Abstract:
A sensor element to determine the temperature or the oxygen concentration of an exhaust gas of an internal combustion engine includes a first solid electrolyte film, a second solid electrolyte film, and a diffusion barrier disposed in a layer plane between the first and the second solid electrolyte film. A gas-impermeable or at least largely gas-impermeable cover layer is provided locally on the diffusion barrier, so that in the regions in which the cover layer is provided on the diffusion barrier, diffusion of the measured gas into or out of the diffusion barrier is at least largely prevented. A corresponding method for manufacturing a sensor element includes ablating a diffusion barrier using a laser in order to adjust the diffusion resistance of the diffusion barrier. A cover layer that, after a sintering process, is gas-impermeable or at least largely gas-impermeable is applied onto a side of the diffusion barrier that faces toward a measured gas located outside the sensor element. The cover layer is ablated in order to adjust the diffusion resistance of the diffusion barrier.

Description:
FIELD OF THE INVENTION 
   The invention relates to a sensor element and a method for manufacturing the sensor element. 
   BACKGROUND INFORMATION 
   A sensor element of this kind and a method for manufacturing the sensor element are described in published German patent document DE 198 17 012, which sensor element has, between a first and a second solid electrolyte film, an annular diffusion barrier that is surrounded by an annular measured gas space. Disposed in the measured gas space are electrodes to which a measured gas, located outside the sensor element, can travel via a gas entry opening installed in the first solid electrolyte film, and through the diffusion barrier. 
   Also known are sensor elements in which gas entry occurs through openings disposed in the layer plane between the first and the second solid electrolyte film. 
   In order to compensate for production-related fluctuations in the diffusion resistance of the diffusion barrier, the diameter of the gas entry hole and thus the inside diameter of the diffusion barrier are modified in controlled fashion. For this purpose, firstly a sensor element is sintered from a charge, and the so-called limit current (pumping current) of this sensor element is ascertained; the desired inside diameter of the diffusion barrier is ascertained on the basis of the measurement result, and the ascertained inside diameter is adjusted, by drilling, for the further sensor elements deriving from the same charge. 
   It is disadvantageous in this context that the method for ascertaining the limit current on the sintered sensor element, and the subsequent adaptation of the inside diameter of the diffusion barrier and the gas entry opening, are time-consuming and cost-intensive. It is additionally disadvantageous that even after the above-described correction of production-related fluctuations in the diffusion resistance, the limit current can vary because the magnitude of the diffusion barrier can be subject to a variation within a charge. 
   SUMMARY 
   The sensor element according to the present invention and the method for manufacturing the sensor element according to the present invention have the advantage that after sintering, the diffusion barrier can be processed with high accuracy in simple, time-saving, and economical fashion in order to compensate for production fluctuations in diffusion resistance related to production engineering. 
   For this purpose, the diffusion barrier that is disposed between a first and a second solid electrolyte film is coated, on its side facing toward the measured gas, with a cover layer. The cover layer is gas-impermeable or at least largely gas-impermeable. The diffusion barrier and/or the cover layer can thus be processed through a gas entry opening that is introduced into the first solid electrolyte film, and the diffusion resistance of the diffusion barrier can thereby be adjusted on the sintered element. 
   Also advantageous is a method in which the diffusion barrier inside the sintered sensor element is ablated using a laser. It is advantageous in this context that during ablation of the diffusion barrier using the laser, the diffusion resistance of the diffusion barrier can be measured by ascertaining the limit current of an electrochemical cell of the sensor element. 
   For purposes of this specification, a “largely gas-impermeable cover layer” is to be understood as a cover layer in which the quantity of measured gas flowing through the cover layer, or of a component of the measured gas flowing through the cover layer, corresponds to at most 10 percent of the quantity of the total measured gas flowing through the diffusion barrier, or of the total component of the measured gas flowing through the diffusion barrier. 
   The cover layer has an orifice through which the measured gas can travel via the diffusion barrier to the electrode, the orifice being disposed, in the context of a cylindrical or hollow-cylindrical diffusion barrier, centeredly on the diffusion barrier, for example as a circular orifice whose center point lies on the axis of symmetry of the diffusion barrier. This ensures that the diffusion path of the measured gas is substantially the same to all regions of the electrode. It is furthermore advantageous if the surface of the diffusion barrier covered by the cover layer is disposed parallel to a layer plane of the sensor element, and if an orifice (gas entry opening) is provided in the first solid electrolyte film, the cover layer being disposed on the side of the diffusion barrier adjoining the gas entry opening. 
