Reverse Non-equilibrium Molecular Dynamics Research Articles

Extending reverse nonequilibrium molecular dynamics to the calculation of mutual diffusion coefficients in molecular fluid mixtures

In a recent publication, a reverse nonequilibrium molecular dynamics method was introduced to calculate the mutual diffusion (binary diffusion, interdiffusion) coefficient of binary Lennard-Jones mixture directly. In this work, modification is made to this method for calculation the mutual diffusion coefficient of complicated binary mixture of molecules containing the same number of particle in calculation. As test, this method is applied to calculate the mutual diffusion coefficient of water/deuterium oxide (H2O/D2O) and benzene/cyclohexane (C6H6/C6H12) systems. Steady mass flows are induced by suitably exchanging molecule positions and velocities in different regions. The analysis of the resulting steady state concentration profiles allows the calculation of mutual diffusion coefficient. The obtained results are qualitative agreement with the references. The excess energy introduced by swapping molecules becomes strong when the studied mixture contains large and/or complicated molecules. For the systems studied, however, a standard thermostat is still able to remove the excess energy efficiently.

Image-based multi-scale simulation and experimental validation of thermal conductivity of lanthanum zirconate

Lanthanum zirconate (La2Zr2O7) is a promising candidate material for thermal barrier coating (TBC) applications due to its low thermal conductivity and high-temperature phase stability. In this work, a novel image-based multi-scale simulation framework combining molecular dynamics (MD) and finite element (FE) calculations is proposed to study the thermal conductivity of La2Zr2O7 coatings. Since there is no experimental data of single crystal La2Zr2O7 thermal conductivity, a reverse non-equilibrium molecular dynamics (reverse NEMD) approach is first employed to compute the temperature-dependent thermal conductivity of single crystal La2Zr2O7. The single crystal data is then passed to a FE model which takes into account of realistic thermal barrier coating microstructures. The predicted thermal conductivities from the FE model are in good agreement with experimental validations using both flash laser technique and pulsed thermal imaging-multilayer analysis. The framework proposed in this work provides a powerful tool for future design of advanced coating systems.

Effect of alkyl functionalization on thermal conductivity of graphene oxide nanosheets: a molecular dynamics study

In this study, the influence of alkyl functional groups on thermal conductivity of graphene oxide (GO) nanosheets is studied using a reverse non-equilibrium molecular dynamics simulation. The different types of alkyl-functionalized GO with functional groups containing six, twelve, sixteen and eighteen carbon atoms are investigated while the functional groups were bonded to the GO nanosheets both covalently and non-covalently. The objective is to investigate the effect of functional groups length as well as functionalization mechanisms on thermal conductivity of nanosheets. The results demonstrate that the thermal conductivity of GO reduces after alkylation and decreases with an increase in the chain length of functional groups. The analysis of the phonon spectra demonstrates that the functionalization of GO disrupts its phonon vibrations which results in a reduction of the thermal conductivity. Furthermore, the investigation of the alkylation mechanisms shows that the major portion of the reduction in the thermal conductivity is due to the non-covalent functionalization. This study can provide helpful information for controlling the thermal properties of alkylated GO for diverse applications such as superhydrophobic coatings, polymer composites, and nanocolloids. It also enhances the understanding of the mechanisms which affects the thermal properties after functionalization process.

Increasing the Thermal Conductivity of Graphene-Polyamide-6,6 Nanocomposites by Surface-Grafted Polymer Chains: Calculation with Molecular Dynamics and Effective-Medium Approximation

By employing reverse nonequilibrium molecular dynamics simulations in a full atomistic resolution, the effect of surface-grafted chains on the thermal conductivity of graphene-polyamide-6.6 (PA) nanocomposites has been investigated. The interfacial thermal conductivity perpendicular to the graphene plane is proportional to the grafting density, while it first increases and then saturates with the grafting length. Meanwhile, the intrinsic in-plane thermal conductivity of graphene drops sharply as the grafting density increases. The maximum overall thermal conductivity of nanocomposites appears at an intermediate grafting density because of these two competing effects. The thermal conductivity of the composite parallel to the graphene plane increases with the grafting density and grafting length which is attributed to better interfacial coupling between graphene and PA. There exists an optimal balance between grafting density and grafting length to obtain the highest interfacial and parallel thermal conductivity. Two empirical formulas are suggested, which quantitatively account for the effects of grafting length and density on the interfacial and parallel thermal conductivity. Combined with effective medium approximation, for ungrafted graphene in random orientation, the model overestimates the thermal conductivity at low graphene volume fraction (f < 10%) compared with experiments, while it underestimates it at high graphene volume fraction (f > 10%). For unoriented grafted graphene, the model matches the experimental results well. In short, this work provides some valuable guides to obtain the nanocomposites with high thermal conductivity by grafting chain on the surface of graphene.

