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Gas Sensors

SEM microphotograph of the sensing electrode (SE) region. (Layers form top to bottom are Pt-Collectorlayer, NiCr2O4 and FYSZ)

Among many possible gas sensors, the solid-state sensors offer the most suitable concept for the mobile systems. The best known representative of this group is the Lambda-Sensor which measures primarily the oxygen partial pressure of a gas mixture and then compares this with the reference gas. This sensor makes use of the potentiometer principle. Potentiometric sensors contain two Pt-electrodes; one is positioned at the side of the exhaust gas and the other at the reference gas. An ionic conductive solid-state electrolyte which is mostly an oxygen conductor is placed between these two electrodes. A typical material for electrolyte is tape-cast partially yttria stabilized zirconia (PYSZ). Since the reference and exhaust gases hold different oxygen partial pressures and this creates different electrochemical potentials on either electrode, the oxygen partial pressure in the exhaust gas can be predicted from the resulting voltage by means of the Nernst equation and with regard to the known oxygen partial pressure of the reference gas. This principle can be utilized vice versa such that when a  a definite voltage is applied between the electrodes, the oxygen partial pressure can be determined through the measured current (i.e. electrical flow). In order to determine different gases other than oxygen, it is necessary to employ suitable electrodes as well as sensing elements for the desired gases. In the case of NOx which contains many oxygen derivative species such as NO, NO2, N2O, etc., it is a crucial fact to use different sensing element arrays to increase the sensitivity of the sensor.

Furthermore, if these sensors are to integrate into a catalytic system, it may be necessary to replace the conventional electrolytes with those which perform more robust and incorporated. The research efforts in institute are therefore focused on finding better suitable electrolyte materials and concepts which are able to contribute more to the sensors characteristics through their microstructure as well as chemistry.

Our research targets also the most promising impedance based sensors. The measurement principle of this sensor is that the variations in the AC-resistance are measured at different frequencies depending on the gas environment, allowing the determination of the total NOx-content. Relying on that fact, this type of sensors is most interesting for the aimed application area. Alternating sensing elements in form of nano-sized and -structured layers can increase selectivity and sensitivity of the sensors. The best candidates for this purpose are binary compounds based on Co, Ni, Pd, and Zn which are to produce by Magnetron Sputtering.

Development of catalytic materials is carried out primarily on powders and coatings synthesized by sol-gel route which is flexible and easy to handle for production of complex oxides. Successful materials are coated by means of jumping beam EB-PVD process for achievement of robust and highly porous layers.