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

Area of Science:

• Materials Science
• Thermoelectric Materials
• Solid State Chemistry

Background:

• Thermoelectric (TE) modules require mechanically robust n- and p-type legs to withstand operational temperature gradients and thermal cycling.
• Mismatched coefficients of thermal expansion (CTEs) between n-type Mg3Sb2 and p-type MgAgSb legs lead to stress and TE performance degradation.
• The oxidation resistance of these promising low-temperature TE materials at elevated temperatures is not well understood.

Purpose of the Study:

• To address the CTE mismatch between n-type Mg3Sb2 and p-type MgAgSb for improved thermoelectric module reliability.
• To investigate the thermal stability and oxidation resistance of modified Mg3Sb2 and MgAgSb in relevant operating conditions.

Main Methods:

• Alloying Mg3Sb2 with Mg3Bi2 to modify its coefficient of linear thermal expansion (CTE).
• Characterization of the CTE of the resulting Mg3Sb1.5Bi0.5 alloy.
• Thermogravimetric analysis (TGA) to assess the oxidation resistance and thermal stability of Mg3Sb1.5Bi0.5 and MgAgSb in air and argon atmospheres.

Main Results:

• Alloying Mg3Sb2 with Mg3Bi2 successfully reduced the CTE from 22.6 × 10^-6 K^-1 to 21.2 × 10^-6 K^-1 for Mg3Sb1.5Bi0.5.
• The CTE of Mg3Sb1.5Bi0.5 (21.2 × 10^-6 K^-1) closely matches that of MgAgSb (21 × 10^-6 K^-1), minimizing thermal expansion mismatch.
• Both Mg3Sb1.5Bi0.5 and MgAgSb demonstrated stability in air and argon below approximately 570 K.

Conclusions:

• The Mg3Sb1.5Bi0.5 alloy exhibits a CTE compatible with MgAgSb, effectively resolving the mismatch issue.
• The demonstrated thermal stability and oxidation resistance suggest suitability for low-temperature thermoelectric applications.
• Mg3Sb1.5Bi0.5 and MgAgSb are proposed as a robust and compatible pair of thermoelectric legs for enhanced low-temperature TE module performance and longevity.