   The gas entry opening is cylindrical in shape, and the diffusion barrier hollow-cylindrical. The cover layer is configured annularly and has a circular orifice. The respective center points and center axes are superimposed on one another. Diameter d 1  of the gas entry opening, inside diameter d 2  of the diffusion barrier, outside diameter d 3  of the diffusion barrier, and diameter d 4  of the circular orifices advantageously satisfy the condition d 4 &lt;d 1 &lt;d 3 . Particularly advantageously, d 1  is in the range from 0.6 mm to 1.8 mm, and/or d 2  is in the range from 0.2 mm to 0.6 mm, and/or d 3  is in the range from 1.8 mm to 3.0 mm, and/or d 4  is in the range from 0.2 mm to 0.6 mm. 
   In an example embodiment, the condition d 2 ≦d 4 &lt;d 1  is satisfied. In this embodiment, gas entry occurs both via the inside radius of the diffusion barrier and via that region of the diffusion barrier not covered by the cover layer. As a result, a portion of the exhaust gas has a shorter diffusion distance, and the diffusion flow through the diffusion barrier is increased. 
   In a further example embodiment, d 4 ≦d 2 , so that the cover layer projects beyond the inside radius of the diffusion barrier and thus protects the diffusion barrier from deposition of damaging constituents of the exhaust gas. The layer thickness of the cover layer is at least as great as the layer thickness of the diffusion barrier, thus ensuring the mechanical stability of the projecting region of the cover layer. 
   As an alternative to the hollow-cylindrical geometry, a linear geometry can also be provided for the gas entry opening, diffusion barrier, and measured gas space; with this geometry the diffusion barrier and measured gas space are disposed one behind another and have approximately the same width and (including the cover layer) the same height. The gas entry opening is introduced, for example, as a gap into the first solid electrolyte film, the width of the gap corresponding to the width of the diffusion barrier. 
   In an alternative example embodiment, a further diffusion barrier is disposed between the cover layer and the first solid electrolyte film. The measured gas, or a component of the measured gas, can thus travel to the electrodes via the orifice in the cover layer and via the diffusion barrier, or directly via the further diffusion barrier. The measured gas diffusing through the further diffusion barrier does not flow through the orifice in the cover layer. The diffusion resistance is thus made up of one component of the further diffusion barrier and one component of the orifice in the cover layer and the diffusion barrier. Division of the diffusion flow into two branches simplifies equalization of the diffusion resistance, since the entire diffusion flow does not pass through the orifice in the cover layer. Advantageously, the inside diameter of the further diffusion barrier is larger than the inside diameter of the gas entry opening. 
   The diffusion barrier is ablated through the gas entry opening using a laser. Prior to ablation, the gas entry opening is also filled at least locally with the diffusion barrier. This simplifies accessibility for the laser, and increases the volume that can be ablated with the laser. 
   In an alternative example method for equalizing the diffusion resistance of the diffusion barrier, the cover layer and/or the diffusion barrier are ablated, using a laser, through the gas entry opening. Ablation is accomplished on the sintered sensor element, i.e., after the sintering process. 
   The limit current is ascertained during ablation of the diffusion barrier, and ablation is monitored on the basis of the ascertained limit current. This is done by applying a voltage to an electrochemical cell of the sensor element, one of the electrodes of the electrochemical cell being disposed behind the diffusion barrier in the diffusion direction. The voltage is sufficiently high that all the oxygen flowing through the diffusion barrier is pumped off by the electrochemical cell from the electrode located behind the diffusion barrier. Ablation is continued until a target value for the limit current is reached. The limit current measurement can also occur before and/or after ablation of the diffusion barrier, or ablation can be interrupted during determination of the limit current. 
   The eroded material resulting from ablation of the diffusion barrier is removed using a gas flow that is directed through an injection nozzle onto the diffusion barrier or onto the corresponding region of the diffusion barrier. In order not to distort the limit current that is to be determined, the gas flow has a defined oxygen partial pressure, e.g., the oxygen partial pressure of air, and is heated to a temperature of approximately 750 degrees Celsius. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a longitudinal cross-section through a first exemplary embodiment of the invention. 
