Reverse Non-equilibrium Molecular Dynamics Method Research Articles

Molecular dynamics study on the thermal properties of DGEBA/DETA/Ag/SWCNT-Ag composite materials.

Traditional conductive adhesives based on epoxy resin system often encounter problems such as high brittleness and low heat resistance. Therefore, it is particularly important to improve the thermal and mechanical properties of the conductive adhesive. In this study, the effects of SWCNT-Ag and SWCNT fillers on the thermal properties of DGEBA/DETA/Ag conductive adhesive system were studied by using molecular dynamics to construct different cross-linking models. The final results show that the addition of SWCNT and SWCNT-Ag can significantly improve the thermal properties of the conductive adhesive. However, the nanosilver particles on the surface of SWCNT-Ag act as a bridge for the connection between SWCNT and Ag in the conductive adhesive. Therefore, SWCNT-Ag has a more positive impact on the thermal properties of DGEBA/DETA/Ag conductive adhesive system. In this paper, the influence of SWCNT-Ag on the thermal properties of traditional DGEBA/DETA/Ag conductive adhesive system was studied by using Materials Studio software. The volume shrinkage, glass transition temperature, thermal expansion coefficient, and thermal conductivity of the material were calculated based on COMPASS force field. The thermal conductivity is calculated by using reverse non-equilibrium molecular dynamics method. Finally, it is found that SWCNT-Ag has a positive effect on the thermal properties of the conductive adhesive system by comparing several groups of calculation data.

Effects of Topological Parameters on Thermal Properties of Carbon Nanotubes via Molecular Dynamics Simulation

Due to their unique properties, carbon nanotubes (CNTs) are finding a growing number of applications across multiple industrial sectors. These properties of CNTs are subject to influence by numerous factors, including the specific chiral structure, length, type of CNTs used, diameter, and temperature. In this topic, the effects of chirality, diameter, and length of single-walled carbon nanotubes (SWNTs) on the thermal properties were studied using the reverse non-equilibrium molecular dynamics (RNEMD) method and the Tersoff interatomic potential of carbon–carbon based on the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). For the shorter SWNTs, the effect of chirality on the thermal conductivity is more obvious than for longer SWNTs. Thermal conductivity increases with increasing chiral angle, and armchair SWNTs have higher thermal conductivity than that of zigzag SWNTs. As the tube length becomes longer, the thermal conductivity increases while the effect of chirality on the thermal conductivity decreases. Furthermore, for SWNTs with longer lengths, the thermal conductivity of zigzag SWNTs is higher than that of the armchair SWNTs. Thermal resistance at the nanotube–nanotube interfaces, particularly the effect of CNT overlap length on thermal resistance, was studied. The simulation results were compared with and in agreement with the experimental and simulation results from the literature. The presented approach could be applied to investigate the properties of other advanced materials.

Thermo-mechanical characteristics of oxide-coated aluminum nano-powder

This paper presents a comprehensive study on the thermo-mechanical characteristics of oxide-coated aluminum nano-powder, which is a crucial property for their application in powder metallurgy, particularly in warm compaction process. A representative volume element (RVE) of oxide-coated aluminum nano-powder is analyzed under the hydrostatic compression test at different temperatures using the Reactive Molecular Dynamics (RMD) method that can predict the thermal conductivity of nano-powders by incorporating the interaction effects of density and temperature. The thermal conductivity is evaluated based on the Müller-Plathe's reverse non-equilibrium molecular dynamics (rNEMD) method, and the size dependence of the method is investigated. The computational model is validated with the experimental data. The results illustrate that the thermal conductivity of oxide-coated aluminum nano-powder is significantly lower than that of the bulk aluminum due to restricted surface contact between individual aluminum particles. Moreover, it is observed that increasing the density and temperature leads to an increase in the thermal conductivity of oxide-coated aluminum nano-powder.

