Exhaust gas sensor having porous, metal-impregnated ceramic element

An improved sensor of the type having a metal oxide ceramic element that undergoes changes in resistivity in response to variations in the partial pressure of oxygen in the gaseous medium to which the metal oxide ceramic element is exposed, the metal oxide ceramic element having electrodes in spaced apart relationship, the electrodes extending from the metal oxide ceramic element to permit the resistance between them to be sensed, and the metal oxide ceramic element being adapted to be immersed in the gaseous medium, wherein the improvement comprises:

• the metal oxide ceramic element is titania (TiO.sub.2) and is porous to enlarge, as compared to a less porous material, the amount of titania element surface area exposed to the gaseous medium, and the titania element surface area has on it, and substantially throughout the volume of the titania ceramic element, a discontinuous charge transfer material comprising a metal having free electrons, whereby, the response time of the titania ceramic element is substantially reduced due to the presence of the charge transfer material, in the lower portion of the temperature range over which the titania ceramic element responds to variations in the partial pressure of oxygen in the gaseous medium;
• the sensor includes a second ceramic element less responsive to variations in the partial pressure of oxygen in the gaseous medium that is the first-mentioned ceramic element, and includes means for electrically connecting the first-mentioned ceramic element and second ceramic element in circuit such that output voltage changes from 33% to 66%, and vice versa, of applied voltage occur in less than about 300 milliseconds when the ceramic elements are at a temperature of about 350.degree. C.