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 ??-Zn₄Sb₃. According to the interatomic potential obtained from first-principles calculation and fitting to the ground-state physical equations, the stress?Cstrain curves of single-crystal bulk ??-Zn₄Sb₃ under different conditions, which include Zn atom vacancy and temperature effects, are presented. From the stress?Cstrain responses, single-crystal bulk ??-Zn₄Sb₃ exhibits typical nonlinear elastic brittleness of thermoelectric materials. With increasing Zn atom vacancy proportion, the mechanical properties of single-crystal bulk ??-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 300?K to 700?K, the crystal structure of the vacancy model of ??-Zn₄Sb₃ maintains stability, and the mechanical properties are degraded slowly. The mechanical properties along the [001] axis are better than along the [010] axis.     

Molecular Dynamics Simulation of Mechanical Properties of Single-Crystal Bismuth Telluride Nanowire

The mechanical properties of single-crystal bismuth telluride nanowires have been studied by molecular dynamics methods. The mechanical behavior of the Bi₂Te₃ nanowire for two principal axes was simulated under different strain rates at low temperature and the results compared with those of bulk Bi₂Te₃. The simulation results show that, due to its marked anisotropy, the nanowire shows quite different failure behaviors in the two directions, with the failure stress for the a-axis (4.7 GPa) being three times that for the c-axis (1.4 GPa). The stress–strain curve of the nanowire is different from that for Bi₂Te₃ bulk, as surface stress induced by atomic rearrangement significantly reduces the strength of the nanowire. The effect of strain rate on the mechanical properties of the nanowire has also been analyzed, showing that the failure stress and failure strain decrease with decreasing strain rate, a behavior not apparent in the bulk Bi₂Te₃ simulation.     

Molecular Dynamics Simulation on Mechanics of Mg2Si Nanofilm

Molecular dynamics simulation has been carried out to study the mechanical properties of Mg₂Si nanofilm. For the binary thermoelectric material Mg₂Si with antifluorite crystal structure, 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 its mechanical properties. In the simulation, the radial distribution function of Mg₂Si nanofilm is computed to validate its crystal structure, and the stress–strain responses of the nanofilm are examined at room temperature. It is found that the mechanical properties of Mg₂Si nanofilm are quite different from those of bulk Mg₂Si due to the impact of surface atoms of the nanostructures. The size effect and the temperature effect on the mechanical properties of Mg₂Si nanofilm are discussed in detail.     

Computational Investigation of the Electronic and Thermoelectric Properties of Strained Bulk Mg2Si

The purpose of this work was to investigate, by numerical simulation, the effect of isotropic and anisotropic strain on the transport properties of Mg₂Si. Analysis of the effects of temperature and charge-carrier concentration on evolution of the energy gap and on the thermoelectric properties of strained Mg₂Si is also reported in this paper. Gap evolution is highly dependent on the type of strain applied to the structure. The Seebeck coefficient (S) and power factor (PF) are strongly modified; a gain of up to 40% can be obtained for S and up to 100% for PF under specific conditions of strain. In most cases the temperature corresponding to the maximum value of PF was found to shift downward under the effect of strain.     

Synchrotron Study of Ag-Doped Mg2Si: Correlation Between Properties and Structure

The crystal structure of Ag-doped Mg₂Si was investigated using synchrotron and neutron powder diffraction analysis, including in situ synchrotron x-ray powder diffraction patterns, recorded during a thermal cycle from room temperature up to 600°C. Rietveld refinement of diffraction patterns indicated that Ag doping results in partial substitution at Si sites. During heating, the Mg₂Si lattice parameters exhibited a shift in the temperature dependence at 300°C to 350°C, which was attributed to Ag precipitation out of Mg₂Si_1?x Ag _x solid solution. In turn, an increase of the Ag present in the Mg₂Si lattice after 350°C could be linked to thermally activated diffusion of Ag from β-AgMg phase. The Ag-dopant migration may explain previously outlined instabilities in the thermopower of Ag-doped Mg₂Si, e.g., the drop of the Seebeck coefficient value after heating to 150°C to 200°C and its subsequent increase after 350°C to 450°C.     

Room-Temperature Mechanical Properties and Slow Crack Growth Behavior of Mg2Si Thermoelectric Materials

Mg₂Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material??s response to such stresses, the mechanical properties of the TE material must be known. The Mg₂Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young??s modulus, shear modulus, and Poisson??s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg₂Si to slow crack growth.     

