Abstract:
A planar sensor element for determining gas components, which includes a layer structure with a heating element integrated therein with a layer-shaped heating conductor. The heating conductor is arranged in a layer plane of the layer structure so that an at least approximately homogeneous distribution of the heating power of the heating element over the cross-section of the layer structure is obtained.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a planar sensor element for determining gas components, in particular for determining the oxygen level in internal combustion engine exhaust gases. 
   BACKGROUND INFORMATION 
   A conventional sensor element is described in, for example, German Patent Application No. 42 31 966 (corresponding to U.S. Pat. No. 5,529,677), which is made of a composite of individual foils arranged in consecutively superimposed layers. Function layers such as electrodes, printed conductors and an electric resistance heating element are arranged between the individual foils. The function layers and the heating element are printed onto the unsintered (green) foils using screen printing, for example. Then, the foils are placed one on top of the other, laminated and subsequently sintered. In the planar sensor element of this type, the resistance heating element is arranged in one of the layers between an external cover foil and an adjacent layer structure. The resistance heating element is embedded between two electrically insulating layers (e.g., Al 2 O 3 ), so that the heating conductor is electrically insulated from the adjacent foils. On its side opposite a side of the cover foil, the layer structure has a considerably greater thickness than the cover foil adjacent to the other side. Due to this highly asymmetrical arrangement of the heating element with respect to the layer sequence of the layer structure, the cover foil heats up much more than the layer structure provided with function layers. The non-homogeneous distribution of the heating power results in increased heat shock sensitivity of the planar sensor element when the temperature varies. 
   SUMMARY OF THE INVENTION 
   The planar sensor element according to the present invention is advantageous in that the heating power is homogeneously distributed over the cross-section of the sensor element. 
   Thus, a resistance of the sensor element to temperature variations and thermal shock is improved. Furthermore, the efficiency of the heating element is enhanced. 
   It is also advantageous if a covering layer structure is made of a single foil having a thickness, in the unsintered (green) state, of 0.6 to 1 mm, preferably 0.8 mm. The layer structure adjacent to the resistance heating element on the opposite side and containing the function layers (function layer-side layer structure) has a total thickness approximately equal to that of the cover foil or the cover foil-side layer structure, according to the number of foils and other layers, such as a cover layer on the external electrode. This means that the thickness of the function layer-side foils d F  for uniform thickness distribution is defined by
 
 d   F =( d   F1   −d   D ): n,  
 
where d F1  is the thickness of the unsintered cover foil, d D  is the thickness of the cover layer or protective layer arranged on the external electrode, and n is the number of function layer-side foils. In this calculation, it is assumed that the insulation layers have at least approximately the same thickness on both sides of the heating element.
 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows an exemplary embodiment of a sensor element according to the present invention. 
   

   DETAILED DESCRIPTION 
   The FIGURE shows a cross section through an exemplary embodiment of a planar sensor element  10 , which may be used, e.g., for determining the oxygen level in exhaust gases of internal combustion engines or combustion systems. The sensor element shown in this embodiment is a lambda-1 sensor (e.g., a Nernst sensor). The design and function of such a sensor are generally known. 
   Sensor element  10  has, when unsintered, an elongated, plate-shaped design, which contains a plurality of layers arranged one on top of the other in a layer structure. The layers, when unsintered (green), are basically formed by oxygen ion-conducting solid electrolyte foils. 
   In this embodiment according to the present invention, sensor element  10  has an electrochemical measuring cell  12  and a heating element  14 . Measuring cell  12  has a function layer-side layer structure  12 ′ with a first foil  16  and a second foil  18 . A reference channel  20  is integrated in second foil  18 . A measuring electrode  22  is arranged on the measuring-gas side surface of foil  16 , and a reference electrode  24  is arranged on the surface associated with reference channel  20 . A porous cover layer  26  having a thickness of approximately 0.1 mm is placed on measuring electrode  22 . 
   Heating element  14  has a heating conductor  30 , embedded in two insulating (insulation) layers  28  and  29 , the two insulation layers  28 ,  29  having essentially the same thickness. An external covering foil  32  follows first insulation layer  29 . In order to seal porous insulation layers  28 ,  29  in a gas-tight manner, a sealing frame  34  is positioned around them, which is manufactured, for example, by printing solid electrolyte material on foils  18 ,  32 , arranged on both sides of insulation layers  28 ,  29 . 
   Heating conductor  30  is in a layer plane  36 , centered with respect to the layer structure above or below. Due to this fact, the layer thickness of foils  16 ,  18  of measuring cell  12  is to be dimensioned according to the thickness of cover foil  32 , taking into account the thickness of cover layer  26 , or vice-versa. In this embodiment, the unsintered cover foil has a thickness of 0.8±0.1 mm. The thickness of porous cover  26  layer is assumed to be approximately 0.1 mm. This results in a layer thickness for first and second foils  16 ,  18  of approximately 0.35±0.05 mm each. The thickness ratio of foils  16 ,  18 ,  32  remains basically preserved even after sintering as a layer thickness ratio, based on a sintering shrinkage of approximately 20%. 
   It is also possible for the two foils  16 ,  18  to have different thicknesses. It is essential, however, that the total thickness of the function layer-side layer structure of the sensor element, considering other layers such as cover layer  26 , for example, be (at least approximately) equal to the thickness of cover foil  32  or a cover foil-side layer structure used instead of cover foil  32 . 
   Foils  16 ,  18 ,  32  are made of stabilized zirconium oxide, for example. In order to achieve densely sintered bonding, sealing frame  34  is made of the same material as the adjacent foils  18  and  32 . Electrodes  22 ,  24  and heating conductor  30  are made of a platinum cermet, for example. Insulation layers  28 ,  29  are made of Al 2 O 3  in this embodiment, an insulation layer  29  being initially printed onto cover foil  32 . Heating element  30  is also applied to insulation layer  29  by printing. Finally, one-half of sealing frame  34 , for example, is also applied around insulation layer  29  by printing. 
   To manufacture the layer structure of measuring cell  12 , the two electrodes  22 ,  24 , with the leads not illustrated in detail, are printed onto foil  16 . The additional insulation layer  28  and the second half of sealing frame  34  are applied onto second foil  18 . 
   The function layer-side and heating element-side layer structures thus formed are laminated with the unsintered foils using binder layers applied between the foils, and are sintered at a temperature of 1400° C., for example. After sintering, the plate-shaped sensor element with a rectangular cross section is obtained. 
   The layer structure described is, however, not limited to the exemplary embodiment with a Nernst type sensor element  10 . The present invention can also be used with a sensor element having more than three foils. Such a sensor is, for example, a broadband sensor in which a pump cell and a concentration cell (Nernst cell) are provided instead of measuring cell  12 .