Molecular dynamics simulation of polyhedron analysis of Cu–Ag alloy under rapid quenching conditions

In this study, the formation mechanism of polyhedron clusters in Cu₅₀Ag₅₀ binary alloy system consisting of 50 000 atoms has been investigated by using molecular dynamics simulations based on embedded atom method (EAM) during the rapid cooling processes. The cluster-type index method (CTIM) has been used to describe the evaluation properties of clusters and the structural development has been investigated by using radial distribution function (RDF). The simulation results show that the amorphous phase is formed by the main bonded pairs of 1551, 1541 and 1431 in the system, and ideal icosahedral (icos) cluster (12 0 12 0) and other basic polyhedron clusters, such as defective icos, Frank–Kasper, Bernal polyhedron, play a critical role under the rapid cooling conditions. The results of our simulations that have been disclosed show that high cooling rate favors the icos and defective icos clusters for model alloy system.     

Molecular dynamics study of roughness and stress evolution using a Lennard–Jones potential

A three-dimensional molecular dynamics (MD) simulation is proposed to study the film growth, roughness and stress evolution during atom deposition on the (100) plane of a fcc regular crystal. We use the cubic system with an x–y periodic boundary condition. At the bottom we have an atomic surface and at the top a reflecting wall. The model uses the Lennard-Jones potential to describe the interatomic forces. The simulation results show that the film grows with the Volmer–Weber mode and exhibits specific curve shape of the stress evolution. The mean biaxial stress obtained during the simulation attains a local tension maximum at a coverage of two monolayers. The stress in the normal direction is smaller than the biaxial stress. The main contribution to the stress in the film arises from the first monolayer. The curves describing roughness possess maximum values at the same substrate coverage. The dependence of the roughness on the temperature is examined.     

Laser surface modification of Al–4Cu–1Mg alloy for enhanced thermal conductivity

The feasibility of enhancing thermal conductivity of Al–4Cu–1Mg alloy by depositing 80Cu–20Mo coating using high-power lasers has been examined. Coatings of 667±2.5 μm thickness were formed with metallurgically sound interface. Results showed an 86% increase in the thermal conductivity of Al–4Cu–1Mg alloy due to laser-deposited 80Cu–20Mo alloy coating. This coating approach can potentially be used on low coefficient of thermal expansion metal matrix composites to enhance their thermal conductivity in electronic devices.     

Employing response surface methodology and neural network to accurately model thermal conductivity of TiO2–water nanofluid using experimental data

In this research, the thermal conductivity of the H₂O–titania nanofluid is modeled versus the particle concentration and temperature via the Artificial Neural Network (ANN) and Response Surface Methodology (RSM). The experimental data include six particle concentrations and five temperatures from 30 to 70 °C. The thermal conductivity augments by the increment in nanoparticle concentration and temperature, such that the maximum thermal conductivity increment happens at the highest temperature and nanoparticle concentration (i.e., T = 70 °C and φ = 1%). It is observed that the impact of temperature on the thermal conductivity is more noticeable than the influence of particle concentration, however, the thermal conductivity demonstrates a more non-linear trend versus nanoparticle volume fraction compared with the temperature. The best structure of the neural network has 2 hidden layers with 2 and 4 neurons, respectively in the 1^st and 2^nd hidden layers. The results show that the prediction precision of the ANN correlation is better than that of the RSM correlation.     

Molecular dynamics simulation of graphene on Cu (111) with different Lennard-Jones parameters

The interaction between graphene and copper (111) surface have been investigated using the molecular dynamics simulations. The range of Lennard-Jones parameters which correspond to the binding energies and the binding distances calculated via ab initio methods was found. The dependencies of the binding energy, the binding distance and the graphene thickness on the parameters of the potential and the rotational angle are presented. We have found minima of the binding energy which can be related to experimentally observed Moiré superstructures.     

