Molecular Dynamics Study Of Thermo-mechanical Properties Of Mg₂Si Thermoelectric Material

Thermoelectric (TE) materials, which can achieve the conversion between heatand electric power directly, have received extensive attentions. Mg2Si-basedcompounds, with a widespread application prospect, have become a hot point forstudy recently all over the world, due to their higher thermoelectric performance,abundant constituent elements, low cost and non-toxicity. Since the devices made bythe thermoelectric material will surfer from the cyclic temperature different andcyclic thermal stress during the possessing of working, the thermodynamic andmechanical properties of thermoelectric material are the significant indicators of theservice behavior. Therefore, the investigations on the thermodynamic and mechanicalproperties of the thermoelectric material are of great importance for the optimaldesign and the reliability evaluation of thermoelectric device.The goal of this paper is to investigate the thermodynamic and mechanicalbehaviors of the single-crystal bulk Mg2Si by using the molecular dynamics method.In order to achieve the goal, the main research works and results in this paper includesome aspects as follows:Since the atomic interaction potential is the fundamental problem for moleculardynamics simulation, one multi-body interatomic potential for Mg2Si, which candescribe the atomic interactions correctly, has been studied and developed based oncharacteristics of crystal structure and the analysis of bonding. Then, the establishedpotential is employed to study the structural parameters, elastic properties, thermalexpansion coefficient, heat capacity and lattice thermal conductivity by using themolecular dynamics method. The calculation results from molecular dynamicssimulation agree well with that from the experiment and first principles calculations.It can be found that the potential exhibits great reliability on investigations of theelastic and thermodynamic properties of Mg2Si.By using the validated interatomic potential, the basic mechanical properties ofthe single-crystal bulk Mg2Si with ideal crystal structure were investigated. Firstly,the experiment of aniaxial tension of single-crystal bulk Mg2Si was simulated at roomtemperature. By analyzing the stress-strain curves and the structural evolution, the basic mechanical properties of single-crystal bulk Mg2Si were obtained. Then thetemperature effect and the strain rates effect on the mechanical properties werestudied and discussed in detail. Moreover, the size effect on mechanical properties ofMg2Si thermoelectric material was also studied by aniaxial tensile simulations withdifferent structural dimensions.Since element Mg is highly active and prone to volatilization at high temperature,Mg vacancy can easily occur during the preparation of this thermoelectric material.And it is difficult to address the Mg vacancy effect on the properties by quantitativeexperimental study. Therefore, in this paper, the molecular dynamics simulation isused to investigatie thermal conductivity and tensile mechanical properties under theeffect of different Mg vacancy concentration. The vacancy Mg atom distributed in thesystem randomly, and the nonequilibrium molecular dynamics method is adopted tosimulate the thermal transport. The change rules of the thermal conductivity withdifferent Mg vacancy concentration can be obtained. Meanwhile, Mg vacancy effecton the tensile mechanical properties of single-crystal bulk Mg2Si is also investigated.The calculation results show that the lattice thermal conductivity of single-crystalbulk Mg2Si drops significantly with increasing Mg vacany concentration. And the Mgvacancy defect also weakens the mechanical properties, but the decreasing scope isrelatively small.The related experimental and theoretical studies show that the lattice thermalconductivity of the materials can be declined effectively by introducing the nanoporesin the structure. Therefore, the tensile tensile experiment of single-crystal bulk Mg2Sicontaining nanopres is performed. The porosity and radius effects on the latticethermal conductivity and tensile mechanical properties are studied. The simulationresults indicate that the appearance of nanopres declines lattice thermal conductivityeffectively. And when the nanopres distribute uniformly, the higher porosity andgreater radius make the lattice thermal conductivity declines more singnificantly.However, the increasing of porosity weakens the elastic modulus of the materials.When the porosity is a constant, the smaller radius could enhance the elastic modulusand ultimate strength.