Engineering Grain Boundaries in Thermoelectric Materials

Grain boundaries have a remarkable effect on the thermal and electrical transport properties of polycrystalline materials but are often ignored by prevailing physical theories. Grain boundaries and interfaces can adversely alter the properties of Solar Cells, Batteries and Thermoelectrics. To devise strategies for improving the thermoelectric performance of materials, it is essential to understand the coupled charge and thermal transport mechanisms including an interfacial Seebeck effect. The inhomogeneous nature of materials, such as that caused by grain boundaries, must be taken into account to rethink engineering strategies based on Mathiessen’s rule which interprets scattering homogeneously.

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.