Molecular‐dynamics simulation of YBa2Cu3O7 at high temperatures

Molecular-dynamics simulation of YBa₂Cu₃O₇ is carried out to study the oxygen-atom distribution at high temperatures as the system undergoes the orthorhombic-to-tetragonal phase transition. While at the transition the one-dimensional arrangement of oxygen atoms in the Cul(0,0,0)-O4(0, 1/2, 0) chains begins to disorder, at temperatures above the transition the simulation also indicates the presence of a few vacancies at the O1(0, 0, z) sites. At still higher temperatures vacancies occur at all the oxygen sites. The simulation utilises an interatomic potential based on an unscreened rigid-ion model which was earlier found to be useful in understanding the structure, phonon density of states, and related properties. The thermal expansion, as obtained from the zero-pressure simulation, is in reasonable agreement with the observations of Jorgensen et al. on the oxygen-deficient system. The melting temperature obtained from the simulation is also in fair agreement with experiment.     

How to improve the accuracy of equilibrium molecular dynamics for computation of thermal conductivity?

Equilibrium molecular dynamics (EMD) simulations through Green-Kubo formula (GKF) have been widely used in the study of thermal conductivity of various materials. However, there exist controversial simulation results which have huge discrepancies with experimental ones in literatures. In this Letter, we demonstrate that the fluctuation in calculated thermal conductivity is due to the uncertainty in determination of the truncation time, which is related to the ensemble and size dependent phonon relaxation time. We thus propose a new scheme in the direct integration of heat current autocorrelation function (HCACF) and a nonzero correction in the double-exponential-fitting of HCACF to describe correctly the contribution to thermal conductivity from low frequency phonons. By using crystalline Silicon (Si) and Germanium (Ge) as examples, we demonstrate that our method can give rise to the values of thermal conductivity in an excellent agreement with experimental ones.     

Molecular dynamics simulation of sulphur nucleation in S–H2S system

The mechanism of sulphur nucleation in S–H₂S system is investigated by molecular dynamics simulation with the ReaxFF reactive force field. The results indicate that the nucleation of sulphur requires certain conditions. The nucleus of sulphur will form once the allotropes of sulphur dissolve from polysulphanes. Separate sulphur atoms aggregate into the cluster in the initial stage of nucleation according to the snowball effect. The cluster of nucleation is judged by the average distance of the neighbour sulphur atoms, which is identified as 2.8 Å through a parametric study. The sustainable process of nucleation depends on whether the cluster can overcome its critical state. The formation of the cluster may accelerate its own nucleation/coalescence and H₂S decomposition.     

Decomposition kinetics in Fe–Cu dilute alloys. Monte Carlo simulation using concentration-dependent interactions

A generalization of the statistical (Monte Carlo) simulation technique for determining the structure of alloys is proposed. It takes into account the dependence of effective interactions between the atoms of a dissolved chemical element on their local concentration. Using the ab initio parametrization of the model, the decomposition of the bcc Fe–Cu alloy accompanied by the formation of Cu nanoprecipitates is studied. It is shown that the concentration dependence of effective interactions significantly affects the decomposition kinetics by displacing its onset to longer times in agreement with the experiment.     

Lattice thermal conductivity of δ-graphyne — A molecular dynamics study

Thermal conductivity of δ-graphyne was investigated using reverse non-equilibrium molecular dynamics simulations. The dependence of the thermal conductivities with the temperature, acetylenic linkages, and external strain were explained by the phonon density of states. Our simulations revealed that as the temperature increased, the thermal conductivity of graphene first increased and then decreased, whereas that of δ-graphyne monotonically decreased. Owing to the presence of the acetylenic linkages, a significant reduction was found in the thermal conductivity of δ-graphyne, which resulted in a phonon vibration mismatch or weakened coupling. Moreover, the temperature profile changed from mono linear to the ladder the number of acetylenic linkages increased. These results play a guidance role in the design and application of thermoelectrics devices using 2D carbon materials.     

