Growth mechanism of anodic tantalum pentoxide formed in phosphoric acid

The formation of anodic tantalum oxide (Ta2O5) in dilute phosphoric acid is quantitatively described using point defect chemistry reactions. Oxide formed in phosphoric acid has a distinct bi-layer structure, where the inner layer is pure Ta2O5, but the outer layer contains phosphate incorporated from the solution. In the point defect model (PDM) presented herein, the inner layer forms directly from, and grows into the metal, due to the production of oxygen vacancies at the metal/oxide interface. The outer layer forms due to the production of tantalum interstitials at the metal/oxide interface and their subsequent migration to the oxide/solution interface, where they hydrolyze to form Ta2O5. The Faradaic impedance is derived for the point defect reactions, and a bi-layer equivalent electrical analog is used to optimize the model to the measured electrochemical impedance spectroscopy (EIS) data. The oxide thickness and ionic current density have been measured separately, and the PDM parameters correctly predict the oxide thickness and ionic current densities due to the production of tantalum interstitials and oxygen vacancies.