带孔硅纳米薄膜热整流及声子散射特性研究

本文基于非平衡的分子动力学模拟方法计算了带有三角形孔的硅纳米薄膜的界面热阻特性,结果表明300-1100 K范围内随着热流方向的改变,在含有三角形孔的硅纳米薄膜中存在热整流效应,热整流系数达28％.同时借助于声子波包动力学模拟方法,获得了不同频率下的纵波声子在三角形孔处的散射特性,结果表明纵波声子在散射过程中产生了横波声子,并且从三角形底部向顶部入射的声子能量透射系数比反向时平均低22％.不对称结构引起的声子透射率的差异是引起热整流效应的主要因素.     

多约束纳米结构的声子热导率模型研究

纳米技术的快速发展使得对微纳尺度导热机理的深入研究变得至关重要. 理论和实验都表明, 在纳米尺度下声子热导率将表现出尺寸效应. 基于声子玻尔兹曼方程和修正声子平均自由程的方法得到了多约束纳米结构的声子热导率模型, 可以描述多个几何约束共同作用下热导率的尺寸效应. 不同几何约束对声子输运的限制作用可以分开计算, 总体影响则通过马西森定则进行耦合. 对于热流方向的约束, 采用扩散近似的方法求解声子玻尔兹曼方程; 对于侧面边界约束, 采用修正平均自由程的方法计算边界散射对热导率的影响. 得到的模型能够预测纳米薄膜(法向和面向)及有限长度方形纳米线的热导率随相应特征尺寸的变化. 与蒙特卡罗模拟及硅纳米结构热导率实验值的对比验证了模型的正确性.     

铝纳米颗粒的热物性及相变行为的分子动力学模拟

采用分子动力学方法模拟了纳米金属铝在粒径为0.8-3.2 nm 时的熔点、密度和声子热导率的变化, 研究了粒径为1.6 nm的铝纳米颗粒的密度、比热和声子热导率随温度的变化. 采用原子嵌入势较好地模拟了纳米金属铝的热物性及相变行为, 根据能量-温度曲线和比热容-温度曲线对铝纳米颗粒的相变温度进行了研究, 并利用表面能理论、尺寸效应理论对铝纳米颗粒熔点的变化进行了分析. 随着纳米粒径的不断增大, 铝纳米颗粒的熔点呈递增状态, 当粒径在2.2-3.2 nm时, 熔点的增幅减缓, 但仍处于递增趋势. 随着纳米粒径的增大, 铝纳米颗粒的密度呈单调递减, 热导率则呈线性单调递增, 且热导率的变化情况符合声子理论. 随着温度的升高, 粒径为1.6 nm的铝纳米颗粒的密度、热导率均减小. 该模拟从微观原子角度对纳米材料的热物性进行了研究, 对设计基于铝纳米颗粒的相变材料具有指导意义.     

硅纳米薄膜中声子弹道扩散导热的蒙特卡罗模拟

通过建立声子散射概率函数描述声子在输运过程中的散射,提出了一种模拟声子弹道扩散导热的蒙特卡罗方法,并将其应用于硅纳米薄膜中的稳态和瞬态弹道扩散导热过程的研究. 提出的蒙特卡罗方法对边界发射的声子束进行跟踪,根据散射概率函数模拟声子束在传播区域内经历的散射过程,并通过统计声子束的分布得到温度分布. 稳态导热过程的模拟发现,尺寸效应会引起边界温度跳跃,其值随着Knudsen数的增大而增大;计算的硅纳米薄膜的热导率随着厚度的增大而增大,与文献中的实验数据和理论模型相符. 通过瞬态导热过程的模拟得到了纳米薄膜内的温度分布随时间的变化,发现瞬态导热过程中的热波现象与空间尺度相关,材料尺寸越小,弹道输运越强,薄膜中的热波现象也越显著. 关键词： 纳米薄膜 弹道扩散导热 蒙特卡罗模拟 尺寸效应     

硅纳米薄膜法向热导率的进一步分析

应用非平衡分子动力学方法进一步研究了平均温度为300K、厚度为2.715nm-43.44nm的单晶硅薄膜的法向热导率,模拟结果表明,薄膜热导率低于同温度下单晶硅的实验值,存在显著的尺寸效应,当膜厚度在20nm以下时,法向热导率随尺度减小而线性减小,当膜厚度大于20nm时法向热导率随尺度呈现二阶多项式变化。法向热导率的变化规律与面向热导率的变化规律类似,表明薄膜厚度和表面晶格结构对声子传热影响的复杂性。     

Ballistic–diffusive phonon transport and size induced anisotropy of thermal conductivity of silicon nanofilms

The effective thermal conductivity of nanofilms is size dependent due to the diffusive–ballistic transport of phonons. In this paper, we investigate the cross-plane phonon transport from the viewpoint of the phonon Boltzmann equation. A predictive model for the size dependent thermal conductivity is proposed and agrees well with the results of molecular dynamics simulation for silicon nanofilms. The ballistic transport has different effects on the heat conduction in the in-plane or cross-plane directions, which causes the anisotropy of thermal conductivity of nanofilms. Such anisotropy is also size dependent and vanishes with the increase of film thickness.     

