Abstract
An oxide film on a metal surface will thicken only if metal, or oxygen, or both, migrate across it. The relative magnitude of the metal and oxygen migrations can be determined using markers, and the possible results are reviewed for the ideal case of a homogeneous oxide grown to uniform thickness on a plane metal surface. Such experiments require markers that are both inert and immobile; criteria are developed to establish that these requirements have been met. Experimentally, it has been shown that the anodic oxidation of tantalum is a system close to the ideal, and that the radioisotope of a noble gas is a good approximation for the required marker. Radioactive noble gases were implanted by means of a mass separator into thin films of anodic tantalum oxide, and the transport numbers of the tantalum and oxygen measured after thickening by further anodization. The transport numbers were found to be 0.243 and 0.757, respectively, for anodizing conditions of . The noble gas concentration profiles were observed to broaden on further anodizing, and this was interpreted as being due to the simultaneous movement of tantalum and oxygen atoms during the charge transfer event.
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