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.     

Kondo效应对磁杂质碳纳米管电输运特性的影响

从碳纳米管中Kondo效应的影响出发, 在有限温度下采用Anderson模型表征碳纳米管/磁杂质系统, 利用Landauer公式对磁杂质碳纳米管的电导和热电势进行研究, 得出和实验结果一致结论. 关键词： Kondo效应 碳纳米管 电输运特性     

Nonlinear transport and heat dissipation in metallic carbon nanotubes

We show that the local temperature dependence of thermalized electron and phonon populations along metallic carbon nanotubes is the main reason behind the nonlinear transport characteristics in the high bias regime. Our model is based on the solution of the Boltzmann transport equation considering both optical and zone boundary phonon emission as well as absorption by charge carriers. It also assumes a local temperature along the nanotube, determined self-consistently with the heat transport equation. By using realistic transport parameters, our results not only reproduce experimental data for electronic transport but also provide a coherent interpretation of thermal breakdown under electric stress. In particular, electron and phonon thermalization prohibits ballistic transport in short nanotubes.     

Electron transport and hot phonons in carbon nanotubes

We demonstrate the key role of phonon occupation in limiting the high-field ballistic transport in metallic carbon nanotubes. In particular, we provide a simple analytic formula for the electron transport scattering length, which we validate by accurate first principles calculations on (6, 6) and (11, 11) nanotubes. The comparison of our results with the scattering lengths fitted from experimental I-V curves indicates the presence of a nonequilibrium optical phonon heating induced by electron transport. We predict an effective temperature for optical phonons of thousands Kelvin.     

Mesoscopic thermal and thermoelectric measurements of individual carbon nanotubes

We discuss the mesoscopic experimental measurements of electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes. The temperature distributions in electrically heated individual multiwalled carbon nanotubes have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in nanotubes indicate the bulk dissipation of electronic energy to phonons. In addition, thermal conductivity of an individual multiwalled nanotube has been measured using a microfabricated suspended device. The observed thermal conductivity is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. Finally, we present thermoelectric power (TEP) of individual single walled carbon nanotubes using a novel mesoscopic device. A strong modulation of TEP as a function of the gate electrode was observed.     

Understanding of temperature and size dependences of effective thermal conductivity of nanotubes

The anomalous thermal transport properties of nanotubes may lead to many important applications, but the mechanisms are still unclear. In this work, we present new governing equations for non-Fourier heat conduction in nanomaterials based on the concept of thermomass. The effective thermal conductivities of nanotubes are therefore predicted which agree very well with the available experimental data. Analysis suggests that the inertial effect of heat and the confined heat flux by nanostructured surfaces are two key mechanisms causing the anomalous temperature and size dependences of effective thermal conductivity of nanotubes.     

Spin-polarized transport in carbon nanotubes

We present transport measurements of ferromagnetically contacted carbon nanotubes. In both single- and multi-walled nanotube devices, a spin valve effect is observed due to spin-polarized transport. In one single-walled nanotube device, the spin-valve effect is suppressed as the influence of Coulomb charging is observed at around 10 K. To help understand the interplay between the Coulomb charging and the spin-polarized transport we investigated the temperature dependence of the carbon nanotube magnetoresistance.     

Rapid transport of gases in carbon nanotubes

We report atomistic simulations for both self- and transport diffusivities of light gases in carbon nanotubes and in two zeolites with comparable pore sizes. We find that transport rates in nanotubes are orders of magnitude faster than in the zeolites we have studied or in any microporous material for which experimental data are available. The exceptionally high transport rates in nanotubes are shown to be a result of the inherent smoothness of the nanotubes. We predict that carbon nanotube membranes will have fluxes that are orders of magnitude greater than crystalline zeolite membranes.     

Mesoscale simulation of aggregation of imogolite nanotubes from potential of mean force interactions

The aggregation of colloidal clay mineral particles plays an important role in controlling the mechanical and transport properties of soils. Interactions and aggregation of plate-like montmorillonite particles were previously studied with the help of Molecular Dynamics (MD) simulation. This paper investigates the aggregation of cylindrical imogolite-like phyllosilicate nanotubes. Nano-scale MD simulations are carried out to find the potential of mean force between two nanotubes. This PMF is then used in a mesoscale simulation that represents interactions between elemental nanotubes through coarse-graining. We investigate the distribution of water molecules around the curved surfaces, and the effects of the surface charge density and tube length on aggregation. Shorter nanotubes were found to form larger stacks.     

Collective diffusion in carbon nanotubes: Crossover between one dimension and three dimensions

Using non-equilibrium molecular dynamics and Monte Carlo methods, we study the collective diffusion of helium in carbon nanotubes. The results show that the collective diffusion coefficient(CDC) increases with the dimension of the channel. The collective diffusion coefficient has a linear relationship with the temperature and the concentration. There exist a ballistic transport in short carbon nanotubes and a diffusive transport in long carbon nanotubes. Fick's law has an invalid region in the nanoscale channel.     

