Thermal conductivity and thermopower of vapor-grown graphite fibers

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Abstract
Values for the thermal conductivity and thermopower of graphite fibers grown by pyrolysis of natural gas are reported between 10 and 370 K. The measurements were made along the fiber axis on both as-grown fibers and fibers heat treated to 3000 °C. It is shown that heat treatment improves the thermal conductivity by a factor of 50: at room temperature highly heat-treated fibers rank among the very best thermal conductors available. We quantitatively interpret our low-temperature thermal conductivity data using the phonon dispersion relation for graphite. Remarkable agreement is achieved between the phonon mean free paths we deduce from our measurements and defect structures we identify in electron micrographs. The thermopower data we report on the highly heat-treated fibers are very similar to those obtained on single-crystal graphite.