查看更多>>摘要：The tetradymite compounds are known as good thermoelectric materials and also strong topological insulators, which exhibit unique electronic structures governed by several essential material features such as containing heavy elements and having small band gap. Here we demonstrate by first-principles calculations that for a given alloyed tetradymite, its topological nature actually shows certain sensitivity on the detailed atomic configurations, even if all of them have the same stoi-chiometric composition. Specifically, some tetradymites are found to be robust topological insulators (or normal insulators) upon arbitrary atomic mutations, which however can induce transition from topologically trivial to non-trivial or vice versa for other tetradymites. The underlying physical mechanisms are discussed in terms of the band gaps and strength of spin-orbit coupling in some exampling tetradymites.
查看更多>>摘要：In this study, we investigated the topological effect caused by chain crossing prohibition in ring-linear blends through dissipative particle dynamics (DPD) simulations. Multipoint segmental repulsive potential (MP-SRP) was used to ensure thermodynamic consistency between the systems that permitted and prohibited chain crossing in DPD polymer simulations. We successfully extracted the topological effect of ring polymers by observing the distribution of penetration in ring-linear blends and the radius of gyration of rings. The topological effect prohibits ring-ring crossing, resulting in a small radius of gyration of the rings. In the DPD polymer system with MP-SRP, the minimum size (number of particles) of a ring with penetration was found to be 30, which is less than half of the value of 80 observed in previous work using the Kremer-Grest bead-spring model.
查看更多>>摘要：Ab initio calculations were used to determine physical properties of AlN(0001) surface under Al coverage. It was shown that Al atoms are adsorbed in T4 sites for very low Al coverage, up to OAl = 1/4 monolayers (ML). For higher Al coverage, OAl = 1/4ML up to OAl = 1ML the adlayer becomes disordered and corrugated vertically. In this coverage range (0 < OAl < 1ML) the Al bonding energy is independent on the coverage and equal to EDFT ads approximately equal to 5.0eV. For higher coverage, 1ML < OAl 7/6ML, the Al adlayer becomes ordered and atomically flat. The Al adsorption energy in this range is EDFTads approximate to 6.0eV, i.e. much higher. For higher coverage, OAl 1.25ML, the Al adatoms are located in the second layer. The adsorption energy is reduced to EDFT and linearly increasing to EDFT ads = 4.88eV for the coverage increase up to OAl = 2ML. Full thermodynamic analysis identified two regions in equilibrium with Al vapor: first at very low pressures where OAl < 10-3ML and the second for higher pressures which is 1ML < OAl < 7/6ML. The second region is relevant for physical vapor transport (PVT) AlN growth. The nitrogen adsorption process, important for PVT AlN growth, takes place at single or double Al layer covered AlN(0001) surface.
查看更多>>摘要：The phase stability, thermo-physical and mechanical properties of C40-type hexagonal structure (Mo0.2Cr0.2Ta0.2Nb0.2X0.2)Si2 (X = W, V) high entropy refractory metal silicides (HERMSs) are systematically calculated through density functional theory. The special quasi-random structure (SQS) approach is approached to simulate the chemical disorder of high entropy silicide. The equilibrium lattice parameters and elastic moduli identified with the results from previous experimental data, indicating SQS approach is suitable for (Mo0.2Cr0.2Ta0.2Nb0.2X0.2)Si2 (X = W, V). The formation enthalpy and stiffness constants together determine the two silicides are thermodynamically and mechanically stable. The thermo-physical properties of two alloys were studied by quasi-harmonic Debye model. The bulk modulus of (MoCrTaNbW)Si2 always leads during 0-2000 K, indicating that it has a higher strength in the application. The stronger covalent bonds derived from the hybridization of V-d and Si-p in (MoCrTaNbV)Si2 resulted in greater brittleness. The polycrystalline moduli are calculated using Voigt-Reuss-Hill (VRH) scheme, which denotes that both two silicides are brittle in nature. Moreover, (MoCrTaNbV)Si2 is more brittle taking pugh ratio, Poisson's ratio and Cauchy pressure into consideration. Based on density of states analysis of these two alloys, the d orbital electron occupancy is critical to determine the mechanical properties of silicides.
查看更多>>摘要：Surface diffusion controlled grain boundary grooving in a polycrystalline thin film is studied using a phase-field model. We validate our model by comparing the simulation results with the classical theory of Mullins and further study the systems with anisotropic interfaces and finite grain sizes. Steady state velocity for grain growth is observed in the former case when grooving becomes extremely slow. The temporal evolution of groove depth shows significant deviation from Mullins' theory in the latter case, due to overlap of groove profiles. We also extend the model in the third dimension to study pitting at grain vertices in a thin film with equal sized hexagonal grains. The results indicate that for a given film thickness there exists a critical grain size above which hole formation occurs due to pitting.
