Electric heating – Heating devices – With heating unit structure
Utility Patent
1999-06-03
2001-01-02
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S553000, C219S542000, C219S548000
Utility Patent
active
06169275
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a ceramic heater and an oxygen sensor using the same. More specifically, the present invention concerns a ceramic heater in which the ratio of the electric resistivity between a heat generating portion and a lead portion of the heat generating resistor is specified and concerns an oxygen sensor using the same. The ceramic heater according to the present invention is useful, particularly, as a heater for use in an automobile oxygen sensor. Further, it can be used also as a glow system for use in internal combustion engines, a ceramic heater for heating semiconductors and a petroleum gasifying heat source used for petroleum fan heaters.
2. Description of the Related Art
A ceramic heater is generally manufactured by printing a paste containing a high melting point metal such as tungsten, molybdenum or platinum as a thick-film to the surface of a ceramic substrate of a desired shape such as a flat plate or a cylinder obtained by pressure molding or extrusion molding to form a heat generating resistor pattern, laminating another ceramic substrate thereon and sintering them integrally. A ceramic heater using alumina as a main ingredient constituting the ceramic substrate and tungsten as the high melting point metal and obtained by integrally sintering them is a typical example thereof. Since the ceramic heater is stable at a high temperature, it has been used, for example, in an application exposed to a high temperature such as an automobile oxygen sensor or a glow plug for use in internal combustion engines.
However, in automobile oxygen sensor applications, it has been required that an oxygen sensor operate rapidly after starting of an engine since regulations for exhaust gases have become severe recently. Hence, the oxygen sensor has to be heated rapidly and the temperature be increased rapidly to working temperature. Accordingly, it is necessary to use a heater having a high temperature elevation rate. Further, for an automobile oxygen sensor used in a severe circumstance in which it is exposed for a long time to a high temperature, it is also required that the heater to be used has an outstandingly excellent durability compared with conventional heaters.
As a ceramic heater of stable performance, Japanese Patent Unexamined Publication Hei 9-52784 discloses a ceramic heater having a heat generating resistor containing rhenium. In this heater, the temperature can be elevated easily and a stable performance can be obtained by the compounding of rhenium. Further, as a heater of high durability with less degradation of the performance even during long time use, Japanese Patent Unexamined Publication Hei 8-315967 discloses a ceramic heater with an alumina ingredient incorporated into a heat generating resistor. In this heater, adhesion between an alumina substrate and a heat generating resistor is improved to prevent defoliation of them thereby improving the durability. Further, Japanese Patent Unexamined Publication Hei 5-34313 discloses a ceramic heater having a heat generating resistor in which the temperature coefficient of resistance varies depending on the portions of the resistor. In this heater, the temperature elevation just after the application of voltage is rapid and a constant temperature is kept with no provision of an additional circuit.
However, in the ceramic heater having the heat generating resistor containing rhenium, as described in Japanese Patent Unexamined Publication Hei 9-52784, no particular consideration is made on the heater in that, after elevation to a predetermined temperature, the temperature is kept at a stationary state. Therefore, depending on circumstances, a control circuit for keeping the temperature within a predetermined range is required. Further, in the ceramic heater with the alumina ingredient incorporated into the heat generating resistor, as described in Japanese Patent Unexamined Publication Hei 8-315967, since the electric resistivity in the lead portion of the heat generating resistor is high, the temperature elevation rate in the heat generating portion is low, and the lead portion also sometimes shows considerable heat generation.
SUMMARY OF THE INVENTION
A ceramic heater according to the present invention comprises a ceramic substrate and a heat generating resistor disposed in the ceramic substrate, wherein the heat generating resistor has a heat generating portion and a lead portion and wherein the ratio of electric resistance of the heat generating portion to the total electric resistance of the heat generating portion and the lead portion at a normal temperature is from 55 to 95%.
For the ceramic substrate described above, it is preferred to use those having a high heat resistance and high strength at a high temperature. Ceramic substrates sandwich the heat generating resistor between them and shield them from atmospheric air to prevent oxidation and deterioration of the heat generating resistor.
Usually, alumina is used for such ceramic substrates. In addition, it may be mullite and spinel. Further, the ceramic substrate may be incorporated with other elements. In a case of a ceramic substrate comprising alumina as a main ingredient, it is particularly preferred to contain alumina by 80 parts by weight (hereinafter simply referred to as parts) or more (more preferably 85 parts or more and, more preferably, 91 parts or more) based on 100 parts by weight of the entire ceramic substrate. The ceramic substrate is excellent in sinterability and durability. Further, the ceramic substrate may contain elements belonging to group IV and group V of the periodic table, as well as oxides thereof.
The ceramic substrate may contain a sintering aid added for easy sintering. As the sintering aid, those mixed generally with a green material which is sintered into a ceramic substrate may be used. For instance, SiO
2
, CaO and MgO, as well as those forming such oxides by heating, for example, CaCO
3
or MgCO
3
can be used. In addition, Y
2
O
3
or oxides of rare elements may also be used.
The heat generating resistor can be formed by printing a pattern of a predetermined shape by sintering a conductive paste mainly containing tungsten, molybdenum and platinum by a thick film printing method on a green material to be formed as a ceramic substrate by sintering and then sintering them integrally. Further, rhodium or the like may be used in admixture with these ingredients. Tungsten, molybdenum, platinum and rhodium described above may also be used alone. By the use of platinum or rhodium alone, the resistance characteristic can be improved.
The heat generating resistor has a heat generating portion and a lead portion. The heat generating resistor in the present invention can be formed, for example, into a shape as shown in
FIGS. 1
,
2
and
3
. In each of the Figures, A is a heat generating portion and B is a lead portion. However, the shapes for the heat generating portion and the lead portion are not restricted only to those in the Figures. By the change of the shape and the ingredient for the heat generating portion and the lead portion, the ratio of the electric resistivity of each of the portions in the heat generating resistor can be controlled.
Further, resistance is measured under a normal atmospheric temperature. Normal temperature is defined as 18° to 30° C. (particularly, 20° to 25° C.). Further, measurement is conducted by a milli-ohm, high tester. Since the electric resistance is different depending on the ingredients and the shape of the heat generating portion and the lead portion as described above, the maximum resistance value measured for the heat generating portion and the lead portion under the conditions described above is determined as the electric resistances for each of them. That is, if the measuring value of the electric resistance is different, for example, between the longitudinal direction and the lateral direction, a greater resistance value is defined as the electric resistance.
Assuming the sum for the entire electric resistance o
Aoyama Toshihiko
Noda Yoshiro
Brinks Hofer Gilson & Lione
Hoang Tu Ba
NGK Spark Plug Co, Ltd.
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