       FIG. 2  shows a section, taken along line II-II in  FIG. 1 , through the first exemplary embodiment of the invention. 
       FIG. 3  shows a portion of a section through a second exemplary embodiment of the invention. 
       FIG. 3   a  shows a portion of a section through a further exemplary embodiment of the invention. 
       FIG. 4  is a plan view, in the direction IV indicated in  FIG. 3 , of the second embodiment of the invention. 
       FIG. 5  shows a cross-sectional view of a third exemplary embodiment of the invention. 
       FIGS. 6   a  and  6   b  show a fourth exemplary embodiment of the invention before and after ablation of a region of the diffusion barrier using a laser. 
   

   DETAILED DESCRIPTION 
     FIGS. 1 and 2  show a first exemplary embodiment of the invention having a sensor element  10  including a first, a second, and a third solid electrolyte film  21 ,  22 ,  23 . Disposed on the outer side of first solid electrolyte film  21  is an annular first electrode  31 , adjoining which is a supply lead  31   a  to first electrode  31 . First electrode  31  is covered with a porous protective layer  45 . 
   Introduced into first solid electrolyte film  21 , inside the opening of first electrode  31 , is a gas entry opening  59  having a first enveloping surface  51  that possesses a diameter d 1 . 
   A hollow-cylindrical diffusion barrier  41  is disposed between first and second solid electrolyte films  21 ,  22 . Diffusion barrier  41  has an inner enveloping surface  52  having a diameter d 2 , and an outer enveloping surface  53  having a diameter d 3 . Outer enveloping surface  53  of diffusion barrier  41  is surrounded by a measured gas space  42  that is likewise hollow-cylindrical in shape. 
   Additionally disposed between first and second solid electrolyte films  21 ,  22  is a reference gas space  43  that extends, proceeding from measured gas space  42 , in the longitudinal direction of sensor element  10  and is filled with a ceramic material of low porosity. A sealing frame  61  is disposed between measured gas space  42  and reference gas space  43 . Sealing frame  61  completely surrounds measured gas space  42  and seals it off from the outside. 
   An annular second electrode  32  is disposed in measured gas space  42  on first solid electrolyte film  21 . Opposite second electrode  32 , an annular third electrode  33  is provided in measured gas space  42  on second solid electrolyte film  22 . A fourth electrode  34  that is exposed to a reference gas is disposed in reference gas space  43 . The reference gas space can also be constituted by the pores of a porously configured fourth electrode or its supply lead, or by a porous ceramic layer. 
   Provided between second and third solid electrolyte films  22 ,  23  is a heater  63  that is electrically insulated from the surrounding solid electrolyte films  22 ,  23  by a heater insulator  64 . Heater  63  and heater insulator  64  are surrounded laterally by a heater sealing frame  62 . 
   The measured gas present outside sensor element  10  can travel, via gas entry opening  59  and via inner enveloping surface  52  of diffusion barrier  41 , into measured gas space  42  and thus to second and third electrodes  32 ,  33 . 
   First and second electrodes  31 ,  32 , and solid electrolyte  21  disposed between first and second electrodes  31 ,  32 , constitute an electrochemical pumping cell with which oxygen is pumped into or out of measured gas space  42 . The oxygen partial pressure is determined by an electrochemical Nernst cell that is constituted by third and fourth electrodes  33 ,  34  and by solid electrolyte  22  located between third and fourth electrodes  33 ,  34 . Based on the signal of the Nernst cell, the voltage applied to the pumping cell is controlled, by a control unit disposed outside the sensor element, in such a way that an oxygen partial pressure of lambda=1 is present in measured gas space  42 . The oxygen partial pressure of the measured gas can be ascertained from the magnitude of the pumping current of the pumping cell. 
   The voltage applied to the pumping cell is selected to be sufficiently high that all the oxygen present in measured gas space  42  is pumped off. The pumping current is thus limited by the quantity of oxygen flowing through diffusion barrier  41 . The pumping current flowing in this context constitutes the so-called limit current. 
