Abstract:
An electrochemical sensor element for determining the oxygen concentration in exhaust gases of internal combustion engines is described. The sensor element has a pump cell, which has a pump electrode situated on a surface of the sensor element facing the gas mixture, and a measuring gas electrode situated in a measuring gas area, the gas mixture entering the measuring gas area through a diffusion resistor. In addition, the sensor element has a reference electrode situated in a reference gas area. Another diffusion resistor is provided between the measuring gas area and the reference gas area.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrochemical sensor element for determining the concentration of a gas component in a gas mixture, in particular for determining the oxygen concentration in exhaust gases of internal combustion engines. 
     BACKGROUND INFORMATION 
     An electrochemical sensor for determining oxygen concentration is described in German Published Patent Application No. 196 47 144, for example. The sensor elements described are used in gas detectors which are used to regulate the air/fuel ratio of combustion mixtures in automotive engines and as broadband lambda probes. A concentration cell is combined with an electrochemical pump cell in these sensor elements. 
     The concentration cell has a measurement electrode situated in a measuring gas area and a reference electrode situated in a reference gas area. The exhaust gas passes through a gas access orifice and a diffusion barrier to enter the measuring gas area. The reference gas area communicates with a reference atmosphere through an opening situated on the side of the sensor element facing away from the measuring gas area. The measuring gas area and the reference gas area are situated in the same layer plane of the sensor element, which is structured as a layered system and are separated by a gas-tight partition. A Nernst voltage develops between the measuring electrode and the reference electrode and can be used to determine the ratio of the oxygen partial pressure in the measuring gas area to the oxygen partial pressure in the reference gas area. 
     The pump cell has a first pump electrode situated in the measuring gas area and a second pump electrode situated on a surface of the sensor element facing the exhaust gas, and it pumps oxygen ions out of the measuring gas area into the exhaust gas, or conversely, out of the exhaust gas and into the measuring gas area. A pump voltage applied to the pump cell is regulated by an external circuit element, so that a predetermined oxygen partial pressure which corresponds to a certain Nernst voltage prevails in the measuring gas area. The pump voltage is selected so that the pump current flowing in the pump cell is limited by the diffusion rate of the oxygen molecules through the diffusion barrier, and the stream of oxygen molecules flowing through the diffusion barrier is proportional to the oxygen concentration in the exhaust gas, so the oxygen partial pressure of the exhaust gas can be determined from the pump current. 
     A disadvantage of the above-described sensor element is that the design of two gas spaces that are separated from one another in a gas-tight manner in one plane of the sensor element, namely the measuring gas area and the reference gas area, is complicated and difficult from the standpoint of the manufacturing technology. 
     SUMMARY OF THE INVENTION 
     The electrochemical sensor element according to the present invention has the advantage that the structure of the sensor element is greatly simplified by providing an additional diffusion resistor between the measuring gas area and the reference gas area. This achieves the result that it is not necessary to form a recess for the reference gas area separated from the measuring gas area in a gas-tight manner. 
     Since the gas component, generally oxygen, is pumped into the reference gas area by an external circuit element via the reference electrode, this achieves the result that a uniform partial pressure of the gas component prevails in the reference gas area, so that the partial pressure of the gas component in the measuring gas area may be determined with a good accuracy via the voltage difference (Nernst voltage) which develops between the measuring gas electrode and the reference electrode. 
     It is also advantageous that the reference gas area communicates with a gas space situated outside the sensor element only via the additional diffusion resistor. This prevents impurities from the reference gas atmosphere, for example, from entering the reference gas area, which could result in damage to the reference electrode and thus impair the sensor function. 
     The sensor design is greatly simplified further by the fact that the measuring gas area and/or the reference gas area is filled in at least some areas by a porous layer forming the diffusion resistor and/or the additional diffusion resistor. 
     Due to the fact that the reference gas area is situated in a channel-shaped area remote from the gas access orifice, this achieves the result that the concentration of the gas component in the reference gas area is influenced only slightly by fluctuations in the concentration of the gas component of the gas mixture. 
     Due to the fact that a measuring gas electrode and the reference electrode are situated in the same layer plane, this yields the manufacturing advantage that the measuring gas electrode and the reference electrode may be applied in a printing step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional diagram of a first embodiment of the sensor element according to the present invention along line I—I in FIG.  2 . 
     FIG. 2 shows a sectional-diagram of the first embodiment of the sensor element according to the present invention along line II—II in FIG.  1 . 
     FIG. 3 shows a sectional diagram of a second embodiment of the sensor element according to the present invention along line III—III in FIG.  4 . 
     FIG. 4 shows a sectional diagram of the second embodiment of the sensor element according to the present invention along line VI—VI in FIG.  2 . 
     FIG. 5 shows a longitudinal section of additional embodiments of the sensor element according to the present invention. 
     FIG. 6 shows another longitudinal section of additional embodiments of the sensor element according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 show as the first embodiment of the present invention a sensor element  10  of a broadband lambda probe designed as a layered system and having a first, second, third and fourth solid electrolyte layer  21 ,  22 ,  23 ,  24 . A gas access orifice  30  is introduced into first and second solid electrolyte layers  21 ,  22 . A recess containing a diffusion resistor  31 , a measuring gas area  40 , an additional diffusion resistor  32  and a reference gas area  41  is provided in second solid electrolyte layer  22 . This recess is designed as a shallow cylindrical area, in the middle of which is provided gas access orifice  30  surrounded by hollow cylindrical diffusion resistor  31  and measuring gas area  40  which is also in the shape of a hollow cylinder, and a channel-shaped area which accommodates an additional diffusion resistor  32 , directly adjacent to the cylindrical area, and reference gas area  41 . 
