Xuqiu Yang

Thermodynamic and mechanical properties of crystalline CoSb3: A molecular dynamics simulation study

Molecular dynamics simulations have been performed to study the fundamental thermodynamic and mechanical properties of single-crystalline skutterudite CoSb3 in the nanometric scale. The several interesting thermodynamic predictions, including linear thermal expansion coefficient, specific heat capacity, thermal conductivity, and temperature dependence of elastic constants, show excellent agreement with data available in the literature. The classic mechanical tests of uniaxial tension and compression are performed respectively at constant temperatures. The CoSb3 single-crystal exhibits nonlinear elastic response during the deformation process and the sustainable stress is very high, demonstrating its outstanding stability. An interesting phenomenon occurs at compression that the stress-strain curve undergoes a transition. The cause of the transition is an atomic reconstruction, which is observed and interpreted on the basis of interatomic interactions. Both of the failure patterns under tension and compres...

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.

Analysis of Thermal Power Generation Capacity for a Skutterudite-Based Thermoelectric Functional Structure

Due to military or other requirements for hypersonic aircraft, the energy supply devices with the advantages of small size and light weight are urgently needed. Compared with the traditional energy supply method, the skutterudite-based thermoelectric (TE) functional structure is expected to generate electrical energy with a smaller structural space in the hypersonic aircraft. This paper mainly focuses on the responded thermal and electrical characteristics of the skutterudite-based TE functional structure (TEFS) under strong heat flux loads. We conduct TE simulations on the transient model of the TEFS with consideration of the heat flux loads and thermal radiation in the hot end and the cooling effect of the phase change material (PCM) in the cold end. We investigate several influential factors on the power generation capacity, such as the phase transition temperature of the PCM, the heat flux loads, the thickness of the TE materials and the thermal conductivity of the frame materials. The results show that better power generation capacity can be achieved with thicker TE materials, lower phase transition temperature and suitable␣thermal conductivity of the frame materials.

Effects of Van der Waals Bonding on the Compressive Mechanical Behavior of Bulk Bi2Te3: A Molecular Dynamics Study

Along the c axis of the crystal lattice, Bi2Te3 has periodic quintuple layers “ Te1 Bi Te2 Bi Te1–” which are connected by Van der Waals bonding. The weak bonding between Te1–Te1 layers substantially affects the mechanical properties of Bi2Te3. In the work discussed in this paper, the molecular dynamics method was used to study the mechanical properties of cuboid single-crystal of bulk Bi2Te3 under compressive loads. The emphasis was on the effects of the Van der Waals bonding on the deformation and failure mechanism. The molecular dynamics simulation results revealed that Van der Waals bonding has a dominant effect on the mechanism of deformation, and fundamentally determines the ultimate stress and fracture strain. Furthermore, the compressive load along the a and c axes lead to quite different failure behavior, which can be distinguished by their specific effects on the deformation of the Van der Waals bonding. However, only models with the load along the a axis dramatically demonstrate the effect of strain rate on the stress–strain curves, in accordance with the poor structural stability.

Effects of Hydrothermally Synthesized BiFeO3 Additive on Electrical Properties of High-Power Piezoelectric Ceramic PMnS-PZN-PZT

Pb(Mn1/3Sb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr, Ti)O3+xwt%BiFeO3 piezoelectric ceramics were prepared by the solid action route with hydrothermally synthesized BiFeO3 additive to introduce Bi3+ and Fe3+ double doping. All ceramics are well sintered and have perovskite structure with coexisting rhombohedral and tetragonal phases. Fe3+ doping creates oxygen vacancies, reducing the dielectric loss. The octahedral tilting are restrained by Bi3+ doping, enhancing tetragonality. With the introduction of BiFeO3, all the properties improved initially, but then began to deteriorate when x > 0.4 because of spontaneous polarization decreasing caused by c-axis orientation reduction. High-power piezoelectric ceramics are obtained, with excellent properties of tanδ = 0.2%, kp = 0.60, d33 = 433 pC/N, and Qm = 745.

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 a0–6a0 and 0.1–5%, respectively, where a0 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.

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.

Ba-Filling Effect on the Uniaxial Tensile and Compressive Mechanical Behavior of Crystalline CoSb3: A Molecular Dynamics Study

Filled skutterudites, which possess application potential, are believed to be a class of novel thermoelectric materials. The contribution of atomic filling to the significant decrease of phonon conductivity is investigated extensively in the literature. However, the filling effect on the fundamental mechanical behavior is not so far very clear. In the present study, molecular dynamics simulations have been performed to investigate the effect of Ba-filling on the uniaxial tensile and compressive mechanical properties of crystalline CoSb3 with a multibody interatomic potential. First, we constructed the fully Ba-filled CoSb3 model according to the ideal lattice structure. For comparison, pure binary CoSb3 was also modeled. Then, the simulation models were relaxed to reach more favorable configurations. Thereafter, the uniaxial tension and compression were carried out by strain-controlling until failure at room temperature. Stress–strain curves were obtained during the whole deformation process. The atomic rearrangements and failure patterns were also examined. The comparison of these mechanical responses between the filled and unfilled CoSb3 was made and analyzed. The results are expected to be helpful for the application of high-performance skutterudites.

Electric Field Induced-Strain Behavior of Single Lead-Free KNBT Piezoelectric Fiber

In this work, electric field-induced strain properties of sodium potassium bismuth titanate single fiber were investigated as a function of temperature. The results show that the normalized strain increases significantly with increasing temperature and reaches the maximum value of 315 pm/V at 160°C. Meanwhile, the negative strain decreases with increasing temperature, and then almost disappears in the vicinity of depolarization temperature. The temperature-dependent X-ray diffraction patterns, Raman spectra, polarization-electric field hysteresis loops, and the associated current-electric field curves show that the giant strain may be attributed to the antiferroelectric-ferroelectric phase transition.

Effect of Nanopores on the Phonon Conductivity of Crystalline CoSb3: A Molecular Dynamics Study

Molecular dynamics simulations have been performed to investigate the effect of nanometer-size pores on the phonon conductivity of single-crystal bulk CoSb3. The cylindrical pores are uniformly distributed along two vertical principal crystallographic directions of a square lattice. Because pore diameter and porosity are two key factors that could affect the performance of the materials, they were varied individually in the ranges a0–6a0 and 0.1–5%, respectively, where a0 is the lattice constant of CoSb3. The simulation results indicate that the phonon conductivity of nanoporous CoSb3 is significantly lower than that of no-pore CoSb3. The reduction of phonon conductivity in this simulation was consistent with the ballistic–diffusive microscopic effective medium model, demonstrating the ballistic character of phonon transport when the phonon mean-free-path is comparable with or larger than the pore size. Reducing pore diameter or increasing porosity are alternative means of effective reduction of the thermal conductivity of CoSb3. These results are expected to provide a useful basis for the design of high-performance skutterudites.