Nonequilibrium Molecular Dynamics Method Research Articles

Investigation of Interface Thermal Resistance between Polymer and Mold Insert in Micro-Injection Molding by Non-Equilibrium Molecular Dynamics.

Micro-injection molding has attracted a wide range of research interests to fabricate polymer products with nanostructures for its advantages of cheap and fast production. The heat transfer between the polymer and the mold insert is important to the performance of products. In this study, the interface thermal resistance (ITR) between the polypropylene (PP) layer and the nickel (Ni) mold insert layer in micro-injection molding was studied by using the method of non-equilibrium molecular dynamics (NEMD) simulation. The relationships among the ITR, the temperature, the packing pressure, the interface morphology, and the interface interaction were investigated. The simulation results showed that the ITR decreased obviously with the increase of the temperature, the packing pressure and the interface interaction. Both rectangle and triangle interface morphologies could enhance the heat transfer compared with the smooth interface. Moreover, the ITR of triangle interface was higher than that of rectangle interface. Based on the analysis of phonon density of states (DOS) for PP-Ni system, it was found that the mismatch between the phonon DOS of the PP atoms and Ni atoms was the main cause of the interface resistance. The frequency distribution of phonon DOS also affected the interface resistance.

Molecular dynamics simulation of small gas molecule permeation through CAU-1 membrane

CAU-1 is one of aluminum-based amine-functionalized Metal–Organic Frameworks (MOFs). Gas permeation and separation behaviors through CAU-1 membrane were simulated by the dual-control plane nonequilibrium molecular dynamics (DCP-NEMD) method. The thickness of membrane was 3.55 nm. Gases CO2, N2, CH4, H2, He, Kr and Xe were chosen for the calculation in both single component and binary mixtures. The permeation process was calculated in grand canonical (μVT) ensemble with periodic boundary conditions (PBC) in x- and y-directions at different temperatures. The calculated permeance of H2, CH4, N2, CO2 and Kr decreased with increasing temperature in both single and binary system, while that of Xe with kinetic molecule of 0.41 nm increased with increasing temperature. It shows Xe permeation is governed by activated diffusion. The simulated separation factors of CO2/N2 and CO2/CH4 of 4.2 and 1.3 respectively were lower than the experimental ones when only considering van der Waals interaction. Further consideration of electrostatic potential leads to improved calculation CO2/N2 and CO2/CH4 separation factor of 23.0 and 12.9 respectively that were consistent with the experimental ones of 26.2 and 14.8. It suggests the necessity of considering the Coulomb interactions between CO2 and NH2- on the pore wall of CAU-1 for permeation of CO2. For H2/N2 and H2/CH4 the ideal selectivities also keep consistent with our experimental results. Interestingly, the simulated separation factor for noble Kr/Xe reaches infinite, predicting that CAU-1 membrane possesses potential separation properties for radioactive Kr/Xe.

Interfacial thermal conductance across hexagonal boron nitride & paraffin based thermal energy storage materials

It is of vital importance to understand the interfacial thermal conductance between the high thermal conductivity fillers and the phase change thermal energy storage materials in order to enhance the energy storage efficiency. The interfacial thermal conductance across low-dimensional hexagonal boron nitride layers (h-BN) and n-octadecane was calculated with reverse non-equilibrium molecular dynamics methods in this paper. The influences of size effect, molecular arrangement of n-octadecane, layer number of h-BN as well as the defects modified with various functional groups on the interfacial thermal conductance within the temperature range from 273.15 to 373.15 K were investigated, respectively. The results showed that the interfacial thermal conductance of all systems increase with rising temperature and the size effect is inconspicuous. And systems with n-octadecane molecules parallel to the h-BN surface possess the highest interfacial thermal conductance with respect to the vertical and amorphous arrangements. In addition, the interfacial thermal conductance will reduce when the layer number of h-BN increases. In terms of the effect of defects, the systems with h-BN containing the defects modified with hydrogen atoms, i.e., lighter atom, possess higher interfacial thermal conductance. At last, for interpreting deeper mechanism, the vibration power spectrum of the velocity autocorrelation function and the components in different directions were also calculated.