The thermal conductivity in hybridised graphene and boron nitride nanoribbons modulated with strain

Thermal transport properties in hybridised graphene and boron nitride ribbons (HGBNRs) under different strains are studied by using reverse nonequilibrium molecular dynamics simulations. It is found that the effect of strains on the thermal conductivity is different for different types of strains. When the tensile and shear strains are applied, the thermal conductivity can be modulated at least up to 50% at room temperature as the strain ε ranges from 0 to 0.2. However, when the compressive and flexural strains are respectively applied, the thermal conductivity is insensitive to the variation of the strain. In addition, it is also found that the thermal conductivity of HGBNRs depends sensitively on the dimension of the hybridised ribbon and the relative amount of h-BN to graphene. A brief analysis of these results is given.

Tunable thermal property in edge hydrogenated AA-stacked bilayer graphene nanoribbons

The effect of edge hydrogenation on the thermal conductivity of AA-stacked bilayer graphene nanoribbons (BGNs) is explored by using the reverse non-equilibrium molecular dynamics method (RNEMD). We study the thermal transport properties of AA-stacked zigzag BGNs (ZBGNs) with different edge hydrogenation degrees in the range of 200–600K. The calculated results show that the interlayer van der Waals interaction has a big significant influence on the thermal conductivity of AA-stacked BGNs compared with monolayer graphene nanoribbons (GNRs) in the same simulation conditions. For AA-stacked ZBGNs with different edge hydrogenation degrees, thermal conductivities firstly increase with the temperature increasing, and then start to decrease when reaching certain maximum always reach maximum values at the temperature of 300K. Furthermore, the thermal conductivities of the simulation models have certain variation when the heat flow direction changes. In the meantime, the thermal rectification phenomenon is most obvious when the length ratio of the edge hydrogenated GNRs versus pristine one is 1. It is interesting that the thermal rectification effect is apparently weaken at high temperatures owing to obvious reduction of phonon power spectrum overlap, which implies that edge hydrogenated AA-stacked BGNs have the potential in future BGNs-based thermal rectification device applications.

Probing Thermal Conductivity of Fullerene C60 Hosting a Single Water Molecule

Encapsulation of a single water molecule inside the fullerene C60 can yield a significant change on its fundamental properties. Through comprehensive reverse nonequilibrium molecular dynamics (RNEMD) simulations, we investigate the thermal conductivity of endohedral C60 fullerene containing a single water molecule. The encapsulated single water molecule enhances the thermal phonon propagation resistance of the fullerene C60 due to its interaction with carbon atoms and brings down the thermal conductivity of the fullerene C60. Further simulations indicate that the thermal conductivity will depend on the external electrical field due to the polarity of the encapsulated water molecule. A larger electrical intensity will lead to a smaller thermal conductivity. A hierarchical model is proposed to quantitatively understand thermal behavior of the single endohedral fullerene and its enabled single-chain structures and shows good agreement with simulations.

Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures.

Although boron nitride nanotubes (BNNT) and hexagonal-BN (hBN) are superb one-dimensional (1D) and 2D thermal conductors respectively, bringing this quality into 3D remains elusive. Here, we focus on pillared boron nitride (PBN) as a class of 3D BN allotropes and demonstrate how the junctions, pillar length and pillar distance control phonon scattering in PBN and impart tailorable thermal conductivity in 3D. Using reverse nonequilibrium molecular dynamics simulations, our results indicate that although a clear phonon scattering at the junctions accounts for the lower thermal conductivity of PBN compared to its parent BNNT and hBN allotropes, it acts as an effective design tool and provides 3D thermo-mutable features that are absent in the parent structures. Propelled by the junction spacing, while one geometrical parameter, e.g., pillar length, controls the thermal transport along the out-of-plane direction of PBN, the other parameter, e.g., pillar distance, dictates the gross cross-sectional area, which is key for design of 3D thermal management systems. Furthermore, the junctions have a more pronounced effect in creating a Kapitza effect in the out-of-plane direction, due to the change in dimensionality of the phonon transport. This work is the first report on thermo-mutable properties of hybrid BN allotropes and can potentially impact thermal management of other hybrid 3D BN architectures.

