Electric lamp and discharge devices – Spark plugs – Shaped electrode chamber – insulator end – shell skirt – baffle...
Reexamination Certificate
1999-05-18
2001-08-14
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Shaped electrode chamber, insulator end, shell skirt, baffle...
Reexamination Certificate
active
06274971
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug and to a method of manufacturing the spark plug.
2. Description of Related Art
A spark plug used for providing ignition in an internal combustion engine such as an automobile engine typically includes a metallic shell; an insulator formed from, for example, an alumina-based ceramic and disposed within the metallic shell; and a center electrode disposed within the insulator. A ground electrode is attached to the metallic shell. The insulator axially projects from the rear opening portion of the metallic shell. A terminal metal piece is disposed inside the projecting portion of the insulator. The terminal metal piece is connected to the center electrode via a resistor, conductive glass seal layers formed in a glass sealing step and the like. Upon application of a high voltage via the terminal metal piece, spark discharge is induced at a gap formed between the ground electrode and the center electrode.
However, if the spark plug is subjected to certain conditions simultaneously, such as high plug temperature, high ambient humidity, and the like, application of high voltage cannot successfully induce spark discharge at the gap, and there may occur a so-called “flashover” phenomenon in which spark discharge is induced between the terminal metal piece and the metallic shell such that the spark runs over the surface of the projecting portion of the insulator. Therefore, in order to prevent the flashover phenomenon, most spark plugs for general use have a glaze layer formed on the surface of their insulators. The glaze layer also serves for smoothing the surface of the insulator so as to prevent contamination, and for increasing chemical or mechanical strength.
The glaze layer is formed on an insulator, through application of glaze slurry onto the surface of an insulator and firing (particularly called “glost firing”). In the case of an alumina-based insulator for use in a spark plug, a glaze layer is formed on a fired insulator through subsequent baking at 1000° to 1100° C. In such a case, there has conventionally been used lead-silicate-glass-based glaze which has a lowered softening point due to incorporation of a relatively large amount of PbO to silicate glass. However, this type of glaze involves the following drawbacks:
(1) Since the glaze has a coefficient of linear expansion lower than that of alumina-based insulating material which serves as a substrate, the obtained glaze layer is susceptible to cracks and the like.
(2) Although the glaze contains a considerable amount of PbO, the glost-firing temperature is still high; i.e., 1000° C. or more. In the manufacture of spark plugs, glost firing is often performed concurrently with a glass sealing step so as to reduce the number of manufacturing steps. Therefore, high glost-firing temperature as described above disadvantageously permits accelerated oxidation of a terminal metal piece and a center electrode. A conceivable measure for further lowering the glost-firing temperature is to add alkali metal oxide such as Na
2
O to the glaze. However, if the amount of alkali metal components is increased excessively, insulation performance is lowered, and the spark plug becomes susceptible to flashover.
(3) In recent years, concerns for environmental protection have increased worldwide, and glaze containing Pb has been used less often. For example, the automobile industry, which uses a large number of spark plugs, is considering the complete abolition of spark plugs using glaze containing Pb, in view of the environmental effect of discarded spark plugs.
SUMMARY OF THE INVENTION
According to a first mode of the present invention, there is provided a spark plug comprising a center electrode, a metallic shell, a ground electrode, an insulator, and a glaze layer. The metallic shell is disposed so as to surround the center electrode. One end of the ground electrode is connected to the metallic shell, while the other end of the ground electrode faces the center electrode. The insulator is disposed between the center electrode and the metallic shell such that the insulator covers the outer surface of the center electrode. The glaze layer is formed so as to cover at least a portion of the surface of the insulator. In order to achieve the above-described objects, glaze which forms the glaze layer predominantly comprises, as elements before undergoing oxidization, Si, B, Zn, and Ba, and two elements selected from among Na, K, and Li (hereinafter these two elements are called “co-added alkali metal components”). Specifically, the glaze contains Si in an amount of 18 to 35% by weight as reduced to SiO
2
, B in an amount of 25 to 40% by weight as reduced to B
2
O
3
, Zn in an amount of 10 to 25% by weight as reduced to ZnO, and Ba in an amount of 7 to 20% by weight as reduced to BaO. Also, each of the two co-added alkali metal components is contained in the glaze in an amount of 3 to 9% by weight, as reduced to Na
2
O, K
2
O, or Li
2
O.
When the insulator to be coated with the glaze is formed of, for example, alumina-based insulating material, the difference between the coefficient of linear expansion of the insulator and that of the glaze of the above-described composition used for the spark plug of the first mode is relatively small, so that the glaze layer is less susceptible to cracks and the like. Also, since the amount of the alkali metal components is set relatively high, the glaze has a softening point lower than that of conventional lead-silicate-glass-based glaze. Consequently, the glost-firing temperature can be as low as 800° to 950° C. Therefore, even in the case where glost firing is performed concurrently with the glass sealing step as mentioned above, the center electrode and the below-described terminal metal piece are less susceptible to oxidation.
Further, notwithstanding the high amount of the alkali metal components, excellent insulation performance can be obtained. In this regard, it is important that two different alkali metal components selected from among Na, K, and Li be added concurrently, rather than just one type of alkali metal component being added. That is, the present inventors conducted studies and found the following: In the case where an alkali metal component is added alone, as the amount thereof increases, the conductivity of glaze drastically increases, leading to considerable impairment of insulation performance. However, when two types of alkali metal components are added in combination, conductivity of glaze does not increase greatly, even when the total amount of the added components is considerably increased, so that excellent insulation performance is secured. As a result, the amount of alkali metal components can be increased while permitting minimal reduction in insulation performance, leading to simultaneous attainment of the two goals, i.e., flashover prevention and lowering of glost-firing temperature. Also, the remaining third alkali metal component and other alkali metal components may be added, so long as the conductivity suppression effect of the co-added alkali metal components is not impaired.
The amount of Si contained in the glaze is set within the range of 18 to 35% by weight as reduced to an oxidized form, SiO
2
. When the insulator to be coated with the glaze is formed of alumina-based insulating material, if the Si content is less than 18% by weight, the coefficient of linear expansion of the glaze becomes excessively high, so that the glaze layer becomes more susceptible to defects such as cracks. By contrast, if the Si content is in excess of 35% by weight, the coefficient of linear expansion of the glaze becomes too low, so that the glaze layer becomes more susceptible to defects such as crazing. Preferably, the Si content is set within the range of 25 to 30% by weight.
The B content is set within the range of 25 to 40% by weight as reduced to B
2
O
3
. If the B content is less than 25% by weight, the softening point of the glaze increases, which can lead to failure to attain glost firi
Masuda Hiroaki
Sugimoto Makoto
Brinks Hofer Gilson & Lione
NGK Spark Plug Co. Ltd.
Patel Ashok
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