   Production-related (e.g., geometric) fluctuations in diffusion barrier  41  can cause changes in, for example, the diffusion distance inside diffusion barrier  41  or the magnitude of diffusion barrier  41 . Under otherwise identical conditions, the oxygen flow through the diffusion barrier, and therefore the limit current, may fluctuate; in the case of an elongation of the diffusion distance, for example, the diffusion flow and thus the limit current are reduced. The ultimate consequence of this is a distortion of the measurement result. Diffusion barrier  41  is therefore, according to the present invention, adjusted to a uniform oxygen flow through the diffusion barrier under uniform external conditions (adjusting diffusion resistance to a predefined target value for the limit current). 
   For that purpose, the sintered sensor element  10  is heated by heater  63  to operating temperature (e.g., 750 degrees Celsius), and is exposed to a measured gas having a defined oxygen concentration (e.g., ambient air). The oxygen flowing into measured gas space  42  is pumped out by the pumping cell (from second electrode  32  to first electrode  31 ), and the corresponding limit current is measured. At the same time material is ablated from sensor element  10 , thereby influencing the oxygen flow flowing through diffusion barrier  41 . Material ablation is continued until a target value for the limit current, which value corresponds to a target value for the oxygen flow flowing through diffusion barrier  41 , is achieved. Diffusion barrier  41  is configured in such a way that, taking into account the production-related fluctuations that are to be expected, the diffusion flow prior to material ablation is less than the target value for the limit current, so that the diffusion distance is decreased by the material ablation and the diffusion flow can thus be increased until the target value is reached. 
   Provision is made for this purpose, in the case of the first exemplary embodiment according to  FIGS. 1 and 2 , to select a larger diameter for gas entry opening  59  than for the inside diameter of diffusion barrier  41 . Diffusion barrier  41  can thus be ablated with a laser through gas entry opening  59 . In order to arrive a defined gas flow with identical diffusion distances, oxygen needs to enter diffusion barrier  41  at inner enveloping surface  52 . Diffusion barrier  41  is therefore coated, on its side facing toward measured gas opening  59 , with a gas-impermeable cover layer  44 . The desired diffusion resistance can thus be adjusted by ablating cover layer  44  and diffusion barrier  41  using a laser. In order to adjust the diffusion resistance, an opening is therefore introduced with the laser into both diffusion barrier  41  and cover layer  44 , so that inside diameters d 4  and d 2  of cover layer  45  and diffusion barrier  41  are approximately equal. 
   The first exemplary embodiment according to  FIGS. 1 and 2  has the following dimensions:
         d 1 =1.0 mm   d 2 =0.4 mm   d 3 =2.0 mm   d 4 =0.4 mm.       

   It is conceivable for the inner opening (enveloping surface  52 ) of diffusion barrier  41  not to be shaped cylindrically, for example, because diffusion barrier  41  is less severely ablated on the side facing toward second solid electrolyte film  22 . In that case the inside radius of cover layer  44  corresponds to the inside radius of diffusion radius  41  at least in the region of diffusion barrier  41  directly bordering cover layer  44 . 
   Cover layer  44  can also extend in the region between diffusion barrier  41  and first solid electrolyte film  21  (outside the gas entry opening). 
   In the case of the exemplary embodiments depicted in the subsequent Figures, elements corresponding to those shown in  FIGS. 1 and 2  are labeled with the same reference characters. 
     FIG. 3  and  FIG. 4  depict a second exemplary embodiment of the invention that differs from the first exemplary embodiment in terms of the configuration of cover layer  44 . In the second exemplary embodiment, cover layer  44  has an inside radius d 4  that is larger than inside radius d 2  of diffusion barrier  41 . In order to equalize the diffusion barrier, it is sufficient to ablate cover layer  44  in such a way that cover layer  44  has an opening  54  centeredly with respect to diffusion barrier  41 . Largely similar diffusion distances to electrodes  32 ,  33  are achieved as a result of opening  54 , centered with respect to diffusion barrier  41 , in cover layer  44 . In an alternative embodiment, diffusion barrier  41  may be cylindrical, i.e., has no opening  52 . 
   The second exemplary embodiment according to  FIGS. 3 and 4  has the following dimensions:
         d 1 =1.0 mm   d 2 =0.3 mm   d 3 =2.0 mm   d 4 =0.5 mm.       

