The present invention is directed to a sensor element for a sensor for determining the concentration of a gas component in a gas mixture, in particular the oxygen concentration in the exhaust gas of internal combustion engines.
A known sensor element for a broadband lambda sensor (e.g., as described in published German patent document DE 199 41 051 has a sensor body composed of solid electrolyte layers in which a cavity or measuring chamber, connected to the exhaust gas via a diffusion barrier, and a reference gas channel exposed to a reference gas are formed. A pump cell for pumping oxygen into the cavity (rich exhaust gas) or from the measuring chamber (lean exhaust gas) includes an external pump electrode situated on the solid electrolyte body and covered by a porous protective layer, and an internal pump electrode situated in the cavity. A concentration cell or Nernst cell includes a measuring electrode or Nernst electrode situated in the measuring chamber and a reference electrode situated in the reference gas channel. The limit current flowing between the pump electrodes when a constant voltage, e.g., 450 mV, is applied to the Nernst electrode and the reference electrode is a measure of the lambda value of the exhaust gas. The sensitivity of this sensor element is set via the limit current determined by the diffusion barrier.
Such a sensor element exhibits a dynamic relationship with the pressure, i.e., the pressure peaks in the exhaust gas appear as output signals of the lambda sensor, although there is no causal relationship between the pressure peaks and the change in gas composition. This is explained by the fact that, when pressure pulses occur in the exhaust gas, an additional amount of exhaust gas is pushed into the cavity, which causes a brief increase in the positive or negative pump current intensity. In particular, in the case of a high oxygen concentration (lean exhaust gas) this fluctuation of the partial pressure in the cavity due to the incoming exhaust gas is very noticeable, and the amplitude of the fluctuations of the lambda sensor output voltage is proportional to the oxygen concentration, i.e., the pump current.
The sensor element according to the present invention has the advantage that, by omitting the measuring chamber and combining the pump electrode and Nernst electrode to form an electrode situated on the external surface of the solid electrolyte, and by designing the diffusion barrier as a finely porous diffusion layer directly covering the electrode, no additional gas mixture influx occurs upon a pressure increase in the gas mixture, which makes it possible to avoid measuring errors due to pressure fluctuations in the gas mixture. In contrast to the annular diffusion barrier in the conventional sensor element, the diffusion layer may be manufactured much more easily, and a layer structure may be implemented. A layer structure permits the inlet surface of the finely porous diffusion layer, for example, to have a somewhat coarsely porous design, which makes it resistant to contamination, for example, by oil ashes. The absence of a measuring chamber or cavity permits the gas inlet bore hole to be omitted and eliminates a heat conduction barrier which would otherwise promote cracking of the solid electrolyte. Situating the electrodes on the external surface of the solid electrolyte, i.e., on the two major surfaces of the sensor element, permits proper heat distribution.
If the porous protective layer on the first electrode is configured as a coarsely porous diffusion layer, the sensor element according to the present invention also has the advantage that the Nernst cell may optionally be formed by the first and'second electrodes. By this change of the reference electrode for the Nernst cell, i.e., using both the electrode coated with the finely porous diffusion layer and the electrode coated with the coarsely porous diffusion layer, two different limit currents may be implemented, making it possible to operate the sensor element in two different measuring ranges. For measurements requiring a low static dependence on pressure, the electrode coated with the coarsely porous diffusion layer is used as the reference electrode of the Nernst cell, while for measurements requiring low dynamic dependence on pressure and temperature, the electrode coated with the finely porous diffusion layer is used, for example, by applying 450 mV relative to the reference electrode. Finely porous is understood here as a diffusion layer in which a limit current in the range of 4 mA flows when the oxygen concentration in the measuring gas is 20%. Coarsely porous is understood as a diffusion layer in which a limit current in the range of 25 mA flows under the same circumstances.
The two measurements may be adjusted by changing the mode of operation. It is also possible to detect and compensate via adjustment the gas type sensitivity of the sensor element or its static dependence on pressure or temperature by measuring in both modes of operation.
According to an example embodiment of the present invention, both electrodes are situated on opposite sides of a solid electrolyte body. The solid electrolyte body is manufacturable in a very simple construction using two thick solid electrolyte sheets. According to an example embodiment of the present invention, one electrode is situated on each solid electrolyte sheet for this purpose. The two solid electrolyte sheets enclose an insulation layer having an integrated electric resistance heater between their surfaces facing away from the electrodes and are connected by this insulation layer and a solid electrolyte frame enclosing the insulation layer. To reduce the internal resistance between the two electrodes functioning as a pump cell, a solid electrolyte web passing through the insulation layer is formed between the two solid electrolyte layers.
The sensor element schematically illustrated in
One of two electrodes 12, 13 is mounted on each external surface of solid electrolyte sheets 111, 112 facing away from insulation layer 14. First electrode 12 situated on solid electrolyte sheet 111 (lower solid electrolyte sheet in
Solid electrolyte body 21 is manufactured also in this case from solid electrolyte sheets or solid electrolyte layers in a layered structure. The two electrodes 12, 13 are situated on opposite sides of a first solid electrolyte layer 211. First solid electrolyte layer 211 is connected to a second solid electrolyte layer 213 via an intermediate layer 212, which is also made of a solid electrolyte. Intermediate layer 212 is provided with a recess 22, in which the electrode facing second solid electrolyte layer 213 and coated with its diffusion layer is situated. In the exemplary embodiment of
Also in this sensor element, diffusion layers 12, 13 may be made of multiple layers to set a limit current that may be carried by electrodes 12, 13. In the case of the finely porous diffusion layer 19, the upper layer facing away from second electrode 13, which forms the inlet surface of finely porous diffusion layer 13 for the gas mixture, i.e., the exhaust gas, may also be manufactured to be somewhat coarsely porous and therefore resistant to contamination, for example, by oil ashes.
Number | Date | Country | Kind |
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103 46 858.7 | Oct 2003 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP04/52372 | 9/30/2004 | WO | 1/19/2007 |