Evolution of Thermoelectric and Oxidation Properties in Lu-Substituted Yb₁₄MnSb₁₁

Abstract

Yb14MnSb11 is one of the state-of-the-art high-temperature p-type thermoelectric materials with reported zTs of 1.2–1.3 at 1273 K. Site substitution of Yb14MnSb11 provides a means to control carrier concentration and impacts the oxidation kinetics. Substitution of Lu3+ for Yb2+ in Yb14MnSb11 single crystals has been shown to provide faster oxidation kinetics compared with the pristine phase, and it has been speculated that this may lead to surface passivation. Polycrystalline samples of Yb14–x Lu x MnSb11 were synthesized from the elements and through a route utilizing YbH2, MnSb, and Yb4Sb3 as reactive precursors. The solubility of Lu was found to be less than x = 0.5, and at that composition and above LuSb impurities are observed in the powder X-ray diffraction. Because Lu3+ is substituting for Yb2+ in a p-type system, there is an increase in both the electrical resistivity and Seebeck coefficient with Lu substitution. As Yb14MnSb11 has been predicted to be optimally doped ∼1.5 × 1021 h+ cm–3, this reduction in carrier concentration in the solid solution of Yb14–x Lu x MnSb11 decreases the peak zT. Oxidation of Yb13.7Lu0.3MnSb11 as a function of temperature was studied. At low temperatures, the Lu-substituted sample may have improved oxidation resistance through forming a passivating oxide film on the surface.

Impact of Lu-Substitution in Yb14–xLuxZnSb11: Thermoelectric Properties and Oxidation Studies

“…which is closer in contrast to the core, although it does show some differences in contrast throughout. The porosity in this region is likely due to the material which melted out of the sample and shows less porosity than oxidized Yb 14– x Lu x MnSb 11 . The core shows a consistent lighter contrast, suggesting that it is made of a higher average Z material than the outer shell.…”

“…The effect of Lu-substitution in Yb 14 ZnSb 11 is in contrast to that observed for Yb 14 MnSb 11 and is attributed to differences in the Yb valency. 6,10 The introduction of Lu into the system helped stabilize the YbSb phase upon oxidation; however, it did not negate the formation and subsequent melting of YbSb 2 , which proves to be problematic for forming…”

“…The Yb 2 O 3 and Sb are likely to result from the oxidation of YbSb 2 after it extruded through the outer layer of the oxidizing pellet. The lack of any YbZnSb 2 phase seen in the molten material presents a key advantage of the Yb 14 ZnSb 11 system over Yb 14 MnSb 11 which was observed to have a YbMnSb 2 –YbSb 2 eutectic upon oxidation. , Figure b shows the diffraction pattern from the white outer layer of the oxidized pellet. This layer consists mostly of Yb 2 O 3 with small amounts of YbSb 2 and Sb.…”

“…In comparison to the oxidation of Lu-substituted Yb 14 MnSb 11 , the sample here shows significantly less material, which has melted out of the sample and the outer oxide coating shows better adherence to the inner core. 10 Figure 11 shows backscattered electron micrographs of the cross section of the oxidized Yb 14−x Lu x ZnSb 11 pellet with (a) showing an overall view at low magnification, (b) showing the sample at higher magnification, and (c) showing a zoomed in view of the outer layer. The sample consists of a darker outer region, which shows some cracking, and is an average of 50− 100 μm thick in most regions, although it extends to ∼200 μm in others.…”

“…Thermochemical studies on single crystals of Yb 14– x RE x MnSb 11 (RE = La–Nd and Sm–Lu) suggested that the substitution of Yb for the late rare earth (RE) elements led to improved oxidation resistance as they quickly formed a more stable oxide shell . Previously, Lu-substitution in polycrystalline Yb 14 MnSb 11 showed some improvements in oxidation resistance; however, the YbMnSb 2 –YbSb 2 eutectic lead to molten material forming underneath and breaking through the oxide shell at high temperatures . In this work, Yb 14 ZnSb 11 is substituted with trivalent RE, Lu.…”