     FIG. 3   a  depicts a further example embodiment of the invention in which inside diameter d 4  of cover layer  44  is smaller than inside diameter d 2  of diffusion barrier  41 , so that cover layer  44  extends over the opening in diffusion barrier  41 . A cavity paste is to be provided for that purpose in the opening in the diffusion barrier, which paste volatilizes upon sintering leaving no residue. The layer heights of cover layer  44  and of diffusion barrier  41  are identical in this embodiment, in order to ensure sufficient stability for the cover layer. The layer height of the cover layer is approximately 100 μm. 
   The embodiment according to  FIG. 3   a  has the following dimensions:
         d 1 =1.0 mm   d 2 =0.5 mm   d 3 =2.0 mm   d 4 =0.3 mm.       

   In the exemplary embodiment according to  FIG. 5 , a further hollow-cylindrical diffusion barrier  41   a , having an inside radius d 5 , is provided between cover layer  44  and first solid electrolyte film  21 . The oxygen flow flowing into measured gas space  42  is thus divided into a portion that flows through further diffusion barrier  41   a  and a portion that flows through opening  54  in cover layer  44  and through diffusion barrier  41 . In order to increase the equalization accuracy, diffusion barrier  41  and further diffusion barrier  41   a  are configured in such a way that the oxygen flow through further diffusion barrier  41   a  is greater than the oxygen flow through diffusion barrier  41 . The portion of the oxygen flow through further diffusion barrier  41   a  amounts to 70 to 80 percent, and the portion of the oxygen flow through diffusion barrier  41  to 20 to 30 percent, of the total oxygen flow. 
   The inside diameter of further diffusion barrier  41   a  is equal to or greater than the inside diameter of gas entry opening  59 . The ablation of cover layer  44 , and (if applicable) of diffusion barrier  41 , thus has no influence on the quantity of oxygen flowing through further diffusion barrier  41   a . The outside diameters of diffusion barrier  41 , of cover layer  44 , and of further diffusion barrier  41   a  are approximately equal. 
   In an alternative embodiment, the cover layer may extend only as far as inner enveloping surface  55  of further diffusion barrier  41   a.    
     FIGS. 6   a  and  6   b  depict a method for adjusting the diffusion resistance in which a cover layer is omitted, and in which gas entry opening  59  is at least locally filled with diffusion barrier  41 . After a sintering process, diffusion barrier  41  is ablated with a laser  81  while the limit current is continuously measured, until the target value for the limit current is reached. 
   In the method for adjusting diffusion resistance for the sensor elements according to  FIGS. 1 to 6   b , in general a laser beam  81  is directed, using an apparatus  80  for generating a laser beam  81 , through gas entry opening  59  in first solid electrolyte film  21  onto cover layer  44  and/or diffusion barrier  41  of sintered sensor element  10  (see  FIGS. 6   a  and  6   b ). In accordance with the method described above, material ablation is continued until the measured limit current corresponds to the predefined target value. The laser ablation of diffusion barrier  41  and/or of cover layer  44  produces eroded material. To remove the eroded material, an injection nozzle  83  is provided through which an air stream  82  (flushing gas) is directed onto diffusion barrier  41  and onto cover layer  44 . The air stream, whose oxygen concentration corresponds to the oxygen concentration of the measured gas surrounding the sensor element, is heated to the operating temperature of the sensor element, for example to a temperature of 750 degrees Celsius, by a corresponding heating apparatus in injection nozzle  83 . 
   In the exemplary embodiments described, the diffusion barriers, measured gas space, and gas entry opening exhibit a cylindrical geometry. The present invention applies equally to sensor elements in which diffusion into the measured gas space takes place linearly. For that purpose, the sensor element has an elongated conduit of largely constant width and height in which the diffusion barrier, and the measured gas space having the electrodes, are disposed (not depicted). A (for example) gap-shaped gas entry opening is provided in the first solid electrolyte film. The measured gas can travel through the gas entry opening and through the diffusion barrier into the measured gas space and to the electrodes. The diffusion direction of the measured gas is substantially parallel to the longitudinal axis of the sensor element. Provided on the diffusion barrier is the cover layer, which can be processed through the gas entry opening using a laser. The diffusion resistance is adjusted by ablating the cover layer and/or the diffusion barrier.