Abstract:
A gas sensor for determining a physical quantity of a gas component, e.g., in an exhaust gas of an internal combustion engine, including a sensor element which contains at least one electrochemical cell. The electrochemical cell includes a first electrode and a second electrode that are arranged at a distance on at least one solid electrolyte, the second electrode is arranged in a reference gas space. A third electrode which is in contact with a gas located in the gas space is provided. The gas component may be exchanged between the gas space and the reference gas space using a voltage applied between the second electrode and the third electrode.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a gas sensor.  
         BACKGROUND INFORMATION  
         [0002]    Gas sensors of similar kind are described for example in German Published Patent Application No. 198 15 700. Such a gas sensor has a potentiometrically driven electrochemical cell including a first and a second electrode and a solid electrolyte arranged between the first and second electrodes. The first electrode is coated with a porous protective film and is in contact with a measuring gas located outside the sensor element. The second electrode is arranged in a reference gas space, which is at least partly filled with a porous material. A heater is be provided for the purpose of heating the sensor element in a measurement area, and is separated from the solid electrolyte coatings that surround it by a heater insulation.  
           [0003]    If differing oxygen partial pressures arise in the measuring gas outside the sensor element and the reference gas in the reference gas space, a Nernst voltage is formed between the first and the second electrode, which may be calculated using electronic evaluation means located outside the sensor element. The Nernst voltage may be used to determine the ratio of the oxygen partial pressures in the measuring gas and the reference gas.  
           [0004]    Moreover, the electronic evaluation means creates an electrical pumping voltage between the first and the second electrode, which causes oxygen to be pumped into the reference gas space via the first and the second electrodes. As a consequence, there is always adequate oxygen partial pressure in the reference gas space, regardless of the operating conditions. An alternating voltage is also applied between the first and the second electrodes to regulate the heater. The electronic evaluation arrangement may be used to calculate the temperature of the sensor element&#39;s measurement area from the temperature-dependent impedance, so that the heater may be switched on or off.  
           [0005]    The disadvantage of the conventional gas sensor is that the generation of a voltage serving to pump the reference gas space at the potentiometrically driven electrochemical cell causes the probe signal to be distorted by polarization effects, particularly at the first electrode. The polarization effects are stronger at low probe temperatures, which are present particularly in the case of a gas sensor arranged on the gas outlet side of a catalytic converter.  
         SUMMARY  
         [0006]    The gas sensor according to the present invention may provide that pumping into and out of the reference gas space may be effected by applying a voltage between a second electrode, arranged in the reference gas space, and a third electrode. In this manner, the function of an electrochemical cell that may be formed by a first and the second electrodes and by a solid electrolyte arranged between the first and the second electrodes may not be disrupted. For this purpose, the third electrode may be arranged on a solid electrolyte and may be in contact with an area containing a gas.  
           [0007]    If a pumping voltage exists between the second and third electrodes such that oxygen is regularly pumped into the reference gas space, which may be filled with a porous material, the level of the oxygen partial pressure in the reference gas space may always be adequate, regardless of operating conditions. Thus the measurement result of the gas sensor obtained with the Nernst voltage may not be distorted by a drop in the oxygen partial pressure in the reference gas space. Contaminants may also be prevented from infiltrating the reference gas space.  
           [0008]    If an alternating voltage is applied between the second and third electrodes to determine the temperature in the measurement area of the sensor element, the measuring function of the electrochemical cell is influenced only minimally, if at all, by the alternating voltage. Moreover, a larger internal resistance may be provided between the second and the third electrode, and/or between the first and the third electrode, for example, by the fact that the third electrode has a smaller surface area than the second and/or first electrode. As a result, the impedance may be calculated more easily using, for example, an analog-to-digital converter contained in the electronic evaluation arrangement.  
           [0009]    The measurement area of the sensor element may be heated with a heater which may include a first and a second heater lead in the supply area. The heater leads may be electrically connected by feedthroughs to contact surfaces on an exterior surface of the sensor element. The first heater lead may be set to a constant potential, the potential of the second heater lead may be varied by the electronic evaluation arrangement. In an example embodiment of the present invention, the third electrode may be electrically connected to the first heater lead, so that no power supply wire may be needed for the third electrode.  
           [0010]    In an example embodiment of the present invention, the third electrode may be arranged on an external surface of the sensor element and may be covered with a gas-permeable protective film.  
           [0011]    A simple construction of the sensor element may be achieved if the sensor element includes a heater having heater insulation that is porous and is in contact with a gas outside the sensor element, and if the third electrode is attached between the heater insulation and an adjacent solid electrolyte, and is in contact with the gas located in the porous heater insulation.  
           [0012]    The construction may be further simplified if the heater has a porous heater insulation and if the first heater lead is arranged at least in part between the heater insulation and one of the adjacent solid electrolytes, so that the first heater lead may be used at least in part as the third electrode. Since the first heater lead may have a constant potential, there may be no danger of capacitive coupling between the heater and an electrochemical cell, and therefore the first heater lead may not need to be insulated from the adjacent solid electrolyte.  
