Engineering thermal and electrical interfaces and grain boundaries in thermoelectric materials
Jeffrey Snyder, Northwestern University, Evanston, USA
To devise strategies for improving the thermeoelctric performance of materials, it is essential to understand the coupled charge and thermal transport mechanisms. In heavily doped semiconductors for example we often expect ionized impurity scattering to dominate electrical transport especially when mobility increasing with temperature is observed. However, the inadequacy of this description in thermoelectric materials such as the new high-performance n-type Mg3Sb2, becomes apparent when trying to consistently explain various experimental observations like the enhanced mobilities in larger grain samples and sharp crossovers to metal-like mobilities that decrease with temperature. The underlying cause of such complications is largely associated with the conventional Mathiessen’s rule that interprets or models all of the charge carrier scattering as homogeneous events. The inhomogeneous nature of materials, such as that caused by grain boundaries, must be taken into account to rethink engineering strategies and further improve thermoelectric materials.
Prevailing models for thermal transport treat interfaces and grain boundaries as structureless even though at the atomic scale they are better described as arrays of linear defects of various types. Allowing for this inherent structure, several fundamental characteristics of heat transport arise, such as diffraction conditions when heat carrying phonons scatter off the periodic, linear defect arrays that should be present in grain boundaries. Furthermore, a dimensionality crossover is observed in diffusive heat transport where phonons with a wavelength longer than the linear defect spacing see the interface simply as a structureless planar defect, and phonons with see the interface as a collection of independently scattering linear defects.
 J. J. Kuo, G. J. Snyder “Grain boundary dominated charge transport in Mg3Sb2-based compounds” Energy & Env. Sci. 11, 429 (2018)
 R. Hanus, G. J. Snyder “Phonon diffraction and dimensionality crossover in phonon interface scattering” (submitted)