关键词： Molecular dynamics simulation   Torsional copper nanorod   Tensile mechanical behaviors   Surface effect  

摘要： The tensile mechanical behaviors of axial torsional copper nanorods with the diameter of 5-6.5nm are investigated systematically by molecular dynamics simulation. When increasing the angle of torsion loading, the initial stress gradually departures from the near-zero state, and the elastic modulus remains essentially constant. The tensile yield is closely related to the surface deformation reflected by the average potential energy of surface atoms (Pe(Surf)). In spite of varied torsion loading, the Pe(Surf) of nanorods are promoted to a similar critical level by the torsion and tension, and then fall abruptly indicating the nanorods yield. For the nanorods with [001] orientation in long axis, the rotation loading improves the Pe(Surf) at the start of tension and makes dislocation nucleation occur easily, leading to the decline of tensile yield strength. For the [110] orientated nanorods, the Pe(Surf) rise induced by the torsion is relatively small and quite close to the range size of the critical level, conducing to the insignificant fluctuation of the tensile yield stress. Meanwhile, lowering the temperature, enlarging the aspect ratio, and shrinking the size can lighten the yield stress descend of [001] orientated nanorods in different extents. At a constant temperature, the Pe(Surf) differences between the Pe(Surf) at yield moment and initial Pe(Surf) without any loadings for all [001] orientated nanorods disperse in a narrow range, no matter how the aspect ratio and size change. This work contributes to understanding the mechanical properties and yield mechanisms of the nanorods under the torsion-tension combined loading.

关键词： thrombosis   hyperhomocysteinemia   vascular morphometry   single nucleotide polymorphisms  

摘要： A model of moderate hyperhomocysteinemia associated with mechanical injury of the musculoskeletal system was developed and experimentally substantiated. The adequacy of this model for studies of morphological and functional regularities is verified. This model can be used for the development of a new concept of evaluation of thrombotic complications of mechanical injury.

关键词： Epoxy resin   Curing kinetics   Thermal-mechanical properties   Composites  

摘要： A novel glycidylamine epoxy resin containing phthalide cardo structure (TGMP) was designed and synthesized based on o-cresolphthalein using a three-step procedure. Its curing systems were prepared using methyl-nadilic anhydride (MNA) and 2-ethyl-4-methylimidazole (EMI-24) as the curing agent and accelerating agent, respectively. The chemical structures of synthetic intermediate, epoxy resin and its curing systems were characterized by Fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra. The curing behavior and kinetics were investigated by differential scanning calorimetry (DSC) and the activation energies were calculated according to the Kissinger equation and Flynn-Wall-Ozawa (FWO) equation. The TGMP/MNA/EMI-24 system is of more reactivity more that of pure TGMP. The Friedman method was used to describe the progress of the curing reaction of the TGMP/MNA/EMI-24 system, finding it follows the autocatalytic reaction model and nth-order reaction mechanism is predominant. The thermal-mechanical properties of the TGMP/MNA/EMI-24 composites were studied by dynamic mechanical analysis (DMA). The tan S curves show double peaks and the maximum glass transition temperature is up to 345 degrees C.

关键词： nanocrystalline Cu   flow stress   grain size   temperature   molecular dynamics  

摘要： Nanocrystalline copper (Cu) is considered to be one of the best interconnected material in integration circuit (IC) industry, because of its ultra-low resistivity and high mechanical stability. Mechanical properties of nanocrystalline Cu are completely different from those of bulk monocrystalline Cu. These properties are of high importance in the assessment of the thermo-mechanical reliability of the interconnected IC structure. To investigate the effects of the grain sizes and temperature on the mechanical properties of nanocrystalline Cu, molecular dynamics simulations of uniaxial tensile test are performed in this study. The results show that the elastic modulus of nanocrystalline Cu with grain sizes of 4.65-12.41 nm gradually increases with the increase of the mean grain sizes, the corresponding flow stress concurrently increases, and the flow stress is proportional to the square-root of the grain size, which satisfies the inverse-Hall-Petch relation. Furthermore, the elastic modulus linearly decreases with the increase of temperature. The coupled effect of the flow stress, strain rate and temperature were elaborated by the Arrhenius hyperbolic sinusoidal model. Meanwhile, the deformation activation energy of nanocrystalline Cu for various grain sizes were obtained. All of the tensile simulation tests confirmed that the mechanism of plastic deformation for nanocrystalline Cu with 4.65-12.41 nm grain sizes is mainly specific to the grain boundary sliding and grain rotation. The dislocation nucleation and migration, which is usually the deformation mechanism of plasticity for macroscopic materials, is no longer the dominant factor for nanocrystalline Cu.

