Sol Weller

22 Studies of the Electrical Resistivity of Chromic Oxide

The dependence of the electrical resistivity of chromic oxide on the oxidation-reduction state of the surface has been investigated. The resistivity of oxidized chromia at 500° varies with the total excess oxygen according to the relation: p = k(Oads)−1.2. On addition of hydrogen at 500° to an oxidized, evacuated sample, the resistivity increases to a maximum and then decreases slightly with increasing equilibrium pressure. Exposure of the dry, reduced material to water vapor in hydrogen results in a reversible increase in resistivity and evolution of hydrogen. This behavior appears to be related to the formation of chromous oxide (or its equivalent) on the surface and to the n-type semiconductor properties of chromic oxide in dry hydrogen. The assumptions in Wagners thermodynamic treatment of the relation between conductivity and oxygen partial pressure have been critically examined. The theory in its elementary form is found to be inapplicable in cases where deviations from stoichiometric composition are relatively large.