Control of defects for optimizing performance in thermoelectric alloys

Conference Dates

September 4-8, 2016


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. Ab initio methods have become extremely powerful but knowing which charge states to include is poorly defined [1]. Examples discussed in the field of thermoelectrics include the lead chalcogenides such as PbSe and 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 defect concentration and therefore carrier concentration [2]. Complexing substitutional and interstitial defects are found in skutterudites and their control can also be used to optimize these excellent thermoelectric materials [3]. 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 [4].

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