Abstract:
A sensor element containing a porous layer is provided for detecting a physical magnitude of a measured gas, such as for determining the concentration of a gas component of an exhaust gas of an internal combustion engine. The porous layer includes pores of a first pore type whose diameters correspond to at least half the layer thickness of the porous layer.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a sensor element.  
         BACKGROUND INFORMATION  
         [0002]    A sensor element is discussed, for example, in German Published Patent Application No. 198 57 471. The sensor element contains a porous layer that is used as a diffusion barrier and an additional porous layer that covers an external pump electrode. To manufacture the porous layers using a screen printing method, a paste containing a finely distributed powdery pore-forming material may be applied onto a ceramic element (green foil (film)). Subsequently, the paste is heated to a temperature at which the pore-forming material volatilizes almost without residue, leaving pores. Theobromine may, for example, be used as a pore-forming material.  
           [0003]    Porous layers may have varying thicknesses, due, for example, to non-uniform application of the paste in the screen printing method, or due to a squeezing of the paste during a lamination process. If, for example, the thickness of a porous layer used as a diffusion barrier deviates from the target value, the diffusion current through the diffusion barrier may change, and the measurement result of the sensor element may thus change, so that expensive methods for the correction of this effect become necessary.  
         SUMMARY OF THE INVENTION  
         [0004]    It is believed that an exemplary sensor element according to the present invention has the advantage that a porous layer situated in the sensor element has a uniform thickness, with a production variance that is negligibly small.  
           [0005]    For this purpose, the porous layer has pores whose diameters correspond approximately to the thickness of the porous layer. The porous layer is manufactured by application of a paste onto a substrate, the paste containing a finely distributed powdery pore-forming material that volatilizes almost without residue during the sintering process. The pore-forming material has particles, the diameters of which correspond approximately to the layer thickness of the paste. In this manner, the paste may be applied in a more uniform fashion, so that a uniform layer thickness may be ensured or at least be more likely, independent of the conditions during the printing method. Moreover, the paste may not be squeezed, for example, by the lamination process.  
           [0006]    If the porous layer has pores of a first type, the diameters of which correspond approximately to the thickness of the porous layer, and pores of a second type, the diameters of which are approximately 10 to 80 percent, for example, 20 to 50 percent, of the diameter of the pores of the first type, it the diffusion current through the diffusion barrier is easily adjustable and is sufficiently limited. A particularly reliable reduction of the scattering of the thickness of the porous layer is achieved in that the diameters of the pores of the first type are at most 20 percent, for example, at most  10  percent, smaller than the thickness of the porous layer.  
           [0007]    In an exemplary embodiment according to the present invention, the portion of pores of the first type in the porous layer is approximately 3 to 10 volume percent, and the portion of the pores of the second type in the porous layer is approximately 10 to 50 volume percent.  
           [0008]    An exemplary method for manufacturing a sensor element according to the present invention permits a manufacturing of the porous layers having a negligibly small manufacturing fluctuation with respect to the thickness of the porous layers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    The FIGURE shows a cross-section of detail of an exemplary sensor element according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    The FIGURE shows a schematic representation of a section through an exemplary sensor element  10  according to the present invention that may be manufactured using ceramic foil technology and screen printing technology. Sensor element  10 , shown in the FIGURE, is a broadband lambda sensor, having a pump cell that operates according to the limiting current principle, and having a measurement cell (Nernst cell). In addition, the sensor element has an integrated resistance heating unit (not shown). However, this design does not limit the invention to the exemplary embodiment shown in the FIGURE. The invention is likewise applicable to other sensor elements having porous layers.  
         [0011]    The sensor element, which is shown only in detail in the FIGURE, contains four or five solid electrolyte layers that are laminated together, of which only a first solid electrolyte layer  21  and a second solid electrolyte layer  22  are shown.  
