Molecular Dynamics Simulations of Diffusive-Ballistic Heat Conduction in Carbon Nanotubes
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1022-II05-03
Molecular Dynamics Simulations of Diffusive-Ballistic Heat Conduction in Carbon Nanotubes Junichiro Shiomi and Shigeo Maruyama Department of Mechanical Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan ABSTRACT Heat conduction of finite-length single-walled carbon nanotubes (SWNTs) has been studied by means of non-equilibrium molecular dynamics (MD) simulations. The length-dependence of the thermal conductivity was quantified for a range of nanotube-lengths at room temperature. The length dependence of thermal conductivity exhibits a gradual transition from nearly pure ballistic heat conduction to diffusive-ballistic heat conduction. The results show that the thermal conductivity profile does not converge even beyond a micrometer nanotube-length. Furthermore, the diameter dependence suggests that the phonon diffusion is reduced with the diameter. INTRODUCTION The growing expectations for single-walled carbon nanotubes (SWNTs) include applications for various electrical and thermal devices due to their unique properties [1]. SWNTs are expected to possess high thermal conductivity due to their strong carbon bonds and the quasi-onedimensional structure [2]. On considering the actual applications, one of the essential tasks is to characterize the thermal properties not only for thermal devices but also for electrical devices since they determine the affordable amount of electrical current through the system. With advances in SWNT synthesis and MEMS techniques, thermal conductivity (or thermal conductance) measurements of individual SWNTs have been recently reported [3, 4]. The reported values of thermal conductivity were around 3500 W/mK, similar to those of individual multi-walled carbon nanotubes [5, 6], and about an order of magnitude larger than that of bulk carbon nanotubes in forms of mats and bundles [7]. Furthermore, experimental measurements with temperature variation [3, 4] have identified a critical temperature (~300 K) above which Umklapp scattering becomes important. The thermal property measurements of SWNTs mentioned above are extremely challenging as there are potential uncertainties residing in the technicality for instance related to the contact resistances between thermal reservoirs and an SWNT. Therefore, there is a great demand for reliable theories and numerical simulations, especially for investigations of detail heat-transfer characteristics that are not accessible in experiments. One of such characteristics with practical importance is the size dependence of thermal conductivity. In general, the size-dependence of the thermal conductivity appears when the system characteristic length is smaller or comparable to the phonon mean free path [8]. For SWNTs, due to the expected long phonon mean free path, the regime of the length effect stretches beyond the realistic length in many applications. The length effect has been demonstrated using MD simulations [9, 10] and the power-law divergence was discussed with analogy to the convectional one-dimensional mod
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