Molecular dynamics simulation of UO2 nanocrystals melting under isolated and periodic boundary conditions

TL;DR

This study uses molecular dynamics simulations to analyze the melting behavior of UO2 nanocrystals under different boundary conditions, assessing potential models and size effects to better understand their thermodynamic properties.

Contribution

It evaluates multiple pair potentials and boundary conditions in MD simulations to accurately predict UO2 nanocrystal melting characteristics, including temperature and density jumps.

Findings

Pressure dependence of melting point shows nonlinear behavior near -20 GPa and 25 GPa.

Surface simulations reproduce melting temperature and density jump but underestimate heat of fusion.

Size dependence of melting properties decreases nonlinearly, with parabolic extrapolation fitting better.

Abstract

Melting of uranium dioxide (UO2) nanocrystals has been studied by molecular dynamics (MD) simulation. Ten recent and widely used sets of pair potentials were assessed in the rigid ion approximation. Both isolated (in vacuum) and periodic boundary conditions (PBC) were explored. Using barostat under PBC the pressure dependences of melting point were obtained. These curves intersected zero near -20 GPa, saturated near 25 GPa and increased nonlinearly in between. Using simulation of surface under isolated boundary conditions (IBC) recommended melting temperature and density jump were successfully reproduced. However, the heat of fusion is still underestimated. These melting characteristics were calculated for nanocrystals of cubic shape in the range of 768-49 152 particles (volume range of 10-1000 nm^3). The obtained reciprocal size dependences decreased nonlinearly. Linear and parabolic extrapolations to macroscopic values are considered. The parabolic one is found to be better suited for analysis of the data on temperature and heat of melting.