Powerful predictability of FPMD simulations for the phase transition behavior of NaCl-MgCl_2 eutectic salt

摘要

First principle molecular dynamics (FPMD) simulations in combination with experimental measurements are applied to investigate the phase transition behavior of NaCl-MgCl2 eutectic salt from 823 to 573 K. First, the essential phase transition indexes of thermal expansion coefficient, density, melting point, and enthalpy of fusion for NaCl-MgCl2 eutectic obtained by simulations and experiments are in good agreement with each other. Second, the micro-structural characteristics during phase transition are depicted by the temperature dependences of angular distribution function, radial distribution function and coordination number. The structural evolution of NaCl-MgCl2 eutectic is illustrated from the rearrangement of polyhedral MgCln2-n clusters, and it is founded that high and low temperature are conducive to the formation of 4-fold and 6-fold coordinated structure, respectively. Third, the ionic self-diffusion coefficients for NaCl-MgCl2 eutectic are predicted and concluded that Mg2+ behaves like a complex than as a free ion resulting in less diffusivity. Therefore, the shear viscosities of molten NaCl-MgCl2 as well as the corresponding pre-exponential factor and activation energy are evaluated considering the solvodynamic mean radius of all the diffusive particles. Ultimately, two figures of merit for molten NaCl-MgCl2 are deduced to evaluate the heat-transfer and heat-loss performance by integrating multiple thermal properties including density, viscosity, specific heat capacity and thermal conductivity. Overall, these simulation results are in satisfactory agreement with available reference data, indicating that the FPMD simulations work extremely well in predicting the thermophysical and structural properties of NaCl-MgCl2, which also provide insights for the applications of binary chloride salts in thermal energy storage.