Nonequilibrium Molecular Dynamics Method Research Articles

Size effect on heat transfer performance of ZnO/Al<SUB align="right">2O<SUB align="right">3 interface materials

The aim of this paper is to provide a spanscale method to investigate and estimate the interfacial thermal conductivity (ITC) of ZnO/Al2O3; interface from the nanoscale to the microscale situation based on the non-equilibrium molecular dynamic (NEMD) method. Comparing the simulated with the experimental data, a fitting model of the ITC from the nanoscale to microscale is built to evaluate its changing rule. The results show that the ITC increased until bulking materials from the nanoscale to microscale. The scope of size effect area reduced to 30 nm from 600 nm and 330 nm comparing with each pure material. It means that the size effect of pure ZnO or Al2O3 materials can be avoided by compounding with Al2O3 or ZnO materials. It implies a potential method for electronic material design of packaging or micro/nano manufacturing.

Multiscale modeling of heat conduction in graphene laminates

We developed a combined atomistic-continuum hierarchical multiscale approach to explore the effective thermal conductivity of graphene laminates. To this aim, we first performed molecular dynamics simulations in order to study the heat conduction at atomistic level. Using the non-equilibrium molecular dynamics method, we evaluated the length dependent thermal conductivity of graphene as well as the thermal contact conductance between two individual graphene sheets. In the next step, based on the results provided by the molecular dynamics simulations, we constructed finite element models of graphene laminates to probe the effective thermal conductivity at macroscopic level. A similar methodology was also developed to study the thermal conductivity of laminates made from hexagonal boron-nitride (h-BN) films. In agreement with recent experimental observations, our multiscale modeling confirms that the flake size is the main factor that affects the thermal conductivity of graphene and h-BN laminates. Provided information by the proposed multiscale approach could be used to guide experimental studies to fabricate laminates with tunable thermal conduction properties.

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.

All-atom molecular-level computational analyses of polyurea/fused-silica interfacial decohesion caused by impinging tensile stress-waves

Purpose – The purpose of this paper is to address the problems of interaction of tensile stress-waves with polyurea/fused-silica and fused-silica/polyurea interfaces, and the potential for the accompanying interfacial decohesion. Design/methodology/approach – The problems are investigated using all-atom non-equilibrium molecular-dynamics methods and tools. Before these methods/tools are employed, previously determined material constitutive relations for polyurea and fused-silica are used, within an acoustic-impedance-matching procedure, to predict the outcome of the interactions of stress-waves with the material-interfaces in question. These predictions pertain solely to the stress-wave/interface interaction aspects resulting in the formation of transmitted and reflected stress- or release-waves, but do not contain any information regarding potential interfacial decohesion. Direct molecular-level simulations confirmed some of these predictions, but also provided direct evidence of the nature and the extent of interfacial decohesion. To properly model the initial state of interfacial cohesion and its degradation during stress-wave-loading, reactive forcefield potentials are utilized. Findings – Direct molecular-level simulations of the polyurea/fused-silica interfacial regions prior to loading revealed local changes in the bonding structure, suggesting the formation of an interphase. This interphase was subsequently found to greatly affect the polyurea/fused-silica decohesion strength. Originality/value – To the authors’ knowledge, the present work is the first public-domain report of the use of the non-equilibrium molecular dynamics and reactive force-field potentials to study the problem of interfacial decohesion caused by the interaction of tensile waves with material interfaces.

Investigation on thermal conductivity of bilayer graphene nanoribbons

We investigated the thermal conductivity of bilayer graphene nanoribbons (BGNs) using nonequilibrium molecular dynamics method (NEMD). The relationships among thermal conductivity, different ways of stacking, size, interfacial temperature and edge shape were studied. Two different stacking ways for BGNs are AA-stacked type and AB-stacked type. The results show that the thermal conductivities of AA-stacked BGNs are slightly higher than those of AB-stacked BGNs under the same simulation conditions because of different crystal structures. The thermal conductivity of zigzag BGNs (ZBGNs) first increases and then decreases with increasing width. However, the thermal conductivity of armchair BGNs (ABGNs) monotonously increases with increasing width. The thermal conductivity of BGNs increases with the length of the simulation system. In addition BGNs show temperature dependence and edge-shape dependence for thermal conductivity. We have explained the simulation results by the inter-layer phonon coupling and the phonon scattering, which have been shown to significantly affect the thermal conductivity of BGNs. The systematic analysis of thermal conductivity of BGNs did not only help to obtain conclusions regarding the achievable thermal performance, but also provided the possibility to design different dimensional BGNs for different BGNs-based thermal and microelectronic components.

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.