Molecular dynamics simulation of laser-induced interconnections of silver nanowires

In this paper, the geometric deformation and atomic behavior evolution of silver nanowires subjected to laser are simulated by non-equilibrium molecular dynamics method. The simulation results show that due to the surface plasma resonance effect of the laser, significant atomic diffusion occurs in the contact area of the two silver nanowires. With the increase of the irradiation time of the laser, the atomic diffusion becomes more and more significant, and finally the two nanowires are connected. Because it is realized in plasma resonance under the action of the connection, and diffusion joint can produce stress due to atomic focus, the stress led to the nanowire outer atomic force by the inward contraction, completely different from the whole heating lead to the phenomenon of melting, diffusion, refactoring, produce similar to the form of cold welding joint, the joint can build complete face-centered cubic lattice structure, so the laser induced nanowires connect will complete nanowires of retention of the original structure and has superior properties of the nanowires interconnect technology will become the trend of The Times, It’s the most important piece of the puzzle for building the next generation of nanodevices.

Equilibrium and nonequilibrium molecular dynamics methods to compute the first normal stress coefficient of a model polymer solution

The first normal pressure (or stress) difference is directly computed from the local values of the pressure tensor components in molecular dynamics simulations of planar Poiseuille flow for a low molecular weight polymeric fluid. The resulting zero shear rate normal pressure difference agrees very well with the value computed using homogeneous shear simulations and the SLLOD algorithm, and less well with the result of the Coleman-Markowitz equation evaluated at equilibrium. This resolves doubts about the effects of homogeneous thermostats in homogeneous nonequilibrium molecular dynamics simulations.

Kapitza resistance at water-graphene interfaces.

Heat transfer across fluid-solid interfaces in nanoconfinement has received significant attention due to its relevance in nanoscale systems. In this study, we investigate the Kapitza resistance at the water-graphene interface with the help of classical molecular dynamics simulation techniques in conjunction with our recently proposed equilibrium molecular dynamics (EMD) method [S. Alosious et al., J. Chem. Phys. 151, 194502 (2019)]. The size effect of the Kapitza resistance on different factors such as the number of graphene layers, the cross-sectional area, and the width of the water block was studied. The Kapitza resistance decreases slightly with an increase in the number of layers, while the influence of the cross-sectional area and the width of the water block is negligible. The variation in the Kapitza resistance as a function of the number of graphene layers is attributed to the large phonon mean free path along the graphene cross-plane. An optimum water-graphene system, which is independent of size effects, was selected, and the same was used to determine the Kapitza resistance using the predicted EMD method. The values obtained from both the EMD and the non-equilibrium molecular dynamics (NEMD) methods were compared for different potentials and water models, and the results are shown to be in good agreement. Our method allows us to compute the Kapitza resistance using EMD simulations, which obviates the need to create a large temperature gradient required for the NEMD method.

Microscopic mechanism of alternating oscillations in a microchannel pulse tube based on molecular dynamics

The non-equilibrium molecular dynamic simulation method was used to analyze the microscopic dynamics process of alternating helium oscillations in a micro-channel pulse tube (MPT). The driving force was provided by a sinusoidal speed piston. A reflect wall was adopted as the power source similar to high-frequency liner compressors. The MPT's instantaneous temperature, pressure, speed mass flow, and time-averaged temperature were statistically analysed. At the micro level, the curve of the MPT's instantaneous properties approximated sinusoidal function. There was an alternating oscillation process with asymmetric property distributions between the MPT's compression and expansion process and the space-time distribution of each parameter's properties were non-uniform. The expansion process had a larger axial pressure gradient than the compression process. The analytical results showed that when the forced oscillation time caused by the piston motion was shorter than the natural oscillation time, the acoustic wave's energy flow density was higher. The cycle's time had little effect on the temperature of the cold end under low charge pressure. When the inflation pressure was higher than 20 bar, the temperature at the cold end was more sensitive to the time. As the time decreased, the temperature at the cold end further decreased and the time-averaged temperature at the cold end increased as the charge pressure increased. The negative effect mainly came from the temperature disturbance during the expansion process. The pressure ratio increased as the time decreased and was not significantly affected by the charge pressure, but it could generate a larger temperature gradient in the MPT's axial direction. In summary, the use of different forms of heat exchangers and phase shifters at the warm end could release or recover extra acoustic power. MPTs with fixed operating modes and dimension parameters should have optimal frequency to obtain the lowest no-load temperature at the cold end. The simulation results improve the understanding of pulse tubes' cooling mechanisms and provide theoretical support for the optimal design of micro-channel pulse tube coolers.

