METHOD OF PRODUCING EXHAUST GAS SENSORS

Abstract: A method for inhibiting the loss of the effectiveness of an exhaust gas sensor that employs a metal oxide ceramic ele­ ment. The ceramic element generates an electrical signal in response to variations in the air/fuel ratio of the mixture supplied to an internal combustion engine. The engine exhaust gases cause the ceramic element to change charac­ teristics in response to variations in the partial pressure of oxygen in the engine's exhaust gases. Exhaust gases ing zinc/phosphorous compounds have been found to cause the ceramic to lose its electrical response due to the deposi­ tion of such compounds on the ceramic element as a glassy coating. The application of organic salt solutions to the exhaust gas sensor metal oxide elements, either before or after failure, has been found to inhibit sensor signal loss. A salt applied as a solution of [Mg(C₂H₃O₂)₂] and/or [Ca(c₂H₃O₂)₂] is currently preferred.

Description: DESCRIPTION. This invention relates to exhaust gas sensors and methods for their production. An exhaust gas sensor of preferred design has a titania sensing element that is immersed in exhaust gases from an internal combustion engine throughout its useful life. The titania is a metal oxide ceramic material that has an electrical resistance that varies as a function of the partial pressure of oxygen in the exhaust gases to which the sensor is exposed. We have found that, when the titania element is immersed in exhaust gases that contain the products of combustion of engine oils that contain zinc dialkyldithiophosphate oil additives, the products of combustion cause zinc phosphate and other zinc/phosphorous compounds to be deposited on the titania element of the exhaust gas sensor. The zinc compounds are semiconductor materials that cause a shunting effect in the titania ceramic structure and this reduces the electrical resistance between electrodes connected to the titania element. The electrical resistance between these terminals is a function of the characteristics of the titania ceramic material under normal circumstances and varies with the oxygen content of the medium that surrounds the titania element. When immersed in exhaust gases from an engine, the titania element has a substantial change in electrical resistance as a result of excursions of the exhaust gases through compositions corresponding to rich and lean air/fuel ratios of the mixture supplied to the engine. The titania element cannot function properly if the exhaust gases contain substantial quantities of zinc/ phosphorous compounds because these are the cause of the formation of a continuous path of semi conductive zinc/phosphorous compounds along the grain boundaries of the titania element. According to the present invention a method of producing an exhaust gas sensor which comprises a metal oxide ceramic sensor element characterised in that the sensor element is treated with an inhibitor material which reduces the formation thereon of compounds of zinc and/or phosphorous as a result of exposure to motor vehicle engine exhaust gases. The invention also includes an exhaust gas sensor comprising a metal oxide sensor element characterised in that the sensor element includes an inhibitor material which reduces the formation of zinc or phosphorous compounds on the sensor element as a result of exposure to motor vehicle engine exhaust gases. The preferred method of the present invention provides an inhibitor coating on the titania element that prevents the formation of zinc/phosphorous compounds along the grain boundaries. The inhibitor coating is not a semiconductive material, reacts with phosphorous in preference to zinc, and does not form a continuous coating when it reacts with the zinc/phosphorous compounds. The method comprises, generally, the step of applying an inhibitor coating to a titania exhaust gas sensor element to prevent loss of the effectiveness of the titania element as an exhaust gas sensor as a result of the deposition thereon of zinc/phosphorous compounds. The inhibitor coating preferably is applied as a water solution of mixed organic salts of an element of Group II of the Periodic Table, especially magnesium or calcium, which forms an inhibitor coating. The inhibitor coating also has been found to be effective when applied to exhaust gas sensors of the zirconia type. The invention will now be described in more detail with reference to preferred embodiments thereof. In accordance with the preferred method of the present invention, a compound of a metal and acetate, an organic material, combines with phosphorous to form a compound that when applied or after heating inhibits the formation of zinc/phosphorous compounds on the surface of a metal oxide ceramic material. The compounds of zinc and phosphorous are of a glassy consistency and when deposited on the surface of the metal oxide ceramic material can cause a loss of effectiveness of an exhaust gas sensor in which the metal oxide ceramic material is used. The