Oxygen-sensing element

Abstract

An oxygen-sensing element particularly for use in the exhaust stream of an internal combustion engine. In one embodiment, the sensing element may be a tubular solid electrolyte body with a transverse web forming a relatively shallow recess at one end of the body and a relatively deep tube at the other. Separate electrodes are formed on the surface of the body proximate the recess and on both opposing sides of the web. A porous plug disposed in the recess covers one of the electrodes. When the electrode on the surface of the body is exposed to the exhaust stream of a fuel-rich air-fuel mixture, the EMF across that electrode and the electrode on the underside of the web, which is exposed to fresh air, is indicative of the oxygen content of the exhaust stream. When an exhaust stream from a fuel-lean air-fuel mixture is present, the magnitude of an oxygen pumping current that flows between the electrodes disposed on opposing sides of the web, in response to a voltage applied across those electrodes, indicates the oxygen content of the exhaust stream. The sensing element body may be conveniently formed from a slip prepared by ball milling yttria-stabilized zirconia powder with water and a binder. The slip may be cast in a mold or dried to form a flowable powder that is molded under pressure to form a green body. After calcining of the green body, electrodes are formed on the body by applying a platinum paste and heating it. A slurry of ball-milled calcined yttria-stabilized zirconia powder is disposed in the recess. The calcined green body and slurry are fired to produce a sensing element in which a tightly fitting porous plug, free of cracks and shrinkage away from the body, is disposed in the recess.