关键词： Monolayers  

摘要： Recently, laser-assisted chemical vapor deposition has been used to synthesize a free-standing, continuous, and stable monolayer amorphous carbon (MAC). MAC is a pure carbon structure composed of randomly distributed five, six, seven, and eight atom rings, which is different from that of disordered graphene. More recently, amorphous MAC-based nanotubes (a-CNT) and nanoscrolls (a-CNS) were proposed. In this work, we have investigated (through fully atomistic reactive molecular dynamics simulations) the mechanical properties and sublimation points of pristine and a-CNT and a-CNS. The results showed that a-CNT and a-CNS have distinct elastic properties and fracture patterns compared to those of their pristine analogs. Both a-CNT and a-CNS presented a non-elastic regime before their total rupture, whereas the CNT and CNS underwent a direct conversion to fractured forms after a critical strain threshold. The critical strain values for the fracture of the a-CNT and a-CNS are about 30% and 25%, respectively, and they are lower than those of the corresponding CNT and CNS cases. Although less resilient to tension, the amorphous tubular structures have similar thermal stability in relation to the pristine cases with sublimation points of 5500 K, 6300 K, 5100 K, and 5900 K for a-CNT, CNT, a-CNS, and CNS, respectively. An interesting result is that the nanostructure behavior is substantially different depending on whether it is a nanotube or a nanoscroll, thus indicating that the topology plays an important role in defining its elastic properties.

关键词： Sodium chloride  

摘要： The lowest band in the charge-transfer-to-solvent ultraviolet absorption spectrum of aqueous chloride ion is studied by experiment and computation. Interestingly, the experiments indicate that at concentrations up to at least 0.25 M, where calculations indicate ion pairing to be significant, there is no notable effect of ionic strength on the spectrum. The experimental spectra are fitted to aid comparison with computations. Classical molecular dynamic simulations are carried out on dilute aqueous Cl-, Na+, and NaCl, producing radial distribution functions in reasonable agreement with experiment and, for NaCl, clear evidence of ion pairing. Clusters are extracted from the simulations for quantum mechanical excited state calculations. Accurate ab initio coupled-cluster benchmark calculations on a small number of representative clusters are carried out and used to identify and validate an efficient protocol based on time-dependent density functional theory. The latter is used to carry out quantum mechanical calculations on thousands of clusters. The resulting computed spectrum is in excellent agreement with experiment for the peak position, with little influence from ion pairing, but is in qualitative disagreement on the width, being only about half as wide. It is concluded that simulation by classical molecular dynamics fails to provide an adequate variety of structures to explain the experimental CTTS spectrum of aqueous Cl-.

关键词： Ultra-confined water   C-S-H   Molecular dynamics   Nanopore   Mechanical performance Construction  

摘要： The properties of water ultra-confined in the calcium-silicate-hydrate (C-S-H) affect the mechanical properties and durability of concrete. In this paper, the structure, bonding, dynamics, as well as mechanical properties of C-S-H with ultra-confined water were characterized based on molecular dynamics simulations. The results suggest that the nanopore size has a significant impact on the distribution of water molecules, coordination number of Si-Ow and Ca-Ow and H-bonds. In terms of dynamic features, the penetration of water may weaken the stability of these chemical bonds. The movements of water and Ca, on the other hand, were restricted by various chemical interactions as well as the ultra-confinement environment and the diffusion behavior of water in the nanopore less than 1 nm is isotropic and quasi-two-dimensional. As the increase of nanopore size, the mechanical attributes of C-S-H decrease remarkably and the C-S-H is more easily to be damaged since the increasing of water reduces the cohesion of C-S-H and increases its brittleness. Hopefully, this study could provide some new insights into the characteristics of C-S-H with ultra-confined water. (C) 2021 Published by Elsevier Ltd.

