Molecular Dynamics Simulation Study Of The Thermoelectric Material CoSb₃

The skutterudite compounds, whose figure of merit ZT has achieved1.7～1.8, is a class of novel medium-temperature thermoelectric materials, and possesses application prospect for thermoelectric generation in medium-temperature range of250～600℃. In the practical service condition, the thermoelectric material of skutterudites and the thermoelectric device will bear the changing temperature loading and thus the changing thermal stress simultaneously, the investigations on its thermodynamic and mechanical properties will play an important role on the design of the thermoelectric devices as well as the reliability evaluation.The present thesis aims to develop the molecular dynamics simulation method for the skutterudite material CoSb3, and thus to study the thermodynamic properties and the fundamental mechanical performances of the ideal structure CoSb3material. A series of related research work has been carries out.Since the interatomic potential is an essential issue for molecular dynamics simulations, the thesis proposed two different potential models to describe the interatomic interactions in the CoSb3compound. First, the two-body potential function, i.e., Morse potential, is adopted, and the potential parameters are determined respectively for Co-Co, Sb-Sb, and Co-Sb, according to the relation between potential and materials'properties. By test, with the two-body potential, the single-crystal CoSb3can remain stable at the low temperature30K, but unfortunately disordering appears when in the medium-temperature range. Considering the limit of the two-body potential model, the thesis then promotes a three-body potential model for the CoSb3compound on the basis of the characteristic of the crystal structure as well as the bonding pattern, which takes the bond-angle interaction into account. By examination, with the three-body potential, CoSb3can keep stable crystal structure at the medium temperature range, and the predicted basic physical properties agree well with the reported experimental and theoretical results.With the proposed three-body potential model, the molecular dynamics study on the fundamental mechanical properties of the ideal structure CoSb3material is executed. First, the uniaxial tension and compression processes are simulated for the single-crystal bulk CoSb3at different temperatures. From the stress-strain curves and atomic evolutions, the basic mechanical properties of the single-crystal bulk CoSb3is obtained, and the general rule of the impact of temperature on the material's mechanical behavior is addressed. Moreover, at different structural dimensions, i.e., bulk, nanofilm, nanowire, and nanoparticle, both the uniaxial tension and compression tests are simulated for the CoSb3thermoelectric material, and the effect of structural scale variation on the material's fundamental mechanical behavior is discussed.Since the sublimation of Sb atoms in CoSb3would cause void defect in the crystal structure, and experiments can hardly control the defects quantitatively, the molecular dynamics simulation is applied to study the effect of Sb void on the lattice thermal conductivity and the fundamental mechanical properties of the single-crystal bulk CoSb3. Assuming that the void Sb atoms are randomly distributed, the thermal transport process is simulated for the void model by a velocity-exchanging method, and the lattice thermal conductivity variation with the void fraction is obtained for the bulk CoSb3. At different void fraction, the uniaxial tension and compression are simulated for the bulk CoSb3, and the uniformity effect of the void distribution is briefly discussed as well. The results show that, with the Sb void in the single-crystal bulk CoSb3, the thermal conductivity would be remarkably reduced, while the ultimate strength would be degraded to a large extent as well.Since the inducing of pores is expected to reduce the thermal conductivity in theory, the molecular dynamics simulations of nanoporous single-crystal CoSb3are carried out, and the impact of nanometer-scale pores on the lattice thermal conductivity and the fundamental mechanical performances of the material are investigated. It is indicated that, introducing nanometer-scale pores is a good way to reduce the lattice thermal conductivity of the material, and reducing pore diameter or increasing porosity are two alternative choices to make such effect more effective; for the porous CoSb3material, reducing pore diameter promotes the material's ultimate strength, and increasing porosity decreases the material's Young's modulus.