高压退火对碳纳米管微结构转变的影响

利用高分辨电子显微镜研究了在高压退火条件下碳纳米管的微结构变化规律，发现在5.5GPa下，770℃退火后即发生了明显的结构改变；在950℃退火25min后发现形成了类碳纳米洋葱和纳米碳条带结构.碳纳米管转变成类碳纳米洋葱可以分成几个步骤，首先碳纳米管破裂，然后形成碳纳米球，最后形成类碳纳米洋葱结构.讨论了高压和退火温度对碳纳米管结构变化的影响. 关键词：     

Transport of liquid mercury under pressure in double-walled carbon nanotubes

In this paper, transport of liquid mercury under pressure through double-walled carbon nanotubes is studied using classical molecular dynamics simulations in conjunction with a pressure control model. The results indicate that wetting of double-walled carbon nanotubes by mercury occurs above a threshold pressure of liquid mercury. Liquid mercury can be transported through the inner tube of double-walled carbon nanotubes with the continuous increase of its pressure. The threshold pressure of liquid mercury decreases and the transport efficiency increases greatly with enlarging the inner tube size. The space between the two walls of double-walled carbon nanotubes can also transport the liquid mercury while the distance between the two walls is much larger than the radius of the inner tube. Transport efficiency of double-walled carbon nanotubes is a little lower than that of single-walled carbon nanotubes while double-walled carbon nanotubes transport liquid more steadily than single-walled carbon nanotubes.     

Nonequilibrium Green's function approach to phonon transport in defective carbon nanotubes

We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective carbon nanotubes with increasing temperature.     

Superconducting properties of carbon nanotubes

Metallic single wall carbon nanotubes have attracted much interest as 1D quantum wires combining a low carrier density and a high mobility. It was believed for a long time that low temperature transport was exclusively dominated by the existence of unscreened Coulomb interactions leading to an insulating behavior at low temperature. However experiments have also shown evidence of superconductivity in carbon nanotubes. We distinguish two fundamentally different physical situations. When carbon nanotubes are connected to superconducting electrodes, they exhibit proximity induced superconductivity with supercurrents which strongly depend on the transmission of the electrodes. On the other hand intrinsic superconductivity was also observed in suspended ropes of carbon nanotubes and recently in doped individual tubes. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsion at low temperature, and enables investigation of superconductivity in a 1D limit never explored before. To cite this article: M. Ferrier et al., C. R. Physique 10 (2009).     

碳纳米管中点缺陷对热导率影响的正交试验模拟分析

在碳纳米管的制备过程中, 各种点缺陷不可避免地存在于其晶格结构中, 对于碳管的热输运性质造成不可忽视的影响. 使用非平衡分子动力学方法, 选用反应经验键序势能, 模拟计算含有缺陷的碳纳米管的热导率. 尝试采用正交试验方法设计算例, 不但减少了计算量, 并且利于分析缺陷类型、 管长和管径三种结构因素对缺陷造成的热导率下降影响的主次和趋势. 重点研究了掺杂、 吸附和空位三类点缺陷的影响, 与无缺陷完整碳纳米管进行比较, 开展缺陷效应分析, 并进一步考察了环境温度等因素的影响. 模拟结果表明, 相对完整无缺陷碳管, 含有点缺陷的碳管热导率显着下降; 在有缺陷存在的情况下, 缺陷的类型对碳管热导率的影响最大, 管径次之, 管长影响相对最小; 缺陷类型对热导率影响力从大到小依次为: 空位 > 掺杂 > 吸附; 不同环境温度下, 点缺陷对碳管热导率的影响不尽相同.     

Adsorption and desorption of an O2 molecule on carbon nanotubes

Adsorption and desorption of an oxygen molecule on carbon nanotubes are investigated using density functional calculations. Several precursor states exist at the edge of armchair nanotubes, whereas an exothermic adsorption takes place at the edge of zigzag nanotubes. We also estimate desorption barriers of a CO molecule from nanotubes as well as fullerenes and amorphous phases. Our calculations suggest that carbon nanotubes can survive selectively during the oxidative etching process with a precise control of annealing temperature, in good agreement with experimental results of purification process of carbon nanotubes.     

碳纳米管在高压下的电学性质与状态方程

 在金刚石压砧装置上，采用电阻测量方法研究了多层碳纳米管在室温下、23 GPa内的电阻与压力的关系。实验结果表明，在1.5～4.6 GPa内，电阻急剧减小，而在5.7 GPa左右，电阻出现一个极大值。这些变化与碳纳米管的结构、形状和层间距离的变化有关。同时，还在活塞-圆筒式p-V关系测量装置上首次研究了多层碳纳米管在室温下、4.5 GPa内的p-V关系，给出了它在第一次和第七次压缩时的状态方程。     

Thermal conduction of one-dimensional carbon nanomaterials and nanoarchitectures

This review summarizes the current studies of the thermal transport properties of one-dimensional(1D) carbon nanomaterials and nanoarchitectures. Considering different hybridization states of carbon, emphases are laid on a variety of 1D carbon nanomaterials, such as diamond nanothreads, penta-graphene nanotubes, supernanotubes, and carbyne. Based on experimental measurements and simulation/calculation results, we discuss the dependence of the thermal conductivity of these 1D carbon nanomaterials on a wide range of factors, including the size effect, temperature influence, strain effect, and others. This review provides an overall understanding of the thermal transport properties of 1D carbon nanomaterials and nanoarchitectures, which paves the way for effective thermal management at nanoscale.     

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

Correlated tunneling in intramolecular carbon nanotube quantum dots

We investigate correlated electronic transport in single-walled carbon nanotubes with two intramolecular tunneling barriers. We suggest that below a characteristic temperature the long-range nature of the Coulomb interaction becomes crucial to determine the temperature dependence of the maximum G(max) of the conductance peak. Correlated sequential tunneling dominates transport yielding the power law G(max) proportional, variant T(alpha(end-end)-1), typical for tunneling between the ends of two Luttinger liquids. Our predictions are in agreement with recent measurements.