单根单壁碳纳米管导热系数随长度变化尺度效应的实验和理论

考虑纳米碳管与基体之间的热损失,采用四焊盘-3ω法测量了室温下基体表面不同长度单根单壁碳纳米管(SWNT)的导热系数.SWNT的导热系数在测试长度范围(05—7μm)内随长度的增大而增大,增加的幅度逐渐减小.考虑二阶3-声子过程的影响,采用改进的WV模型预测了SWNT导热系数随长度的变化规律.理论预测的声子平均自由程～175nm.导热系数的测量结果与室温下不同长度SWNT的实验结果相吻合.理论预测结果与实验结果均说明SWNT导热系数随长度变化具有尺度效应. 关键词： ω法')" href="#">3ω法 单壁碳纳米管 导热系数 二阶3-声子过程     

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes(SWNTs) is discussed in this paper, using a non-equilibrium molecular dynamics(NEMD) method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

碳纳米管的热导率:从弹道到扩散输运

采用非平衡分子动力学方法研究了300 K和1000 K时（5,5）碳纳米管热导率随长度的变化.在室温下,碳纳米管长度小于40 nm时热导率与长度呈线性关系,此时导热处于弹道输运阶段,单位面积弹道热导为5.88×10⁹ Wm^-2K^-1．随着碳纳米管长度的增加,其热导率逐渐增加,但增加速度随长度逐渐减小,此时导热处于弹道—扩散输运阶段,并随长度的增加从以弹道输运为主向以扩散输运为主转变．长度大于10 μm时由于弹道输运可以忽略,导热近似达到完全 关键词： 碳纳米管 热导率 弹道输运 低维导热     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Preparation of single-walled nanotubes with the help of a Ni/Cr-based catalyst

The high-yield synthesis of single-walled carbon nanotubes (SWNTs) is carried out in an electric-arc discharge using the Ni-Cr alloy as a catalyst. A new method of introducing the catalyst into the plasma hot region is used in the synthesis. In this method, the anode with a sandwich structure consists of two longitudinal graphite rods of a rectangular cross section, between which the Ni-Cr alloy in the form of a foil having a thickness approximately equal to 0.2 mm is placed. The obtained samples are investigated using transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetry. According to the results of TEM observations, SWNTs are tied into bundles with a length of several micrometers and a diameter of about 10 nm. The Raman spectra indicate that the diameter distribution of SWNTs lies between 1.2 and 1.5 nm with a peak at approximately 1.24 nm. The SWNT content in the obtained samples is approximately 20%. Heat treatment at various temperatures with a dosed air supply leads to a noticeable mass loss of the sample and to a change in its composition. For example, thus heating to 600 K causes a mass loss of about 40%, leading to an increase in the content of SWNTs up to 35% without their noticeable destruction. Further heating above 600 K leads to a virtually complete thermal decomposition of SWNTs.     

Thermal Conductivity of Carbon Nanotubes Embedded in Solids

A carbon-nanotube-atom fixed and activated scheme of non-equilibrium molecular dynamics simulations is put forward to extract the thermal conductivity of carbon nanotubes （CNTs） embedded in solid argon. Though a 6.5% volume fraction of CNTs increases the composite thermal conductivity to about twice as much as that of the pure basal material, the thermal conductivity of CNTs embedded in solids is found to be decreased by 1/8-1/5 with reference to that of pure ones. The decrease of the intrinsic thermal conductivity of the solid-embedded CNTs and the thermal interface resistance are demonstrated to be responsible for the results.     

Heat Conduction and Characteristic Size of Fractal Porous Media

Based on fractal theory, two types of random Sierpinski carpets （RSCs） and their periodic structures are generated to model the structures of natural porous media, and the heat conduction in these structures is simulated by the finite volume method. The calculated results indicate that in a certain range of length scales, the size and spatial arrangement of pores have significant influence on the effective thermal conductivity, and the heat conduction presents the aeolotropic characteristic. Above the length scale, however, the influence of size and spatial arrangement of pores on the effective thermal conductivity reduces gradually with the increasing characteristic size of porous media, the aeolotropic characteristic is weakened gradually. It is concluded that the periodicity in structures of porous media is not equal to the periodicity in heat conduction.     

