Design and development of compact, high-performance NOx sensors for preservation of environment

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The purpose of the present study is the development of new type high-performance NOx sensors with the aim of preservation of environment. Four types of solid-state NOx sensors based on ionic conductors or oxide semiconductors were proposed here and their sensing properties were examined. (1) The stabilized zirconia-based electrochemical devices using the oxide sensing electrodes were tested for NOx sensing properties at high temperature. Among the many metal oxides examined, CdCr2O4 or WO3 was found to give the best sensing properties to NO and NO2 at 500-700℃. The EMF response of the device was linear to the logarithm of NOx concentration. (2) The stabilized zirconia-based devices using oxide electrode for selective amperometric detection of NO at high temperature. The CdCr2O4-attached tubular device showed quick and selective response to NO over NO2 at 500 and 550℃, if the potential of sensing electrode was polarized properly at + 100 mV vs. the reference Pt electrode. The 90% respon … More se time of the divide to 100 ppm NO was as short as ca. 20 s at 500℃. The current response was well correlated to the NO concentration in the range of 0 - 200 ppm. (3) A new solid-state amperometric sensor for NO2 was fabricated by combining an NaィイD1+ィエD1 conductor (NASICON) based device with an NaNO2 layer deposited on the counter electrode. With the sensing electrode polarized at - 150 mV relative to the reference Au electrode at 150℃, the electric current flowing through the device under exposure to NO2 was linear to the NO2 concentration in the range from 10 ppb to 1 ppm. (4) A search for semiconducting oxides was carried out to design a sensor selective to NO over NO2. Bi2O3 was very selective to NO through not very sensitive among the various oxides examined, and NiO was proved to be the most effective promoter to Bi2O3 for enhancing NO sensitivity. The sensor element using 5wt%NiO-Bi2O3 showed fairly good sensing properties to NO in the range of 0 - 400 ppm in sensitivity, selectivity over NO2, and response rate, at 300℃. Less