Abstract:
A sensor element is described, having a heating device, which is used for determining at least one gas component of an exhaust gas of an internal combustion engine. On at least one outer surface of sensor element a heat-conducting layer is applied, at least in a plurality of places, which has a higher thermal conductivity than the outer surface of sensor element.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a sensor element.  
         BACKGROUND INFORMATION  
         [0002]    A conventional sensor element may be used, for example, in gas sensors which determine the oxygen content in the exhaust gas of internal combustion engines and may be used for regulating the air/fuel ratios of combustion mixtures in these internal combustion engines. The sensor element may be secured in position in the housing of the gas sensor by a sealed packing. The gas sensor may be mounted in a measuring port of an exhaust pipe. The sensor element may include at least one electrochemical cell, which may include a first and a second electrode, as well as a solid electrolyte positioned between the first and the second electrode. The electrochemical cell may be heated by a heating device to a temperature such as 500 to 800° C.  
           [0003]    An exhaust probe is described in German Patent Application No. 198 34 276, having a sensor element constructed with a planar technique and having a layered structure. In its measuring region, the sensor element may include an electrochemical cell which is heated by a heating device that may also be positioned in the measuring region. An electrode of the electrochemical cell and also the heating device may be electrically connected, by supply leads arranged in a supply lead region of the sensor element, to contact surfaces arranged at the end of the sensor element facing away from the measuring region. The heating device may be positioned between a first and a second solid electrolyte foil and may include, in the measuring region, a heater which may be separated from the surrounding solid electrolyte foils by a heater insulation.  
           [0004]    In the measuring region as well as in the transition region of measuring region and supply lead region, high temperature gradients may appear at the outer surfaces of the sensor element, which may lead to high compressive or tensile stresses, and thereby may finally lead to cracks in the ceramic.  
         SUMMARY  
         [0005]    In an example sensor element according to the present invention, the temperature gradient on the outer surfaces of the sensor element may be minimized by a heat-conducting layer, so that cracks caused by temperature-related compressive and tensile stresses may be avoided. These compressive and tensile stresses may result from a nonhomogeneous temperature distribution in the sensor element which may be the result of heating the sensor element by the heating device and of the temperatures present in the operation outside the sensor element. The heat-conducting layer may effect a temperature adjustment among regions having different temperatures, whereby the temperature gradient, and thereby the mechanical tensions may be minimized.  
           [0006]    Further developments and improvements may be possible.  
           [0007]    In a sensor element produced by planar technique, if the heat-conducting layer is positioned on an outer surface parallel to the layer plane of the heating device, the heat-conducting layer may be applied with the use of the same technique. The outer surface lying closer to the heating device may be furnished with a heat-conducting layer, since at this outer surface the temperature gradients, and thus the mechanical tensions, may be at their highest.  
           [0008]    If the sensor element has a measuring region and a supply lead region, and if the measuring region is heated by the heating device, then the heat-conducting layer may be provided on an outer surface of the sensor element, at least in a plurality of places in the measuring region and/or in the transition region between measuring region and supply lead region, since high mechanical tensions may appear in these regions by the heating process.  
           [0009]    The heat-conducting layer may be provided, for example, in the region of the edges of the sensor element, since in these regions the susceptibility to cracks may be greatest, on account of the mechanical tensions. The heat-conducting layer may extend on the outer surface or the outer surfaces along the directions of the temperature gradients to the edges of the sensor element. Thus, with respect to a planar sensor element, the heat-conducting layer may, for example, have strips starting from the projection of the middle of the heating device onto the layer plane of an outer surface of the sensor element, which may extend star-shaped all the way to the edges enclosing the outer surface. This may save material of the heat-conducting layer without substantially limiting the heat adjustment between the colder edges of the outer surface and the warmer middle of the outer surface. Furthermore, the heat-conducting layer may be structured as a grid. By using these measures, material of the heat-conducting layer may also be saved.  
           [0010]    Good heat-conducting capability of the heat-conducting layer may be ensured when the heat-conducting layer contains a metal, e.g., platinum, and may have a thickness in the range of 5 to 50 μm. In order to stabilize it, the heat-conducting layer may have a ceramic material, for instance Al 2 O 3 .  
           [0011]    In order to prevent the heat-conducting layer from being worn away, for instance, by outer influences or vaporized by the high temperatures, the heat-conducting layer may be covered by a protective layer which may include a ceramic material such as Al 2 O 3  and/or ZrO 2 . The protective layer may be designed as closed porous, and may have a thickness of 10 to 100 μm. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a cross section through a measuring region of an exemplary embodiment of a sensor element according to the present invention.  
