Abstract:
A gas sensor for determining the concentration of gas components in gas mixtures is used in particular to measure the concentration of nitrogen oxides in exhaust gases of internal combustion engines or in the interiors of motor vehicles. It contains an electrochemical measuring cell which includes a first electrode situated on a solid electrolyte and an additional electrode. The electrode is made of an oxide material containing lanthanum which at least catalytically decomposes nitrogen oxides. The pump current flowing between the electrodes is used as a measure of the nitrogen oxide concentration in the gas mixture.

Description:
FIELD OF THE INVENTION 
   The invention relates to a gas sensor for determining gas components. 
   BACKGROUND INFORMATION 
   A sensor element for determining the NO x , concentration in gas mixtures is known from German Published Patent Application No. 196 52 968 which is based on the interaction of two electrochemical pump cells. Two internal pump electrodes that are interconnected with a common external pump electrode arranged in a reference gas channel are located in a measuring gas space of the sensor element. The first of the two pump cells in the direction of inflow of the gas mixture causes oxygen to be transported from the measuring gas space into the reference gas channel. The internal pump electrode of this first oxygen-transporting pump cell is covered with a multilayered structure made of a mixed-conducting metal oxide layer and an electrically insulating aluminum oxide layer, which selectively removes the oxygen present in the gas mixture without changing the concentration of nitrogen oxides. These are then decomposed at the internal pump electrode of the second pump cell and the oxygen released in doing so is pumped off. The pump current of the second pump cell is used as a measure of the concentration of nitrogen oxides contained in the gas mixture. 
   A gas sensor for determining the NO x , concentration in gas mixtures is also known from European Published Patent Application No. 678 740. It includes two measuring gas spaces with one pump cell each, which are arranged adjacent to each other in a layer plane of a planar ceramic substrate. The measuring gas flows through a diffusion opening into the first measuring gas space in which a first pump cell is located. The first pump cell is used in the first measuring gas space to set a predetermined oxygen partial pressure by pumping oxygen in or out. A concentration cell also situated in the first measuring gas space makes it possible to maintain a constantly low oxygen partial pressure in the first measuring gas space by determining the electrical voltage (electromotive force) present at the electrodes of the concentration cell. Via an additional diffusion opening, the gas mixture set to a constant oxygen partial pressure enters the second measuring space. An additional pump cell is situated in the second measuring gas space. Its internal pump electrode is made of rhodium and it makes it possible to decompose nitrogen oxides to N 2  and O 2 . The reduced oxygen arising at the internal pump electrode is pumped off via an applied pump voltage. The pump current of the second pump cell is proportional to the nitrogen oxide concentration of the gas mixture. 
   In both cases, a constantly low oxygen partial pressure of the gas mixture in the sensor element must be set in an elaborate manner before the nitrogen oxides can be determined with the internal pump electrodes used in them. 
   SUMMARY OF THE INVENTION 
   The gas sensor according to the present invention has the advantage that an electrochemical measuring cell, the NO x -sensitive electrode (which is made of a material that makes it possible to determine the nitrogen oxide concentration in a gas mixture reliably even at high oxygen partial pressures), may be used to determine the nitrogen oxide concentration in the measuring gas. This may make it unnecessary to install oxygen-transporting pump cells into the sensor element and therefore may considerably simplify the sensor design. 
   The use of the NO x -sensitive pump cell according to the invention may make it possible to omit the incorporation of a measuring gas space and a reference gas channel into the sensor element on which the gas sensor is based since the NO x -sensitive pump electrode as well as the counter-electrode may be directly exposed to the exhaust gas. Of particular advantage may be a sandwich arrangement of both electrodes one above the other on the wide surface area of the sensor element. 
   If a reference gas channel is provided in the sensor element, a reference electrode situated there together with the NO x -sensitive electrode may make it possible to determine the nitrogen dioxide concentration of the gas mixture simultaneously using a voltage measurement as an alternative. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section through the wide surface area of a sensor element on which the gas sensor according to an exemplary embodiment of the invention is based. 
       FIGS. 2 and 3  show a sensor element according to alternative exemplary embodiments. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a basic structure of an exemplary embodiment of a planar sensor element  10  of an electrochemical gas sensor. Sensor element  10  may have a plurality of oxygen ion-conducting solid electrolyte layers  11   a ,  11   b ,  11   c ,  11   d ,  11   e  and  11   f . Solid electrolyte layers  11   a – 11   f  may be designed as ceramic films and may form a planar ceramic body. The integrated form of the planar ceramic body of sensor element  10  may be produced by laminating together the ceramic films printed with functional layers and subsequently sintering the laminated structure. Each of solid electrolyte layers  11   a – 11   f  may be made of oxygen ion-conducting solid electrolyte material such as, for example, ZrO 2  partially or fully stabilized with Y 2 O 3 . 
   Sensor element  10  may contain a measuring gas space  13  and, for example, a reference gas channel  19  in an additional layer plane  11   d , in which one end of the reference gas channel may lead out from the planar body of sensor element  10  and may be in contact with an atmosphere of air. 
