Annealing effect on the thermoelectric properties of multiwall carbon nanotubes

In this article, it is demonstrated that transport properties and thus thermoelectric performance of multiwall carbon nanotubes (MWCNTs) can be enhanced significantly by annealing. Two different samples of MWCNTs with small and large diameters were investigated. The as-prepared MWCNT samples were treated at different conditions of annealing. The pristine and annealed MWCNTs were investigated using x-ray diffraction, Raman spectroscopy and scanning electron microscope in order to investigate the microstructure, defect content and morphology before and after the annealing process. The electrical conductivity, Seebeck coefficient and thermal conductivity were measured over the temperature range of (300–450 K) and were found to correlate with the tube diameter and the annealing conditions. The electrical conductivity and Seebeck coefficient measurements show that the conduction mechanism in MWCNTs is thermal activation as the samples have degenerate semiconductor-like behavior. Upon annealing, the Seebeck coefficient increases, followed by a reduction in the thermal conductivity. The thermoelectric power factor was enhanced by annealing to realize the highest value of 1.06 μW m−1 K−2 at 475 K for the sample with the larger diameter. The thermoelectric figure of merit ZT was calculated to determine the performance of the materials. ZT of the sample of the larger tube's diameter was enhanced by about 247% at 450 K.