Enhanced High-Temperature Thermoelectric Performance of Yb14–xCaxMnSb11

The high temperature p-type thermoelectric material Yb14MnSb11 has been of increasing research interest since its high temperature thermoelectric properties were first measured in 2006. Subsequent substitutions of Zn, Al, and La into the structure have shown that this material can be further optimized by altering the carrier concentration or by reduction of spin-disorder scattering. Here the properties of the Yb14–xCaxMnSb11 solid solution series where isovalent Ca2+ is substituted for Yb2+ will be presented. Crystals of the Yb14–xCaxMnSb11 solid solution series were made by Sn-flux (x = 2, 4, 6, 8) with the following ratio of elements: (14–x)­Yb: xCa: 6 Mn: 11Sb: 86Sn, and their structures determined by single crystal X-ray diffraction. The density of the material significantly decreases by over 2 g/cm3 as more Ca is added (from x = 1 to 8), because of the lighter mass of Ca. The resulting lower density is beneficial from a device manufacturing perspective where there is often a trade-off with the specific power per kilogram. The compounds crystallize in the Ca14AlSb11 structure type. The Ca substitution contributes to systematic lengthening the Mn–Sb bond while shortening the Sb–Sb bond in the 3 atom linear unit with increasing amounts of Ca. Temperature dependent thermoelectric properties, Seebeck, electrical resistivity, and thermal conductivity were measured from room temperature to 1273 K. Substitution of Yb with Ca improves the Seebeck coefficient while decreasing the thermal conductivity, along with decreasing the carrier concentration in this p-type material resulting in an enhanced thermoelectric figure of merit, zT, compared to Yb14MnSb11.