Open‐Circuit Transient Analysis of the Anodic Oxidation of Tungsten and Niobium

Abstract

The anodic oxidation of tungsten and niobium is studied using open‐circuit transients to determine the field dependence of both the current density and the dielectric constant of the oxide film. The resolution of the transient recording and analysis procedures has been increased in order to improve the sensitivity of the technique to the form of the field dependence of the dielectric constant. The results of the analysis are related to an elastic dielectric model of the oxide film by assuming that the field dependence of the dielectric constant is a result of electrostriction. The Clausius‐Mossotti relation is used to express the dependence of the dielectric constant of the film on its density and to calculate the fractional change in film thickness required to produce the observed field dependence of the dielectric constant. The results are compared with ellipsometric measurements of the strain induced by electrostriction in the two systems. In the tungsten system the open‐circuit transient results agree closely with published ellipsometric measurements, and in the niobium system they agree well with the ellipsometric results obtained in this study. It is concluded that for tungsten and niobium the elastic dielectric model provides a quantitative description of the field dependence of the strain, the optical anisotropy, the dielectric constant, and the ionic conductivity of the oxide film.