Thermal conductivity (kappa) of isolated carbon nanotubes (CNTs) is higher than the kappa of diamond; however, in this Letter we show that the kappa of a packed bed of three-dimensional random networks of single and multiwall CNTs is smaller than that of thermally insulating amorphous polymers. The thermoelectric power (Sigma) of the random network of CNTs was also measured. The Sigma of a single wall nanotube network is very similar to that of isolated nanotubes and, in contrast with kappa, Sigma shows a strong dependence on the tube diameter.

Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

The surface phonon-polaritons contribution to the thermal conductivity of a nano thin film of silicon dioxide is investigated based on the Maxwell equations and the Boltzmann transport equation. It is shown that: (1) a small difference between the permittivities of the substrate and superstrate of the film can generate giant propagation lengths and therefore remarkably enhances its thermal conductivity with respect to values obtained for a freestanding one. (2) The propagation of surface phonon-polaritons is present in a broad band of frequencies and exhibits its largest propagation lengths at the frequency where the absorption of energy is minimal. (3) The increase of the thermal conductivity of the film as its thickness decreases is higher when it is deposited on potassium bromide instead of being suspended in air. The difference in the thermal conductivity for these two systems increases with increasing temperature and reducing the film thickness. A thermal conductivity as high as 2.5 W/m K is obtained for a 30 nm-thick thin film at room temperature, which is about 1.8 times larger than its bulk phonon value. The obtained results show that the propagation of surface phonon-polaritons has the potential not only to offset the reduction of the phonon thermal conductivity of a nano thin film, when its sizes are scaled down, but also to enhance it, by choosing properly the permittivity of its substrate. V C 2013 American Institute of Physics. [http://dx.

We provide a derivation allowing the calculation of thermal conductance at interfaces by equilibrium molecular dynamics simulations and illustrate our approach by studying thermal conduction mechanisms in Si/Ge superlattices. Thermal conductance calculations of superlattices with period thicknesses ranging from 0.5 to 60 nm are presented as well as the temperature dependence. Results have been compared to complementary Green-Kubo thermal conductivity calculations demonstrating that thermal conductivity of perfect superlattices can be directly deduced from interfacial conductance in the investigated period range. This confirms the predominant role of interfaces in materials with large phonon mean free paths.

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International audienceWe present a microscopic approach for estimating the frequency vs. wave-vector dependentphonon transmission across a solid-solid interface. We show that the spectral properties of theheat flux can be generally deduced from the equilibrium displacements fluctuations of thecontact atoms. We have applied and demonstrated our formalism with molecular dynamicssimulations to predict the angular and mode dependent phonon transport in silicon andgermanium thin films. This notably unveils the existence of confined interface mode at thethermal contacts. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816738

Among a broad range of carbonaceous materials, such as exfoliated graphite, [1] graphene or diamond, carbon nanotubes (CNTs) are widely used as a thermal filler because of their exceptional intrinsic thermal conductivity (TC) and aspect ratios, which are larger than 1000. The TC of single-walled CNTs (SWNTs) has been reported [2] as high as 6000 W m À1 K À1, and that of multi-walled CNTs (MWNTs) was experimentally measured at 3075 W m À1 K À1 at room temperature, [3] which remains above the performances of diamond (TC ¼ 2200 W m À1 K À1 ). [4,5] Therefore the improvement of the TC of composites based on CNTs was extensively investigated over the past years. [6,7] A recent work revealed that a TC of 0.28 W m À1 K À1 or a 40% increase had been reached in composites with a 10% weight fraction (wt%) of CNTs dispersed in polyvinylacetate matrix by using the classical sonication method.[8] Another optimized configuration was proposed by Haddon and co-workers, who brought into play a hybrid filler based on the combination of SWNTs and graphite nanoplatelets.[9] An improved TC of 1.7 W m À1 K À1 -about a fivefold increase-was obtained for epoxy composites. The hybrid loading mass fraction was as high as 10%, including 7 wt% graphite nanoplatelets and 3 wt% SWNTs.Those wt% appear to be far larger than the one of percolation, which should be smaller than 0.1% in CNT-reinforced composites.[10] The percolation should yield a significant TC augmentation that has not yet been observed. A previous investigation on CNT-based nanofluids emphasized that the TC of the mixture remains thirty times lower than the expected theoretical value and much worse at low mass fractions.[11] This unsatisfying behavior was attributed to interfacial contact resistances, [12] which several teams tried to reduce, but with little success. We have also evaluated the TC of CNT pellets in vacuum from the number of thermal contacts between CNTs, and obtained stringent theoretical limitations that are confirmed by experiment.[13] It is very plausible that the TC improvement at percolation was never achieved in practice due to insufficient CNT dispersion leading to the predominance of thermal contact resistances.[11] In many cases indeed, no efficient percolating networks are visible. [8] In this communication, we demonstrate that epoxy composites with extremely low CNT wt% corresponding to the percolation can have the predicted TC values, which are as high as those previously obtained at a wt% one order of magnitude larger.This new field of properties can indeed be reached by a microarchitecture involving Al 2 O 3 microparticles and MWNTs. These multiscale fillers belong to a new generation of hybrid materials, where alumina microparticles provide efficient structures by dispersing the CNT network within the polymer matrix. To further improve the dispersion, we have also implemented mechanical dispersion of the Al 2 O 3 -CNTs hybrid fillers without any chemical pretreatment. The result is a drastic decrease in the number of thermal contacts between the C...

Using atomistic Green’s function calculations, we find that the phonon thermal conductivity of pellets composed of ∼μm long carbon nanotubes has an upper bound of a few W/m K. This is in striking contrast with the extremely high thermal conductivity of individual nanotubes (∼3000 W/m K). We show that, at room temperature, this upper bound does not depend on the nanotube diameter. Conversely, for low temperatures, an inverse proportionality with nanotube diameter is predicted. We present concrete results as a function of nanotube length and chirality, pellet density, and temperature. These results imply that carbon nanotube pellets belong to the category of thermal insulators, contrasting with the good conducting properties of parallel nanotube arrays, or individual nanotubes.