Lowering thermal conductivity in thermoelectric Ti2−xNiCoSnSb half Heusler high entropy alloys

Ti 2− x NiCoSnSb ( x = 0.125, 0.250, 0.375, and 0.500) half Heusler (HH) high-entropy thermoelectric alloys were synthesized by the arc melting—ball milling—spark plasma sintering route. The impact of secondary phase content on the thermoelectric properties in these alloys was examined. Ni-rich intermetallic (Ni 3 Sn 2 , Ni 3 Sn 4 ) compounds were observed; the intermetallic content increased for lower Ti content, e.g., Ti 1.5 NiCoSnSb. A Ni-rich full Heusler (FH) secondary phase was also observed. These results were consistent with first-principles calculations that show that the formation enthalpy of Ti 1.5 NiCoSnSb was higher than that of Ti 2 NiCoSnSb and the full Heusler (FH) TiNi 2 Sn phase. In lower Ti content samples, the electrical conductivity increased, and lattice thermal conductivity decreased at the expense of thermopower owing to higher FH and the Ni 3 Sn 2 phase content. Ti 1.5 NiCoSnSb exhibited lower lattice thermal conductivity of 3.5 W/mK, compared to 5.4 W/mK at 823 K for Ti 2 NiCoSnSb due to increased phonon scattering at HH/Ni 3 Sn 2 interfaces. But considering the decreasing power factor with lower Ti content, the maximum ZT obtained in Ti 1.875 NiCoSnSb (0.171 at 973 K) was only marginally higher than the value for Ti 2 NiCoSnSb. Further, compositional tuning is hence necessary to maximize the power factor. Graphical abstract