Ultralow thermal conductivity and thermoelectric properties of carbon nanotubes doped Ca3Co4O9+δ

In recent years, many efforts have been made aiming to explore low thermal conductivity materials for thermoelectric application. In this paper, we present an effective way to design an oxide-based thermoelectrics with ultralow thermal conductivity. The thermoelectric properties of carbon nanotubes doped Ca3Co4O9+δ (CCO) have been studied. The lowest thermal conductivity among oxides thermoelectric materials reported so far has been successfully reached. The 6 wt% CNTs doped CCO has a thermal conductivity of 0.3 W K−1 m−1 at room temperature, which is a 91% decrease, compared to that of pristine CCO. Such ultralow thermal conductivity is even lower than the disordered crystal limit of 0.319 W K−1 m−1, which can be attributed to increasing phonon scattering at nano-defects induced by the CNTs׳ nanoinclusions, porous structure, and CCO/CNTs׳ boundaries. We reveal that the dramatic reduction in phonon mean free path should be the dominating factor in depressing the thermal conductivity of nanocomposite materials. It is remarkable that the CNTs׳ doping also significantly enhances the magnitude of the thermopower.