Orientation dependent thermal conductivity in graphyne nanoribbons

We investigate the thermal conductivity in the armchair and zigzag graphyne nanoribbons by using the nonequilibrium molecular dynamics method. The results show that a strong orientation dependence of the thermal conductivity is observed. The thermal conductivity in the armchair graphyne nanoribbon is larger than that in the zigzag graphyne nanoribbon with the same width and length at room temperature. Furthermore, the orientation dependence is increased when the width of the graphyne nanoribbons decreases. We disclose the underlying mechanism for this intrinsic orientation dependence to be the more phonon transport channels and higher phonon velocity in armchair graphyne nanoribbon throughout the overall frequency range.

Probability of self-healing in damaged graphene bombarded by fullerene

Using non-equilibrium molecular dynamics method, we study the self-healing behavior of graphene after bombarded by fullerene (C60) through controlling the environmental temperature and the incident velocity of C60. The self-healing probability depends on the size of graphene, the velocity of fullerene (C60), and the temperature of heat baths. It is suggested that the self-healing in damaged graphene originates from thermal fluctuation. Our results can offer additional insights for further understanding self-healing mechanisms and bombardment phenomena in low dimensional materials. Additionally, controlling the bombardment between the graphene and the fullerene (C60) may also lead to some potential applications in the surface cleaning of graphene and the production of nanopore.

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes (SWNTs) is discussed in this paper, using a non-equilibrium molecular dynamics (NEMD) method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.

Anharmonic force constants extracted from first-principles molecular dynamics: applications to heat transfer simulations

A systematic method to calculate anharmonic force constants of crystals is presented. The method employs the direct-method approach, where anharmonic force constants are extracted from the trajectory of first-principles molecular dynamics simulations at high temperature. The method is applied to Si where accurate cubic and quartic force constants are obtained. We observe that higher-order correction is crucial to obtain accurate force constants from the trajectory with large atomic displacements. The calculated harmonic and anharmonic force constants are, then, combined with the Boltzmann transport equation (BTE) and non-equilibrium molecular dynamics (NEMD) methods in calculating the thermal conductivity. The BTE approach successfully predicts the lattice thermal conductivity of bulk Si, whereas NEMD shows considerable underestimates. To evaluate the linear extrapolation method employed in NEMD to estimate bulk values, we analyze the size dependence in NEMD based on BTE calculations. We observe strong nonlinearity in the size dependence of NEMD in Si, which can be ascribed to acoustic phonons having long mean-free-paths and carrying considerable heat. Subsequently, we also apply the whole method to a thermoelectric material Mg2Si and demonstrate the reliability of the NEMD method for systems with low thermal conductivities.

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 rectification in pristine-hydrogenated carbon nanotube junction: A molecular dynamics study

Using non-equilibrium molecular dynamics method, we investigate thermal rectification (TR) in hybrid pristine carbon nanotube (PCNT) and hydrogenated carbon nanotube (HCNT) structures. The interface thermal resistance of the junction is dependent on the direction of thermal transport, leading to TR. We show that by selecting nanotubes of smaller diameters, and/or increasing the hydrogen coverage of HCNT, the TR can be amplified. The observed TR does not decrease by increasing the system length, which presents PCNT/HCNT system as a promising thermal rectifier at room temperature.

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.

(Invited) The Effects of Doping Pattern on Lattice Thermal Conductivity of Silicon Nanowires

The thermal conductivity of silicon nanowire doped with Germanium in different geometrical distributions was investigated with nonequilibrium molecular dynamics method. Five different geometrical arrangements of the impurities that influence the thermal conductivity properties of silicon nanowires were investigated, which include centralization doping, uniform doping, non-uniform doping, random doping and cubic pattern doping. The results demonstrate that the uniformity of the impurity positions has a substantial influence on the thermal conductivity of silicon nanowires under the same impurity concentration.

