Hard-sphere heat conductivity via nonequilibrium molecular dynamics

We use an Evans-Gillan driving forceF ^d, together with isokinetic and isoenergetic constraint forcesF ^c, to drive steady heat currents in periodic systems of 4 and 32 hard spheres. The additional driving and constraint forces produce curved trajectories as well as additional streaming and collisional contributions to the momentum and energy fluxes. Here we develop an analytic treatment of the collisions so that the simulation becomes approximately ten times faster than our previous numerical treatment. At low field strengths, for less than 0.4, where is the hard-sphere diameter, the 32-sphere conductivity is consistent with Alder, Gass, and Wainwright's 108-sphere value. At higher field strengths the conductivity varies roughly as ^1/2, in parallel with the logarithmic dependence found previously for three hard disks.     

Homogeneous periodic heat flow via nonequilibrium molecular dynamics

Two nonequilibrium methods for simulating homogeneous periodic heat flow are applied to 108 three-dimensional soft spheres in both the fluid and face-centered cubic solid phases. Both nonequilibrium methods use irreversible thermodynamics to express heat conductivity in terms of the work required to generate heat flow. The Evans-Gillan method, derived from Green-Kubo theory, correctly reproduces Ashurst's heat conductivities. An approach based on Gauss' principle of least constraint, in which the heat flow is constrained to a fixed value, fails this test. Heat flow is an inhomogeneous, nonlinear function of particle velocities and coordinates. Thus, Gauss' principle cannot be relied upon for treating inhomogeneous nonlinear nonholonomic constraints.Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract #W-7405-Eng-48. Work performed at U.C. Davis-Livermore with the support of the Army Research Office and the Air Force Office of Scientific Research.     

Two-dimensional gas of disks: Thermal conductivity

The phenomenon of heat conduction in a two-dimensional gas ofN hard disks is studied in the hydrostatic regime by means of nonequilibrium molecular dynamics (N ranging from 100 to 8000). For systems withN1500 the temperature and density profiles observed are in excellent agreement with the continuous theory, but the conductivityk differs from the one derived from Enskog's theory in a systematic way. This difference seems to slowly decrease with increasing density.     

基于量子修正的石墨烯纳米带热导率分子动力学表征方法

本文提出了基于量子修正的非平衡态分子动力学模型,可用于石墨烯纳米带热导率的表征.利用该模型对不同温度下,不同手性及宽度的石墨烯纳米带热导率进行了研究,结果发现:相较于经典分子动力学模型给出的热导率随温度升高而单调下降的结论,在低于Debye温度的情况下,量子修正模型的计算结果出现了反常现象.本文研究还发现,石墨烯纳米带的热导率呈现出明显的边缘效应及尺度效应:锯齿型石墨烯纳米带的热导率明显高于扶手椅型石墨烯纳米带;全温段的热导率及热导率在低温段随温度变化的斜率均随宽度的增加而增大.最后,文章用Boltzmann声子散射理论对低温段的温度效应及尺度效应进行了阐释,其理论分析结果说明文章所建模型适合在全温段范围内对不同宽度和不同手性的热导率进行精确计算,可为石墨烯纳米带在传热散热领域的应用提供理论计算和分析依据.     

Calculations of thermal conductivity of complex (dusty) plasmas using homogenous nonequilibrium molecular simulations

The heat conductivity of three-dimensional Yukawa dusty plasma liquids (YDPLs) has been investigated by employing a homogenous nonequilibrium molecular dynamics (HNEMD) technique at a low normalized force field strength (F*). The obtained results for plasma heat conductivity with suitable normalizations are measured over a wide range of various plasma states of the Coulomb coupling (Γ) and screening length (κ) in a canonical ensemble (NVT). The calculations for lattice correlations (Ψ) show that our YDPLs system remains in a nonideal strongly coupled regime for a complete range of Γ. It has been shown that the presented Yukawa system obeys a simple analytical temperature demonstration of λ₀ with a normalized Einstein frequency. The employed HNEMD algorithm is found to have a more efficient method than that of different earlier numerical methods and it gives more satisfactory results for lower to intermediate Γ with small system sizes at low F*. The obtained simulation results at nearly equilibrium F* (=?0.002) are in reasonable agreement with different earlier numerical results and with the present reference set of data showed deviations within less than ±15% for most of the present data points and generally underpredicted the λ₀ by 2–22%, depending on (Γ, κ).     