         [0012]    On first solid electrolyte layer  21 , a first electrode  31  (outer pump electrode) and a second electrode  32  (inner pump electrode) are situated on an external surface of sensor element  10 . A porous protective layer  42  is situated over first electrode  31 . Second electrode  32  is of annular construction, and is situated in a measured gas chamber  35  in which a third electrode  33  (measurement electrode) is situated opposite second electrode  32 , on second solid electrolyte layer  22 . Measured gas chamber  35  is sealed laterally by a sealing frame  23 , which may be made, for example, of a solid electrolyte. First and second electrode  31 ,  32  together form the pump cell. Third electrode  33  operates together with a fourth electrode reference electrode (not shown), which is situated in a reference gas chamber (not shown), which may be connected, for example, with the air as a reference atmosphere.  
         [0013]    In the layer plane between first and second solid electrolyte layers  21 ,  22 , a diffusion channel extends, in which a porous diffusion barrier  41  is situated. Diffusion barrier  41  is placed in annular fashion around a gas inlet opening  36  in first solid electrolyte layer  21 . The measured gas, situated outside sensor element  10 , may flow to second and third electrodes  32 ,  33 , situated in measured gas chamber  35 , through gas inlet opening  36  and diffusion barrier  41 .  
         [0014]    For the manufacturing of the exemplary sensor element  10  according to the present invention, ceramic foils are used that are made of a solid electrolyte that conducts oxygen ions, for example, zirconium dioxide stabilized with Y 2 O 3 . The solid electrolyte foils may be printed with the electrodes and the associated printed conductors, as well as with additional functional layers, for example, using the screen printing technique, and, after the sintering, form solid electrolyte layers  21 ,  22 . The electrodes and the printed conductors may be made of a platinum cermet.  
         [0015]    On first solid electrolyte foil  21 , for example, first electrode  31  and pastes forming porous protective layer  42  may be printed. On the side opposite first electrode  31  of first solid electrolyte layer  21 , pastes are printed that form second electrode  32 , diffusion barrier  41 , measured gas chamber  35 , third electrode  33 , and sealing frame  23 . The pastes for measured gas chamber  35 , and, if necessary, gas inlet opening  36 , are cavity pastes, which may be made, for example, of glassy coal, which burns out or vaporizes during the later sintering process, forming hollow spaces  35 ,  36  between first and second solid electrolyte foils  21 ,  22 . The finally printed solid electrolyte foils are laminated together and sintered.  
         [0016]    To produce the pores in the porous layers, for example, diffusion barrier  41  and protective layer  42 , a paste is used that contains a ceramic powder and a pore-forming powder. The finely distributed particles of the pore-forming powder burn out during the sintering, thus producing an open porosity. The paste that forms porous layer  41 ,  42  contains pore-forming material of a first and of a second pore type. The pore-forming material of the first pore type is selected such that the diameter of the particles of the pore-forming powder of the first pore type correspond approximately to the layer thickness of the ceramic paste that is applied onto the solid electrolyte foil and that forms the porous layer. The diameter of the particles of the pore-forming powder of the second pore type is from approximately 20 to 50 percent of the diameter of the particles of the pore-forming powder of the first pore type. In an alternative exemplary embodiment according to the present invention, at least approximately 90 percent of the pores of the second type are smaller than approximately 80 percent of the diameter of the pores of the first type, that is, d 90  of the pores of the second type is smaller than approximately 80 percent of the diameter of the pores of the first type.  
         [0017]    In the exemplary embodiment shown in the FIGURE, the distance of the first second solid electrolyte layer from the second solid electrolyte layer is 20 μm. The diameter of the particles of the pore-forming material of the first pore type is selected at approximately 20 to 22 μm, and the diameter of the particles of the pore-forming material of the second pore type is selected at approximately 2 to 10 μm. After the sintering process, due to the sintering shrinkage the diameter of the pores of the first type in diffusion barrier  41  is in the range from approximately 18 to 20 μm, and the diameter of the pores of the second type is from approximately 2.2 to 9 μm. The d 90  of the pores of the second type is approximately 8 μm, so that approximately 90 percent of the pores of the second type have a diameter less than or equal to approximately 8 μm. The diameter of a pore of the first or of the second type is the extension of a pore in the direction perpendicular to the plane of the porous layer. The portion of the pores of the first type in diffusion barrier  41  is approximately 5 percent by volume, and the portion of the pores of the second type is approximately 20 percent by volume.