         [0013]    [0013]FIGS. 2 a  through  2   g  shows top views of a large surface of various exemplary embodiments of the sensor element according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 and FIGS. 2 a  through  2   g  show, as exemplary embodiments of the present invention, a sensor element  10  of a so-called lambda probe having a measuring region  15  and a supply lead region  16 . Sensor element  10  is constructed as a layer system and has a first, second, third and fourth solid electrolyte layer  21 ,  22 ,  23 ,  24 . On first solid electrolyte layer  21  a first electrode  31  is applied on an outer surface of sensor element  10  in measuring region  15 , and it is coated with an electrode protective layer  33 . Electrode protective layer  33  may be designed to be porous, so that first electrode  31  is exposed to a measuring gas surrounding sensor element  10 . On the side opposite first electrode  31  of first solid electrolyte foil  21  a second electrode  32  is applied. Second electrode  32  is positioned in a reference gas chamber  34  put into second solid electrolyte foil  22 . Reference gas chamber  34  may be filled with a porous material.  
         [0015]    In order to heat measuring region  15  of sensor element  10 , a heating device  40  is provided between third and fourth solid electrolyte layers  23 ,  24 , which has a heater  41  that is electrically insulated from the surrounding solid electrolyte layers  23 ,  24  by a heating insulation  42 . Heater  41  and heater insulation  42  are surrounded on their sides by a sealing frame  43 , which, for example, may be made of an ion-conducting material. In one example embodiment, heater  41  may not be, or at least not fully electrically insulated from surrounding solid electrolyte layers  23 ,  24 , or heater insulation  42  may be brought right to the side surfaces of sensor element  10 , so that sealing frame  43  may be dispensed with.  
         [0016]    On the outer surface of fourth solid electrolyte layer  24 , a heat conducting layer  51  may be applied, for example, by a screen-printing technique. Heat-conducting layer  51  is coated with protective layer  52 , also, for instance, by a screen-printing technique. Heat-conducting layer  51  is made of platinum, and has a thickness of 5 to 50 μm, e.g. 12 μm. The protective layer is made of a ceramic material such as Al 2 O 3 , ZrO 2  or of a mixture of Al 2 O 3  and ZrO 2 , and has a thickness of 10 to 100 μm, e.g. 30 μm.  
         [0017]    In FIGS. 2 a  through  2   g , exemplary embodiments of the present inventions are shown. A top view of fourth solid electrolyte layer  24  and heat-conducting layer  51  is shown, protective layer  52  not being shown so as to make clearer the position of heat-conducting layer  51 . Protective layer  52  is arranged so that heat-conducting layer  51  is completely covered. The position of heater  41 , which is positioned not on the outer surface of sensor element  10 , but in the layer plane between third and fourth solid electrolyte layers  23 ,  24 , is shown in FIG. 2 a  by dotted lines. The position of heater  41  in FIGS. 2 b  through  2   g  corresponds to the position of heater  41  in FIG. 2 a.    
         [0018]    In the exemplary embodiment shown in FIG. 2 a , heat-conducting layer  51  completely covers measuring region  15  and the transition region between measuring region  15  and supply lead region  16  of sensor element  10 . In the exemplary embodiment shown in FIG. 2 b  or  2   c , measuring region  15  or the transition region, respectively, are covered.  
         [0019]    [0019]FIG. 2 d  shows an exemplary embodiment in which heat-conducting layer  51  is provided in the region of the edges of the outer surface of fourth solid electrolyte foil  24 . In the exemplary embodiment shown in FIG. 2 e , heat-conducting layer  51  has strips arranged in a star shape, which run from the center of measuring region  15  of the outer surface of sensor element  10  to the edges of the outer surface, and thereby may make possible a temperature adjustment between the center and the edges of the outer surface in measuring region  15 . The exemplary embodiment in FIG. 2 f  represents a combination of the embodiments of FIGS. 2 d  and  2   e . In the exemplary embodiment shown in FIG. 2 g , heat-conducting layer  51  has a grid-type structure.  
         [0020]    Heat-conducting layer  51  may be brought right up to the edge of the outer surface of sensor element  10  without formation of a separation from the edge. It may also be provided that heat-conducting layer  51  has a small distance from the edge of the outer surface, and that protective layer  52  fills the gap between heat-conducting layer  51  and the edge, and thereby covers heat-conducting layer  51  also on its sides. The distance of the heat-conducting layer from the edge may need to remain so small that no substantial temperature gradients may arise in the edge region. This may be safely ensured if, for example, the distance of heat-conducting layer  51  is not greater than 0.5 mm.  
         [0021]    It should be pointed out that the arrangement, according to the present invention, of heat-conducting layer  51  on an outer surface of sensor element  10  is not limited to the special type shown in FIG. 1, but may be generally used for sensor elements in which mechanical tensions appear at the outer surface, on account of temperature gradients.  
         [0022]    In a further exemplary embodiment, several of the outer surfaces of the sensor element may be furnished with a heat-conducting layer.