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A5: Nanoscale and low dimensional effects
Thermoelectric devices can perform a direct conversion between thermal and electrical energies. Their efficiencies are determined by the dimensionless figure of merit [ZT=S2σT/(κel+κph)], in which the Seebeck coefficient (S), electric conductivity (σ), electronic (κel) and phononic (κph) thermal conductivities can be calculated by using the Boltzmann formalism [1]. The inherent correlation between these transport parameters makes difficult to improve the value of ZT. Recently, nanowire heterostructures, such as Si/Ge [2] and PbTe/BiTe [3], have demonstrated better ZT in comparison with their corresponding pure structures, mainly due to the reduction of long-wavelength phonons that are responsible of the lattice thermal conduction at low temperatures. However, the presence of composite constituents, grain boundaries, as well as device interfaces in these heterostructures requires performing the theoretical modeling in the real space, which is practically impossible to carry out for truly macroscopic devices by means of the current computing capacity. In this work, we report a fully real-space study of ZT by using a renormalization plus convolution method for the Kubo-Greenwood formula [4] and its extension for the lattice thermal conductivity [5]. Analytical results are obtained for periodic nanowires. Clear improvements of ZT are observed in segmented and branched nanowires in comparison to periodic ones. Finally, the theoretical results are compared with the available experimental data.
[1] T. M. Tritt (Ed.), Thermal Conductivity – Theory, Properties and Applications (Kluwer/Plenum Pub., New York, 2004) p. 3.
[2] E. K. Lee, et al., Nano Lett. 12, 2918 (2012).
[3] H. Fang, T. Feng, H. Yang, X. Ruan and Y. Wu, Nano Lett. 13, 2058 (2013).
[4] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004).
[5] C. Wang, F. Salazar and V. Sanchez, Nano Lett.8, 4205 (2008).