Surface diffusion, surface free energy, and grain-boundary free energy of uranium dioxide

Abstract

Measurements of the growth kinetics of grain-boundary grooves were made on stoichiometric UO 2 annealed in a helium atmosphere at 1660–1940°C. Vaporphase diffusion (vapor transport) was found to be the dominant mass-transport mechanism. Analysis of the results shows that mass-transfer methods are not applicable to measurements of surface diffusivity of UO 2 at temperatures ⪢ 1660°C, but are useful in obtaining fundamental information on the surface and grain-boundary free energies. A survey of the published data on surface diffusion obtained at 1200–1700°C using grain-boundary and scratch-decay techniques revealed that the activation energy Q s is about 0.9 of the heat of evaporation, and the Arrhenius preexponential factor D 0 is about 10 6–10 8 cm 2/sec. As has been demonstrated, these values require a small correction to account for the contribution of vapor transport. The published surface-diffusion coefficients were corrected using mass-transport theories and the results are discussed.