Unraveling the Mg Loss Mechanism and Degradation Kinetics in Thermoelectric n‐Type Mg ₂ Si‐Mg ₂ Sn Solid Solutions

Mg‐based materials have revolutionized the thermoelectric landscape between 300 K and 900 K, showcasing performance beyond commercial‐grade materials for electrical power generation and thermal management applications. However, the long‐term stability of these materials is questioned based on the reactivity and volatility of Mg. Consequently, elucidating degradation mechanisms and quantifying their kinetics are pivotal for identifying suitable application conditions and establishing efficient measures for enhanced stability. Therefore, the behavior of high‐performance n‐type Mg 2 Si 0.4 Sn 0.6 specimens at high temperatures is analyzed utilizing in situ measurements of the Seebeck coefficient, the electrical conductivity, and the Hall coefficient in combination with a defect chemistry model. From this and a two‐band transport modeling, an approximately exponential decay of majority carrier concentration – linked to Mg loss – is confirmed. A microscopic multi‐step model based on Mg diffusion is developed, rationalizing the degradation of n‐type Mg 2 (Si,Sn). Mg loss rates at different temperatures are evaluated by an Arrhenius law, enabling quantification of an effective activation energy associated with the degradation process. It is deduced that Mg sublimation is the rate‐limiting step of the degradation mechanism and estimated that for ≈ 700 K, unprotected Mg 2 (Si,Sn)‐based thermoelectric devices can operate in vacuum without significant performance loss for over a year.