Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-03-07
2004-09-21
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C219S541000, C219S544000
Reexamination Certificate
active
06794614
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic heater, and more particularly to a ceramic heater applied to a glow plug used, for example, to accelerate startup of a diesel engine or applied to, among others, a heater used to ignite a kerosene fan heater.
2. Description of the Related Art
By virtue of its high strength at room temperature as well as at high temperature and small coefficient of thermal expansion, a silicon nitride ceramic heater is widely used in a glow plug or a like device.
FIG. 7
shows an example of a silicon nitride ceramic heater
72
for use as a glow plug. The ceramic heater
72
is configured such that a turned (U-shaped) heating element (hereinafter also referred to as a heating element)
76
formed of electrically conductive ceramic is embedded in a ceramic substrate
75
formed of silicon nitride ceramic at a portion biased toward a front end
72
a
. Junction wires
78
and
79
, which are formed of a high-melting-point metal, such as tungsten or molybdenum, each have one end connected to a corresponding end portion
76
c
(corresponding leg end portion) of the U-shaped heating element
76
. The remaining end portions of the junction wires
78
and
79
are exposed on the side surface of the ceramic heater
72
in the vicinity of a rear end
72
c
of the ceramic heater
72
, thereby serving as a pair of lead wire connection terminals (hereinafter also referred to as terminals)
81
. A metallization layer (not shown) is formed on the surface of the ceramic substrate
75
in the vicinity of the lead wire connection terminals
81
. Lead wires
15
are jointed to the corresponding terminals
81
by use of an Ag-based active brazing metal. This is a general joint structure for the ceramic heater
72
.
In order to meet demand for a reduction in size, the ceramic heater
72
itself is shortened, with a resultant reduction in the distance between the front end
72
a
and lead wire joints where the lead wires
15
and the lead wire connection terminals
81
are connected. Thus, for the case where the ceramic heater
72
is installed as a glow plug in a subsidiary chamber of an engine, the temperature of the lead wire joints (hereinafter also referred to as joints) was once 200° C. at the highest, but in recent years the lead wire joints have been exposed to a high temperature of 300° C. or higher.
3. Problems to be Solved by the Invention
However, exposure of the joints to such high temperature has raised the following problem. Namely, a problem arises in a conventional joint structure using an Ag-based brazing metal in that the joint between a lead wire and a lead wire connection terminal suffers separation (unjoining), which is considered to be caused by migration.
One measure for coping with the problem is, for example, to impart a high melting point to an Ag-based brazing metal by employing an Ag rich composition so as to enhance heat resistance of lead wire joints. However, since a glow plug is exposed to severe heat cycles in the course of use, in order to ease generation of thermal stress in ceramic caused by a difference in thermal expansion coefficient between ceramic and an Ag-based brazing metal, such a joint structure is desirably configured such that copper, which is easily deformable, is present in the form of a buffer plate at an intermediate portion of a layer of brazing metal (hereinafter also referred to as a brazing metal layer). The joint structure is not compatible with an Ag-rich composition, for the following reason. An Ag-rich composition induces a eutectic reaction between Ag and copper; thus, a buffering effect cannot be expected. Also, use of a nickel buffer plate is not compatible with Ti contained as an activation metal in a brazing metal and thus is not applicable to the joining work. If Ti is contained in a brazing metal, Ti reacts strongly with Ni to form a layer of an intermetallic compound, thereby impairing joining strength.
Further, a technique has been proposed for preventing migration in joining by use of an Au-based brazing metal, which contains a predominant amount of gold (Au). However, this technique fails to meet the demand for reduction in cost. Further, few combinations of an Au-based brazing metal and an activation metal to be contained therein improve wettability in brazing to ceramic. Therefore, joining by use of an Au-based brazing metal is not practicable.
SUMMARY OF THE INVENTION
The prevent invention has been accomplished in view of the above-described problems, and an object of the invention is to provide a joint structure which does not impair joining strength induced by exposure to heat cycles, does not increase cost, and does not cause migration.
The above-described object has been achieved in a first aspect of the invention by providing a ceramic heater comprising a heating element embedded in an insulating ceramic substrate, and a lead wire joined to a lead wire connection terminal (electrode leading-out portion), which is connected to the heating element while electrical continuity is established therebetween, by means of a brazing metal which contains a predominant amount of copper.
A brazing metal which contains a predominant amount of copper exhibits excellent migration resistance and can retard generation of residual stress stemming from the difference in thermal expansion between electrically conductive ceramic and a lead wire, by virtue of copper's easy deformability, thereby exhibiting only slight impairment in joining strength even upon exposure to heat cycles. Therefore, the ceramic heater of the present invention, in which lead wires are joined to lead wire connection terminals by use of such a brazing metal, can assume a joint structure which is free from the occurrence of migration without an increase in cost. As a result, the ceramic heater can assume a joint structure of high durability, heat resistance, and reliability.
In order to utilize such characteristics of copper, in a second aspect of the invention, preferably, the brazing metal contains copper in an amount of not less than 85% by mass. Also, in a third aspect of the invention, preferably, the brazing metal contains Ti or Si as an activation metal to thereby avoid the necessity of forming a metallization layer. Si effectively enhances wettability in brazing to metal or ceramic. However, a brazing metal which contains a large amount of Si suffers low ductility in the course of production thereof. In view of these phenomena, preferably, Si is contained in an amount of 0.1-5% by mass. Ti effectively enhances wettability in brazing to ceramic and contributes most to enhancement of wettability. However, when the Ti content is excessive, a brazing metal layer formed by joining exhibits increased hardness and thus becomes brittle. In view of these phenomena, in a fourth aspect of the invention, preferably, the Ti or Si content of the brazing metal is 0.1-5% by mass.
A fifth aspect of the invention is directed to the ceramic heater as described in any one of the first through fourth aspects, wherein a pad is formed on the lead wire so as to serve as a joining surface to be joined to the lead wire connection terminal, the lead wire being joined to the lead wire connection terminal via the pad. Joining via such a pad is particularly preferred when a lead wire has a circular cross section, since reliability of joining is enhanced. Notably, the pad may be formed of an Fe—Ni alloy plate, an Fe—Ni—Co alloy plate, an Ni plate, or a like plate and welded to an end portion of a lead wire. Alternatively, an end portion of a lead wire may be rolled into a planate or flat shape.
In a sixth aspect of the invention, the thickness of a layer of the brazing metal is 30-400 &mgr;m. This thickness range of the brazing metal layer is suited for reducing residual stress in ceramic by absorbing the difference in thermal expansion between ceramic and a lead wire as observed after joining, by utilizing the of easy plastic deformability of copper. The lower limit of the thickness range is far thicker th
Okinaka Manabu
Taniguchi Masato
Jeffery John A.
NGK Spark Plug Co. Ltd.
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