Guodong Li

Molecular Dynamics Simulation of the Mechanical Properties of Single-Crystal Bulk Mg2Si

This paper reports a study of the mechanical behavior of single-crystal bulk Mg2Si by the molecular dynamics method in view of the effect of temperature and strain rate. A modified Morse potential energy function in which the bond-angle deformation has been taken into account is developed and employed to describe the atomic interactions to shed light on the mechanical properties. The stress–strain response of single-crystal bulk Mg2Si under uniaxial tensile loading is investigated at different temperatures (300xa0K to 1200xa0K) and different strain rates, respectively. Through comparison of the mechanical properties of single-crystal bulk Mg2Si at different temperatures and strain rates, it is found that the elastic modulus gradually decreases with increasing temperature, while the strain rate seems to have little effect on the mechanical properties of single-crystal bulk Mg2Si. Moreover, in contrast to conventional polycrystalline bulk Mg2Si, single-crystal bulk Mg2Si exhibits much better mechanical performance.

Effects of Nanoparticle Size on the Thermoelectric and Mechanical Properties of Skutterudite Nanocomposites

Nano-Co4Sb11.5Te0.5/Co4Sb11.5Te0.5 composites with different nanoparticle sizes and contents were obtained by ultrasonic dispersion followed by ball-milling and spark plasma sintering (SPS) processes. It was found that the nanoparticles obviously grew in size after SPS; most of them were larger than 200xa0nm in the 5%50xa0h and 10%50xa0h samples, while they were 100xa0nm to 200xa0nm in the 3%100xa0h and 5%100xa0h samples. The thermoelectric (TE) and mechanical properties of the nanocomposites with different sizes and contents were then characterized. Our results show that the TE properties were not noticeably affected, while the flexural strength and fracture toughness increased remarkably with the nanoparticle content, and the nanoparticle size had a notable effect on the mechanical properties of the nanocomposites; that is, the smaller the nanoparticles, the greater their reinforcing and toughening effects.

An Investigation on the Coupled Thermal–Mechanical–Electrical Response of Automobile Thermoelectric Materials and Devices

Thermoelectric (TE) materials, which can directly convert heat to electrical energy, possess wide application potential for power generation from waste heat. As TE devices in vehicle exhaust power generation systems work in the long term in a service environment with coupled thermal–mechanical–electrical conditions, the reliability of their mechanical strength and conversion efficiency is an important issue for their commercial application. Based on semiconductor TE devices wih multiple p–n couples and the working environment of a vehicle exhaust power generation system, the service conditions of the TE devices are simulated by using the finite-element method. The working temperature on the hot side is set according to experimental measurements, and two cooling methods, i.e., an independent and shared water tank, are adopted on the cold side. The conversion efficiency and thermal stresses of the TE devices are calculated and discussed. Numerical results are obtained, and the mechanism of the influence on the conversion efficiency and mechanical properties of the TE materials is revealed, aiming to provide theoretical guidance for optimization of the design and commercial application of vehicle TE devices.

Effects of Disordered Atoms and Nanopores on Mechanical Properties of β-Zn4Sb3: a Molecular Dynamics Study

Effects of disordered Zn atoms and nanopores on mechanical properties of β-Zn4Sb3 are studied by using the molecular dynamics (MD) method. Due to the influence of disordered Zn atoms in β-Zn4Sb3, the elastic modulus decreases from 90.85xa0GPa to 68.17xa0GPa, a decrease of 24.96%. The ultimate tensile stress decreases from 18.25xa0GPa to 9.96xa0GPa, a decrease of 45.42%. The fracture strain decreases from 32.7% to 20.8%, a decrease of 36.39%. Due to the influence of nanopores, the elastic modulus decreases with growing porosity, and the relationship between the elastic modulus and porosity leads to a scaling law. It seems that the porous radius and porous distribution are also important factors influencing the ultimate tensile stress and fracture strain, in addition to the porosity. However, our simulation results demonstrate that disordered Zn atoms and nanopores reduce the structural stability, dramatically decreasing the mechanical properties of β-Zn4Sb3.

Influence of Nanopores on the Tensile/Compressive Mechanical Behavior of Crystalline CoSb3: A Molecular Dynamics Study

Recently, many experimental studies have reported that inserting nanopores into thermoelectric materials can both remarkably reduce the thermal conductivity and significantly improve the thermoelectric performance of the target material. Research on nanoporous materials has thus been attracting much attention worldwide. However, most of the studies mainly focus on the preparation of nanoporous material and the effect of different geometrical sizes of nanopores on thermal conductivity and thermoelectric properties of the nanoporous material. In this paper, the mechanical behavior of crystalline CoSb3 with nanopores under uniaxial tensile/compression is studied by means of the molecular dynamics method. The emphasis is on the influence of porosity, temperature and strain rate on the tensile/compressive mechanical behavior of nanoporous CoSb3. The simulation results show that both failure patterns under tension/compression are typical brittle fractures. The elastic modulus decreases with the growing porosity, and the porosity and the elastic modulus are inversely proportional to each other. The increase of temperature results in a linear degradation of the elastic modulus and the ultimate strength. The elastic modulus and the ultimate strength under uniaxial compression are greater than those under uniaxial tension. The present study sheds light on the future application of nanoporous CoSb3 thermoelectric materials.

