Molecular Dynamics Study On The Local Structures And Transport Properties Of Molten Alkali Chlorides

Electrolytic process is one of the important methods to produce metallic magnesium.Due to its high melting point,poor conductivity,strong volatility and hydrolysis,magnesium chloride can not be used solely as the electrolyte.Some other halides are added to improve the performance of magnesium electrolyte.As the main components of magnesium electrolyte,alkali metal chlorides can improve properties of molten magnesium electrolyte effectively.Investigating the structures and properties of molten alkali metal chlorides is the foundation of magnesium electrolyte design and optimization.In this paper,the densities,local structures and some transport properties of molten unitary alkali metal chlorides and their binary mixtures have been studied fully through molecular dynamics simulations.Firstly,the densities,local structures,self-diffusion coefficients,shear viscosities,thermal conductivities and ionic conductivities of molten alkali metal chlorides at near melting temperatures,1100K,1200K,1300K,1400K and 1500K have been calculated.The densities of molten alkali metal chlorides under the same temperature meet the following relationship,LiCl<KCl<NaCl<RbCl<CsCl.As the temperatures rise,the densities of molten alkali metal chlorides decrease.For alkali metal chlorides,after melting,the long-range order of crystal disappears.The melts only show short-range order,however,on the micro level,the melts have the tendency to maintain the octahedron geometry of the crystalline state.Compared with the crystals,the coordination numbers of anion-cation pairs decrease and those of anion-anion as well as cation-cation pairs increase for molten alkali metal chlorides.The coordination numbers of the ion pairs increase with the increase of the cationic radius.The cations and anions in the melts exhibit an alternative arrangement,reflecting the characteristic of charge ordering for molten alkali metal chlorides.In a certain alkali metal chloride melt,the interactions between unlike ions are stronger,the corresponding coordination structures are also more stable,the micro distributions of anion-anion and cation-cation pairs are similar,and the same kind of ion pairs from different alkali metal chloride melts also show the same trend on micro distribution.The temperature has no significant impact on the local structures of molten alkali metal chlorides.The ions of molten alkali metal chlorides have relatively large self-diffusion coefficients,and the self-diffusion coefficients for all ions in different systems increase as the temperatures increase.At the same temperature,the self-diffusion coefficients are affected by ionic mass and radius.For each melt,the rule is D(Li+)>D(Cl-),D(Na+)>D(Cl-),D(K+)?D(Cl-,D(Rb+)<D(Cl-,D(Cs+)<D(Cl-),while for all the cations from different systems,the relationship is D(Li+)>D(Na+)>D(K+)>D(Rb+)>D(Cs+).Molten alkali metal chlorides have relatively low shear viscosity,high ionic conductivity and good thermal conductivity.As the temperatures increase,the shear viscosity and thermal conductivity decrease,while the ionic conductivity increase.The ionic conductivities and thermal conductivities for all the alkali metal chloride melts at the same temperature obey the following rule,LiCl>NaCl>KCl>RbCl>CsCl.The potential parameters for molten binary alkali metal chlorides have been constructed based on the successful calculations for structures and transport properties of molten unitary alkali metal chlorides,and the structures of molten LiCl-NaCl,LiCl-KCl,LiCl-RbCl,LiCl-CsCl,NaCl-KCl,NaCl-RbCl and NaCl-CsCl at 1100K have been calculated across the full composition range.The addition of LiCl into other alkali metal chloride melts can weaken the interactions between unlike ions and also the interactions between cations,meanwhile,promote the closed packings of both anion-cation pairs and cation-cation pairs.While the interactions between anions are strengthened and the corresponding closed packings are hindered.For molten binary alkali metal chlorides,the anions tend to combine with the smaller cations,and the tendency becomes more obvious as the difference between cationic radii becomes larger.According to the simulated results at different temperatures and different compositions,the expressions of densities,shear viscosities and ionic conductivities on temperatures and compositions for molten LiCI-NaCl,LiCl-KCl,LiCl-RbCl,LiCl-CsCl,NaCl-KCl,NaCl-RbCl and NaCl-CsCl have been fitted.The results show that,as the temperatures rise,densities and shear viscosities of molten binary alkali metal chlorides decrease,while the ionic conductivities increase.The addition of LiCl can reduce the densities of molten NaCl,KCl,RbCl and CsCl,while the addition of NaCl can only reduce the densities of molten RbCl and CsCl,but increase the density of molten KCI.Besides,the additions of LiCl or NaCl can also improve the fluidities and ionic conductivities of other melts.Compared with NaCl,LiCl has more significant effects on improving the transport properties of other alkali metal chloride melts.