           [0013]    In an example embodiment of the present invention, the third electrode may be arranged in a gas space that may be located in the sensor element, for example, in the coating layer of the reference gas space, and may be connected to a gas region outside the sensor element.  
           [0014]    The present invention is illustrated in the drawings and explained in the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cross-sectional view through a measurement area of a first example embodiment of a sensor element according to the present invention.  
         [0016]    [0016]FIG. 2 is a plan view of the example embodiment illustrated in FIG. 1.  
         [0017]    [0017]FIG. 3 is a cross-sectional view through a measurement area of a second example embodiment of the sensor element according to the present invention.  
         [0018]    [0018]FIG. 4 is a longitudinal cross-sectional view through the measurement area of the second example embodiment corresponding to section line illustrated IV-IV in FIG. 3.  
         [0019]    [0019]FIG. 5 is a cross-sectional view through the measurement area of a third example embodiment of the sensor element according to the present invention.  
         [0020]    [0020]FIG. 6 is a longitudinal cross-sectional view corresponding to a section line VI-VI illustrated in FIG. 5 through the third example embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The first example embodiment of the present invention, illustrated in FIGS. 1 and 2, has the form of a sensor element  10  of a lambda probe including a measuring area  15  and a supply area  16 . Sensor element  10  may be constructed as a layered system and may include first, second, third and fourth layers of solid electrolyte  21 ,  22 ,  23 ,  24 . A first electrode  31 , coated with protective film  41 , may be attached to first electrolyte layer  21  on an external surface of sensor element  10  in measurement area  15 . Protective film  41  may be porous, so that first electrode  31  is exposed to a measuring gas. A second electrode  32  may be attached to the side of first solid electrolyte film  21  facing electrode  31 . Second electrode  32  may be arranged in a reference gas space provided in second solid electrolyte film  22 . Reference gas space  51  may be filled with a porous material.  
         [0022]    In order to heat measurement area  15  of sensor element  10 , a heater  61  may be provided between third and fourth solid electrolyte layers  23 ,  24  and may be insulated from the surrounding solid electrolyte films by heater insulation  62 . Heater  61  and heater insulation  62  may be enclosed laterally by a sealing body  63 , which may be made from an ion-conducting material.  
         [0023]    A third electrode  33  may be attached to the external surface of fourth solid electrolyte film  24  and may be coated with additional protective film  42 . Additional protective film  42  may be porous, so that third electrode  33  may be in contact with the measuring gas in a gas space  52 . Gas space  52  may be the area adjacent to third electrode  33  outside sensor element  10 . Third electrode  33  has a smaller surface with respect to the large surface area of sensor element  10  than first and/or second electrodes  31 ,  32 . Third electrode  33  may be electrically connected to a first contact surface  71  by a lead  33   a  arranged in supply area  16 . First contact surface  71  may be arranged on the side of sensor element  10  facing away from measurement area  15  and may serve as the contact for the sensor element. A second contact surface  72  may be provided adjacent to first contact surface  71 . First and second contact surfaces  71 ,  72  may be electrically connected respectively to a first and a second heater lead by a first and a second feedthrough  75 ,  76 , and lead to heater  61 . In this manner, third electrode  33  may be electrically connected to first heater lead via lead  33   a , contact surface  71  and first feedthrough  75 .  
         [0024]    A constant potential, for example, a ground potential may be applied to first heater lead and thereby also to third electrode  33  by electronic evaluation arrangement arranged outside the sensor element. A voltage may be applied at heater  61  by a change in the potential at second contact surface  72  to heat measurement area  15  of sensor element  10 . The potential of second electrode  32  may be selected so that oxygen may be pumped from third electrode  33  to second electrode  32  and thus also into reference gas space  51  caused by a voltage gradient between second and third electrodes  32 ,  33 . In this manner, it may be assured that the oxygen partial pressure in reference gas space  51  is always adequate.  
         [0025]    The second example embodiment of the present invention, illustrated in FIG. 3 and FIG. 4, has the form of a sensor element  110  including measurement area  115  and supply area  116 . Sensor element  110  may also be constructed as a layered system and may include first, second, third, and fourth solid electrolyte layers  121 ,  122 ,  123 ,  124 . A first electrode  131  may be attached to first solid electrolyte film  121 , and may be coated with porous protective film  141 . A second electrode  132  may be attached to the side of first solid electrolyte film  121  facing first electrode  131 . Second electrode  132  may be arranged in a reference gas space  151  provided in second solid electrolyte film  122 .  
         [0026]    In order to heat measurement area  115  of sensor element  110 , as in the first example embodiment, a heater  161  may be provided between third and fourth solid electrolyte layers  123 ,  124 , and may be insulated from the surrounding solid electrolyte layers by heater insulation  162 . Heater  161  and heater insulation  162  may be enclosed laterally by a sealing body  163 .  
