Graphene-Mediated Grain Growth for Enhanced Carrier Mobility and Thermoelectric Performance in Mg ₃ (Sb,Bi) ₂

Mg 3 (Sb,Bi) 2 -based thermoelectric materials are among the most promising n-type candidates for mid- to low-temperature energy conversion, but their near-room-temperature performance is strongly limited by severe grain boundary scattering in polycrystals. In this work, high-mobility graphene was introduced into n-type Mg 3.2 Sb 1.195 Bi 0.795 Te 0.01 to regulate the microstructure and enhance carrier transport properties. Graphene addition promotes significant grain growth, increasing the average grain size from 9.1 to 16.3 μm, corresponding to an increase of nearly 79%. The reduced grain boundary density weakens grain boundary carrier scattering and increases the room-temperature carrier mobility by ∼46% from 104 to 152 cm 2 V −1 s −1 . As a result, the room-temperature power factor is enhanced to ∼18.2 μW cm −1 K −2, and a maximum power factor of ∼21.5 μW cm −1 K −2 is achieved at 500 K. Additionally, graphene addition introduces additional phonon scattering, resulting in a reduced lattice thermal conductivity of ∼0.5 W m −1 K −1 at high temperatures. Consequently, the Mg 3.2 Sb 1.195 Bi 0.795 Te 0.01 -G 0.05 sample achieves a peak zT of ∼1.46 at 650 K and an average zT of ∼1.13 over 300–725 K. Furthermore, the Vickers hardness remains nearly unchanged at ∼0.6 GPa, demonstrating that the improved thermoelectric performance is achieved without compromising mechanical robustness. This work demonstrates that graphene-mediated grain boundary engineering offers an effective strategy for overcoming grain boundary limitations and improving the thermoelectric performance of polycrystalline Mg 3 (Sb,Bi) 2 .