Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg 3 Sb 2 Thermoelectrics.

N-type Mg 3 Sb 2 -based thermoelectric materials show great promise in power generation due to their mechanical robustness, low cost of Mg, and high figure of merit (ZT) over a wide range of temperatures. However, their poor thermal stability hinders their practical applications. Here, MgB 2 is introduced to improve the thermal stability of n-type Mg 3 Sb 2 . Enabled by MgB 2 decomposition, extra Mg can be released into the matrix for Mg compensation thermodynamically, and secondary phases of Mg─B compounds can kinetically prevent Mg diffusion along grain boundaries. These synergetic effects inhibit the formation of Mg vacancies at elevated temperatures, thereby enhancing the thermal stability of n-type Mg 3 Sb 2 . Consequently, the Mg 3.05 (Sb 0.75 Bi 0.25 ) 1.99 Te 0.01 (MgB 2 ) 0.03 sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg 3 Sb 2 can be maintained by adding MgB 2 , reaching a high average ZT of ≈1.1 within 300-723 K. An eight-pair Mg 3 Sb 2 -GeTe-based thermoelectric device is also fabricated, achieving an energy conversion efficiency of ≈5.7% at a temperature difference of 438 K with good thermal stability. This work paves a new way to enhance the long-term thermal stability of n-type Mg 3 Sb 2 -based alloys and other thermoelectrics for practical applications.