The Kinetics of Segregation and Its Application to the Fe(100)-S System

Abstract

The iron single crystal, the bulk sulfur concentration of which had been adjusted between 23 ppm and 66 ppm, was heated at 1023 K in vacuum, and the segregation rate of sulfur at the (100) surface was measured with good reproducibility by using AES. The result was compared with the theoretical equation.
Because the equilibrium concentration of sulfur at the surface was constant, unaffected by the bulk sulfur concentration and annealing temperatures, the number of the surface sites to which sulfur could segregate was assumed to be constant. Assuming further that the segregation rate is proportional to the number of vacant sites at the surface and the bulk concentration of the segregant beneath the surface, the theoretical equation of the segregation rate has been derived. This equation shows that at the early stage of segregation, the concentration at the surface increases in proportion to the annealing time, and that the segregation rate is proportional to the bulk concentration and independent of the diffusion coefficient of the segregant.
From the experiments, the concentration of sulfur at the surface was observed to increase in proportion to the annealing time and then reached a constant value. The segregation rate became greater as the bulk sulfur concentration increased. This tendency coincides well with that of the theoretical equation.
The concentration profile of the segregant near the surface influenced the segregation rate at the first stage. Therefore, in order to observe the segregation behavior with good reproducibility, it was necessary prior to measurements to sufficiently anneal the sample at the measuring temperature and sputter the surface by the argon ion until the concentration at the surface became equal to the bulk concentration.