Heat transport and ionic dynamics in pure and mixed molten salts

Heat transfer in liquids is challenging as it combines competing effects of oscillations, which dominate heat capacity, and diffusion, which enables hydrodynamic flow. This is compounded by the junior state of liquid thermodynamics theories. Nevertheless, molten salts play an increasingly important role in industrial and energy applications and there is a pressing need to understand the mechanisms behind their transport processes. We use molecular dynamics simulations to investigate the heat transport of three molten salts: LiCl, KCl, and their mixture. In multi-component fluids, the distinction between conduction and convection is confounded by the presence of partial mass currents. We therefore calculate different susceptibilities, which include and exclude heat carried by partial currents. We find that, while the heat advected by mass currents in the mixture increases with increasing temperature, the heat transferred by collective vibrational motion decreases with increasing temperature. This causes a maximum in the heat transport in the mixtures only—in pure salts the vibrational and diffusive components decrease monotonically with temperature. We attribute this anomaly to the extra freedom of collective ionic motion in mixtures—in pure salts the motion of cations and anions is bound by conservation of momentum. In mixtures, the introduction of a new species releases this condition and enables a coherent but diffusive collective motion. We tentatively ascribe this motion to “diffusive” phonons, previously used to explain similar anomalies in solid thermal conductivity. These results indicate the rich relationship between ionic dynamics and transport in molten salt mixtures.