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     

电场和温度对聚合物空间电荷陷阱性能的影响

模拟分子的结构和行为有助于更深刻地分析空间电荷陷阱性能变化的微观机理.利用Materials studio软件建立聚乙烯模型,通过分子链段运动产生的能量和自由体积变化对微观结构和电荷陷阱进行分析.结果表明:温度由298 K逐渐升高至363 K的过程中,聚合物分子热运动加剧导致的滑移扩散现象,使自由体积和陷阱能级在363 K处分别出现1542.07 ~3和0.66 eV的最大值和最小值.然而在Z轴方向施加0.0007 Hartree/Bohr(1 Hartree/Bohr=5.2×10~(11)V/m)电场作用时,由于电致伸缩产生Maxwell应力,使分子链段出现局部有序排列,增大范德瓦耳斯能至-360.18 kcal/mol(1 kcal/mol=4.18 kJ/mol),而自由体积降低了279.77 ~3,导致陷阱能级减小0.45 eV.当363 K和0.0007 Hartree/Bohr联合作用时,聚乙烯的陷阱能级相比同温无电场作用降低0.17 eV.分子模拟结果与实测结果相符.利用分子热运动和电致伸缩效应,初步探讨了材料自由体积和范德瓦耳斯相互作用能变化的微观机理,证实分子链段运动改变了微观结构,从而影响电荷陷阱特性.并且与温度相比,电场作用会使材料产生更低能级的空间电荷陷阱.     

TiO2/ZnO纳米薄膜界面热导率的分子动力学模拟

采用平衡分子动力学方法及Buckingham势研究了金红石型TiO₂薄膜与闪锌矿型ZnO薄膜构筑的纳米薄膜界面沿晶面[0001](z轴方向)的热导率.通过优化分子模拟初始条件中的截断半径r_c和时间步后,计算并分析了平衡温度、薄膜厚度、薄膜截面大小对热导率的影响.研究表明,薄膜热导率受薄膜温度和厚度的影响很大,当温度由300 K升高600 K时,薄膜的热导率逐渐减小;当薄膜厚度由1.8 nm增大到5 nm时,热导率会逐渐增大;并在此基础 关键词： 热导率 分子动力学 2/ZnO纳米薄膜界面')" href="#">TiO₂/ZnO纳米薄膜界面 数值模拟     

多壁碳纳米管热物性参数的理论计算

本文计算了多壁碳纳米管的声子色散关系,分析了多壁碳纳米管的声子振动模式和热物理性质.结果表明:多壁碳纳米管的层间相互作用使得声子频率升高,因而使得比热容和热导率降低.在极低温区层间相互作用对热物性参数的影响明显.对于(5,5)@(10,10)管,不考虑层间相互作用时热导率的计算值在10 K处可相差60%.随着层数的增加,多壁管热导率值降低,并且趋向定值.     

二元室温金属流体热导率的理论计算与实验研究

室温金属流体在芯片散热领域中正日益显示重要价值,但相关材料的热物性比较缺乏.本文基于Faber-Ziman理论和Wiedemann-Franz-Lorenz定律,采用Ashcroft-Langrethp硬球模型偏结构因子以及空核心模型赝势计算了液态二元镓铟及钠钾合金的热导率.结果表明:相比于钠钾合金的热导率随原子组分浓度呈抛物线变化,镓铟合金的热导率与原子组分浓度呈单调变化关系,同时镓铟合金的热导率随着温度升高而升高.作为对比,本文同时也对镓铟合金的热导率进行了实验测定.结合所获数据及钠钾合金文献数据对比,表明计算结果与实验值符合得较好.本文工作为进一步研究二元合金热导率打下了基础.     

非简谐振动对SiC类石墨烯热输运性质的影响

考虑到原子的非简谐振动,应用固体物理理论和方法,计算了SiC类石墨烯的简谐系数和非简谐系数,得到它的德拜温度、热容量和热导率等随温度的变化规律,探讨了原子非简谐振动对它的热输运性质的影响.结果表明:SiC类石墨烯的德拜温度随温度的升高而在117-126 K之间线性增大,定容比热随温度升高而非线性增大,热导率随温度升高而非线性减小,温度较低时变化较快,而温度较高时变化较慢,并随着温度升高而趋于常量;考虑到非简谐振动后,SiC类石墨烯的德拜温度、定容比热和热导率的值分别大于、小于和大于简谐近似的相应值,温度愈高,其差值愈大,即温度愈高,非简谐效应的影响愈显著;二维平面状的SiC类石墨烯的定容比热和热导率随温度的变化规律,与三维块状SiC晶体总体趋势相同,只是具体数值不同.     

