Introduction

Thermoelectric materials directly convert heat fluxes into useable electricity and are therefore discussed as a key-enabler in waste heat recovery. For this vision, the main challenge is to develop thermoelectric materials with sufficiently high conversion efficiencies, expressed by the material's figure of merit zT = a²sT/k, where a, s, k and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. a²s  is called power factor. It is assumed that zT@1.5 is necessary for most technical applications.^[1]

Nanostructuring of thermoelectric materials has been demonstrated experimentally and theoretically to greatly improve the figure of merit by reducing the lattice contribution to the thermal conductivity.^[2,3] Different types of scattering centers for the heat carrying phonons were implemented as design concepts for thermoelectric materials, such as nanoscale precipitates or interfaces.^[4,5] To effectively scatter the broad spectrum of phonon wavelengths, a hierarchical design of the nano- and microstructure was developed which led to record-high zT values.^[6]

References

[1] L. E. Bell, Science 2008, 321, 1457.

[2] J. P. Heremans, M. S. Dresselhaus, L. E. Bell, D. T. Morelli, Nature Nanotech. 2013, 8, 471.

[3] J. Yang, H.-L. Yip, A. K.-Y. Jen, Adv. Energy Mat. 2013, 3, 549.

[4] N. Mingo, D. Hauser, N. P. Kobayashi, M. Plissonnier, A. Shakouri, Nano Lett. 2009, 9, 711.

[5] B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen, Z. Ren, Science 2008, 320, 634.

[6] K. Biswas, J. He, I. D. Blum, C.-I. Wu, T. P. Hogan, D. N. Seidman, V. P. Dravid, M. G. Kanatzidis, Nature 2012, 489, 414.