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

Ti2−xNiCoSnSb (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 (Ni3Sn2, Ni3Sn4) compounds were observed; the intermetallic content increased for lower Ti content, e.g., Ti1.5NiCoSnSb. 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 Ti1.5NiCoSnSb was higher than that of Ti2NiCoSnSb and the full Heusler (FH) TiNi2Sn 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 Ni3Sn2 phase content. Ti1.5NiCoSnSb exhibited lower lattice thermal conductivity of 3.5 W/mK, compared to 5.4 W/mK at 823 K for Ti2NiCoSnSb due to increased phonon scattering at HH/Ni3Sn2 interfaces. But considering the decreasing power factor with lower Ti content, the maximum ZT obtained in Ti1.875NiCoSnSb (0.171 at 973 K) was only marginally higher than the value for Ti2NiCoSnSb. Further, compositional tuning is hence necessary to maximize the power factor. [ABSTRACT FROM AUTHOR]