Heat conduction in graphene flakes with inhomogeneous mass interface

Using nonequilibrium molecular dynamics simulations, we study the heat conduction in graphene flakes composed by two regions. One region is mass-loaded and the other one is intact. It is found that the mass interface between the two regions greatly decreases the thermal conductivity, but it would not bring thermal rectification effect. The dependence of thermal conductivity upon the heat flux and the mass difference ratio are studied to confirm the generality of the result. The interfacial scattering of solitons is studied to explain the absence of rectification effect.Comment: 5 pages, 4 figure

Thermal rectification in asymmetric U-shaped graphene flakes

In this paper, we study the thermal rectification in asymmetric U-shaped graphene flakes by using nonequilibrium molecular dynamics simulations. The graphene flakes are composed by a beam and two arms. It is found that the heat flux runs preferentially from the wide arm to the narrow arm which indicates a strong rectification effect. The dependence of the rectification ratio upon the heat flux, the length and the width of the beam, the length and width of the two arms are studied. The result suggests a possible route to manage heat dissipation in U-shaped graphene based nanoelectronic devices.Comment: 3 pages, 4 figure

Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?

Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes. Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments. Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surface

Hierarchical simulations of hybrid polymer-solid materials

Complex polymer-solid materials have gained a lot of attention during the last 2-3 decades due to the fundamental physical problems and the broad spectrum of technological applications in which they are involved. Therefore, significant progress concerning the simulations of such hybrid soft-hard nanostructured systems has been made in the last few years. Simulation techniques vary from quantum to microscopic (atomistic) up to mesoscopic (coarse-grained) level. Here we give a short overview of simulation approaches on model polymer-solid interfacial systems for all different levels of description. In addition, we also present a brief outlook concerning the open questions in this field, from the point of view of both physical problems and computational methodologies

Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite

Thermal Conductivity and Thermal Rectification in Carbon Nanotubes - Reverse Non-Equilibrium Molecular Dynamics Simulations

The purpose of this research is an investigation of the thermal conductivity () and thermal rectification of carbon nanotubes as well as the different factors which have an influence on these quantities. As computational tool we have used reverse non-equilibrium molecular dynamics (RNEMD) simulations. In chapter 1 we have briefly discussed the importance of research in nanoscale science. Furthermore the motivation for this work has been explained. In chapter 2 we have investigated the thermal conductivity of single-walled and multi-walled carbon nanotubes by RNEMD as a function of the tube length (L), temperature and chiral index. We found that the thermal conductivity in the ballistic-diffusive regime follows a L law. The exponent is insensitive to the diameter of the carbon nanotube; at room temperature has been derived for short carbon nanotubes. The temperature dependence of the thermal conductivity shows a peak between 250 and 500 K. We have also defined and shortly discussed the phenomenon of thermal rectification in mass-graded and extra-mass-loaded nanotubes. In chapter 3 the thermal rectification in nanotubes with a mass gradient has been studied in more detail. We predict a preferred heat flow from light to heavy atoms which differs from the preferential direction in one-dimensional (1D) monoatomic systems. This behavior of nanotubes is explained by anharmonicities caused by transverse motions which are stronger at the low mass end. The present simulations show an enhanced rectification with increasing tube length, diameter and mass gradient. Implications of the present findings for applied topics are mentioned concisely. In chapter 4 we have extended our work on thermal rectification from mass-graded quasi-one-dimensional nanotubes to the other model systems. Mass-graded polyacetylene-like chains behave like single-file chains as long as the mass gradient is hold by the backbone atoms. The thermal rectification in nanotubes with a gradient in the bond force constant (kr) has been studied, too. They show a preferred heat transfer from the region with large kr to the domain with small kr. Thermal rectification has been studied also in planar (2D) and 3D mass-graded systems where the heat flow followed a preferred direction similar to that observed in nanotubes. Additionally, a more realistic system has been implemented. Here a different number of carbon nanotubes have been grafted on both sides of a graphene sheet. We have found that the transfer of the vibrational energy as well as the generation of low-energy modes at atoms with large masses is responsible for the sign of the thermal rectification. In chapter 5 the thermal conductivity of carbon nanotubes (CNTs) with chirality indices (5,0), (10,0), (5,5) and (10,10) has been studied by reverse non-equilibrium molecular dynamics simulations as a function of different bondlength alternation patterns (r). The r dependence of the bond force constant (krx) in the MD force field has been determined with the help of an electronic band structure approach. From these calculations it follows that the r dependence of krx in tubes with not too small diameter can be mapped by a simple linear bondlength–bondorder correlation. A bondlength alternation with an overall reduction in the length of the nanotube causes an enhancement of while an alternation scheme leading to an elongation of the tube is coupled to a reduction of the thermal conductivity. This effect is more pronounced in CNTs with larger diameters

Tribenzotriquinacene receptors for C60 fullerene rotors: towards C3 symmetrical chiral stators for unidirectionally operating nanoratchets

The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (7) for C-60 fullerene, possessing C-3 point group symmetry, by threefold condensation of C-2-symmetric 1,2-diketone synthons (5) and a hexaaminotribenzotriquinacene core (6) is described. The chiral diketone was synthesized in a five-step reaction sequence starting from C-2h-symmetric 2,6-di-tert-butylanthracene. The highly diastereo-discriminating Diels-Alder reaction of 2,6-di-tertbutylanthracene with fumaric acid di(-)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by H-1 and C-13 NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7% ee, based on the ee (99.9%) of the chiral synthon (1). The absolute configuration of 7 could thus be established as all-S [(2S,7S,16S,21S,30S,35S)-(7)]. Spectroscopic titration experiments reveal that the host forms 1: 1 complexes with either pure fullerene (C-60) or fullerene derivatives, such as rotor 1'-(4-nitrophenyl)-3'-(4-N,N-dimethylaminophenyl)-pyrazolino[4',5':1,2][60]fullerene (R). The complex stability constants of the complexes dissolved in CHCl3/CS2 (1:1 vol.%) are K([C-60 subset of 7])= 319(+/- 156) M-1 and K([R subset of 7])= 110(+/- 50) M-1. With molecular dynamics simulations using a first-principles parameterized force field the asymmetry of the rotational potential for [R subset of 7] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet