Homogeneous and heterogeneous melting behavior of bulk and nanometer-sized Cu systems: a numerical study

Molecular dynamics simulations have been used to investigate the solid–liquid transition of different Cu systems. These consisted of surface-free crystalline bulks and semi-crystals terminating with a free surface as well as of particles and wires with different shape and size in the mesoscale regime. The characteristic melting points of the various systems were attained by gradual heating starting from 300 K. Apart from surface-free bulk systems, where the phase transition at the limit of superheating is homogeneous, melting displays heterogeneous character. This is due to the existence of surface layers with structural and energetic properties different from the ones of bulk-like interior. Simulations point out a significant depression of both the melting point and latent heat of fusion for nanometer-sized systems respect to semi-crystals. Below the characteristic melting point, free surfaces are involved in pre-melting processes determining the formation of a solid–liquid interface. The onset of melting is related to the formation of a critical amount of lattice defects and this provides a common basis for the rationalization of homogeneous and heterogeneous melting processes despite their intrinsic differences.