Abstract:
A measuring sensor is described for determining a physical property of a measured gas, especially for determining the oxygen concentration or the pollutant concentration in the exhaust gas of internal combustion engines, which has a sensor element that is exposable to the measured gas which is at least partially coated with a protective layer that protects against harmful components in the measured gas. In order to achieve producing a “contamination protection”, that is cost-effective from a manufacturing technology point of view, particularly against silicon compounds and phosphorus compounds, the protective layer ( 26 ) is made of highly active γ- or δ-aluminum oxide (Al 2 O 3 ) having additives of compounds of the alkaline metals group, the alkaline earths group, the IV B subgroup or the lanthanides group.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is based on a sensing element for determining a physical property of a gas mixture, in particular for determining the oxygen concentration or the pollutant concentration in the exhaust gas of internal combustion engines.  
       BACKGROUND INFORMATION  
       [0002]     In measuring sensors that are exposed to the exhaust gas of internal combustion engines, during driving operation, as a result of combustion residues and compounds contained in the exhaust gas, such as heavy metals, for example, lead, manganese, zinc, cadmium, magnesium, cerium, phosphorus, silicon, which originate with additives or residual pollutants in fuels, lubricating oils and gaskets of the internal combustion engine, deposits form on the sensor element which, in the long run, lead to glazing or clogging of the diffusion barrier and protective layers or to poisoning, that is, passivating of the catalytic activity, of the electrodes of the sensor element. The so-called contamination of the measuring sensor interferes with the function of the sensor element, since the electrodes preferably made up of platinum or platinum alloys lose their catalytic function.  
         [0003]     In order to counter the so-called probe contamination due to lead residues, in one known measuring sensor of this type, (European Patent No. 0 159 905) the outer element of the sensor element is coated with a protective lining made up of a heat-resistant metal oxide, in which the lead-trapping, stable metals from the group platinum, ruthenium, paddaium, nickel, gold and alloys of the same are uniformly distributed.  
       SUMMARY OF THE INVENTION  
       [0004]     The measuring sensor according to the present invention has the advantage that the novel protective layer extremely effectively prevents a “contamination” of the electrodes of the measuring sensor by silicon and phosphorus compounds as well as other particulate and gaseous harmful components in the measured gas, and may be manufactured cost-effectively from a production technology point of view. Because of its extreme porosity, the protective layer is not able to be applied in great layer thickness, for instance, greater than 250 μm, which increases its effectiveness against contaminants, without the functional properties of the sensor element being impaired. The added solid in the protective layer may be introduced, for instance, as oxides, carbonates, acetates or nitrates of Ca, Al, La, Mg, Li, Ti, Zr. the measuring sensor according to the present invention may be designed as a finger probe or as a planar probe.  
         [0005]     The method according to the present invention, for producing the protective layer on the sensor element of the present invention, has the advantage that the viscosity and the solid content of the flowable substance (slip) or spreadable, dough-like substance (paste) that are prepared in a water-based manner, may be optimized for the respectively selected method of application. As an inorganic binding agent, aluminum nitrate or an aluminum hydroxide gel are preferably added, and as organic binding agent preferably water-soluble polymers, such as PVP (polyvinylpyrolidon), hydroxyethylcellulose, tylose or Zaponlack, or water-dispersable polymers, such as polyvinyl alcohols (PVA) or polyvinyl acetates (PVAe) By selection of the method of application as well as of the drying profile and burn-off profile, one may determine the desired layer thickness, layer porosity, layer adhesion and the layer stability of the protective layer.  
         [0006]     The adhesion of the protective layer to the sensor element, and the durability of the protective layer may, in principle, be improved by a high burn-off temperature during the production method. It is true that, in this context, one must put up with a loss in activity of the protective layer. In order to achieve an advantageously great layer adhesion at a low baking temperature, which does not effect any impairing of the protective function of the protective layer, the surface of the sensor element is appropriately prepared before the application of the protective layer, according to additional embodiments of the method according to the present invention.  
         [0007]     According to one advantageous specific embodiment of the present invention, the sensor element is produced having a surrounding frame on the protective lining and is sintered, and the substance is printed in the frame, such as in a silk-screen printing method, painted in or dripped in. The frame is preferably made of desnsely sintering zirconium oxide (ZrO 2 ).  
