Microstructural effects on thermoelectric efficiency: A case study on magnesium silicide

Carrier transport and conversion efficiency of thermoelectric materials is not only governed by the intrinsic bulk material properties, but often heavily influenced by microstructural effects. Using Mg2Si—a very attractive, non-toxic and abundant material—as an example, the authors investigate the microstructural effects on thermoelectric transport in the material. Combining microstructural analysis (scanning electron microscopy, X-ray diffraction) with high-temperature transport measurements (Seebeck coefficient, electrical and thermal conductivity, charge carrier mobility), it is verified that small amounts of impurity phases can heavily impair electrical transport. Further electron microscopy studies confirm MgO functioning as a scattering center at the grain boundaries of the material. The effect of the impurity phases on the transport properties can be understood in a simple transport model based on a single parabolic band including grain boundary scattering. The model can be employed to assess material parameters such as the phonon deformation potential, the barrier height of the grain boundaries and the average size of the grains. Furthermore, the detrimental effect of MgO on thermoelectric efficiency is quantified. Compared with Sb-doped Mg2Si with optimized properties, a few per cent MgO decreases the average thermoelectric figure of merit by 30%.