关键词： Free energy landscapes   Computer simulation   Arrhenius equation   Molecular dynamics   Kinetics and dynamics  

摘要： Using atomistic molecular dynamics simulations, we study the temperature dependence of the mechanical unfolding of a model supramolecular complex, a dimer of interlocked calixarene capsules. This system shows reversible transitions between two conformations that are stabilized by different networks of hydrogen bonds. We study the forced dissociation and formation of these networks as a function of temperature and find a strong impact of the nonequilibrium conditions imposed by pulling the system mechanically. The kinetics of the transition between the two conformations is ideally suited to investigate the range of validity of the stochastic models employed in the analysis of force dependent kinetic rates obtained from experimental or simulation data. These models usually assume activated dynamics for the relevant transitions, and therefore, the analytical expressions for the kinetic rates are of an Arrhenius form. A study of the temperature- and force-dependent kinetics by simulation allows an analysis of the transition rates without any model assumption. We find that the temperature dependence of the rates is well described by an Arrhenius law for each value of the force. This enables us to determine the activation free energy and the bare kinetic rate as a function of force independent of each other. In accord with the common model assumptions, we find that the activation free energy decreases with increasing force. The force dependence of the bare rates is compatible with the results of model calculations in the low force regime, and deviations are observed at high forces.

关键词： Self organization   Molecular structure   Alkyls   Molecular interactions   Macrocycles  

摘要： The intermolecular interactions underlying self-assembly are important for designing supramolecular architectures with defined function and composition over various length scales. Here, an integrated quantum mechanical/molecular dynamics (QM/MD) approach is introduced to explore the intermolecular interactions governing the two-dimensional self-assembly of a shape-persistent macrocycle, that is, tricarbazolo triazolophane (tricarb), on highly ordered pyrolytic graphite (HOPG). Three structural motifs (row, zig-zag, and rosette) for the self-association and growth of tricarb oligomers are analyzed using dispersion-corrected density functional theory to reveal their relative stability. QM analysis shows that the major factor in the self-assembly, via H-bonding contributions to the interaction energies, originates from favorable local dipole orientations between triazole units in the zig-zag and rosette motifs. Closing of the macrocyclic loop leads to the formation of a tertiary structure, which induces stability for the rosette formation relative to other motifs. MD simulations of these motifs on HOPG demonstrate the stability of the rosette, corroborating the QM results. In addition, the effect of long alkyl chains on the self-assembly dynamics is explored by simulating preexisting honeycomb domains of tricarbs with various alkyl chain lengths, without and with the presence of an explicit solvent. MD simulations reveal that alkyl chains mediate the formation of a honeycomb pattern, as observed experimentally.

关键词： Extracellular freezing   Elastic modulus   Diameter change   Pines sylvestris   Alnus glutinosa   Tilia x europaea  

摘要： When freezing occurs in trees in the autumn living cells are able to withstand the rapid dehydration that they experience due to the chemical properties of ice that fills apoplastic spaces. Elasticity and hydraulic permeability of living tissue are important properties influencing the frost tolerance of trees, because of their effects on how rapidly the dehydration occurs. Stem diameter change measurements have been used to quantify these tissue properties during the summer, but no such attempt has yet been made during winter when the apoplast is frozen. Here the dynamics of xylem, phloem and whole stem diameter changes for three tree species were simulated by formulating a mechanistic model using temperature of ice as a driver for water exchange between symplast and apoplast in accordance with theory of extracellular freezing. Hence in our model formulation, a decrease in the temperature of the apoplastic ice leads to a decrease in the apoplastic water potential drawing water from the living cells, which could be also observed in diameter change measurements in the field. The model was successful in explaining diameter changes in the case of all tree species when modelled diameter changes were fitted against measured diameter changes. Estimates for the elastic modulus, the time constant of diameter changes and the hydraulic permeability at tissue level were obtained by the fitting of three model parameters. The elastic modulus of the living tissue was found to exhibit a negative temperature dependency, while the time constant of diameter changes was found to exhibit a positive temperature dependency. Overall, this modelling approach offers an easy and non-destructive way of gaining valuable information about physiological properties of trees and their tissues in the winter. However, a complete understanding of the parameters estimated by the model requires further investigation into the physical processes that result in winter diameter changes.

关键词： Elastic energy   Fracture mechanics   Molecular dynamics   Force field   Stress strain relations   Corrosion   Elastic modulus  

摘要： Using molecular dynamics simulations, we study the fracture initiation of grain boundaries in NiCr2O4 and FeCr2O4 of spinel structure. These compounds are representative of corrosion layers of nickel-chromium-iron austenitic stainless alloys. Uniaxial deformation is applied to several symmetric tilt-, twist-, and random-grain boundaries until complete decohesion is reached, in order to measure the critical cleavage stresses. We find that this mechanical quantity depends on the chemical composition and the structure of the grain boundaries. A correlation between the critical stress and the misorientation angle of the grain boundaries can be established. It is also found than the twist- and the random-grain boundaries exhibit the weakest resistance. Furthermore, these simulations show a localization of the elastic response in the vicinity of the grain boundary plane. Therefore, we were able to compare our simulation results with those provided by the theoretical model based on the universal binding energy relation that relates the critical stress to the cleavage grain boundary energy. It shows that this model provides similar critical stresses than the molecular dynamics simulations if the localization distance parameter is defined as the distance related to the deformation of the grain boundary thickness during the deformation.