Synergistic Optimization of Electrical and Thermal Transport in Mg3+δ(Sb, Bi)2 through Ga-Modified Grain Boundaries

The n-type Mg 3+δ (Sb,Bi) 2 ‐based system has recently emerged as a breakthrough class of thermoelectric (TE) materials, drawing considerable interest for its eco‐friendly composition and potential to replace conventional commercial counterparts. However, the precise regulation of grain boundaries in Mg-based materials—akin to wielding an accurate scalpel—so as to extremely optimize thermoelectric performance and device properties remains ill-defined. In this study, the incorporation of Ga into the Mg 3 (Sb,Bi) 2 matrix via high‐energy ball milling (HBM) and spark plasma sintering (SPS) yielded an ultralow lattice thermal conductivity of 0.41 W m -1 K -1 at 300 K, a superior figure of merit ( ZT ) exceeding 1.84 at 673 K, and a high average ZT ( ZT avg ) of 1.55 across 300—773 K. The lattice thermal conductivity of Ga‐modified Mg 3+δ (Sb,Bi) 2 is markedly reduced over the entire temperature range, primarily due to the enhanced Kapitza thermal resistivity (ρ Kapitza ) resulting from the introduction of a liquid‐like phase at grain boundaries (GBs), which strengthens phonon scattering. while, Ohmic-like metal–semiconductor junctions form at the interfaces between the Ga/Bi secondary phases and the Mg 3+δ (Sb,Bi) 2 matrix lead to superior power factor. The high performance of Ga‐Mg 3+δ (Sb,Bi) 2 enabled a two‐pair module based on Mg 3.2 Ga 0.04 Sb 1.5 Bi 0.49 Te 0.01 / MgAgSb to achieve a conversion efficiency ( η ) of ∼6.0% at ΔT = 300 K. As a result, this work demonstrates significant theoretical and practical value in areas such as thermal management and thermoelectric material design.