1, California Institute of Technology, Pasadena, California, United States
Experimental evidence shows that grain boundaries are responsible for the thermally-activated conductivity in some thermoelectric materials, such as Mg3Sb2. Existing grain-boundary models using the Matthiessen’s rule on the carrier scattering rate fail to explain the thermally-activated conductivity in n-type Mg3Sb2-based materials. We establish a model describing the carrier conductivity (σ) and Seebeck coefficient (S) of polycrystalline thermoelectric materials. The key factor is to treat the depletion region induced by the grain boundary as a secondary phase, which takes into account the relatively larger depletion width in semiconductors, as compared with classical metals. The model is successfully applied to explain both the temperature dependency (i.e. σ-T) and energy dependency (i.e. log|S|-logσ) of Mg3Sb2-based compounds. We discuss how the model can be extended to other thermoelectric materials.