Polaron effects on the thermal conductivity of zigzag carbon nanotubes

Thermal conductivity of metallic zigzag carbon nanotube is investigated in the context of Holstein model. Green's function approach is implemented to calculate the electronic contribution of thermal conductivity as a function of radius of carbon nanotube, temperature and electron phonon coupling strength. Our results show that electronic thermal conductivity increases as a function of temperature at low temperature and gets a maximum value then decays at high temperature. Also the effect of radius of both metallic and semiconductor zigzag carbon nanotube on the thermal conductivity is studied. Our results show thermal conductivity increases when CNT diameter increases and decreases with electron phonon interaction strength.     

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.     

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.     

The temperature variation of the thermal conductivity of benzene-derived carbon fibers

The temperature variation of the in-plane thermal conductivity of benzene-derived carbon fibers (BDF) measured from 5 to 300 K is reported and discussed. Very high thermal conductivity values — comparable to that of the best HOPG heat treated at the same temperature — are found. The data confirms the high structural order previously reported for BDF.     

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.     

疏导式结构在头锥热防护中的应用

针对高超声速飞行器工作时头锥恶劣的热环境,为了保证飞行器头锥的尖锐外形, 提出疏导式热防护结构,利用内置高导热碳材料结构为飞行器头锥提供热防护. 采用流固耦合方法对头锥疏导式防热结构进行了分析,验证了头锥内置高导热碳材料具有较好防热效果, 其中来流马赫数(Ma)为9时头锥前缘壁面最高温度下降了21.9%,尾部最低温度升高了15.2%, 实现了热流由高温区向低温区的转移,削弱了头锥的热载荷,强化了头锥的热防护能力. 本文对外蒙皮结构参数、材料参数以及内部高导热碳材料导热率对头锥热防护性能的影响进行了分析, 其中头锥最高温度随着蒙皮材料导热系数的增加而降低到一个稳定值; 随着蒙皮材料表面黑度的增加而降低;随着蒙皮厚度的增加而升高;随着高导热碳材料导热系数的 增加而呈抛物线下降.     

Transport coefficients of helium and argon-helium plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of helium, and mixtures of argon and helium, at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the mixtures. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30000 K. The results are compared with those of previously published studies. Significant discrepancies are found; these are attributed to the improved values of the collision integrals used here in calculating the transport coefficients     

Thermal properties of single-walled carbon nanotube crystal

In this work, the thermal properties of a single-walled carbon nanotube (SWCNT) crystal are studied. The thermal conductivity of the SWCNT crystal is found to have a linear dependence on temperature in the temperature range from 1.9 K to 100.0 K. In addition, a peak (658 W/mK) is found at a temperature of about 100.0 K. The thermal conductivity decreases gradually to a value of 480 W/mK and keeps almost a constant in the temperature range from 100.0 K to 300.0 K. Meanwhile, the specific heat shows an obvious linear relationship with temperature in the temperature range from 1.9 K to 300.0 K. We discuss the possible mechanisms for these unique thermal properties of the single-walled carbon nanotube crystal.     

碳纳米管热传导的分子动力学模拟研究

采用改进的经验键序作用势描述碳原子间的相互作用,应用分子动力学方法和Green-Kubo函数计算了碳纳米管的热导率.在模拟中,使用了重叠计算的方法来计算热流相关函数,大大减少了模拟步数.计算结果表明,碳纳米管的热导率以原子间作用力相互做功所引起的热流形式为主;热导率的值随着直径的增加而减小;在室温下,热导率的值随着温度的增加而增加,达到室温后逐渐收敛于定值.计算的单壁碳纳米管热导率在1000W/mK至4000W/mK之间,计算结果与实验结果基本符合. 关键词： 分子动力学 碳纳米管 热导率     

Nonuniversal critical phenomena along the lambda line of4He

We present a new calculation of the critical thermal conductivity near the lambda line of⁴He. Static and dynamic effects are properly separated and are calculated within modelF in three dimensions by means of renormalized field theory up to two-loop order. The result provides an appropriate basis for analyzing forthcoming thermal conductivity data and for determining the temperature and pressure dependence of the effective dynamic parameters of⁴He in the entire critical region. This can be used to predict other nonuniversal dynamic critical phenomena above and below the lambda line.     

