Abstract:
An electrochemical sensor for determining a gas concentration of a measuring gas using a sensor element that has at least one electrode situated on an ion-conducting solid electrolyte body, an electrode lead leading to the electrode. The electrode lead is made of a material possessing no ionic conductivity or an ionic conductivity that is significantly less in comparison with the material of the electrode, and/or having a low resistance.

Description:
FIELD OF THE INVENTION 
     The present invention is based on an electrochemical sensor. 
     BACKGROUND INFORMATION 
     The sensors of this species must be heated in the active range to temperatures of more than ca. 350° C. to achieve the necessary ionic conductivity of the solid electrolyte body. To increase the measuring accuracy of the sensor, it is known to control and, if necessary, to adjust the operating temperature of the measuring cell, i.e., of the solid electrolyte body in the measuring region. To this end, it is known to assign a heating device to the sensor, the heating device being capable of being switched on and off as a function of an operating temperature measured at the measuring cell. 
     To determine the operating temperature of the measuring cell, it is known to apply an a.c. voltage to the sensor and to use a measuring device to determine a total alternating-current resistance made up of the conjugate impedances of the solid electrolyte body and of the corresponding electrodes and electrode leads. The temperature-dependent internal resistance of the solid electrolyte body in the measuring region and, as such, its temperature in the measuring region can be deduced from the total resistance. 
     In the known method, it is disadvantageous that the measuring device, which determines the temperature-dependent resistance of the solid electrolyte body, uses a constant resistance of the electrodes and the electrode leads as a baseline. However, the resistance of the electrode leads and the electrodes is subject to a relatively high degree of scatter due to manufacture. 
     The measuring device adds this not insignificant scatter error to a temperature-dependent change in the resistance of the solid electrolyte body in the measuring region and provides a corresponding faulty control signal for the heating device of the sensor. As a result, the sensor is adjusted to an incorrect operating temperature. 
     It is further disadvantageous that, in the lead region, the solid electrolyte body forms an additional internal resistance that is connected in parallel to the internal resistance of the solid electrolyte body in the region of the electrodes (measuring region) and also makes a not insignificant contribution to the total resistance. If, in addition, the temperature in the lead region is higher than in the measuring region, the internal resistance of the solid electrolyte body in the lead region is reduced, and it makes a contribution to the total resistance that is dependent on the temperature of the solid electrolyte body in the lead region. As a result, the sensor is likewise adjusted to an incorrect operating temperature. 
     To avoid the effect of the internal resistance in the lead region, it is known from German Published Patent Application No. 198 37 607 to provide the lead of an electrode opposite the lead region of the solid electrolyte body with an electrically insulating layer. This design has the disadvantage that the use of at least one insulating layer additionally requires at least one printing step and is, therefore, expensive from a standpoint of production engineering. 
     SUMMARY OF THE INVENTION 
     In comparison with the related art, the electrochemical sensor according to the present invention has the advantage of an improved regulation of the operating temperature, thereby enabling the sensor to function more precisely and more uniformly. 
     An exemplary embodiment and/or exemplary method of the present invention provides that the internal resistance of a solid electrolyte body in a lead region between the electrode leads situated on the solid electrolyte body is significantly higher than the internal resistance of the solid electrolyte body in a measuring region between the corresponding electrodes. Thus, the contribution to the total resistance made by the internal resistance in the lead region of the solid electrolyte body, which is connected in parallel to the internal resistance in the measuring region of the solid electrolyte body, is significantly reduced. Thus, the influence of the internal resistance in the lead region on the temperature regulation may be negligible. Additionally, from a standpoint of production engineering, an electrically insulating layer may be dispensed with so that a printing step may no longer be required. 
     According to the present invention, the resistance of at least one electrode lead makes a small contribution to the total resistance. Furthermore, the electrode lead is made of a material having a smaller degree of processing scatter with respect to its resistance. Thus, the effect of the resistance of the electrode lead on the total resistance is smaller. The present invention additionally improves the regulation of the operating temperature of the sensor. 
     Designing the internal pump electrode lead and/or the reference electrode lead using a material having a lesser ionic conductivity or no ionic conductivity in comparison with the electrode in question has the additional advantage that the resistive coupling of the particular electrode leads that can lead to a loading effect of the pump voltage on the measuring voltage of the sensor cell is prevented. As a result, the lambda=1−ripple is decreased or even prevented, thereby further improving the control dynamic response of the sensor. 
     An additional advantage results from designing the external pump electrode lead and/or the internal pump electrode lead using a material having a low resistance in comparison with the material of the electrode in question. As a result, the drop in the pump voltage in the external pump electrode lead and/or internal pump electrode is reduced, thereby improving pump function. 
     A particular embodiment of the present invention provides that the reference electrode lead is situated in the layer plane of the heater, thereby eliminating at least one printing step. In a further embodiment of the present invention, the heater and reference electrode lead are produced from the same material, thereby resulting in an additional advantage from a standpoint of production engineering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an exploded view of a first exemplary embodiment of a sensor. 
