Gas sensing properties of YMnO^sub 3^ based materials for the detection of NO^sub x^ and CO

This paper deals with chemoresistive behavior of hexagonal YMnO3 produced by gel combustion. Stoichiometric, defective and doped compositions were studied and both reducing (CO) and oxidizing test gases (NOx) were used. The materials were characterized by means of X-ray diffraction, CO conversion tests, and oxygen temperature programmable desorption (TPD) and temperature programmable reduction (TPR) with H2. Experiments indicate that, besides the oxygen chemisorption, the direct adsorption of the target gases at the material surface plays an important role in resistive gas sensing. A model based on these assumptions was developed and excellently fits the transient response of the sensors under test to NO2 in the absence of oxygen. All the tested materials behave as p-type semiconductors and have a fast reversible response to NO2 which is optimum at temperatures close to 180 °C (30% @10 ppm NO2, after about 1 min). In particular, it was found that the best sensor performance in NO2 detection is obtained with defective materials in terms of response, while an interesting trade-off between temperature response and gas response is obtained with Pd doped materials. Regarding CO detection, materials impregnated with Pd showed the best performance both in terms of speed and of response, with an optimum temperature of 300 °C.