Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2001-03-21
2004-06-01
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C219S544000, C123S14500A
Reexamination Certificate
active
06744016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic heater for use in, for example, a glow plug, as well as to a method for manufacturing the ceramic heater.
2. Description of the Related Art
A conventional ceramic heater for use in, for example, a ceramic glow plug includes an insulating ceramic substrate and a resistance heating member embedded in the ceramic substrate and formed of, for example, a conductive ceramic material. Because of excellent thermal shock resistance and high temperature strength, silicon nitride ceramic is a popular material for the ceramic substrate.
As in the case of manufacture of many other ceramics, manufacture of silicon nitride ceramic employs a sintering aid. A sintering aid melts into a liquid phase during firing to thereby accelerate densification of a ceramic sintered body to be obtained, and plays a role in forming a grain boundary phase for bonding silicon nitride main phase (grains) of the ceramic sintered body. Sintering aids which are commonly used in manufacturing silicon nitride ceramic include magnesium oxide (MgO) and a combination of alumina (Al
2
O
3
) and yttria (Y
2
O
3
). However, these sintering aids involve a drawback in that a vitric grain boundary phase, whose softening point is low, tends to be formed in firing, and thus high temperature strength, particularly strength at 1200° C. or higher, of an obtained sintered body tends to be impaired. When a rare-earth metal oxide and silica are added as sintering aids, the grain boundary phase can be crystallized to thereby contribute to enhancement of high temperature strength of a sintered body. However, since a liquid phase generated during firing exhibits poor fluidity, the sintering speed lowers. As a result, nonuniform sintering tends to occur, and thus variations in room temperature strength tend to occur among sintered bodies.
In order to cope with the above problems, Japanese Patent No. 2735721 discloses a silicon nitride ceramic heater whose manufacture employs a rare-earth metal oxide and alumina as sintering aids. Conceivably, addition of Al
2
O
3
appropriately improves the sintering speed and increases the strength of the grain boundary phase.
However, in the technique disclosed in the above publication, since Al
2
O
3
is added uniformly to the entirety of the ceramic sintered body, the softening point of the grain boundary phase tends to lower. Thus, high temperature strength unavoidably lowers. Also, since an Al component is contained in the form of Al
2
O
3
, the grain boundary phase tends to be vitrified, causing an adverse effect on the intended improvement of high temperature strength.
SUMMARY OF THE INVENTION
The present invention provides a ceramic heater comprising a silicon nitride ceramic substrate (hereinafter referred to as either a ceramic substrate or a substrate) and a resistance heating member embedded in the silicon nitride ceramic substrate. An Al-thickened layer is formed in a surface layer portion of the silicon nitride ceramic substrate. The Al-thickened layer has an Al concentration higher than that of an internal layer portion of the silicon nitride ceramic substrate.
According to the above configuration, the Al-thickened layer is formed merely in a surface layer portion of the ceramic substrate. Thus, even when the Al component causes lowering of the softening point of the grain boundary phase, the influence is limited to the surface layer portion of the ceramic substrate. Therefore, the high temperature strength of the ceramic substrate is unlikely to be impaired. Formation of the Al-thickened layer suppresses growth of silicon nitride main phase (hereinafter, referred to as may be called merely main phase) grains in the surface layer portion of the ceramic substrate, so that abnormally grown grains which provide a starting point of fracture are hardly produced. Therefore, the Al-thickened layer prevents variations in strength, particularly room temperature strength, among ceramic substrates.
Preferably, the thickness of the Al-thickened layer is 50 &mgr;m to 1000 &mgr;m. When the thickness is less than 50 &mgr;m, variations in room temperature strength among silicon nitride ceramic substrates may fail to be effectively suppressed. When the thickness is in excess of 1000 &mgr;m, sufficient high temperature strength may fail to be imparted to the ceramic substrate. More preferably, the thickness is 50 &mgr;m to 500 &mgr;m. Preferably, the Al-thickened layer assumes an average Al concentration of 0.1% to 5% by weight. When the Al concentration is less than 1% by weight, growth of main phase grains in the surface layer portion of the ceramic substrate may fail to be effectively suppressed, potentially causing variations in room temperature strength among ceramic substrates. When the Al concentration is in excess of 5% by weight, the high temperature strength of the Al-thickened layer itself may be impaired, potentially failing to attain enhancement of strength of the ceramic substrate intended by means of the Al-thickened layer.
When growth of main phase grains in the Al-thickened layer is effectively suppressed, the average grain size of the silicon nitride main phase in the Al-thickened layer becomes smaller than that of the silicon nitride main phase in an internal layer portion of the ceramic substrate. Preferably, the average grain size of the silicon nitride main phase in the Al-thickened layer is 0.1 &mgr;m to 1 &mgr;m, and that of the silicon nitride main phase in the internal layer portion is 0.2 &mgr;m to 5 &mgr;m. In either case, when the average grain size is below the lower limit, preparation of material powder for attainment of the average grain size becomes very difficult. When the average grain size is in excess of the upper limit, the strength of the ceramic substrate may become insufficient In order to suppress formation of a starting point of fracture, the maximum grain size of the main phase in the Al-thickened layer is preferably not greater than 10 &mgr;m. Herein, the grain size is defined as follows. Various pairs of parallel lines are drawn tangent to the contour of a crystal grain observed on a sectional microstructure of the ceramic substrate, in such a manner as not to traverse the crystal grain. The distance between the parallel lines of each pair is measured. The maximum distance is defined as the grain size of the crystal grain.
The silicon nitride ceramic substrate assumes, for example, a microstructure such that Si
3
N
4
grains are bonded by means of a grain boundary phase (bonding phase) derived from a sintering aid component, which will be described later. Preferably, the main phase is predominantly composed of an Si
3
N
4
phase which contains &bgr;-Si
3
N
4
in an amount of not less than 70% by volume (preferably, not less than 90% by volume). In this case, the Si
3
N
4
phase may be such that a portion of Si or N atoms may be replaced with Al or oxygen atoms and such that metallic atoms of, for example, Li, Ca, Mg, or Y, are incorporated into the phase in the form of solid solution. Examples of such a phase construction are sialon expressed by the following formulas.
&bgr;-sialon: Si
6
-Si
6
Al
z
O
z
N
8-z
(z=0 to 4.2) &agr;-sialon: M
x
(Si,Al)
12
(O,N)
16
(x=0 to 2)
where M represents Li, Mg, Ca, Y, or R (a rare-earth element other than La and Ce). Herein, unless otherwise specified, the term “predominant” or “predominantly” used in relation to content means that the content of a substance in question is contained in an amount of not less than 50% by weight.
Preferably, a predominate portion of the Al component in the ceramic substrate is present in the form of an inorganic compound other than Al
2
O
3
. Specifically, it is preferable that the Al component be integrated into the main phase through replacing a silicon component as mentioned above and that the Al component present in the grain boundary phase assume the form of nitride or oxynitride or the form of composite nitride, composite oxide, or composite oxynitride with another sintering aid
Konishi Masahiro
Watanabe Shindo
Brinks Hofer Gilson & Lione
Jeffery John A.
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