     FIG. 2 shows an exploded view of an additional exemplary embodiment of a sensor. 
     FIG. 3 shows a top view of an electrode including an electrode lead of a sensor. 
     FIG. 4 shows a top view of an electrode including an electrode lead as well as a heater. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an electrochemical sensor for analyzing gases, in the form of a planar sensor element  10 . Sensor element  10  including a measuring region  61  and a lead region  62  has electrical connection contacts  60 , a first solid electrolyte foil  11  designated as a heating foil, an insulating layer  12 , a heater  13 , an additional insulating layer  14 , a second solid electrolyte foil  20  designated as a reference gas duct foil, as well as a reference electrode  21  having a reference electrode lead  22 . Formed in reference gas duct foil  20  is a reference gas duct  29 , which is connected via an opening in the lead region to air as a reference gaseous atmosphere. Above reference electrode  21  and reference electrode lead  22 , the sensor element further has a third solid electrolyte foil  23  designated as a measuring foil, a measuring electrode  26  including measuring electrode lead  27 , as well as a porous protective layer  28 . 
     FIG. 2 shows an additional exemplary embodiment of an electrochemical sensor for analyzing gases. This sensor is a so-called broadband probe having two cells  37 ,  38 . First cell  37  is a concentration cell that functions according to the Nernst principle. The operating mode of first cell  37  corresponds with the sensor described in FIG.  1 . Therefore, the same reference numerals are used for the same elements in FIG.  2 . Second cell  38  is an electrochemical pump cell that is laminated together with first cell  37  and that cooperates with the concentration cell in a method known per se, according to the functional principle of the broadband probe. Situated in the junction region between first cell  37  and second cell  38  is an intermediate layer  35  and a filler layer  34  for forming a space (not further represented) for accommodating diffusion barrier  30 . Second cell  38  has an internal pump electrode  31 , including an internal pump electrode lead  32 , a fourth solid electrolyte foil  33  designated as a pump foil, an external pump electrode  40 , including an external pump electrode lead  41 , and a porous protective layer  42 . Measuring electrode lead  27  and internal pump electrode lead  32  run together in lead region  62  of sensor element  10 . 
     FIG. 3 shows a large surface of a solid electrolyte foil  49  having an electrode  50  and an electrode lead  51 , which can, for example, form measuring electrode  26 , including measuring electrode lead  27 , or reference electrode  21 , including reference electrode lead  22 , of the sensor shown in FIG.  1 . The electrode  50  shown in FIG. 3, including electrode lead  51 , can, for example, also represent external pump electrode  40 , including external pump electrode lead  41 , internal pump electrode  31 , including internal pump electrode lead  32 , measuring electrode  26 , including measuring electrode lead  27 , or reference electrode  21 , including reference electrode lead  22 , of the sensor shown in FIG.  2 . 
     Electrode lead  51  is made of an electrically conductive material, preferably platinum, and has a ceramic component for mechanical stabilization of 7% by volume Al 2 O 3 , for example. Electrode  50  is made of a catalytic material, preferably platinum, and a ceramic material, preferably 20% by volume ZrO 2  stabilized with Y 2 O 3 . In an additional embodiment, electrode  50  further has a porosity produced by a pore-forming material. The junction between electrode  50  and electrode lead  51  is produced by a wedge-shaped junction region  52  having an overlap zone. Electrode  50  and electrode lead  51  are produced according to a method known per se, e.g. by screen printing. 
     The described design can be used in any combination for every electrode shown in FIGS. 1 and 2 and for the respective electrode leads. It is conceivable to also use the described design of electrode  50  including electrode lead  51  for other electrochemical sensors of this type. 
     In the exemplary embodiment for the broadband probe (FIG.  2 ), internal pump electrode lead  32  and/or reference electrode lead  22  are produced using Al 2 O 3  as the ceramic component to reduce the lambda=1−ripple. In comparison with the ZrO 2  stabilized with Y 2 O 3 , which is suitable as the ceramic material for electrode  21 ,  31 , the Al 2 O 3  possesses no ionic conductivity. As a result, there is no ionic conduction between electrode leads  22 ,  32 , thereby increasing the internal resistance in this region. 
     A further exemplary embodiment of a broadband probe (FIG. 2) is that to reduce the drop in pump voltage in the lead region, external pump electrode lead  41  features a material having a low resistance in comparison with the material of external pump electrode  40 . This is achieved in that the proportion of electrically conductive material, e.g. platinum, is higher in the cermet material of external pump electrode lead  41  than in external pump electrode  40 . 
     FIG. 4 represents an additional specific embodiment in which electrode  50  and electrode lead  51 , including a junction region  52 , are situated in a layer plane in which a heater  55  embedded in the solid electrolyte body is located. For this purpose, heater  55 , electrode  50 , and electrode lead  55  are pressed onto first insulation layer  12 , for example. In a preferred embodiment, heater  55  is produced from the same material as electrode lead  51 .