Pore-size dependence of the heat conduction in porous silicon and phonon spectral energy density analysis

Semiconductor devices made of silicon material have been widely used due to its excellent thermoelectrical properties. Here, the silicon material with aligned distributed rectangular-shaped holes is proposed for the manufacture of semiconductor device. Comprehensive understanding the heat conduction is of great significance to improve the efficiency of the thermoelectrical materials. This letter investigates the thermal conductivity of nanoscale porous silicon structures by adopting the nonequilibrium molecular dynamics method. The results demonstrate that the temperature is sensitive to the sizes of the rectangular-shaped holes. Additionally, it is found that the effective thermal conductivity significantly decreases with the increase of the dimensions of the holes. Our work reveals that the key to reduce the effective thermal conductivity is to disturb the distribution of heat flux. Furthermore, the phonon spectral energy density method is used to obtain the phonon dispersion and phonon energy in the frequency domain.

Nonequilibrium versus equilibrium molecular dynamics for calculating the thermal conductivity of nanofluids

Both equilibrium and nonequilibrium molecular dynamics (EMD and NEMD, respectively) methods have been used to predict the thermal conductivity of nanofluids. However, there are considerable discrepancies among the results of these two methods. In this study, by estimating the effects of different mechanisms including the Brownian motion of nanoparticles, the micro-convection in the base fluid, the nanolayers around the nanoparticles, and the thermal boundary resistance at the surface of nanoparticle, we determine upper and lower physical limits for the thermal conductivity of a nanofluid with spherical nanoparticles. The prediction of the NEMD simulations is in the acceptable range, while the result of the EMD simulations is higher than the upper bound. Since the prediction of the EMD method is not physically justifiable, we conclude the inadequacy of the traditional EMD method in calculating the thermal conductivity of nanofluids. Consequently, we recommend the researchers to use a modified version of the EMD method or the NEMD method for new studies in this field. We also apply the NEMD method to investigate the effects of the shape of nanoparticles and the formation of percolation networks in enhancing the thermal conductivity of nanofluids. The interference of the effects of nanolayer and thermal boundary resistance on the thermal conductivity of nanofluids is a new phenomenon we introduce in this study.

Effects of temperature and wax binder on thermal conductivity of RDX: A molecular dynamics study

In this work, based on the ReaxFF-lg reactive force field, the effects of temperature and wax binder on RDX thermal conductivity were studied by using molecular dynamics simulation (MD). The non-equilibrium molecular dynamics (NEMD) method was used to calculate the heat transfer process of RDX crystals with different sizes and at different temperatures and for RDX/wax mixtures with different ratios. The contribution of acoustic and optical phonons to the thermal conductivity was calculated using a thermal conductivity decomposition procedure. The heat transfer process of the RDX supercell and the RDX/wax mixtures were analyzed via calculation of the vibrational density of states. The thermal conductivity of the RDX increased with increasing temperature while the temperature was below 298 K, and decreased slightly with increasing temperature while the temperature was above 323 K. The interfacial thermal resistance between RDX and wax was the main factor that led to a decrease in the thermal conductivity of the RDX/wax mixed explosives. Acoustic phonons played a major role for heat transfer in RDX and mixed RDX explosives. An increased temperature resulted in more frequent expansion or torsion of the bonds in the optical modes, resulting in a slight reduction in the thermal conductivity.