metal oxide ceramic material may be titanium dioxide (titania) or zirconium dioxide (zirconia) and these materials have found widespread use in exhaust gas sensors required in feedback fuel control systems for internal combustion engines. The exhaust gas sensors are positioned in the exhaust gases that emanate from the engine and, thus, are exposed to the various products of combustion. Metal oxide ceramic materials in the exhaust gas sensors are porous to some degree and have grain boundaries within them. The titania sensor has an electrical resistance that varies as a function of the oxygen content of the exhaust gases to which it is exposed, whereas the zirconia sensor hap an electromotive force that is generated across its opposite surfaces that is a function of the difference in oxygen partial pressures of the gases to which its respective opposite sides are exposed. The undesirable zinc/phosphorous compounds are deposited on the metal oxide ceramic material as a result of the combustion of zinc dialkyldithiophosphates normally contained in engine oils. This undesirable deposit prevents the ceramic material and usual noble metals on the exhaust gas sensors from functioning as they should in producing the desired electrical signal variations in response to changing oxygen partial pressures in the exhaust gases. Combustion of a lean air/fuel mixture in an engine produces exhaust gases that contain a large amount of oxygen as compared to exhaust gases produced by the combustion of a rich air/fuel mixture. The purpose of the exhaust gas sensor is to detect transitions between the lean and rich mixture conditions so that the average of the mixture supplied to the engine can be held at or near the stoichiometric level. The grain boundaries in the metal oxide ceramic material of the exhaust gas sensor are the locations where deposits tend to accumulate. The glassy coating formed by zinc/phosphorous is semiconductive and is- quite continuous along these grain boundaries. -As a result, in the titania exhaust gas sensor, the zinc/phosphorous compounds tend to prevent an electrical resistance change, between the usual electrodes located in spaced relationship in the titania element, in response to exhaust gas compositional changes relative to the stoichiometric value of air and fuel supplied to the engine. In other words, the titania exhaust gas sensor fails to function as required to detect air/fuel mixture excursions about stoichiometry. This is due to the glassy zinc/phosphorous compounds deposited on the titania. The zirconia sensor has a noble metal (platinum or platinum alloy) coating on each of its surfaces that are used to detect the electromotive force produced by the zirconia in response to differing partial pressures of oxygen in the respective exhaust and reference (air) gases to which its opposite sides are exposed, as is well known. The zirconia is an electrolyte that allows oxygen ions to migrate through it to result in production of the electromotive force. When the glassy zinc/phosphorous compounds form on the surface of the zirconia ceramic exposed to the engine exhaust gases, the glassy coating can become substantially continuous and, thereby, can inhibit oxygen ion migration through the zirconia solid electrolyte. The oxygen ions migrate through the zirconia due to interstitial vacancies that exist within its crystal structure. In the most commonly used engine exhaust gas sensors of the zirconia type, the interstitial vacancies are formed by the controlled addition of yttrium to the zirconia powder that is sintered to form the tubular zirconia element of the exhaust gas sensor. The glassy coating on the ceramic material of the exhaust gas sensor can result in exhaust gas sensor failure, as described above. As previously stated, the glassy coating is the result of the combustion of zinc dialkyldithio phosphates. These zinc/phosphorous additives are contained in varying proportions in engine oils marketed in most countries. The zinc dialkyldithiophosphates decompose to form the zinc/phosphorous compounds that are deposited on the exhaust gas sensor ceramic elements. The zinc/phosphorous compounds can be complex in the mixture of these that accumulates on the exhaust gas sensor metal oxide ceramic elements, but the compounds are primarily comprised of zinc phosphate. The greater the burning of oil in an engine and the greater the content of the zinc dialkyldithiophosphates in the oil, the faster the zinc phosphates accumulate on the metal oxide ceramic elements of the sensors in the absence of use of the method of the present invention. In the method of the present invention, the formation of zinc/phosphorous compounds on the surface of an exhaust gas sensor ceramic element is inhibited by the application to the ceramic element of a coating of an organic salt dissolved in distilled water, such as a water solution of a mixture of magnesium acetate [Mg(C2H302)2] and calcium acetate [Ca(C2H302)2] in equal parts. The acetate, upon heating