Achieving a High zT_avg in n-Type Sb-Doped Mg₂Si_0.3Sn_0.7 via High-Pressure-Modulated Microstructures

Mg 2 Si-based compounds are cost-effective and environmentally friendly thermoelectric materials. However, the current Mg 2 (Si,Sn) solid solutions still suffer from the low figure of merit zT (or the low energy conversion efficiency), especially the low averaged zT value ( zT avg < 1.0). In this study, we synthesize the Mg 2 (Si 0.3 Sn 0.7 ) 1– x Sb x ( x = 0, 0.5, 0.75, 1, and 1.5%) solid solutions through the combination of high-pressure, high-temperature (HPHT) synthesis and spark plasma sintering (SPS). The high-pressure instrument effectively inhibits the unfavorable oxidation of Mg. This HPHT + SPS methodology improves the defect formation efficiency of Sb-doped Mg 2 Si 0.3 Sn 0.7, leading to an increased carrier concentration and enhanced electrical conductivity. Moreover, as a benefit from the pressure-induced conduction band convergence and Sb-flattened conduction band, the density of states’ effective mass ( m d *) significantly increases to ∼3.3 m e, maintaining high Seebeck coefficients even at a high carrier concentration. The synergetic effects of doping and m d * increase the peak power factor to exceed 50 μW cm –1 K –2 . Notably, due to the HPHT-modified microstructures, the hierarchy phonon scatterings are established to suppress the lattice thermal conductivity to as low as 1.29 W m –1 K –1 at 568 K; the Sb point defects, dislocations, and grain boundaries/pores can scatter the short-, medium-, and long-wavelength phonons, respectively. Ultimately, zT of the optimized Mg 2 (Si 0.3 Sn 0.7 ) 0.99 Sb 0.01 sample increases significantly in the whole temperature region; the zT peak is 1.37 at 673 K, and a high zT plateau of ∼1.35 is realized between 568 and 723 K. Thus, the notable average zT over the range of 323–723 K is 1.08. Our work demonstrates that the high-pressure-induced defect concentration, effective doping, and microstructure modifications facilitate the thermoelectric property improvement in the Mg 2 (Si,Sn)-based compounds.