固态金属中声子热传递的分子动力学模拟研究

固态金属中的热传递是声子和自由电子共同作用的结果。自由电子引起的热导率可以通过电导率,利用Wiedemann-Franz定律得到,声子引起的热导率目前仍然不能进行实验测量,只能借助其他方法来研究。本文采用非平衡分子动力学(NEMD)方法,用镶嵌原子方法(EAM)势能模型,模拟计算了不同厚度(1.760-10.56nm)金属镍薄膜中由于声子-声子作用引起的热导率。然后根据纳米厚度金属薄膜的热导率借助关联式推到宏观尺度下由于声子-声子作用引起的热导率。结果表明,对于纳米厚度金属薄膜,由于声子-声子作用引起的热导率比块体金属镍的热导率小一个数量级;薄膜厚度越小,声子-声子作用引起的热导率越小;对于块体金属镍,由于声子-声子作用引起的热导率约占其总热导率的33.0%左右。     

Effect of normal processes on thermal conductivity of germanium,silicon and diamond

The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch — KK-S model and (b) between different phonon branches — KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.      

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

Phonon considerations in the reduction of thermal conductivity in phononic crystals

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity.     

Performance investigation of nanoscale thermal cloak by the perforated silicon film

In recent years, nanoscale thermal cloak, as a representative of nanoscale heat flux regulation devices, has attracted a lot of attention from researchers. However, the existing design methods are relatively complicated and all adopt constant temperature boundaries, the temperature changes constantly in the real environment, which greatly hinders its engineering applications. In this paper, inspired by phonon localization theory, we construct a nanoscale thermal cloak by a perforated silicon membrane and evaluate its cloaking performance and dynamic response. Results show that when the perforated area is fixed, the more the number of holes, the better the cloaking performance. In addition, the nanoscale thermal cloak still exhibits good cloaking performance in the dynamic environment. Finally, the cloaking mechanism is analyzed by calculating the phonon density of states (PDOS) and mode participation rate (MPR), and the reduction of thermal conductivity in the functional region is attributed to phonon localization.     

Interface-facilitated energy transport in coupled Frenkel–Kontorova chains

The role of interface couplings on the energy transport of two coupled Frenkel–Kontorova (FK) chains is explored through numerical simulations. In general, it is expected that the interface couplings result in the suppression of heat conduction through the coupled system due to the additional interface phonon–phonon scattering. In the present paper, it is found that the thermal conductivity increases with increasing intensity of interface interactions for weak inter-chain couplings, whereas the heat conduction is suppressed by the interface interaction in the case of strong inter-chain couplings. Based on the phonon spectral energy density method, we demonstrate that the enhancement of energy transport results from the excited phonon modes (in addition to the intrinsic phonon modes), while the strong interface phonon–phonon scattering results in the suppressed energy transport.     

Thermal conductivity of hydrogenated amorphous silicon

The thermal conductivity of amorphous silicon thin films is measured in one dimension steady state condition. The experimental method is based on heating the sample front surface and monitoring the temperatures at its two sides. The experiments were carried out in conditions ensuring one-direction heat flow from top to bottom throughout the sample thickness. Sputtered a-Si:H films prepared with different conditions are used in order to investigate the dependence of thermal conductivity on material properties (i.e. hydrogen content and microstructure). The results show that, firstly, amorphous silicon is a very bad thermal conductor material. Secondly, the disorder in the film network plays an important role in thermal conduction. The highly disordered film exhibits the lowest thermal conductivity.     

A molecular dynamics simulation: the effect of finite size on the thermal conductivity in a single crystal silicon

Non-equilibrium molecular dynamics (NEMD) simulations are performed to calculate thermal conductivity. The environment-dependent interatomic potential (EDIP) potential on crystal silicon is adopted as a model system. The issues are related to nonlinear response, local thermal equilibrium and statistical averaging. The simulation results by non-equilibrium molecular dynamics show that the calculated thermal conductivity decreases almost linearly as the film thickness reduced at the nanometre scale. The effect of size on the thermal conductivity is also obtained by a theoretic analysis of the kinetic theory and formulas of the heat capacity. The analysis reveals that the contributions of phonon mean free path (MFP) and phonon number in a finite cell to thermal conductivity are very important.     

