考虑界面散射的金属纳米线热导率修正

理论分析了声子和电子输运对Cu, Ag金属纳米线热导率的贡献. 采用镶嵌原子作用势模型描述纳米尺寸下金属原子间的相互作用, 应用平衡分子动力学方法和Green-Kubo函数模拟了金属纳米线的声子热导率; 采用玻尔兹曼输运理论和Wiedemann-Franz定律计算电子热导率; 并通过散射失配模型和Mayadas-Shatzkes模型引入晶界散射的影响. 在此基础上, 考察分析了纳米线尺度和温度的影响. 研究结果表明: Cu, Ag纳米线热导率的变化规律相似; 电子输运对金属纳米线的导热占主导地位, 而声子热导率的贡献也不容忽视; 晶界散射导致热导率减小, 尤其对电子热导率作用显著; 纳米线总热导率随着温度的升高而降低; 随着截面尺寸减小而减小, 但声子热导率所占份额有所增加. 关键词： 纳米线 热导率 表面散射 晶界散射     

Phonon thermal transport in InAs nanowires with different size and growth directions

We investigate the phonon thermal transport properties in InAs nanowires with different size and growth directions by using nonequilibrium molecular dynamics methods. The results show a remarkable anisotropy for the thermal conductivity in InAs nanowire. It is found that the thermal conductivity along [110] growth direction is about three times larger than that along [100] or [111] direction. With the increase of temperature, the thermal conductivity along [110] direction decreases significantly. However, the thermal conductivity along other two directions is not sensitive to temperature. Moreover, we find a crossover from ballistic to ballistic-diffusive thermal transport for a certain length of InAs nanowire. A brief physical analysis of these results is given. It is suggested that the anisotropy of thermal conductivity is common for nanowires with zinc blende 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.     

Thermal conductivity of VLS-grown rough Si nanowires with various surface roughnesses and diameters

In this paper, we synthesize VLS-grown rough Si nanowires using Mn as a catalyst with various surface roughnesses and diameters and measured their thermal conductivities. We grew the nanowires by a combination vapor-liquid-solid and vapor-solid mechanism for longitudinal and radial growth, respectively. The surface roughness was controlled from smooth up to about 37 nm by the radial growth. Our measurements showed that the thermal conductivity of rough surface Si nanowires is significantly lower than that of smooth surface nanowires and decreased with increasing surface roughness even though the diameter of the smooth nanowire was lower than that of the rough nanowires. Considering both nanowires were grown via the same growth mechanism, these outcomes clearly demonstrate that the rough surface induces phonon scattering and reduces thermal conductivity with this nanoscale-hole-free nanowires. Control of roughness induced phonon scattering in Si nanowires holds promise for novel thermoelectric devices with high figures of merit.     

Effects of vacancy defects and axial strain on thermal conductivity of silicon nanowires: A reverse nonequilibrium molecular dynamics simulation

Thermal conductivity of silicon nanowires (SiNWs) is evaluated using the reverse nonequilibrium molecular dynamics simulation. The Stillinger–Weber (SW) and Tersoff interatomic potentials are employed to simulate thermal conductivity of SiNWs. In this work, the influence of random vacancy defects, axial strain, temperature and length on thermal conductivity and effective mean free path of SiNWs is investigated. It is found that by raising the percent of random vacancy defects, thermal conductivity of SiNWs decreases linearly for the results obtained form SW potential and nonlinearly for those obtained from Tersoff interatomic potential. Dependence of the thermal conductivity on axial strain is also studied. Results show that thermal conductivity increases as compressive strain increases and decreases as tensile strain increases. Influence of temperature is also predicted. It is found that the thermal conductivity of SiNWs decreases with increasing the mean temperature. Most of the simulations are performed for 4 UC×4 UC×40 UC silicon nanowires using ssp boundary condition.     

Size dependent ultrasonic properties of InN nanowires

The ultrasonic properties of single crystalline indium nitride nanowires (InN NWs) are studied for wire size (diameter) 6–100 nm at 300 K following the interaction potential model. Ultrasonic attenuation, ultrasonic velocity, acoustic coupling constant and thermal relaxation time are calculated using higher order elastic constants and thermal conductivity of the nanowires. The analysis of size dependent thermal relaxation time and ultrasonic properties shows that above the 20 nm diameter, InN nanowire tends towards its bulk material property. The third order polynomial is found to be best fit for size variation of thermal relaxation time. The ultrasonic attenuation as a function of size of the nanowires is found to be mainly affected by the thermal conductivity of the nanowires of different sizes.     

