AMADEus Seminar - Pr. Jeffrey Snyder - Friday 13 November 2015, 11:00 am - ICMCB (Salle Patio PLACAMAT))

Abstract: Processing influences many aspects of thermal and electrical transport properties in materials. Non-equilibrium
microstructures and defects can strongly scatter phonons and electrons in many ways. Even the equilibrium defects
and disorder inherent in materials has a profound effect on transport properties. Using near-equilibrium alloys we
can combine our atomic level understanding of point defects (using ab initio methods) with phase relations
determined from equilibrium thermodynamics to predict alloy structures at varying composition and temperature.
Then using simple but effective models for semiconductors, the thermal and electrical transport properties can be
explained and improvements predicted.
Defects are important because achieving the maximum performance of a typical thermoelectric semiconductor
requires optimization of the carrier concentration, which is entirely controlled by defects. Here we will discuss the
chemical control afforded by extrinsic (impurity atom) defects. However, often intrinsic defects dominate or
interact with and change the effect of extrinsic defects. Examples discussed in the field of thermoelectrics include
AZn2Sb2 where A is any of the isovalent +2 element Ca, Yb, Sr, Eu (Figure). While all these A element donate the
same 2 electrons to the valence band, the electronegativity of the A cation determines the vacancy formation energy
which in turn control the vacancy defect concentration and therefore carrier concentration to optimize
thermoelectric performance [1]. Complexing substitutional and interstitial defects are found in skutterudites and
their control can also be used to optimize these excellent thermoelectric materials [2]. Defects from alloys also
reduce lattice thermal conductivity but this benefit to thermoelectric performance must also be weighed against
the detriment of the reduction in charge carrier mobility [3].
References
[1] Gregory S. Pomrehn, Alex Zevalkink, W. G. Zeier, A. van de Walle and G. J. Snyder, Angewandte Chemistry, 126, 3490 (2014)
[2] Yinglu Tang, Yuting Qiu, Lili Xi, Xun Shi, Wenqing Zhang, Lidong Chen, Ssu-Ming Tseng, Sinn-wen Chen and G. Jeffrey Snyder, Advanced
Functional Materials 23, 3194 (2013); Energy and Environmental Science 7, 812 (2014)
[3] Heng Wang, Yanzhong Pei, Aaron D. LaLonde and G. Jeffrey Snyder, Advanced Functional Materials23, 1586 (2013)