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.     

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     

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     

纳米孔模板浸润法制备聚乙烯纳米线阵列的热导率的实验研究

采用纳米孔模板润湿技术制备了直径为200 nm的低密度聚乙烯(LDPE)纳米线阵列, 并利用纳秒激光闪光法测量了20—80℃时LDPE纳米线阵列的热导率. 测量得到室温时LDPE纳米线阵列的热导率为2.2 W/mK, 大约比其体材料的热导率高1个数量级, 并且纳米线阵列的热导率随温度的升高略有增加. 忽略纳米线之间的声子散射, 估算得到室温下单根LDPE纳米线的热导率高于5 W/mK. 本文制备LDPE纳米线热导率的提高源自其分子链定向度增加导致的低维导热效应的增强, 纳米线的分子链定向度受工艺过程中流体剪切、振动、分子链迁移运动、 纳米孔约束等几种因素的综合影响.     

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.     

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.     

Measuring the thermal conductivity of a single carbon nanotube

Although the thermal properties of millimeter-sized carbon nanotube mats and packed carbon nanofibers have been readily measured, measurements for a single nanotube are extremely difficult. Here, we report a novel method that can reliably measure the thermal conductivity of a single carbon nanotube using a suspended sample-attached T-type nanosensor. Our experimental results show that the thermal conductivity of a carbon nanotube at room temperature increases as its diameter decreases, and exceeds 2000 W/mK for a diameter of 9.8 nm. The temperature dependence of the thermal conductivity for a carbon nanotube with a diameter of 16.1 nm appears to have an asymptote near 320 K. The present method is, in principle, applicable to any kind of a single nanofiber, nanowire, and even single-walled carbon nanotube.     

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.     

纳米ZnO／石蜡复合相变材料的热物理性质研究

本文通过将纳米氧化锌（ZnO）颗粒加入熔融的石蜡（PW）并进行搅拌和超声制备了一种纳米ZnO／PW复合相变储能材料。为使纳米氧化锌在基体物质中分散均匀,在制备过程中使用了搅拌和超声以制备均匀的复合材料。使用扫描电镜观察其微观结构表明氧化锌在石蜡中分散良好。对所得ZnO／PW复合相变材料的相变温度、相变焓及导热系数等热物...     

A study on the DC-electrical and thermal conductivities of epoxy/ZnO composites doped with carbon black

Thermo-electrical characterizations of hybrid polymer composites, made of epoxy matrix filled with various zinc oxide (ZnO) concentrations (0, 4.9, 9.9, 14.9, and 19.9 wt%), and reinforced with conductive carbon black (CB) nanoparticles (0.1 wt%), have been investigated as a function of ZnO concentration and temperature. Both the measured DC-electrical and thermal conductivities showed ZnO concentration and temperature dependencies. Increasing the temperature and filler concentrations were reflected in a negative temperature coefficient of resistivity and enhancement of the electrical conductivity as well. The observed increase in the DC conductivity and decrease in the determined activation energy were explained based on the concept of existing paths and connections between the ZnO particles and the conductive CB nanoparticles. Alteration of ZnO concentration with a fixed content of CB nanoparticles and/or temperature was found to be crucial in the thermal conductivity behavior. The addition of CB nanoparticles to the epoxy/ZnO matrix was found to enhance the electrical conduction resulting from the electronic and impurity contributions. Also, the thermal conductivity enhancement was mostly attributed to the heat transferred by phonons and electrons hopping to higher energy levels throughout the thermal processes. Scanning electron microscopy and energy-dispersive spectroscopy were used to observe the morphology and elements’ distribution in the composites. The observed thermal conductivity behavior was found to correlate well with that of the DC-electrical conductivity as a function of the ZnO content. The overall enhancements in both the measured DC- and thermal conductivities of the prepared hybrid composites are mainly produced through mutual interactions between the filling conductive particles and also from electrons tunneling in the composite's bulk as well.     

Effect of surface roughness on thermal conductivity of VLS-grown rough Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanowires

The recent demonstration of thermal conductivity of rough electrolessly etched Si nanowire (Hochbaum et al., Nature, 451:163, 2008) attracted a lot of interest, because it could not be explained by the existing theory; thermal conductivity of rough Si nanowires falls below the boundary scattering of the thermal conductivity. However, nanoscale pores presented in the nanowires (Hochbaum et al., Nano Letters, 9:3550–3554, 2009) hinder one to be fully convinced that the surface roughness solely made a contribution to the significant reduction in thermal conductivity. In this study, we synthesized vapor–liquid–solid (VLS) grown rough Si_1−x Ge _x nanowire and measured and theoretically simulated thermal conductivity of the nanowire. The thermal conductivity of rough Si_0.96Ge_0.04 nanowire is an order of magnitude lower than that of bulk Si_0.96Ge_0.04 and around a factor of four times lower than that of smooth Si_0.96Ge_0.04 nanowire. This significant reduction could be explained by the fact that the surface roughness scatters medium-wavelength phonons, whereas the long-wavelength phonons are scattered by phonon boundary scattering, and the short-wavelength phonons are scattered by alloy scattering.     

Effect of C60 fullerene additions on the thermal conductivity of low-density polyethylene films

The thermal conductivity of LDPE + C₆₀ nanocomposites with a fullerene concentration up to 10 wt % is studied in the temperature range 20–80°C. This conductivity is found to nonlinearly decrease with increasing fullerene concentration. The decrease in the thermal conductivity of the composites is considered to be caused by a decrease in the phonon mean free path as a result of an increase in the number of scattering centers. The temperature dependence of the thermal conductivity is found to have a maximum.     

