Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-09-17
2003-10-21
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S425000, C205S781000
Reexamination Certificate
active
06635162
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor for measuring the concentration of a gas component such as NOx gas or combustible gas (e.g., HC or CO) in exhaust gas emitted from an internal combustion engine of a vehicle, such as an automobile, ship, or airplane, or from an industrial combustion engine, or in combustion gas emitted from, for example, a boiler.
2. Description of the Related Art
Recently, with exhaust gas regulations being tightened, studies have been conducted on engine control and catalyst control realized through direct measurement of the concentration of NOx, HC, or CO gas in exhaust gas emitted from, for example, an engine. An NOx gas concentration sensor for such an application is disclosed in, for example, SAE paper No. 960334, pp. 137-142, 1996. The NOx gas concentration sensor assumes the form of a laminate of solid electrolyte layers, each formed of a zirconia sheet, and includes a first diffusion passage, a first cavity portion, which communicates with the atmosphere under measurement via the first diffusion passage, a second diffusion passage, and a second cavity portion, which communicates with the first cavity portion via the second diffusion passage. The sensor further includes a first oxygen pump cell and an oxygen sensor cell, which is exposed to the interior of the first cavity portion, and a second oxygen pump cell, which is exposed to the interior of the second cavity portion. The oxygen sensor cell is adapted to measure oxygen concentration in the first cavity portion. On the basis of the measured oxygen concentration, the first oxygen pump cell pumps out oxygen from the first cavity portion, thereby diffusing a gas having a controlled oxygen concentration into the second cavity. A predetermined voltage is applied to a pair of electrodes of the second oxygen pump cell, causing NOx to dissociate into ions on one of the paired electrodes that is exposed to the interior of the second cavity portion. The thus-generated oxygen ions pass through the solid electrolyte that constitutes the second oxygen pump cell. As a result, a limiting current flows between the paired electrodes. On the basis of the limiting current, NOx gas concentration is determined. The paired electrodes of the second oxygen pump cell are disposed such that one electrode is exposed to the interior of the second cavity portion, while the other electrode is exposed to the atmosphere.
According to the above-described conventional gas sensor, oxygen concentration is lowered in the first cavity portion, and the concentration of NOx in the gas diffused into the second cavity portion is determined according to a limiting-current process. Since a detection output (current flowing between the paired electrodes of the second oxygen pump cell) with respect to gas to be measured (hereinafter referred to as “gas under measurement”) is very small (of the order of several &mgr;A), accurate measurement of such a small current is difficult. In order to detect such a small current, a sensor unit must be of high precision and thus becomes expensive. Also, the structure of the gas sensor becomes complex; specifically, the first and second cavity portions, the first and second oxygen pump cells, and the oxygen sensor cell are provided independently of one another.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a gas sensor that produces a large gas sensor output even with respect to a low-concentration gas to be detected and that has a simple structure.
Aspects of the present invention are described below. First aspect: a cavity portion, whose oxygen concentration is controlled at a constant level; an active electrode having a relatively high catalytic capability with respect to NOx or combustible gas; an inactive electrode having a relatively low catalytic capability with respect to NOx or combustible gas; and an oxygen concentration cell, which is disposed so as to be exposed to the interior of the cavity portion. Second aspect: the active electrode and the inactive electrode are disposed so as to be exposed to the interior of the same cavity portion. Third aspect: the active electrode contains one or more elements from the platinum group, which includes Pt, Rh, Pd, Ir, and Ru; and the inactive electrode contains one or more elements selected from the transition metals, which include Au, Ni, Co, Cr, Fe, Mn, Cu, Ti, and Zn, so that the catalytic capability with respect to NOx or combustible gas becomes lower than that of the active electrode. Fourth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; and the oxygen-concentration-sensing electrode and the inactive electrode are implemented in the form of a common electrode. Fifth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; the oxygen concentration reference electrode and the inactive electrode are implemented in the form of a common electrode; and the common electrode is disposed outside the cavity portion. Sixth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; and the oxygen-concentration-sensing electrode and the active electrode are implemented in the form of a common electrode. Seventh aspect: The cavity portion includes a first cavity portion and a second cavity portion, which communicates with the first cavity portion across a diffusion resistance and to which the oxygen concentration cell is exposed; an oxygen-concentration-sensing electrode, which is exposed to the interior of the first cavity portion in order to detect oxygen concentration in gas that diffuses from the first cavity portion into the second cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; an oxygen pump cell, which is exposed to the interior of the first cavity portion and pumps out oxygen from and/or pumps oxygen into the first cavity portion on the basis of the differential in electric potential between the oxygen-concentration-sensing electrode and the oxygen concentration reference electrode; and the active electrode and the inactive electrode are disposed within the second cavity portion. Eighth aspect: NOx or combustible gas concentration is determined by means of the oxygen concentration cell, which is exposed to the interior of the cavity portion whose oxygen concentration is held constant.
Features of preferred embodiments of the present invention will next be described. Preferably, a gas sensor according to the present invention assumes a laminate structure composed of thin sheets of solid electrolyte. An oxygen concentration cell includes an active electrode, an inactive electrode, and an oxygen-ion-conductive solid electrolyte layer on which the active and inactive electrodes are formed. The active and inactive electrodes have a reversible catalytic function (catalytic function related to oxygen dissociation) in relation to at least a dissociation reaction of oxygen molecules for injecting oxygen into the solid electrolyte layer and a recombination reaction of oxygen to cause the solid electrolyte layer to release oxygen. Preferably, in order to hold constant oxygen concentration in a cavity portion, to the interio
Ishida Noboru
Nadanami Norihiko
Oshima Takafumi
Sugaya Satoshi
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
Tung T.
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