SURFACE SELF-DIFFUSION STUDIES ON THE W(112) PLANE BY THE FIELD-EMISSION METHOD

The surface self-diffusion anisotropy of tungsten on the W(112) plane has been measured along (parallel-to) and across (perpendicular-to) the channels, which exists on this plane, by using the field emission fluctuation method with two forms of probe holes. For most measurements the slit probe-hole dimensions, 0.4 x 4 mm, correspond to 40 x 400 angstrom at the emitter and the round probe-hole dimension, 1 mm, corresponded to 100 angstrom. In the round probe hole the thermal activation energy is E = 13.0 kcal and the prefactor of the surface self-diffusion coefficient D0 = 4.8 x 10(-7 cm2 s-1; in the long narrow rectangular slit E(parallel-to) = 13.2 kcal, D0(parallel-to) = 7.1 x 10(-7) cm2 s-1 and E(perpendicular-to) = 6.2 kcal, D0(perpendicular-to) = 1.8 x 10(-9) cm2 s-1 at T = 500-700 K. The experimental data suggest that a W atom diffusing across channels should not climb over the channel wall, which would almost certainly require a much higher activation energy, but displace an atom in the wall. This process can require a much smaller activation energy, but since it involves concerted motion of at least two atoms it will also have a much smaller prefactor.