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)

Analyzing thermoelectric transport in n-type Mg2Si0. 4Sn0. 6 and correlation with microstructural effects: An insight on the role of Mg

Fundamental material parameters governing the carrier transport in thermoelectric materials are affected by microstructural characteristics. We have investigated the effects of compaction duration on microstructure and the thermoelectric properties of Sb doped Mg2Si0.4Sn0.6. The transport properties show drastic changes with increasing compaction duration from 10 min to 40 min. A TEM-EDS analysis on samples sintered for 20 min and 40 min highlights Mg depleted grain boundaries and local compositional inhomogeneities but gives no indications for dopant loss. The transport properties were analyzed using a single parabolic band (SPB) model, and the observed changes can be attributed to carrier (n) loss, diminished carrier mobility (μ_0) and a reduction in lattice thermal conductivity (κ_lat). Comparatively stronger carrier scattering in longer sintered sample is a combined effect of increasing electron-phonon interaction (higher E_Def) and local compositional inhomogeneities in the material which are both linked to Mg depletion. The transport behavior of these samples can be fully captured by the SPB model only after the addition of grain boundary scattering in conjunction to acoustic phonon and alloy scattering. Furthermore, compensation between a lower κ_lat and μ_0 of the longer sintered sample led to a similar zT_max = 1.3±0.18 and an only marginally reduced performance parameter β. While it is evident that Mg deficiency modifies the transport properties, the thermoelectric performance is only mildly affected and a Mg2(Si,Sn) based TE device can therefore withstand some Mg loss without a deterioration of its performance

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