表面共振圆环结构对Si纳米线热传导的调控

利用声子的波动性,在纳米线表面引入共振结构,可以有效阻碍声子输运.为进一步优化共振结构,本文基于平衡态分子动力学(EMD)方法,研究表面共振圆环结构的高度和宽度对Si纳米线热输运性质的影响.结果表明：随着共振圆环高度的增加,Si纳米线的热导率逐渐减小,最大减幅可达61.9%.当高度达到2nm以后,热导率基本不再变化.共振圆环宽度则对热导率的影响较小.声子色散关系中出现的平带,证实了共振圆环引起的声子共振效应;最低共振频率的变化说明了共振圆环中的声子波长决定了共振圆环高度对纳米线热导率的最大影响程度.研究进一步发现,随着共振圆环高度的增加,声学支声子对热导率贡献的比重变小.本文研究结果对高效热电材料和隔热材料的微纳结构设计提供了一种新的思路.     

Anisotropy and temperature dependences of thermal conductivity for silicon nanowires

The anisotropy and temperature dependences of thermal conductivity for silicon nanowires with diameters higher than 50 nm is investigated using the Callaway three-mode model. Contributions to the thermal conductivity from the boundary and bulk mechanisms of phonon scattering are calculated at room temperature. The relationship between the thermal conductivity and nanowire diameter is analyzed in symmetrical directions and at room temperature.     

Novel epoxy-silicone thermolytic transparent packaging adhesives chemical modified by ZnO nanowires for HB LEDs

A novel high transparent thermolytic epoxy-silicone for high-brightness light-emitting diode (HB-LED) is introduced, which was synthesized by polymerization using silicone matrix via diglycidyl ether bisphenol-A epoxy resin (DGEBA) as reinforcing agent, and filling ZnO nanowires to modify thermal conductivity and control refractive index of the hybrid material. The interactions of ZnO nanowires with polymers are mediated by the ligands attached to the nanoparticles. Thus, the ligands markedly influence the properties of ZnO nanowires/epoxy-silicone composites. The refractive indices of the prepared hybrid adhesives can be tuned by the ZnO nanowires from 1.4711 to 1.5605. Light transmittance can be increased by 20% from 80 to 95%. The thermal conductivity of the transparent packaging adhesives is 0.89–0.90 W/mK.     

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.     

Silver nanowires

Free silver nanowires were produced in aqueous electrolyte by a novel chemical reaction. Their diameters are about 27 nm, the lengths range up to more than 70 μm, yielding extreme length to thickness-ratios up to 2500. Their structure was identified by TEM analysis (SAED) and HRTEM to consist of a lattice aligned bundle of five monocrystalline rods of triangular cross-section forming an almost regular pentagonal cross-section. It is demonstrated that, for application purposes, single free nanowires can be mounted between contact areas. This manipulation is enabled by observing the nanowires in real time at atmosphere by Zsigmondy-Siedentopf farfield darkfield microscopy. Experimental results are presented concerning electrical dc–conductivity and optical plasmon polariton excitation, the latter obtained from a single free wire without substrate and a single wire deposited on quartz glass. We also report about a present research cooperation with the Graz group of Aussenegg and Krenn which is devoted to investigate plasmon propagation in our Ag nanowires and to prove application possibilities as information guide fibers – in analogy to optical fibers – which may be integrated into micro- and nanoelectronic circuits.     

Heat transport in ZnO/PMMA nanocomposites

The thermal conductivity of zinc oxide-polymethyl methacrylate (PMMA) composites has been measured. Using theoretical models, the thermal resistance of ZnO-NW/PMMA interfaces (NW is the nanowire) and the critical particle radius above which the composite thermal conductivity can be increased have been estimated. Based on these measurements, the dependence of the thermal conductivity of zinc oxide nanowires on their diameter has been determined.     

Phonon heat transport in silicon nanowires

Thermal conductivity of silicon and porous silicon nanowires based on the equation of phonon radiative transport is theoretically evaluated. The thermal conductivities of silicon nanowires with square cross-sections are found to match molecular dynamics simulation results reasonably well. It is shown that the results of meso-porous silicon nanowires are about two orders of magnitude lower than that of silicon nanowires in a wide range of temperature (50 K-300 K). Received 24 April 2001 and Received in final form 23 December 2001     

