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A5: Nanoscale and low dimensional effects
The efficiency of thermoelectric materials is determined by the dimensionless figure of merit, which is given by ZT=S2σT/ (Ke + Kl), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the surrounding temperature, Ke and Kl are the electronic and lattice phonon thermal conductivity, respectively. According to the equation, ZT is inverse proportion to the thermal conductivity (Ke + Kl). When the dimension of materials is reduced (or comparable) to the mean free path of phonons, the phonon scatterings of surfaces and boundaries play a crucial role in the reduction of thermal conductivity. Bi0.5Sb1.5Te3 is a typical thermoelectric material with figure of merit (zT) about 1 at room temperature, it becomes a good candidate for the studying of size effect on thermoelectric properties [1]. Single crystalline thermoelectric Bi0.5Sb1.5Te3 nanowires were grown by stress-induced method from the Bi0.5Sb1.5Te3 deposited film prepared by pulse laser deposition (PLD). The line width dependence of thermoelectric properties is studied for nanowires with line width ranging from 80 to 350 nm. The thermal conductivity of 350 nm at 300 K is 1.5 W/mK which is about the same as that of its counterpart bulk, however a five time reduction of thermal conductivity ~0.3 W/mK was found in the 80 nm single nanowire. This result confirms the above hypothesis that the restricted size or low dimension can radically reduce the thermal conductivity of phonons by phonon-surface interactions. The strategy is a promising method to improve the figure of merit of the existing thermoelectric materials.
[1] Xie, W. J. et. al., Appl. Phys. Lett. 94, 102111(2009)