Capacitive gas sensor of mixed oxide CoO-In₂O₃ to selectively detect nitrogen monoxide

Selective detection of nitric oxide, NO, based on the capacitance change of a mixed-oxide capacitor was investigated. Since the capacitance of an oxide mixture consisting of two kinds of semiconductive oxides depends on the NO concentration, the NO concentration can be estimated from the capacitance of these mixed-oxide capacitors. In particular, the capacitance of the physical mixture of CoO and In₂O₃ was highly sensitive and selective to NO. Although the sensitivity to NO was decreased, the period for recovery was extremely shortened by increasing the operating temperature. In considering the sensitivity and response characteristics, the optimum operating temperature of CoO-In₂O₃ seems to exist around 500 K. Whereas the sensitivity to NO diluted with N₂ was higher than that in air, the sensitivity to NO was almost independent of the oxygen partial pressure over a wide range. Furthermore, the capacitance of CoO-In₂O₃ increased with increasing concentration of NO in the range from 1 to 1000 ppm. The sensitivity of CoO-In₂O₃ to CO₂, CO, and SO₂ was far smaller than that to NO. Consequently, a mixed oxide of CoO-In₂O₃ is one of the promising materials for the detection of NO in exhaust gases.