Standing or lying C70s encapsulated in carbon nanotubes with different diameters

Single-walled carbon nanotubes (SWNTs) encapsulating C₇₀s, so-called C₇₀ peapods, were synthesized in high yield by a vapor-phase doping method. Raman spectra, high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) measurement indicate that the tube diameter is one of the important factors to determine the orientation of C₇₀ molecules inside the SWNTs. SWNTs with different diameters give different alignment of C₇₀ molecules. The lying orientation is favorable over the standing orientation in thin nanotube, i.e. 1.36 nm nanotubes, whereas the standing orientation is favorable in thick nanotubes, i.e. 1.49 and 1.61 nm nanotubes.     

Thermal Conductivity Measurement of Submicron-Thick Aluminium Oxide Thin Films by a Transient Thermo-Reflectance Technique

Thermal conductivity of submicron-thick aluminium oxide thin films prepared by middle frequency magnetron sputtering is measured using a transient thermo-reflectance technique. A three-layer model based on transmission line theory and the genetic algorithm optimization method are employed to obtain the thermal conductivity of thin films and the interracial thermal resistance. The results show that the average thermal conductivity of 330- 1000nm aluminium oxide thin films is 3.3 Wm^-1K^-1 at room temperature. No significant thickness dependence is found. The uncertainty of the measurement is less than 10%.     

催化剂比例对单壁碳纳米管制备的影响

采用电弧放电法以Y/Ni为催化剂制备了单壁碳纳米管 ,研究了不同催化剂比例对制备产物的影响。获得了不同激发波长下 (476 5～ 1 0 6 4nm)单壁碳纳米管的拉曼光谱 ,采用图表法对径向呼吸模的谱峰进行了认定。结果表明 :样品中碳管的直径分布在 1 2～ 1 6nm之间 ,直径在 1 43nm附近的碳管居多。催化剂的比例只是影响碳管的产额 ,对其直径分布的影响很小     

Adjustable thermal conductivity in carbon nanotube nanofluids

Homogeneous and stable nanofluids have been produced by suspending well dispersible multi-walled carbon nanotubes (CNTs) into ethylene glycol base fluid. CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. Thermal conductivity enhancement was adjusted by ball milling and cutting the treated CNTs suspended in the nanofluids to relatively straight CNTs with an appropriate length distribution. Our findings indicate that the straightness ratio, aspect ratio, and aggregation have collective influence on the thermal conductivity of CNT nanofluids.     

The effective thermal conductivity of porous media based on statistical self-similarity

A fractal model is presented based on the thermal-electrical analogy technique and statistical self-similarity of fractal saturated porous media. A dimensionless effective thermal conductivity of saturated fractal porous media is studied by the relationship between the dimensionless effective thermal conductivity and the geometrical parameters of porous media with no empirical constant. Through this study, it is shown that the dimensionless effective thermal conductivity decreases with the increase of porosity (?) and pore area fractal dimension (Df) when ks/kg>1. The opposite trends is observed when ks/kg<1. In addition, the dimensionless effective thermal conductivity decreases with increasing tortuous fractal dimension (Dt). The model predictions are compared with existing experimental data and the results show that they are in good agreement with existing experimental data.     

Specific heat and thermal conductivity of low-stress amorphous Si-N membranes

We present values of the specific heat and thermal conductivity from 3-300 K of low-stress amorphous silicon-nitride thin-films determined from measurements using a membrane-based microcalorimeter. The thermal conductivity has a temperature dependence often seen in amorphous solids, but the magnitude is large, with the expected plateau occurring at significantly higher temperatures than seen in other amorphous systems. Specific heat measurements show that the expected ‘peak’ in the vibrational spectrum also occurs at relatively high temperatures. The estimated phonon mean-free-path at 300 K is ≈5 Å, comparable to the inter-atomic spacing, as seen in other amorphous solids. Below ≈ 20 K the mean free path is comparable to or exceeds the thickness of the membrane, indicating that surface scattering dominates the thermal transport. This surface scattering is found to be either specular or diffuse, depending on details of the membrane processing, which affects both the thermal conductivity and specific heat below 10 K.     

