Thermo‐Electro‐Magnetic Interactions and ≈1 nm Fe Interfacial Layer Realize High Average <i>ZT</i> in Fe/Mg₃(Sb,Bi)₂ Below 300 °C

Abstract Mg 3 (Sb,Bi) 2 alloys offer exceptional near‐room temperature thermoelectric (TE) performance comparable to Bi 2 Te 3 . However, the low carrier mobility due to Mg vacancies and relatively high lattice thermal conductivity adversely affect overall TE performance. Herein, carrier mobility and thermal conductivity of Mg 3 (Sb,Bi) 2 are decoupled by modulating both the phase interface and grain boundaries through incorporating different sizes of iron (Fe). Ferromagnetic micrometer‐sized Fe particles enhance the figure of merit ( ZT ) more than superparamagnetic nano‐sized counterparts. Magnetic moments of Fe induce the charge density overlap near phase boundaries and ≈1 nm Fe interfacial layer lowers grain‐boundary barriers, leading to sharply increased carrier mobility. Moreover, the additional magnon‐phonon scattering reduces lattice thermal conductivity by over 40%. Consequently, Fe/Mg 3 (Sb,Bi) 2 composite achieves a high average ZT of 1.4 over room temperature to 573 K. The fabricated Mg 3 (Sb,Bi) 2 ‐CdSb module demonstrates a high conversion efficiency of 8.4% under a 275 K temperature gradient, among the best for Mg 3 (Sb,Bi) 2 ‐based modules. This work uncovers the role of thermo‐electro‐magnetic interactions in bolstering TE performance and inspires the development of low‐cost, high‐efficiency TE modules for low‐grade heat recovery.