     A ring-shaped measuring gas electrode  50  having a supply lead  50   a  is provided on first solid electrolyte layer  21  in measurement gap area  40 , and a reference electrode  51  having a supply lead  51   a  is provided in reference gas area  41 . A ring-shaped pump electrode  52  is applied to the outside surface of first solid electrolyte layer  21 . Third and fourth solid electrolyte layers  23 ,  24  are adjacent to second solid electrolyte layer  22 . A heating element  57  having a heating insulation  58  is provided between third and fourth solid electrolyte layers  23 ,  24 . 
     Pump electrode  52  together with measuring gas electrode  50  forms a pump cell which pumps oxygen into or out of measuring gas area  40  through an external circuit element. The pump voltage applied to the pump cell through the external circuit element is regulated so that a predetermined oxygen partial pressure prevails in measuring gas area  40 . An oxygen partial pressure of λ=1 is preferably set, i.e., the oxygen partial pressure corresponds to the stoichiometric air/fuel ratio. 
     The oxygen partial pressure prevailing in measuring gas area  40  is determined by a Nernst cell which is formed by measuring gas electrode  50  and reference electrode  51 . A Nernst voltage produced by different oxygen partial pressures in measuring gas area  40  and in reference gas area  41  is measured using the Nernst cell and is used to regulate the pump voltage. This requires that a sufficiently constant oxygen partial pressure prevails in reference gas area  41 . Therefore, a low pump current between measuring gas electrode  50  and reference electrode  51  or between pump electrode  52  and reference electrode  51  is produced by the external circuit element. Oxygen is pumped into reference gas area  41  by this pump current of 5 to 50 μA, for example. Depending on the oxygen partial pressure in the exhaust gas, the oxygen partial pressure in reference gas area  41  may vary, for example, in the range of 0.01 bar with rich exhaust gas to 0.3 bar with very lean exhaust gas. However, the effect of these fluctuations on the Nernst voltage is negligible. 
     The gas in reference gas area  41  may enter measuring gas area  40  through additional diffusion resistor  32 . No additional connection to a gas space located outside the sensor element, such as the air atmosphere, is necessary. Because of the low pump current, the oxygen partial pressure in measuring gas area  40  is altered only to a negligible extent by venting of reference gas area  41  into measuring gas area  40 . 
     According to one embodiment, reference electrode  51  is mounted on third solid electrolyte layer  23  in reference gas space  41 . 
     FIGS. 3 and 4 show as the second embodiment of the present invention a sensor element  110  of a broadband lambda probe having a first and a second solid electrolyte layer  121 ,  122  into which is introduced a gas access orifice  130 , and a third and fourth solid electrolyte layer  123 ,  124  between which is provided a heating element  157  having a heating insulation  158 . Second solid electrolyte layer  122  has a recess containing a diffusion resistor  131 , a measuring gas area  140  having a measuring gas electrode  150 , an additional diffusion resistor  132 , and a reference gas area  141  having a reference electrode  151 . A pump electrode  152  is mounted on the outside surface of first solid electrolyte layer  121 . 
     The second embodiment differs from the first embodiment in that additional diffusion resistor  132  and reference gas area  141  in the form of concentric hollow cylinders are also situated in a shallow cylindrical recess in addition to diffusion resistor  131  and measuring gas area  140 . In addition, reference gas area  141  situated in the area of reference electrode  151  is filled completely by a porous material which forms additional diffusion resistor  132 . Accordingly in this embodiment, reference gas area  141  is understood to refer to the area of the porous material adjacent to reference electrode  151 . 
     According to one embodiment, the reference electrode in reference gas space  141  is applied to third solid electrolyte layer  123 . It is also possible for the additional diffusion resistor not to cover reference electrode  151  at all or to cover it only in some areas. 
     FIG. 5 shows a third embodiment of the present invention which differs from the second embodiment in that reference electrode  151  in reference gas area  141  is applied to third solid electrolyte layer  123 , and another measuring gas electrode  153  is applied to the third solid electrolyte layer and is opposite measuring gas electrode  150  in measuring gas area  140 . 
     In this embodiment, additional measuring gas electrode  153  may be electrically connected to measuring gas electrode  150  in the supply lead area, for example. It is also possible for the Nernst cell to be formed by additional measuring gas electrode  153  and reference electrode  151  and for the pump cell to be formed by measuring gas electrode  150  and pump electrode  152 . In this case, the pump current may flow between measuring gas electrode  150  and additional measuring gas electrode  153  and/or between pump electrode  152  and additional measuring gas electrode  153  to maintain the required oxygen partial pressure in reference gas area  141 . 
     FIG. 6 shows a fourth embodiment which differs from the second embodiment in that the recess in the second solid electrolyte layer containing diffusion resistor  131 , measuring gas area  140 , additional diffusion resistor  132 , and reference gas area  141  is filled completely by a porous material. In this embodiment, measuring gas area  140  is understood to be the area of the porous material adjacent to measuring gas electrode  150 , and reference gas area  141  is understood to be the area of the porous material adjacent to reference electrode  151 . Diffusion resistor  131  is formed by the porous material situated between gas access orifice  130  and measuring gas electrode  150 , and additional diffusion resistor  132  is formed by the porous material situated between measuring gas electrode  150  and reference electrode  151 .