Anodic oxidation of tungsten in non-aqueous electrolyte; 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 N= 4.080 and extinction coefficient K = 3.205 at a wavelength of 632.8 nm. At an anodizing current density of 172 {mu}A/cm{sup 2} 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 Nz = 2.062 and transverse to the field is Nx = 2.176. When the field is removed, the oxide film retains a high degree of anisotropy, with Nz = 2.115 and Nx = 2.196. 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. In this paper the results are compared with the predictions of the theory of electrostriction, and with the results of studies of other oxides.