Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-02-03
2004-01-13
Olsen, Kaj K. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S429000
Reexamination Certificate
active
06676817
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an electrolyte, particularly to an electrolyte body or layer for use in a cell that flows or rather transfers ions such as oxygen ions, lithium ions, sodium ions between the electrodes of the cell. Specifically the invention provides a gas sensor with a solid electrolyte for detecting or rather measuring a concentration of a specific gas component such as O
2
, CO
2
, NOx, HC, H
2
O and H
2
. More specifically, this invention provides a gas sensor with an electrochemical cell using an oxygen ion conductive solid electrolyte that is capable of transferring or conducting oxygen to thereby detect a specific gas component existing in an exhausted gas emitted from an internal combustion engine, and a method for fabricating the gas sensor. Further, the invention provides a new oxygen-ion conductive solid electrolyte material and a strenuous gas-sensor structure using the same.
2. Description of the Related Art
Conventionally, various gas sensors using a solid electrolyte such as zirconia have been proposed for internal combustion engine control. For instance, a so-called lambda sensor that employs a cylindrical and bottom-closed solid electrolyte has been widely used for detecting oxygen in a gas exhausted from an internal combustion engine. On the other hand, a so-called thick-film gas sensor that utilizes a thick electrolyte film or rather layer formed on a ceramic substrate or rod as a sensing element has been proposed, which sensor enables prompt activation of a gas-sensing mechanism as compared to the lambda sensor. This is because its heat-propagation efficiency is comparatively high compared to the lambda sensor. The thick-film gas sensor may include an insulating ceramic substrate or rod in which a heating wire is embedded and insulated from the electrolyte film, and which substrate is co-fired with the electrolyte film so as to form an integral or unitary ceramic laminate as a gas sensor.
In a conventional process of fabricating the thick-film gas sensor, a green or rather unfired oxygen-ion conductive solid electrolyte layer including zirconia particles therein and unfired metal electrode wires formed thereon is superposed on an unfired alumina substrate, and then the layer and the substrate are co-fired to form the unitary laminate. However, this process encounters a problem in that since the alumina substrate and the zirconia differ in thermal coefficient and thermal expansion and the zirconia undergoes a phase transition with firing temperature variations, a volume change and/or thermal stress is induced in the laminate. This causes difficulties in attaining a high-quality oxygen-ion conductive solid electrolyte layer of zirconia firmly bonded on the alumina substrate without losing the required performance by co-firing. Otherwise, cracks are induced in the resultant oxygen-ion conductive solid electrolyte layer formed on the laminate in a thermal cycle environment ranging, e.g., from −20° C. up to 1100° C. (hereinafter referred to as “thermal cycle”).
Suppression of cracking in the laminate and firmly bonding the oxygen-ion conductive solid electrolyte layer thereon are disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 61-51557, 61-172054, and 6-300731. However, these disclosures are still unsatisfactory in bringing about a good solid electrolyte ceramic that can be used as a thick film layer of a gas sensing element formable with an insulating ceramic substrate or rod.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the above-mentioned problems and to provide a new or improved electrolyte body or layer capable of sufficiently suppressing the formation of cracks during fabrication and/or use including use in a high temperature gas environment.
Another object of the invention is to provide a tough laminate comprised of an electrolyte ceramic layer and a ceramic substrate, for use in a gas sensor.
Yet another object is to provide a method of fabricating a solid electrolyte body, layer and/or laminate that withstands and perform well in a high temperature gas environment.
In terms of practical applications of a solid electrolyte of zirconia including a partially or wholly stabilized one, it has been conventionally believed that inclusion of insulating ceramic material into the solid electrolyte material should be less than about 5% by mass (=% by weight). Otherwise, such alumina inclusion degrades the electrochemical function of the fired solid electrolyte.
However, the present inventors have found that, even if the solid electrolyte ceramic contains the insulating ceramic grains in a remarkably increased amount, e.g., several tens of percent by weight (namely, 10-80% by weight), the fired solid electrolyte having such a large amount of the insulating grains therein can function sufficiently as a solid electrolyte for a gas sensor cell and is comparable to a conventional one containing alumina in an amount of less than 10% by weight. Especially when a laminate is formed using a partially or wholly stabilized zirconia electrolyte containing several tens of percent of alumina grains of high purity, according to the invention, the laminate works better in some physical values than a conventional one that contains a lesser amount of alumina.
Specifically, an average grain size of the oxygen-ion conductive solid electrolyte other than the insulating grains after firing becomes not more than 2.5 &mgr;m, according to the invention, when an average particle size of the alumina, zirconia and yttria powders contained in a green or rather unfired body or layer is less than 1 &mgr;m.
These findings are quite useful for developing a good solid electrolyte body and/or a thick-film laminated structure (namely a laminate of at least one electrolyte layer and another material layer) for use in a high temperature gas sensor. Typically, such a laminate has a total thickness of 10 &mgr;m-150 &mgr;m at an electrolyte portion that is coated or bonded integrally on a thick strenuous insulating substrate or rod of alumina by co-firing. The alumina substrate is preferred when alumina grains with high purity is included in the electrolyte portion that is coated or bonded with metal electrodes such as platinum, forming the laminate of a gas sensor.
According to a first aspect of the first embodiment according to the invention, a solid electrolyte ceramic body is provided containing insulating ceramic grains and partially or wholly stabilized electrolyte ceramic, wherein the ceramic body contains 10% to 80% by weight of the insulating ceramic grains having an average grain size of not larger than 1 &mgr;m distributed in the partially or wholly stabilized electrolyte ceramic. The rest in the insulating body, in other words, more than 20% by weight of the ceramic body is the partially or wholly stabilized electrolyte. Herein, it is important, when the electrolyte is used for a cell for a gas sensor, that the electrolyte body is formed so that the stabilized zirconia grains are continuously connected to each other in a manner surrounding the insulating grains. Otherwise, the cell having electrodes cannot transfer ions from one electrode to another. If the average grain size of the insulating ceramic grains are formed, a mechanical or electrical performance of the electrolyte body tends to be lost.
In a second aspect of the first embodiment, the solid electrolyte ceramic body importantly has insulating ceramic grains having an average purity of more than 99%. The preferable purity of the insulating ceramic grain is more than 99.9% and the most preferable purity is more than 99.99% or more than 99.995% when it is measured at the center of the grain, because high-purity insulating grains tend not to combine with other ceramic material. Although there are various candidates for the high-purity insulating grains, alumina grains having such purity as described above are best in the case that the solid electrolyte ceramic is required to transfer oxyg
Adachi Yutaka
Aoki Ryohei
Awano Sinya
Imaeda Koichi
Kojima Takao
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
Sughrue & Mion, PLLC
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