Photothermal Characterization of Thermal Transport in Carbon Nanotubes and Nanostructured Polymers

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Bougher, Thomas Lloyd
Cola, Baratunde A.
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In this work we aim to improve thermal transport in three types of materials: vertical arrays of polymer nanotubes, polymer thin films, and vertical arrays of carbon nanotubes (CNTs). In addition to developing new materials, we examine ways to extend several existing photothermal techniques for better measurements of nanostructured materials. Vertically aligned arrays of polymer nanotubes are synthesized through electrochemical polymerization, and melt processing. In both cases the vertically aligned arrays are created using a nanoporous template with pore sizes ranging from 50 to 200 nm. It is found that the nanopores induce alignment of polymer chains in the direction of the pore axis, which creates large enhancements in thermal conductivity. For amorphous polythiophene nanofibers the thermal conductivity was up to 4.4 W/m-K, up to 20 times higher than bulk. Polyethylene nanofibers were melt infiltrated into 200 nm templates, and the thermal conductivity was as high as ~10 W/m-K based on in-template measurements. The vertical arrays of polymer nanotubes are joined to opposing surfaces to create thermal interface materials (TIMs) and the total thermal resistance was as low as 10 mm2-K/W shown to be thermally stable at elevated temperatures. In addition the thermal conductivity of conjugated polymer thin films is measured using Time Domain Thermoreflectance (TDTR) and the photoacoustic technique for thin (~100 nm) films and thick (~10 μm) films and it is found that Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), highly anisotropic electrical conductivity leads to only small increases in through-plane thermal conductivity, despite high in-plane electrical conductivity of 2000 S/cm. In poly(3-hexylthiophene-2,5-diyl) the addition of 10 wt. % of a heavy dopant molecule was found to decrease the thermal conductivity by nearly a factor of two. Individual carbon nanotubes possess high thermal conductivity although when grown in a vertical array their thermal conductivity is drastically reduced. A method for the direct measurement of the effective thermal conductivity of CNT arrays is presented using the photoacoustic technique. In addition, new data analysis is applied to time domain thermoreflectance to measure the contact resistance between CNT free tips and opposing substrates. Lastly, vertically aligned CNT forests are infiltrated with PEDOT:PSS to reduce the contact resistance and increase the thermal conductivity. The aqueous PEDOT:PSS dispersion is found to infiltrate CNT forests well in most cases and dramatically reduce the total thermal resistance, to below 2 mm2-K/W in some cases. This work provides a number of materials and metrology developments that will enable better thermal management nanostructured organic materials.
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