THERMOELECTRIC PROPERTIES OF HALF-HEUSLER BASED MULTIELEMENT ZrTiNiFeSnSb and M2Ni1.5Fe0.5SnSb (M = Ti, Zr) ALLOYS

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.