关键词： Molecular dynamics  

摘要： Bombyx morisilk fibroin (B. moriSF) is emerging as a promising biomaterial for its application in biomedical field such as hard and soft tissue engineered grafts considering it is a load-bearing polymer with biocompatible and biodegradable characteristics.B. moriSF being a biopolymer is made up of amino acids (residues) comprising crystalline and amorphous domains at nanoscale. In general, mechanical behavior of a biomaterial is actively governed by its chemical environment. Extent of water present in silk can significantly affect its mechanical behavior and alter structural functions at the molecular level. Hence, it is essential to understand the mechanical behavior and molecular deformation mechanics ofB. moriSF at the building block level under varying hydration conditions. In current investigation, tensile mechanical behavior of two characteristic computational models ofB. moriSF, along the fiber axis, under varying hydration levels (0 to 70 wt%) has been studied using molecular dynamics simulations. Elastic modulus values in the range of 7 to 11 GPa and 8 to 13 GPa were obtained for the two models for varying hydration levels. Analyses reveal that the hydrophobic effect has a dominating role at lower hydration levels resulting in an enhanced interaction between protein chains. This results in mechanical strengthening ofB. moriSF nanostructure. However, at higher hydration levels, the osmotic pressure plays a dominating role, resulting into screening of interatomic interaction between protein chains thus giving a weakening effect. Overall, this work contributes toward developing an understanding of mechanistic interactions between different protein phases (amorphous and crystalline regions) and water in hydratedB. moriSF nanostructure, which is useful for explorations in silk-based biomaterials.

关键词： Molecular dynamics   Deposition   Indentation   Composition   Dislocation  

摘要： Molecular dynamics (MD) simulations have been employed to simulate the deposition process of Cu and Ni atoms on the Ni (001) substrate with the different compositions in the CuxNi100-x film. Then, the mechanical characteristics of CuxNi100-x/Ni substrate are investigated during the nanoindentation process. The deformation behavior in the nanoindentation process is also investigated when the parameter for velocity, temperature changes. The lattice constant of CuNi alloy increases as Cu content in CuNi alloy rising. In the case of different contents of Cu atoms, the force, hardness of the CuxNi100-x/Ni substrate reduce as increasing the content of Cu. The shear strain and von Mises stress region increase as increasing the temperature and the loading velocity after the loading process. The plastic and elastic deformation of the CuxNi100-x/Ni substrate formed during nanoindentation. The force and hardness of the CuxNi100-x/Ni substrate reduce as increasing the temperature. In addition, with the higher loading speed, the force and hardness values become larger. The dislocation density increases as increasing the indentation depth after the loading process.

关键词： Thermo-mechanical refrigeration   Isobaric expansion engine   Refrigerants selection   Waste heat   VCR  

摘要： Refrigeration and air conditioning systems consume about 17% of the world-wide electricity and their conventional refrigerants cause ozone depletion and global warming. In this study a novel thermomechanical refrigeration (TMR) system is developed and analyzed that is powered, instead of electricity, by thermal energy from waste heat or renewable sources in the ultra-low temperature range of 60- 100 degrees C. A novel isobaric expander-compressor unit (ECU) is designed and combined with vapor compression refrigeration cycle to constitute the TMR system. The technological solutions (mainly towards simplification of the design) are crucial components of the study novelty. The suitable refrigerants for the system are systematically investigated, analyzed and selected from a list of 43 refrigerants. Nine fluids for the power loop (the isobaric expansion cycle) and nine fluids for the cooling loop (the thermal refrigeration cycle) were selected and compared based on their mode of operation (subcritical and supercritical), environmental effects and safety class. It is found that the HFO refrigerants such as R1234yf and R1234ze have acceptable performance with no ODP and very low GWP. Natural refrigerants R717 (ammonia) has the best performance in subcritical mode with toxicity as the main drawback. At heat source temperatures less than 85 degrees C, the system operation in subcritical mode is more efficient and more compact than in the supercritical mode. Thorough analysis and recommendations are made for the size of the ECU in terms of the diameters of the expander and the compressor. (c) 2020 Elsevier Ltd and IIR. All rights reserved.

