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1267-DD10-23

Effect of Vacancies on the Thermoelectric Properties of Mg2Si containing Sb and Bi substitution T. Dasgupta, C. Stiewe, R. Hassdorf, L. Boettcher and E. Mueller Institute of Materials Research, German Aerospace Center (DLR), D-51170 Cologne, Germany ABSTRACT Mg2Si1-xRx (R-Sb,Bi) compositions with x=0.025, 0.05 for Sb and x=0.01,0.025, 0.05 for Bi were synthesized by induction melting of the constituent elements followed by compaction by hot pressing. Phase identification by x-ray diffraction (XRD) indicate a biphasic nature for Bi substituted compositions for x≥.025 while solid solubility for the different Sb substitutions. Abundance distribution plots of the Seebeck (S) coefficient show asymmetry in the Sb compositions and is not observed in the Bi substituted samples. This indicates presence of vacancies only in the Sb substituted compositions. Electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ) values were measured from 300K to 773 K. The observed trends in the absolute values of σ and S can be explained based on doping/second phase influence in Bi substitutions and due to effect of vacancies for Sb substitutions. The (κL) (phonon component) values show a significant decrease for the Sb substitutions due to the presence of vacancies. Calculation of the thermoelectric figure of merit (ZT) show a ZTmax of 0.56 for x=0.025 Bi composition compared to a ZTmax of ~ 0.05 for Mg2Si.

INTRODUCTION Mg2Si and its solid solutions are promising n type thermoelectric materials with reported ZT of greater than unity [1-3]. The high ZT in these materials arise due to a combination of lowering of κL (due to alloy disorder scattering) and increase of power factor (S2σ) (due to band degeneracy of the conduction band minima) [4]. Attempts for further improvement in the ZT in this material system are worthwhile due to its thermal stability, non toxicity and low price of constituent raw materials. Recently, a report on Sb substituted Mg2Si (Mg2Si1-ySby) showed significant reduction of κ (by a factor of 6 at room temperature) at high Sb concentrations (0≤y≤0.4) [5]. This was attributed to the formation of vacancies at the Mg site which act as additional phonon scattering centers. Our recent work on the high temperature thermoelectric measurements on this system indicated improvements in ZT due to presence of Mg site vacancies [6]. Thus, in this paper, the effect of Bi and Sb substitutions in Mg2Si on the formation of Mg site vacancy formation and the high temperature thermoelectric properties are investigated.

EXPERIMENT Polycrystalline ingots of Mg2Si1-xRx (R-Sb,Bi) with x=0.025, 0.05 for Sb and x=0.01,0.025, 0.05 for Bi were synthesized by direct melting of the elemental constituents in the desired stoichiometric ratio in an induction furnace. Mg was taken in excess of 3 at% to account for the loss during melting. The reaction was carried out in a pressure tight BN coated graphite crucible at a temperature of 1423 K under argon. The resulting ingots were hand crushed and compacted by hot