The present invention is directed to a sensor element for determining a physical property of a measuring gas, in particular the pressure or the concentration of a gas component in a gas mixture, in particular in the exhaust gas from an internal combustion engine.
In a known electrochemical sensor for determining the oxygen content in gas mixtures having a heating device for generating the operating temperature of the sensor element (German Published Patent Application No. 198 15 700), the volume provided with pores via which the reference electrode is connected to a reference gas channel conveying the reference gas is formed as a layer plane between the reference channel and the reference electrode and is used for improved thermal coupling between the reference electrode and the resistance heating element of the heating device, providing uniform heat distribution. In addition, the porous layer reduces increased mechanical stresses which occur on the edge of the reference gas channel toward the adjacent solid electrolyte and which may result in stress cracks in the solid electrolyte body. Furthermore, the large-surface contact of the reference electrode with the adjacent porous layer achieves improved adhesion because the reference electrode is thus held pressed between neighboring films during lamination of the solid electrolyte body which is made up of films.
In sensor elements of this type, functionality is reduced over the long term due to aging processes. This affects in particular the external sensor areas exposed to the exhaust gas of internal combustion engines where electrodes are situated. The presence of foreign substances in the exhaust gas, such as acidic exhaust gas components, e.g., phosphorus or sulfur compounds, neutral particles, and oil ashes including compounds containing Ca, P, Zn, Mn, Fe as well as lead and silicon compounds can result in deposits on, or direct chemical interactions with, the electrodes with the consequence of changed electrode activity, electrode poisoning or electrode passivation.
However, the reference gas, in particular when the ambient air in the engine compartment of a motor vehicle is used as a reference gas, also contains contaminants—even if only a small amount—which result in accelerated aging of the reference electrode. Sources of such contaminants in the reference gas or in the reference air are insulating and sealing materials as well as detergents and lubricants which are used in the motor vehicle's engine compartment.
The sensor element according to the present invention has the advantage that through the selection of the volume material, through which the reference gas is applied to the reference electrode, with respect to its physical and chemical properties, in particular with respect to its affinity for binding the foreign substances normally present in the reference gas, the foreign substances are bound in the porous volume or enter into a chemical reaction in the porous volume and are thus unable to interact with the electrode surface of the reference electrode. Since the reference electrode is generally situated in a reference channel formed in the interior of the solid electrolyte, the requirements placed on the mechanical strength of the volume material are not high.
According to an advantageous specific embodiment of the present invention, the porous volume is designed as a porous protective layer that covers the open surfaces of the reference electrode situated on the solid electrolyte. In this connection, the protective layer is applied in the form of a paste in a specific work operation and then sintered in a cofiring process.
According to an advantageous specific embodiment of the present invention, the porous volume completely fills at least one channel segment of a reference gas channel in which the reference electrode is situated, the channel segment being located upstream from the reference electrode. In this case also, the volume material is introduced into the reference channel in the form of a paste and then sintered by cofiring so that the channel cross section is completely filled. In both cases, the porosity and layer thickness are optimized in such a way that a free gas exchange is ensured between the reference electrode and the reference gas channel without impairing the sensor function. For example, the porosity of the filling volume is 20% to 60% and the layer thickness of the porous protective layer is 5 microns to 50 microns.
The sensor element shown in two different sectional views in
On first solid electrolyte layer 111, an external electrode 12 is applied to an outer surface of solid electrolyte body 11, the external electrode being covered by a protective layer 13. Protective layer 13 is designed to be porous so that external electrode 12 is exposed to the exhaust gas surrounding the sensor element through protective layer 13. A reference electrode 14 is applied to the surface of first solid electrolyte layer 11 facing away from external electrode 12. Reference electrode 14 is situated in a reference gas channel 15, which is introduced into second solid electrolyte layer 112 and is covered from above by first solid electrolyte layer 111 and from below by third solid electrolyte layer 113.
An electric resistance heater 16 is provided between third solid electrolyte layer 113 and fourth solid electrolyte layer 114 for heating the electrode area, the resistance heater having a preferably meandering heating surface 17 and two printed conductors (not shown) leading to heating surface 17 for the supply of current. Heating surface 17 and the feed conductors are embedded in an electrical insulation 18 which is laterally surrounded by a sealing frame 19. It is of course possible to omit sealing frame 19 and to route insulation 18 to the lateral surfaces of solid electrolyte body 11.
A reference gas is applied to reference gas channel 15, atmospheric air withdrawn from the engine compartment of a motor vehicle equipped with the internal combustion engine preferably being used as a reference gas. In order to protect reference electrode 14 against contamination caused by foreign substances or pollutants contained in the reference air, reference electrode 14 is not directly exposed to the reference gas or reference air but is instead exposed through a porous volume, the volume material of which is selected with respect to its physical and chemical properties in such a way that the foreign substances contained in the reference gas are bound in the volume and/or subjected to a chemical reaction. Sources of such a contamination of the reference air are insulation and sealing materials as well as detergents and lubricants which are normally used in the motor vehicle's engine compartment. The porosity of the volume is optimized so that a free gas exchange may occur between reference electrode 14 and reference gas channel 15. The volume material selected with regard to its affinity for binding the foreign substances contained in the reference gas binds these foreign substances or exposes them to a chemical reaction in the volume at the time the reference gas is diffused through the volume, so that foreign substances do not interact with the electrode surface of reference electrode 14 and are unable to cause reference electrode 14 to age prematurely.
The volume is preferably made up as follows for use of the sensor element in the exhaust gas of an internal combustion engine:
30-70% yttrium oxide (Y2O3)/zirconium oxide (ZrO2)
30-70% aluminum oxide (Al2O3)
0-20% lithium oxide (Li2O3)
0-20% calcium oxide (CaO)
0-20% magnesium oxide (MgO)
0-20% titanium oxide (TiO2)
0-20% cerium oxide (CeO2)
In the exemplary embodiment according to
In the exemplary embodiment of
The present invention is not limited to the described sensor element for a lambda sensor operating according to the Nernst principle. The protection according to the present invention of reference electrode 14 against harmful contaminations in the reference gas may also be implemented in sensor elements for planar wideband sensors as described in DE 199 41 051 A1 or for λ=1 or lambda sensors designed as finger sensors as described in DE 43 12 506 A1. The present invention may also be used to the same advantage in sensor elements having a reference electrode 14 for pressure measurement in a gas, in particular in the exhaust gas of an internal combustion engine.
Number | Date | Country | Kind |
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10 2004 027 630.7 | Jun 2004 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP05/51905 | 4/27/2005 | WO | 6/8/2007 |