关键词： Mechanical behavior   Zirconia nanopillars   Phase transformation   Molecular dynamics  

摘要： A novel stress-strain relation with two stages of linear elastic deformation is observed in [001]-oriented tetragonal zirconia nanopillars subjected to tensile loading via molecular dynamics simulation. This phenomenon results from a phase transformation from the tetragonal structure to the monoclinic one. Detailed explorations including the crystallographic structural analysis and the atomic strain calculation have been made to further elucidate the stress-strain curve. The lattice orientation strongly affects the plastic deformation mechanism, i.e., the [001]and [011]-oriented nanopillars experience the phase transformation under tensile loading, while the brittle fracture is induced for the orientation lying along the [111] direction. Complementary uniaxial compressive tests are performed to study the loading direction dependence. The deformation mechanism differs for the nanopillars with the same lattice orientation but different loading directions;for instance, under compressive loading, the plastic deformation behavior for [001]-oriented nanopillar is governed by intense dislocation activity rather than phase transformation. Additionally, a significant temperature effect is observed, with Young's modulus decreasing linearly from 323.97 to 283.55 GPa as the temperature increases from 300 to 1400 K. This work will help deepen the understanding of the tetragonal-to-monoclinic transformation and nanoscale mechanical behavior of zirconia.

关键词： Mg2Si thermoelectric material   Nanotwin   Mechanical properties   Molecular dynamics  

摘要： The excellent thermoelectric and mechanical properties of thermoelectric materials are the important guarantee for their industrial application. The introduction of twin boundary is an effective way to improve the thermoelectric properties of thermoelectric materials. However, our previous work shows the twin boundary weakens the mechanical properties of Mg2Si, and the role of twin size on mechanical properties remains unexplored so far. To determine the size effect of nanotwins on mechanical properties of Mg2Si, we applied molecular dynamics simulation to investigate the shear stress responses of nanotwinned Mg2Si with different sizes under the shear loading along its most plausible slip system (1-1-1)/<1 1-2>. The results show that the shear modulus and model size of nanotwinned Mg2Si increase and approach to the value of single crystal Mg2Si with the increasing twin size. While the ultimate strength does not change significantly with the increasing twin size. Under pure shear loading, the Mg-Si bond at the twin boundary breaks, leading to the failure of nanotwinned Mg2Si. The twin size has little influence on the failure of nanotwinned Mg2Si. Furthermore, the material failure mechanism and the weakening effect of temperature on materials are also discussed.

关键词： Graphene/Epoxy nanocomposites   Differential scanning calorimetry   Mechanical properties   Nanoindentation  

摘要： New graphene/polymer nanocomposites were prepared using graphene nanoplatelets (GNPs) and the epoxy system Epilok 60-566/Curamine 32-494. The GNPs were first dispersed into the curamine hardener using bath ultrasonication, followed by the addition of the epoxy resin. The cure kinetics were studied by DSC under non-isothermal and under isothermal conditions. The kinetic parameters of the curing process were determined using the non-isothermal Kissinger and Ozawa-Flynn-Wall models. The degree of curing increased with the addition of GNPs, while the activation energy decreased by 13.7 % for the primary amine reaction and by 6.6 % for the secondary amine reaction with epoxy groups as obtained from Kissinger. An increase in thermal stability by the addition of GNPs was identified in the range of 360-580 degrees C using TGA. In terms of mechanical properties, addition of an optimum amount of 0.5 %wt of GNPs in the hardener improved the Young's Modulus by 37 %. Nanoindentation measurements showed 9.4 % improvement in hardness at 0.7 %wt.

关键词： Soot formation   MD simulation   n-decane   Pyrolysis   Mechanism  

摘要： The chemical-kinetic mechanism of soot formation for the n-decane pyrolysis was comprehensively investigated from theoretical perspective. We found the formation of carbonaceous nanoparticle with a multi-layer closed cylindrical structure needs four processes including initial rings formation, PAH growth, particle nucleation via coalescence of PAH and particle growth by coagulation or surface reactions. The mechanisms of HACA, CAHM and CHRCR play crucial roles in the processes of the formation and growth of PAH molecules. In addition, the temperature effect was discussed and found the higher temperature can accelerate the formation of soot particle, which is in agreement with the experiment findings.