Phonon Thermal Properties of Heterobilayers with a Molecular Dynamics Study

Two-dimensional transition metal dichalcogenide materials, such as the heterobilayers by piling up the monolayers in out-of-plane direction, behave unique performances in applications of nanoelectronic components. In this work, the in-plane thermal conductivities of MoS2–MoSe2, MoS2–WS2, and MoS2–WSe2 heterobilayers are investigated with nonequilibrium molecular dynamics method by considering the effects of system dimensions, temperature, coupling strength, strain, and misorientation. The results show that the thermal conductivity decreases as the temperature rises from 100 K to 500 K. The effect of coupling strength on the thermal conductivity is not significant. Both strain and misorientation could result in a reduction in the thermal conductivity, and the effect of tensile strain is stronger than that of compressive strain. An analysis based on phonon spectral energy density is performed to further understand phonon thermal properties in heterobilayers. This work provides a fundamental basis for regulating thermal transport in two-dimensional heterobilayer-based nanoelectronics.

Thermal protection of a hypersonic vehicle by modulating stagnation-point heat flux

The heat flux at the stagnation-point is usually used to evaluate the heat transfer efficiency between high-temperature gas and a hypersonic flying vehicle. Controlling the stagnation-point heat flux is of critical important for the design of a thermal protection system for a hypersonic flying vehicle. In this work, we demonstrate that the nonequilibrium molecular dynamic (NEMD) method can well describe the shock wave and the stagnation-point heat flux. In particular, our results show that the heat flux is more sensitive to the change of the peak amplitude of the hypersonic flow in the bimodal velocity probability density function (PDF) at the stagnation point. Decreasing the peak amplitude of the hypersonic flow in the PDF can effectively reduce the stagnation-point heat flux. Three types of the outer shapes for the flying objects are employed to compare the heat flux at the stagnation-point. The NEMD simulation results confirmed that the heat flux can be decreased 16.8% for the spherical cavity outer shape compared with the flat shape in the leading edge.

Influence of thermostatting on nonequilibrium molecular dynamics simulations of heat conduction in solids.

Nonequilibrium molecular dynamics (NEMD) has been extensively used to study thermal transport at various length scales in many materials. In this method, two local thermostats at different temperatures are used to generate a nonequilibrium steady state with a constant heat flux. Conventionally, the thermal conductivity of a finite system is calculated as the ratio between the heat flux and the temperature gradient extracted from the linear part of the temperature profile away from the local thermostats. Here, we show that, with a proper choice of the thermostat, the nonlinear part of the temperature profile should actually not be excluded in thermal transport calculations. We compare NEMD results against those from the atomistic Green's function method in the ballistic regime and those from the homogeneous nonequilibrium molecular dynamics method in the ballistic-to-diffusive regime. These comparisons suggest that in all the transport regimes, one should directly calculate the thermal conductance from the temperature difference between the heat source and sink and, if needed, convert it into the thermal conductivity by multiplying it with the system length. Furthermore, we find that the Langevin thermostat outperforms the Nosé-Hoover (chain) thermostat in NEMD simulations because of its stochastic and local nature. We show that this is particularly important for studying asymmetric carbon-based nanostructures, for which the Nosé-Hoover thermostat can produce artifacts leading to unphysical thermal rectification.

