关键词： Molecular dynamics  

摘要： While spectroscopic data on small hydrocarbons in interstellar media in combination with crossed molecular beam (CMB) experiments have provided a wealth of information on astrochemically relevant species, much of the underlying mechanistic pathways of their formation remain elusive. Therefore, in this work, the chemical reaction mechanisms of C(P-3(J)) + C6H6 and C+(P-2) + C6H6 systems using the quantum mechanical molecular dynamics (QMMD) technique at the PBE0-D3(BJ) level of theory is investigated, mimicking a CMB experiment. Both the dynamics of the reactions as well as the electronic structure for the purpose of the reaction network are evaluated. The method is validated for the first reaction by comparison to the available experimental data. The reaction scheme for the C(P-3(J)) + C6H6 system covers the literature data, e.g. the major products are the 1,2-didehydrocycloheptatrienyl radical (C7H5) and benzocyclopropenyl radical (C6H5-CH), and it reveals the existence of less common pathways for the first time. The chemistry of the C+(P-2(J)) + C6H6 system is found to be much richer, and we have found that this is because of more exothermic reactions in this system in comparison to those in the C(P-3(J)) + C6H6 system. Moreover, using the QMMD simulation, a number of reaction paths have been revealed that produce three distinct classes of reaction products with different ring sizes. All in all, at all the collision energies and orientations, the major product is the heptagon molecular ion for the ionic system. It is also revealed that the collision orientation has a dominant effect on the reaction products in both systems, while the collision energy mostly affects the charged system. These simulations both prove the applicability of this approach to simulate crossed molecular beams, and provide fundamental information on reactions relevant for the interstellar medium.

关键词： Gallium nitride  

摘要： Chemical mechanical polishing (CMP) is a key manufacturing process for applying gallium nitride (GaN), especially the Ga-face GaN, to semiconductor devices such as laser diodes. However, the CMP efficiency for GaN is very low due to its high hardness and chemical stability. Experimentally, OH radicals appear able to improve the CMP efficiency of GaN polished by a SiO2 abrasive grain, whereas the mechanisms of the OH-radical-assisted CMP process remain unclear because experimental elucidation of the complex chemical reactions occurring among GaN substrate, abrasive grain, and OH radicals is difficult. In this work, we used our previously developed tight-binding quantum chemical molecular dynamics simulator to study the OH-radical-assisted CMP process of the widely employed Ga-face GaN substrate polished by an amorphous SiO2 abrasive grain in an effort to understand how OH radicals assist the CMP process and then aid the development of next-generation CMP techniques. Our simulations revealed that the OH-radical-assisted CMP process of GaN occurs via the following three basic reaction steps: (i) first, all hydrogen terminations on the GaN surface are replaced by OH terminations through continuous reactions with OH radicals;(ii) after the substrate is fully terminated by OH, the hydrogen atoms of these OH terminations are removed by reacting with newly added OH radicals, which forms H2O molecules and leaves energetic oxygen atoms with dangling bonds on the surface;and (iii) finally, these energetic oxygen atoms intrude inside the substrate with concomitant dissociation of Ga-N bonds and the generation of N-2 and gallium hydroxide molecules, which accumulatively lead to the removal of N and Ga atoms from the substrate.

关键词： mechanical properties   graphene   molecular dynamics   interfacial shear strength  

摘要： Due to its extraordinary properties, graphene has been widely used as reinforcing nanofillers to enhance the mechanical properties of polymer- or metal-based composites. However, the weak interfacial interaction between the matrix and graphene is still a major bottleneck that considerably hinders its reinforcing effectiveness and efficiency. This study presents an atomistic study via molecular dynamics simulation on a chemical modification strategy where the aluminium (Al) substrate is modified with Al2O3 (with or without covalent bonds formed between Al2O3 and graphene) or Al4C3 to achieve significantly improved interfacial shear strength and overall mechanical properties of graphene-reinforced aluminium (Al/Gr) composites. Numerical results show that this strategy works very well and among the three cases considered, modifying Al substrate by Al2O3 without covalent bonds formed at the interface between Al2O3 and graphene produces the strongest interfacial interaction and the best mechanical properties. In the presence of covalent bonds, however, the reinforcing effect is adversely affected due to the sp(2)-sp(3) bond transformation which partially degrades graphene. The present work provides, for the first time, valuable insight into the role of substrate surface modification on the mechanical performance of Al/Gr nanocomposites.

关键词： Adsorption   Layers   Silicon   Wear   Materials  

摘要： Reactive molecular dynamics (ReaxFF) simulations are performed to explore the atomistic mechanism of chemical mechanical polishing (CMP) processes on a Si(110) surface polished with an a-SiO2 particle. The Si surface is oxidized by reacting with water before the CMP process, and the O atoms of the oxidized Si surface mainly exist in the form of Si-O- dangling bonds and Si-O-Si bonds. In the CMP process, the insertion of O atoms into the surface, the formation of interfacial Si-O-Si and Si-Si bridge bonds, and the adsorption of H atoms on the surface-saturated Si atoms can all cause the surface bond breakage and even the Si atomic removal. The contributions of the four different kinds of tribochemical wear mechanisms to the surface wear decrease in turn and are much larger than that of mechanical wear. The results indicate that the material removal in the actual Si CMP process is the combined results of multiple atomic-level wear mechanisms. Furthermore, we find that the oxide layer of the Si surface plays an important role in the surface wear mainly by providing O atoms to insert into the surface, rather than by providing additional reaction pathways to form interfacial Si-O-Si bridge bonds. This work provides new and further insights into the process and mechanism of silicon removal during CMP.

