Electrochimica Acta, Vol.52, No.5, 2134-2145, 2007
Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium
Anodizing of mobium has been investigated to develop mobium solid electrolytic capacitors. Chemically polished niobium specimens were anodized in a diluted phosphoric acid solution, initially galvanostatically at i(a) = 4 Am-2 up to E-a = 100 V, and then potentiostatically at E-a = 100 V for t(pa) = 43.2 ks. During the galvanostatic anodizing, the anode potential increased almost linearly with time, while, during potentiostatic anodizing, the anodic current decreased up to t(pa) = 3.6 ks, and then increased slowly before decreasing again after t(pa) = 30.0 ks. Images of FE-SEM and in situ AFM showed that nuclei of imperfections were formed at the ridge of cell structures before t(pa) = 3.6 ks. After formation, the imperfection nuclei grew, showing cracking and rolling-up of the anodic oxide film, and crystalline oxide was formed at the center of imperfections after t(pa) = 3.6 ks. The growth of imperfections caused increases in the anodic current between tpa = 3.6 and 30.0 ks. Long-term anodizing caused a coalescence of the imperfections, leading to decreases in the anodic current after t(pa) = 30.0 ks. As the imperfections grew, the dielectric dispersion of the anodic oxide films became serious, showing a bias voltage dependence of the parallel equivalent capacitance, C-p, and a dielectric dissipation factor, tan delta. The mechanism of formation and growth of the imperfections, and the correlation between the structure and dielectric properties of anodic oxide films is discussed. (c) 2006 Elsevier Ltd. All rights reserved.