烃类水合物导热特性的分子动力学模拟

采用分子动力学模拟方法Green-Kubo理论计算了263.15 K、3 MPa,sⅠ乙烷水合物、乙烯水合物的导热,给出密度和热导率值。从主客体分子和晶体结构（致密性、规整程度）对导热的影响等角度研究了烃类水合物（甲烷水合物、乙烷水合物、乙烯水合物）导热的特性。结果显示化学性质相似、分子量相差不大的烃类形成的水合物,其导热具有相似的温度压力依赖关系和晶体结构相关关系。对于sⅠ型水合物,水分子对水合物导热的影响远远超过客体分子对导热的影响。水合物的分子量越大,水合物密度越大,热导率越大。水合物晶体越致密、晶格越规整,热导率越大。     

烃类水合物导热特性的分子动力学模拟

采用分子动力学模拟方法 Green-Kubo理论计算了263.15 K、3 MPa,sⅠ乙烷水合物、乙烯水合物的导热,给出密度和热导率值。从主客体分子和晶体结构(致密性、规整程度)对导热的影响等角度研究了烃类水合物(甲烷水合物、乙烷水合物、乙烯水合物)导热的特性。结果显示化学性质相似、分子量相差不大的烃类形成的水合物,其导热具有相似的温度压力依赖关系和晶体结构相关关系。对于sⅠ型水合物,水分子对水合物导热的影响远远超过客体分子对导热的影响。水合物的分子量越大,水合物密度越大,热导率越大。水合物晶体越致密、晶格越规整,热导率越大。     

客体分子数对甲烷水合物导热性能影响的分子动力学模拟

本文通过采用EMD方法Green-Kubo理论计算263.15 K 晶穴占有率0-100% sI甲烷水合物导热系数,研究客体分子数对甲烷水合物导热性能的影响。模拟结果显示,甲烷水合物的低导热性能由主体分子构建的笼型结构决定。而在相同温压条件下,随着客体分子甲烷进入晶胞数目增多,晶穴占有率增大后,密度增大,同时客体分子对声子的散射也增强,二者均导致导热性能增强。     

独立探头3ω法表征甲烷水合物热导率和热扩散率

甲烷水合物热物性参数的测量一般是基于时域信号测量,测量方法没有考虑探测器与试样之间的接触热阻。基于频域信号测量原理,研发的3ω独立探头大大拓展了该方法的应用范围。建立了低温高压甲烷水合物合成测量系统。利用独立探头3ω法实时测量甲烷水合物热导率、热扩散率、探头和甲烷水合物之间的接触热阻。分析了甲烷水合物热导率、热扩散率随温度的变化规律;比较了测量值与国内外学者测量数据的不同;发现接触热阻对甲烷水合物热导率有显著影响。     

独立探头3ω法表征甲烷水合物热导率和热扩散率

甲烷水合物热物性参数的测量一般是基于时域信号测量,测量方法没有考虑探测器与试样之间的接触热阻。基于频域信号测量原理,研发的3ω独立探头大大拓展了该方法的应用范围。建立了低温高压甲烷水合物合成测量系统。利用独立探头3ω法实时测量甲烷水合物热导率、热扩散率、探头和甲烷水合物之间的接触热阻。分析了甲烷水合物热导率、热扩散率随温度的变化规律;比较了测量值与国内外学者测量数据的不同;发现接触热阻对甲烷水合物热导率有显著影响。     

碳化硅材料热导率计算研究进展

从声子散射机制出发,介绍了Si C热导率的温度特性和微观导热机理。综述了Si C单晶热导率的2种主要计算方法。Boltzmann-弛豫时间近似(RTA)适用于各个温度段的热导率计算,而分子动力学方法更适用于高温热导率计算。分子动力学方法相比于Boltzmann-RTA方法的优点在于它可以考虑所有高次项的非谐作用。介绍了3种Si C陶瓷热导率近似计算模型,包括界面热阻模型、Debye-Callaway模型及多相系统热导率模型。下一步研究的主要方向仍然是优化计算模型及减少拟合参数。     