Transferability of interatomic potentials with insights into the structure–property relationship of SiO2–CaO–MgO–Al2O3 melts

ABSTRACTMolecular dynamics (MD) simulations were carried out to validate the transferability of Matsui, Kawamura, Miyake, and Guillot potentials on the composition–structure–property relationship of liquid SiO2–CaO–MgO–Al2O3 (CMAS) melts in a wide range of chemical compositions at 1773K. Results revealed that Matsui, Miyake, and Guillot potentials were able to reproduce experimentally observed local atomic structures and properties while Kawamura potential cannot. Even though an accurate trend can be obtained, the values of properties obtained with Miyake, Matsui and Guillot potentials varied obviously with each other. Viscosity obtained using Einstein–Stokes method is about one magnitude lower than experimental viscosity, while viscosities obtained with Green–Kubo and reverse nonequilibrium molecular dynamics methods underestimate the viscosity by about two magnitudes. The increase in CaO/SiO2 ratio under fixed MgO/Al2O3 ratio and the increase in MgO/Al2O3 ratio under fixed CaO/SiO2 ratio can both decrease the polymerisation degree of system, which finally decreases the viscosity. The influence of adding a Morse term to BMH potential is very slightly. This paper provides the foundation to further simulate the atomic structure of CMAS system and further optimise the Born–Mayer–Higgins potential for various oxide systems.

Investigation of thermal properties of DNA structure with precise atomic arrangement via equilibrium and non-equilibrium molecular dynamics approaches

Background and ObjectiveThermal conductivity of Deoxyribonucleic acid molecules is important for nanotechnology applications. Theoretical simulations based on simple models predict thermal conductivity for these molecular structures. MethodsIn this work, we calculate the thermal properties of Deoxyribonucleic acid with precise atomic arrangement via equilibrium and non-equilibrium molecular dynamics approaches. In these methods, each Deoxyribonucleic acid molecule is represented by C, N, O, and P atoms and implemented dreidng potential to describe their atomic interactions. ResultsOur calculated rate for thermal conductivity via equilibrium and non-equilibrium molecular dynamics methods is 0.381 W/m K and 0.373 W/m K, respectively. By comparing results from these two methods, it was found that the results from equilibrium and non-equilibrium molecular dynamics methods are identical, approximately. On the other hand, the number of DNA molecules and the equilibrium temperature of the simulated structures were important factors in their thermal conductivity rates, and their thermal conductivity was calculated at 0.323 W/m K–0.381 W/m K intervals for equilibrium and 0.303 W/m K–0.373 W/m K interval for non-equilibrium calculations. ConclusionsThese results are in good agreement with thermal conductivity calculation with other research groups.

Thermal transport properties at interface of fatty acid esters enhanced with carbon-based nanoadditives

There has been growing interest in the development and use of sustainable, bio-based phase change materials (PCMs), as an alternative to conventional paraffinic PCMs. Among possible choices, two fatty acid esters, i.e., methyl palmitate and methyl 9,10-dihydroxystearate, are investigated in this study. The physical properties of these two bio-based PCMs are first characterized using the molecular dynamics method. Upon validating the results with the experimental test data, the reverse non-equilibrium molecular dynamics (RNEMD) method is employed to evaluate the thermal performance of these two PCMs in both pristine and enhanced conditions. The interplay between the molecular structure and thermal properties is then determined using a radial distribution function analysis. To address the issue of low thermal conductivity of these two PCMs, the possibility of enhancing them with carbon-based nanoaddivities, such as graphene, is investigated. The interface between the PCM matrix and the graphene filler is modeled at nanoscale and the interfacial thermal conductance is characterized using the RNEMD simulations. A separate set of vibration power spectra analyses are conducted to shed light on the thermal energy transfer mechanisms across the interface of bio-based PCMs and carbon-based nanoadditives. The outcome of this study is expected to transform the design and implementation of PCMs for sustainable energy storage technologies.

Molecular Dynamics Modeling of Thermal Conductivity of Silicon/Germanium Nanowires

The thermal conductivity of silicon/germanium nanowires with different geometry and composition has beenstudied by using the nonequilibrium molecular dynamics method. The thermal conductivity of the Si1-xGexnanowire is shown to firstly decrease, reaches a minimum at x=0.4 and then to increase, as the germaniumcontent x grows. It was found that in the tubular Si nanowires the thermal conductivity decreases monotonouslywith increasing radius of the cylindrical void. The phonon spectra were calculated and the mechanisms of phononscattering in the investigated nanowires were analyzed.

