Chemical plating method, electrolytic cell and automotive...

Chemistry: electrical and wave energy – Apparatus – Electrolytic

Reexamination Certificate

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C204S421000, C427S105000, C427S125000, C427S264000, C427S265000, C427S376700, C427S404000, C427S419100, C427S443200

Reexamination Certificate

active

06315880

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to solid state electrolytic cells and to oxygen sensors utilizing them. It has particular utility as a highly stable, rapid response lambda oxygen sensor in an automotive exhaust system.
Solid state electrolytic cells are well known. A particularly useful cell includes a solid electrolyte which selectively transmits oxygen and which includes catalytic electrodes on opposed sides of the solid electrolyte. Such cells are widely used as automotive lambda (stoichiometric) exhaust gas sensors, where they produce a voltage signal which is highly dependent on the amount of oxygen in the exhaust gas stream. It will be understood, however, that the usefulness of the invention is not limited to such sensors. For example, multiple such cells can be connected as non-stoichiometric, pumping oxygen sensors. See, for example, Kondo et al., U.S. Pat. No. 5,480,535. In other uses, when connected as a current generator, such cells act as fuel cells, and when an external voltage is applied, they can act as oxygen generators which produce exceptionally pure oxygen.
A common configuration of an automotive lambda exhaust gas sensor is a small thimble-shaped body of compacted zirconia (zirconium dioxide) stabilized with about 2-10 mole percent yttria (Y
2
O
3
) and, optionally, 0-20 mole percent alumina (Al
2
O
3
). The catalytic electrodes can be painted on as a platinum ink. Commonly, the outer electrode is formed by vacuum sputtering a thin film onto substantially the entire outer surface of the thimble. The sputtering process is expensive and inefficient, the electrodes are of varying thickness from one axial end of the thimble to the other, and the resulting sensors are unpredictable and have high reject rates.
The basic operation and known problems of an automotive lambda exhaust gas sensor are described, for example, in Topp et al., U.S. Pat. No. 3,978,006, Burgett et al., U.S. Pat. No. 3,844,920, Romine et al., U.S. Pat. No. 4,186,071, and Berg et al., U.S. Pat. No. 4,253,934. As set out in these patents, it is desirable for the sensor to have switching times on the order of under 200 milliseconds when the air-to-fuel ratio fed to the engine switches from lean to rich or rich to lean with respect to the stoichiometric ratio. It is also desirable for the sensor to produce smooth switches of at least about 200 to 300 millivolts when the air-fuel ratio switches. In recent years, the time required for an oxygen sensor to reach its operating temperature has also been recognized as a significant problem, and heated oxygen sensors have become standard. It is thus also desirable to produce an oxygen sensor which is well suited to introduction of a heater into the sensor structure. The background of heated oxygen sensors is well set out, for example, in Ker et al., U.S. Pat. No. 4,824,550.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a solid-state electrolytic cell which is simple and inexpensive to manufacture.
Another object is to provide such a cell which, when utilized as a lambda oxygen sensor produces rapid response times and high signal strength.
Another object is to provide such a cell which is reliable and reproducible.
Another object is to provide such a cell which is easily adaptable to use with a heater.
Another object is to provide a simple, reliable, high-performance oxygen sensor which incorporates such a cell.
These and other objects will become apparent to those skilled in the art in light of the following disclosure and accompanying drawings.
In accordance with one aspect of the invention, generally stated, a solid electrolyte cell is provided comprising a solid electrolyte body having a first side and a second side, a first electrode on the first side of the body, the first side of the body having a porous surface comprising a plurality of substantially spherical recesses, a first electrode substantially covering the first side of the body, the first electrode comprising a thin layer of conductive catalytic material extending into the recesses to mechanically lock the layer to the first surface, and a second electrode on the second surface of the body. The cell is preferably an oxygen sensor installed in the exhaust system of a combustion system, most preferably of an internal combustion engine. In a preferred embodiment, the cell is a lambda oxygen sensor formed as a thimble, the first surface being the outside of the thimble. The layer is plated on the first surface and is of substantially uniform thickness from a closed axial end of the thimble to near an open axial end of the thimble.
Preferably the solid electrolyte is a yttria-stabilized zirconia, having an yttria content of about two to ten percent yttria, most preferably having a mole percentage of yttrium of about 3-6% and a mole percentage of alumina of zero to twenty percent. The electrodes are preferably formed of platinum, rhodium, or palladium, most preferably platinum.
In accordance with another aspect of the invention, a method is provided of forming a solid electrolyte cell, the method comprising a step of forming a solid electrolyte body including a porous layer on a first surface of the body, a step of activating the first surface of the porous layer to form a plurality of growth points for a conductive layer on the first surface, a step of forming a first electrode by plating a conductive layer on the activated first surface of the body, and a step of forming a second electrode on a second surface of the body. Preferably, the porous layer comprises substantially spherical recesses which are formed by coating the body with a slurry of solid electrolyte including in the slurry spray-dried balls of the electrolyte. On firing the body, the spray-dried balls are densified to form small balls of solid electrolyte at the bottoms of substantially spherical recesses. The plating process preferably includes activating the first surface by dipping the porous layer of the body in a solution of platinum salt in a volatile solvent, such as acetone, and allowing the solution to wick into the porous layer. The preferred platinum “salt” is hexachloroplatinic acid, and the term “salt” as used throughout the specification and claims will be understood to include acids. The solvent preferably wets the ceramic. The body is then fired to drive off the solvent and reduce the platinum salt to a 0.01 to 0.5 micron layer of platinum, with numerous unplated areas. The activated body is then plated by electroless plating procedures to grow a coating of about one to ten microns of platinum on the first surface. The coating is permeable to oxygen at the intersections of crystals emanating from individual activation sites. The platinum coating is mechanically locked into the spherical recesses during the plating process.
In accordance with another aspect of the invention, a method is provided of forming a solid electrolyte cell, the method comprising a step of forming a body including an elongate body formed of a solid electrolyte compact, thereafter a step of drilling an axial cavity in the body, and thereafter a step of firing the body to densify it. Preferably the body is formed by uniaxially compressing a zirconia powder into a thimble having a tapered bore, and then drilling out the tapered bore to form a substantially cylindrical cavity.
In accordance with another aspect of the invention, an oxygen sensor is provided including a thimble-shaped electrolytic cell having an interior defined by a substantially cylindrical wall, an electrical contact on the wall, an elongate electrical terminal extending from outside the cell into the interior of the cell, the terminal including a pair of arms, at least one of the arms engaging the contact on the wall, and an elongate electrical heater extending into the interior of the cell, the terminal arms embracing the heater and positioning the heater in the cell.


REFERENCES:
patent: 3400054 (1968-09-01), Ruka et al.
patent: 3562911 (1971-02-01), Walter et al.
patent: 3844920 (1974-10-01), Burgett et al.
patent: 3978006

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