ZrCoSb based half-Heusler (HH) alloys have been widely studied as a p-type thermoelectric (TE) material for power generation applications in the mid-temperature regime. However, their intrinsically high thermal conductivity has been found to be detrimental for the improvement in their thermoelectric figure-of-merit (ZT), which presently is far below unity. In the current work, a state-of-the-art ZT ∼1.1 at 873 K was realized in an optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys by employing compositional modulation i.e. grain-by-grain compositional variations, which leads to a substantial increase in its power factor coupled with a concurrent decrease in its thermal conductivity. Significant reduction in the phonon mean-free-path is observed on Hf substitution, which is comparable to the average crystallite size (∼25 nm), thus leading to a very low thermal conductivity of ∼2.2 W m−1K−1 at 873 K, which is amongst the lowest reported in HH alloys. The TE device characteristics, estimated using cumulative temperature dependence model for quantitative evaluation of TE performance, yielded an output power density of ∼10 Wcm−2 with a leg efficiency of ∼10% in the optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys, which is comparable to the reported efficiencies of other state-of-the-art TE materials. Keywords: Half Heuslers, Iso-electronic, Waste heat recovery, Mid-temperature, Efficiency, Compositional modulation
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