查看更多>>摘要：Chain stiffness plays a critical role in influencing the thermodynamic and dynamic properties of polymers. Here, all-atom molecular dynamics simulations are employed to study the effects of chain tacticity and temperature on polymer stiffness. Our results show that local stiffness is enhanced by introducing an asymmetric stereochemical unit with opposite chirality joined by all -trans series on polymer chains. The free energy landscape of adjacent dihedral pairs can explain the various chain stiffness of polymer chain with different tacticities. In literature, the temperature dependence of chain stiffness measured by small-angle neutron scattering (SANS)-based experiments contradicts with rotational isomeric state (RIS) predictions and recent single chain simulation results. Our results show that chain stiffness of isotactic PP and PB-1 melt is almost temperature independent, which is consistent with SANS-based measurements. By considering the chain density of polymer melt which is ignored by original RIS theory and single-chain simulations, we point that RIS theory and SANS-based experiments can be harmonic, but the chain density must be explicitly considered for polymer melt.
查看更多>>摘要：To meet the requirements of modern technical and scientific applications, the development and investigation of high-performance materials with locally defined properties gets more and more into focus of actual research. These materials often contain phases with at least one stoichiometric component. To investigate such materials within grand potential-based phase-field simulations, a specialized method is needed, which allows to model the required Gibbs energy formulations for the stoichiometric phases. In this work, a modeling approach is introduced, based on the equilibrium conditions of the involved phases, to generate a Gibbs energy formulation for the stoichiometric (Nb, Ti)(3) Si phase in Nb-Si-Ti. With this formulation, the eutectic reaction in the ternary NbSi-10Ti system is investigated within two- and three-dimensional phase-field simulations, with eutectic and off-eutectic melt compositions. The solidified microstructures are qualitatively assessed by measuring the average front undercoolings, the adjusting phase fractions and the concentrations during the simulations. The validation of the results shows the ability of the generated material model to simulate the obtained system and proves the suitability of the presented approach to model stoichiometric phases.
Porsev, Vitaly V.Bandura, Andrei, VEvarestov, Robert A.
11页
查看更多>>摘要：A general algorithm based on the line symmetry group theory is proposed for ab initio modeling of onedimensional (1D) helical nanoobjects using computer codes designed for periodic structure calculations. The key point of the algorithm is the interpolation between the properties of a sequence of periodic structures to obtain the properties of 1D helical nanoobjects without translational periodicity. A special selection of periodic structures avoids excessively long periods, thereby providing control over computational costs. The developed approach gives an opportunity to simulate the continuous dependences of mechanical and electronic properties on the torsional strain for any helical 1D nanoobject using high-level quantum chemical methods. The superposition of torsional and axial deformations can be used to generate the two-parameter maps of the desired properties. The proposed algorithm is applied to obtain the structure and properties of selected nanohelicenes, which are the helicenes infinitely continued along the helical (screw) axis direction. The CRYSTAL17 computer code based on atomic basis set has been used for this purpose. The main advantage of this program in comparison with other widely available ab initio simulation codes is the built-in accounting for helical symmetry operations in 1D periodic systems. Performed calculations show that nanohelicenes have several local minima of potential energy, differing in the order of the helical axis. The irrational order of the helical axis found at all energy minima indicates the absence of translational symmetry in these structures. Two-parameter electronic band gap maps and the dependences of Young's moduli on the torsion angle for the selected nanohelicenes are calculated and discussed.
查看更多>>摘要：Heusler compounds as an impressive group of materials with a vast potential for different applications such as future energy, thermoelectric and spintronics are great interest to materials scientists. Dynamical, electronic and optical properties of Fe2TaAl and Fe2TaGa full-Heusler alloys have been studied in the framework of the density functional theory (DFT). The main goal of this manuscript is tuning of the electronic and optical properties of compounds by applying hydrostatic pressure. The electronic results show that both compounds are semiconductors with energy gap about 0.84 and 0.64 eV, respectively. Also, an indirect to direct gap transition occurs for Fe2TaAl and Fe2TaGa at 5 and 80 GPa pressure, respectively. Calculated phonon dispersion illustrates that both compounds are stable at the range of applied pressure. Also, a blue shift can be observed for optical conductivity spectra by increasing pressure. The results illustrate that electronic and optical properties of mentioned Heusler alloys can be tuned moderately by using hydrostatic pressure.
查看更多>>摘要：As a typical polytypic compound, silicon carbide has various atomic arrangements; however, the role of atomic arrangements in the plastic deformation mechanisms remains unclear. In this article, the relationship between atomic arrangements and mechanical properties in single crystal and polycrystalline SiC is revealed by firstprinciple calculations and molecular dynamics simulations. We found that the twin boundary can delay the fracture of the single crystal 6H-SiC chemical bonds, could account for its excellent tensile toughness. The plastic deformation of polycrystalline SiC is dominated by dislocation nucleation and propagation under compressive strain, and by intergranular fracture under tensile strain. In addition, when the temperature exceeds 1200 K, the yield stress of polycrystalline SiC gradually stabilizes. Hexagonal SiC (2H, 4H and 6H) have stronger strain resistance than cubic one (3C) due to the hierarchical resistance of twinning and stacking fault structures to external stress. These results can provide insight into the relationship between atomic arrangements and stress response in SiC materials.
