Improved Power Factor and Mechanical Properties of Composites of Yb₁₄MgSb₁₁ with Iron

Composite phases have been shown to improve both the thermoelectric efficiency and mechanical properties of materials. Here, we demonstrate an improved thermoelectric figure of merit, power factor, and mechanical properties for the high-temperature p-type Zintl phase Yb14MgSb11. Composites with 0, 1, 2, 3, 4, 6, and 8 vol % 6–10 μm reduced Fe powder were prepared via a fast, scalable, mechanical milling and spark plasma sintering procedure. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that Fe is not incorporated into the Yb14MgSb11 structure. First-order reversal curves and scanning electron microscopy images show that the Fe inclusions are larger and closer together with increasing Fe content. Thermogravimetric and differential scanning calorimetry show that the composites are stable up to 1273 K. The elastic constants of the 8 vol % Fe composite were measured by resonant ultrasound spectroscopy and show that Yb14MgSb11 becomes stiffer with increasing Fe volume % and SEM after indentations show crack arresting occurs at the Fe interface. Thermoelectric properties on dense pellets are measured from 300 K – 1273 K. The thermoelectric power factor (PF = S2/ρ) increases with increasing Fe content, with the 8 vol % Fe resulting in 40% higher PF than pristine Yb14MgSb11. The increase in PF is attributed to a systematic reduction in electrical resistivity. Peak thermoelectric figure of merit [zT = (S2T)/(κρ)] is observed at 3 vol % Fe, an 11% improvement in zT compared to Yb14MgSb11. Yb14MgSb11 composites with Fe are compatible with Ce0.9Fe3.5Co0.5Sb12 for thermoelectric generator couple segmentation.