Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2000-05-03
2001-08-14
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S270000, C219S548000, C219S553000
Reexamination Certificate
active
06274853
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heating resistor, a heating resistor for use in a ceramic heater, and a ceramic heater using the heating resistor. More particularly, the present invention relates to a heating resistor having excellent heat resistance and being useful in applications where oxidation thereof raises no problem; a heating resistor for use in a ceramic heater to be used in special applications requiring heating, such as a water heater and a glow plug of a diesel engine; and a ceramic heater which includes the heating resistor for use in a ceramic heater embedded in a substrate thereof.
2. Description of the Related Art
Conventionally, there has been known a highly heat-resistant sintered silicon nitride compact having in grain boundaries a crystal phase which is composed of, for example, disilicate having a high melting point, or melilite having an even higher melting point. Also, in manufacture of a heating resistor composed of conductive ceramic such as WC, MoSi
2
, or TiN and silicon nitride ceramic, there is obtained a sintered compact which has a glass phase in grain boundaries thereof as a result of the use of a sintering aid such as MgO or an Al
2
O
3
—Y
2
O
3
system, which sintering aid forms a liquid phase at relatively low temperature. Such a sintered compact has been used in various applications.
In a ceramic heater having a heating resistor embedded in a substrate thereof, the surface temperature of the heater becomes considerably high while the ceramic heater is being energized. Particularly, in a high-temperature-type glow plug, the surface temperature rises to about 1400° C., and the temperature of a portion of the interior rises to 1500° C. or higher.
The aforementioned sintered compact having melilite in grain boundaries has excellent mechanical strength at high temperature, but involves disadvantages in that oxidation tends to occur at relatively low temperature and that cracking may arise from volume expansion. In a sintered compact having a crystal phase such as a melilite phase or a disilicate phase, the crystal phase has a high melting point, but, in the case of a certain composition, a vitreous substance of relatively low melting point remains in grain boundaries in a small amount. As a result, even when such a crystal phase is precipitated in a heating resistor which contains a conductive component, the vitreous substance conceivably causes impairment in working durability. Such impairment has not yet been verified.
In manufacture of a heating resistor, when a sintering aid, such as MgO or an Al
2
O
3
—Y
2
O
3
system, which forms a liquid phase at relatively low temperature is used, the sintering aid remains in the form of a glass phase of low melting point in the grain boundaries of silicon nitride ceramic after firing is completed. The remaining glass phase impairs mechanical strength at high temperature and working durability of a heating resistor and a ceramic heater using the heating resistor. Particularly, in the case of a high-temperature-type glow plug, which involves a temperature rise to 1500° C. or higher at a certain portion thereof, a heater may break due to impairment in strength, or a heating resistor may break due to decomposition of the glass phase in grain boundaries caused by a potential difference induced during application of electricity. In order to suppress the decomposition, there has been proposed a method in which nitrogen is introduced into the grain boundaries so as to increase viscosity of the grain boundaries through formation of oxynitride glass. However, the method fails to solve the problem sufficiently.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention, which solves the above-mentioned problems involved in a conventional heating resistor, is to provide a heating resistor which contains a predominant crystal phase of melilite or disilicate in grain boundaries, which is sufficiently densified, and, particularly, a heating resistor which can sufficiently endure use in a particular application where oxidation thereof raises no problem.
Another object of the present invention is to provide a heating resistor for use in a ceramic heater, having a specific composition wherein a crystal phase of a high melting point is established as a predominant phase in grain boundaries to thereby provide excellent mechanical strength at high temperature and sufficient working durability, as well as to provide a ceramic heater having the heating resistor embedded in a substrate thereof.
According to a first aspect of the present invention, there is provided a heating resistor formed of a silicon nitride ceramic containing silicon nitride in a predominant amount, a conductive component, and a rare earth element (RE), wherein the rare earth element is contained in an amount of 1-6 mol % as reduced to RE
2
O
3
; the mole ratio (hereinafter referred to as the SiO
2
/RE
2
O
3
ratio) of residual oxygen to RE
2
O
3
is 1-5, the amount of residual oxygen being expressed on a silicon oxide (SiO
2
) basis and being obtained by subtracting the amount of oxygen contained in the rare earth element oxide from the total amount of oxygen contained in the heating resistor; and a predominant phase of the heating resistor other than the conductive component and the silicon nitride is melilite (RE
2
O
3
.Si
3
N
4
).
When RE
2
O
3
is contained in the silicon nitride ceramic in an amount of less than 1 mol %, the silicon nitride ceramic encounters difficulty in densification. In contrast, when RE
2
O
3
is contained in excess of 6 mol %, even when the SiO
2
/RE
2
O
3
ratio falls within the preferred range, the mechanical strength tends to be impaired. Particularly, in the case of application to a member of a ceramic heater, the durability at high temperature is impaired. That is, when RE
2
O
3
is contained in an amount as specified in the present invention, a sintered compact of high strength and excellent durability can be obtained constantly.
When the SiO
2
/RE
2
O
3
ratio is less than 1, particularly in the case of RE
2
O
3
being contained in a small amount, the strength and working durability are impaired greatly in application to, for example, a ceramic heater. In contrast, when the SiO
2
/RE
2
O
3
ratio is in excess of 5, the durability at high temperature tends to be impaired.
The heating resistor according to the first aspect comprises a predominant phase of melilite as well as the conductive component and silicon nitride. The melilite has a melting point as high as over 1800° C., thus enabling formation of a sintered compact of excellent heat resistance. However, as mentioned above, the melilite is likely to be oxidized at low temperature. Thus, this sintered compact is usefully applied to, for example, a heating member embedded in a ceramic heater and a water heater, since oxidation thereof raises no problem in such applications. Meanwhile, the silicon nitride ceramic constituting the heating resistor may be formed of a ceramic substantially containing silicon nitride only, but may also be formed of a ceramic containing silicon nitride and a Si—Al—O—N ceramic phase, for example.
According to a second aspect of the present invention, there is provided a heating resistor formed of a silicon nitride ceramic containing silicon nitride in a predominant amount, a conductive component, and a rare earth element (RE), wherein the rare earth element oxide is contained in an amount of 1-6 mol % as reduced to RE
2
O
3
; the mole ratio (SiO
2
/RE
2
O
3
ratio) of residual oxygen to RE
2
O
3
is 2-5, the amount of residual oxygen being expressed on a silicon oxide (SiO
2
) basis and being obtained by subtracting the amount of oxygen contained in the rare earth element oxide from the total amount of oxygen contained in the heating resistor; and a predominant phase of the heating resistor other than the conductive component and the silicon nitride is disilicate (RE
2
Si
2
O
7
).
When RE
2
O
3
is contained in the silicon nitride ceramic in an amount of les
Konishi Masahiro
Watanabe Sindo
Hoang Tu Ba
NGK Spark Plug Co. Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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