STABILIZED ZIRCONIA­BASED GAS SENSORS 109 among oxides examined. However, since methane is not regulated as an air pollutant, ZnO was cho- sen as SE material because it provides rather high sensitivity to propene but negligible sensitivity to methane. It was also found that ZnO possesses low sensitivity to NO 2 but is still sensitive to NO. In order to compensate for the NO sensitivity and convert NO to NO 2 at high temperatures, 1.5 wt% Pt was imbedded into the ZnO SE to improve its oxidation catalytic activity. Sample gas containing NO can diffuse through the porous structure of the SE, oxidizing NO to NO 2 in the presence of the dispersed Pt in the SE. Ensuing tests confirmed that the sensitivity to NO was decreased. However, the sensitivity to propene was still low compared to that of the sensor using the SnO 2 SE. In order to en- hance C 3 H 6 sensitivity, an applied potential of +50 mV was set during impedance measurements. From a practical point of view, it is very important to examine the influence of coexisting combustible gases on the changing oxygen level in terms of sensor performance in humidified atmosphere. The complex impedance-based zirconia sensor with the ZnO SE was insensitive to CO 2 (10­20 vol%), H 2 O (0.5­2 vol%) under a changing oxygen level from 5 to 10 vol% with polarization of +50 mV (Nakatou and Miura 2006). The C 3 H 6 sensitivity was also found to be independent of the total flow rate of the sample gas. Consequently, the results obtained confirmed that the complex impedance-based YSZ sensor with the ZnO SE (+1.5 wt% Pt) is capable of detecting C 3 H 6 concentrations selectively at high temperatures under harsh operating conditions. Thus, over just 7 years, impedance-based zirconia gas sensors have been developed into a new niche among zirconia-based gas sensors, and it is our strong belief that future research will open up opportuni-