Abstract
Measurements using field emission techniques of the activation energy for surface selfdiffusion of several of the refractory transition metals when carbon or silicon is present on the surface show large increases which are dependent on the degree of surface coverage. Maximum values obtained were: 8.5 eV for carbon on tungsten, 7.0 eV for silicon on tungsten, 4.9 eV for carbon on tantalum, 4.5 eV for carbon on molybdenum and 2.8 eV for silicon on molybdenum. In addition, two anomalous effects have been observed in which surface changes occur at critical temperatures, (a) Sharp discontinuities occur in the plots of activation energy versus temperature for carbon on tungsten at about 2300 °K and for silicon on tungsten at about 2000 °K. In both cases the activation energy drops from the respective high value to that for the clean substrate material of 3.0 eV. Concomitant with this transition the emission patterns change in appearance from those typical of a contaminated surface to those typical of a clean surface, (b) For carbon on tungsten and silicon on tungsten, (433) planes are observed which decrease in size with temperature and suddenly disappear at a very sharp critical temperature. It is suggested that the presence of these impurities causes a restructuring of the surface layers even when present in much less than stoichiometric amounts and that surface phase changes occur independent of bulk changes.
Published Version
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