关键词： Mathematical model   Viral dynamics   Influenza  

摘要： When analysing in vitro data, growth kinetics of influenza virus strains are often compared by computing their growth rates, which are sometimes used as proxies for fitness. However, analogous to mathematical models for epidemics, the growth rate can be defined as a function of mechanistic traits: the basic reproduction number (the average number of cells each infected cell infects) and the mean generation time (the average length of a replication cycle). Fitting a model to previously published and newly generated data from experiments in human lung cells, we compared estimates of growth rate, reproduction number and generation time for six influenza A strains. Of four strains in previously published data, A/Canada/RV733/2003 (seasonal H1N1) had the lowest basic reproduction number, followed by A/Mexico/INDRE4487/2009 (pandemic H1N1), then A/Indonesia/05/2005 (spill-over H5N1) and A/Anhui/1/2013 (spill-over H7N9). This ordering of strains was preserved for both generation time and growth rate, suggesting a positive biological correlation between these quantities which have not been previously observed. We further investigated these potential correlations using data from reassortant viruses with different internal proteins (from A/England/195/2009 (pandemic H1N1) and A/Turkey/05/2005 (H5N1)), and the same surface proteins (from A/Puerto Rico/8/34 (lab-adapted H1N1)). Similar correlations between traits were observed for these viruses, confirming our initial findings and suggesting that these patterns were related to the degree of human adaptation of internal genes. Also, the model predicted that strains with a smaller basic reproduction number, shorter generation time and slower growth rate underwent more replication cycles by the time of peak viral load, potentially accumulating mutations more quickly. These results illustrate the utility of mathematical models in inferring traits driving observed differences in in vitro growth of influenza strains.

关键词： cashmere fibers   microwave vacuum drying   drying models   effective moisture diffusivity   mechanical properties  

摘要： Drying behavior of low temperature microwave vacuum-dried cashmere fibers and the drying rate at different times were investigated with different microwave powers, vacuum degrees and maximum drying temperatures to specify the optimum drying conditions in consideration of performance and drying efficiency in this study. The results show that higher microwave power can reduce the total drying time and significantly increase the drying rate in the initial stage. The degree of vacuum has a negligible effect on the drying behavior, and the influence of the three factors on effective moisture diffusivity is consistent with the drying behavior. Compared with hot-air-dried fibers, the mechanical properties of low temperature microwave-dried cashmere fibers could be significantly improved. In addition, excessive drying as delayed or overheated treatment will damage the fiber. The Page model is the most appropriate to describe the microwave vacuum drying of cashmere fibers.

关键词： Ni-based single crystal superalloys   Molecular dynamics simulation   Cyclic deformation   Fatigue behavior   Microstructural evolution  

摘要： The fatigue performance and deformation mechanism of Ni-based single crystal superalloys under cyclic tension-compression loading are studied by molecular dynamics simulations. The effects of temperature and strain rate on the cyclic deformation of superalloys are discussed. The results show that there are three different cyclic deformation mechanisms at different temperature ranges. In the low temperature range, dislocations and stacking faults shearing gamma' phase is the main deformation mechanism. In the high temperature range, Orowan bypassing and climbing mechanism dominates the deformation. While in the medium temperature range, the deformation mechanism gradually transforms from the dislocation and stacking faults shearing the gamma' phase to the Orowan bypassing and climbing. The simulation results also reveal that the superalloys exhibit a cyclic saturation stage after the initial cyclic hardening. The cyclic saturation stage is a dynamic equilibrium of dislocation proliferation and annihilation, indicating that the superalloys have excellent fatigue mechanical properties. Moreover, the dislocation density and the proportion of stacking faults increase with increasing strain rate, resulting in faster cyclic stability and higher stress amplitude for the superalloys. These results provide important information for understanding the fatigue mechanisms of superalloys from an atomistic perspective.