Improving inherent thermal conductivity of epoxy resins based on contribution components of thermal conductivity: A molecular dynamics study

The inherent thermal transport properties of epoxy resin are vital factors to be studied in applications. In the present work, we constructed the amorphous crosslinking structure, the step crosslinking structure and the ordered crosslinking structure based on molecular dynamics theory, and used the reverse nonequilibrium molecular dynamics (RNEMD) method to calculate the thermal conductivity (TC) of different structures. Based on the numerical verification of the reliability of the method, the contribution of bonding to the TC was enhanced by increasing the molecular arrangement orderliness to reduce phonon scattering, taking the composition of the TC contribution as the guide. Meanwhile, the contribution of non-bonding interaction to the TC was enhanced by using compression process to achieve TC increase in the epoxy resin intrinsic structure, followed by exploring the effect of cross-linking degree on the TC of different structures. The results showed that the TC of amorphous epoxy resin decreased with the increase of crosslinking degree, and the relative high-frequency VDOS increased, but the influence of crosslinking degree on TC was minor in the range. For amorphous epoxy resin four-sided pressurization varied in the range of 0–7 Gpa, the non-bond energy percentage increased in the range of 0–60%, and the increase of non-bond energy made its contribution to TC increased significantly. The epoxy resin achieved 215% improvement in intrinsic TC by changing the molecular arrangement orderliness. The ordered structure TC varied from 0.3 to 3.5 W/(m⋅K) in the pressure variation range of 0–7 GPa. As the structure becomes more ordered, the TC increased significantly with the degree of cross-linking. The study provided the theoretical basis for the realization of resin preparation and resin thermal property evaluation with regulated intrinsic TC.

Porosity effect on the thermal and mechanical properties of U-50Zr alloy: A molecular dynamics study

The U-50Zr alloy is a promising candidate for helical cruciform fuel application, for the superior performance in neutronics and heat transfer properties. Under in-pile conditions, the formation and aggregation of fission gas products would introduce porosity effect, and further degrade the mechanical-thermal physical properties. However, no porosity model has been established for U-50Zr alloy. In this work, the Young's modulus and lattice conductivity for U-50Zr were predicted via molecular dynamics (MD) simulations with modified embedded atom method potential. The effective Young's modulus was obtained by multi-dimensional elongation tests and Voigt averaging scheme, and the phonon conductivity was computed with reverse non-equilibrium molecular dynamics method. The electron conductivity was estimated by Wiedemann-Franz Law with published resistivity data. To fulfill porosity effect modeling, void generation technique was utilized into the MD models, and effective medium theory was incorporated into the approximations of electron conductivity. Good agreements with experimental data were achieved to verify the research methodology. The semi-empirical models on porosity effects for U-50Zr were established, which could provide preliminary predictions of Young's modulus and thermal conductivity.

Interfacial thermal transport properties and its effect on thermal conductivity of functionalized BNNS/epoxy composites

• The thermal conductivity of functionalized BNNS and its interfacial thermal resistance with epoxy were calculated by molecular dynamics. • The mechanism of functionalization on reducing the interfacial thermal resistance of BNNS-epoxy was studied according to the phonon vibration density of state. • The influence of the BNNS content, size and functionalization rate on the thermal conductivity of composites was analyzed. • Under the synergistic effect of reduced BNNS thermal conductivity and enhanced BNNS-epoxy interfacial thermal conductance, there is a critical size for the effect of functionalized BNNS on the thermal conductivity of composite. The miniaturization and high-performance development of integrated circuits poses a major thermal challenge to microelectronic packaging, and there is an urgent need to study high thermal conductivity composites to improve the thermal management performance of high-power devices. In this paper, aiming at the thermal conductivity of BNNS/epoxy composites, the reverse non-equilibrium molecular dynamics method is used to study the effect of covalent and non-covalent functionalization on BNNS thermal conductivity and BNNS-epoxy interfacial thermal resistance. The strengthening mechanism of functionalization on interfacial thermal conductance is discussed according to phonon vibration density of states. And the effect of BNNS content, size, and functionalization rate on the thermal conductivity of composites is studied. The results found that the intrinsic thermal conductivity of BNNS reduced while the interfacial thermal conductance of BNNS-epoxy enhanced through functionalization. Under the synergistic effect of the two, there is a critical size for the effect of functionalized BNNS on the thermal conductivity of composite. When the BNNS is smaller than the critical size, functionalization can enhancing the thermal conductivity of the composite. The results can provide important theoretical guidance for the preparation of high thermal conductivity BNNS/epoxy composites.