关键词： Molecular dynamics  

摘要： In this work, the structural and dynamical properties of Zn2+ in pure liquid ammonia have been investigated via a quantum mechanical charge field molecular dynamics (QMCF MD) simulation approach. The average equilibrium Zn-N distance in the first solvation shell resulting from the radial distribution analysis is 2.15 angstrom. Even though the complex displays a high rate of ligand exchange compared to Zn2+ in aqueous solution, the result imply that NH3 is the preferred ligand compared to H2O. This result of this simulation studies provide a fundamental understanding of the characteristic of this metal ion in pure liquid ammonia, which is inter alia essential in the development of applications for CO2 capture based on Zn2+-containing NH3 solutions. (C) 2020 Published by Elsevier B.V.

关键词： Glass content   Coal fly ash   Sintering kinetics   Property   Direct sintering  

摘要： In this paper, coal fly ash and waste glass were used as raw materials to prepare CaO-Al2O3-SiO2 system glass-ceramics by direct sintering. The effects of glass content on the kinetics, phase, microstructure and mechanical properties of glass-ceramics were studied by means of liquid phase sintering formula, X-ray diffraction, scanning electron microscope and the corresponding property tests. The results showed that as glass content increased, the activation energy of glass-ceramics first decreased and then increased due to the precipitation of tridymite and augite, and the surface of glass-ceramics gradually became smooth and dense. The water absorption and total porosity of the glass-ceramics were decreased with increase of glass content,while the flexural strength and bulk density were increased. When the amount of waste glass added is 40% and the sintering temperature is 1100 degrees C, the glass-ceramics have dense surface and high bending strength, which is suitable for high-strength building glass-ceramics. The experimental results of this paper provide a basis for direct sintering industrial production of glass-ceramics.

关键词： Vertical heterostructure   Gr/WSe2   Molecular dynamics   Tensile properties   Phase transformation   Functional device  

摘要： Assembling layered materials in the form of vertically stacked heterostructures has emerged as a facile way to leverage the properties of each component and fabricate novel highly-tailored architectures with advanced functionalities. In this study, we employ molecular dynamics (MD) simulations to characterize the temperaturedependent tensile mechanical properties of graphene/WSe2 vertical heterostructure along both the armchair and the zigzag directions utilizing a hybrid scheme of various interatomic potentials. It is found that the tensile strength of the WSe2 monolayer can be significantly improved when supported by graphene benefiting from the synergistic effects of their individual strengths. The comparisons of the predicted tensile properties of the heterostructure with that of the mixing rule exhibit excellent conformity. We found that while the bare WSe2 is more resilient to fracture along the armchair direction, the graphene supported WSe2 layer endures larger tensile stress and strain for zigzag loading. This study further reveals fascinating chirality in mechanical properties and failure mechanism of the heterostructure. Fracture initiates from the graphene layer for armchair loading, whereas the WSe2 sheet becomes more prone to fracture under zigzag loading. Crack propagates along the zigzag direction and results in the complete tearing of the specimen for armchair loading, while the crack propagation is confined in a random small region for the zigzag loading. The WSe2 nanosheet experiences major atomic rearrangements and undergoes a phase transformation from trigonal (h-WSe2) to the distorted octahedral phase (t-WSe2) during zigzag loading. Increasing temperature is found to play a crucial role in deteriorating the mechanical properties of the heterostructure. This study offers a comprehensive characterization of the tensile properties of Gr/WSe2 vertical heterostructure and discloses important deformation mechanisms that will enable the efficient util

关键词： Molecular dynamics  

摘要： High energy ball milling of metallic powders leads to high reactivity in the milled mixture. The reaction is often faster and starts at a lower temperature. However, the mechanisms responsible for this high reactivity are not yet completely understood. The aim of this study is to evaluate one of the possible activating factors of this heightened reactivity: nano-scale mixing of the reagents. Molecular dynamics was used to analyze the role of an amorphous Ni-Al mixing layer, mimicking the powder microstructure after milling, between two Ni layers. The impact of temperature and stoichiometry was investigated in relation to the formation of the B 2-NiAl intermetallic compound. At low temperatures, pre-mixing does not seem to slow down the diffusion of Ni atoms in an amorphous Al region. Homogeneous nucleation was observed in this peculiar milled microstructure. These two phenomena explain why the nano-scale mixing observed experimentally after high energy milling is indeed an activating factor in the reactivity of metallic systems such as Ni-Al.