Method of manufacturing an oxygen sensor

Abstract

An oxygen sensor is disposed downstream from a catalyst for purifying exhaust gas from an internal combustion engine and which can suppress an influence of unburnt hydrocarbon on an output voltage. After forming a platinum thin film on the outer periphery of a zirconia ceramic body, only a detection electrode of the ceramic body is dipped in a silver nitrate aqueous solution of 0.1 mol/l, and the silver nitrate is pyrolyzed through a heat treatment. Subsequently, a platinum reference electrode is formed on the inner periphery of the ceramic body. To protect the silver-doped detection electrode, a protective layer is formed on the surface of the detection electrode. By the exposure to combustion gas and through aging, a detection element is formed, and set into a metal case together with a cylindrical heater, to complete an oxygen sensor to be disposed downstream from a CNG engine catalyst.

What is claimed is:  
     
         1 . An oxygen sensor for being disposed downstream from a catalyst for purifying an exhaust gas from an internal combustion engine using fuel which contains hydrocarbon having a ratio of hydrogen to carbon of at least 3:1, the oxygen sensor having a detection electrode on one exterior surface of a solid electrolytic body having an oxygen ion conductivity and having a reference electrode on an opposed exterior surface on the solid electrolytic body, and the oxygen sensor further comprising: 
 an output inhibiting means for controlling an output voltage of the oxygen sensor, and the output voltage varying in accordance with a concentration of oxygen and at least one of hydrogen, carbon monoxide and hydrocarbon, in such a manner that the output voltage which depends on the concentration of hydrocarbon is prevented from causing the output voltage to exceed a reference level by which it is determined whether an air/fuel ratio is one of lean and rich.    
     
     
         2 . The oxygen sensor according to  claim 1 , wherein the output inhibiting means inhibits the output voltage which is dependent on the concentration of hydrocarbon from causing the output voltage to exceed the reference level for determining the air/fuel ratio to be one of rich and lean within the range of a sensor active temperature.  
     
     
         3 . The oxygen sensor according to  claim 1 , wherein the output inhibiting means inhibits the output voltage which is dependent on the concentration of hydrocarbon from causing the output voltage to exceed the reference level for determining the air/fuel ratio to be one of rich and lean at at least 400° C.  
     
     
         4 . The oxygen sensor according to  claim 1  wherein the reference level is in a range between about 400 mV to about 600 mV.  
     
     
         5 . The oxygen sensor according to  claim 1 , wherein the output inhibiting means comprises the detection electrode, and the detection electrode is mainly composed of a noble metal which has a catalytic action to promote burning of any unburnt hydrocarbon-containing fuel and the detection electrode contains, at least on an exposed surface thereof, at least one element selected from the group consisting of silver, copper, gold, and lead.  
     
     
         6 . The oxygen sensor according to  claim 1 , wherein the output inhibiting means comprises the detection electrode, the detection electrode is composed mainly of a noble metal which has a catalytic action to promote burning of any unburnt hydrocarbon-containing fuel, and the noble metal element has an average particle diameter of at least 2 μm.  
     
     
         7 . The oxygen output sensor according to  claim 1 , wherein, when the sensor temperature is around 400° C. and in an atmosphere containing 3000 ppm methane, 1200 ppm oxygen, with the remainder being non-combustible gasses, the output voltage is below the reference level; and 
 the output voltage is at least at the reference level when the sensor temperature is around 400° C. and in an atmosphere containing 3300 ppm hydrogen, 1000 ppm oxygen, with the remainder being non-combustible gasses.  
 
     
     
         8 . The oxygen sensor according to  claim 1 , wherein even after the oxygen sensor is exposed, at 900° C. for 1000 hours, to the exhaust gas from the internal combustion engine using the hydrocarbon-containing fuel with the ratio of hydrogen to carbon of at least 3:1, the oxygen sensor maintains characteristics that the output voltage does not exceed the reference level under the condition that the sensor temperature is 400° C. in the atmosphere containing 3000 ppm methane, 1200 ppm oxygen, with the remainder being non-combustible gasses, and that the output voltage is at least at the reference level under the conditions that the sensor temperature is 400° C. in the atmosphere containing 3300 ppm hydrogen, 1000 ppm oxygen, with the remainder being non-combustible gasses.  
     
     
         9 . An oxygen sensor comprising: 
 a solid electrolytic body having opposed first and second surfaces, and the solid electrolytic body being formed from a material having an oxygen ion conductivity;    a detection electrode being disposed on the first surface, and the detection electrode being composed substantially of a noble metal and having a catalyst inhibitor for inhibiting a catalyst activity of the detection electrode such that an output voltage which depends on the concentration of hydrocarbon is prevented from causing the output voltage to exceed a reference level by which it is determined whether an air/fuel ratio is one of lean and rich; and    a reference electrode being disposed on the second surface, and the reference electrode being composed substantially of a noble metal.    
     
     
         10 . The oxygen sensor of  claim 9 , wherein the catalyst inhibitor is a dopant combined with the detection electrode, and the dopant is selected from the group comprising gold, silver, copper, and lead.  
     
     
         11 . The oxygen sensor of  claim 9 , wherein the catalyst inhibitor comprises the detection electrode which is heated to at least 1200° C.  
     
     
         12 . The oxygen sensor of  claim 9 , wherein the catalyst inhibitor comprises the detection electrode made from iridium.  
     
     
         13 . The oxygen sensor of  claim 9 , wherein the noble metal is platinum.  
     
     
         14 . An oxygen sensor comprising: 
 a solid electrolytic body having opposed first and second surfaces, and the solid electrolytic body being formed from a material having an oxygen ion conductivity;    a detection electrode being disposed on the first surface, and the detection electrode being composed substantially of a noble metal;    a reference electrode being disposed on the second surface, and the reference electrode being composed substantially of a noble metal; and    a gas limiting means for limiting the flow of hydrocarbons to the detection electrode, whereby the gas limiting means functions as an output inhibiting means for controlling an output voltage of the oxygen sensor, and the output voltage varying in accordance with a concentration of oxygen and at least one of hydrogen, carbon monoxide and hydrocarbon, in such a manner that the output voltage which depends on the concentration of hydrocarbon is prevented from causing the output voltage to exceed a reference level by which it is determined whether an air/fuel ratio is one of lean and rich.    
     
     
         15 . The oxygen sensor of  claim 14 , wherein the gas limiting means is a protective layer disposed about the detecting electrode and has a porosity which allows oxygen is to pass therethrough but limits the flow of hydrocarbon therethrough.  
     
     
         16 . The oxygen sensor of  claim 15 , wherein the protective layer is composed of magnesium aluminate and has a thickness of about 200 μm.