Molecular dynamics simulation of latent track formation in α-quartz

The latent ion track in α-quartz is studied by molecular dynamics simulations. The latent track is created by depositing electron energies into a cylindrical region with a radius of 3nm. In this study, the electron stopping power varies from 3.0keV/nm to 12.0keV/nm, and a continuous latent track is observed for all the simulated values of electron stopping power except 3.0keV/nm. The simulation results indicate that the threshold electron stopping power for a continous latent track lies between 3.0keV/nm and 3.7 keV/nm. In addition, the coordination defects produced in the latent track are analyzed for all the simulation conditions, and the results show that the latent track in α-quartz consists of an O-rich amorphous phase and Si-rich point defects. At the end of this paper, the influence of the energy deposition model on the latent track in α-quartz is investigated. The results indicate that different energy deposition models reveal similar latent track properties. However, the values of the threshold electron stopping power and the ion track radius are dependent on the choice of energy deposition model.     

Novel Cu–Cr alloy matrix CNT composites with enhanced thermal conductivity

Carbon nanotubes (CNTs) are incorporated into the Cu–Cr matrix to fabricate bulk CNT/Cu–Cr composites by means of a powder metallurgy method, and their thermal conductivity behavior is investigated. It is found that the formation of Cr₃C₂ interfacial layer improves the interfacial bonding between CNTs and Cu–Cr matrix, producing a reduction of interfacial thermal resistance, and subsequently enhancing the thermal conductivity of the composites. The thermal conductivity of the composites increases by 12 % and 17 % with addition of 5 vol.% and 10 vol.% CNTs, respectively. The experimental results are also theoretically analyzed using an effective medium approximation (EMA) model, and it is found that the EMA model combined with a Debye model can provide a satisfactory agreement to the experimental data.     

Molecular dynamics study of bimetallic Fe–Cu Janus nanoparticles formation by electrical explosion of wires

Bimetallic nanoparticles comprised of two elements which are immiscible in the bulk present a unique combination of physical–chemical properties that strongly depend on the atomic arrangement within the particle. In this study, molecular dynamics (MD) simulations of bimetallic Fe–Cu nanoparticles formation by high-velocity collision of individual metal nanoparticles (IMNPs) were performed. Physically these conditions model fast electrical explosion of two metal wires (Fe and Cu). By varying the size, temperature and velocity of colliding IMNPs, the conditions under which phase-segregated Janus nanoparticles are formed were determined. The model predictions showed good agreement with the experimental results. The present work is a step forward to understanding the formation mechanisms of bimetallic nanoparticles with different chemical configurations.     

The investigation of solid–solid phase transformation at CuAlNi alloy using molecular dynamics simulation

In this study the thermodynamic and structural properties of a CuAlNi model alloy (3A) system were investigated using a molecular dynamics (MD) simulation method. The interactions between atoms were modelled by the Sutton-Chen embedded atom method (SCEAM) based on many-body interactions. It was observed that at the end of thermal process the thermo-elastic phase transformation occurred in the model alloy system. In order to analyse the structures obtained from MD simulation, techniques such as thermodynamic parameters and radial distribution function (RDF) were used. The local atomic order in the model alloy was analysed using Honeycutt–Andersen (HA) method.     

