(Lecture, Jun 26) Complex Thermoelectric Materials
time: 2017-06-20

Topic: Complex Thermoelectric Materials
Speaker: G. Jeffrey Snyder, Northewstern University
Time: 10:00, Jun 26, 2017
Venue: Room 205, Building 14, Wushan Campus

[Biography]
G. Jeffrey Snyder is a Professor of Materials Science and Engineering at Northwestern University in Evanston Illinois. His interests are focused on the materials physics and chemistry for thermoelectric engineering, such as band engineering, design of complex Zintl compounds and use of nanostructured composites. His interdisciplinary approach stems from studies of Solid State Chemistry at Cornell University and the Max Planck Institute for solid state research, Applied Physics at Stanford University and thermoelectric materials & device engineering at NASA/Jet Propulsion Laboratory and California Institute of Technology (Caltech).

[Abstract]
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 [1] with higher Thermoelectric figure of merit, zT, are being actively studied. 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 by alloying to achieve high valley degeneracy which leads to an extraordinary peak zT of about 2 at 750K [2].
Complex crystal structures that enable relatively low thermal conductivity have lead to several new classes of thermoelectric materials. Ca3AlSb3, Ca5Al2Sb6 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 [3] and Cu2Se [4] 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 [5]
Finally, the incorporation of nanometer sized microstructure reduces thermal conductivity from long mean-free-path phonons. This principle has been successfully demonstrated in (Bi,Sb)2Te3 alloys with arrays of dislocations at grain boundaries [6]. 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 [7]
[1] G. J. Snyder, E. S. Toberer. “Complex thermoelectric materials” Nature Materials 7, p 105 – 114 (2008)
[2] Y. Z. Pei, G. J. Snyder, et al. Convergence of Electronic Bands for High Performance Bulk ThermoelectricsNature 473, p 66 (2011); Advanced Materials 23, 5674 (2011)
[3] H. Liu, X. Shi, G. J. Snyder, et al. “Liquid-like Copper Ion Thermoelectric Materials” Nature Materials, 11, 422 (2012); APL Materials, 1, 052107 (2013)
[4] G. J. Snyder, et al., Disordered Zinc in Zn4Sb3 with Phonon Glass, Electron Crystal Thermoelectric Properties Nature Materials, Vol 3, p. 458 (2004); J. Mater. Chem., 20, 9877 (2010)
[5] E. S. Toberer. A. F. May, G. J. Snyder, “Zintl Chemistry for Designing High Efficiency Thermoelectric Materials” Chemistry of Materials 22, p 624 (2010)
[6] S. I. Kim, H. S. Kim, G. J. Snyder, S. W. Kim et al. “Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics” Science, 348, 6230 (2015)
[7] Nicholas A. Heinz, T. Ikeda, Y. Pei and G. J. Snyder Applying quantitative microstructure control in advanced functional composites Advanced Functional Materials 24, 2135 (2014)