In this study we introduce our numerical and experimental works for the thermal conductivity reduction by using a porous material. Recently thermal conductivity reduction has been one of the key technologies to enhance the figure of merit (ZT) of a thermoelectric material. We carry out numerical calculations of heat conduction in porous materials, such as, phonon Boltzmann transport (BTE), molecular dynamics simulations (MD), in order to investigate the mechanism of the thermal conductivity reduction of a porous material. In the BTE, we applied the periodic boundary conditions with constant heat flux to calculate the effective thermal conductivity of porous materials. In the MD simulation, we calculated phonon properties of Si by using the Stillinger-Weber potential at constant temperature with periodic boundary conditions in the x,y and z directions. Phonon dispersion curves of single crystal of Si calculated from MD results by time-space 2D FFT are agreed well with reference data. Moreover, the effects of nano-porous structures on both the phonon group velocity and the phonon density of states (DOS) are discussed. At last, we made a porous p-type Bi2Te3 by using self-assembly. The average diameter of the holes was 20 nm, and the average pitch of the hexagonally arranged holes was 50 nm. The measured cross-plane thermal conductivity is 0.25W/(m·K). The thermal conductivity of the thin film is extremely lower than that of the bulk material without any major decrease in the electrical conductivity. The figure of merit of Bi0.4Te3Sb1.6 is enhanced to 1.8 at room temperature (300K) by the formation of a porous thin film.

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