碳纳米管电缆式复合材料的热导率

通过非平衡分子动力学方法, 对单壁碳管填充金纳米线的碳纳米管电缆式复合材料开展热导率的模拟分析. 采用Tersoff势函数描述碳-碳原子间的相互作用, Lennard-Jones长程作用势描述碳-金原子间的相互作用, 嵌入原子势函数描述金-金原子间相互作用. 研究结果表明: 相同尺寸下, 金纳米线的电子热导率相较于空碳管以及电缆式复合材料的声子热导率小很多, 对复合材料总热导率的贡献可以忽略; 由于管内金纳米线的存在, 其与碳管的相互作用使得碳管碳原子倾向于沿着轴向振动, 声子间U散射随之减少, 声子平均自由程增加, 导致复合材料热导率明显大于空碳管, 在100–500 K温度范围内高出约20%–45%, 但增大幅度随温升呈降低趋势; 复合材料热导率随着管长增加而增大, 变化趋势和空碳管相似, 但其增长幅度更大; 复合材料和空碳管的热导率随管径增大而减小, 且变化幅度基本一致. 关键词： 碳纳米管 纳米线 电缆式复合材料 导热     

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.     

Understanding length dependences of effective thermal conductivity of nanowires

We have studied the length dependence of effective thermal conductivity of silicon nanowires by a thermon gas model and MD simulations. After modifications of the force term by considering the resistance enhancements from thermon gas interactions with the confined surfaces and the ends (inlet and outlet), the theoretical predictions of effective thermal conductivity agree well with the results of MD simulations in the length range of 4 to 550 nm. The result suggests that the resistance enhancement effect by thermon–boundary interactions, instead of the heat inertia, plays the dominating role in the non-Fourier heat conduction in silicon nanowires.     

Thermal conductivity of periodic array of intermolecular junctions of silicon nanowires

Recent studies on intramolecular junctions of silicon nanowires and carbon nanotubes have revealed a wealth of intriguing phenomena. However, the thermal properties of the intramolecular junctions of silicon nanowires (SiNWs) are not yet well understood. In this study periodic arrays of intramolecular junctions with different lattice orientations are investigated, and their thermal conductivities are calculated using nonequilibrium molecular dynamics (NEMD) simulations. Different from the X-shaped and Y-shaped junctions of carbon nanotubes, no distinct jump is found in the temperature profile at the junctions. Compared with straight pristine SiNWs of the same length, the thermal conductivity of the periodic array of intramolecular junctions is reduced. The underlying mechanism of the observed behavior is analyzed by the phonon spectral density of the atomic velocities. The dependence of temperature on the thermal conductivity of this junction array structure is discussed.     

基于第一性原理的Mn纳米线掺杂二维SnSe热电性能研究

本文提出了一种在二维SnSe中掺杂一维Mn纳米线的2D-1D复合结构,并系统地研究了其热电性能。结果表明,一维Mn纳米线将电子态汇聚在纳米线附近,提高了材料的各向异性,降低了电子在某一方向上的散射效应,导致了较高的迁移率和电导率。自旋向上和向下的电子态发生简并,导致了较高的塞贝克系数和电导率。此外,Mn纳米线将晶格热导率降低了约0.17 W·m^?1·K^?1。在200至650 K的温度范围内,3Mn-SnSe具有0.73至3.78的极高ZT值,比本征二维SnSe平均提高了约39.2%。     

基于第一性原理的Mn纳米线掺杂二维SnSe热电性能研究

本文提出了一种在二维SnSe中掺杂一维Mn纳米线的2D-1D复合结构,并系统地研究了其热电性能。结果表明,一维Mn纳米线将电子态汇聚在纳米线附近,提高了材料的各向异性,降低了电子在某一方向上的散射效应,导致了较高的迁移率和电导率。自旋向上和向下的电子态发生简并,导致了较高的塞贝克系数和电导率。此外,Mn纳米线将晶格热导率降低了约0.17 W·m^?1·K^?1。在200至650 K的温度范围内,3Mn-SnSe具有0.73至3.78的极高ZT值,比本征二维SnSe平均提高了约39.2%。     

Lattice thermal conductivity crossovers in semiconductor nanowires

For binary compound semiconductor nanowires, we find a striking relationship between the nanowire's thermal conductivity kappa(nwire), the bulk material's thermal conductivity kappa(bulk), and the mass ratio of the material's constituent atoms, r, as kappa(bulk)/kappa(nwire) (alpha) (1+1/r)(-3/2). A significant consequence is the presence of crossovers in which a material with higher bulk thermal conductivity than the rest is no longer the best nanowire thermal conductor. We show that this behavior stems from a change in the dominant phonon scattering mechanism with decreasing nanowire size. The results have important implications for nanoscale heat dissipation, thermoelectricity, and thermal conductivity of nanocomposites.