Thermal Transport in MWNT Sheet: Extremely High Radiation From the Carbon Nanotube Surface
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Thermal Transport in MWNT Sheet: Extremely High Radiation From the Carbon Nanotube Surface. A. E. Aliev1, C. Guthy2, M. Zhang1, A. A. Zakhidov1, J. E. Fisher2, and R. H. Baughman1 1 NanoTech Institute, University of Texas at Dallas, Richardson, TX, 75083 2 Dept. of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, 19104 ABSTRACT Laser flash and self-heating 3ω techniques were employed to determine the anisotropic thermal conductivity and thermal diffusivity of highly oriented free standing multiwalled carbon nanotube (MWNT) sheet drawn from a sidewall of a MWNT forest that was grown by chemical-vapor deposition. The thermal conductivity and the thermal diffusivity along the alignment are 50±5 W/m·K and 45±5 mm2/s, respectively, and are mostly limited by intrinsic defects of individual nanotubes and phononphonon interaction within bundles which form the supporting matrix of the MWNT sheet. The long tubetube overlapping substantially decreases the electrical and thermal interconnection resistances which are usually dominate in randomly deposited mat-like nanotube assemblies. The extremely large surface area of the MWNT sheet leads to excessive heat radiation that dose not allow to transfer the heat energy by means of phonons to distances > 2 mm.
INTRODUCTION Recently we developed a new dry-state technique to produce highly-oriented, free-standing multiwalled carbon nanotube (MWNT) sheet that appears to be very attractive for thermal transport applications [1]. The good alignment, two dimensional structure of the MWNT sheet and long overlapping of individual tubes give promising features for effective heat transport. Randomly deposited nanotubes in mat-like paper or polymer composites do not provide proper pathways for electron and phonon transport. The point interconnections between tubes in these types of assemblies pose the main obstacle to high electron and phonon conductivities. In contrast, a bundle pulled off from the MWNT forest side presents the perfectly aligned assembly that dose not suffer from weak tube-tube interconnections within the sample. However, the bundle-electrode interface at short electrode distances becomes now another irresistible drawback for effective transport. The problem arises from the week Van der Waals interaction between nanotubes that prevents good electrical or thermal contact to the central part of bundles. In this respect the thin, good aligned MWNT sheet presents a perfect structure for electron and phonon propagation and can be easily utilized for many applications. This work presents the comprehensive study of the anisotropic thermal conductivity and the thermal diffusivity of MWNT sheets using two methods: laser flash and self-heating 3ω methods. The goal of this study is to clarify the main mechanisms that prevent the effective thermal transport in the suspended highly aligned assembly of MWNT. Taken into account the complicated nature of studied matter the both used methods were thoroughly calibrated on the same specimens of well-
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