Molecular dynamics study for the thermal conductivity of diatomic liquid

The thermal conductivity of diatomic liquids was analyzed using a nonequilibrium molecular dynamics (NEMD) method. Five liquids, namely, O₂, CO, CS₂, Cl₂ and Br₂, were assumed. The two-center Lennard-Jones (2CLJ) model was used to express the intermolecular potential acting on liquid molecules. First, the equation of state of each liquid was obtained using MD simulation, and the critical temperature, density and pressure of each liquid were determined. Heat conduction of each liquid at various liquid states [metastable (ρ=1.9ρ_cr), saturated (ρ=2.1ρ_cr), and stable (ρ=2.3ρ_cr)] at T=0.7T_cr was simulated and the thermal conductivity was estimated. These values were compared with experimental results and it was confirmed that the simulated results were consistent with the experimental data within 10%. Obtained thermal conductivities at saturated state were reduced by the critical temperature, density and mass of molecules and these values were compared with each other. It was found that the reduced thermal conductivity increased with the increase in the molecular elongation. Detailed analysis of the molecular contribution to the thermal conductivity revealed that the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation while the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation. Moreover, by comparing the reduced thermal conductivity at various states, it was found that the increase of thermal conductivity with the increase in the density, or pressure, was caused by the increase of the contribution of energy transfer due to molecular interaction.     

Study of lattice thermal conductivity of alpha-zirconium by molecular dynamics simulation

The non-equilibrium molecular dynamics method is adapted to calculate the phonon thermal conductivity of alphazirconium. By exchanging velocities of atoms in different regions, the stable heat flux and the temperature gradient are established to calculate the thermal conductivity. The phonon thermal conductivities under different conditions, such as different heat exchange frequencies, different temperatures, different crystallographic orientations, and crossing grain boundary (GB), are studied in detail with considering the finite size effect. It turns out that the phonon thermal conductivity decreases with the increase of temperature, and displays anisotropies along different crystallographic orientations. The phonon thermal conductivity in [0001] direction (close-packed plane) is largest, while the values in other two directions of [2īī0] and [01ī0] are relatively close. In the region near GB, there is a sharp temperature drop, and the phonon thermal conductivity is about one-tenth of that of the single crystal at 550 K, suggesting that the GB may act as a thermal barrier in the crystal.     

Lorentz gas shear viscosity via nonequilibrium molecular dynamics and Boltzmann's equation

When nonequilibrium molecular dynamics is used to impose isothermal shear on a two-body periodic system of hard disks or spheres, the equations of motion reduce to those describing a Lorentz gas under shear. In this shearing Lorentz gas a single particle moves, isothermally, through a spatially periodic shearing crystal of infinitely massive scatterers. The curvilinear trajectories are calculated analytically and used to measure the dilute Lorentz gas viscosity at several strain rates. Simulations and solutions of Boltzmann's equation exhibit shear thinning resembling that found inN-body nonequilibrium simulations. For the three-dimensional Lorentz gas we obtained an exact expression for the viscosity which is valid at all strain rates. In two dimensions this is not possible due to the anisotropy of the scattering.     

Effect of electrostatics techniques on the estimation of thermal conductivity via equilibrium molecular dynamics simulation: application to methane hydrate

Equilibrium molecular dynamics (MD) simulations for three system sizes of fully occupied methane hydrate have been performed at around 265 K to estimate the thermal conductivity using the Ewald, Lekner, reaction field, shifted-force and undamped Fennell–Gezelter methods. The TIP4P water model was used in conjunction with a fully atomistic methane potential with which it had been parameterized from quantum simulation. The thermal conductivity was evaluated by integration of the heat flux autocorrelation function (ACF) derived from the Green–Kubo formalism; this approach vas validated by estimation of the average phonon mean free path. The thermal conductivities predicted by non-periodic techniques were in reasonable agreement with the experimental results of 0.62 and 0.68 W/m K, although it was found that the estimates by the non-periodic techniques were up to 25% larger than those of Lekner and Ewald estimates, particularly for larger systems. The results for the Lekner method exhibited the least variation with respect to system size. A decomposition of the heat flux vector into its respective contributions revealed the importance of electrostatic interactions, and how different electrostatic treatments affect the contribution to the thermal conductivity.     

