Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-05-01
2003-11-25
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A, C313S141000, C313S143000
Reexamination Certificate
active
06653601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic heater to be used in a glow plug for preheating a diesel engine or in a like device, to a method for manufacturing the same, and to a glow plug using the same.
2. Description of the Related Art
A conventionally known ceramic heater for the above-mentioned applications is configured such that a resistance-heating member formed of an electrically conductive ceramic is embedded in an insulating ceramic substrate. In such a ceramic heater, electricity is supplied to the resistance-heating member via metallic leads formed of tungsten or a like metal. However, use of the metallic leads involves a corresponding increase in the number of components, possibly resulting in an increase in the number of manufacturing steps and thus an increase in cost. In order to cope with the problem, Japanese Patent No. 3044632 discloses an all-ceramic-type heater structure in which (1) a first resistor portion serves as a major resistance-heating portion, and (2) a second resistor portion, which is formed of an electrically conductive ceramic having electrical resistivity lower than that used to form the first resistor portion, serves as an electricity conduction path to the first resistor portion. This structure eliminates the use of metallic leads.
The integration of resistor portions that have different electrical resistivities facilitates implementation of a ceramic heater having a so-called self-saturation-type heat generation characteristic. These ceramic heaters function in the following manner. At an initial stage of electricity supply, a large current flows to the first resistor portion via the second resistor portion, thereby promptly increasing the temperature. And when the temperature rises to be near a target temperature, the current flow is controlled by means of an increase in electric resistance of the second resistor portion. Japanese Patent Application Laid-Open (kokai) No. 2000-130754 also discloses this effect as well as a ceramic heater structure in which electricity is supplied, via metallic leads, to a ceramic resistor configured such that two resistor portions of different electrical resistivities are joined together.
In the above-described ceramic heaters, a joint interface between ceramic resistors formed of different materials is inevitably formed. Usually, electrically conductive ceramics that have different electrical resistivities also have considerably different coefficients of linear expansion. Accordingly, in an application involving frequent repetition of temperature rise and cooling (as in the case of a glow plug), thermal stress induced by the above-mentioned difference in coefficient of linear expansion tends to concentrate at the joint interface between the different resistor portions. Particularly, in the case of the structure disclosed in Japanese Patent No. 3044632, in which the resistor portions are joined via a flat interface that is perpendicular to the axis, the area of the joint interface is small, and thus the above-described stress concentration is likely to fracture the resistor along the joint interface. To cope with this drawback, Japanese Patent Application Laid-Open (kokai) No. 2000-130754 proposes a structure in which a circular recess is formed on an end part of the first resistor portion, and a protrusion is formed on an end part of the second resistor portion so as to be fitted into the recess, thereby increasing the area of the joint interface and thus enhancing the strength of the joint.
Although the conventional ceramic heaters are generally thought to be acceptable, they are not without shortcomings. These shortcomings include the following. (1) Since the protrusion and the recess must be formed independently on the corresponding joint interfaces, when the resistor is to be formed through injection molding and firing, the two resistor portions must be formed independently of each other by use of completely different molds, potentially resulting in an increase in the number of manufacturing steps and mold cost. Moreover, a mold for forming the resistor portion on which the recess is to be formed must be combined with a core for forming the recess which can move toward and away from the mold; therefore, the mold is likely to become expensive.
(2) The conventionally-configured ceramic resistor generates heat such that temperature is high at a front end part of the first resistor portion and drops rearward along the axial direction. Thus, a steep temperature gradient is likely to be developed along the axial direction (the joining direction) between the first resistor portion, which generates a relatively high amount of heat, and the second resistor portion, which is at a relatively low temperature. In the ceramic heater disclosed in the above-mentioned publication, the cross-sectional ratio between the first resistor portion and the second resistor portion, which are formed from different kinds of ceramic, changes abruptly in a stepwise fashion at a joint where the protrusion and the recess are engaged. Therefore, when the above-mentioned temperature gradient arises, the effect of alleviating thermal stress concentration at the joint cannot be expected to be strong.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a ceramic heater that can be manufactured at low cost. The ceramic heater has a ceramic resistor in the form of a joined body consisting of different kinds of resistor portions. A second object of the present invention is to provide a ceramic heater in which a joint portion between different kinds of resistor portions exhibits excellent strength and durability. The present invention also provides a glow plug using such a ceramic heater.
A ceramic heater of the present invention includes a rodlike heater body which is configured such that a ceramic resistor formed of an electrically conductive ceramic is embedded in a ceramic substrate formed of an insulating ceramic. The heater body is also configured such that the ceramic resistor comprises a first resistor portion, which is disposed at a front end portion of the heater body and formed of a first electrically conductive ceramic, and a pair of second resistor portions, which are disposed on the rear side of the first resistor portion in such a manner as to extend along the direction of the axis of the heater body, whose front end parts are joined to corresponding end parts of the first resistor portion as viewed along the direction of electricity supply. The second resistor portions are formed of a second electrically conductive ceramic having an electrical resistivity that is lower than that of the first electrically conductive ceramic. The ceramic resistor assumes the form of a joined body consisting of resistor portions of different resistivities, for a reason similar to that described previously in relation to the conventional ceramic heaters.
To achieve the above-described first object, a first configuration of a ceramic heater according to the present invention includes at least a portion of a joint interface between the first resistor portion and the second resistor portion being deviated from a plane perpendicularly intersecting the axis of the heater body, and the joint interface is formed of a plane, a curved surface, or a combination thereof perpendicularly intersecting a reference plane defined as a plane including the axis of the heater body and the axis of the second resistor portion.
Since at least a portion of the joint interface between the resistor portions deviates from a plane perpendicularly intersecting the axis of the heater body, the area of the joint is increased as compared with the case where the joint interface assumes a simple plane perpendicularly intersecting the axis of the heater body, thereby enhancing the joining strength of the two resistor portions. With a plane including the axes of the second resistor portions being defined as a reference plane, the joint interface is formed of a plane, a curved surface, or a combination
Hotta Nobuyuki
Sato Haruhiko
Taniguchi Masato
Dahbour Fadi H.
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
Walberg Teresa
LandOfFree
Ceramic heater, glow plug using the same, and method for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ceramic heater, glow plug using the same, and method for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ceramic heater, glow plug using the same, and method for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3153735