Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2001-01-11
2003-04-15
Paik, Sang (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C501S089000
Reexamination Certificate
active
06548787
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic heater having a heating element formed on a ceramic substrate (hereinafter simply referred to as a substrate), and more particularly, it relates to a ceramic heater usefully applied to an electric or electronic apparatus.
2. Description of the Prior Art
In general, ceramics having an excellent insulation property and a high degree of freedom in design of a heater circuit is applied to various types of heater substrates. In particular, an alumina sintered body, having high mechanical strength among ceramic materials with thermal conductivity reaching 30 W/m·K, relatively excellent in thermal conductivity and thermal shock resistance and obtained at a low cost, is widely employed. When the alumina sintered body is applied to a A substrate, however, the substrate cannot follow abrupt temperature change of a heating element and may be broken due to a thermal shock.
Japanese Patent Laying-Open No. 4-324276 (1992) discloses a ceramic heater employing aluminum nitride having thermal conductivity of at least 160 W/m·K. A substrate having such a degree of thermal conductivity is not broken by abrupt temperature change dissimilarly to the substrate of alumina. This gazette describes that the uniform heating property of the overall heater can be secured by stacking about four layers of aluminum nitride and forming heating elements having different shapes on the respective layers while locating an electrode substantially at the center of the substrate for uniformizing temperature distribution in the ceramic heater.
Japanese Patent Laying-Open No. 9-197861 (1997) discloses employment of aluminum nitride for a substrate of a heater for a fixing device. According to this prior art, a substrate having thermal conductivity of at least 50 W/m·K, preferably at least 200 W/m·K can be obtained by setting the mean particle diameter of aluminum nitride particles to not more than 6.0 &mgr;m, optimizing combination of sintering agents and performing sintering at a temperature of not more than 1800° C., preferably not more than 1700° C. This gazette describes that the substrate having excellent thermal conductivity is employed for the heater for a fixing device thereby efficiently transferring heat of a heating element to paper or toner and improving a fixing rate.
In addition, Japanese Patent Laying-Open No. 11-95583 (1999) discloses the use of silicon nitride for a substrate of a heater for a fixing device. This prior art reduces the thickness of the substrate itself by employing silicon nitride having a relatively high strength with a flexural strength of 490 to 980 N/mm
2
and a thermal conductivity of at least 40 W/m·K, preferably at least 80 W/m·K, and reducing the heat capacity thereof, thereby reducing the power consumption. This gazette describes that silicon nitride has a lower thermal conductivity than aluminum nitride and hence the heat of a heating element is not readily transmitted to a connector of a current feeding part and oxidation of an electrode of the heating element can be prevented for avoiding a contact failure.
When thermal conductivity of a substrate is increased, the quantity of diffusion to parts other than a heating part is also increased although heat propagation efficiency from a heating element is improved, to consequently increase power consumption. In order to prevent oxidation of a contact between an electrode of the heating element and a connector of a feeding part, therefore, it is effective that a uniform heating property around the substrate is excellent and a temperature around the electrode of the heating element is lower by at least several % than that of the heating element region.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ceramic heater having an increased mechanical strength of a substrate and an improved thermal shock resistance.
Another object of the present invention is to provide a ceramic heater capable of controlling the thermal conductivity of a substrate and reducing the steepness of a temperature gradient from a heating element to an electrode thereby preventing oxidation of a contact between the electrode of the heating element and a connector of a current feeding part.
In a ceramic heater according to the present invention, a ceramic substrate provided with an electrode and a heating element on its surface is formed in a shape satisfying A/B≧20 assuming that A represents the distance from a contact between the heating element and the electrode to an end of the substrate closer to the electrode and B represents the thickness of the substrate, and the thermal conductivity of the substrate is adjusted to 30 to 80 W/m·K.
The main component forming the substrate is aluminum nitride, silicon nitride or silicon carbide, and a subsidiary component having thermal conductivity of not more than 50 W/m·K is added thereto.
If the main component of the ceramic is aluminum nitride, 5 to 100 parts by weight of aluminum oxide, 1 to 20 parts by weight of silicon and/or a silicon compound in terms of silicon dioxide or 5 to 100 parts by weight of zirconium and/or a zirconium compound in terms of zirconium oxide is added to 100 parts by weight of aluminum nitride, in order to adjust thermal conductivity thereof.
In order to obtain a ceramic sintered body having high mechanical strength, 1 to 10 parts by weight of an alkaline earth element and/or a rare earth element of the periodic table is introduced as a sintering agent with respect to 100 parts by weight of aluminum nitride. Calcium (Ca) is preferably selected as the alkaline earth element of the periodic table, while neodymium (Nd) or ytterbium (Yb) are preferably selected as the rare earth element of the periodic table.
The material for the substrate of the ceramic heater according to the present invention is preferably mainly composed of aluminum nitride (AlN), silicon nitride (Si
3
N
4
) or silicon carbide (SiC). While a substrate having thermal conductivity exceeding 100 W/m·K can be obtained by sintering material powder of such ceramic with addition of not more than several % of a proper sintering agent, the thermal conductivity of the substrate can be reduced to 30 to 80 W/m·K by adding a subsidiary component having thermal conductivity of not more than 50 W/m·K to the material powder.
If the thermal conductivity of the substrate is less than 30 W/m·K, there is a high possibility that the substrate itself is unpreferably broken by a thermal shock due to abrupt temperature increase of the heating element as energized. If the thermal conductivity of the substrate exceeds 80 W/m·K, the heat of the heating element is propagated to the overall substrate to unpreferably increase the quantity of diffusion to parts other than a heating part while also increasing power consumption, although a uniform heating property is excellent.
When adding aluminum oxide (Al
2
O
3
) to aluminum nitride (AlN), it is preferable to add 5 to 100 parts by weight of the former with respect to 100 parts by weight of the latter. The added aluminum oxide solidly dissolves oxygen in aluminum nitride in the sintered body thereby reducing the thermal conductivity while aluminum oxide having thermal conductivity of about 20 W/m·K itself is present in a grain boundary phase of aluminum nitride to effectively reduce the thermal conductivity of the ceramic sintered body. If the content of aluminum oxide is less than 5 parts by weight, the thermal conductivity may exceed 80 W/m·K. If the content of aluminum oxide exceeds 100 parts by weight, aluminum nitride reacts with aluminum oxide to form aluminum oxynitride. This substance has extremely low thermal conductivity, and hence the thermal conductivity of the overall substrate may be less than 30 W/m·K in this case.
Silicon and/or a silicon compound can be added to aluminum nitride (AlN) for adjusting the thermal conductivity. Silicon dioxide (SiO
2
), silicon nitride (Si
3
N
4
) or silicon carbide (SiC) may be employed as the added silicon compound. Such a substan
Nagao Syunji
Nakata Hirohiko
Natsuhara Masuhiro
Fasse W. F.
Fasse W. G.
Paik Sang
Sumitomo Electric Industries Ltd.
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