Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-08-07
2002-11-12
Warden, Sr., Robert J. (Department: 1744)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S424000, C204S429000, C204S292000
Reexamination Certificate
active
06478941
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensor installed in an exhaust system of an automotive internal combustion engine to detect an oxygen concentration in the exhaust gas, or an air-fuel ratio, or the like.
The present invention relates to a gas sensing element used for controlling an air-fuel ratio of an internal combustion engine and a method for manufacturing the gas sensing element.
In general, to control the air-fuel ratio, a gas sensor is installed in an exhaust system of an automotive internal combustion engine.
The gas sensor comprises a gas sensing element provided at its front end for detecting an oxygen concentration. The gas sensing element comprises a solid electrolytic sintered body having oxygen ion conductance, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The measured gas side electrode is covered by a porous electrode protective layer.
In many cases, the electrode protective layer is a ceramic coating layer, or a double layer consisting of a ceramic coating layer and a &ggr;-A1203 layer provided on this ceramic coating layer.
According to this type of gas sensing elements, a measured gas reaches a measured gas side electrode through the ceramic coating layer or the double layer of the ceramic coating layer and the &ggr;-A1203 layer. The gas sensing element produces a sensor output.
Recent radically changing circumstances, such as enhancement of emission control laws and regulations as well as requirement to high power internal combustion engines, forces automotive manufacturers to develop automotive engines capable of precisely controlling the combustion.
To realize this, it is essentially important to provide excellent gas sensors having sensing properties stable under severe operating conditions and durable for a long-term use.
FIG. 5
shows a characteristic curve representing a relationship between air-fuel ratio and voltage, as important sensor output characteristics of a gas sensing element used for combustion control of an internal combustion engine. In
FIG. 5
, point &lgr; is referred to as a specific air-fuel ratio where the voltage causes steep changes. In
FIG. 5
, a reference voltage is a criteria used for judging whether a fuel injection amount should be increased or decreased in the combustion control of an internal combustion engine. In general, the reference voltage is set to 0.45V.
More specifically, when a sensor output is larger than the reference voltage, the fuel injection amount is reduced to form an air/fuel mixture whose air-fuel ratio is shifted to a lean side. On the contrary, when a sensor output is less than the reference voltage, the fuel injection amount is increased to form a relatively rich air/fuel mixture. Through such a feedback control, the air-fuel ratio of the controlled engine can be always kept in a window of a ternary catalyst.
Accordingly, to precisely perform the air-fuel ratio control, it is essentially important to stabilize the point &lgr; (hereinafter, referred to as control &lgr;).
In other words, the control &lgr; should be stable during a long-term use of a gas sensing element and should be constant regardless of any environmental change of the gas sensing element.
When a gas sensing element is installed in an exhaust system of an internal combustion engine, a sensor output is produced in the following manner.
First, an exhaust gas containing unburnt components reaches a measured gas side electrode. Then, an equilibrium oxygen concentration is obtained through a catalytic reaction caused on the measured gas side electrode. The sensor output is produced as a signal representing a difference between the equilibrium oxygen concentration thus obtained and an oxygen concentration in the air serving as a reference gas.
Accordingly, it becomes possible to increase the measuring accuracy of a gas sensing element when an electrode having excellent activity is used as a measured gas side electrode of a gas sensing element.
The following is a method for activating a measured gas side electrode disclosed in Unexamined Japanese patent publication No. 10-104194.
First, a measured gas side electrode is formed on a surface of a solid electrolytic body by baking in the air at the temperature range from 1,000° C. to 1,400° C. Then, a heat treatment is applied to the measured gas side electrode thus formed in an atmosphere containing H
2
.
Subsequently, a heat treatment in an inert atmosphere and a heat treatment in a non-oxidative atmosphere including moisture vapor are applied to the measured gas side electrode.
By combining these treatments, the catalytic activity of the measured gas side electrode can be enhanced.
However, according to the above-described conventional method, it was difficult to provide a gas sensing element having a measured gas side electrode which can assure a sufficiently stable control &lgr; even in a severe high-temperature environment or in a poisonous environment containing Si compounds.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the prior art, the present invention has an object to provide a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
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 side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas, wherein a crystal face strength ratio of the measured gas side electrode according to X-ray diffraction is 0.7≦{I(
200
)/I(
111
)} or 0.6≦{I(
220
)/I(
111
)}.
The first gas sensing element of the present invention is characterized in that the measured gas side electrode has a crystal face strength ratio according to X-ray diffraction satisfying the above-described conditions.
If I(
200
)/I(
111
) is less than 0.7, a ratio of an active surface to an entire electrode surface will reduce to 0.5 or less and it will be difficult to assure satisfactory catalytic activity and stability for smoothly promoting an equilibrating reaction of exhaust gas.
If I(
220
)/I(
111
) is less than 0.6, it will be difficult to assure satisfactory catalytic activity and stability.
To obtain crystal grains and an electrode film which are stable in energy level and easily fabricable, a preferable upper limit of the crystal face strength ratio is 1.0 in view of the fact that the total area of active faces (
200
) and (
220
) can be maximized and because according to this condition the crystal face orientation can satisfy the requirement that the solid electrolytic body causes no alteration.
Next, functions and effects of the present invention will be explained hereinafter.
Inventors of this invention enthusiastically conducted research and development for stabilizing the activity of a measured gas side electrode, i.e., stabilization of control &lgr;. And, as a result of the research and development, the inventors have found the fact that a crystal face of the measured gas side electrode greatly contributes to activation and stability of a gas sensing element.
The measured gas side electrode is made of an electrode material containing noble metals which possess catalytic properties and usually have a face-centered cubic structure.
In a crystal lattice of such metals, specific crystal faces, i.e., faces (
100
) and (
110
), have a lower surface density of atoms compared with other face (
111
) dominant in this crystal lattice.
Due to lower surface densities, these faces (
100
) and (
110
) promote adsorption of various exhaust components. Thus, these crystal faces can smoothly adsorb unburnt exhaust components and residual oxygen
E Gang
Fujii Namitsugu
Hotta Yasumichi
Kobayashi Kiyomi
Nixon & Vanderhye P.C.
Olsen Kaj K.
Warden, Sr. Robert J.
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