查看更多>>摘要：Quantitative simulations of the carbon-induced internal friction in ferrite/martensite Fe-C and Fe-Ni-C alloys are performed by combining a mean-field elastic model and the atomistic kinetic Monte Carlo based on a pair interaction model to describe the composition-dependent carbon migration. The simulation is validated by experimental data and a thermo-kinetic theory. Our results predict that (i) additional peaks occur in the internal friction profiles of Fe-C and Fe-Ni-C due to C-C and Ni-C pair interactions; (ii) the Ni-alloying shifts the internal friction peak to lower temperature than in Fe-C alloys; (iii) the peak temperature is not simply related to the most frequent carbon jumps during the relaxation process; (iv) the internal friction behavior in martensite depends on the excitation direction with respect to the carbon ordering direction.
查看更多>>摘要：Co-based Heusler phases, for example Co(Cr,Fe,Al), are considered as potential half-metallic ferromagnets with full spin polarization at the Fermi level and are therefore not only interesting from a fundamental point of view but also for their prospective technological exploitation. For successful materials design via selection of candidates towards desired properties as well as practical synthesis, knowledge of the relevant thermodynamic phase diagrams is an important aspect. However, the materials of interest constitute quaternary systems, where such knowledge is scarce. The importance of thermodynamic understanding is underlined by previous experimental findings, which report that the phase formation of the desired structurally ordered Co(Cr,Fe,Al) is not straightforward due to thermodynamic instabilities. In this work, we revisit the quaternary region of the CoCrCoAl-CoFe section of the thermodynamic phase diagram through DFT and thermodynamic calculations. The regular solution model indicates the formation of an immiscibility region centered around a pseudoternary critical point at T-c = 1412 K and the composition CoFe0.2Cr0.4Al0.4. FP-LAPW ab initio calculations of the enthalpy of mixing show an energetically favorable minimum at the CoCr0.125Fe0.25Al0.625 composition and a local maximum around CoFe0.125Cr0.375Al0.5 which corresponds to the region of immiscibility and matches the area of the binodal/spinodal decomposition resulting from the regular solution model. We discuss three types of substitution: Al for Cr, Al for Fe, and Al for Cr/Fe with special focus on the formation of different structural motifs, specifically on the order- and disorder, depending on the Cr/Fe ratio. Chemical bonding and magnetic properties of selected compositions are calculated and analyzed. Our results underline the significance of careful consideration of thermodynamic phase formation for materials design and consequently for machine learning approaches towards practical realization of functional materials.
查看更多>>摘要：Self-assembly behavior of semiflexible polymers on rigid spherical shell is investigated using the molecular dynamics (MD) simulation method. We find that the conformation of semiflexible polymers depends on the length of polymers and bending energy of polymer chains. For moderate adhesive strength and bending energy, the polymers aggregate into multiple clusters, each of which contains several parallel chains. Simultaneously, the self-assembly conformations, volley-ball, helical and tennis-ball state, form on spherical shell depend on the number of clusters. We quantify the self-assembly conformation and analysis its physical mechanics of formation. This work may provide a theoretical foundation for the future theories of conformation prediction and material fabrication.
查看更多>>摘要：Amorphous carbon (a-C) films can transform to graphene under the catalysis of some transition metals, while the transformation mechanisms are still not clear for the experimental techniques unable dynamically tracing the diffusion of atoms. In this work, an atomic-level simulation method-molecular dynamics simulation was adopted to conquer the disadvantage of experimental techniques. Using this method, the microstructure evo-lution processes of a-C films with different thicknesses on the Cu(0 0 1), Cu(1 1 0) and Cu(1 1 1) surfaces were studied. It was found that the a-C films directly transformed to single-layer or multilayer graphene without the dissolving and precipitating processes of carbon atoms, and the final product strongly depends on the surface density of a-C films. Specifically, the a-C films transformed to a piece of equal-surface-density single-layer or multilayer graphene. However, for the limited relaxation time, the formed graphene contain different amount of defects; and the quality analysis of graphene implies that the Cu(0 0 1) crystal plane owns higher catalyzing ability than the other two planes. Detailed microstructure analysis indicates the a-C films gradually transformed to graphene by breaking the vertical bonds and reconstructing the horizontal bonds. And the variation of the bonds make the six-carbon (C6) rings to be the main rings and to parallel the Cu substrate.
Vo Van HoangNguyen Hoang GiangTo Quy DongBubanja, Vladimir...
7页
查看更多>>摘要：Atomic structure and rippling of the amorphous two-dimensional SiC nanoribbons (a-2D SiC NRs) are studied by means of molecular dynamics (MD) simulations. Amorphous models are obtained by cooling from the melt. Atomic structure of a-2D SiC NRs is analyzed in details. We find that atomic structure of glassy 2D SiC nano ribbons is disordered not only by containing various structural defects including rings of various sizes and shapes but also by the non-alternative distributions of Si and C atoms inside the rings. This structure is quite different from that of the crystalline counterparts although the 6-fold rings also dominate in the amorphous models. Moreover, size effects on rippling of a-2D SiC NRs are analyzed. On the other hand, wrinkled disordered atomic structure of amorphous 2D SiC NRs has a tendency to reduce the rippling amplitude compared to that of the crystalline counterparts. We find a significant amount of 5-fold rings occurred in the amorphous 2D SiC nano ribbons that are obtained by cooling from the melts. These 5-fold rings stabilize the glassy 2D structure.
NSTL
Elsevier