Thermal conductivity of semiconductor (bismuth–telluride)–semimetal (antimony) superlattice nanostructures

In the present study, the thermal conductivity of superlattice bismuth–telluride (semiconductor)–antimony (semimetal) (Bi2Te3–Sb) nanostructures (nanowires and nanotubes) has been modeled using an incoherent particle model, approximating all the scattering to be diffuse and gray, and applying a Matthiessen-type simplification. The effect of varying the ratio of the superlattice nanowire segment lengths (L) of Sb and Bi2Te3 has also been studied assuming: (i) LSb = LBi2Te3LBi2Te3; (ii) LSb = 0.25 × LBi2Te3LBi2Te3; (iii) LSb = 0.5 × LBi2Te3LBi2Te3; (iv) LSb = 2.0 × LBi2Te3LBi2Te3; and (v) LSb = 4.0 × LBi2Te3LBi2Te3. It is shown that thermal conductivity of the superlattice nanowires reduces either with a reduction of segment lengths (LSb and LBi2Te3LBi2Te3) or with a reduction of nanowire diameter. Specifically, the thermal conductivity is lower than 2 W m−1 K−1 (the bulk value for Bi2Te3), even when the nanowire diameters (10 nm) are 10 times larger than the mean free path (1 nm) of Bi2Te3, provided the individual segment lengths (LSb and LBi2Te3LBi2Te3) are lower than the mean free path limit. The thermal conductivity of either superlattice nanowires or superlattice nanotubes was also observed to decrease, as the segment length of semimetal (Sb) is lowered relative to the segment length of semiconductor (Bi2Te3). In the case of superlattice nanotubes, a reduction in wall thickness caused a corresponding reduction in thermal conductivity as well. For example, with a fixed outer diameter value of 5 nm, the thermal conductivity of the nanotubes can be lowered by ∼33% by decreasing the tube wall thickness from 0.75 to 0.1 nm. Our predictions also suggest that for a given value of the segment lengths of LSb and LBi2Te3LBi2Te3, nanotubes exhibit a lower thermal conductivity than nanowires. This therefore suggests that nanotubes of superlattice structures of Sb and Bi2Te3 should exhibit a higher thermoelectric figure of merit (ZT) than nanowires under corresponding conditions.