关键词： High entropy alloy   Molecular dynamics   Nanoindentation   Scratch test   Mechanical property   Tribological property  

摘要： In this work, an improved molecular dynamics (MD) model is developed to simulate the nanoindentation and tribological tests of additively manufactured high entropy alloys (HEA) AlCoCrFe coated on an aluminum substrate. The model shows that in the interface region between the HEA coating and Al substrate, as the laser heating temperature increases during the HEA coating additive manufacturing process, more Al in the substrate is melted to react with other elements in the coating layer, which is qualitatively in agreement with experiment in literature. Using the simulated nanoindentation tests, the calculated Young's modulus of pure Al and Al with HEA coating is 79.93 GPa and 119.30 GPa, respectively. In both our simulations and the experimental results in the literature, the hardness of Al with the HEA coating layer is about 10 times higher than the Al hardness, indicating that HEA can significantly improve the hardness of the metallic substrate. Using the simulated tribological scratch tests, the computed wear tracks are qualitatively in agreement with experimental images in literature. Both our model and experiment show that the Al with HEA coating has a much smaller wear track than that of Al, due to less plastic deformation, confirmed by a dislocation analysis. The computed average coefficient of friction of Al is 0.62 and Al with HEA coating is 0.14. This work demonstrates that the HEA coating significantly improves the mechanical and tribology properties, which are in excellent agreement with the experiments reported in the literature.

关键词： Sulfide precipitation   CFD   Supersaturation level   Gas-liquid mass transfer   Heavy metal wastewater  

摘要： The production of fine particles from extremely high supersaturation has challenged the application of sulfide precipitation in treating heavy metal wastewater due to the difficulty of solid-liquid separation. To this end, a gas-liquid sulfide precipitation reactor for the removal of Cu2+ was designed by controlling the mass transfer and supersaturation levels during sulfidation processes. Particularly, a computational fluid dynamics (CFD) model of the reactor, integrating sulfidation reaction kinetics with two-phase flow hydrodynamics, was first built, followed by examining the effects of H2S(g) bubble diameter and flow rate. Based on the CFD simulation, the rate-limiting step of the gas-liquid sulfide precipitation reaction is the gas-liquid mass transfer process. Either reducing H2S(g) bubble diameter or increasing H2S(g) flow rate can result in the control of reaction rate and supersaturation level in the system. In order to validate the CFD simulations, we measured Cu2+ concentrations during the sulfidation process with the batch experiments. The agreement between computational and experimental results indicated that our mechanistic model can provide a protocol for the design and optimization of the reaction system, allowing one to visualize the time-dependent reaction process and evaluate the performance of a reactor.

关键词： Graphene reinforced polycarbonate nanocomposites   Coarse-grained molecular dynamics   Heterogeneous deformation   Viscoelastic property  

摘要： Incorporating graphene nanosheets into a polymer matrix is a promising way to utilize the remarkable electronic, thermal, and mechanical properties of graphene. However, the underlying mechanisms near the graphenepolymer interface remain poorly understood. In this study, we employ coarse-grained molecular dynamics (MD) simulations to investigate the nanoscale mechanisms present in graphene-reinforced polycarbonate (GRPC) nanocomposites and the effect of those mechanisms on GRPC's mechanical properties. With a mean-squared displacement analysis, we find that the polymer chains near the GRPC interface exhibit lower mobility than the chains further from the graphene sheet. We also show that the embedding of graphene increases Young's modulus and yield strength of bulk PC. Through non-equilibrium MD simulations and a close look into the deformation mechanisms, we find that early strain localization arises in GRPC with voids being concentrated further away from the graphene sheet. These results indicate that graphene nanosheets promote the heterogeneous deformation of GRPC. Additionally, to gain deeper insight into the mechanical, interfacial, and viscoelastic properties of GRPC, we study the effects of varying PC chain lengths and interfacial interactions as well as the comparative performance of GRPC and PC under small amplitude oscillatory shear tests. We find that increasing the interfacial interaction leads to an increase in both storage and loss moduli, whereas varying chain length has minor influence on the dynamic modulus of GRPC. This study contributes to the fundamental understanding of the nanoscale failure mechanisms and structure-property relationships of graphene reinforced polymer nanocomposites.

关键词： Multi-abrasive   SiC substrate   FAP   Removal behaviors   MD simulation  

摘要： Molecular dynamics (MD) as a powerful simulation tool revealed the mechanistic removal behaviors of SiC substrates using a fixed abrasive polishing (FAP) with randomly distributed multi-abrasive particles in a single cycle scratching at nano-scales. This study presents features of surface morphology, subsurface damage and temperature distribution of SiC substrates in nano-abrading. It is demonstrated that the exposed height and abrasive distribution of multi abrasives in a single pad asperity (SPA) dominates the removal behaviors of SiC substrates. MD simulation reveals that the random distribution of diamond abrasives in FAP pad would worsen the processing quality. Our investigation sheds new insights into the mechanical removal mechanisms of SiC in FAP at an asperity-scale.