Thermal conductivity of carbon nanoring linked graphene sheets:A molecular dynamics investigation

Improving the thermal conduction across graphene sheets is of great importance for their applications in thermal management. In this paper, thermal transport across a hybrid structure formed by two graphene nanoribbons and carbon nanorings(CNRs) was investigated by molecular dynamics simulations. The effects of linker diameter, number, and height on thermal conductivity of the CNRs–graphene hybrid structures were studied respectively, and the CNRs were found effective in transmitting the phonon modes of GNRs. The hybrid structure with 2 linkers showed the highest thermal conductivity of 68.8 W·m~(-1)·K~(-1). Our work presents important insight into fundamental principles governing the thermal conduction across CNR junctions and provides useful guideline for designing CNR–graphene structure with superior thermal conductivity.     

Insight into lattice thermal impedance via equilibrium molecular dynamics: case study on Al

Using results of equilibrium molecular dynamics simulation in conjunction with the Green–Kubo formalism, we present a general treatment of thermal impedance of a crystal lattice with a monatomic unit cell. The treatment is based on an analytical expression for the heat current autocorrelation function which reveals, in a monatomic lattice, an energy gap between the origin of the phonon states and the beginning of the energy spectrum of the so-called acoustic short-range phonon modes. Although, we consider here the f.c.c. Al model as a case example, the analytical expression is shown to be consistent for different models of elemental f.c.c. crystals over a wide temperature range. Furthermore, we predict a frequency ‘window’ where the thermal waves can be generated in a monatomic lattice by an external periodic temperature perturbation.     

Molecular dynamics studies on the thermal conductivity of single-walled carbon nanotubes

We have studied the thermal conductivity of single-walled carbon nanotubes (SWCNTs) using the NEMD method. The results indicate that the thermal conductivity values are not profoundly influenced by the specific simulation-technique used in the MD simulations. Some possible reasons, which could be responsible for the discrepancy on thermal conductivity values of SWCNTs in the literatures, are discussed.      

分形结构纳米复合材料热导率的分子动力学模拟研究

提出了一基于Sierpinski分形结构的Si/Ge纳米复合材料结构,以调控纳米复合材料的热导率.采用非平衡分子动力学方法模拟研究了分形结构Si/Ge纳米复合材料的导热性能,给出了硅原子百分比、轴向长度以及截面尺寸对分形结构纳米复合材料热导率的影响规律,并与传统矩形结构进行了对比.研究结果表明,分形结构纳米复合材料增强了Si/Ge界面散射作用,使得热导率低于传统矩形结构,这为提高材料的热电效率提供了有效途径.Si原子百分比、截面尺寸、轴向长度皆对分形结构纳米复合材料热导率存在着重要影响.纳米复合材料热导率随着Si原子百分比的增加呈先减小后增加的趋势,随轴向长度的增加则呈单调增大趋势.     

Layer and size dependence of thermal conductivity in multilayer graphene nanoribbons

Using nonequilibrium molecular dynamics method (NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers which can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons.     