Thermal conductivity of monolayer hexagonal boron nitride nanoribbons

Using reverse nonequilibrium molecular dynamics simulations (RNEMD), the thermal conductivity of monolayer hexagonal boron nitride (h-BN) nanoribbons as a function of length, width and edge chirality are investigated. While width effects on the thermal conductivity are not considerable, by increasing the length of ribbons their thermal conductivity significantly increases. The thermal conductivity of infinitely long armchair and zigzag nanoribbons are respectively predicted to be 277.78W/mK and 588.24W/mK, which are about one order of magnitude less than those of graphene. Moreover, we have studied the impact of monovanacies and Stone–Wales defects. While both monovacancies and Stone–Wales defects drastically lower the thermal conductivity of BN ribbons, the Stone–Wales defects have a more sever impact on the thermal transport properties of h-BN ribbons.

Reverse Non-Equilibrium Molecular Dynamics Demonstrate That Surface Passivation Controls Thermal Transport at Semiconductor-Solvent Interfaces.

We examine the role played by surface structure and passivation in thermal transport at semiconductor/organic interfaces. Such interfaces dominate thermal transport in semiconductor nanomaterials owing to material dimensions much smaller than the bulk phonon mean free path. Utilizing reverse nonequilibrium molecular dynamics simulations, we calculate the interfacial thermal conductance (G) between a hexane solvent and chemically passivated wurtzite CdSe surfaces. In particular, we examine the dependence of G on the CdSe slab thickness, the particular exposed crystal facet, and the extent of surface passivation. Our results indicate a nonmonotonic dependence of G on ligand-grafting density, with interfaces generally exhibiting higher thermal conductance for increasing surface coverage up to ∼0.08 ligands/Å(2) (75-100% of a monolayer, depending on the particular exposed facet) and decreasing for still higher coverages. By analyzing orientational ordering and solvent penetration into the ligand layer, we show that a balance of competing effects is responsible for this nonmonotonic dependence. Although the various unpassivated CdSe surfaces exhibit similar G values, the crystal structure of an exposed facet nevertheless plays an important role in determining the interfacial thermal conductance of passivated surfaces, as the density of binding sites on a surface determines the ligand-grafting densities that may ultimately be achieved. We demonstrate that surface passivation can increase G relative to a bare surface by roughly 1 order of magnitude and that, for a given extent of passivation, thermal conductance can vary by up to a factor of ∼2 between different surfaces, suggesting that appropriately tailored nanostructures may direct heat flow in an anisotropic fashion for interface-limited thermal transport.

Effects of vacancy defects and axial strain on thermal conductivity of silicon nanowires: A reverse nonequilibrium molecular dynamics simulation

Thermal conductivity of silicon nanowires (SiNWs) is evaluated using the reverse nonequilibrium molecular dynamics simulation. The Stillinger–Weber (SW) and Tersoff interatomic potentials are employed to simulate thermal conductivity of SiNWs. In this work, the influence of random vacancy defects, axial strain, temperature and length on thermal conductivity and effective mean free path of SiNWs is investigated. It is found that by raising the percent of random vacancy defects, thermal conductivity of SiNWs decreases linearly for the results obtained form SW potential and nonlinearly for those obtained from Tersoff interatomic potential. Dependence of the thermal conductivity on axial strain is also studied. Results show that thermal conductivity increases as compressive strain increases and decreases as tensile strain increases. Influence of temperature is also predicted. It is found that the thermal conductivity of SiNWs decreases with increasing the mean temperature. Most of the simulations are performed for 4 UC×4 UC×40 UC silicon nanowires using ssp boundary condition.

Thermal conductivities of graphyne nanotubes from atomistic simulations

Thermal conductivities (TCs) of graphyne-n nanotubes (GNT-n) are investigated by reverse non-equilibrium molecular dynamics simulations. Dependences of the generation number n, diameter d, and length L on TCs of GNT-n are derived with scaling relations and explained from the analysis of phonon density of states. Simulation results reveal that with the increase of the generation number n, TC decreases and scales as λ∼n-0.57. The diameter d has a weak impact on TC and a universal scaling law of λ∼d0.03 at d>5nm is derived for all GNT-n. With the increase of the length, the scaling relation between TC and L has a crossover. After the crossover, the scaling exponents are 0.16, 0.07, 0.05, 0.04, 0.03 for n=1–5, respectively, which are much smaller than the scaling exponent of 0.48 for the carbon nanotube (CNT). TC values of GNT-n are estimated to be 92.4, 43.6, 30.4, 27.4, 23.0W/(mK) for n=1–5, respectively, at L=2.6μm by extrapolation, which are two orders of magnitude smaller than 2820.6W/(mK) of the CNT with the same length. This implies the graphyne-n nanotubes may be more promising thermoelectric materials than the CNT.