关键词： MoS2   nanotubes   Stillinger-Weber   molecular dynamics  

摘要： We investigate the tensile properties of MoS2 nanotubes by the implementation of a set of molecular dynamics runs, using a recently developed version of the Stillinger-Weber (SW) potential. The nanotubes considered are of the H and T polytypes, with zigzag and armchair chirality. We found that only nanotubes of diameter greater or equal than 30 angstrom are stable when modelled by the SW potential. Zigzag nanotubes have a larger elastic modulus than armchair nanotubes of the same polytype and diameter, and T nanotubes deform more easily than H nanotubes. We also found that elastic modulus, tensile strength, and point of rupture depend on diameter when the diameters are less than 60 angstrom. We investigate the role of defects on the mechanical response, finding that, while elastic modules is not appreciably affected by the presence of vacancies, tensile strength, and point of rupture value decrease significantly, in particular when the defect is a vacancy of a whole MoS2 unit. The presence of a defect also affects the nature of the rupture, with the fracture becoming brittle. Increasing the temperature makes Young's modulus and tensile strength decrease. The mechanical response of chiral nanotubes is also investigated.

关键词： Photovoltaic technology   Solar-powered agricultural robots   Electric tractors   Mechanized agriculture  

摘要： With an uprising trend in cutting agriculture's reliance on fossil fuels because of their limited supply and associated adverse impacts on the environment, the use of infrastructures adapted with alternative energy sources would be of crucial necessity. Among all renewable sources, solar energy has the highest compatibility with agricultural activities. The emergence of photovoltaic (PV) solar energy conversion technology in agriculture diminishes the need for oil-based fuels in this sector, offering a more affordable and sustainable electricity generation technique, and causing a remarkable reduction in greenhouse gas (GHG) emissions. In modern farm activities, the need for electrification has been raised, leading to the creation of a great opportunity for the employment of PV technology in this sector. The current study investigates the integration of PV technology with electric farm tractors and agricultural robots by dis-cussing research works and commercial case studies. The results indicate that two major challenges against the widespread deployment of modern solar-powered electric farm machinery are high initial costs mainly associated with PV modules and battery storage units, along with deficiencies in electricity storage technologies. Due to operating in outdoor conditions, the effect of environmental parameters on the performance of PV modules integrated with agricultural machines including surface temperature, dust accumulation, shading, and air humidity should also be considered. Further technical improve-ments, cost reductions, and securing government incentives can facilitate the real-world deployment of these sustainable machines. This study is expected to promote the integration of PV technology with modern electric agricultural machinery and to encourage farmers, producers, planners, and decision-makers to employ this technique in mechanized agriculture by giving them an insight into the latest advances along with major challenges. (c) 2021 Elsevie

关键词： Atoms  

摘要： The influence of the number of atoms, N = 3000, 5000, 7000, and 9000 atoms, at temperature T = 300 K and temperatures T = 300, 500, 700, 900, 1100, 1300, and 1500 K at N = 9000 atoms, on microscopic structure, phase transition temperature, and mechanical property of bulk aluminium in an amorphous state is studied by the molecular dynamics method with the Sutton-Chen embedded interaction potential and the periodic boundary condition. Structural results are analyzed through the radial distribution function, the total energy of the system, the size, and the common neighbor analysis. The phase transition temperature is determined by the relationship between the total energy of the system and temperature. The mechanical property is derived from the deformation along the Z-axis. It can be noted that when the number of atoms increases, the first peak's position for radial distribution function changes, the first peak's height decreases, the number of FCC and HCP structural units decreases, the number of Amor structural units increases, and the total energy of system increases. It can be seen that when temperature increases, the first peak's position changes, the first peak's height decreases, the number of FCC and HCP structural units decreases, the number of Amor structural units increases, and the total energy of the system decreases. The obtained results are very useful for experimental studies in the future.