Synthesis of p-type Mg2Si1-xSnx with x = 0-1 and optimization of the synthesis parameters

Mg2Si is a promising thermoelectric material in the mid-temperature region 500 – 800 K. Development of Mg2Si based thermoelectric generators requires both good n- and p-type materials. While the thermoelectric properties n-type Mg2(Si,Sn) materials are good, those of the corresponding p-type are not as much. Therefore, optimizing p-type solid solution of magnesium silicide and magnesium stannide is highly desired. We employ high energy ball milling for efficient synthesis of p-type Mg2(Si,Sn) and investigate the effect of milling time, sintering temperature, and holding time on the thermoelectric properties of p-type Mg2Si1-xSnx with x = 0-1. We can show the synthesis of p-type Mg2(Si,Sn) for the whole compositions using Li as a dopant. We have also studied the effect of the synthesis parameters (milling time, sintering temperature, and holding time) on the phase purity, functional homogeneity and thermoelectric properties. The phase purity increases with longer milling time. The functional homogeneity decreases with higher sintering temperature and longer holding time. The optimum synthesis condition for x = 0.6 leads to zTmax0.6 at 700 K, which is one of the highest value reported for p-type Mg2(Si,Sn)

Raman Spectroscopic Study of the Optical Phonons of Mg2Si1−x Snx Solid Solutions

Mg2Si1−x Snx solid solutions are prepared in the composition range from x  = 0 to 1 by mechanical alloying using high‐energy ball milling followed by direct‐current sintering at 600–800 °C. X‐ray diffraction analysis confirms that the samples obtained are uniform solid solutions. Raman spectroscopy shows two first‐order phonon Raman bands and a distribution of Raman signals due to second‐order phonon Raman scattering. The first‐order Raman bands are assigned to the nondegenerate Raman‐forbidden F1u (LO) mode and the triply degenerate Raman‐allowed F2g mode. Both modes exhibit a linear shift of the phonon frequency with a composition showing a clear one‐mode behavior for this solid solution. The findings are discussed in the context of existing theories for the occurrence of the one‐mode or two‐mode behavior of phonons in solid solutions

Experimental investigation of the predicted band structure modification of Mg2X (X: Si, Sn) thermoelectric materials due to scandium addition

Modification of the electronic band structure via doping is an effective way to improve the thermoelectric properties of a material. Theoretical calculations from a previous study have predicted that Sc substitution on the Mg site in Mg2X materials drastically increase their Seebeck coefficient. Herein, we experimentally studied the influence of scandium substitution on the thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Sn. We found that the thermoelectric properties of these materials are unaffected by Sc addition, and we did not find hints for a modification of the electronic band structure. The SEM-energy dispersive X-ray analysis revealed that the scandium does not substitute Mg but forms a secondary phase (Sc-Si) in Mg2Si0.4Sn0.6 and remains inert in Mg2Sn, respectively. Thus, this study proves that scandium is an ineffective dopant for Mg2X materials