ICT2014 Abstracts

The conversion efficiency of and the transport in 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 example we study microstructural effects on the thermoelectric transport in the material. Combining microstructural analysis (SEM, XRD) with high temperature transport measurements (S, sigma, kappa, mobility) we show that small amounts of impurity phases can heavily degrade the electrical transport. Further electron microscopy studies confirm MgO acting as 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 plus grain boundary scattering. From the model we can estimate the deformation potential, the barrier height of the grain boundaries and the crystallite size of the employed powder. We can also quantify the detrimental effect of MgO on the thermoelectric efficiency. Compared to Sb-doped Mg₂Si with optimized properties we find that a few percent MgO can decrease the thermoelectric performance by 30%. The findings and the analysis reported here are relevant for the whole Mg₂Si _1-xSn_x family and for other material systems.