Patent Publication Number: US-2004040846-A1

Title: Sensor element

Description:
BACKGROUND INFORMATION  
       [0001] The present invention relates to a sensor element for determining a gas component, in particular for determining the oxygen concentration in exhaust gases of internal combustion engines according to the preamble of the independent claims.  
       [0002] Such a sensor element is already described in German Patent Application 198 38 456 A1, for example. This sensor element, which is known as a broadband lambda probe by those skilled in the art, has a measurement gas space which is incorporated into the sensor element and is connected to the exhaust gas outside the sensor element via a gas inlet opening; a first and a second electrode are situated one opposite the other in this measurement gas space. A diffusion barrier having a porous material is provided between the electrodes and the gas inlet opening. The area between the two electrodes is designed as a cavity.  
       [0003] One disadvantage of such sensor elements is that the cavity between the two diametrically opposed electrodes may be compressed during the manufacturing process, thus having a negative effect on access of gas to the electrodes or suppressing it entirely. Furthermore, the first and second electrodes may come in contact, causing a short circuit and thus impairing sensor function.  
       [0004] German Patent Application 43 42 005 A1 also describes a sensor element having a measurement gas space which is incorporated into the sensor element and is connected to the exhaust gas outside the sensor element via a gas inlet opening and in which an electrode is situated. The measurement gas space here is filled completely, i.e., including the area of the electrode, with a diffusion barrier made of a porous material having a uniform porosity.  
       [0005] Since the measurement gas space in such a sensor element is filled up in the area of the electrode, this prevents compression of the measurement gas space during the manufacturing process. However, it is a disadvantage of these sensor elements that due to the diffusion barrier located in the area of the electrodes, the gas exchange between the areas of the electrode facing the gas inlet opening and the areas of the electrode facing away from the gas inlet opening is hindered, so that the load on the electrode is not uniform.  
       ADVANTAGES OF THE INVENTION  
       [0006] The sensor element according to the present invention as characterized in the independent claims has the advantage that collapse of the measurement gas space in the manufacturing process is prevented by at least one spacer element in the measurement gas space, while at the same time ensuring adequate gas exchange between the various areas of an electrode situated in the measurement gas space.  
       [0007] To do so, the measurement gas space is filled in at least some areas with a porous material which has a higher pore content than a diffusion barrier situated between a gas inlet opening and the measurement gas space. In an alternative embodiment, some areas of the measurement gas space may have at least one spacer element which has a closed porosity or no porosity at all, for example, and which allows access to the areas of the electrode not covered by the spacer element. In another alternative, a spacer element is designed so that the magnitude of the diffusion flow of the measurement gas and/or a component of the measurement gas from the gas inlet opening to the electrode is limited essentially by the diffusion barrier.  
       [0008] Advantageous embodiments and refinements of the sensor element characterized in the independent claims are possible through the measures characterized in the dependent claims.  
       [0009] If the porosity of the spacer element is selected so that the pore content of the spacer element is at least 30% higher than the pore content of the diffusion barrier (pore contents given in vol %) and/or the pore content of the spacer element is 60 to 80 vol %, then an adequate gas exchange in the measurement gas space is ensured especially reliably. A short circuit between two electrodes situated in the measurement gas space may be prevented especially effectively if at least approximately all of the area between the two electrodes is filled up by the spacer element.  
       [0010] In another advantageous embodiment, additional spacer elements resembling supporting posts are provided in the measurement gas space and are, for example, uniformly distributed on the side of the measurement gas space facing away from the diffusion barrier. The spacer elements preferably cover a total of at most 50% of the area of the electrode situated in the measurement gas space. Such an arrangement of the spacer elements reliably ensures that the gas exchange in the measurement gas space will not be hindered by the spacer elements.  
       [0011] It is also especially advantageous if the spacer element contains a catalytically active material, e.g., platinum, thus ensuring that a thermodynamic equilibrium will be established among the constituents of the gas.  
       [0012] If two electrodes are provided in the measurement gas space and both are connected to the spacer element, then in another advantageous embodiment of the present invention, a material that insulates with respect to electron conduction is advantageously selected for the spacer element to prevent an unwanted electric connection between the two electrodes. If the spacer element contains an electron-conducting material such as catalytically active platinum, then the electron-conducting material must be insulated from at least one of the electrodes by an electrically insulating material in order to prevent a short circuit.  
       [0013] In a method according to the present invention for manufacturing the spacer element, the spacer element is formed by a paste in the unsintered state. The paste is applied to a green film, i.e., a solid electrolyte layer in the unsintered state, e.g., by a screen printing technique, and sintered, if necessary, after a lamination operation. The paste contains a ceramic powder and a pore-forming substance, where the average particle radius of the ceramic powder and the pore-forming substance differs by no more than 20%, and the volume content of the ceramic powder is approximately the same as that of the pore-forming substance in the paste. This achieves an optimum space-filling effect and a mutual support of the particles of the ceramic powder, so that it is possible to manufacture a spacer element having a high porosity. Glass carbon, theobromine, flame carbon and/or other carbon compounds having an average particle diameter in the range of 2 to 30 μm have proven suitable for the pore-forming substance. 
     
