Niobium anodic oxide films: Effect of incorporated electrolyte species on DC and AC ionic current

Ellipsometric measurements on anodic oxide films grown on niobium in phosphoric acid were found to show behavior characteristic of films consisting of two layers, but films grown in chloride solution showed the behavior typical of uniform films. It was concluded that as with tantalum, phosphate but not chloride ions are taken up in the outer part uf the film, which grows by metal ion motion. The logarithm of the steady-state current density was linear in the field in the oxide for films grown in chloride solution and nonlinear for films formed in phosphate solution. It is concluded that, as with tantalum, the nonlinearity for some electrolyte solutions is caused by incorporation of electrolyte species in the outer part of the films rather than being a fundamental feature of the ionic transport process. The small signal ac response of the ionic current density was studied fur both electrolyte solutions. These measurements gave information on steady-state and stepped field kinetics and on the distribution of relaxation times involved in the transients observed when the current or field is suddenly changed. The steady-state and stepped field admittivities from the ac + de measurements were normalized by the dielectric permittivities of the films grown in the different solutions. (C) 2000 The Electrochemical Society. S0013-4651(99)09-050-3. All rights reserved.