Role of grain boundaries on the thermal properties of carbon nanotubes

Abstract Carbon nanotubes (CNTs) do not exist in their pristine form naturally and they have certain types of defects. In comparison to all other types of defects, grain boundary (GB) defects may affect the thermal conductivity of CNTs significantly and the study of thermal properties of CNTs containing GB is essential. In this work, Fourier’s law and Reverse Non-Equilibrium Molecular Dynamics (RNEMD) method have been applied on CNTs based on adaptive intermolecular reactive empirical bond order force field potential to calculate their thermal conductivities. The current results reveal the deviation of thermal conductivities of defective CNTs pristine ones.

Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

There is an ongoing debate concerning the best rheological model for liquid flows in elastohydrodynamic lubrication (EHL). Due to the small contact area and high relative velocities of bounding solids, the lubricant experiences pressures in excess of 500 MPa and strain rates that are typically $$10^5{-}10^7\,\text {s}^{-1}$$ . The high pressures lead to a dramatic rise in Newtonian viscosity $$\eta _{N}$$ and the high rates lead to large shear stresses and pronounced shear thinning. This paper presents detailed simulations of a model EHL fluid, squalane, using nonequilibrium molecular dynamics methods to extract the scaling of its viscosity with shear rate ( $$10^5{-}10^{10}\,\text {s}^{-1}$$ ) over a wide range of pressure P (0.1 MPa to 1.2 GPa), and temperature T ( $$150{-}373$$ K). Simulation results are consistent with a broad range of equilibrium and nonequilibrium experiments. At high T and low P, where $$\eta _{N}$$ is low, the response can be fit to a power law, as in the common Carreau model. Shear thinning becomes steeper as $$\eta _{N}$$ increases, and for $$\eta _{N}\gtrsim 1$$ Pa s, shear thinning is consistent with the thermally activated flow assumed by another common model, Eyring theory. Simulations for a bi-disperse Lennard–Jones (LJ) system show that the transition from Carreau to Eyring is generic. For both squalane and the LJ system, the viscosity decreases by only about a decade in the Carreau regime, but may fall by many orders of magnitude in the Eyring regime. Shear thinning is often assumed to reflect changing molecular alignment, but the alignment of squalane molecules saturates after the viscosity has dropped by only about a factor of three. In contrast, thermal activation describes shear thinning by six or more decades in viscosity. Changes in the diagonal elements of the stress tensor with rate and shear stress are also studied.

Molecular dynamics study on transport properties of supercritical working fluids: Literature review and case study

Transport properties of working fluids are of great importance of analyses of supercritical thermodynamic cycles. However, at the present stage, the lack of available data poses critical challenges to the application of cutting-edge working fluids in supercritical thermodynamic cycles. Accordingly, molecular dynamics simulation emerges as a powerful tool to explore the transport properties of supercritical working fluids. Particularly, when an appropriate force field is applied to the molecular system, transport properties with an acceptable accuracy can be obtained through molecular dynamics simulation due to its solid foundation of statistical thermodynamics. Thereby, the expensive supercritical experiments can be avoided and there’s a key advantage of molecular dynamics simulation in cost and safety over experiments. In this study, a comprehensive review on molecular dynamics simulation applied in transport properties prediction of supercritical working fluids is proposed. The review first covers an introduction of transport properties calculation through molecular dynamics simulation. Pros and cons of both equilibrium and non-equilibrium molecular dynamics are proposed as well. Besides, the knowledge gap, as well as the development prospect is also proposed. Finally, in order to provide a clear understanding and evaluate the predictive ability, a case study for one of the supercritical working fluids is proposed. Specially, the studied working condition covers a region where the applicability of existing predictive models are invalid. The study suggests that molecular dynamics simulation have the ability to predict transport properties in supercritical region. For thermal conductivity, non-equilibrium molecular dynamics methods are recommended, while equilibrium molecular dynamics for viscosity.