Construction and mechanism analysis on nanoscale thermal cloak by in-situ annealing silicon carbide film

In recent years,there is a strong interest in thermal cloaking at the nanoscale,which has been achieved by using graphene and crystalline silicon films to build the nanoscale thermal cloak according to the classical macroscopic thermal cloak model.Silicon carbide,as a representative of the third-generation semiconductor material,has splendid properties,such as the high thermal conductivity and the high wear resistance.Therefore,in the present study,we build a nanoscale thermal cloak based on silicon carbide.The cloaking performance and the perturbation of the functional area to the external temperature filed are analyzed by the ratio of thermal cloaking and the response temperature,respectively.It is demonstrated that silicon carbide can also be used to build the nanoscale thermal cloak.Besides,we explore the influence of inner and outer radius on cloaking performance.Finally,the potential mechanism of the designed nanoscale thermal cloak is investigated by calculating and analyzing the phonon density of states(PDOS)and mode participation rate(MPR)within the structure.We find that the main reason for the decrease in the thermal conductivity of the functional area is phonon localization.This study extends the preparation method of nanoscale thermal cloaks and can provide a reference for the development of other nanoscale devices.     

The effect of normal phonon-phonon scattering processes on the maximum thermal conductivity of isotopically pure silicon crystals

The effect of normal phonon-phonon scattering processes on the thermal conductivity of silicon crystals with various degrees of isotope disorder is considered. The redistribution of phonon momentum in normal scattering processes is taken into account within each oscillation branch (the Callaway generalized model), as well as between different oscillation branches of the phonon spectrum (the Herring mechanism). The values of the parameters are obtained that determine the phonon momentum relaxation in anharmonic scattering processes. The contributions of the drift motion of longitudinal and transverse phonons to the thermal conductivity are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons in the Herring relaxation model represents an efficient mechanism that limits the maximum thermal conductivity in isotopically pure silicon crystals. The dependence of the maximum thermal conductivity on the degree of isotope disorder is calculated. The maximum thermal conductivity of isotopically pure silicon crystals is estimated for two variants of phonon momentum relaxation in normal phonon-phonon scattering processes.     

Modification of the lattice thermal conductivity in silicon quantum wires due to spatial confinement of acoustic phonons

Lattice thermal conductivity in silicon quantum wires is theoretically investigated. The bulk of heat in silicon structures is carried by acoustic phonons within a small region in the first Brillouin zone. Our formalism rigorously takes into account modification of these acoustic phonon modes and phonon group velocities in free- and clamped-surface wires due to spatial confinement. From our numerical results, we predict a significant decrease (more than an order of magnitude) of the lattice thermal conductivity in cylindrical quantum wires with diameter D =  200 Å. The decrease is about two times stronger in quantum wires than in quantum wells of corresponding dimensions. Our theoretical results are in qualitative agreement with experimentally observed drop of the lattice thermal conductivity in silicon low-dimensional structures.     

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF₂, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.     

单晶硅薄膜法向热导率分子动力学研究

采用非平衡分子动力学方法（NEMD）研究了平均温度为 500K、厚度为 2～32nm的单晶硅薄膜的法向热导率。模拟结果表明,薄膜热导率显著低于对应温度下的体硅单晶的实验值,并随膜厚度减小以接近线性的规律减小。用声子气动力论模型的分析结果与NEMD模拟相一致,表明纳米单晶硅薄膜中声子平均自由程显著减小。     

Tuning Thermal Conductivity in Si Nanowires with Patterned Structures

Tuning the thermal conductivity of silicon nanowires(Si-NWs)is essential for realization of future thermoelectric devices.The corresponding management of thermal transport is strongly related to the scattering of phonons,which are the primary heat carriers in Si-NWs.Using the molecular dynamics method,we find that the scattering of phonons from internal body defects is stronger than that from surface structures in the low-porosity range.Based on our simulations,we propose the concept of an exponential decay in thermal conductivity with porosity,specifically in the low-porosity range.In contrast,the thermal conductivity of Si-NWs with a higher porosity approaches the amorphous limit,and is insensitive to specific phonon scattering processes.Our findings contribute to a better understanding of the tuning of thermal conductivity in Si-NWs by means of patterned nanostructures,and may provide valuable insights into the optimal design of one-dimensional thermoelectric materials.