氮化铝陶瓷低温热导率的实验研究

根据轴向稳态热流法原理 ,实验研究了 Al N陶瓷的低温热导率 ,为高温超导制冷机直接冷却提供了必要的实验数据。 Al N的热导率在 30～ 170 K之间 ,随温度的升高而增大 ,在 10 0 K时达到 5 8.36 W/ (m· K)。根据平均自由程理论 ,对影响氮化铝陶瓷热导率的微观因素进行了分析 ,影响本次测试氮化铝样品低温热导率的主要因素为声子与缺陷之间的散射。     

红外热成像法测量薄膜热物理参数

研究了一种利用稳态红外热成像法实现自悬浮薄膜面内热导率测量的方法。从一维热传导方程出发,建立了稳态时薄膜表面温度分布的理论模型,利用稳态理论模型,只需测量薄膜边缘温度及厚度,便可同时得到面内热导率、发射率及热流,无需测量薄膜对可见光的吸收率。仿真表明,当薄膜的温升不超过5K时,可以保证拟合得到的面内热导率与理论值的误差低于3%,薄膜样品x方向的最小尺寸为6mm。对厚度为900nm的自悬浮聚酰亚胺薄膜进行实验测量,拟合得到的面内热导率为2.04W/mK、红外发射率为0.92、x=0处的热流为1.77×10~4W/m~2。实验结果与查阅文献的测量值一致,证明方法可以用来测量自悬浮薄膜的面内热导率。     

非简谐振动对SiC类石墨烯热输运性质的影响

考虑到原子的非简谐振动,应用固体物理理论和方法,计算了SiC类石墨烯的简谐系数和非简谐系数,得到它的德拜温度、热容量和热导率等随温度的变化规律,探讨了原子非简谐振动对它的热输运性质的影响.结果表明:SiC类石墨烯的德拜温度随温度的升高而在117-126 K之间线性增大,定容比热随温度升高而非线性增大,热导率随温度升高而非线性减小,温度较低时变化较快,而温度较高时变化较慢,并随着温度升高而趋于常量;考虑到非简谐振动后,SiC类石墨烯的德拜温度、定容比热和热导率的值分别大于、小于和大于简谐近似的相应值,温度愈高,其差值愈大,即温度愈高,非简谐效应的影响愈显著;二维平面状的SiC类石墨烯的定容比热和热导率随温度的变化规律,与三维块状SiC晶体总体趋势相同,只是具体数值不同.     

碳纳米管Y形分子结的热导率与热整流现象

基于碳纳米管Y形分子结的结构重构, 通过非平衡分子动力学方法和量子修正, 模拟分析了Y形分子结的热导率和热整流现象. 研究表明: 相对单根完整碳管, Y形分子结在不同温度下导致热导率大约12%–85%的下降; Y结主干向分支方向的导热能力强于分支向主干方向的导热能力; Y结降低热导率的作用随着温度的升高逐渐减小; Y结的热整流效果随着温度的上升先减弱后增强. 关键词： 碳纳米管 热导率 热整流     

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.     

Temperature-Dependent Debye Temperature and Specific Capacity of Graphene

The shot-range interaction and the atomic anharmonic vibration are both considered,and then the analytic functions of the Debye temperature,the specific capacity and the thermal conductivity of graphene with the temperature are obtained.The influence of anharmonic vibration on these thermal physical properties is also investigated.Some theoretical results are given.If only the harmonic approximation is considered,the Debye temperature of the graphene is unrelated to the temperature.If the anharmonic terms are considered,it increases slowly with the increasing temperature.The molar heat capacity of the graphene increases nonlinearly with the increasing temperature.The mean free path of phonons and the thermal conductivity of the graphene decrease nonlinearly with the increasing temperature.The relative changes of the Debye temperature,the specific heat capacity and the thermal conductivity caused by the anharmonic terms increase with the increasing temperature.The anharmonic effect of atomic vibration becomes more significant under higher temperature.     

Thermal Conductivity of poly-p-phenylene-2,6-benzobisoxazole Film along the In-Plane Axis in the 10–300 K Temperature Range

The thermal conductivities of compression molded thin films of poly-p-phenylene-2,6-benzobisoxazole (PBO) were measured in directions along an in-plane axis in the 10–300?K temperature range by a steady-state heat flow method, with interest in the use of the material for superconductivity applications. The thermal conductivities of the PBO films increased from 0.3?W/mK to 9.0?W/mK with increasing temperature from 10?K to 300?K and these were much higher than those of polyimide films, epoxy resin and glass fiber reinforced plastics at all temperatures. The 9.0?W/mK at 300?K was 60% of that of stainless steel (SUS304). It was 6?W/mK at 150?K, which was half that of SUS304 and was 3.3?W/mK at 77?K, which was 33% of that of SUS304. The thermal conductivities of the PBO films were lower than those of a cloth of high strength ultrahigh molecular weight polyethylene fiber reinforced plastics in the 30?K–180?K temperature range and were almost equivalent to its values in the 180?K–300?K temperature range. The main contribution to the thermal conduction in the PBO films was from thermal phonon conduction along the molecular chains. Although many kinds of high thermal conductivity polymeric materials have been prepared by a uni-directional drawing process or by adding high thermal conductive additives, the PBO film showed high thermal conductivity without a uni-directional drawing process or high thermal conductive additive.     

