Anodic Oxidation of Tungsten in Non‐Aqueous Electrolyte: I . Optical Anisotropy of the Oxide Film

Ellipsometry is used to study the optical properties of the oxide film grown on tungsten by anodization in acetic acid electrolyte. The tungsten substrate was found to have refractive index and extinction coefficient at a wavelength of 632.8 nm. At an anodizing current density of 172 μA/cm² the field in the oxide is 5.38 MV/cm. With the anodizing field applied, the index of refraction of the film in the field direction is and transverse to the field is. When the field is removed, the oxide film retains a high degree of anisotropy, with and . The fractional decrease in thickness is 1.1% when the field is removed. The analysis of data makes extensive use of correlation between optical and electrochemical measurements. The results are compared with the predictions of the theory of electrostriction, and with the results of studies of other oxides.