Thermodynamic description of the quaternary Mg(NO3)2–KNO3–NaNO3–LiNO3 system and investigation on the novel Mg(NO3)2 based nitrate salts with low temperature

Abstract

The thermodynamic calculation for the quaternary Mg(NO3)2–KNO3–NaNO3–LiNO3 system was performed based on the CALPHAD approach to design novel magnesium nitrate-based molten salts with low melting temperatures. Thermodynamic evaluations and optimizations of the Mg(NO3)2–NaNO3, Mg(NO3)2–LiNO3, and Mg(NO3)2–KNO3 systems were critically carried out in this study. The modified quasi–chemical model (MQM) was used to describe the liquid phases, and the double salt 2KNO3·Mg(NO3)2 was treated as a stoichiometric compound. The self-consistent thermodynamic database for the quaternary Mg(NO3)2–KNO3–NaNO3–LiNO3 system was established. For this quaternary system and its magnesium nitrate-based ternary subsystems, the phase diagrams were calculated using the Kohler-Toop and Kohler interpolation techniques, and the calculated eutectic points were experimentally verified by differential scanning calorimetry (DSC). The results show that the experimentally-measured melting temperatures of these eutectic points are consistent with the calculated values. The thermophysical properties of these eutectic mixtures, including the enthalpy of fusion, the specific heat capacity and the thermal stability, were further determined through thermal analysis. Two novel magnesium nitrate-based eutectic mixtures are expected to serve as potential heat transfer and storage material candidates applied in the medium and high temperatures–thermal energy storage.