Low-Lattice Thermal Conductivity in Zr-Doped Ti₂NiCoSnSb Thermoelectric Double Half-Heusler Alloys

The effect of doping on the thermoelectric propertiesof Half-Heusler(HH) high-entropy alloy (HEA) Ti2NiCoSnSb was studied.Lower thermal conductivity was observed with increased Sb doping.Mass scattering by heavy (Ta, Zr) and light (Al) dopants was studiedto further lower the thermal conductivity. Dopants at the level ofup to 50% at the Ti site were studied. A high HH phase content wasobtained in the Zr-doped samples, and a low-lattice thermal conductivityof 1.9 W/(m center dot K) was observed. This value is one of the lowestreported lattice thermal conductivities in HH alloys. The poor solubilityof Ta led to undissolved Ta in the samples, which enhanced the electricalproperties. In the case of Al doping, the NiAl phase raised the powerfactor value of Ti1.8Al0.2NiCoSn0.5Sb1.5 to 2.2 x 10(-3) W/(m center dot K-2), which is almost twice the corresponding value reportedfor Ti2NiCoSnSb. Interestingly, a maximum ZT of 0.29 was found in all of the doped systems, although the transportmechanism and microstructure varied widely with the type of dopant.An optimum dopant level of 25% of Zr, 7.5% of Ta, and 10% of Al isnecessary to obtain the maximum ZT in these alloys.Compared to HH systems, the HH high-entropy alloy (HEA) systems providea larger composition field for tuning the transport properties bysimultaneous doping of multiple elements to lower the thermal conductivity.