Theoretical investigation of the thermodynamic stability of nanoscale systems-III: Thin film with an IPB

Thermodynamic stability of nanoscale thin films is analyzed theoretically using equilibrium statistical mechanics. A model binary system with a miscibility gap is treated in fcc using the pair approximation of the Cluster Variation Method. Each film has equal total numbers of A and B atoms, and has an interphase boundary (IPB) around the center of a film at low temperatures. The composition profile across the film is calculated for different temperatures and thicknesses. The result reveals that the composition inside a film is practically the same as that of the corresponding bulk fcc system, and the surface composition is close to that of the two-dimensional surface lattice. For a fixed temperature, the mutual solubility of two components increase as the film thickness decreases, and becomes totally miscible for very thin films. For a given thickness, the miscibility increases with the temperature, and the critical temperature T_c of phase separation is determined as a function of film thickness.