Molecular dynamics study of equilibrium concentration profiles and the gradient energy coefficient in Cu-Pb nanodroplets

Atomistic simulations were used to equilibrate the concentration profiles in 5.3 nm liquid droplets for an embedded atom method model of Cu-Pb. The strong tendency of Pb to surface segregate establishes a nonuniform profile, which decays over a length comparable to the particle radius. With the free energy vs composition determined from separate Monte Carlo simulations, the composition profile can be analyzed in terms of the Cahn-Hilliard diffuse interface model and the gradient energy coefficient, kappa, can be obtained. Results from three different temperatures indicate that kappa lies in the range 1.0-1.4x10(-10) J/m. In addition to the gradient energy coefficient, various aspects of phase equilibria at the nanoscale have been investigated. The critical point of the liquid-liquid miscibility was found to decrease by approximate to 50 K for the 5.3 nm particle size, the melting point of pure Cu is suppressed by approximate to 110 K, and the liquidus slope is shallower than that of the bulk system.