A molecular dynamics simulation study on enhancement of thermal conductivity of bitumen by introduction of carbon nanotubes

• Thermal conductivity of bitumen was predicted by molecular dynamics simulation. • Carbon nanotubes can enhance the thermal conduction of bitumen/CNTs composites. • Interfacial thermal resistance inhibits the heat transfer at composite interface. • Conduction efficiency was affected by the length, diameter and alignment of CNTs. The thermal conductivity of bituminous concrete determines its ability to transfer heat in road pavement, and thus affects the microclimate near pavement surfaces. In this work, the reverse nonequilibrium molecular dynamics (RNEMD) simulation method was employed to study the thermal conductivity of bituminous materials for the first time. Three-dimensional bitumen and bitumen/carbon nanotubes (CNTs) composite models were constructed at the atomic level, and then followed by geometry optimization and simulated annealing. By imposing an energy flux on the molecular system, the temperature gradient and thermal conductivity were calculated. The simulated thermal conductivity shows a good agreement with test results, which suggests that the RNEMD method is suitable for predicting the thermal conductivity of bituminous materials. To investigate the effects of CNTs on the thermal conductivity of bitumen/CNTs composite, the length, diameter, dispersion and alignment of CNTs in composite were investigated. Simulation results indicated that CNTs can greatly improve the thermal conductivity of bitumen/CNTs composite, especially with long CNTs. Due to the interfacial thermal resistance, the conduction efficiency of composite failed to match the intrinsic thermal conductivity of CNTs. In addition, good dispersion and formation of interconnecting paths of CNTs are conducive to heat transport and could greatly improve the thermal conductivity of the composite.

Effects of branching and polydispersity on thermal conductivity of paraffin waxes

• Increased branching of paraffin molecules leads to decreased thermal conductivity. • Polydispersity has a minor effect on thermal conductivity at 250 K, but none at 450 K. • For branched paraffin, there is a strong concomitance of crystallinity and thermal conductivity. • Increased thermal conductivity of polydisperse paraffin is explained purely by increasing average chain length. Paraffin waxes are promising phase change materials, abundantly available at very low cost. Having large latent heat, these materials can be used for thermal energy storage. However, when used in heat batteries, paraffin’s low thermal conductivity prevents fast charging and discharging. This calls for the design of tailored hybrid materials with improved properties, the present study concentrates on properties of pure paraffin wax. Using fully atomistic molecular-dynamics (MD) simulations, we study the effects of polydispersity and branching on the thermal conductivity of paraffin waxes, in molten (450 K) and solid (250 K) state. Both branching and polydispersity affect the density and especially the crystallinity of the solid. Branching has a pronounced effect on crystallisation caused by inhibited alignment of the polymer backbones while the effect of polydispersity is less pronounced. The thermal conductivity (TC) has been simulated using the reverse non-equilibrium molecular-dynamics method, as well as the equilibrium Green-Kubo approach. Increased branching, added to backbones comprised of twenty monomers, results in decreasing TC of up to 30%, polydispersity only has an effect in the semi-crystalline state. Comparison to available experiments shows good agreement which validates the model details, applied force field and the calculation methods. We show that at comparable computational costs, the reverse non-equilibrium MD approach produces more reliable results for TC, as compared to the equilibrium Green-Kubo method. The major contribution to TC by acoustic phonon transport along the backbone was shown by analysing extreme cases. The phonon density of states (PDOS) of samples with high branching or with small chain length displayed diminished peaks in the acoustic range as compared to the PDOS of samples with low branching or larger chain length, respectively. The suggested MD approach can definitely be used to investigate specific material modifications aimed at increasing the overall TC.

A Study of the Effects of Graphene Nanosheets on the Thermal Conductivity of Nanofluid (Argon-Graphene) Using Reverse Nonequilibrium Molecular Dynamics Method

A Study of the Effects of Graphene Nanosheets on the Thermal Conductivity of Nanofluid (Argon-Graphene) Using Reverse Nonequilibrium Molecular Dynamics Method

