Probing High-Temperature Oxidation of Thermoelectric Phases Yb₁₄MSb₁₁ (M = Mg, Mn, Zn)

Yb₁₄MSb₁₁ (M = Mg, Mn, Zn) are p-type Zintl phases with high thermoelectric efficiencies at 1000 °C and melting points above 1200 °C under vacuum and/or inert atmosphere. In a thermoelectric generator, even within a vacuum jacket, small amounts of oxygen may be present, and therefore, elucidating chemical reactions in the presence of air or oxygen provides a framework for engineering design. The oxidation of Yb₁₄MSb₁₁ was investigated from room temperature to 1000 °C in dry air with thermogravimetric/differential scanning calorimetry (TG/DSC) on small pellets and visually after heat treatment to 1000 °C under ambient conditions on large pellets. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and powder X-ray diffraction provide identification of the oxidation products. In the presence of dry air, Yb₁₄MSb₁₁ initially oxidizes initially slowly at room temperature with a sweeping exotherm and weight gain with rapid oxidation at 400 °C, after which the exotherm signal plateaus at about 600 °C, with M = Zn showing the smallest overall exothermic curve. All samples showed a paired endo-/exotherm at 785-803 °C, consistent with the melting/solidification of YbSb₂, which in the case of M = Mg, Mn extrudes from the sample. The various sections of the pellets─outer layer, inner layer, and core are analyzed, and oxidation reactions are proposed. After cycling to 1000 °C, the outer layer is composed of Yb₂O₃ with small amounts of the corresponding metal oxides. The inner layer shows delamination by inward diffusion of oxygen and outward diffusion of Sb or Sb oxide-containing phases, and the core shows Yb₁₄MSb₁₁. Yb₁₄ZnSb₁₁ shows the best resistance to oxidation and may provide a promising material for further passivation optimization.