Abstract:
Thermoelectrics are used to directly convert heat into electricity and vice versa. They have
attracted increased interest from the materials science community and grown quickly in recent
decades. Efficient thermal to electrical conversion requires high electrical conductivity (σ), a
high Seebeck coefficient (S = ΔV/ΔT), and low thermal conductivity (κ). Radioisotope
thermoelectric generators use thermoelectric materials such oxides, SiGe, and half-Heusler
(hH) alloys. Half-Heusler alloys with MgAgAs crystal structure have outstanding electrical
and mechanical properties, but have high κ and require prolonged annealing times.
Traditionally, nanostructuring and mass defects were used to reduce κ. Recently, researchers
have delved into high entropy materials to mitigate κ through lattice scattering. Higher
configurational entropy in multielement hH alloys contributes to lower thermal conductivity.
In this thesis, new half-Heusler type HEA alloys MNiSn (M = Zr, Ti) based ZrTiNiFeSnSb,
Zr2Ni1.5Fe0.5SnSb and Ti2Ni1.5Fe0.5SnSb are investigated using arc melting followed by heat
treatment and quenching. Scanning electron microscopy (SEM), Electron Probe Microanalysis
(EPMA), X-ray diffractometry (XRD), Transmission electron microscopy (TEM) techniques
were used to investigate alloy’s microstructure, composition, and phase information.
Theoretical calculations, support experimentally obtained reduced lattice thermal conductivity
and suggest that large anharmonicity derives low lattice thermal conductivity while semi metallic and semiconducting nature of these hH alloys help in obtaining high Seebeck
coefficients by entropy engineering and compositional tuning. Owing to these improved
thermoelectric parameters, by tuning the valence electron count of ZrTiNiFeSnSb, which is
bipolar, the thermoelectric figure of merit (ZT) of 0.23 and 0.21 were observed in
Zr2Ni1.5Fe0.5SnSb and Ti2Ni1.5Fe0.5SnSb hH type high entropy alloys at 974 K, implying that
these alloys could be potential n-type thermoelectric materials for energy conversion at high
temperatures.