Investigation of the thermal conductivity of diamond film on aluminum nitride ceramic

Diamond films were successfully synthesized on aluminum nitride(AlN) ceramic substrates by hot-filament chemical vapor deposition (HFCVD) method. The thermal conductivity of the diamond film/aluminum nitride ceramic (DF/AlN) composites was studied by photothermal deflection (PTD) technique. It has reached 2.04 W/cm K, 73% greater than that of AlN ceramic. Compared with the measurement of scanning electron microscopy (SEM) and Raman spectroscopy, the influence of diamond films on the thermal conductivity of the composites was pointed out. The adhesion and the stresses were also studied. The unusual stability and very good adhesion of the diamond film on AlN ceramic substrate obtained is attributed to the formation of aluminum carbide. Received: 24 March 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Thermal properties of continuously spun carbon nanotube fibres

As indicated by theory and experimental measurements individual carbon nanotubes (CNTs) have very high values of thermal conductivity. One of the challenges is to achieve high thermal conductivity in macroscopic assemblies of CNTs such as fibres, films and composites, paving the way to a wide range of applications. CNT fibres have tremendous potential in succeeding as the future materials for a variety of applications when properties at the nanoscale are translated to their macroscopic assemblies. In this paper we report the measurements of thermal conductivity of continuously spun CNT fibres and its dependence on temperature. Thermal conductivity measurements were performed using in-house built temperature sensing microscope probe. Specific thermal conductivity of CNT fibres showed an order of magnitude advantage over the traditional materials used for heat dissipation.     

Thermal conductivity for single-walled carbon nanotubes from Einstein relation in molecular dynamics

Equilibrium molecular dynamics based Einstein relation with an appropriate definition for integrated heat current (i.e., with modified energy moment) are combined to quantify the thermal conductivity of individual single-walled carbon nanotubes, armchair, zigzag and chiral tubes. The thermal conductivity has been investigated as a function of three parameters, tube radius, length and chirality at and near room temperature with Brenner potential model. Thermal conductivity is found to have unusually high value and varies with radius, length and chirality of tubes. Also the thermal conductivity at temperature range from 50 to 100 K is found to have a maximum value. For 12.1 nm tube length, the thermal conductivity has converging trend which its value dependents on the tube radius and chirality. Tubes with large radius have lower values of thermal conductivity. Furthermore, the results show that armchair tubes have large values of the thermal conductivity comparing with zigzag and chiral tubes. It seems possible to uncover carbon nanotubes thermal properties based on measurements having heat dependence by adding another methods for calculations.     

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400 K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.     

Growth of boron nano-junctions

In this work, we demonstrate the synthesis of various types of boron nanowire junctions in a self-assembled manner by simple closed-tube thermal vapor transfer method. The Y-type boron nano-junctions and lateral boron-silicon alloy nano-junctions were grown on Si substrates, based on the oxide assisted VLS growth mode at a relatively low processing temperature regime and the VLS growth mode at the high processing temperature regime, respectively.     

Role of graphene waviness on the thermal conductivity of graphene composites

The waviness effect of graphene nanoplates (GNPs) on the thermal conductivity of GNP-based composites is investigated. Two types of wrinkled GNPs (w-GNPs) and flat GNPs (f-GNPs) are used to fabricate GNP/epoxy composites. Thermal conductivity enhancement is observed in both types of composites. However, under the same processing, f-GNPs exhibit a higher thermal conductivity enhancement than w-GNPs. We finally introduce a concept, the waviness factor, to theoretically analyze the thermal conductivity considering the waviness effect of GNPs. The theoretical predictions are found to show good agreement with experimental observations.     

The Effects of Copper and Polytetrafluoroethylene (PTFE) on Thermal Conductivity and Tribological Behavior of Polyoxymethylene (POM) Composites

The effects of copper and polytetrafluoroethylene (PTFE) on thermal conductivity and tribological behavior of polyoxymethylene (POM) composites were investigated by a hot disk thermal analyzer and an M-2000 friction and abrasion testing machine. The results indicated that the incorporation of 3 wt% copper particles into POM had little effect on the thermal conductivity of POM composites, but led to the decreased friction coefficient and wear rate of composites. As the copper content was increased, the thermal conductivity increased and reached 0.477 W m^?1 K^?1 for POM-25% Cu composite, an increase of 35.9% compared with that of unfilled POM, while the friction coefficient and wear rate of composites also increased. The incorporation of PTFE into POM-Cu composites had a negligible effect on the thermal conductivity of composites, but helped in the formation of a continuous and uniform transfer film and resulted in the reduction in the friction coefficient and wear rate of composites. The POM-15% Cu-10% PTFE composite, with a value of wear rate similar to unfilled POM possessed higher thermal conductivity and lower friction coefficient.     

Thermal conductivity of the pine-biocarbon-preform/copper composite

The thermal conductivity of composites of a new type prepared by infiltration under vacuum of melted copper into empty sap channels (aligned with the sample length) of high-porosity biocarbon preforms of white pine tree wood has been studied in the temperature range 5–300 K. The biocarbon preforms have been prepared by pyrolysis of tree wood in an argon flow at two carbonization temperatures of 1000 and 2400°C. From the experimental values of the composite thermal conductivities, the fraction due to the thermal conductivity of the embedded copper is isolated and found to be substantially lower than that of the original copper used in preparation of the composites. The decrease in the thermal conductivity of copper in the composite is assigned to defects in its structure, namely, breaks in the copper filling the sap channels, as well as the radial ones, also filled by copper. A possibility of decreasing the thermal conductivity of copper in a composite due to its doping by the impurities present in the carbon preform is discussed.