Thermal transport in P3HT nanostructures and nanocomposites

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Smith, Matthew Kirby
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Consumers continue to pressure electronic device manufacturers to produce smaller and higher performance products. By decreasing size and increasing computational power, electronic devices get hotter and fail faster and heat removal is an increasingly difficult challenge. Thermal interface materials (TIMs) are used to maximize heat transport across the interface connecting heat generating components (such as processors, batteries, and LEDs) to the heat sinking components (such as heat sinks, heat pipes, and cold plates) and they are often the bottleneck to heat removal. The focus of this work is to explore the thermal transport properties of three new poly(3-hexylthiophene-2,5-diyl) (P3HT) based nanostructured material systems that show potential for use as TIMs; vertically aligned P3HT nanotubes and nanofibers, vertically aligned P3HT/multi-walled carbon nanotube (MWCNT) composite nanofibers, and P3HT infiltrated laser-induced graphene (LIG) foam composite thin films. Solution processed P3HT nanotubes are fabricated using a simple template technique where polymer solution is cast onto nanoporous anodic aluminum oxide (AAO) templates. The thermal conductivity of the P3HT nanotubes is measured using the photoacoustic (PA) technique and is approximately 1 W/m-K, or ~5x that of bulk P3HT film, indicating preferential chain alignment along the long axis of the tube. The solution processed nanotube arrays proved to have poor mechanical stability and a melt processing technique is introduced to produce P3HT nanofiber arrays. The nanofiber thermal conductivity is measured as a function of P3HT molecular weight and nanofiber diameter, where the highest known P3HT thermal conductivity (~7 W/m-K) is observed in 100 nm diameter fibers. In addition, polarized Raman spectroscopy is used to demonstrated preferential polymer chain alignment along the nanofiber’s long axis. A novel method to fabricate P3HT/MWCNT composite fibers is introduced to further improve thermal transport in the melt processed nanofibers. AAO templates are first infiltrated with MWCNT through sonication in MWCNT dispersion and then subsequently filled with polymer melt. TEM imaging reveals strong preferential alignment of MWCNT along the fibers long axis and the processing conditions are varied to achieve P3HT/MWCNT composite fibers with MWCNT wt% of up to 55%. Composite fiber thermal conductivity peaks at a value of 4.7 ±1.1 W/m-K for the 200 nm diameter 24 wt% MWCNT fibers, or roughly twice the value of pure P3HT 200 nm melt processed fiber. Higher MWCNT weight percentages results in pore blocking and the inability of the P3HT melt to fully infiltrate the MWCNT filled nanopores, which significantly reduces fiber thermal transport. Lastly, a new material system is introduced where P3HT solution is infiltrated into a three-dimensional graphene foam network. The P3HT/LIG composites exhibit an unexpected increase in LIG foam thermal conductivity, from 0.68 W/m-K for bare LIG to 1.72 W/m-K for LIG foam that is fully infiltrated with P3HT. It is hypothesized that the dramatic increase in thermal conductivity arises from the P3HT’s ability to serve as a bonding agent between broken graphene ligaments, which significantly reduces thermal contact resistances within the graphene foam skeleton.
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