Mechanical Compatibility between Mg(Sb,Bi) 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 MgSb 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-MgSb 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 MgSb by alloying it with MgBi. The addition of Bi to MgSb reduces the coefficient of linear thermal expansion from 22.6 × 10 to 21.2 × 10 K for MgSbBi, which is in excellent agreement with that of MgAgSb (21 × 10 K). Furthermore, thermogravimetric data indicate that both MgSbBi and MgAgSb are stable in air and Ar at temperatures below ∼570 K. The results suggest the compatibility and robustness of MgSbBi and MgAgSb as a pair of thermoelectric legs for low-temperature TE modules.