Topics of Heat Transfer Related to Single-walled Carbon Nanotubes

Using an alcohol catalytic CVD method shown to produce high-quality single-walled carbon nanotubes (SWNTs), films of vertically aligned (VA-)SWNTs were synthesized on quartz substrates. The VA-SWNTs can be removed from the substrate and transferred onto an arbitrary surface—without disturbing the vertical alignment—using a hot-water assisted technique. This ability makes experimental measurements of the anisotropic properties of SWNTs considerably less challenging. A series of molecular dynamics simulations have been performed to investigate a variety of heat conduction characteristics of SWNTs. Investigations of stationary heat conduction identifies diffusive-ballistic heat conduction regime in a wide range of nanotube-lengths. Furthermore, studies on non-stationary heat conduction show that the extensive ballistic phonon transport gives rise to wave-like non-Fourier heat conduction. Finally, several case studies are presented for SWNT heat transfer in more practical situations. INTRODUCTION Single-walled carbon nanotubes (SWNTs) are rolled-up tubes of single-layer graphite, which have a diameter on the order of 1 nm. They have been shown to have remarkable electrical, optical, mechanical and thermal properties [1], making them not only interesting from a purely scientific perspective, but also highly desirable for development of many new applications. As a result, SWNTs have been the focus of numerous investigations in numerous fields of science and engineering. However, one challenge facing many of these studies is control over the morphology of synthesized SWNTs. A significant development in morphology control came when the alcohol catalytic CVD technique [2] was combined with a simple dip-coat preparation of catalytic metal particles. The result was vertically aligned (VA-)SWNT films [3, 4] synthesized on quartz substrates. These films can exceed 30 μm in thickness, and consist of thin, vertically aligned SWNT bundles [5]. Their use in various applications is becoming more and more promising, and VA-SWNT synthesis has recently been achieved by several other groups [6-12]. In the heat transfer community, the expected high thermal conductivity of SWNTs [13] has attracted a number of studies over the past few years. Recent activities in our group directed at experimentally measuring the thermal conductivity of our aligned SWNT films will be discussed. With advances in SWNT synthesis and MEMS techniques, thermal conductivity (or thermal conductance) measurements of individual SWNTs have been recently reported [14, 15]. However, the thermal property measurements of SWNTs in experiments are extremely challenging as there are potential uncertainties residing in the technicality related to, for instance, the contact resistances between thermal reservoirs and an SWNT. Therefore, the demands for reliable theories and numerical simulations are greater than ever for validation of experimental results and for detailed investigation of heatconduction characteristics that are not accessible in experiments. Molecular dynamics (MD) simulations have been a strong tool for the investigation of thermal conductivity of SWNTs. The diffusive-ballistic phonon transport regime covers a wide range of nanotube-lengths in actual applications due to the extraordinary long phonon mean free path at room temperature. This gives rise to various unique stationary and non-stationary heat conduction characteristics. A unique aspect of the heat conduction of an SWNT can be seen in the length dependence of the thermal conductivity. 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 [16, 17] and the power-law divergence was discussed with analogy to conventional one-dimensional