The efficiency of thermal to electrical conversion depends on high electrical
conductivity (σ), high Seebeck coefficient (S = ΔV
ΔT
⁄ ), and low thermal conductivity (κ).
Radioisotope thermoelectric generators in spacecraft require high-temperature thermoelectric
materials like oxides, SiGe, and half-Heusler (HH) alloys. HH alloys with MgAgAs crystal
structure despite excellent electrical and mechanical properties, have high κ and need long
annealing times. Traditionally, nanostructuring and mass defects have been employed to lower
κ. Recently, high entropy materials are explored to lower κ through lattice scattering. Higher
configurational entropy in multicomponent HH Ti2NiCoSnSb helped obtain a single phase
without annealing. The effect of nanostructuring in Ti2NiCoSnSb was studied by mechanical
alloying as well as milling of the crushed powders of vacuum arc melted samples. While
nanostructuring has helped lower the κ to 3.5 W/mK at room temperature, the S value was
considerably reduced.
Lowering of Ti content and increasing the Ni, Co content led to precipitation of
intermetallic Ni3Sn2 and full Heusler Ni2TiSn secondary phases, respectively. The intermetallic
phase improved the electrical conductivity and lowered κl at the expense of S value in the Ti2-
xNiCoSnSb samples In the Ti2(NiCo)1+xSnSb alloys, the power factor increased till optimum
doping of 2.5% Ni and Co due to full Heusler (FH) phase and thermal conductivity lowered till
optimum doping of 5% Ni and Co due to phonon scattering at HH/FH interfaces. The effect of
doping was understood through the introduction of lighter element Al and heavier elements Zr
and Ta, respectively. Based on the structure-property optimizations, compositional tuning by
multi-element doping was performed. Thermoelectric figure of merit of 0.692 was observed in
Ti1.6Al0.2Ta0.2NiCoSn0.5Sb1.5 double half Heusler alloy at 823 K. The precipitating AlCoNiTa
type FH phase helped increase the power factor and lower thermal conductivity by scattering
phonons at the HH/FH interface in these alloys.
Nb2FeCoSnSb double half Heusler exhibited ZT similar to Ti2NiCoSnSb. The effect of
Sb content in Ti2NiCoSnSb based alloys has been extensively studied (Karati et al., 2020,
2022b). For comparison, the effect of processing route and Sb content on thermoelectric
properties was studied here. The impact of varying Fe, Co content on the thermoelectric
properties was also studied. First principles calculations helped understand the phase evolution
and resultant electronic band structures in these alloys.