Molecular Dynamics Study of the Mechanical Properties of Single-Crystal Bulk β-Zn4Sb3: Vacancy and Temperature Effects

The molecular dynamics method is employed to study the mechanical properties of single-crystal bulk β-Zn4Sb3. According to the interatomic potential obtained from first-principles calculation and fitting to the ground-state physical equations, the stress–strain curves of single-crystal bulk β-Zn4Sb3 under different conditions, which include Zn atom vacancy and temperature effects, are presented. From the stress–strain responses, single-crystal bulk β-Zn4Sb3 exhibits typical nonlinear elastic brittleness of thermoelectric materials. With increasing Zn atom vacancy proportion, the mechanical properties of single-crystal bulk β-Zn4Sb3 gradually degrade. When the Zn atom vacancy proportion reaches 10% in the vacancy model, the elastic modulus and the ultimate stress are found to decrease by 30% and 50% compared with the full-occupancy model. With increasing temperature from 300xa0K to 700xa0K, the crystal structure of the vacancy model of β-Zn4Sb3 maintains stability, and the mechanical properties are degraded slowly. The mechanical properties along the [001] axis are better than along the [010] axis.

The influence of Zn vacancy on thermal conductivity of β-Zn4Sb3: A molecular dynamics study

In our present work, the effect of the Zn-vacancy concentration on the lattice thermal conductivity of β-Zn4Sb3 at room temperature is studied by using the nonequilibrium molecular dynamics approach. Along both the x- and z- axes, the heat flux and the temperature gradient exponentially decay and increase respectively. The lattice thermal conductivity of the single crystal bulk β-Zn4Sb3 rapidly decreases when there exists Zn atom vacancy, and then the lattice thermal conductivity slowly falls further with the growing Zn atom vacancy proportion, which suggests that the Zn atom vacancy (nv) to the lattice thermal conductivity (kvac) leads to a scaling law of kv ∼ nv−α This phenomenon is attributed to the fact that the existence of vacancy scattering can significantly decrease the mean free path. When the vacancy proportion of Zn atom reaches 10%, that is the vacancy model of β-Zn4Sb3, the lattice thermal conductivity of β-Zn4Sb3 is 1.32 W/mk and 1.62 W/mk along the x- and z- axes respectively, which drops b...

Use of Molecular Dynamics Simulations to Study the Effectsof Nanopores and Vacancies on the Mechanical Propertiesof Bi2Te3

The effects of nanopores and vacancies on the mechanical properties of Bi2Te3 have been studied. Cuboid single-crystal bulk Bi2Te3 with atoms removed was chosen for molecular dynamics simulations. The mechanisms of action of the two defects can be distinguished by their specific effects on the crystal structure of the bulk Bi2Te3. The mechanical properties of the nanoporous Bi2Te3 are affected by porosity (ϕ), surface-to-volume ratio (ρ), and minimum cross-section length (Lmin). The elastic modulus remains unchanged at 52.86xa0GPa for constant porosity of 0.7% whereas the ultimate stress and fracture strain gradually decrease with growing ρ or decreasing Lmin. The lattice stability of Bi2Te3 gradually weakens as the proportion of vacancies increases; this leads to increasing potential energy and poorer mechanical properties of Bi2Te3. When the proportion of Bi vacancies is increased from 0% to 8%, the elastic modulus decreases from 57.17xa0GPa to 36.32xa0GPa, a reduction of 36.47%, the ultimate stress decreases from 6.40xa0GPa to 3.61xa0GPa, a reduction of 43.59%, and the fracture strain decreases from 22.4% to 13.8%, a reduction of 38.39%.

Effects of MAss Fluctuation on Thermal Transport Properties in Bulk Bi2Te3

In this paper, we applied large-scale molecular dynamics and lattice dynamics to study the influence of mass fluctuation on thermal transport properties in bulk Bi2Te3, namely thermal conductivity (К), phonon density of state (PDOS), group velocity (vg), and mean free path (l). The results show that total atomic mass change can affect the relevant vibrational frequency on the micro level and heat transfer rate in the macro statistic, hence leading to the strength variation of the anharmonic phonon processes (Umklapp scattering) in the defect-free Bi2Te3 bulk. Moreover, it is interesting to find that the anharmonicity of Bi2Te3 can be also influenced by atomic differences of the structure such as the mass distribution in the primitive cell. Considering the asymmetry of the crystal structure and interatomic forces, it can be concluded by phonon frequency, lifetime, and velocity calculation that acoustic-optical phonon scattering shows the structure-sensitivity to the mass distribution and complicates the heat transfer mechanism, hence resulting in the low lattice thermal conductivity of Bi2Te3. This study is helpful for designing the material with tailored thermal conductivity via atomic substitution.

Modal Analysis and Study of the Vibration Characteristics of the Thermoelectric Modules of Vehicle Exhaust Power-Generation Systems

Thermoelectric (TE) materials and modules are important components of vehicle exhaust power-generation systems. The road and the engine, the main sources of vibration of TE modules, have substantial effects on the vibration characteristics of TE modules. In this work, modal analysis and the vibration characteristics of TE modules were investigated in detail. On the basis of the TE modules and their service environment, simulations for modal analysis were performed by use of the finite-element method, and the natural frequencies and mode shapes of the TE modules were obtained. The numerical results were used to compare the natural frequencies of TE modules under different contact stiffness with the range of excitation frequencies of road and engine, in an attempt to prevent severe resonance. The effects on the vibration characteristics of geometric dimensions, service temperature, and thermal stress of the TE modules are also discussed in detail. The results reveal the vibration characteristics of the TE modules and provide theoretical guidance for structure optimization in the design of vehicle exhaust power-generation systems.