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 90.85 GPa to 68.17 GPa, a decrease of 24.96%. The ultimate tensile stress decreases from 18.25 GPa to 9.96 GPa, 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.     

Molecular Dynamics Study of the Effects of Nanopores on the Tensile Mechanical Properties of Crystalline CoSb3

The effects of nanometer-size pores on the uniaxial tensile mechanical properties of single-crystal bulk CoSb₃ were investigated by classical molecular dynamics simulation. The pores were assumed to be cylindrical and uniformly distributed along two vertical principal crystallographic directions of a square lattice. The dependence of the effects of pores on pore diameter and porosity was examined separately, by varying pore diameter and porosity in the ranges a ₀–6a ₀ and 0.1–5%, respectively, where a ₀ is the lattice constant of CoSb₃. The results from simulation indicate that, at constant porosity, Young’s modulus remains almost constant whereas ultimate strength decreases as pore diameter increases. At constant pore diameter, Young’s modulus decreases monotonically as porosity increases exponentially; interestingly, variation of the ultimate strength is negligible. Numerically, the mechanical performance of systems containing nanopores is still desirable, although no better than that of the no-pore system. The results provide useful information for realistic application of skutterudites.     

Use of Molecular Dynamics Simulations to Study the Effects of Nanopores and Vacancies on the Mechanical Properties of 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 (?), surface-to-volume ratio (ρ), and minimum cross-section length (L _min). The elastic modulus remains unchanged at 52.86 GPa for constant porosity of 0.7% whereas the ultimate stress and fracture strain gradually decrease with growing ρ or decreasing L _min. 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 57.17 GPa to 36.32 GPa, a reduction of 36.47%, the ultimate stress decreases from 6.40 GPa to 3.61 GPa, a reduction of 43.59%, and the fracture strain decreases from 22.4% to 13.8%, a reduction of 38.39%.     

使用分子动学方法研究碳纳米管的力学性质

用分子动力学理论对单壁碳纳米管的机械性质进行了计算和分析，通过对直径为1.2nm，长度为4.7nm的拓扑结构为扶手椅形式的单壁碳纳米管进行分子动力学计算，得出其杨氏模量为3.62TPa，拉伸强度为9.6GPa。从计算结果可以看出，单壁碳纳米管的杨氏模量和拉伸强度很高，比普通金属材料大1-2个数量级，这说明碳纳米管是一种机械性能非常优良的材料。     

The Influence of Grain Boundary Scattering on Thermoelectric Properties of Mg2Si and Mg2Si0.8Sn0.2

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg₂Si and Mg₂Si_0.8Sn_0.2 samples were measured in the temperature range from 300 K to 850 K together with the Hall coefficients at room temperature. The features of the complex band structure and scattering mechanisms were analyzed based on experimental data within the relaxation-time approximation. Based on the obtained model parameters, the possibility of improvement of the thermoelectric figure of merit due to nanostructuring and grain boundary scattering was theoretically analyzed for both Mg₂Si and the solid solution.     

Molecular Dynamics Study of Mechanical Properties of β-Zn4Sb3 Nanowires: Temperature,Strain Rate,and Size Effect

Recent experimental studies reveal that β-Zn₄Sb₃ nanowires have greater potential commercial application compared with conventional bulk materials. Mechanical stability of β-Zn₄Sb₃ nanowires is an important foundation for their commercial application. In the present work, mechanical properties of β-Zn₄Sb₃ nanowires are studied by using the molecular dynamics method, which theoretically reveals the influence mechanism of various effects (temperature, strain rate, and size effect) on the mechanical behavior of β-Zn₄Sb₃ nanowires. The simulation results show that mechanical properties of β-Zn₄Sb₃ nanowires decrease remarkably compared with bulk β-Zn₄Sb₃. Due to the disordered Zn atoms in β-Zn₄Sb₃, mechanical properties of β-Zn₄Sb₃ nanowires show a weak temperature softening effect, weak strain rate hardening effect, and slight size dependence. The present simulations of β-Zn₄Sb₃ nanowires represent a fundamental study and shed light on the understanding of the essential mechanical behavior of β-Zn₄Sb₃ nanowires.     

Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg₂Si (Mg₂Si:Al _m , m?=?0, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg₂Si was synthesized by solid-state reaction (SSR) at 773?K for 6?h, and Al-doped Mg₂Si powders were obtained by mechanical alloying (MA) for 24?h. Mg₂Si:Al _m were fully consolidated by hot pressing (HP) at 1073?K for 1?h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 10¹⁶ cm^?3 to 10¹⁹ cm^?3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg₂Si:Al_0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823?K.     

