Mixtures of Al 2 O 3 -ZnO-V 2 O 5 have been explored for use as active sensing materials in Taguchi-type NO x sensors. The electrical resistance and Seebeck effect of these semiconducting oxide mixtures have been measured as a function of the composition. The sensitivity and selectivity of the sensor to NO and NO 2 gases against CO 2 were studied in order to find the optimal composition of the oxides. The presence of p-n junctions may play an important role in the conductivity and sensitivity of the sensor materials. The experimental results indicate that sensors based on mixtures of Al 2 O 3 -ZnO-V 2 O 5 exhibit sensitive and fast response to 20 to 300 ppm of NO and/or NO 2 in the temperature range of 200 to 400°C, the sensitivity to NO is usually larger than that to NO 2 . The sensors are almost insensitive to even 0.01 atm CO 2 gas. A preliminary discussion on the operation principle of the sensors is presented.
