The Melting Mechanisms of UO_2 Nanocrystals: A Molecular Dynamics Simulation

摘要

A molecular dynamics simulation of the melting of UO_2 nanocrystals of the optimal truncated octahedron shape in the size range from 4317 to 381174 ions was carried out. The dependence of the melting temperature of the nanocrystals on the size was obtained at the time of evolution of the model system up to 150 ns. This dependence was characterized by nonlinearity caused by a change in the melting mechanism corresponding to the transition from larger nanocrystals to smaller ones. The nanocrystals containing more than 20 000 ions undergo the phase transition as a result of the formation of the surface melt on a single (111) face, which caused the irreversible propagation of the melt into the bulk. Smaller crystallites could melt at temperatures below the crystallization temperature of the (111) face, provided that two adjacent faces melt simultaneously due to the thermal activation. The difference in melting temperatures of the largest and smallestmodel crystals was 320K, which corresponds to the experimental estimates.