Electrostriction in Anodic Oxides: Exchange of Elastic and Electrostatic Energy

Abstract

The classical theory of electrostriction is put into a form which emphasizes the electrostrictive coupling between electrostatic and elastic energy. The predictions of the theory for capacitors with elastic dielectrics are compared with experimental charging curves on anodic films of tungsten oxide. The experimental curves show that the electrostatic energy stored can exceed the external work done on a charging cycle. This is possible because anodic oxides are grown in a high field, and when the anodizing field is removed the oxide films acquire elastic energy which can be reconverted partially to electrostatic energy on a subsequent charging cycle. The apparent conflict between theory and experiment is not because there is anything unusual about the coupling between electrostatic and elastic energy in anodic oxides, it is because the theory assumes that the elastic energy is zero at zero field and expands the energy density accordingly. The permittivity variation deduced from charging curves on tungsten oxide compares closely with the permittivity variation deduced from open‐circuit transients using an effective field model with the permittivity expanded about the anodizing field. If the ionic current is controlled by an effective field, changes in relative permittivity will affect the current density, and electrostriction will have a significant impact on the ionic transport process whenever the field in the oxide is changed.