Effect of incorporated electrolyte species on the ac response of the high field ionic conduction process in anodic oxide films on tantalum

Anodic oxide films on tantalum consist of an outer layer, grown by metal ion motion, which usually takes up anions from the electrolyte, and an inner layer, grown by oxygen transport. By using the recent observation that films grown in solutions of KCl do not take up chloride ions, measurements of the ac response of the ionic conduction process in films grown in phosphoric acid and dilute sulphuric acid were analyzed to obtain the admittivities of the two parts of the films separately. Plots of the real vs, the imaginary part of the small signal ionic admittivity for films grown in solutions of KCl were found to be arcs of circles with centres above the real axis implying a distribution of relaxation times in the process responsible for the dependence of the ionic current on the history of the film. Thus the presence of a distribution, rather than two or more discrete values, is not due to the presence of incorporated electrolyte but to the amorphous nature of the oxide. With phosphoric acid, the incorporated phosphate in the outer layer affects the permittivity and the ionic conduction properties more strongly than incorporated sulphate. Kinetic parameters were compared to those from steady state and stepped field kinetic data and related to the permittivities of the layers. (C) 1998 Elsevier Science Ltd. All rights reserved.