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 Yb14-xCexMnSb11 (x similar to 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 similar to 44% at 600 K over Yb14MnSb11, and adjusting the stoichiometry toward Yb13.6Ce0.4MnSb11 leads to increases in resistivity and the Seebeck coefficient with a reduction in thermal conductivity. Yb13.6Ce0.4MnSb11 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 Yb13.6Ce0.4MnSb11 and Yb(14)MnSb(11 )do not oxidize until 700 K.