Kinetics of the Adsorption of Water and Electrical Conduction in Amorphous Ferric Oxide

Abstract

Abstract Amorphous ferric oxide was prepared by precipitation in a mixture of ammonium hydroxide and ferric nitrate at 90°C, after which it was calcinated in air at a constant rate of 10°C/min up to 200°C. The rate of the adsorption on the amorphous oxide was measured by a gravimetric method. An equation of the Elovich type was obtained: dq⁄dt=k′p0.63exp(−α′q⁄RTp0.5), where k′ and α′ are constants; q, the amount of adsorbed water, and p, the pressure. This kinetics can be interpreted by a model of a two-stage adsorption process, the van der Waals adsorption and the charge-transfer adsorption, the latter stage being assumed to be rate-determining. The effect of adsorbed water on the direct-current conductivity of the oxide, i, is formulated as i=i0exp(aq). Assuming that the adsorbed water is acting as a donor-type impurity and that the ionization energy decreases with the concentration of impurity, the relation between i and q can be derived from the theory of the semiconduction of an n-type conductor.