of the compound, decomposes to form an oxide and is an organic material that in combination with hydrogen forms a weak acid. This is the presently preferred method. The application of this coating to the ceramic element of an oxygen sensor, whether over the usual noble metals already deposited thereon or not, inhibits the stabilization or adhesion of tinc/phosphorous compounds to the sensor. When the zinc/phosphorous compounds in the exhaust gases reach the sensor, the organic salt (which upon heating of the sensor or otherwise may have decomposed to a mixture of metal oxides) will give up its metal component. In the preferred method, either magnesium or calcium or both are substituted for the zinc in the zinc/phosphorous compounds that contact the organic-salt-coated metal oxide ceramic element of the sensor. It is desirable that the organic salt be applied to the sensor metal oxide element when the sensor is first manufactured, but the inventor has found that exhaust gas sensors of the titania type can be rejuvenated after failure due to zinc/phosphorous compound deposition on the titania, ' immersion of the titania elements thereof in a solution of equal parts of a mixture of 1.0 molar solutions of [Mg(C2H3O2)2] and [Ca(C2H3O2)2) and by heating thereafter 8500 for five minutes. These sensors returned to their original working condition after immersion in this organic salt solution and it was found to be due to nonconductive and thermodynamically stable calcium/magnesium phosphorous compounds present on the ceramic elements of the sensor. These calcium/magnesium/phosphorous compounds are of a nonconductive and discontinuous character and do not inhibit the normal function of the exhaust gas sensors. Phosphorous compounds on the sensor ceramic elements are not harmful unless they prevent the generation of the exhaust gas sense electrical signals desired. Zinc/phosphorous compounds are harmful, but neither magnesium nor calcium compounds with phosphorous are harmful to sensor operation. The zinc/phosphorous compounds in exhaust gases and the organic salts on the metal oxide ceramic elements react to form other compounds. Phosphorous compounds are formed having metals other than zinc as a constituent. The organic radical compounds, which may have decomposed to an oxide or other element or radical, reacts with the zinc and becomes a innocuous compound from the standpoint of its presence, if present, on the metal oxide ceramic materials in the exhaust gas sensor. The method of the invention inhibits the loss of effectiveness of the metal oxide ceramic material of an exhaust gas oxygen sensor by preventing the formation of zinc/phosphorous compounds on the ceramic material. Sulfur compounds also may form on ceramic material used in an exhaust gas environment, the method of the invention can be used to prevent this from occurring as well.

Claims: CLAIMS. 1. A method of producing an exhaust gas sensor which comprises a metal oxide ceramic sensor element characterised in that the sensor element is treated with an inhibitor material which reduces the formation thereon of compounds of zinc and/or phosphorous as a result of exposure to motor vehicle engine exhaust gases. 2. A method according to claim 1 wherein the inhibitor material is abe to react chemically with phosphorous without forming a semiconductive compound. 3. A method according to Claim 1 or 2 wherein the inhibitor material contains an organic salt of a Group IT metal. 4. A method according to Claims 1,2, or 3 wherein the inhibitor material is an organic solution containing an alkaline earth metal that forms an oxide upon the decomposition of the organic solution by heating in the presence of air or other oxygen containing environment. 5. A method according to Claims 1,2,3, or 4 wherein the inhibitor material is an organic salt of magnesium. 6. A method according to Claims 1,2,3, or 4 wherein the inhibitor material is an organic salt calcium. 7. A method according to Claim 5 wherein the inhibitor material is the product of the decomposition of an aqueous solution of magnesium united with a material that is a weak acid when in combination with Itydrogen. 8. A method according to Claim 5 wherein the inhibitor material is tho product of the decomposition of an aqueous solution of calcium united with a material that is a weak acid when in combination with hydrogen. 9. A method according to Claims 1,2, or 3 wherein the inhibitor material is applied as a solution containing a material selected from the oup consisting of magnesium acetate (Mg (H2C302)2) and calcium acetate (Ca(H2C3O2)2 ). 10. An exhaust gas sensor comprising an metal oxide sensor element characterised in that the sensor element includes an inhibitor material which reduces the formation of zinc or phosphorous compounds on the sensor element as a result of exposure to motor vehicle engine exhaust gases.

Inventors: ACHARI, ACHYUTA
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1980
Main classification: G01N27
IPC codes: G01N27