Simplified force field for molecular dynamics simulations of amorphous SiO2 for solar applications

Mixtures of molten-salts (e.g. NaNO3, KNO3) with α-SiO2 nanoparticles have potential applications in concentrated solar power generation both as a thermal energy storage material and heat transfer fluid. In this study, a set of Lennard-Jones parameters for amorphous silica is proposed, aimed at understanding the molten-salt/amorphous-silica interface at the nanoscale through molecular dynamics simulations. Current force fields used for the simulation of α-SiO2 do not have validated rules for cross-term interactions or are computationally expensive. In this work, a novel set of Lennard-Jones parameters capable of accurately simulating the structure of amorphous SiO2 is presented. Its advantage is that it has precise validated mixing rules, which can be used to model cross term (different species) interactions, which are dominant in simulations involving interfaces. The proposed potential is validated by experimental results and data from previous studies: all the calculated properties (i.e. radial distribution, angular distribution, mean bond length, coordination number, density, thermal expansion coefficient and thermal conductivity) are in good agreement with the data in the literature. Additionally, the molten salt-amorphous silica interface is investigated. The existence of a compressed liquid layer at the liquid-nanoparticle interface is observed. The density of the molten salt mixture is found to be significantly higher and the self-diffusivity lower, in the proximity of the interface. Finally, the Na+ and K+ ions appear to be ordered in the proximity of the interface, indicating the existence of an ordered double-layer.