Thermal Conductivity of Single Wall Carbon

Thermal Conductivity of Single Wall Carbon - Thermal...

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Thermal Conductivity of Single Wall Carbon Nanotubes: Diameter and Annealing Dependence M.C. Llaguno, J. Hone*, A.T. Johnson, I.E. Fischer Department of Physics and Astronomy, Laboratory for the Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104,USA '"Department of Physics, California Institute of Technology Pasadena, California 91125, USA Abstract. The thermal conductivity, K(T), of bulk single-wall carbon nanotubes (SWNT's) displays a linear temperature dependence at low T that has been attributed to ID quantization of phonons. To explore this issue further, we have measured the K(T) of samples with varying average tube diameters. We observe linear K(T) up to higher temperatures in samples with smaller diameters, in agreement with a quantization picture. In addition, we have examined the effect of annealing on K(T). We observe an enhancement in K(T) for annealed samples which we attribute to healing of defects and removal of impurities. These measurements demonstrate how the thermal properties of an SWNT material can be controlled by manipulating its intrinsic nanoscale properties. INTRODUCTION The high thermal conductivity of single wall carbon nanotubes (SWNT's) makes them ideal for thermal management applications. To illustrate, in magnetic field aligned films of SWNT's, a value of 250 W/m-K has been measured at room temperature[l]. For single tubes, theoretical calculations of the thermal conductivity give even higher values of about 10,000 W/m-K[2]. In addition, the unique structure of carbon nanotubes allows for the study of low-dimensional phonons. Specifically, the cylindrical geometry of a tube enforces periodic boundary conditions on the circumferential wave vector, resulting in the formation of ID phonon 'subbands'- analogous to the electronic subbands. In an isolated SWNT, there are four acoustic phonon modes, with the first optical subband contributing at an energy of a few meV[3]. In heat capacity measurements, the quantized phonon spectrum was observed as a deviation from linear behavior at around 8 K[4] which corresponds to a first subband energy of 4 meV. Similarly, the thermal conductivity of nanotubes should exhibit a linear T-dependence at low T and then a nonlinear trend above a crossover temperature that is directly related to the subband splitting. Previous measurements
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This note was uploaded on 03/18/2012 for the course PHYSICS 303 taught by Professor Ihn during the Spring '12 term at Swiss Federal Institute of Technology Zurich.

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Thermal Conductivity of Single Wall Carbon - Thermal...

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