Kinetics of Surface Segregation and Phase Separation in Nanostructures

摘要

Phase separating AB alloy systems can be divided into two groups: strongly and weakly segregating binary alloys. In a strongly segregating system desegregation can never be established, whereas in weakly segregation system, both segregation and desegregation can occur depending on the overall composition of the system. In order to get information about the time evolution of a slab, we used a numerical solution of Martin's deterministic kinetic equations. In case of strongly segregating spinodal systems an interesting interplay between surface segregation, spinoda! decomposition and phase separation has been found. Furthermore, the effect of the slab thickness (d) was investigated and the change of the solubility as a function of d has been studied. In case of weakly segregating spinodal systems also a segregation of A atoms starts at the surface first and the segregation generated spinodal wave is also developed (A/B...B/A morphology). Later on, the surface structure makes a sudden "reversal". This means that a morphology B/A...A/B develops with a small segregation of A atoms on the surfaces of the B-rich phase. The observed surface reversal is an inherent property of the weakly segregating system and the characteristic time-evolution of the surface concentration is observable experimentally in thin film samples.