抄録
The segregation behavior of sulfur, oxygen and phosphorus at the (100) surface of the iron single crystal, the bulk sulfur concentration of which had been adjusted to 23∼66 mass ppm, was investigated by using AES-LEED from the segregation kinetic point of view.
When the specimen was annealed between 973 K and 1123 K in vacuum, sulfur, oxygen and phosphorus initially segregated to the surface. As the annealing was prolonged, however, the concentration of sulfur at the surface increased, and at first phosphorus and then oxygen disappeared from the surface. So finally only sulfur existed at the surface.
This phenomenon can be explained as follows. At the first stage, sulfur, oxygen and phosphorus occupy surface sites simultaneously. The surface activities of these elements, however, are in the order of sulfur>oxygen>phosphorus. Therefore, as the concentration of sulfur at the surface increases, sulfur replaces first phosphorus and then oxygen. Finally, only sulfur occupies the surface sites.
The concentration of sulfur at the surface increased in proportion to the annealing time and then reached a constant value. The segregation rate became greater as the bulk sulfur concentration increased.
The activation energy of the segregation rate increased as the bulk sulfur concentration decreased. This is because sulfur is trapped by the impurities such as manganese and magnesium which have strong affinity for sulfur and this effect becomes more remarkable as the bulk sulfur concentration decreases.
When the concentration of sulfur at the surface reached an equilibrium value, the structure of the surface was c(2×2) and two iron atoms combined with one sulfur atom at the surface.
