This work reports the effect of high temperature annealing on thermal diffusivity along the alignment direction of multiwalled carbon nanotube arrays annealed for two hours in an inert ambient at 1900°C. Thermal diffusivity measurements were performed on as-grown and annealed samples using a photothermoelectric technique, which was adapted here for temperature dependent measurements in a vacuum cryostat between 80K and 300K. Experimental results show more than two-fold increase in thermal diffusivity along the carbon nanotubes alignment direction of the annealed sample as compared with the as grown specimen. Transmission electron microscopy studies indicate that metallic catalyst impurities embedded during the growth of the samples were successfully removed by the annealing process, although no significant improvement in the carbon nanotube walls graphitization was observed. This work demonstrates the effectiveness of the annealing method to restore the thermal properties of as grown carbon nanotube material and indicates that metallic impurity scattering may play an important role in thermal conductivity reduction of carbon nanotubes grown by chemical vapor deposition. However, the thermal diffusivity of the annealed sample (at 300K) is still one order of magnitude smaller than the in-plane thermal diffusivity of graphite. It is suggested that temperature annealing at even higher temperatures may lead to improved graphitization of the carbon nanotube walls and further recovery of the thermal transport properties.
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