Oxygen-18 Tracer Measurements of Anion Diffusion in Uranium Dioxide Thin Films
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Oxygen 18 was used as a tracer to quantify anion diffusion in thin-film UO2 using secondary ion mass spectroscopy to measure one-dimensional depth profiles. Both thermal and heavy ion bombardment (1.8 MeV Kr+) treatments were employed over a temperature range from 295 K to 623 K. Textured and single-crystal thin-film samples were grown using reactive-gas magnetron sputtering at ambient temperature. Both microstructures resulted in similar thermal activation energies, Ea = 0.2 eV. This activation energy is significantly lower than the known value for intrinsic anion vacancy-self diffusion in stoichiometric UO2.00 (Ea = 2.5 eV). We attribute this to an interstitialcy mechanism in our hyper-stoichiometric films. The activation energy for irradiated textured films was approximately half that of thermal diffusion, consistent with the chemical rate theory. The opposite was true for the single-crystal microstructure (irradiated Ea = 0.36 eV). This may be due to radiation-induced changes in the microstructure. The mixing parameter was quantified on the anion sublattice as well, ξ = 2.1 ± 0.2 Å5eV−1.
The assistance of T. Spila (SIMS), R. Haasch (XPS), D. Jeffers (ion accelerator), and M. Sardela (XRD) is gratefully acknowledged. The National Academy for Nuclear Training (NANT) fellowship program is greatly acknowledged as well. This work was supported by the US Department of Energy Nuclear Energy Research Initiative under grant no. DEFG-07-14891. In addition, the work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois, which are partially supported by the US Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471.
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