Materials Research Lectures
COMPLEX THERMOELECTRIC MATERIALS
The widespread use of thermoelectric generators has been limited by the low material efficiency of the thermoelectric material. A number of strategies for Complex Thermoelectric Materials  with higher Thermoelectric figure of merit, zT, are being actively studied at Caltech. Complex electronic band structures provide mechanisms to achieve high zT in thermoelectric materials through band structure engineering. High zT is obtained p-type PbTe and PbSe which contains both light and heavy valence bands that can be engineered to achieve high valley degeneracy which leads to an extraordinary peak zT of nearly 1.8 at 750K .
Complex crystal structures that enable relatively low thermal conductivity have lead to several new classes of thermoelectric materials. Ca3AlSb3, Ca3AlSb3 and Yb14AlSb11 are complex Zintl compounds containing differently connected AlSb4 tetrahedra that obtain zT near 1 at high temperatures. Fast diffusing or 'liquid-like' elements in the complex materials Zn4Sb3  and Cu2Se  provide additional mechanisms to scatter and otherwise inhibit phonon heat conductivity. The principles of Zintl chemistry facilitates the search for new complex materials and the tuning of known thermoelectric materials with earth abundant, non-toxic elements 
Finally, the incorporation of nanometer sized particles reduces thermal conductivity from long mean-free-path phonons. This principle has been successfully demonstrated in PbTe with large nanoscale precipitates (>100nm) that can be independently doped with La (n-type) or Na (p-type). The synthesis of nanoscale composites can be controlled with the aid of equilibrium phase diagrams (experimental or theoretically determined) to produce microstructure of varying composition and length scale .
 G. J. Snyder, E. S. Toberer. "Complex thermoelectric materials" Nature Materials 7, p 105 - 114 (2008)
 Y. Z. Pei, G. J. Snyder, et al. "Convergence of Electronic Bands for High Performance Bulk Thermoelectrics"Nature 473, p 66 (2011)
 H. Liu, X. Shi, G. J. Snyder, et al. "Liquid-like Copper Ion Thermoelectric Materials" Nature Materials, doi:10.1038/nmat3273 (2012)
 G. J. Snyder, et al., "Disordered Zinc in Zn4Sb3 with Phonon Glass, Electron Crystal Thermoelectric Properties" Nature Materials, Vol 3, p. 458 (2004)
 E. S. Toberer. A. F. May, G. J. Snyder, "Zintl Chemistry for Designing High Efficiency Thermoelectric Materials" Chemistry of Materials 22, p 624 (2010)
 D.L. Medlin and G.J. Snyder "Interfaces in Bulk Thermoelectric Materials" Current Opinion in Colloid & Interface Science 14, 226 (2009)
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