Abstract
The anodic behavior of tantalum in phosphoric acid solutions has been investigated by measuring AC impedance at the metal-solution interface and also by AES analysis of the anodic film. Anodic polarization caused an increase in direct current and AC resistance at 108Hz, but a decrease in capacitance and tan δ. These data indicate that an insulating film having a growth ratio of 12Å/V was formed with potential increase. The AES analysis revealed that the tantalum oxide film anodized in 0.1M/l H3PO4 includes P ions as deep as the half of the film thickness; in the film anodized in more concentrated phosphoric acid solutions, P ions were found to penetrate deeper than in the above film. Concerning the behavior of P ions in the film there were no differences between phosphoric acids of the first and of the third dissociation. Film growth took place both at the interface of metal/oxide and of oxide/electrolyte. The oxide film, therefore, is supposed to consist of three layers, that is, two layers with and without P ions and a tantalum excess oxide layer.
