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New types of potentiometric NOx sensors suitable for use on automobiles were developed by using stabilized zirconia as a base solid electrolyte. It was found that the sensor with sensing electrodes of metal oxides (CdMn2O4 or NiCr2O4) showed excellent response to NOx in the concentrations between 20 and 4000 ppm at temperatures higher than 600 °C. The electromotive force of the sensors was almost linear to the logarithm of the NOx concentrations with positive slope for NO2 and negative slope for NO. Especially, the sensor fitted with CdMn2Os gave excellent responses to NO at 600 °C, while the sensor fitted with NiCr2O4 showed high sensitive to NO2 at 650 °C. The sensors were insensitive to CO, CO2, CH4, C3H6, O2 and water vapor. The sensors were fabricated as planar types with the reference electrode exposed to the sample gas, so that the sensors were simple in structure and easy to manufacture.

We have been developing the mixed-potential type NOx sensor which can detect the total-NOX concentration directly in exhausts for automobiles. It has been confirmed that the sensor is capable of detecting wide concentrations of total-NOx from 20 to 1000 ppm under the condition from rich-burn (A/F=12) to lean-burn without any interference from reducing gases, such as HC and CO. In addition, it has been confirmed that the sensor output is correlated fairly well to NOx concentrations from the analyzer in the engine test at any rotations. The results obtained here indicate that the present sensor has great possibility of being utilized as an on-board NOx sensor for practical use.

An NOx sensor based on mixed potential was made by laminating YSZ green sheets, on which an NOx sensing electrode, an NOx conversion electrode, a Pt heater, and a pair of thermocouples were printed and sintered. The NOx sensor was redesigned to reduce the number of lead wires from the sensor body. The oxygen sensor in the inner cavity of the sensor was also eliminated to improve controllability and reliability. Sensing characteristics in model gases have demonstrated that NOx concentrations as low as 1 ppm can still be detected by the simplified sensor. Even though the sensor shows a slight interference from HC, overall performance for sensing low levels of NOx in model gases is good. Its performance in exhaust from a stoichiometric engine (λ≦1 ) also shows that this NOx sensor is operable 1 under more realistic conditions.

The new total-NOx sensors were made by laminating YSZ green sheets, on which electrodes including an NOx sensing electrode, an oxygen pump, an NOx conversion electrode, a Pt heater and a pair of thermocouples were printed and fired. The exhaust gas containing NOx(NO and NO2), HC and CO were introduced into the cavity inside the sensor body, then HC and CO were oxidized to non-interference gases. NO was subsequently converted to NO2 by the NOx conversion electrode at the second half cavity. Consequently, NO and NO2 in the exhaust can be detected in total-NOx concentration as low as 10 ppm by the sensing electrode based on the mixed potential.