CHROMIUM-OXIDE GAS SENSORS FOR THE DETECTION OF HYDROGEN, OXYGEN AND NITROGEN-OXIDE

The gas sensitivity of chromium oxide, a semiconductor metal oxide, to hydrogen in air and to oxygen in hydrogen is presented and the operation principle is described. It is shown that when this oxide is doped with Pd promoter, it has high sensitivity to hydrogen concentrations of a few ppm in air. When this oxide is doped with Pt and In it acquires room-temperature sensitivity. Sensors tested in flowing forming gas (50-100 cm3 min-1) detect oxygen down to a few ppm concentration. It is shown that Pd-doped sensors have a switching characteristic at hydrogen/oxygen stoichiometric ratios of 1:1, resulting in a very sharp change in their conductance. It is found that the role of In, an n-type promoter, is to transform Cr2O3 from a high-resistivity p-type to a low-resistivity n-type oxide. Pt provides room-temperature sensitivity due to its thin surface oxide layer. In flowing forming gas the sensors detect oxygen concentrations of 50 ppm at room temperature, but at about 100-degrees-C the oxygen sensitivity increases and a concentration of a few ppm is detectable.