Simulation was used to investigate the kinetics of self-diffusion in the bulk and on the surface of pristine vitreous silica. Multibody potential functions were used to describe interatomic forces. The activation energies were found to be similar for Si and O self-diffusion and did not vary appreciably between self-diffusion in the bulk and on the surface. For both species, in the bulk and on the surface, the average activation energies ranged from 113 to 115kcal/mol. The oxygen self-diffusivity was found to be only slightly higher than silicon self-diffusivity in the bulk and on the surface. The self-diffusion coefficients in the top 3 to 7Å of the surface were found to be higher than those for bulk simulations by less than a factor of two. Self-diffusion on the surface was observed to occur via the motion of SiO3 and SiO4 polyhedra over several angstroms while bulk self-diffusion involved significant neighbor exchange over similar length-scales.

Vitreous Silica Bulk and Surface Self-Diffusion Analysis by Molecular Dynamics. Litton, D.A., Garofalini, S.H.: Journal of Non-Crystalline Solids, 1997, 217[2-3], 250-63