Detrimental influence of nanostructuring on the thermoelectric properties of magnesium silicide

Abstract

Nanostructuring techniques have steered the performance of many thermoelectric (TE) compounds towards significant improvement in performance in the last two decades. In this paper, we present a comprehensive study on the effect of bulk nanostructuring in magnesium silicide (Mg2Si) through simulation of thermoelectric properties using a multi-band semi-classical approach. It is shown that the magnitude of reduction in lattice thermal conductivity in nanostructured Mg2Si is comparable to that of reduction in charge carrier mobility for any chosen range of the grain sizes. The results are justified through a comparison with experimental data for both n-type and p-type Mg2Si characteristics versus temperature as well as doping concentration. In order to understand the underlying reasons for the detrimental effect of nanostructuring in Mg2Si, analogous calculations were performed on the well-known TE system of nanostructured Si0.8Ge0.2 and the results are compared. Model calculations show that in nanostructured Mg2Si a grain size of 20 nm results in approximately 40% reduction in lattice thermal conductivity, whereas the reduction in electrical conductivity is nearly 50% of its value in crystalline structures. For the case of nanostructured Si0.8Ge0.2, the loss in electrical conductivity was found to be a mere 20% of its magnitude in crystalline structures. The differential electrical and thermal conductivities versus charge carrier and phonon energies were calculated, respectively, and it was shown that the enhancement in Seebeck coefficient due to the energy filtering effect is also marginal. Therefore, it is conclusively shown that bulk nanostructuring in Mg2Si is not an efficient method to enhance ZT.