Molecular dynamics simulation of the melting of a twist Σ=5 grain boundary

The existence of a possible grain boundary disordering transition of the melting type in a =5 (001) twist boundary of aluminium bicrystal below the melting temperature was investigated using a constant pressure molecular dynamics simulation. The calculated melting temperature T _cm of the bulk Al is about 960 K. The total internal energy, the structure factor, and the pair distribution function were calculated at different layers across the grain boundary. The mean atomic volume, the grain boundary energy, and the thermal expansion coefficients were also calculated using the same simulation method. This simulation also allows us to image the grain boundary structure at different temperatures. The equilibrium grain boundary structure at 300 K retains the periodicity of the coincident site lattice, so that the lowest energy structure corresponds to the coincident site arrangement of the two ideal crystals. With increasing temperature, the total internal energy of the atoms for both the perfect crystal and the grain boundary increases, as do the number of layers in the grain boundary. The grain boundary core exists and the perfect crystal structure still exists outside the grain boundary at 0.9375 T _cm. However, two atomic layers of the equilibrium grain boundary structure at 0.9375 T _cm lose the coincident site lattice periodicity and attain a structure with liquid-like disorder. Therefore, partial melting of the grain boundary has occurred at the temperature above 0.9375 T _cm which is in agreement with the experimental results.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Effect of Ni and vacancy concentration on initial formation of Cu precipitate in Fe–Cu–Ni ternary alloys by molecular dynamics simulation

In the present work, the effects of Ni atoms and vacancy concentrations(0.1%, 0.5%, 1.0%) on the formation process of Cu solute clusters are investigated for Fe–1.24%Cu–0.62%Ni alloys by molecular dynamics(MD) simulations. The presence of Ni is beneficial to the nucleation of Cu precipitates and has little effect on coarsening rate in the later stage of aging. This result is caused by reducing the diffusion coefficient of Cu clusters and the dynamic migration of Ni atoms. Additionally, there are little effects of Ni on Cu precipitates as the vacancy concentration reaches up to 1.0%,thereby explaining the embrittlement for reactor pressure vessel(RPV) steel. As a result, the findings can hopefully provide the important information about the essential mechanism of Cu cluster formation and a better understanding of ageing phenomenon of RPV steel. Furthermore, these original results are analyzed with a simple model of Cu diffusion, which suggests that the same behavior could be observed in Cu-containing alloys.     

Molecular dynamics simulations of the melting of Al–Ni nanowires

Molecular dynamics (MD) simulations were performed to investigate the influence of nickel (Ni) composition and nanowire thickness on the thermal properties of Al-x%Ni (at%) nanowires using the embedded atom model (EAM) potential. The melting of the nanowire was characterised by studying the temperature dependence of the cohesive energy and mean square displacement. The effect of the nanowire thickness on the cohesive energy, melting temperature, heat capacity as well as latent heat was studied in canonical ensemble. Moreover, the crystal stability of Al, Al-20%Ni, Al-40%Ni, Al-60%Ni, Al-80%Ni, Al₃Ni, Ni₃Al and Ni nanowires was studied at different temperatures using mean square displacement and cohesive energy.     

Investigation of thermal conductivity and rheological properties of vegetable oil based hybrid nanofluids containing Cu–Zn hybrid nanoparticles

Vegetable oils (Ground nut) are being investigated to serve as a possible substitute for non-biodegradable mineral oils, which are currently being used as metal-cutting fluids in machining processes. In this study, thermophysical properties of hybrid nanofluids (vegetable oil) to be used as metalworking cutting fluids are investigated. In-situ synthesis of copper (Cu) and zinc (Zn) combined hybrid particles is performed by mechanical alloying with compositions of 50:50, 75:25, and 25:75 by weight. Characterizations of the synthesized powder were carried out using X-ray diffraction, a particle size analyzer, FE-SEM, and TEM. Hybrid nanofluids with all the three combinations of hybrid nanoparticles were prepared by dispersing them into a base fluid (vegetable oil). The thermophysical properties, such as thermal conductivity and viscosity, were studied for various volume concentrations and at a range of temperatures. Experimental results have shown enhancement in thermal conductivity in all cases and also an increase in viscosity. The enhancement in viscosity is similar in all three combinations, as the particle size and shape are almost identical. The enhancement in thermal conductivity is higher in Cu–Zn (50:50), resulting in better enhancement in thermal conductivity due to the Brownian motion of the particles and higher thermal conductivity of the nanoparticles incorporated.