         [0008]     According to one advantageous specific embodiment of the present invention, the sensor element, in addition to the surrounding frame, is produced to have columns that project from the surface of the protective lining inside the frame, and is sintered. In this context, the columns are preferably made of the same material as the protective lining. The columns bring about a greater stability as well as a mechanical anchoring of the protective layer on the protective lining of the sensor element.  
         [0009]     According to an alternative specific embodiment of the present invention, the sensor element is made having a very porous adhesive layer that covers the protective lining and is sintered, and the substance is printed onto the adhesive layer, painted on, rolled on or dripped on. As the material for the very porous protective layer, preferably zirconium oxide (ZrO 2 ) is used, having a substantially higher proportion of aluminum oxide (Al 2 O 3 ) than the protective lining and a higher proportion of a pore-forming material than the protective lining. When the protective layer is applied onto the adhesive layer, because of the great porosity of the adhesive layer, the protective layer penetrates into it and is firmly anchored mechanically to it by the subsequent drying and burn-out process and thereby also to the sensor element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows, sectionally, a longitudinal section of a sensor element of a measuring sensor.  
         [0011]     FIGS.  2  to  4  show in each case an enlarged representation of cutout A in  FIG. 1 , having an exemplary embodiment for the pretreatment of the electrode protective lining in the measuring sensor according to  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0012]     Sensor element  30 , shown in  FIG. 1  sectionally schematically in longitudinal section for a measuring sensor developed, for example, as a planar broadband lambda probe for determining the oxygen concentration in the exhaust gas of internal combustion engines, is known as to construction and mode of operation, and is described in detail, for example, in German Patent No. 199 41 051. It has a ceramic element  10  made of a composite foil, on whose surface there is situated an outer electrode  11 , which, in turn, is covered by a porous protective lining  12 . Porous protective lining  12  is made of a zirconium oxide (ZrO 2 ) that has a low proportion of aluminum oxide (Al 2 O 3 ). Outer electrode  11  is printed onto a first solid electrolyte layer  13 , developed as a foil, made of yttrium-stabilized zirconium oxide (ZrO 2 ), and is connected to a printed circuit trace  111  that runs on the surface of solid electrolyte layer  13 , for applying a voltage potential. On the lower side of solid electrolyte layer  13 , an inner electrode  14  is printed on, lying opposite outer electrode  11 . Both electrodes  11 ,  14  are made of platinum or a platinum alloy. Onto the lower side of first solid electrolyte layer  13  a second solid electrolyte layer  15 , also designed as a foil, is printed on, using silk-screen printing of a paste-like ceramic material. In this second solid electrolyte layer  15  are formed, in a known manner, a measuring gas chamber  16 , in which inner electrode  14  is positioned together with a measuring electrode  17 , and a reference gas channel  18  in which a reference gas electrode  19  is situated. Reference gas channel  18  is subjected to a reference gas, such as environmental air, while measured gas chamber  16  is able to be exposed to the exhaust gas of the internal combustion engine via a diffusion barrier  20  and a bore  21  that is inserted into first solid electrolyte layer  13  and connected upstream of diffusion barrier  20 . Measuring electrode  17  and reference electrode  19  as well as diffusion barrier  20  are printed onto a third solid electrolyte layer  22 , which is also developed as a ceramic foil. Third solid electrolyte layer  22  is laminated together with a substrate  23 , which may also be made up of a solid electrolyte. On the surface of substrate  23  that faces third solid electrolyte layer  22 , there is a resistance heater  24  which is embedded in an insulation  25  made, for example, of aluminum oxide (Al 2 O 3 ). Sensor element  30 , thus constructed, is inserted into a housing of the measuring sensor in a gas-tight manner, and is connected via connecting lines to a connecting plug, so as to connect a control unit. The complete construction of the measuring sensor having the sensor element integrated into the housing is described in German Patent No. 197 41 203.  
