Papers by Keyword: Molecular Dynamic Simulation

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Authors: Qing Hua Zeng, Ai Bing Yu, Gao Qing Max Lu
Abstract: Polymer nanocomposites are recognized as the next generation of polymer composites due to their exceptional properties. Understanding the molecular origin of the reinforcement mechanism is crucial to the development of such promising materials. This paper reports our recent molecular dynamic study on clay-based polyurethane nanocomposites. The effect of clay platelets on phase separation behavior of polyurethane, at the clay-polyurethane interface, is quantified in terms of molecular interactions, structure and dynamics. The results show that the nanoconfinement of polyurethane chains in clay gallery impedes the development of phase separation commonly observed in bulk polyurethane. The absence of phase separation of intercalated polyurethane is believed to be related to the competitive interactions among clay platelet, polyurethane and surfactant.
Authors: Seyed Vahid Hosseini, Mehrdad Vahdati, Ali Shokuhfar
Abstract: Today, there is a need to understand the micro mechanism of material removal to achieve a better roughness in ultra precision machining (UPM). The conventional finite element method becomes impossible to use because the target region and grids are very tiny. In addition, FEM cannot consider the micro property of the material such as atomic defect and dislocation. As an alternative, molecular dynamics (MD) simulation is significantly implemented in the field of nano-machining and nano-tribological problems to investigate deformation mechanism like work hardening, stick-slip phenomenon, frictional resistance and surface roughness [1]. One of the machining parameters than can affect nano-cutting deformation and the machined surface quality is tool nose radius [2]. In this paper molecular dynamics simulations of the nano-metric cutting on single-crystal copper were performed with the embedded atom method (EAM). To investigate the effect of tool nose radius, a comparison was done between a sharp tool with no edge radius and tools with a variety of edge radii. Tool forces, coefficient of friction, specific energy and nature of material removal with distribution of dislocations were simulated. Results show that in the nano-machining process, the tool nose radius cannot be ignored compared with the depth of cut and the edge of tool can change micro mechanism of chip formation. It appears that a large edge radius (relative to the depth of cut) of the tool used in nano-metric cutting, provides a high hydrostatic pressure. Thus, the trust force and frictional force of the tool is raised. In addition, increasing the tool edge radius and the density of generated dislocation in work-piece is scaled up that is comparable with TEM photographs [6].
Authors: Shuang Xu, Ya Fang Guo
Abstract: Molecular dynamics (MD) simulations with an EAM potential are carried out to study the strain rate effects on the tensile deformation of single-crystal copper films. The stress, the atomic energy, as well as the atomic configurations of the systems are presented to explore the strain rate effects on copper films. It is found that yield stress increases with loading rate. Meanwhile, deformation mechanisms with different strain rates are analyzed in the present work. At lower strain rate, slips along {111} planes are primarily responsible for the plastic deformation in nano-Cu films. As strain rate increased, the motion of dislocations becomes easier, a transition of the deformation mechanism from sequential propagation of slips along well-defined slip planes to complex cross-slip.
Authors: Lan He, Kai Leung Yung, Yun Wen Shen, Yan Xu
Abstract: The rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel with different surface roughness are investigated by molecular dynamics (MD) simulations. Simulation results show the surface roughness has great impact on the rheological properties and phase orientation of LCP melts in the nanochannel (cross section is 12nm). As the amplitude of serrations increases, the shear viscosity increases nonlinearly and the value of orientational order parameter decreases. When the serration amplitude is larger than 1.1nm, a phase transition (from nematic to isotropic phase) of LCP melt happens, which makes flowing in nanochannels more difficult. On the other hand, the influence of serration period on the shear viscosity and orientational order parameter are found not so obvious. Findings in this study will be helpful for injection molding plastic products with nanofeatures.
Authors: Kyung Soo Kim, Seung Cheol Lee, Kwang Real Lee, Pil Ryung Cha
Abstract: Developments of tetrahedral amorphous carbon (ta-C) films having low residual compressive stress are essential to extend the applicability of the films. The annealing of the ta-C films was known to be an effective way for the reduction the stress of the films. However, the effects of annealing on the atomic structure of ta-C films have not been fully understood. The atomic structure changes by the annealing were studied using molecular dynamics simulation. The simulation showed that the annealing caused an increase of the atomic volume of ta-C film, which explained the stress reduction partially. However, the tendency of the stress reduction was different to high and low stress films. The annealing substantially reduced the stresses of high stress films compared to those of low stress films. Atomic structure analysis showed that the reason for the asymmetric stress reduction resulted from the relaxation of highly distorted bonds that existed in as-deposited films.
Authors: M. Montorsi, Maria Cristina Menziani, Cristina Siligardi, Tiziano Manfredini, A.N. Cormack
Authors: Li An Chen, Hong Jing Li
Abstract: Recently, there were many accident of small dam break of reservoir in China, which reflects a problem in construction and management of small reservoir. By studying over 306 reservoirs which have been reinforced, the main problems to the safety operation of reservoir were concluded. The main 17 problems and 10 questions were discussed in this paper and the answers and suggestions are also made.
Authors: Li Li Zhou, Chong Xing, You Lin Peng
Abstract: The molecular dynamics simulation studies on the microstructure evolution properties of amorphous Ca7Mg3 alloy during the isothermal annealing have been performed. The simulated structure factor S(q) of Ca7Mg3 is well agreed with the experimental data. Results indicate that the icosahedron structure plays dominate role in the structure transformation, and the smaller Mg atoms are much more probable to be the central atoms of the basic clusters of icosahedron.
Authors: Jens Ribbe, Guido Schmitz, Sergiy V. Divinski
Abstract: Grain boundary (GB) diffusion of 59Fe in high purity polycrystalline copper was measured using the radiotracer technique and precision parallel sectioning in an extended temperature interval. The results of diffusion measurements below 900 K are consistent with Harrison's C kinetics and yield the GB diffusion coefficient of Fe in Cu with the pre-exponential factor of 5.610-6 m2/s and the activation enthalpy of 121 kJ/mol. Unexpectedly strong GB segregation of Fe in Cu hindered reliable determination of the Fe diffusivity in the B kinetics. Additionally, unconventional penetration profiles were measured for GB diffusion of 59Fe in Cu at high temperatures. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials was performed to shed light into the observed features. A strong effect of the Fe coverage on GB structure and kinetics in pure Cu is predicted at increased temperatures above 900 K.
Authors: Duk Yong Yoon, Young Kyu Cho, Hyun Min Jang
Abstract: Flat surfaces and grain boundaries lying on low crystal planes are singular corresponding to the cusps in the polar (Wulff) plots of their energy against their orientation. The theoretical analysis of the entropy effect at high temperatures shows that these interfaces undergo roughening transitions. The molecular dynamics simulations also show disordering to liquid-like structures at high temperatures that can be interpreted as the roughening transition. Experimentally, singular flat surfaces and grain boundaries become curved at high temperatures or with additives, indicating their roughening transition. The grain boundaries in polycrystals are often faceted with hill-and-valley shapes and their defaceting at high temperatures also show their roughening transition.
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