VITREOUS SILICA BULK AND SURFACE SELF-DIFFUSION ANALYSIS BY MOLECULAR-DYNAMICS

Molecular dynamics simulation was used to investigate the kinetics of self-diffusion in the bulk and on the surface of pristine vitreous sil ica. Multibody potential functions were used to describe interatomic f orces, Activation energies were found to be similar for Si and O self- diffusion and did not vary appreciably between self-diffusion in the b ulk and an the surface. For both species in the bulk and on the surfac e, average activation energies ranged from 113-115 kcal/mol. Oxygen se lf-diffusivity was found to be only slightly higher than silicon self- diffusivity in the bulk and on the surface. Self-diffusion coefficient s in the top 3-7 Angstrom of the surface were found to be higher than those of bulk simulations by less than a factor of two. Self-diffusion on the surface was observed to occur by motion of SiO3 and SiO4 polyh edra over several angstroms while bulk self-diffusion involved signifi cant neighbor exchange over similar length scales, Surface diffusion t hrough a vapor phase (via desorption and re-adsorption at a distant lo cation on the surface) was not included in the surface data. (C) 1997 Elsevier Science B.V.