    
    
     DRAWING  
     [0014] Exemplary embodiments of the present invention are illustrated in the drawing and explained in the following description.  
     [0015]FIG. 1 shows as the first exemplary embodiment a sensor element according to the present invention in a sectional diagram;  
     [0016]FIG. 2 shows a section of the first exemplary embodiment corresponding to sectional line II-II in FIG. 1;  
     [0017]FIG. 3 shows as the second exemplary embodiment the sensor element according to the present invention in a sectional diagram;  
     [0018] and FIG. 4 shows a section of the second exemplary embodiment corresponding to sectional line IV-IV in FIG. 3. 
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
     [0019]FIGS. 1 and 2 show as the first exemplary embodiment of the present invention a sensor element  10 , which is used to detect a gas component, e.g., oxygen in the exhaust gas of an internal combustion engine. Sensor element  10  is constructed as a layered system having a first, second, third, fourth, and fifth solid electrolyte layer  21 ,  22 ,  23 ,  24 ,  25 . A gas inlet opening  43  is incorporated into first and second solid electrolyte layers  21 ,  22 . A measurement gas space  41  is provided in the second solid electrolyte layer and a diffusion barrier  44  is provided between measurement gas space  41  and gas inlet opening  43 . The exhaust gas is able to pass through gas inlet opening  43  and diffusion barrier  44  to enter measurement gas space  41 . Measurement gas space  41  is separated by third solid electrolyte layer  23  from a reference gas space  42 , which is incorporated into fourth solid electrolyte layer  24 , contains a reference gas and is connected to a reference atmosphere situated outside of sensor element  10 , for example. Between fourth and fifth solid electrolyte layers  24  and  25  there is a heater  45  which is electrically insulated from the surrounding solid electrolyte layers  24 ,  25  by a heater insulation  46 .  
     [0020] A first electrode  31  is applied to first solid electrolyte layer  21  in measurement gas space  41 , forming a pumping cell together with a third electrode  33  applied to an outside surface of sensor element  10  and the area of first solid electrolyte layer  21  between first and third electrodes  31 ,  33 . Third electrode  33  is coated with a porous protective layer  35 . In measurement gas space  41 , a second electrode  32  is applied to third solid electrolyte layer  23  on the side opposite first electrode  31  and forms a Nernst cell together with a fourth electrode  34  situated in reference gas space  42  and the area of third solid electrolyte layer  23  situated between second and fourth electrodes  32 ,  34 .  
     [0021] To prevent measurement gas space  41  from being compressed in the manufacture of sensor element  10 , thereby short-circuiting first and second electrodes  31 ,  32  or reducing the area of first and/or second electrodes  31 ,  32  accessible to the measurement gas, measurement gas space  1  is filled with a porous material which functions as spacer element  50 . Spacer element  50  has a pore content of 60 to 85 vol %, preferably 70 vol %. The pore content of diffusion barrier  44 , however, is lower than the pore content of spacer element  50 , and is 20 to 80 vol %, preferably 50 vol %.  
     [0022] Sensor element  10  is manufactured in a known manner by applying the various function layers such as electrodes  31 ,  32 ,  33 ,  34 , protective layer  35 , diffusion barrier  44 , and spacer element  50  in the form of pastes by screen printing, for example, to the various green films, i.e., the unsintered solid electrolyte layers. Then the printed green films are laminated together and sintered. The pastes may contain pore-forming substances such as glass carbon, theobromine, flame carbon, and/or other carbon compounds. The pore-forming substances burn up in sintering and leave behind a cavity.  
     [0023] A paste containing a ceramic powder and a powdered pore-forming substance with approximately equal volume amounts is used for spacer element  50 . The average diameter of the particles of the ceramic powder and the pore-forming substance in the paste is also approximately the same, namely in the range from 2 μm to 30 μm, preferably 10 μm.  
     [0024]FIGS. 3 and 4 illustrate a second exemplary embodiment of the present invention which differs from the first exemplary embodiment in that eight spacer elements  51  like supporting posts are provided in measurement gas space  41 , filling up only a partial area of measurement gas space  41  and not necessarily being porous. Spacer elements  51  are positioned at equal intervals on the side of measurement gas space  41  facing away from diffusion barrier  44  and have a rectangular cross section. Spacer elements  51  cover only approximately 20% of the area of first and second electrodes  31 ,  32 , thus ensuring adequate access of the measurement gas to first and second electrodes  31 ,  32 .  
     [0025] Spacer element  50 ,  51  of the first and second exemplary embodiments is preferably made of a material that does not conduct electrons such as Al 2 O 3  or ZrO 2 . For special applications, it may also be necessary for spacer element  50 ,  51  not to be ion conducting (Al 2 O 3 ).  
     [0026] In an alternative instance of the first and second exemplary embodiments, spacer element  50 ,  51  has a catalytically active substance, preferably platinum. First and second electrodes  31  and  32  should be prevented from being connected electrically by the catalytically active substance. For this purpose, an insulation layer, for example, may be provided between spacer element  50 ,  51  and first and second electrodes  31 ,  32 , or the catalytically active material is situated at a distance from first and/or second electrodes  31 ,  32  in the spacer element.