Processus simultane d'évaporation libre et de diffusion de surface

Abstract

If a conical metal tip is annealed in vacuum, the radius at the apex increases with time by surface diffusion. This was calculated by Nichols and Mullins and recently verified by the authors. This phenomenon is now calculated in case of a simultaneous action of evaporation and surface diffusion. By evaporation the blunting rate should be lowered until it becomes zero for a critical value of the curvature radius. Numerical data for different temperatures and cone angles are calculated for some metals (W, Mo, Pt, Ni, Cu). Measured are profile changes of Mo tips during annealing. The lowering of the blunting rate and the existence of a limit radius are confirmed. An evaporation can also occur as a consequence of a surface reaction. The measured radius changes of tungsten tips annealled in 1.5 × 10 −5 torr and 2.5 × 10 −5 torr of oxygen agree fairly well with the predictions. Surface self-diffusion measurements at high temperatures may be erroneous if evaporation is not considered, an example for Mo at 2150°K is given.