FPMD Simulations On The Structure Evolution And Thermal Energy Storage Properties Of Magnesium–based Chloride Salts

There are abundant MgCl₂ salts in salt lakes in China and they are extremely cheap,and the chloride salt is characterized as high-efficiency energy storage material and heat transfer medium,high heat capacity,high thermal conductivity,low viscosity as well as keep stable under wider operating temperature ranges.Therefore,the research on the heat transfer and storage characteristics of magnesium-based chloride salts for effective utilization has attracted people’s attention.However,the existing chloride salt test methods need further develop.On the one hand,the existing testing methods for chloride salts are based on nitrates lacking the consideration of the corrosion and vapor pressure.On the other hand,the chloride salt has inherent problems such as the extreme condition during the measurements and corrosive which resulting in the personnel safety issues and serious damage to equipment.Thus,it is necessary to develop a safe,accurate and reliable method to study low-cost and high-performance magnesium-based chloride.In this thesis,first-principles molecular dynamics（FPMD）simulation supplemented by experimental measurement methods are used to study magnesium-based chlorides,which provides a novel method for the development and utilization of efficient heat transfer and storage media.The following aspects are carried out:1)The thermophysical properties of MgCl₂-NaCl（44:56 mol%）and MgCl₂-KCl（32:58 mol%）are studied.First,the thermophysical properties such as thermal expansion coefficient,density,melting point,heat of fusion and specific heat capacity of the eutectic salt are obtained.Second,the volume change of magnesium-based chloride during the phase transition is studied,which the phase change interval of magnesium-based chloride is inferred from the mutation value of the thermal expansion coefficient,bulk modulus,density and other characteristics with the temperature change.FPMD simulation can explore the mechanism between the experiment and the thermal properties,and predict the thermal storage characteristics of the magnesium-based chloride accurately.2)The micro-structural characteristics during phase transition are depicted by the temperature dependences of angular distribution function,radial distribution function and coordination number.By the angular distribution function,the stable polyhedron MgCl₉9))^2-99))varies with the distance between the first and second peaks of the Cl–Mg–Cl and Mg–Cl–Mg bond angles in the MgCl₂–NaCl salt.Then,the partial static structure factors of neutron scattering in MgCl₂–KCl salt were explored,and it was found that at the correlation between anion and cation from 1.5（?）^–1 to 2.0（?）^–1 would weaken with the increasing temperature.The relationship between the probability distribution of coordination number and temperature has been studied,and the results show that low temperature contributes to the formation of high-coordination structures,and vice versa.3)Ultimately,the thermodynamic characteristics of magnesium-based chlorides,such as viscosity,specific heat capacity,and thermal conductivity,are analyzed using FPMD simulation and experimental measurement.The results show that the thermal conductivity of MgCl₂-NaCl salt is higher while its viscosity is also higher than MgCl₂-KCl salt.Subsequently,the FOM_ht and FOM_hl,which combined density,viscosity,specific heat capacity,and thermal conductivity together,are studied to evaluate the heat transfer and storage characteristics of magnesium-based chloride salts.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 chloride salt,which also provide insights for the applications of binary chloride salts in thermal energy storage.