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A5: Nanoscale and low dimensional effects
Thermal conductivity can be reduced by using nano-structured materials due to the reduction of phonon transport. The figure of merit (ZT) can be improved by the reduction of the thermal conductivity and the energy conversion efficiency from thermal to electrical energy is enhanced. It is important to predict the thermal conductivity of the nano-structured materials to realize the thermoelectric materials with high-ZT. We roughly predict the lattice thermal conductivity of the nano-structured bismuth telluride as a function of the phonon mean free paths by using a spectral phonon gas model. In this case, only the physical properties of bulk single crystal of bismuth telluride are used as parameters for the calculation. The anisotropic heat conductions of bismuth telluride are considered in the model taking into account of the anisotropy of elastic stiffness. Phonon mean free paths are assumed to be cut-off by nano-sized grains. As a result, both the cumulative lattice thermal conductivities of bismuth telluride along the a-axis and c-axis are obtained similar values in the cut-off phonon free path less than a few 100 nm.