Surface Self-diffusion of Tungsten

Abstract

HERE we describe briefly the results of some recent experiments on the diffusion of tungsten atoms over an atomically clean surface of tungsten. The method used was that of following the decay with time of a series of sinusoidal grooves in the surface of the specimen at various temperatures. Mullins¹ has shown that the depth of the grooves at a time t is given by where ω = (2φ/the wavelength of the groove). The terms in the exponent are the contributions to the smoothing process due to evaporation, volume diffusion and surface diffusion respectively. A, B and C are functions of temperature, so that by careful choice of temperature and wavelength it is possible to make surface diffusion the dominant mechanism of mass transfer, corrections being made for the other two processes. B is given by D_s = D₀ exp (−Q_S/RT), where Q_s is the activation energy for surface diffusion, v_s is the specific surface free energy, Ω the atomic volume and N the number of atoms per unit surface area. Thus from the decay constant in equation (1) it is possible to obtain the surface self-diffusion coefficient, D_s.