二氧化碳水合物导热和热扩散特性

热导率和热扩散率是天然气水合物资源开采关键性基础热物性数据,采用反应釜内壁衬有氟塑料材料,低过冷度,让水合物在反应釜内逐层生成的合成方法,获得可直接用于导热测试的二氧化碳水合物样品。采用瞬变平面热源法原位测试了温度264.68～282.04 K、压力1.5～3 MPa二氧化碳水合物热导率、热扩散率,并测试了二氧化碳水合物在268.05 K、0.6 MPa左右发生自保护效应过程中热导率、热扩散率,获得了晶态下和自保护效应过程中的二氧化碳水合物热导率、热扩散率变化特性。测试结果将为天然气水合物资源的开发利用提供基础数据和理论依据。     

二维氮化铝材料传热性能的模拟研究

二维氮化铝材料是一种新型Ⅲ-Ⅴ族二维材料,具有与石墨烯相似的分子结构和材料性能,受到了广泛的关注,然而其导热性能尚未被充分探讨。应用分子动力学模拟的方法研究了单层二维氮化铝在不同温度的热稳定性和导热性能,并分析了其声子频谱。结果表明,单层二维氮化铝材料可以在极高温度（3500 K）下保持结构稳定性,同时在常温情况热导率可达264.2 W·m^-1·K^-1;在500 K以上温度时,声子色散现象使得该材料热导率明显降低。为二维氮化铝材料导热过程的调控和高温导热材料的应用提供了理论指导。     

二氧化碳水合物导热和热扩散特性

热导率和热扩散率是天然气水合物资源开采关键性基础热物性数据,采用反应釜内壁衬有氟塑料材料,低过冷度,让水合物在反应釜内逐层生成的合成方法,获得可直接用于导热测试的二氧化碳水合物样品。采用瞬变平面热源法原位测试了温度264.68~282.04 K、压力1.5~3 MPa二氧化碳水合物热导率、热扩散率,并测试了二氧化碳水合物在268.05 K、0.6 MPa左右发生自保护效应过程中热导率、热扩散率,获得了晶态下和自保护效应过程中的二氧化碳水合物热导率、热扩散率变化特性。测试结果将为天然气水合物资源的开发利用提供基础数据和理论依据。     

二维氮化铝材料传热性能的模拟研究

二维氮化铝材料是一种新型Ⅲ-Ⅴ族二维材料,具有与石墨烯相似的分子结构和材料性能,受到了广泛的关注,然而其导热性能尚未被充分探讨。应用分子动力学模拟的方法研究了单层二维氮化铝在不同温度的热稳定性和导热性能,并分析了其声子频谱。结果表明,单层二维氮化铝材料可以在极高温度(3500 K)下保持结构稳定性,同时在常温情况热导率可达264.2 W·m~(-1)·K~(-1);在500 K以上温度时,声子色散现象使得该材料热导率明显降低。为二维氮化铝材料导热过程的调控和高温导热材料的应用提供了理论指导。     

Effects of quantum statistics of phonons on the thermal conductivity of silicon and germanium nanoribbons

We present molecular dynamics simulation of phonon thermal conductivity of semiconductor nanoribbons with an account for phonon quantum statistics. In our semiquantum molecular dynamics simulation, dynamics of the system is described with the use of classical Newtonian equations of motion where the effect of phonon quantum statistics is introduced through random Langevin-like forces with a specific power spectral density (color noise). The color noise describes interaction of the molecular system with the thermostat. The thermal transport of silicon and germanium nanoribbons with atomically smooth (perfect) and rough (porous) edges are studied. We show that the existence of rough (porous) edges and the quantum statistics of phonon change drastically the low-temperature thermal conductivity of the nanoribbon in comparison with that of the perfect nanoribbon with atomically smooth edges and classical phonon dynamics and statistics. The rough-edge phonon scattering and weak anharmonicity of the considered lattice produce a weakly pronounced maximum of thermal conductivity of the nanoribbon at low temperature.     

Spectral phonon thermal properties in graphene nanoribbons

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.     

