Bismuth antimony telluride (Bi_(0.4)Te_(3.0)Sb_(1.6)) nano-porous thin films have been prepared and measured their thermal conductivities. The thin films exhibit an average grain size of 50 nm and random crystal orientation. The cross-plane thermal conductivity is measured by a differential 3ω method at temperature range from 100 to 300 K, and the determined thermal conductivities are from 0.09 to 0.18 W/(mK). As compared with bulk alloys at the same atomic composition, the nano-porous thin films exhibit an eightfold reduction in the thermal conductivity. For more detail analysis, the reduction of the thermal conductivity is examined by a simplified phonon gas model on a single crystal of bulk Bi_2Te_3. The analytical model fairly agreed with the experimental results, and thus we consider that the thermal conductivity is reduced by the strong phonon scattering at the nano-pores.
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