A Molecular Dynamics Simulation Study of the Melting and Solidification Processes of Hafnium Nanoparticles

Abstract In this study, the atomic structural and dynamic evolution of hafnium (Hf) nanoparticles (NP) during melting and cooling processes has been comprehensively investigated using classical molecular dynamics (MD) simulations. To observe the effect of size on thermal properties such as the melting point (1490–2170 ± 5 K), solidification temperature (1230–1550 ± 5 K), and heat of fusion, spherical NPs of six different sizes, ranging from 2 to 10 nm in diameter, have been studied. The results support the common rule that there is an inverse linear relationship between the thermal properties and the inverse size of the NPs. The melting points obtained from MD simulations for the NPs show good agreement with the results obtained from a theoretical model based on Gibbs free energy calculations. The melting phenomenon of Hf NPs occurs in two stages. First, a liquid-like layer forms on the surface of the particle due to pre-melting. This liquid layer spreads into the NP as the temperature increases until it reaches a critical thickness (∼3a0 for NP-4 and ∼5a0 for NP-6). The entire NP then melts, including the solid-like region that remains inside it. The solidification behavior of melted NP indicates that the system transforms into a polycrystalline structure consisting mainly of hexagonal close-packed (hcp) crystal structures and containing a small amount of icosahedral structures.