Enhancement of the Thermal Stability and Thermoelectric Properties of Yb₁₄MnSb₁₁ by Ce Substitution

Yb₁₄MnSb₁₁ is a p-type high-temperature thermoelectric material with operational temperatures as high as 1273 K. Rare-earth (RE) substitution into this phase has been shown to increase the melting point further while also decreasing the sublimation rate. Solid solutions of 3+ RE elements with Yb²⁺ in Yb₁₄MnSb₁₁ have shown to have increased stability against oxidation. Ce is an abundant RE element, and the substitution of Ce³⁺ on the Yb²⁺ sites should increase the thermoelectric efficiency of the material due to a decrease in carrier concentration. Samples of Yb_14–xCe_xMnSb₁₁ (x ∼ 0.4) were synthesized using ball milling, followed by annealing and consolidation via spark plasma sintering. The systematic addition of a small increase of excess Mn and the resulting compositions were investigated. Small amounts of impurities in the samples, such as Yb₂O₃ and Mn, are correlated with negative attributes in the resistivity data. Hall effect measurements revealed a reduced carrier concentration of ∼44% at 600 K over Yb₁₄MnSb₁₁, and adjusting the stoichiometry toward Yb_13.6Ce_0.4MnSb₁₁ leads to increases in resistivity and the Seebeck coefficient with a reduction in thermal conductivity. Yb_13.6Ce_0.4MnSb₁₁ shows an improved average zT_avg = 0.80 when compared to Yb₁₄MnSb₁₁ (0.71) and no degradation when exposed to ambient air for 77 days at room temperature. Thermogravimetric analysis of air oxidation shows that Yb_13.6Ce_0.4MnSb₁₁ and Yb₁₄MnSb₁₁ do not oxidize until 700 K.