolve in the dispersing medium such as water to increase the viscosity of the slurry. If the visocisty of the slurry extremely increases (e.g., exceeds 1000 mPa·s), it is difficult to form a coating layer having a uniform thickness, and the possibility of air bubbles entrapment increases. A proper selection of the alkali metal component content and the B component content within the respective specific ranges
Nishikawa Kenichi
Sugimoto Makoto
Dong Dalei
Morgan & Lewis & Bockius, LLP
NGK Spark Plug Co. Ltd.
O'Shea Sandra
LandOfFree
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