Figure of merit zT mapping of p-Mg2Si1−xSnx with respect to carrier concentration.

Mechanical alloying by high energy ball milling is an attractive solid -state technique for synthesizing a diverse range of equilibrium and non-equilibrium phase materials. We have studied the synthesis of n -type thermoelectric Mg 2 Si 0.4 Sn 0.6 solid solution, aiming for a fundamental understanding of the mechanisms underlying this synthesis technique. The investigations on powders by XRD and SEM show that milling leads to welding of Mg and Sn but fracturing of Si. This fractured Si diffuses into the ductile matrix on longer milling times resulting in a phase mixture close to the nominal starting composition after 35h of milling. However, single phase pure material was only achievable after sintering, hence the synthesis of Mg 2 (Si,Sn) is a two -step process. Furthermore, a thorough study on the effect of varying synthesis parameters on the thermoelectric properties was performed. No strong influence of milling time on the thermoelectric properties was observed and just 2h of milling followed by compaction was sufficient to obtain a pellet with optimal thermoelectric properties. Moreover, increasing sinter temperature/time deteriorated carrier concentration hence degrading the electronic properties. Thus, optimized thermoelectric properties were obtained for the powder consolidated at 973K/20minutes. Mg 2 Si 0.4 Sn 0.6 synthesized by mechanical alloying achieved a thermoelectric figure of merit zT max~1 .4.

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Magnesium silicide and its solid solutions are among the most attractive materials for thermoelectric generators in the temperature range of 500-800 K. However, while n-type Mg 2 (Si,Ge,Sn) materials show excellent thermoelectric performance, the corresponding p-type solid solutions are still inferior, mainly due to less favorable properties of the valence bands compared to the conduction bands. Here, Li doped Mg 2 Ge with a thermoelectric figure of merit zT of 0.5 at 700 K is reported, which is four times higher than that of p-type Mg 2 Si and double than that of p-type Mg 2 Sn. The reason for the excellent properties is an unusual temperature dependence of Seebeck coefficient and electrical conductivity compared to a standard highly doped semiconductor. The properties cannot be captured assuming a rigid band structure but well reproduced assuming two parabolic valence bands with a strong temperature dependent interband separation. According to the analysis, the difference in energy between the two bands decrease with temperature, leading to a band convergence at around 650 K and finally to an inversion of the band positions. The finding of a combination of a light and a heavy band that are non-rigid with temperature can pave the way for further optimization of p-type Mg 2 (Si,Ge,Sn).