A reverse nonequilibrium molecular dynamics method for calculating the mutual diffusion coefficient for binary fluids

This work introduces a nonequilibrium molecular dynamics method for calculating the mutual diffusion coefficient for mixtures. The method is based on the idea of the reverse nonequilibrium molecular dynamics algorithms, and artificially generates a mass flux through the Lennard–Jones mixtures by suitably exchanging particle positions and velocities in different regions. The analysis of the resulting steady-state concentration profiles allows the calculation of the mutual diffusion coefficient. As a test, this method is applied to the calculation of the mutual diffusion coefficient of Ar/Kr systems, and reasonable results are obtained. At the same time, it is observed that the mutual diffusion coefficient strongly depends on the temperature and composition of the mixture. The method can be easily extended to other fluid mixtures and be adopted when studying the nonequilibrium fluid mixtures.

Theoretical determination of anisotropic thermal conductivity for initially defect-free and defective TATB single crystals.

The anisotropic thermal conductivity was determined for initially defect-free and defective crystals of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a material that exhibits a graphitic-like packing structure with stacked single-molecule-thick layers, using the reverse non-equilibrium molecular dynamics method and an established TATB molecular dynamics force field. Thermal conduction in TATB is predicted to be substantially higher and more anisotropic than in other related organic molecular explosives, with conduction along directions nominally in the plane of the molecular layers at least 68% greater than conduction along the direction exactly perpendicular to the layers. Finite-size effects along the conduction directions were assessed. The conductivity along directions nominally in the plane of the molecular layers was found to be insensitive to the supercell length along the conduction direction-a result commensurate with the estimated phonon mean free path, ∼6 Å. A small decrease in the conductivity normal to the layers was found for longer supercells and is likely due to increased phonon scattering as a result of dynamic structural transitions in the crystal. The thermal conductivity of TATB crystals containing vacancy defects was also determined and the variation of conductivity with crystal density was found to be both linear and anisotropic, with the introduction of vacancy defects leading to a greater percentage reduction in conduction for the direction perpendicular to the molecular layers.

Thermal transport properties of defective graphene: A molecular dynamics investigation

In this work the thermal transport properties of graphene nanoribbons with randomly distributed vacancy defects are investigated by the reverse non-equilibrium molecular dynamics method. We find that the thermal conductivity of the graphene nanoribbons decreases as the defect coverage increases and is saturated in a high defect ratio range. Further analysis reveals a strong mismatch in the phonon spectrum between the unsaturated carbon atoms in 2-fold coordination around the defects and the saturated carbon atoms in 3-fold coordination, which induces high interfacial thermal resistance in defective graphene and suppresses the thermal conductivity. The defects induce a complicated bonding transform from sp2 to hybrid sp—sp2 network and trigger vibration mode density redistribution, by which the phonon spectrum conversion and strong phonon scattering at defect sites are explained. These results shed new light on the understanding of the thermal transport behavior of graphene-based nanomaterials with new structural configurations and pave the way for future designs of thermal management phononic devices.

Phonon scattering and thermal conductivity of pillared graphene structures with carbon nanotube-graphene intramolecular junctions

We present results of a reverse non-equilibrium molecular dynamics study of thermal transport in single-walled carbon nanotube (SWCNT)-graphene junctions comprised of carbon-carbon (C-C) bonds with either sp2 or mixed sp2/sp3 hybridization. In both cases, a finite interfacial thermal resistance is observed at the SWCNT-graphene junctions for thermal transport in the out-of-plane direction. The interfacial thermal resistance at the junctions is attributed to the combined effects of scattering of the phonons at the SWCNT-graphene junctions due to the presence of distorted sp2 bonds in the junction region and the change in dimensionality of the medium along the phonon transport path as the phonons propagate from SWCNT pillars (quasi-1D) to graphene sheet (2D) and then again to SWCNTs. Moreover, the thermal resistance is found to depend on the C-C bond hybridization at the intramolecular junctions with mixed sp2/sp3 hybridization showing a higher interfacial resistance when compared to pure sp2 bonding. Thermal conductivity of typical SWCNT-graphene unit cells was observed to increase nearly linearly with an increase in cell dimensions, and then reaches a plateau as the pillar height and the inter-pillar distance approach the critical length for ballistic thermal transport in SWCNT and single layer graphene. These results indicate that the thermal transport characteristics of the three-dimensional SWCNT-graphene (hybrid) structures can be tuned by controlling the unit cell size.