查看更多>>摘要：A new 3D phase-field model is developed to investigate the evolution of the domain structure and phase transformation in a ferroelectric material near the morphotropic phase boundary (MPB) under the influence of external stimuli. The model combines the Landau-Ginzburg-Devonshire (LGD) theory with a general multiphase approach and enables to analytically compute the microstructure inside the grain structures with coexisting phases. Together with experimental analysis, a numerical energy overlap was employed to find the phase-dependent phenomenological coefficients. Domain structures and phase transformations in single crystals, as well as in the polycrystalline systems with or without an external electric field have been investigated in a MPB ferroelectric material, by employing phase-field simulation. The simulated results reveal the relation between the polarization switching and the phase transformation under an applied electric field, and the role of the external electric field orientation and amplitude in the domain structure and the phase transformation fraction.
查看更多>>摘要：The coupling between size and surface effects on the mechanical and piezoelectric properties of wurtzite-type piezoelectric materials at the nanoscale is still not entirely exploited. The influence of surface orientation and thickness of wurtzite GaN nanofilms on their electromechanical properties (mechanical, piezoelectric and dielectric properties) is therefore investigated in this study by means of a multiscale analysis. The density functional theory (DFT) is utilized to study the atoms reconstruction phenomenon near surface region. Molecular dynamics (MD) simulation is employed to examine the effects of different thicknesses and surface orientations on the electromechanical properties. A continuum mechanics model incorporating the effects of both thicknesses and surface orientations is then developed and can be helpful for investigating the structure-property of GaN nanofilms. It is found through MD simulations that the elastic constant decreases in the out-of-plane direction but increases in the in-plane direction, regardless of surface orientations. This can be justified by DFT-based calculations, which show that the change of surface electron density of the nanofilm causes the reconstruction of surface atoms, leading to the extension of bond length in the out -of-plane direction and the shortening of bond length in the in-plane direction and thereby the difference between surface elasticity and bulk elasticity. The change of surface elasticity further affects the surface piezoelectric and dielectric properties. Also, the developed continuum mechanics model can reproduce the thickness-and orientation-dependent behaviors of the electromechanical parameters by fitting MD results. Finally, the developed continuum mechanics model is applied for predicting the thickness dependence of the piezopotential coefficient, showing that the piezopotential increases by 10% when the nanofilm thickness is 3 times the crystal constant.
查看更多>>摘要：Using techniques widely available in digital image processing, machine learning and spatial statistics, this paper proposes a novel workflow that generates two dimensional spatial models using objects extracted from digital micro-graphs of material micro-structure that can be used in statistical reconstruction modeling within a numerical procedure, such as finite element analysis. This paper also reviews the relevant image processing techniques, spatial statistical theories and reconstruction (modeling) algorithms with unique contributions. As an end-to-end illustration, the workflow is applied to a two dimensional, digital micro-graph of hydrogen porosity taken of as-fabricated AlSi10Mg manufactured using laser powder bed fusion adopted from the literature.
查看更多>>摘要：In linear elastic fracture mechanics, the critical stress intensity factor (CSIF) is related to the surface energy calculated from the bond energy in the reference configuration. For nanomaterials, the difference between the thus computed CSIF and that found from other methods is attributed to lattice trapping. We show here that the energy release rate (and hence the CSIF) determined from the energy of bonds on the crack surface in the current configuration agrees with that estimated by the traditional methods employing the fracture stress and the initial crack length. We demonstrate this by using molecular dynamics simulations with the Tersoff potential by studying crack propagation in a pre-cracked monolayer boronitrene.
查看更多>>摘要：We conducted density functional theory based quantum molecular dynamics (QMD) and force field based classical molecular dynamics (CMD) simulations to compute the shock Hugoniot of polyurethane and polyurea. The QMD results are in reasonable agreement with the available experimental data. However, limited experimental data for polyurea (up to a shock pressure of 30 GPa) is available in the literature. We used QMD to predict the Hugoniot states of polyurea up to a shock pressure of 75 GPa. The subsequent postmortem analysis of the shocked polymer samples provided significant insights into the shock-induced dissociation of polymer structures. Comparison of experimental and QMD derived Hugoniot states with those obtained from CMD demonstrates the limits of CMD in shock simulations. Furthermore, we conducted CMD simulations at lower shock pressures to investigate the size effects of the computed shock response.
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Elsevier