Field of the Invention
This invention relates to the detection of gases and more particularly, to a sensor element and method for making such a sensor element. More specifically, the present invention relates an improvement of an oxygen sensor proposed in our copending U.S. patent application No. 136,404, now U.S. Pat. No. 4,314,996 dated Feb. 9, 1982.
In the U.S. patent application, we have proposed an oxygen sensor which comprises a substrate made of perovskite compounds of the formula, ABO.sub.3, in which A represents an element of a lanthanum family, an alkaline earth metal or a mixture thereof and B represents a transition metal. This type of perovskite compound can serve as a catalyst for combustion and has such a characteristic that its electrical resistance abruptly varies in the vicinity of a stoichiometric point of combustion of both types of gases when applied as a sensor element.
Further, there have also been proposed gas sensor elements for detecting reducing gases and vapors such as alcohols or carbon monoxide or for determining oxygen concentration such as, for example, in U.S. Pat. No. 3,951,603. The sensor element of this patent makes use of a complex metal oxide of a perovskite crystal structure of the general formula A.sub.1-x A'.sub.x BO.sub.3-.delta. represents at least one element such as a rare earth metal, yttrium or hafnium, A' represents at least one element such as an alkaline earth metal or lithium, B is at least one element such as a transition metal, x is in the range of 0.ltoreq.x.ltoreq.1, and .delta. is a non-stoichiometricx parameter. This sensor element is applied to an atmospheric gas composition which has a large partial pressure of oxygen and its operation mechanism is substantially the same as that of gas sensors using ordinary oxide semiconductors.
Among various types of perovskite compounds, compounds which contain cobalt (Co) in the B site of the above formulas have experimentally been confirmed to show very low resistance in an oxidizing gas-rich atmosphere. In addition, the Co-containing compounds show abrupt variation in resistance in the vicinity of an equivalence or stoichiometric point of both types of gases and are thus very useful as a material for gas sensor element. However, this type of perovskite compound is disadvantageous in that once exposed to an atmosphere where reducing gases are rich or in excess, the compound does not become low in resistance even after the atmosphere has been changed to an oxidizing gas-rich atmosphere. Additionally, long-term exposure to a reducing gas atmosphere may result in breakage in structure of the perovskite compound.