Mechanical Compatibility between Mg₃(Sb,Bi)₂ and MgAgSb in Thermoelectric Modules

Thermoelectric (TE) modules are exposed to temperature gradients and repeated thermal cycles during their operation; therefore, mechanically robust n- and p-type legs are required to ensure their structural integrity. The difference in the coefficients of thermal expansion (CTEs) of the two legs of a TE module can cause stress buildup and the deterioration of performance with frequent thermal cycles. Recently, n-type Mg₃Sb₂ and p-type MgAgSb have become two promising components of low-temperature TE modules because of to their high TE performance, nontoxicity, and abundance. However, the CTEs of n-Mg₃Sb₂ and p-MgAgSb differ by approximately 10%. Furthermore, the oxidation resistances of these materials at increased temperatures are unclear. This work manipulates the thermal expansion of Mg₃Sb₂ by alloying it with Mg₃Bi₂. The addition of Bi to Mg₃Sb₂ reduces the coefficient of linear thermal expansion from 22.6 × 10^-6 to 21.2 × 10^-6 K^-1 for Mg₃Sb_1.5Bi_0.5, which is in excellent agreement with that of MgAgSb (21 × 10^-6 K^-1). Furthermore, thermogravimetric data indicate that both Mg₃Sb_1.5Bi_0.5 and MgAgSb are stable in air and Ar at temperatures below ∼570 K. The results suggest the compatibility and robustness of Mg₃Sb_1.5Bi_0.5 and MgAgSb as a pair of thermoelectric legs for low-temperature TE modules.