Bubble Diffusion and the Motion of Point Defects near Surfaces. (Diffusion Theory for Discrete Media, Part IV

Abstract

AbstractThe theory of diffusion of inert‐gas bubbles in solids is important in a variety of fields, including reactor technology, ion bombardment, powder metallurgy, and the study of large defects. Bubble diffusion also provides a convenient frame of reference for discussing the motion near surfaces of point defects, i.e. of surface defects, vacancies, and vapourized atoms or molecules. It is shown that the bubble diffusion coefficient, Db, for a surface‐diffusion mechanism is equal to (3/2 π a4) Ds, where a is the bubble radius in units of λ, the mean atomic spacing, and that this expression is independent of the surface‐diffusion jump distance. The formation of vacancies at a surface should occur at a frequency given by Γfv = Dvol α/f pvλ2 per area λ2 of the surface, where αv is the vacancy condensation efficiency, f is the correlation factor, and pv (in units of λ) is the atom layer spacing next to the surface. With this assumption, Db for a volume‐diffusion mechanism will be equal to (3/2 π a3 f) Dvol. The frequency at which vapourized atoms or molecules are formed is shown to be given by a relation very similar to that for vacancy formation. The corresponding expression for Db, i.e. that for a vapourization‐condensation mechanism, is of the form (3/4 π a3) (Dg Cg λ3), where Dg is the vapour diffusion coefficient and Cg is the equilibrium vapour concentration in atoms/cm3.