Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-09-26
2003-07-08
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S408000, C204S424000, C204S425000
Reexamination Certificate
active
06589410
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydrocarbon sensor.
BACKGROUND ART
Hitherto, a hydrocarbon sensor using barium-cerium oxide as a solid electrolyte has been proposed (Japanese Laid-open Patent No. 9-127055).
A schematic structure of this hydrocarbon sensor is shown in FIG.
12
. Reference numeral
101
is a solid electrolyte layer composed of barium-cerium oxide, and a cathode
102
and an anode
103
are formed on its surface by thick film printing process. On the solid electrolyte layer
101
, a ceramic substrate
104
made of forsterrite is adhered and fixed with an adhesive of inorganic material (inorganic adhesive)
108
. A heater
109
is formed on the surface of the ceramic substrate
104
by a thick film printing process. In part of the adhesion layer adhered by the inorganic adhesive
108
, a diffusion rate-determining hole
111
(area indicated by dotted line in
FIG. 12
) for introducing hydrocarbon gas (HC) is provided.
The operation of this hydrocarbon sensor is explained. As shown in
FIG. 12
, a constant voltage is applied to the cathode
102
and anode
103
. An ammeter for detecting the output is provided in the circuit. The solid electrolyte layer
101
is heated by a heater
109
in order to activate. In this state, when the HC passes through the diffusion rate-determining hole
111
to reach the anode
103
, the HC is decomposed, and protons conduct through the solid electrolyte layer
101
. As a result, current I flows in the circuit. The magnitude of this current I increases in proportion to the amount of protons, that is, the concentration of the HC. Therefore, the HC concentration can be detected from the output of the ammeter.
The hydrocarbon sensor having such structure is capable of obtaining a linear output corresponding to the HC concentration, and is free from effect of oxygen if coexisting while the HC concentration is low. In other words, the hydrocarbon sensor excellent in gas selectivity is obtained.
However, when applying such hydrocarbon sensor in HC detection in automobile emission, the solid electrolyte layer
101
must be activated. Accordingly, when the current flows in the heater
109
, the ceramic substrate
104
on which the heater
109
is formed is subject to a significant stress, and known that the yield is lowered. In this mechanism, the following reasons are considered.
1) The coefficient of thermal expansion of the ceramic substrate
104
(forsterrite) on which the heater
109
is formed is 11 to 11.5×10
−6
/° C., whereas the coefficient of thermal expansion of the solid electrolyte layer
101
is about 10×10
−6
/° C., and the difference of the two is more than 1×10
−6
/° C.
2) The HC in the emission is a reducing atmosphere, and the ceramic substrate
104
is exposed to it, and part of composition (MgO forsterrite) of the ceramic substrate
104
is reduced, and the strength is reduced.
3) The platinum paste of the thick film for forming the heater
109
is a porous and uneven composition, and the current flow is concentrated in this area, and the temperature becomes high locally.
4) The heater
109
is formed at one side of the ceramic substrate
104
only, and this formed side is an exposed structure, and when the current flows in the heater
109
is this state, a temperature difference occurs suddenly between the heater forming side and the back side (the adhesion side with the solid electrolyte layer), and a large stress occurs.
The conventional hydrocarbon sensor also has other problem, that is, when the hydrocarbon sensor is installed in the automobile emission, the emission temperature varies with the engine running state, and the temperature of the hydrocarbon sensor may vary as much as 620±30° C. That is, the temperature variation width is 60° C. The output current of the hydrocarbon sensor varies not only with the HC concentration, but also with the temperature, and such temperature variation may cause to lower the accuracy of hydrocarbon sensor. This is because the temperature regulation precision of the heater
109
is not sufficient.
SUMMARY OF THE INVENTION
The invention solves these problems, and it is hence an object thereof to present a hydrocarbon sensor excellent in yield and high in detection precision.
To solve the problems, the hydrocarbon sensor of the invention comprises a solid electrolyte layer composed of barium-cerium oxide, a pair of electrodes provided on the solid electrolyte layer, a ceramic substrate having a coefficient of thermal expansion nearly same as that of the solid electrolyte layer, and a heater provided on the ceramic substrate, in which the solid electrolyte layer and ceramic substrate are bonded to each other.
Further, at the heater forming side of the ceramic substrate, an auxiliary substrate having a coefficient of thermal expansion nearly same as that of the ceramic substrate is provided.
Further, the heater comprises control means for controlling on/off switching of the heater, comparing means for comparing the resistance value of the heater and the target resistance value of the heater being predetermined corresponding to the temperature, and judging means for suppressing the output from the control means depending on the signal from the comparing means.
In this constitution, the hydrocarbon sensor excellent in yield and high in detection precision is obtained.
REFERENCES:
patent: 3597345 (1971-08-01), Hickam et al.
patent: 4505807 (1985-03-01), Yamada
patent: 4897174 (1990-01-01), Wang et al.
patent: 4961835 (1990-10-01), Kobayashi et al.
patent: 5108577 (1992-04-01), Mase et al.
patent: 5395506 (1995-03-01), Duce et al.
patent: 5474665 (1995-12-01), Friese et al.
patent: 5935398 (1999-08-01), Taniguchi et al.
patent: 6007688 (1999-12-01), Kojima et al.
patent: 6007697 (1999-12-01), Yagi et al.
patent: 772042 (1997-05-01), None
patent: 4-89562 (1992-03-01), None
patent: 7-248307 (1995-09-01), None
patent: 9-127055 (1997-05-01), None
patent: 6-347441 (1997-12-01), None
patent: 9-318592 (1997-12-01), None
Gatewood, “Thermal Stresses”, (1957), pp. 138-140.
Shoji Rihito
Tamai Takashi
Taniguchi Noboru
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Tung T.
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