A molecular dynamics study on interfacial heat transport of alkanethiol surfactant coated nanofluids-effect of chain length and stiffness

The thermal transport across the alkanethiol surfactant layer at the nanoparticle/base fluid interface in nanofluids was investigated by molecular dynamics simulation, with consideration of the conformation of the surfactant layer with different surfactant chain lengths and backbone stiffness. The variation of temperature drop at nanoparticle-surfactant interface reveals that the interfacial thermal conductance was mediated by the chain length, possibly due to the difference in the adsorption density of surfactant on the surface of the nanoparticles, because of the blocking effect from the bending of the long alkyl chains. The intrinsic thermal conductivity of the surfactant layer increased with decreasing chain length and increasing chain stiffness because of the phonon scattering effect from the bending and cross-linking of the alkyl chains. We quantified the modes of heat flow across the surfactant layer and found that the contribution of intramolecular bonded interaction was much higher than that of atomic translation and nonbonded interaction separately. By analysing the variation of bonded interaction contrition with chain length and stiffness, it is demonstrated that the increased thermal conductivities benefited from the enhanced thermal transfer through the covalent bonds of surfactant molecules. The results can provide insights into the design of thermally conductive surfactants.     

介孔材料MCM-41的导热研究

文章根据二氧化硅介孔材料MCM-41纳米孔结构特点,首先建立和验证了纳米结构单元模型,然后使用平衡分子动力学方法模拟了孔壁热导率;接着耦合孔隙内气体导热,开展了一维传热分析,最终提炼出MCM-41的有效热导率表达式;并对壁厚、孔径和孔隙率对热导率的影响进行了分析.研究结果表明,MCM-41具有良好的绝热性能,其有效热导率随孔隙率增大近似呈线性减小,且表现出各向异性;导热性能沿孔道长度方向表现出准一维特性. 关键词： 有效热导率 介孔材料 MCM-41 平衡分子动力学     

Normal heat conductivity of the one-dimensional lattice with periodic potential of nearest-neighbor interaction

The process of heat conduction in a chain with a periodic potential of nearest-neighbor interaction is investigated by means of molecular dynamics simulation. It is demonstrated that the periodic potential of nearest-neighbor interaction allows one to obtain normal heat conductivity in an isolated one-dimensional chain with conserved momentum. The system exhibits a transition from infinite to normal heat conductivity with the growth of its temperature. The physical reason for normal heat conductivity is the excitation of high-frequency stationary localized rotational modes. These modes absorb the momentum and facilitate locking of the heat flux.     

Effect of triangular vacancy defect on thermal conductivity and thermal rectification in graphene nanoribbons

We investigate the thermal transport properties of armchair graphene nanoribbons (AGNRs) possessing various sizes of triangular vacancy defect within a temperature range of 200–600 K by using classical molecular dynamics simulation. The results show that the thermal conductivities of the graphene nanoribbons decrease with increasing sizes of triangular vacancy defects in both directions across the whole temperature range tested, and the presence of the defect can decrease the thermal conductivity by more than 40% as the number of removed cluster atoms is increased to 25 (1.56% for vacancy concentration) owing to the effect of phonon–defect scattering. In the meantime, we find the thermal conductivity of defective graphene nanoribbons is insensitive to the temperature change at higher vacancy concentrations. Furthermore, the dependence of temperatures and various sizes of triangular vacancy defect for the thermal rectification ration are also detected. This work implies a possible route to achieve thermal rectifier for 2D materials by defect engineering.     

Molecular dynamics simulation of thermal conductivity of Cu–Ar nanofluid using EAM potential for Cu–Cu interactions

Mechanism of heat conduction in copper-argon nanofluids is studied by molecular dynamics simulation and the thermal conductivity was obtained using the Green–Kubo method. While the interatomic potential between argon atoms is described using the well-known Lennard–Jones (L–J) potential, a more accurate embedded atom method (EAM) potential is used in describing the interatomic interaction between copper atoms. It is found that the heat current autocorrelation function obtained using L–J potential to describe the copper-copper interatomic interaction fluctuates periodically due to periodic oscillation of the instantaneous microscopic heat fluxes. Thermal conductivities of nanofluids using EAM potentials were calculated with different volume fractions but the same nanoparticle size. The results show that thermal conductivity of nanofluids are almost a linear function of the volume fraction and slightly higher than the results predicted by the conventional effective media theory for a well-dispersed solution. A solid-like base fluid liquid layer with a thickness of 0.6 nm was found in the simulation and this layer is believed to account for the small discrepancy between the results of MD simulation and the conventional effective media theory.     

考虑界面接触热阻的一维复合结构的热整流机理

建立了考虑变截面、变热导率及界面接触热阻效应的组合热整流结构的温度场及热整流系数的理论模型和有限元解.数值算例证明了本文模型及算法的可靠性,进而通过参数影响研究确定了若干几何及材料参数对结构热整流系数的影响规律,揭示界面接触热阻对热整流效果的影响机理.研究结果表明长度比、截面半径变化率、热导率、边界条件温差和界面接触热阻等因素必须通过优化设计才能得到最大的热整流系数,同时界面接触热阻的引入也为调控热整流系数提供了一条新的途径.     

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.