         [0013]     In order to prevent a “contamination” of sensor element  30  by residues contained in the exhaust gas and contaminated particulate or gaseous components, especially silicon compounds and phosphorus compounds, the sensor element is covered with a protective layer  26  which covers the porous protective lining  12  and, just as outer electrode  11  and porous protective lining  12 , is left open in the area of bore  21  in first solid electrolyte layer  13 . Protective layer  26  is made up of highly active γ- or δ-aluminum oxide (Al 2 O 3 ) having additions of compounds of the alkaline metal groups, alkaline earth metal groups, IV B subgroup or lanthanide group, arid has great porosity and a large layer thickness, such as &gt;250 μm. The additives are oxides, carbonates, acetates or nitrates of the elements named, such as lithium oxide.  
         [0014]     Protective layer  26  on sensor element  30  of the measuring sensor is produced as follows:  
         [0015]     The named components of protective layer  26 , that is, aluminum oxide as a fine-particle, voluminous powder, and the additives, such as an alkaline earth oxide, are prepared with components of an organic and inorganic binding agent and a pore-forming substance in a water-based manner to form a pourable substance (a slip) or a paintable, dough-like substance (a paste). The viscosity (500-6000 mPas) and the solid matter content (35 wt.-% or less) of the substance is adapted to the type of the subsequent application method or the desired layer thickness of protective layer  26 . The substance thus prepared is applied to protective lining  12  of sensor element  30 . The application may be undertaken by dipping, rolling spraying, painting, dripping or printing. In the case of a finger-shaped sensor element, one has advantageously available the dipping, rolling or spraying method, and in the case of a planar sensor element  30 , as shown in  FIG. 1 , the dipping, painting, dripping or printing method is available. After the substance is applied, sensor element  30  is subjected to a temperature between 20° C. (room temperature) and 180° C., whereby the applied substance dries. After that, sensor element  30  is subjected to a temperature between 450° C. and 1150° C., whereby the binding agent components and the pore-forming components burn off, and protective layer  26  sinters onto protective lining  12 . The choice of application method, and the selected drying profile and burning-off profile are determinants for the layer thickness, the layer porosity and the layer adhesion and layer stability of protective layer  26  thus produced.  
         [0016]     In principle, the layer adhesion may be improved by raising the baking temperature, in connection with which, however, one must be put up with a loss in activity of protective layer  26 . In order to achieve a strong layer adhesion at an acceptable baking temperature, which does not lead to an impairment of the effectiveness of protective layer  26 , before applying the substance prepared as described above to sensor element  30 , the surface of protective lining  26  is suitably prepared.  
         [0017]     In the section of sensor element  30  shown in  FIG. 2 , sensor element  30  is produced having a surrounding frame  27 , which is preferably made up of a densely sintered zirconium oxide paste, and is sintered. Thereafter, the substance prepared as described above for protective layer  26  is then imprinted, painted or dripped into frame  27 , and sensor element  30  is then treated as was described.  
         [0018]     In the exemplary embodiment of sensor element  30  shown in  FIG. 3 , sensor element  30  is additionally produced having pillars  28  that extend from the surface of protective lining  12 , within frame  27 , and is sintered. Pillars  28  are made of the same material as protective lining  12 . Now, again, the substance is imprinted, painted or dripped into frame  27 , and then sensor element  30  is treated as was described.  
         [0019]     In the exemplary embodiment of sensor element  30  shown in  FIG. 4 , sensor element  30  is additionally produced having an adhesive layer  29  that is applied to porous protective lining  12 , and is sintered. Adhesive layer  29 , in the same way as porous protective lining  12 , is made up of zirconium oxide (ZrO 2 ) having proportions of aluminum oxide (Al 2 O 3 ) and a pore-forming material proportion, however, the pore-forming material proportion being substantially increased and the aluminum oxide proportion being dimensioned greater. Because of the increased proportion of pore-forming material, finished sintered adhesion layer  29  is porous. On this porous adhesion layer, the substance for protective layer  26 , that was prepared as described above, is printed on, painted on or dripped on, and then sensor element  30  is treated as described above. During the application of the substance onto adhesive layer  29 , the substance penetrates porous adhesive layer  29 , and protective layer  26  is mechanically firmly anchored in adhesive layer  29  during drying and sintering.  
         [0020]     The application of protective layer  26 , according to the present invention, is not limited to a planar sensor element  30  as shown in  FIG. 1 . It may be applied in the same way in the case of so-called finger probes, which are known, for instance, as stoichiometric or Nernst lambda probes.