The Roles of Grain Boundaries in Thermoelectric Transports

Grain boundary scattering was long recognized and widely utilized to regulate the transports of charge carriers and phonons in thermoelectric materials; nevertheless, the understanding of grain boundaries in lots of literatures seems somewhat lopsided and way too simplified, without distinguishing the exact microstructural characters for each individual case. In this perspective, we first review the well-known functions of grain boundaries on the electrical and thermal transport properties based on some representative thermoelectric series. Next, we try to deepen the understanding of the roles of grain boundaries in the following two aspects: (1) whether the grain boundaries are large-angle or small-angle ones, and what the difference between them in affecting electrical/thermal performance is; (2) whether the grain boundaries are clean and sharp as lattice discontinuity defects, if not, how can the grain boundaries as an individual phase affect the thermoelectric transports. At last, we suggest that further investigations engaging more detailed microstructural information of grain boundaries are urgently necessary in order to better realize a positive trade-off between lattice thermal conductivity and electrical power factor for an overall enhanced thermoelectric performance.