Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg3 Sb2 Thermoelectrics

Abstract

N-type Mg₃ Sb₂ -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, MgB₂ is introduced to improve the thermal stability of n-type Mg₃ Sb₂ . Enabled by MgB₂ 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 Mg₃ Sb₂ . Consequently, the Mg_3.05 (Sb_0.75 Bi_0.25 )_1.99 Te_0.01 (MgB₂ )_0.03 sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg₃ Sb₂ can be maintained by adding MgB₂ , reaching a high average ZT of ≈1.1 within 300-723 K. An eight-pair Mg₃ Sb₂ -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 Mg₃ Sb₂ -based alloys and other thermoelectrics for practical applications.

Keywords: Mg vacancies; MgB2; grain boundary engineering; thermal stability; thermoelectric devices.