十八烷酸热传导机制的尺度效应研究

十八烷酸的热导率具有显著的尺寸依赖性,然而只有宏观热输运性质得到充分研究,限制了微尺度特征结构的复合材料热性能的进一步提高。因此,为满足新型复合相变材料的研究需求,十八烷酸的纳米尺度热输运特性亟待解决。基于分子动力学模拟,系统性地研究了块体、纳米线和纳米链三种形态的十八烷酸热导率的演变规律,通过平衡法模拟的热导率分别为0.4546、0.2213和0.0085 W·m^-1·K^-1;结合声子态密度和重叠能分析,发现在较低频率声子振动的衰减导致纳米线热导率低于块体,显著降低的声子重叠能严重阻碍纳米链的声子输运导致极低热导率。     

三维石墨烯-碳纳米管复合结构热导率的分子动力学模拟

采用非平衡分子动力学方法模拟了三维石墨烯-碳纳米管复合结构的法向热导率。结果表明相比于多层石墨烯,其法向热导率提高了一个量级,其界面热阻相比碳纳米管的接触热阻降低了一个量级,但是石墨烯和碳纳米管的界面形变又阻碍了三维石墨烯-碳纳米管复合热导率的进一步提高。通过其振动态密度和重叠能进一步探究了三维石墨烯-碳纳米管复合结构结构能量的传递及声子的局域化情况。结果表明,碳管的添加激发了更多中高频声子振动参与传热,但是依然是低频声子占据主导;验证了界面处的形变是阻止法向热导率进一步提升的主要因素。     

High and anisotropic thermal conductivity of body-centered tetragonal C4 calculated using molecular dynamics

The thermal properties of body-centered tetragonal C₄ (bct-C₄), a new allotrope of carbon, were investigated using molecular dynamics (MD) simulations. The calculations gave a high and anisotropic thermal conductivity that is the first of its kind. The cross-plane thermal conductivity is 1209 W/(m K) at room temperature, which is even higher than that of diamond. The thermal conductivity decreases as the temperature increases from 80 to 400 K. The density of states of bct-C₄ was analyzed, which has a prominent peak at 36 THz. The relaxation times were calculated by fitting a heat flux autocorrelation function. The results showed that the acoustic phonons play the dominant role in the heat conduction, with a contribution of more than 99%. The relaxation times decrease with increasing temperature, as does the contribution of the acoustic phonons. Finally, the thermal conductivity based on lattice dynamics agreed well with that from the MD method, with which the group velocity and mean free path were deduced. This outstanding thermal property makes bct-C₄ a promising substitute for diamond, especially as thermal interface materials in microelectronic packaging.     

The dependence of lattice thermal conductivity on phonon modes in pyrochlore-related Ln2Sn2O7 (Ln = La,Gd)

Rare-earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln₂Sn₂O₇ (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore-type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La₂Sn₂O₇ and Gd₂Sn₂O₇. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare-earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.     

A quantum mechanical formulation of electron transport induced wind forces in metallic single-walled carbon nanotubes

In this paper the forces induced on the atoms of the lattice due an electric current (electron transport induced wind forces) are calculated based on quantum mechanics. These forces are calculated in metallic single-walled armchair carbon nanotubes (SWCNTs) from the momentum transfer between the charge carriers and the lattice in a quantum mechanical framework. Energy and phonon dispersion relations are the main input for the formulation proposed. Scattering of electrons with longitudinal acoustic and longitudinal optical phonons are considered to be the only scattering mechanisms that are responsible for the momentum exchange. The current-voltage characteristics are also predicted using the same framework and show good agreement with experimental data. The study shows that using the constant effective charge number for SWCNT is inaccurate at higher electric field forces due to saturation of the lowest energy subband. The thermal effect on the effective charge number appears to be very important, due to the increasing scattering probabilities at higher temperatures.     

Phonon-limited electron mobility in III-nitride heterojunctions

Low-field electron mobility limited by phonons in GaN-, InN- and AlN-based heterojunctions (HJs) for temperatures T < 300 K is studied within the framework of Boltzmann transport formalism by an iterative method. Electrons are assumed to occupy the lowest subband and scattered by the inelastic polar LO phonons and by quasi-elastic acoustic phonons through the deformation potential and piezoelectric couplings. Numerical calculations of the energy dependence of the first-order perturbation distribution function ϕ(E) for the polar LO phonons and of the acoustic phonon relaxation rates τ^− 1(E) bring out the characteristic features of phonon scattering in nitride HJs. The temperature and concentration dependences of phonon-limited mobility are compared with the low-temperature and high-energy relaxation time approximations, commonly used for ϕ. Calculations, taking into account other relevant scattering mechanisms, obtain good agreement with available Hall mobility data for GaN/AlGaN HJ.