Grain growth of NpO and UO nanocrystals.

We report on the crystallite growth of nanometric NpO and UO powders. The AnO nanoparticles (An = U and Np) were synthesized by hydrothermal decomposition of the corresponding actinide(iv) oxalates. NpO powder was isothermally annealed between 950 °C and 1150 °C and UO between 650 °C and 1000 °C. The crystallite growth was then followed by high-temperature X-ray diffraction (HT-XRD). The activation energies for the growth of crystallites of UO and NpO were determined to be 264(26) kJ mol and 442(32) kJ mol, respectively, with a growth exponent = 4. The value of the exponent and the low activation energy suggest that the crystalline growth is rate-controlled by the mobility of the pores, which migrate by atomic diffusion along the pore surfaces. We could thus estimate the cation self-diffusion coefficient along the surface in UO, NpO and PuO. While data for surface diffusion coefficients for NpO and PuO are lacking in the literature, the comparison with literature data for UO supports further the hypothesis of a surface diffusion controlled growth mechanism.