A Method for Creating Thermal and Angular Momentum Fluxes in Nonperiodic Simulations.

We present a new reverse nonequilibrium molecular dynamics method that can be used with nonperiodic simulation cells. This method applies thermal and/or angular momentum fluxes between two arbitrary regions of the simulation and is capable of creating stable temperature and angular velocity gradients while conserving total energy and angular momentum. One particularly useful application is the exchange of kinetic energy between two concentric spherical regions, which can be used to generate thermal transport between nanoparticles and the solvent that surrounds them. The rotational couple to the solvent (a measure of interfacial friction) is also available via this method. As tests of the new method, we have computed the thermal conductivities of gold nanoparticles and water clusters, the interfacial thermal conductivity (G) of a solvated gold nanoparticle, and the interfacial friction of a variety of solvated gold nanostructures.

Thermal conductivity of graphene nanoribbons with defects and nitrogen doping

Thermal conductivity of defective graphene nanoribbons doped with nitrogen for different distributions around the defect edge at nanoscale is investigated using the reverse non-equilibrium molecular dynamics (RNEMD) method, which explores ways to improve thermal management. In addition, thermal conductivity of graphene nanoribbons with both defects and nearby nitrogen doping is investigated in comparison to that of nanoribbons with defects alone. The simulation results are analyzed from three perspectives: phonon match, concentration of N doping, and distribution of N doping. This approach reveals that a coupling effect is the cause of the observed results. Nitrogen doped graphene nanoribbons (both perfect and defective variants) perform better with thermal management than do graphene nanoribbons with defects alone, which is of considerable interest. Based on these investigations, a guide for graphene-interconnected circuits design is implied.

Thermal Resistance across Interfaces Comprising Dimensionally Mismatched Carbon Nanotube-Graphene Junctions in 3D Carbon Nanomaterials

In the present study, reverse nonequilibrium molecular dynamics is employed to study thermal resistance across interfaces comprising dimensionally mismatched junctions of single layer graphene floors with (6,6) single-walled carbon nanotube (SWCNT) pillars in 3D carbon nanomaterials. Results obtained from unit cell analysis indicate the presence of notable interfacial thermal resistance in the out-of-plane direction (along the longitudinal axis of the SWCNTs) but negligible resistance in the in-plane direction along the graphene floor. The interfacial thermal resistance in the out-of-plane direction is understood to be due to the change in dimensionality as well as phonon spectra mismatch as the phonons propagate from SWCNTs to the graphene sheet and then back again to the SWCNTs. The thermal conductivity of the unit cells was observed to increase nearly linearly with an increase in cell size, that is, pillar height as well as interpillar distance, and approaches a plateau as the pillar height and the interpillar distance approach the critical lengths for ballistic thermal transport in SWCNT and single layer graphene. The results indicate that the thermal transport characteristics of these SWCNT-graphene hybrid structures can be tuned by controlling the SWCNT-graphene junction characteristics as well as the unit cell dimensions.

Thermal conductivity of Si nanowires with faulted stacking layers

Faulted stacking layers are ubiquitously observed during the crystal growth of semiconducting nanowires (NWs). In this paper, we employ the reverse non-equilibrium molecular dynamics simulation to elucidate the effect of various faulted stacking layers on the thermal conductivity (TC) of silicon (Si) NWs. We find that the stacking faults can greatly reduce the TC of the Si NW. Among the different stacking faults that are parallel to the NW's axis, the 9R polytype structure, the intrinsic and extrinsic stacking faults (iSFs and eSFs) exert more pronounced effects in the reduction of TC than the twin boundary (TB). However, for the perpendicularly aligned faulted stacking layers, the eSFs and 9R polytype structures are observed to induce a larger reduction to the TC of the NW than the TB and iSFs. For all considered NWs, the TC does not show a strong relation with the increasing number of faulted stacking layers. Our studies suggest the possibility of tuning the thermal properties of Si NWs by altering the crystal structure via the different faulted stacking layers.