Anodization of Tantalum Over the Temperature Range 0° to 250°C

Tantalum was anodized at constant current in an aqueous electrolyte over the temperature range 0°–250°C. The voltage build‐up was linear up to a voltage at which crystallization of the oxide occurred. The "crystallization voltage" decreased with increasing temperature. The change of the voltage build‐up rates with temperature was not compatible with the single barrier law for high field ionic conduction. In spite of the extended temperature range, it was not possible to distinguish between ionic current ( )‐field ( ) relations of the form In and In . Current efficiencies were determined from charge and optical film thickness. The current efficiencies were ( a ) essentially constant up to a formation temperature of about 170°C and ( b ) decreased by about 30% between 170° and 250°C. Capacitance and loss angle of "equal charge" films increased with anodization temperatures higher than about 180°C. Oxygen dissolution in the tantalum substrate was suggested as the likely cause.