Predicting thermoelectric figure of merit in complex materials: What do we need to know?

In a complex nanostructured material or composite, the thermoelectric figure of merit, ZT(T), is strongly dependent on the morphology and transport properties of interfacial areas that connect individual crystal grains or composite constituent phases. Despite the active efforts to improve ZT(T), relatively few experimental and theoretical investigations have been focused on probing the interfacial transport properties of relevant materials. We developed a finite element method based mesoscale-level simulation approach to evaluate the effective values of thermal and electrical conductivities, and Seebeck coefficient in structurally complex materials. This approach was tested on three popular nanocrystalline thermoelectric systems: n-type Si, n-type Si0.80Ge0.20, and p-type BiSbTe, providing excellent agreement between the simulated and previously measured values of ZT(T). The interfacial thermoelectric properties of these material systems were quantified in the process and compared to simple models. Furthermore, the sensitivity of ZT(T) to changes in the system morphology was elucidated as well.