Abstract
The pulsed polarization measurement technique using conventional thimble type lambda probes is suitable for low ppm NOx detection in exhaust gas applications. To evaluate the underlying sensor mechanism, the unknown influence of the reference atmosphere on the NO sensing behavior is investigated in this study. Besides answering questions with respect to the underlying principle, this investigation can resolve the main question of whether a simplified sensor element without reference may be also suitable for NO sensing using the pulsed polarization measurement technique. With an adequate sensor setup, the reference atmosphere of the thimble type lambda probe is changed completely after a certain diffusion time. Thus, the sensor response regarding NO is compared with and without different gas atmospheres on both electrodes. It is shown that there is still a very good NO sensitivity even without reference air, although the NO response is reduced due to non-existing overlying mixed potential type voltage, which is otherwise caused by different atmospheres on both electrodes. Considering these results, we see an opportunity to simplify the standard NOx sensor design by omitting the reference electrode.
Highlights
NOx detection in the low ppm range for vehicle control or turbine emissions is of high interest sinceNOx is a precursor for smog and acid rain
Besides conventional NOx mixed potential type sensors based on yttria-stabilized zirconia [4,5,6,7,8,9,10], another new method uses conventional thimble type lambda probes for NOx sensing
If the oxygen partial pressure, pO2exhaust, at the exhaust electrode is changed, a distinct voltage response, UNernst, between both electrodes of the lambda probe forms, because the inner reference electrode potential is defined by the well-known reference atmosphere
Summary
NOx is a precursor for smog and acid rain. Due to that fact, various materials and sensing principles are under research, e.g., see reviews [1,2]. Besides conventional NOx mixed potential type sensors based on yttria-stabilized zirconia [4,5,6,7,8,9,10], another new method uses conventional thimble type lambda probes (as described for instance in [11]) for NOx sensing. A pulsed polarization technique is applied [12,13,14]. If no air reference is required, a simpler sensor element without air reference but operated in the pulsed polarization technique might be feasible for NOx detection. In order to clarify this, measurements are conducted using a thimble type lambda probe in a set up that allows one to change the reference, so that the same atmosphere is applied to both electrodes
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