   Moreover, a resistance heater  40  may be embedded in the ceramic body of sensor element  10  between two electrical insulation layers. The resistance heater may be used to heat sensor element  10  to the required operating temperature. 
   In addition, sensor element  10  may have a gas inlet opening  21  which may conduct the measuring gas into first measuring space  13 . Gas inlet opening  21  may be, for example, situated in the same layer as measuring gas space  13 . A first diffusion barrier  23  of, for example, porous ceramic material may be formed at the inlet to first measuring gas space  13  downstream of gas inlet opening  21  in the direction of diffusion of the measuring gas. 
   An internal NO x -sensitive electrode  31  may be situated in measuring gas space  13 . Associated external electrode  32  may be located in reference gas channel  19 . Both electrodes  31 ,  32  may be interconnected with a pump cell. Electrode  32  may be made of a catalytically active material, for example, platinum. The electrode material for electrode  32  may be used as a cermet in order to sinter it to the ceramic films. Electrodes  31 ,  32  are contacted via printed conductors, which are guided between solid electrolyte layers  11   a  and  11   b  and are connected to the wide surface area of the sensor element via throughplating. 
   In order to ensure that the nitrogen oxides contained in the gas mixture are completely decomposed into nitrogen and oxygen on NO x -sensitive electrode  31 , NO x -sensitive electrode  31  may be made of a catalytically active, oxide material, for example, from a lanthanum-containing perovskite of the composition La 1−x  Sr x  CO 1−y  Cu y  O 3−δ . Traditionally, electrodes of this type may be produced from rhodium or a platinum-rhodium alloy. The latter may only allow a reliable determination of the concentration of nitrogen oxides at very low oxygen concentrations of, for example, 0.02 ppm in the gas mixture and therefore may be usable only in sensors that remove the greatest proportion of the oxygen contained in the gas mixture electrochemically (see European Published Patent Application No. 678 740). 
   The electrode material of an exemplary embodiment of the present invention, which may include a lanthanum-containing perovskite, may permit a determination of the concentration of nitrogen oxides even at a 2 to 20% concentration of oxygen in the gas mixture. Although the nitrogen oxides may be present in these oxygen-rich gas mixtures at an unfavorable ratio compared to oxygen of 1:1000 to 1:10000, a linear dependence of the pump current flowing in the pump cell on the nitrogen oxide concentration may be observed when lanthanum-containing perovskite is used. The oxygen present in the gas mixture may be observed only in the form of a slightly elevated baseline which may be hardly subject to change even when the oxygen concentration varies greatly. 
   This characteristic may be all the more unexpected since previously a perovskite of the same composition may have only been used as an oxygen-selective protective layer for pump cells for the removal of molecular oxygen from gas mixtures (see German Published Patent Application No. 196 52 968). Only molecular oxygen may be absorbed on this protective layer while it may be impossible to catalytically decompose nitrogen oxides. However, the protective layer may be present as an electrically insulating metallic oxide layer and no pump voltage may be applied to it. 
   The high measuring accuracy of the lanthanum-containing perovskite used according to an exemplary embodiment of the present invention as an NO x -sensitive electrode  31  even at high oxygen concentrations in the gas mixture may make it possible as an alternative to arrange this electrode on the wide surface area of sensor element  10  directly exposed to the gas mixture and thus eliminate the incorporation of a measuring gas space  13  in the sensor element. If, for example, external electrode  32  is also formed on the wide surface area of the sensor element exposed to the gas mixture, the sensor design may be further simplified since a reference gas channel may be omitted also. A sensor element of this design is shown in  FIG. 2 . To protect against contamination, electrodes  31 ,  32  may be additionally provided with a porous gas-permeable protective layer  35  made of CeO 2 , for example. 
   An exemplary embodiment of the invention may provide that electrodes  31 ,  32  are not situated adjacent to each other on the wide surface area of the sensor element, as shown in  FIG. 2 , but rather on top of each other as in a sandwich and separated by a porous, gas-permeable and oxygen ion-conducting solid electrolyte layer  37 . An exemplary embodiment of this type is shown in  FIG. 3 . The arrangement of a measuring and a reference electrode in superimposed layers on the wide surface area of a sensor element may also be customary in mixed potential sensors. 
   If the incorporation of a reference gas channel in the sensor element is not omitted, a reference electrode  33  situated in it according to the exemplary embodiment illustrated in  FIG. 3  may, for example, be interconnected with NO x -sensitive electrode  31  to form a concentration cell. This may allow the simultaneous determination of the nitrogen oxide concentration by an amperometric method using the pump cell consisting of electrodes  31  and  32  and by a potentiometric method using the potential difference formed between electrodes  31  and  33 . 
   The use of a lanthanum-containing perovskite may not be limited to the explained exemplary embodiments but rather this material may also be used in conventional nitrogen oxide sensors having one or more measuring gas spaces and one or more pump cells and concentration cells. 
   As a result of the good oxygen tolerance of the sensor element of the exemplary embodiment of the present invention even at atmospheric oxygen concentrations, it may also be conceivable to use the sensor element in air quality sensors in addition to determining nitrogen oxide concentrations in exhaust gases of internal combustion engines.