高温还原氧化石墨烯膜及其导热性能

采用氧化还原法制备了结构致密且具有较高柔韧性的石墨烯薄膜,探究了薄膜经过较高退火温度还原后结构变化,并通过T型稳态法测量了其热导率,研究了还原温度对薄膜热导率和力学性能的影响。结果表明,高温还原有助于氧化石墨烯中含氧官能团的去除和sp2杂化碳晶格的恢复,并且温度越高还原效果越好。当还原温度高达2800℃时,在200~350 K温度范围内石墨烯膜的热导率在336.9~436 W·m^-1·K^-1之间,伴随着温度的升高,热导率有先增大后减小的趋势。     

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.     

Molecular dynamics studies on the thermal conductivity of single-walled carbon nanotubes

We have studied the thermal conductivity of single-walled carbon nanotubes (SWCNTs) using the NEMD method. The results indicate that the thermal conductivity values are not profoundly influenced by the specific simulation-technique used in the MD simulations. Some possible reasons, which could be responsible for the discrepancy on thermal conductivity values of SWCNTs in the literatures, are discussed.      

Thermal conductivity of stretched and annealed poly (p-phenylene sulfide) films

The temperature variation of the thermal conductivities of as-prepared, mechanically stretched, as-prepared and then annealed PPS samples are presented. Unusually high thermal conductivity values are observed as compared to other polymeric materials. In agreement with the X-ray diffraction observations, the thermal conductivity of the oriented film is higher than that of the as-prepared or annealed films. The differences observed after stretching are comparable to those previously reported for polyethylene and polyacetylene films for the same draw ratios. These unusually high thermal conductivity values justify the use of PPS in devices where efficient heat dissipation, associated with electrical insulation, is required.     

Thermal conductivity of isolated and interacting carbon nanotubes: comparing results from molecular dynamics and the boltzmann transport equation

We investigate the thermal conductivity of single-wall carbon nanotubes (SWCNT) either isolated or in contact with external media by using equilibrium molecular dynamics and the Boltzmann transport equation. We show that, contrary to existing controversies, both methods yield a finite value of the thermal conductivity for infinitely long tubes, as opposed to the case of 1D, momentum-conserving systems. Acoustic and flexure modes with mean free paths of the order of a few microns are identified as major contributors to the high value of SWCNT conductivity. We also find that the interaction with an external medium may substantially decrease the lifetime of the low-frequency vibrations, reducing the thermal conductivity by up to 2 orders of magnitude.     

Thermal lens investigation in amorphous SiN

In this work we report the use of thermal lens (TLs) technique to determine the thermo-optical parameters, such as thermal conductivity (K), thermal diffusivity (D) and the optical path dependence with temperature (ds/dT) of amorphous SiN. Our results indicate that ds/dT is positive at room temperature. Moreover, the obtained values of thermal diffusivity and thermal conductivity are in good agreement with that found in the literature.     

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.     

Dimensional crossover of thermal conductance in graphene nanoribbons: a first-principles approach

First-principles density-functional calculations are performed to investigate the thermal transport properties in graphene nanoribbons (GNRs). The dimensional crossover of thermal conductance from one to two dimensions (2D) is clearly demonstrated with increasing ribbon width. The thermal conductance of GNRs of a few nanometers width already exhibits an approximate low-temperature dependence of T(1.5), like that of 2D graphene sheets which is attributed to the quadratic nature of the dispersion relation for the out-of-plane acoustic phonon modes. Using a zone-folding method, we heuristically derive the dimensional crossover of thermal conductance with the increase of ribbon width. Combining our calculations with the experimental phonon mean-free path, some typical values of thermal conductivity at room temperature are estimated for GNRs and for 2D graphene sheet. Our findings clarify the issue of the low-temperature dependence of thermal transport in GNRs and suggest a calibration range of thermal conductivity for experimental measurements in graphene-based materials.     

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.     

Conductivité thermique de mousses cellulaires de carbone: Mesures par méthode flash et interprétation

This paper deals with the thermal diffusivity measurement of cellular carbon foams by the flash method. The thermal conductivity is obtained from the thermal diffusivity by using specific heat and density of carbon foam. These materials constitute non-homogeneous porous semitransparent media, hence the classic experimental set-up of the flash method must be adapted. A study is carried out in order to show the influence of the sample thickness, of the temperature, of the enclosed gas pressure and of the gas composition on the effective thermal conductivity. Identification functions are established in the monodimensional and bidimensional cases. Results indicate the dependence of the equivalent thermal conductivity upon the sample thickness. This result, attributed to radiative transfer, is confirmed by modelling. Radiative thermal conductivities are obtained with a Monte-Carlo code. Experimental and theoretical results are compared. It is demonstrated that the concept of effective conductivity is not relevant to these materials.