Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
2001-09-27
2004-01-06
Cygan, Michael (Department: 2855)
Measuring and testing
Gas analysis
Detector detail
C204S429000
Reexamination Certificate
active
06672137
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns an oxygen sensor comprising a sensor device for detecting an oxygen density and a method of manufacturing the sensor device.
BACKGROUND OF THE INVENTION
When a oxygen sensor comprising a sensor device is exposed as an air/fuel ratio sensor to an exhaust gas upon using lead-containing fuels, electrodes are contaminated with lead, phosphorus, silicon or the like and degraded with lapse of time to no more provide sufficient electromotive force. Sensors coping with the problem of deterioration for the durability of electrodes are disclosed, for example, in Japanese Patent Publication No. 90176/1994 and Japanese Laid-Open No. 113480/1997. However, a sensor capable of completely preventing contamination to electrodes in a low temperature region which is most likely to undergo the effect of lead or the like contained in exhaust gases has not yet been obtained.
This invention intends to provide an oxygen sensor having a sensor device capable of preventing an electrode from contamination by lead or the like and having excellent durability even in contact with exhaust gases at a relatively low temperature, as well as a method of manufacturing such a sensor device.
DISCLOSURE OF THE INVENTION
The oxygen sensor of the present invention is an oxygen sensor comprising a sensor device including a detection electrode, an electrode protection layer formed on the surface of the detection electrode and a contamination preventive layer formed on the surface of the electrode protection layer, wherein the contamination preventive layer comprises a composite powder having a ceramic powder having a large grain size (referred to hereinafter as coarse powder) covered therearound with a ceramic powder having a small grain size (referred to hereinafter as fine powder), and hollows not filled with the fine powder are scattered in gaps among such composite powders.
By forming the contamination preventive layer as described above, contamination materials are trapped by the fine powder and prevented from reaching the electrode in the oxygen sensor, thus preventing a deterioration by contamination in the performance of the oxygen sensor. On the other hand, the fine powder is carried on the coarse powder, thus preventing the problem of removal of a contamination preventive layer from a sensor device surface such as a layer composed exclusively of the fine powder upon high heat setting during continuous use at high temperatures. Further, the fine powder is carried so as to cover the surface of the coarse powder, while suitable hollows having a similar size to that of the coarse powder are formed in gaps among the coarse powders, so that the gaps among the coarse powders are not completely filled with the fine powder, and therefore, the contamination preventive layer cannot be clogged even upon accumulation of comtamination materials thereon, to prevent a reduction in the response of the sensor.
The grain size distribution of primary particles of the ceramic powder constituting the contamination preventive layer as described above has at least two peaks, and if the peak on the side of the smallest grain size is 10 &mgr;m or less and the peak on the side of the largest grain size is 0.1 &mgr;m or more, a desirable contamination preventive layer having a high contamination preventing effect is given.
In this case, the peak on the side of the small grain size is preferably at 1 &mgr;m or less and can be at 0.05 &mgr;m or less and, particularly, at 0.01 &mgr;m or less. Further, the peak on the side of the large grain size is preferably at 1 &mgr;m or more and can be, particularly, 10 &mgr;m or more.
For good biting of the coarse particle, the electrode protection layer under the contamination preventive layer is formed preferably by spray coating.
The “ceramic powder” contained in the “contamination preventive layer” is preferably selected from the powder of oxide which is chemically stable in exhaust gases at high temperature such as titania, alumina, silica, and composite oxide containing aluminum atoms such as spinel and mullite. Powder other than the oxide may also be used so long as it is chemically stable. In this case, two or more kinds of ceramic powders of different compositions may be mixed. When the ceramic powder in one composition is a fine powder while the ceramic powder in the other composition is a coarse powder, the degree of freedom is extended in the selection of the powder to facilitate the provision of the powder having a desired grain size distribution, and such ceramic powders can be used conveniently by selecting a ceramic powder having a high contamination-preventing effect as the fine powder and a ceramic powder having high-temperature durability as the coarse powder.
The two or more kinds of ceramic powders of different compositions, preferably, contain a titania powder having a peak in the grain size distribution at 1 &mgr;m or less and a ceramic powder other than titania having a peak in the grain size distribution at 10 &mgr;m or more.
Titania is considered to be excellent in the ability to adsorb contamination materials. In particular, anatase type titania can be used for easily providing a powder having a small grain diameter and a high contamination preventing effect.
Preferred ceramic powder other than titania is, particularly, less heat shrinkable ceramic powder such as a composite oxide containing aluminum atoms such as spinel or mullite.
Further, a titania powder having a peak from 0.003 to 0.5 &mgr;m is combined particularly preferably with a ceramic powder other than titania having a peak from 15 to 50 &mgr;m in order to form suitable gaps in the contamination preventive layer. Such powders can be incorporated to provide a contamination preventive layer which can sufficiently adsorb contamination materials, is not removed by thermal shrinkage from the electrode protection layer and is further superior in durability with less drop in response.
That is, when a powder having a peak of small grain sizes in the range of 1 &mgr;m or less, preferably 0.003 to 0.5 &mgr;m is used in combination with a powder having a peak of large grain sizes in the range of 10 &mgr;m or more, preferably 15 to 50 &mgr;m, the contamination preventive layer as shown in FIGS.
1
(
a
) & (
b
) having suitable hollows with a similar size to that of coarse powder therein is formed from composite powders comprising a large number of particles of the powder with small grain sizes adhering to the surfaces of particles of the powder having large grain sizes, so the contamination preventive layer can sufficiently keep air-permeability, can certainly adsorb contamination materials, and can be rendered highly durable.
As the coarse powder and fine powder, powders identical in the composition but different in the crystal phase can also be selected. It is particularly preferable that an anatase type titania powder is used as the fine powder and a rutile type titania powder as the coarse powder. Both powders are titania powders but are different in the crystal phase, and these are provided as fine and coarse powders having a narrow distribution of grain sizes and are thus suitable for forming a contamination preventive layer excellent in air-permeability. For the contamination preventive effect, the grain diameter in the peak of the grain size distribution of the anatase type titania powder is preferably 0.5 &mgr;m or less, more preferably in the range of 0.003 to 0.5 &mgr;m. For the contamination preventive effect, the grain diameter in the peak of the grain size distribution of the rutile type titania powder is preferably 1 &mgr;m or more, more preferably in the range of 3 to 8 &mgr;m. By combination of the anatase type titania powder having a very small grain diameter of about 0.003 to 0.5 &mgr;m with the rutile type titania having a larger grain size, the contamination preventive layer excellent in the action of capturing poisoning materials can be formed. Further, by using the ceramic powders having the same composition, the composite powders can be easily formed to
Atsumi Takayoshi
Isomura Hiroshi
Shiono Koji
Takagi Masamine
Cygan Michael
Morgan & Lewis & Bockius, LLP
NGK Spark Plug Co. Ltd.
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