The Migration of Oxygen during the Anodic Oxidation of Tantalum

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Abstract
Oxygen‐18 concentration profiles in thin oxide films have been measured by combining a sectioning technique for locating the isotope with a nuclear method for detecting it. The system studied was the anodic oxidation of tantalum, and the oxide was sectioned by slow dissolution in concentrated HF almost saturated with ; the 18O present was detected by bombarding the oxide with 3.042 MeV protons and counting neutrons emitted in reaction. On anodizing first in 16O electrolyte and then in 18O electrolyte, the 18O incorporated last was found outside the 16O layer incorporated first. There was, however, a small mixing of the populations at the boundary between them, and the degree of mixing was found to be proportional to the square root of the thickness added in the 18O electrolyte. The results could be analyzed very well in terms of forced diffusion (under the influence of the electric field during anodization) from a constant source (the electrolyte) into a semi‐infinite medium (the oxide), thus confirming that oxygen indeed migrates during the anodic oxidation of tantalum.

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