Molecular dynamics simulation of the thermophysical properties and phase change behaviors of aluminum nanoparticles

Abstract

With the development of energy storage technology, phase change materials which can be used to store thermal energy have received much attention in recent years. The nano-metallic materials are universally used as phase change materials due to their many desirable thermophysical properites. In this paper, the molecular dynamics simulation method is adopted to simulate the variations of melting point, density and phonon thermal conductivity of the nano aluminum with grain size ranging from 0.8 nm to 3.2 nm. The variations of density, specific heat capacity and phonon thermal conductivity with temperature of aluminum nanoparticles at a grain size of 1.6 nm are also studied. By using the embedded-atom potential, the thermophysical properties and phase change behaviors of aluminum nanoparticles are stimulated. The phase transition temperature of aluminum nanoparticles is studied based on the energy-temperature curve and the specific heat capacity-temperature curve. The surface energy theory and the size effect theory are applied to the analysis of the variation of the melting point of the aluminum nanoparticles, and the results show that the melting point increases as grain size augments, and it increases slowly when its grain size is between 2.2 nm and 3.2 nm but still holds the trend of increase. In order to obtain accurate thermal conductivity, the Green-Kubo method is adopted to calculate the phonon thermal conductivity of aluminum nanoparticle. As the grain size of aluminum nanoparticles increases, its density monotonically decreases, and the thermal conductivity monotonically increases linearly, which is in line with the theory of phonon. Similarly, with the increase of temperature, the density and thermal conductivity of aluminum nanoparticles of 1.6 nm in grain size both decrease. Moreover, the density of aluminum nanoparticle is generally lower than that of its bulk material. The study also shows that the heat transfer manner of aluminum nanoparticle is based on ballistic-diffusive heat conduction instead of the traditional diffusive heat conduction when it is in a nanoscale. The simulation studies the thermophysical properties of nanoparticles from the atomic perspective, and is of significance for guiding the design of the phase change materials based on the aluminum nanoparticles for thermal energy storage.