Enhanced thermoelectric performance in p-type ZrCoSb based half-Heusler alloys employing nanostructuring and compositional modulation

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-xHfxCoSb_(0.9)Sn_(0.1) 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~2.2Wm^(-1)K^(-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 Zr_(1-x)Hf_(x)CoSb_(0.9)Sn_(0.1) HH alloys,which is comparable to the reported efficiencies of other state-of-the-art TE materials.