         [0027]    The second example embodiment differs from the first example embodiment essentially in that heater insulation  162  is porous, and that a third electrode  133  with lead  133   a  is provided between heater insulation  162  and third solid electrolyte layer  123 . Porous heater insulation  162  is in contact with a gas atmosphere outside sensor element  110 , for example, via contact  175  or via a channel on the side of sensor element facing away from measuring area  115 . As in the first example embodiment, third electrode  133  is electrically connected to a first heater lead via a feedthrough  175  and is at constant potential. The wiring scheme of electrodes  131 ,  132 ,  133  as well as of heater  161  and its leads is the same as for the first example embodiment, and therefore does not require further description.  
         [0028]    In a further example embodiment, first heater lead may be arranged at least partly between the heater insulation and the third solid electrolyte layer, and may serve in these areas as a third electrode. Thereby, the possibility of pumping oxygen into the reference gas space via the first heater lead and the second electrode may be assured.  
         [0029]    A third example embodiment of the present invention, illustrated in FIG. 5 and FIG. 6, has the form of a sensor element  210  including measurement area  215  and supply area  216 . Sensor element  210  may include first, second, third, and fourth solid electrolyte layers  221 ,  222 ,  223 ,  224 . A first electrode  231  may be attached to first solid electrolyte layer  221 , and may be coated with porous protective film  241 . A second electrode  232  may be attached to the side of first solid electrolyte film  221  facing first electrode  231 . Second electrode  232  may be arranged in a reference gas space  251  provided in second solid electrolyte film  222 .  
         [0030]    In order to heat measurement area  215  of sensor element  210 , as in the first and second example embodiments, a heater  261  may be provided between third and fourth solid electrolyte layers  223 ,  224 , and may be electrically insulated from the surrounding solid electrolyte layers by heater insulation  262 . Heater  261  and heater insulation  262  may be enclosed laterally by a sealing body  263 .  
         [0031]    The third example embodiment differs from the first and second example embodiments essentially in that a third electrode  233  with lead  233   a  is provided in an additional gas space  252 , which is included in second solid electrolyte film  222  in addition to reference gas space  251  that is filled with a porous material. Reference gas space  251  and additional gas space  252  are combined in a common gas channel  253  in supply area  216  of sensor element  210 . This channel is in contact with a reference gas atmosphere outside sensor element  210  on the side of sensor element  210  facing away from measuring area  215 . Reference gas space  251  and additional gas space  252  are configured so that (even without pumping into the reference gas space) the diffusion current of the gas outside sensor element  210  to third electrode  233  is greater than that to second electrode  232 . This may be assured, for example, by filling reference gas space  251  with a porous material, while additional gas space  252  is configured as a cavity, or if the porous material in reference gas space  251  has a smaller percentage of porosity than a porous material provided in additional gas space  252 .  
         [0032]    As in the first example embodiment, third electrode  233  is electrically connected to a first heater lead via a feedthrough  275  and is at constant potential. The wiring scheme of electrodes  231 ,  232 ,  233  as well as heater  261  and its leads is the same as for the first example embodiment, and therefore does not require further description.  
         [0033]    In a further improvement of the third example embodiment, the additional gas space may be arranged in a solid electrolyte layer other than second solid electrolyte layer  222 . The additional gas space may also be in contact with a gas space arranged outside the sensor element via a channel that is not connected with the reference gas space.  
         [0034]    In another example embodiment, a fourth electrode may be provided in the reference gas space on the third solid electrolyte film, and may be electrically connected to the second electrode. This enables the reference gas space also to be filled by pumping via the fourth electrode.  
         [0035]    In the example embodiments, reference gas space  51 ,  151 ,  251  may be in contact with a reference gas located outside sensor element  10 ,  110 ,  210  via an aperture in the side of sensor element  10 ,  110 ,  210  facing away from measurement area  15 ,  115 ,  215 . Reference gas space  51 ,  151 ,  251  may be in contact with the measuring gas via an appropriately selected aperture.  
         [0036]    The pumping current into reference gas space  51 ,  151 ,  251  may be selected such that it is greater than the diffusion current into reference gas channel  51 ,  151 ,  251  in the area of second electrode  32 ,  132 ,  232 . A stable reference was achieved with a pumping current greater by a factor of  4  than the diffusion current. The porous material that fills reference gas space  51 ,  151 ,  251  was selected for the example embodiments described such that with a typical partial pressure differential between reference gas space  51 ,  151 ,  251  and the gas atmosphere present prevailing outside sensor element  10 ,  110 ,  210 , the diffusion current is 5 μA. It proved sufficient to set the pumping current to a value from 5 to 50 μA by appropriate selection of the voltage differential between second electrode  32 ,  132 ,  232  and third electrode  33 ,  133 ,  233 .  
         [0037]    The gas sensor described may be suited for installation on the gas outlet side of a catalytic converter.