Influence of Current Density on the Structure and Dielectric Properties of Anodic Oxide Films on Niobium

Abstract

Niobium specimens with chemical polishing were anodized in a phosphoric acid solution galvanostatically at i_a,ini=1, 10, 100 and 1000Am⁻² up to E_a=100V, and then potentiostatically at E_a=100V for t_pa=7.2ks. During galvanostatic anodizing, the anode potential increased almost linearly with time on all the specimens, while, during potentiostatic anodizing, the anodic current decreased with time. The current density at t_pa=7.2ks was higher at lower i_a,ini in the range of 0.02 to 0.25Am⁻². The spectra of Rutherford backscattering spectroscopy (RBS) and glow discharge optical emission spectroscopy (GD-OES) showed that higher i_a,ini causes film thickness to decrease and the amount of incorporated phosphorus to increase. Micro imperfections were formed in the film at the ridge of the convex network structure produced by chemical polishing, and the number of imperfections decreased with increasing i_a,ini. Parallel equivalent capacitance, C_p, and the dielectric dissipation factor, tan δ, of anodic oxide films decreased with increasing i_a,ini. The mechanism of decrease in the number of imperfections in anodic oxide with increasing i_a,ini is discussed in term of film thickness, phosphorus incorporation and Nb⁵⁺ transport number.