Optimized Thermoelectric Properties of Sb-Doped Mg_{2(1+<i>z</i>)}Si_{0.5–<i>y</i>}Sn_0.5Sb_<i>y</i> through Adjustment of the Mg Content

Abstract

Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds are low-cost and environmentally friendly thermoelectric materials expected to be applied as power generators in the intermediate temperature range. Optimization of the thermoelectric properties of Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds can be accomplished by the precise control and adjustment of the Mg content. A series of Mg_{2(1+<i>z</i>)}Si_{0.5–<i>y</i>}Sn_0.5Sb_<i>y</i> (0 ≤ <i>y</i> ≤ 0.015 and 0 ≤ <i>z</i> ≤ 0.15) compounds with controlled Mg content were synthesized by a two-step solid-state reaction method, followed by a spark plasma sintering technique. On the basis of optimized thermoelectric properties via doping with Sb, the effect of a variable content of Mg spanning from understoichiometry to overstoichiometry has been systematically explored. The results indicate that when the actual Mg content exceeds the stoichiometric amount, the dominant point defects in Mg_{2(1+<i>z</i>)}Si_0.49Sn_0.5Sb_0.01 compounds are interstitial Mg and Si/Sn vacancies. At the same time, the electron concentration is enhanced with increasing content of Mg. However, when the actual Mg content is substoichiometric, the point defects consist mainly of Mg vacancies that tend to counteract the doping effect of Sb. Thus, the electron concentration of the nominal Mg₂Si_0.49Sn_0.5Sb_0.01 compound (in reality a 2 mol % deficiency of Mg) is markedly lower compared with the Mg_2.10Si_0.49Sn_0.5Sb_0.01 compound, which actually had a 2 mol % excess of Mg. Furthermore, a modest overstoichiometry of Mg enhances the power factor and improves the dimensionless figure of merit. The highest value of <i>ZT</i> = 1.25 at 800 K among the compounds was obtained on Mg_2.20Si_0.49Sn_0.5Sb_0.01, which had an actual Mg excess of 5.5 mol %. The study suggests that point defects, such as interstitial Mg and Si/Sn vacancies, which are created by an overstoichiometric content of Mg, have a positive effect on the electron concentration and thermoelectric properties of n-type Mg₂Si_1–<i>x</i>Sn_<i>x</i>-based compounds. This research has also established an essential foundation for further optimization of the thermoelectric properties of Mg₂Si_1–<i>x</i>Sn_<i>x</i> compounds