Change in the Structure and Dielectric Properties of Niobium Anodic Oxide Films during Potentiostatic Anodizing

Niobium specimens with chemical polishing were anodized in a phosphoric acid solution galvanostatically up to Ea = 100 V, and then potentiostatically at Ea = 100 V. During galvanostatic anodizing, anode potential increased almost linearly with time, while, during potentiostatic anodizing，anodic current decreased with time before tpa = 3.6 ks，and then increased slowly before decreasing again at tpa = 32.4ks. Galvanostatic anodizing allowed compact oxide films to grow at a steady rate, and also micro imperfections to form in the film at the ridge of convex network structure produced by chemical polishing. The imperfections grew during potentiostatic anodizing, showing the cracking and rolling-up of the oxide film as well as the formation of crystalline oxide at the center of the imperfections. Long-term anodizing lead to the coalescence of the imperfections and eventually the covering of all surfaces with imperfections. Parallel equivalent capacitance, Cp, of anodic oxide films decreased with tpa before 3.6 ks, and increased after 3.6 ks, w ile the dielectric dissipation factor, tan δ, remained to a small value before 3.6 ks, and increased with tpa after 3.6 ks. Measurements with higher bias voltages caused less dielectric dispersion of anodic oxide films. The mechanism of structural change of anodic oxide films during potentiostatic anodizing, and the correlation between the structure and dielectric properties of anodic oxide films are discussed.