ICT2014 Abstracts

The electronic structure calculations using density functional theory (DFT) predicted that the hypothetical type-VIII clathrate Si₄₆ has a significant number of valleys around both the valence and the conduction bands.[1] The large density of states near the Fermi level in this material can enhance the thermoelectric power factor of both p-type and n-type clathrate Si₄₆-VIII. We employed an integrated approach combining the data extracted from DFT, lattice dynamics, molecular dynamics, Holland-Callaway method, and multiband Boltzmann transport theory to calculate the thermoelectric transport properties of bulk and nanostructured Si₄₆-VIII as functions of temperature and carrier concentration. The contribution of each valley to the total thermoelectric power factor was calculated separately. The GH and NH points in the band edges with the degeneracy value of 6 and 12, respectively, showed the largest contributions to the power factor. The figure-of-merit of bulk nanostructured p-type and n-type Si₄₆-VIII was predicted to be in the order of 2.5 and 2, respectively, at 1000 C for average crystallite size of 20 nm. The reduction of the thermal conductivity and the electrical conductivity mostly occur for crystallite sizes less than 30 nm and 5 nm, respectively. Therefore, the spectrum of phonon mean free path is much broader than that of the charge carriers. To guide the experimental characterization of this hypothetical material, the Infrared and Raman active modes were calculated providing references for their structural identifications.