Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-06-11
2003-10-14
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S424000, C204S428000, C073S023320
Reexamination Certificate
active
06632338
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensing element incorporated in a gas sensor used for controlling an air-fuel ratio of the gas mixture introduced into a combustion chamber of an internal combustion engine.
In an automotive vehicle, a gas sensor is provided in an exhaust pipe of an internal combustion engine to control the air-fuel ratio of the gas mixture introduced into a combustion chamber of the engine.
For example, a practical gas sensing element incorporated in the gas sensor is required to detect a concentration of oxygen gas contained in the exhaust gas. The gas sensing element may be formed by a ZrO2 solid electrolytic body which is capable of generating an electromotive force in accordance with the oxygen concentration.
Japanese Patent No. HEI 2-15017 discloses a conventional gas sensing element whose output is responsive to an electromotive force.
According to this type of conventional oxygen concentration detector, a gas sensing element is provided at a distal end of this detector. The gas sensing element is configured into a cup-shaped multilayered structure comprising a reference gas sensing electrode, a solid electrolytic sintered body, a measured gas sensing electrode, and an electrode protective layer which are successively stacked in this order. A heater is accommodated in an inside chamber of the cup-shaped sensing element.
The electrode protective layer is a ceramic coating layer, or a double layer consisting of a ceramic coating layer and a &ggr;-Al2O3 layer provided on this ceramic coating layer.
Exhaust gas reaches the measured gas sensing electrode through the ceramic coating layer or the double layer of the ceramic coating layer and the &ggr;-Al2O3 layer.
Recently enacted regulations relating to exhaust gas emissions have forced automotive manufacturers to develop automotive engines having the capability of precisely controlling the combustion. To realize this, it is essentially important to provide an excellent gas sensor having sensing properties stable under severe exhaust gas temperatures and durable for a long time.
One of the main factors causing change in sensing properties is deterioration of the gas sensing element subjected to poisonous substances such as Pb, S etc. contained in gasoline, additives K, Na, P, Ca, Zn etc. involved in gasoline or oil and the Si component contained in a seal member of the internal combustion engine.
U.S. Pat. No. 5,766,434 (corresponding to JP10-221296) proposes a gas sensing element comprising a measured gas sensing electrode covered by a protective layer including coarse particles and fine particles structurally arranged in such a manner that interparticle cavities formed between the coarse particles are filled with the fine particles.
This conventional technique intends to adequately set a porosity and a pore diameter of the protective layer by filling the coarse interparticle cavities with the fine particles. Anti-poisoning properties can be improved. Obtained gas choking effect will cause unburnt components in the exhaust gas to react with residual oxygen in an equilibrium condition. The waveform of a sensor output will be sharpened.
The above-described prior art effectively prevents blinding or plugging of the protective layer subjected to crystalline or glass substances formed by gasoline or oil compounds. Accordingly, the gas sensing element can be effectively protected from poisonous substances.
However, the above-described prior art cannot bring satisfactory effects against poisonous substances of gas phase, such as Si components, contained in the seal member or the like of the internal combustion engine.
Poisoning mechanism of Si components is as follows.
Si components of gas phase residing in the vicinity of a gas sensing element penetrate the protective layer and subsequently reach the measured gas sensing electrode. The measured gas sensing electrode, when covered by Si components, loses its activity. As a result, Si components deteriorate the sensing properties of the gas sensing element.
SUMMARY OF THE INVENTION
In view of the above-described problems, the present invention has an object to prevent poisonous substances, such as gaseous Si components, from reaching the measured gas sensing electrode. In other words, an object of the present invention is to prevent the measured gas sensing electrode from being covered by poisonous substances. Thus, the present invention provides a gas sensing element capable of maintaining a stable sensor output for a long term.
To accomplish the above and other related objects, the present invention provides a first gas sensing element comprising a solid electrolytic body, a reference gas sensing electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas sensing electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. A ceramic porous protective layer is provided on a surface of the measured gas sensing electrode. The protective layer comprises coarse particles and fine particles structurally arranged in such a manner that inter particle cavities formed between the coarse particles are filled with the fine particles. And, at least either of the coarse particles and the fine particles contain at least one selected from the group consisting of &ggr;-Al203, &thgr;-Al203, &dgr;-Al203 and solid solution having the same crystal structure as those of &ggr;-Al203, &thgr;-Al203, and &dgr;-Al203.
The first gas sensing element of the present invention is characterized in that at least either of the coarse particles and the fine particles constituting the protective layer contain at least one selected from the group consisting of &ggr;-Al2O3, &thgr;-Al2O3, &dgr;-Al2O3 and solid solution having the same crystal structure as those of &ggr;-Al2O3, &thgr;-Al2O3, &dgr;-Al2O3. More specifically, the solid solution is an alumina solid solution having the crystal structure of either a tetragonal system or a monoclinic system which are the same crystal structures as those of &ggr;-Al2O3, &thgr;-Al2O3, &dgr;-Al2O3.
The coarse particles and the fine particles can be made of the same material or can be made of different materials.
For example, practical examples of the solid solution are &ggr;-(Al
1-x
La
x
)
2
O
3
, &thgr;-(Al
1-x
La
x
)
2
O
3
, and &dgr;-(Al
1-x
La
x
)
2
O
3
.
The protective layer covers the entire surface of the measured gas sensing electrode. It is also possible to provide the protective layer so as to cover the surface of the solid electrolytic body together with the measured gas sensing electrode.
Furthermore, it is possible to provide another layer between the protective layer and the measured gas sensing electrode (refer to FIG.
1
).
Effects of the first gas sensing element of the present invention will be explained hereinafter.
The protective layer of the first gas sensing element consists of coarse particles and fine particles. Accordingly, interparticle cavities formed between the coarse particles are filled with the fine particles. It becomes possible to sufficiently reduce a porosity and a pore diameter of the protective layer.
Furthermore, in a process of forming the protective layer, the coarse particles cooperatively constitute bridges which effectively prevent generation of cracks in the protective layer. Thus, the present invention makes it possible to easily obtain a protective layer having a satisfactory thickness.
The coarse particles and fine particles constituting the protective layer contain a material having a large specific surface, such as &ggr;-Al2O3, &thgr;-Al2O3, and &dgr;-Al2O3.
Accordingly, it becomes possible to enlarge a substantial contact area of the protective layer to be exposed to poisonous gaseous substances. Thus, the protective layer surely traps the poisonous substances.
In this manner, the present invention provides a protective layer which can surely trap incoming poisonous gaseous substances, thereby surely preventing the poisonous substances from reaching the measured gas sensing
Fujii Namitsugu
Hotta Yasumichi
Kobayashi Kiyomi
Satou Motoaki
Denso Corporation
Nguyen Nam
Nixon & Vanderhye PC
Olsen Kaj K.
LandOfFree
Gas sensing element incorporated in a gas sensor for an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Gas sensing element incorporated in a gas sensor for an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Gas sensing element incorporated in a gas sensor for an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3114758