Influence of Zn2+ impurities on structure and thermophysical properties of MgCl2-NaCl-KCl molten salt: MD simulation and experimental research

Molten chloride salts are considered promising media for heat transfer and energy storage in thermal energy storage (TES) systems. Zinc ions are significant impurities in chloride salts with magnesium, but their influence mechanism on structure and properties of salts remains limited. This study explores the influence of impurity Zn²⁺ on the structure and thermophysical properties of the molten MgCl₂-NaCl-KCl salt by molecular dynamics simulations (MD) and experiment. The properties calculated by MD based on Born-Mayer-Huggins (BMH) potential have good agreement with experimental data. With the increment of Zn²⁺, the distance between zinc ion and other cations decreases, and zinc ions tend to form [ZnCl₅]^3- complex, so the overall structure tends to be compact, resulting in the increment of density and slightly decrement of specific heat capacity. Since zinc ion complex hinders ion migration, the self-diffusion coefficients of ions are reduced by Zn²⁺ increasing, and then the thermal conductivity decreases and viscosity increases. The correlations between thermophysical properties, impurity content, and temperature are obtained, and the density and viscosity have linear increment with Zn²⁺ increasing, while specific heat capacity and thermal conductivity have linear decrement. Additionally, zinc ion complex disrupts the short-range ordered structure therefore lowers the melting point of the salt. This research highlights that the chloride salts should be kept with minimal zinc ion impurities to optimize the performance.