PREDICTION OF FLUID SLIP AT GRAPHENE AND CARBON NANOTUBE INTERFACES

The hydrodynamic boundary condition is now a subject of greater interest than ever before, even though the problem of formulating the correct boundary condition has existed from the beginning of the 19th century. Since then, many researchers have attempted to formulate a general boundary condition for fluid-solid interfaces. The 21st century has seen revolutionary advancement in nanoscale science and technology, which in turn, poses many fundamental questions about the nature of fluid flow in nanometric pores such as carbon nanotubes (CNTs) and aquaporins. Among them, one of the most important is the boundary condition. In this work, based on a statistical mechanics approach, we present a method to calculate the intrinsic interfacial friction coefficient between a fluid and solid at a planar and cylindrical interface, which determines the slip and boundary condition. We apply the method in conjunction with equilibrium molecular dynamics (EMD) simulation technique to fluids such as argon, methane and water flowing in planar graphene nanoslit pores and CNTs. We compare our model predictions against direct non-equilibrium molecular dynamics (NEMD) simulations and find excellent agreement. We identify several limitations of generally used NEMD methods to predict the slip and boundary condition and show that great care needs to be taken in analyzing the results of NEMD slip data for high-slip systems. We suggest some procedures to increase the reliability of the slip estimates. We also study the shear rate and external field dependent behavior of slip in Couette and Hagen-Poiseuille type flows. The slip length remains constant (indicating a linear response of the fluid to the external perturbation) only for low shear rates/external fields and as the field increases, the slip length increases rapidly. At these high fields the Navier-slip model breaks down. We attempt to resolve the highly debated issue of flow rates of water in carbon nanotubes, the values of which are scattered over 1 to 5 orders of magnitude in literature. We accurately predict these flow rates using both the CNT diameter dependent interfacial friction coefficient between water-CNTs and NEMD simulations streaming velocity profiles. Very narrow tubes show higher flow rate enhancements and as the tube diameter increases, the flow rates approach classical Navier-Stokes predictions with the no slip boundary condition. As the diameter of the tube increases, the slip length decreases monotonically and asymptotically approaches a constant value, which is equal to the slip length on a planar graphene surface. Our model gives the linear regime slip length which corresponds to experimental condition flow rates, which is otherwise cumbersome to find using NEMD simulation techniques. The proposed method is robust, general and can be used to find the slip and boundary condition accurately at any fluid-solid interface.

Computation of the thermal conductivity using methods based on classical and quantum molecular dynamics

The thermal conductivity of a model for solid argon is investigated using nonequilibrium molecular dynamics methods, as well as the traditional Boltzmann transport equation approach with input from molecular dynamics calculations, both with classical and quantum thermostats. A surprising result is that, at low temperatures, only the classical molecular dynamics technique is in agreement with the experimental data. We argue that this agreement is due to a compensation of errors and raise the issue of an appropriate method for calculating thermal conductivities at low (below Debye) temperatures.

The effect of defects on negative differential thermal resistance in symmetric graphene nanoribbons

Effect of defects on negative differential thermal resistance (NDTR) is investigated using non-equilibrium molecular dynamics method for the symmetric graphene nanoribbons (GNRs) with defects. Remarkably, it is found that NDTR will disappear as the number of defects increases. In addition, heat current of GNRs with defects is larger than that of the pristine GNRs when the temperature difference is larger. The results are explained qualitatively utilizing the theory of phonon spectra mismatch. The work may be useful in designing thermal-nano devices on the basis of GNRs.

Thermal Conductivity of the Partly Covered Inner Tube in a Double-Walled Carbon Nanotube with Varied Coverage Ratios

We present a study of the thermal conductivity of a partly covered inner tube in a double-walled carbon nanotube with varied covering ratios using a non-equilibrium molecular dynamics method. Our results show that the thermal conductivity of the inner tube changes non-linearly and non-monotonically with the increasing coverage ratio, forming a V-shaped curve. Minimal conductivity occurs at the coverage ratio of 58%, with its value being 69% of the maximal conductivity, which appears in the full coverage case. We analyze three mutually competitive mechanisms that result in this thermal conductivity behavior with the assistance of a phonon spectrum calculation.

Transient kinetics of graphene bombarded by fullerene

Using non-equilibrium molecular dynamics method, we study the transient kinetics of graphene bombarded by fullerene through controlling the temperature and velocity. Our results show that fullerene (C60) with low velocity cannot pass through graphene at any temperature. However C60 with high velocity can pass through graphene at any temperature. Between low velocity and high velocity, we find that the probability of C60 passing through graphene increases with temperature, the reason is that the probability of destroying carbon-carbon bond at high temperature is higher than at low temperature. In this paper, we also discuss the potential applications in the surface cleaning of graphene and the production of nanopore.

Investigation on the Heat Conduction in Si/3C-SiC/Graphene Film

Based on the nonequilibrium Molecular Dynamics method, interfacial thermal resistances of Si/3C-SiC/grphene composite films are investigated. The dependencies of interfacial thermal resistances of Si/3C-SiC and 3C-SiC/grphene on temperatures and the thickness of buffer layers are simulated separately. The results indicate that the interfacial thermal resistances of Si/3C-SiC and 3C-SiC/grphene increase with the increase of temperatures at the range of 100~700K, and converge to 3.4×10-9 Km2/W. In the Si/3C-SiC/grphene composite film, 3C-SiC connects Si substrate with grphene sheets. The results show the relationships between interfacial thermal resistances and the thickness are not prominent, and the maximum value of interfacial thermal resistance locates at 24×3.35 Å.