Oxygen diffusion in hypostoichiometric uranium dioxide
The tracer oxygen diffusivity in UO/sub 2-x/ has been measured along the lower two phase boundary. The diffusion couple consisted of two matched hypostoichiometric UO/sub 2/ wafers, one enriched with /sup 18/O and the other normal. These two were pressed together with a bond of liquid U in between. After a diffusion anneal the /sup 18/O concentration profile was determined by ion microprobe mass analysis, from which diffusion coefficients were obtained. The results showed much higher diffusion coefficients than those of stoichiometric UO/sub 2/. This directly proved that the major defect species in UO/sub 2-x/ is the anion vacancy. Activation energy of anion vacancy migration was measured to be 11.7 +- 3.0 kcal/mole. A diffusion model established for UO/sub 2/ and UO/sub 2+-x/ showed that in stoichiometric UO/sub 2/ both interstitials and vacancies contribute significantly to oxygen diffusion and neither can be neglected; at 1400/sup 0/C their contributions are about equal. This model was extended to nearly stoichiometric UO/sub 2+-x/ to predict oxygen diffusion coefficients in these stoichiometry ranges. Also deduced from the model were the Frenkel defect energy and entropy of 85.6 +- 9.2 kcal/mole and 18.2 +- 7.3 eu, respectively. Using these values, the contribution of Frenkel disorder to the excess enthalpy of UO/sub 2/ was evaluated. Calculation showed that Frenkel disorder accounts for 87% of the excess enthalpy at 3000/sup 0/K. A simple two band model for electronic excitation, with a band gap of 2.0 eV and effective electron mass of 7.6 m/sub e/, accounted for the remainder of the excess enthalpy.
- Research Organization:
- California Univ., Berkeley (USA)
- OSTI ID:
- 6267524
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
OXYGEN
DIFFUSION
URANIUM DIOXIDE
FRENKEL DEFECTS
ION MOBILITY
MATHEMATICAL MODELS
OXYGEN 18
PHYSICAL PROPERTIES
TRACER TECHNIQUES
ACTINIDE COMPOUNDS
CHALCOGENIDES
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
ELEMENTS
EVEN-EVEN NUCLEI
ISOTOPE APPLICATIONS
ISOTOPES
LIGHT NUCLEI
MOBILITY
NONMETALS
NUCLEI
OXIDES
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
PARTICLE MOBILITY
POINT DEFECTS
STABLE ISOTOPES
URANIUM COMPOUNDS
URANIUM OXIDES
VACANCIES
360202* - Ceramics
Cermets
& Refractories- Structure & Phase Studies