Effect of particle size on melting of aluminum at nano scales

Molecular-dynamics simulations are performed using isobaric-isoenthalpic (NPH) ensembles to predict the melting of nanosized aluminum particles in the range of 2-9 nm and to investigate the effect of surface charge development on the melting. Five different potential functions (the Lennard-Jones, glue, embedded-atom, Streitz-Mintmire, and Sutton-Chen potentials) are implemented, and the results are evaluated using the particle-size dependence of the melting phenomenon as a benchmark. A combination of structural and thermodynamic parameters, including the potential energy, Lindemann index, translational-order parameter, and radial-distribution functions, are used to characterize the melting process. Both bulk and particle melting are considered. The former features sharp changes in structural and thermodynamic properties across the melting point, as opposed to the smooth variations seen in particle melting in which surface premelting plays an important role. The melting temperature of a nanoparticle increases monotonically with increasing size, from 473 K at 2 nm to a bulk value of 937 K at approximately 8 nm. Two-body potentials like the Lennard-Jones potential fail to predict the thermodynamic melting phenomenon. The Sutton-Chen potential, fitted to match structural properties, also fails to capture the size dependence of particle melting. The many-body glue and Streitz-Mintmire potentials accurately predict melting temperature as a function of particle size. The effect of surface charges on melting is found to be insignificant for nanosized aluminum particles.