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

Abstract

Yb14 MSb11 (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 Yb14 MSb11 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, Yb14 MSb11 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 Yb14 MSb11. Yb14ZnSb11 shows the best resistance to oxidation and may provide a promising material for further passivation optimization.

Evolution of Thermoelectric and Oxidation Properties in Lu-Substituted Yb14MnSb11

“…In addition to the brown shell, there is a mass of black material seen on the left side of the sample which appears to have melted out of the sample. This is very similar to what was observed for the oxidation of pristine Yb 14 MnSb 11 in both the shell appearance and the melted mass …”

“…As there is a small exotherm on heating at low temperatures it is likely that there is oxidation occurring at the surface of the polished Yb 14– x Lu x MnSb 11 even at these low temperatures, but the oxide film formed must be passivating enough to prevent significant mass gain. A comparison of the TG/DSC for the oxidation of Yb 14 MnSb 11 and Yb 14– x Lu x MnSb 11 is shown in Figure S5 . The DSC of the unsubstituted Yb 14 MnSb 11 shows an additional exothermic peak in the large sweeping exotherm at 550 °C which may correspond to the exotherm seen upon cooling in the Lu-substituted samples.…”

“…A comparison of the TG/DSC for the oxidation of Yb 14 MnSb 11 and Yb 14−x Lu x MnSb 11 is shown in Figure S5. 21 The DSC of the unsubstituted Yb 14 MnSb 11 shows an additional exothermic peak in the large sweeping exotherm at 550 °C which may correspond to the exotherm seen upon cooling in the Lu-substituted samples. In addition to this difference, the unsubstituted samples have several sharp exotherms at high temperatures from reactions with the crucible.…”

“…20 We have studied the oxidation of Yb 14 MnSb 11 and have shown that it oxidizes with the sublimation of antimony oxides and forms Yb 2 O 3 on the surface and YbSb 2 at ∼800 °C which extrudes from the sample along with the presence of YbMnSb 2 . 21 This work investigates the synthesis, thermoelectric, and oxidation properties of polycrystalline Yb 14−x Lu x MnSb 11 . The case of Lu substitution was chosen for the oxidation study as it was expected that the mixed (Yb,Lu) 2 O 3 might provide the most stable oxide coating as a previous study showed that Yb 13.7 Lu 0.3 MnSb 11 formed a oxide scale at ∼500 °C which appears to passivate the sample.…”

“…Investigation of the Ce-substituted Yb 14 MnSb 11 shows significant air stability at room temperature with no oxidation until 430 °C and provides a slightly better average zT . We have studied the oxidation of Yb 14 MnSb 11 and have shown that it oxidizes with the sublimation of antimony oxides and forms Yb 2 O 3 on the surface and YbSb 2 at ∼800 °C which extrudes from the sample along with the presence of YbMnSb 2 . This work investigates the synthesis, thermoelectric, and oxidation properties of polycrystalline Yb 14– x Lu x MnSb 11 .…”