Ellipsometric Analysis of Changes in Surface Oxide Films on Niobium during Cathodic and Anodic Polarization

Abstract

In order to gain a better understanding of the changes in surface films with the cathodic formation and anodic decomposition of hydride, the changes in ellipsometric parameters have been measured in situ on a Nb electrode under cathodic and anodic polarization in a 0.5 kmol·m⁻³-H₂SO₄ solution. Optical constants of film-free surfaces of Nb metal and Nb hydride were determined by tribo-ellipsometry and used to calculate the thickness and optical constants of the films from the measured values of the parameters.
The Nb electrode with an anodic oxide film was first polarized to a cathodic potential. The transformation of the film from the one with a higher optical constant to the one with a lower optical constant and the formation of hydride layer under the film were observed simultaneously. Although its thickness decreased, the film on the hydride layer was found to remain after prolonged cathodic polarization.
Then, the Nb electrode with a hydride layer was polarized to an anodic potential. The decomposition of hydride layer and the formation of anodic oxide film occurred immediately. The anodic oxide film formed on the Nb hydride layer in this case should be the same as that formed directly on Nb metal because the former had the same optical constant as the latter.