Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-05-21
2001-02-13
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S144000, C313S145000
Reexamination Certificate
active
06188166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine, and more particularly to a resistor spark plug including a resistor for preventing generation of radio noise.
2. Description of the Related Art
An existing resistor spark plug has the following structure. A terminal metal piece is inserted into one end of an axially extending through-hole of an insulator and is fixed thereto. A center electrode is inserted into the through-hole from the other end and is fixed thereto. A resistor is disposed within the through-hole to be located between the terminal metal piece and the center electrode. The resistor is formed of a oil mixture of glass and a conductive material such as carbon black or metal. However, since the metal content of the resistor is not very high, in many cases, direct connection with the terminal metal piece or the center electrode is difficult to achieve. Therefore, in general, a conductive glass seal layer formed of a mixture of glass and a relatively large amount of metal is disposed between the resistor and the terminal metal piece and between the resistor and the center electrode in order to increase the bonding strength.
Such a resistor spark plug is manufactured as follows. After a center electrode is inserted into and fixed to a through-hole of an insulator, powder of conductive glass is charged into the through-hole. Subsequently, powder of resistor composition material is charged into the through-hole, and powder of conductive glass is again charged into the through-hole. Finally, a terminal metal piece is press-fitted into the through-hole from an end opposite the center electrode, to thereby obtain an assembled unit. Thus, within the through-hole of the insulator, a layer of conductive glass powder, a layer of resistor composition powder, and another layer of conductive glass powder are successively layered from the side of the center electrode. The assembled unit is placed in a heating furnace to be heated to a temperature above the melting point of glass. Subsequently, the terminal metal piece is pushed toward the center electrode to compress the respective layers, so that the layers become a conductive glass seal layer on the center electrode side, a resistor, and a conductive glass seal layer on the terminal metal piece side. Thus is completed a structure in which the terminal metal piece and the center electrode are connected to the resistor via the respective conductive glass seal layers.
When a resistor spark plug is manufactured in the above-described manner, during the heating/compressing step, a tip end portion of the terminal metal piece is pushed into a layer of conductive glass powder that has been softened through heating, and finally the terminal metal piece is joined to the conductive glass seal layer in a state in which the tip end portion of the terminal metal piece is located in the conductive glass seal layer. In order to obtain a strong joint, it is important that the clearance between the outer circumferential surface of the tip end portion of the terminal metal piece and the inner surface of the through-hole of the insulator is sufficiently filled with the conductive glass seal layer. However, since the clearance is generally small, and the viscosity of softened conductive glass is not very high, the charge of glass is frequently insufficient. In this case, the bonding or bonding strength between the terminal metal piece and the conductive glass seal layer is insufficient, with a resultant possibility of the terminal metal piece coming off upon receipt of an impact. Further, the bonding strength between the terminal metal piece and the conductive glass seal layer easily deteriorates upon repeated application of high voltage to the spark plug.
In order to increase the bonding strength between the terminal metal piece and the conductive glass seal layer to thereby solve the above-described problem, in a generally used spark plug, a thread or knurl is formed on the outer circumferential surface of the tip end of the terminal metal piece, which is to be inserted into the conductive glass seal layer, to thereby increase the bonding strength between the terminal metal piece and the conductive glass seal layer by means of an anchor effect. However, when such a thread or knurl is formed on the outer circumferential surface of the tip end of the terminal metal piece, the charging of conductive glass into the clearance between the terminal metal piece and the insulator becomes more difficult, so that in some cases, the bonding strength, rather than being increased, is decreased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug having a structure which increases the bonding strength between a conductive glass seal layer and a terminal metal piece to thereby prevent occurrence of failures such as coming off a terminal metal piece and deterioration in the bonding strength between the terminal metal piece and the conductive glass seal layer.
To achieve the above object, according to a first aspect of the present invention, there is provided a spark plug comprising a metallic shell having a ground electrode, an insulator disposed within the metallic shell and having an axially extending through-hole, a center electrode disposed within the through-hole of the insulator, a terminal metal piece disposed within the through-hole of the insulator, and a conductive coupling layer disposed within the through-hole and located between the center electrode and the terminal metal piece.
The conductive coupling layer comprises a conductive glass seal layer formed at least on a side in contact with the terminal metal piece. A surface layer region of the terminal metal piece that comes into contact with the conductive glass seal layer is formed of a metal layer mainly made of at least one metal selected from the group consisting of Zn, Sn, Pb, Rh, Pd, Pt, Cu, Au, Sb, and Ag. In the present specification, the name of each element is represented by its symbol.
In the spark plug according to the present invention, since a metal layer of the above-described material is formed on the surface of the terminal metal piece that comes into contact with the conductive glass seal layer, the bonding strength between the terminal metal piece and the conductive glass seal layer is increased. As a result, it becomes possible to prevent problems such as coming off of the terminal metal piece, which would otherwise occur upon application of impact on the spark plug. Further, even when high voltage is repeatedly applied to the spark plug, the bonding strength between the terminal metal piece and the conductive glass seal layer hardly deteriorates. The reason why the bonding strength can be increased through formation of the metal layer is conceivably that the wettability of the terminal metal piece with respect to the glass material portion within the conductive glass seal layer is improved through formation of the metal layer.
The metal content of the conductive glass seal layer may be set to 35 to 70 wt. %. Specifically, the conductive glass seal layer may contain at least either Cu or Fe as a main component. When the metal content is less than 35 wt. %, the conductivity of the layer is poor with a resultant possibility that electrical connection cannot be attained between the terminal metal piece and the conductive glass seal layer. In contrast, when the metal content is in excess of 70 wt. %, the sealing performance may become poor.
The metal layer may be formed through chemical plating such as electroplating or electroless plating. Alternatively, the metal layer may be formed through vapor phase deposition such as vacuum deposition, ion plating, or sputtering.
The metal layer preferably has a thickness of 0.1 &mgr;m or greater. When the thickness is less than 0.1 &mgr;m, in some cases the effect of the metal layer in increasing the bonding strength between the terminal metal piece and the conductive glass seal layer cannot be obtained. More preferably,
Nishikawa Kenichi
Tanaka Yutaka
Day Michael H.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
NGK Spark Plug Co. Ltd.
Santiago Mariceli
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