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

Mg₂Si-based compounds are cost-effective and environmentally friendly thermoelectric materials. However, the current Mg₂(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 Mg₂(Si_0.3Sn_0.7)_1-xSb_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 Mg₂Si_0.3Sn_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 ∼3.3m_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 Mg₂(Si_0.3Sn_0.7)_0.99Sb_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 Mg₂(Si,Sn)-based compounds.