Finite-size effects on thermal property predictions of molten salts

It is of great importance to obtain the accurate thermal properties of molten salts over the entire operating temperature for system design in concentrated solar power or other thermal storage systems. In this work, we explore size effects on density, viscosity and thermal conductivity predictions of molten nitrate and chloride salts by molecular dynamics. Equilibrium molecular dynamics and reverse nonequilibrium molecular dynamics methods are applied to calculate thermal conductivities of NaNO3 and NaCl regarding different simulation cell sizes at the specific temperatures to study size effects on thermal conductivity predictions of melts. The results show that the size effects are neglected on density and viscosity predictions while a linear positive relationship between 1/λ and 1/Lz is found in the reverse nonequilibrium molecular dynamics method. Through analyzing the phenomenon from a view of microstructure, phonon-phonon-scattering rates are found to be the key factors. The influences of temperature and particle species on size effects are further discussed. Subsequently, appropriate simulation cell sizes are selected for NaNO3, KNO3 and Hitec salt, and the calculated thermal properties are in good agreement with the experimental values. The mean relative errors and fitting formula for density, thermal conductivity, and viscosity for the three salts are also obtained.

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.

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.

Improving thermal conductivity of epoxy resin by filling boron nitride nanomaterials: A molecular dynamics investigation

Several kinds of inorganic nanofillers especially boron nitride (BN) are introduced to improve the thermal conducting properties of epoxy resin (EP). In this work, the influence of BN nanofillers on properties of EP composites is comprehensively investigated by using molecular dynamics (MD) technique. The thermal conductivity of BN/EP composites is calculated by reverse non-equilibrium molecular dynamics (RNEMD) method, then the effects of shapes (nanoparticle, nanotube and nanosheet), volume fractions and aspect ratios of BN nanofillers on the thermal properties are explored. To further elaborate the heat-conducting mechanism of EP composites, temperature gradient distribution and phononic density of states (PDOS) are analyzed. Moreover, the effect of BN volume fractions on the thermal conductivity of the composites is investigated by RNEMD simulation and Nielsen's theoretical model. The results indicate that the improving effectiveness of different BN nanofillers is ranked as nanosheet > nanotube > nanoparticle. This research provides new ideas for improving the thermal conductivity of composites.

An interlayer/intralayer coupling mechanism for the thermal characteristics of polycrystalline few-layer graphene

In this paper, the dependence of thermal conductivities of polycrystalline few-layer graphene (PFG) on the layer number and in-plane strain is systematically studied using the reverse non-equilibrium molecular dynamics method. The thermal conductivities are shown to decrease with the increased layer number, but the dependency on the layer number is anomalously less than what has been observed in single-crystalline few-layer graphene. Through a detailed analysis of the in-plane and out-of-plane phonon density of states, it is found that such a weak dependence can be explained by an interlayer/intralayer coupling mechanism, which is sensitive to the presence of grain boundaries. It is also found that this coupling mechanism can also be greatly influenced by in-plane tension, which indicates that thermal conductivities of PFG can be manipulated using in-plane tensile strains in addition to grain boundaries. The present study provides fundamental understanding and valuable guidelines for the design of graphene based flexible devices for efficient thermal management.

Application of Reverse Nonequilibrium Molecular Dynamics to the Calculation of the Mutual Diffusion Coefficient of Alkane Mixtures.

In a recent publication, a reverse nonequilibrium molecular dynamics (RNEMD) method was presented for computing the mutual diffusion coefficient of liquid mixtures. A concentration gradient and a subsequent mass flux are induced in the system by suitably exchanging molecules in different regions. The algorithm has been successfully tested on Lennard-Jones mixtures and molecular fluid mixtures with molecules having the same number of particles. In this work, a modification is made to the RNEMD method to determine the mutual diffusion coefficient of binary liquid mixtures with molecules having different sizes and masses. To migrate molecules of a different type, the splitting method has been used in this work. Investigation of the resulting steady-state mass fraction profile allows the evaluation of the mutual diffusion coefficient. For validation, the mutual diffusion coefficients of ethane-propane and ethane-pentane liquid mixtures at different compositions and temperatures have been obtained using this method. The mutual diffusion coefficients obtained from the RNEMD simulations are within the error bars of values obtained by equilibrium molecular dynamics for the identical model and conditions. The excess energy released due to the exchange of molecules is efficiently removed by strongly coupling a local thermostat in the region around the insertion point. There is no heating of the analysis region.

Principles and Mechanisms of Strain-Dependent Thermal Conductivity of Polycrystalline Graphene with Varying Grain Sizes and Surface Hydrogenation

In this paper, the thermal conductivities (κ) of polycrystalline graphene (PG) with varying average grain size are investigated using reverse nonequilibrium molecular dynamics method. Due to the pr...

