Collaborative Research: Hydrodynamic Thermal Transport in Graphitic Materials

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Graphitic materials exhibit some of the highest thermal conductivity values found in solids. This feature has resulted in existing and emerging applications of these materials for transferring heat and keeping operating devices cool. However, the actual mechanisms behind their high thermal conductivity are not well understood. Recent theoretical studies have suggested that frequent scattering among a peculiar group of high-population phonons in graphitic materials does not either cause resistance directly or limit their thermal conductivity contribution. This unusual phonon transport behavior has been referred as hydrodynamic phonon transport. Observation of hydrodynamic phonon transport has never been experimentally observed in any materials at a sufficiently high temperature that is relevant to technological applications. Hydrodynamic phonon transport can have practical implications in the design and thermal modeling of graphitic materials. Thus, a set of theoretically guided advanced experiments are conducted in this research to verify the existence of the unusual thermal transport behavior in graphitic materials. Also, the research findings and methods are integrated into three undergraduate and graduate courses and two demonstration modules for general public. The objective of this research is to elucidate the influence of hydrodynamic phonon transport on the thermal transport properties of graphitic materials. This will be accomplished through theoretical and experimental efforts, including first principles based theoretical studies of phonon transport, nanoscale thermal transport measurements of the intrinsic thermal conductance, and ultrafast opto-thermal measurements of second sound in isotopically purified nanotube, graphene, and thin graphite samples. These theoretical and experimental efforts can further lead to a better understanding of the quantum theory of energy transport by lattice vibrations. If successful, the research can lead to a new approach to simulate phonon transport. In addition, it can advance the frontier of nanoscale thermal transport measurements by overcoming a critical challenge in probing the intrinsic thermal transport properties of nanostructures. Moreover, it can result in a cutting edge ultrafast thermal transport method for probing phonon transport.