Further Electrical Effects of the Adsorption of Water Vapour by Anodized Aluminium

Abstract

Measurements have been made of the equivalent parallel capacitance and resistance of porous anodized aluminium oxide layers. The measurements, made in air at relative humidities from 0.0 to 0.85 and at frequencies from 50 c/s to 1 Mc/s, extended up to 433 days after formation of the anodic layer. The influence of the humidity and of the frequency of measurement on the dielectric properties of these layers may be explained satisfactorily on the basis of an equivalent circuit. The variation in properties found among layers anodized in the same conditions is due to differences in conductivity arising from varying amounts of stray ions incorporated in the layers. Ageing is primarily due to a decrease in conductivity, and not to structural changes of the oxide layer. The ageing may be reversed by brief immersion of the layers in electrolytes. The results indicate that, in addition to the dielectric losses arising from the main porous structure, losses also occur within the amorphous alumina, showing that this is permeable to water. This can account for measurements of the surface area of porous anodic layers, and of the humidity dependence of the losses in `non-porous' anodic layers.