Point defect effects on the thermal conductivity of [formula omitted]-SiC by molecular dynamics simulations

The point defect effects on phonon thermal conductivity of bulk crystalline β-SiC at 750 K and 1098 K are calculated with reverse non-equilibrium molecular dynamics method. Quantum correction is used to correct the corresponding temperatures. By extrapolation and quantum correction, the results of thermal conductivity of bulk β-SiC with point defects are achieved and corrected. For the eight types of point defects studied in this work, the thermal conductivity of SiC decreases with the increase of the concentration of certain point defects. The thermal conductivity drops rapidly at low concentration condition and gradually reaches certain value at high concentration condition. At high defect concentration condition, the temperature does not affect the thermal conductivity too much. The additional thermal resistivity is proportional to the concentration of all types of the point defects we studied. In these point defects, the interstitial SiTC decreases the thermal conductivity of β-SiC the most. On the other hand, the interstitial CTSi affects the thermal conductivity the least.

Thermal conductivity of silicon using reverse non-equilibrium molecular dynamics

Simulations are performed using the reverse non-equilibrium molecular dynamics (rNEMD) method and the Stillinger-Weber (SW) potential to determine the input parameters for achieving ±1% convergence of the calculated thermal conductivity of silicon. These parameters are then used to investigate the effects of the interatomic potentials of SW, Tersoff II, Environment Dependent Interatomic Potential (EDIP), Second Nearest Neighbor, Modified Embedded-Atom Method (MEAM), and Highly Optimized Empirical Potential MEAM on determining the bulk thermal conductivity as a function of temperature (400–1000 K). At temperatures > 400 K, data collection and swap periods of 15 ns and 150 fs, system size ≥6 × 6 UC2 and system lengths ≥192 UC are adequate for ±1% convergence with all potentials, regardless of the time step size (0.1–0.5 fs). This is also true at 400 K, except for the SW potential, which requires a data collection period ≥30 ns. The calculated bulk thermal conductivities using the rNEMD method and the EDIP potential are close to, but lower than experimental values. The 10% difference at 400 K increases gradually to 20% at 1000 K.

Influence of graphene oxide nanosheets on the stability and thermal conductivity of nanofluids

Many theoretical and experimental studies on heat transfer and flow behavior of nanofluids have been conducted, and the results show that nanofluids significantly enhance heat transfer. However, less attention has been paid to obtain the thermal conductivity of nanofluids and their stability using molecular simulations which are applied by investigators to explain the molecular mechanisms of nanoscale phenomena. In this work, the stability of water–ethylene glycol-based graphene oxide (GO) nanofluids was investigated by classical molecular dynamics simulations in which the kinetic energy, radial distribution function and intensity diagrams were obtained. The obtained results confirmed the stability of nanofluids. Also, the thermal conductivity of nanofluids was studied by reverse non-equilibrium molecular dynamics method at different ratios of water–ethylene glycol as base fluids and various amounts of graphene oxide as nanoparticles. The results show that the thermal conductivity of nanofluids increases with the amount of graphene oxide nanosheets. For example, the thermal conductivity of water–ethylene glycol (75/25%)-based nanofluid containing 3, 4 and 5% of GO nanosheets was increased by 24, 28 and 33%, respectively, at 46.7 °C. Finally, the theoretical models on heat transfer and flow behavior of nanofluids were employed to validate the molecular simulation results. The obtained thermal conductivity results are in good agreement with theoretical models.

Decreased Thermal Conductivity of Polyethylene Chain Influenced by Short Chain Branching

In this paper, we have studied the effect of short branches (side chains) on the thermal conductivity (TC) of a polyethylene (PE) chain. With a reverse nonequilibrium molecular dynamics (RNEMD) method, TCs of the pristine PE chain and the PE-ethyl chain are simulated and compared. It shows that the branch has a positive effect to decrease the TC of a PE chain. The TC of the PE-ethyl chain decreases with the number density increase of branches, until the density becomes larger than about eight ethyl per 200 segments, where the TC saturates to be only about 40% that of a pristine PE chain. Because of different weights, different branches will cause a different decrease of TCs, and a heavy branch will lead to a lower TC than a light one. This study is expected to provide some fundamental guidance to obtain a polymer with a low TC.