Effect of Biaxial Strain on Electronic and Thermoelectric Properties of Mg2Si

The electronic and thermoelectric properties of biaxially strained magnesium silicide Mg₂Si are analyzed by means of first-principle calculations and semiclassical Boltzmann theory. Electron and hole doping are examined for different doping concentrations and temperatures. Under strain the degeneracy of the electronic orbitals near the band edges is removed, the orbital bands are warped, and the energy gap closes up. These characteristics are rationalized in the light of the electron density transfers upon strain. The electrical conductivity increases with the biaxial strain, whereas neither the Seebeck coefficient nor the power factor (PF) follow this trend. Detailed analysis of the evolution of these thermoelectric properties is given in terms of the in-plane and cross-plane components. Interestingly, the maximum value of the PF is shifted towards lower temperatures when increasingly intensive strain is applied.     

Solid-State Synthesis and Thermoelectric Properties of Sb-Doped Mg2Si Materials

Sb-doped magnesium silicide compounds have been prepared through ball milling and solid-state reaction. Materials produced were near-stoichiometric. The structural modifications have been studied with powder x-ray diffraction. Highly dense pellets of Mg₂Si_1?x Sb _x (0 ≤ x ≤ 0.04) were fabricated via hot pressing and studied in terms of Seebeck coefficient, electrical and thermal conductivity, and free carrier concentration as a function of Sb concentration. Their thermoelectric performance in the high temperature range is presented, and the maximum value of the dimensionless figure of merit was found to be 0.46 at 810 K, for the Mg₂Si_0.915Sb_0.015 member.     

Effects of Al/Sb Double Doping on the Thermoelectric Properties of Mg2Si0.75Sn0.25

Al/Sb double-doped Mg₂Si_0.75Sn_0.25 materials were prepared by liquid–solid reaction synthesis and the hot-pressing technique. The effects of Al/Sb double doping on the thermoelectric properties were investigated at temperatures between room temperature and 900 K, and the resistivity and Hall coefficient were investigated at 80 K to 900 K. Al/Sb double-doped samples were found to be n-type semiconductors in the investigated temperature range. The absolute Seebeck coefficient (α), resistivity (ρ), and thermal conductivity (κ) for Al/Sb double-doped samples at room temperature were in the ranges of 152.5 μV K^?1 to 109.2 μV K^?1, 2.92 × 10^?5 Ω m to 1.29 × 10^?5 Ω m, and 2.50 W K^?1 m^?1 to 2.86 W K^?1 m^?1, respectively. The absolute values of α increased with increasing temperature up to a maximum, and decreased thereafter. This could be attributed to mixed carrier conduction in the intrinsic region. κ decreased linearly with increasing temperature to a minimum near the intrinsic region, then increased rapidly because of bipolar components. The highest ZT value measured was 0.94 at 850 K for Mg_1.9975Al_0.0025Si_0.75Sn_0.2425Sb_0.0075. Sb doping was effective for enhancement of ZT, because of a remarkable increase in the carrier concentration. However, Al doping was almost ineffective for enhancing ZT.     

Mg2Si薄膜在Si(100)衬底上的外延生长研究

采用直流磁控溅射系统,在Si(100)衬底上制备了外延Mg2Si半导体薄膜。通过XRD和场发射扫描电子显微镜(FESEM)对Mg2Si薄膜的晶体结构和表面形貌进行表征,理论分析了Mg2Si薄膜的消光特性对Mg2Si薄膜外延取向的影响,得到了Mg2Si薄膜的外延取向特性。结果表明,在Si(100)衬底上,外延Mg2Si薄膜具有Mg2Si(220)的择优生长特性。     

溅射功率对Mg2Si薄膜制备的影响

采用射频磁控溅射系统,在Si(111)衬底上制备了不同溅射功率下的Mg2Si薄膜。通过X线衍射(XRD)和冷场发射电子显微镜镜(FESEM)对Mg2Si薄膜的晶体结构和表面形貌进行了表征,理论分析了Mg2Si薄膜在Si(111)衬底上的外延生长关系,得到了Mg2Si薄膜的外延生长特性。研究结果表明,在80～110 W的溅射功率范围内,Mg2Si薄膜具有Mg2Si(220)的外延择优生长特性,并且随着溅射功率的增加Mg2Si(220)衍射峰先增强后变弱,在100W功率下Mg2Si(220)衍射峰最强。