Roles of interstitial Mg in improving thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6 solid solutions

Abstract

While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg 2 (Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg 2(1+x) Si 0.38 Sn 0.6 Sb 0.02 (0.05 # x # 0.12) as a function of Mg excess between 300 K and 730 K. The presence of interstitial Mg was corroborated by X-ray powder diffraction, X-ray photoelectron spectroscopy, Hall coefficient measurement, and compositional analysis. The electrical properties have been analyzed in the framework of a single parabolic band model to gain more insight on the roles of Mg interstitials. We found that increasing Mg excess content increased the carrier concentration, electronic effective mass, and electrical conductivity, while it decreased the Seebeck coefficient and led to a non-monotonic change in the lattice thermal conductivity. As a result, a maximum ZT $ 0.85 was attained at 700 K for Mg excess x ¼ 0.1, a $60% enhancement compared to that of the sample x ¼ 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg 2 (Si,Sn)-based materials.

Quantitative analysis of interstitial Mg in Mg2Si studied by single crystal X-ray diffraction

“…If this prediction is correct, we can prepare n-type and p-type Mg 2 Si by varying the amount of Mg i . Experimentally, n-type Mg-excess Mg 2 Si was prepared [11,12]. With increasing excess Mg, the electrical conductivity increased due to the increase of electron carrier density.…”

“…Magnesium silicide (Mg 2 Si) has attracted much attention as one of the promising thermoelectric materials due to the lightweight property and high natural abundance [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Mg 2 Si belongs to the anti-fluorite type structure wherein the 8c (1/4 1/4 1/4) and the 4a (0 0 0) sites are occupied by Mg and Si, respectively [19].…”

Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction

“…The reduced Fermi levels (x F ) and carrier effective mass (m*) were roughly estimated from the measured a and n H based on a single parabolic band model with acoustic phonon scattering. 49 Although the calculation using the single parabolic band model may introduce error because of the nonparabolic nature of the valence band and complex scattering processes, the trends in the values of x F and m* can provide information about the band structure upon the introduction of Sb. The increase in n H with increasing x is accompanied by a gradual increase in x F , indicating the progressive movement of the Fermi level deeper into the valence band.…”

Variations of thermoelectric properties of Mg2.2Si1−Sn−0.013Sb0.013 materials with different Si/Sn ratios

“…In this paper, Mg 2.2 Si 1 À y Sn y À 0.013 Sb 0.013 (0.3ry r0.7) compounds were prepared by B 2 O 3 "flux" method followed by hot pressing [25,29]. Generally speaking, the electrical transport properties of the undoped thermoelectric semiconductors are not good, and it is indispensable to optimize the thermoelectric properties by doping.…”

“…New interest has been sparked by band convergence [19,23] as a method to give rise to large values of the Seebeck coefficient with no adverse effect on the carrier mobility. Meanwhile, the effect of intrinsic defects, such as Mg vacancy [24], Mg interstitial [25,26] has recently gained popularity due to their remarkable influence on the thermoelectric properties. It is now generally accepted that wide regions of solid solution exist in the system Mg 2 Si-Mg 2 Sn [3].…”