碳纳米管热传导的分子动力学模拟研究

采用改进的经验键序作用势描述碳原子间的相互作用,应用分子动力学方法和Green-Kubo函数计算了碳纳米管的热导率.在模拟中,使用了重叠计算的方法来计算热流相关函数,大大减少了模拟步数.计算结果表明,碳纳米管的热导率以原子间作用力相互做功所引起的热流形式为主;热导率的值随着直径的增加而减小;在室温下,热导率的值随着温度的增加而增加,达到室温后逐渐收敛于定值.计算的单壁碳纳米管热导率在1000W/mK至4000W/mK之间,计算结果与实验结果基本符合. 关键词： 分子动力学 碳纳米管 热导率     

氮掺杂和空位对石墨烯纳米带热导率影响的分子动力学模拟

石墨烯是近年纳米材料研究领域的一个热点,其独特的热学性质受到了广泛关注,为了实现对石墨烯传热特性的预期与可控,利用氮掺杂和空位缺陷对石墨烯进行改性.采用非平衡态分子动力学方法研究了扶手形石墨烯纳米带中氮掺杂浓度、位置及空位缺陷对热导率影响并从理论上分析了热导率变化原因.研究表明氮掺杂后石墨烯纳米带热导率急剧下降,氮浓度达到30%时,热导率下降了75.8%;氮掺杂位置从冷浴向热浴移动过程中,热导率先近似的呈线性下降后上升;同时发现单原子三角形氮掺杂结构比多原子平行氮掺杂结构对热传递抑制作用强;空位缺陷的存在降低了石墨烯纳米带热导率,空位缺陷位置从冷浴向热浴移动过程中,热导率先下降后上升,空位缺陷距离冷浴边缘长度相对于整个石墨烯纳米带长度的3/10时,热导率达到最小.石墨烯纳米带热导率降低的原因主要源于结构中声子平均自由程和声子移动速度随着氮掺杂浓度、位置及空位缺陷位置的改变发生了明显变化.这些结果有利于纳米尺度下对石墨烯传热过程进行调控及为新材料的合成应用提供了理论支持.     

硅功能化石墨烯热导率的分子动力学模拟

采用Tersoff势函数与Lennard-Jones势函数,结合速度形式的Verlet算法和Fourier定律,对单层和两层硅功能化石墨烯沿长度方向的导热性能进行了正向非平衡态分子动力学模拟.通过模拟发现,硅原子的加入改变了石墨烯声子的模式、平均自由程和移动速度,使得单层硅功能化石墨烯模型的热导率随着硅原子数目的增加而急剧地减小.在300 K至1000 K温度变化范围内,单层硅功能化石墨烯的热导率呈下降趋势,具有明显的温度效应.对双层硅功能化石墨烯而言,少量的硅原子嵌入,起到了提高热导率的作用,但当硅原子数目达到一定数量后,材料的导热性能下降.     

不同硼掺杂几何形态下石墨烯纳米带热导率与热整流的分子动力学研究

通过非平衡态分子动力学方法,研究了锯齿形石墨烯纳米带中掺杂原子硼的两种不同位置排列（三角形硼掺杂和平行硼掺杂）对热导率和热整流的影响并从理论上分析了其变化的原因。研究表明这两种硼掺杂模型在不同温度下导致石墨烯纳米带热导率大约54％－63％的下降;同时发现平行硼掺杂结构对热传递的抑制作用强于三角形硼掺杂结构;硼掺杂结构降低热导率的作用随着温度的升高逐渐减小;三角形硼掺杂结构两个方向上的热导率值具有较大差异,这种结构下的热整流随着温度的上升呈现减弱的趋势;而平行硼掺杂结构两个方向上的热导率值近乎相等,热整流现象表现不明显。     

不同硼掺杂几何形态下石墨烯纳米带热导率与热整流的分子动力学研究

通过非平衡态分子动力学方法,研究了锯齿形石墨烯纳米带中掺杂原子硼的两种不同位置排列（三角形硼掺杂和平行硼掺杂）对热导率和热整流的影响并从理论上分析了其变化的原因。研究表明这两种硼掺杂模型在不同温度下导致石墨烯纳米带热导率大约54%-63%的下降;同时发现平行硼掺杂结构对热传递的抑制作用强于三角形硼掺杂结构;硼掺杂结构降低热导率的作用随着温度的升高逐渐减小;三角形硼掺杂结构两个方向上的热导率值具有较大差异,这种结构下的热整流随着温度的上升呈现减弱的趋势;而平行硼掺杂结构两个方向上的热导率值近乎相等,热整流现象表现不明显.