Thermal conductance of hydrophilic and hydrophobic interfaces

Using time-domain thermoreflectance, we have measured the transport of thermally excited vibrational energy across planar interfaces between water and solids that have been chemically functionalized with a self-assembled monolayer (SAM). The Kapitza length--i.e., the thermal conductivity of water divided by the thermal conductance per unit area of the interface--is analogous to the "slip length" for water flowing tangentially past a solid surface. We find that the Kapitza length at hydrophobic interfaces (10-12 nm) is a factor of 2-3 larger than the Kapitza length at hydrophilic interfaces (3-6 nm). If a vapor layer is present at the hydrophobic interface, and this vapor layer has a thermal conductivity that is comparable to bulk water vapor, then our experimental results constrain the thickness of the vapor layer to be less than 0.25 nm.     

Thermal conductivity of isotopically enriched Si: revisited

The thermal conductivity of isotopically enriched ²⁸Si (enrichment better than 99.9%) was redetermined independently in three laboratories by high precision experiments on a total of four samples of different shape and degree of isotope enrichment in the range from 5 to 300 K with particular emphasis on the range near room temperature. The results obtained in the different laboratories are in good agreement with each other. They indicate that at room temperature the thermal conductivity of isotopically enriched ²⁸Si exceeds the thermal conductivity of Si with a natural, unmodified isotope mixture by 10±2%. This finding is in disagreement with an earlier report by Ruf et al. At ∼26 K the thermal conductivity of ²⁸Si reaches a maximum. The maximum value depends on sample shape and the degree of isotope enrichment and exceeds the thermal conductivity of natural Si by a factor of ∼8 for a 99.982% ²⁸Si enriched sample. The thermal conductivity of Si with natural isotope composition is consistently found to be ∼3% lower than the values recommended in the literature.     

以Y/Ni为催化剂制备的单壁碳纳米管的拉曼光谱研究

采用电弧放电法以Y/Ni为催化制备了单壁碳纳米管（SWNTs），对样品进行了扫描电镜、透射电镜和拉曼光谱的研究。所制备的样品中单壁碳纳米管的含量较高。对单壁碳纳米管的共振拉曼散射增强效应进行了观察，随激光波长的不同，单壁碳纳米管的拉曼光谱也随之变化，尤其是低频区径向呼吸模的变化比较明显。利用布里渊区折叠法计算了单壁碳纳米管的电子态密度曲线，根据SWNTs电子态密度尖峰之间的能量差、管子的直径和呼吸模频率建立了一个图表，并对SWNTs的呼吸模进行了归属。分析结果表明：样品中单壁碳纳米管的直径分布在0.79-1.76nm范围，金属管和半导体管均存在，并且直径在1.45nm附近的碳管居多。     

Coating of carbon nanotubes on flexible substrate and its adhesion study

The primary goal of this project was to develop a flexible transparent conductor with 100 Ω/sq and 90% transmittance in the wavelength range of 400-700 nm on a flexible substrate. The best result achieved so far was 110 Ω/sq at 88% transmittance using purified single-walled carbon nanotubes (SWNTs) coated on a polyethylene naphthalate (PEN) substrate. The secondary goal was to simplify the overall coating procedure; we successfully reduced the process from five (prior art method) to three steps utilizing a sonication method. We also found that the use of metallic SWNTs significantly improved the conductivity and transmittance compared with the use of mixed SWNTs, i.e., unseparated SWNTs. Furthermore, a possible adhesion mechanism between SWNTs and the surface of PEN was studied; we concluded that a π-π stacking effect and a hydrophobic interaction are the major contributing factors for SWNTs to adhere to the surface of the substrate.     

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.     

Growth of single-wall carbon nanotubes dependent on laser power density and ambient gas pressure during room-temperature CO2 laser vaporization

Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of 20-ms CO₂ laser pulses onto a graphite–Co/Ni target at room temperature. We investigated the effect of laser power density (10–150 kW/cm²) and ambient Ar gas pressure (150–760 Torr) on the abundance of SWNTs with lengths of up to about 200 nm in soot-like carbonaceous deposits. For a constant power density (30 kW/cm²), depending on the Ar gas pressure, SWNTs with diameters of 1.2–1.4 nm were synthesized. Expansion behavior and temperature-fall rates of clusters and/or particles in laser plumes were also analyzed by high-speed video imaging and temporally and spatially resolved emission spectroscopy. The temperature-fall rates were estimated to be 171–427 K/ms. The SWNT growth on the time scale of a few milliseconds appeared to be related to some features of condensing clusters and/or particles, including resident densities, collision frequencies and temperatures. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001