Thermoelectric Properties of Ti₀.₅Zr₀.₂₅Hf₀.₂₅CoₓNi₁₋ₓSn, Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₁₋ₓSbₓ Half-Heusler Alloys

Abstract

Energy is essential in today’s world as it is used in society on a day to day basis for a variety of applications. However, more than half of the energy produced in the world today is wasted as heat. Due to the climate change and depletion of non-renewable natural resources, both the environment and society would benefit from waste heat recovery. This can be accomplished with thermoelectric materials.

Half Heusler compounds are promising thermoelectric materials due to their large electrical conductivity and Seebeck coefficient values. However, high thermal conductivity has made it difficult to obtain large figures of merit. The highest ZT reported for p-type and n-type Half-Heusler compounds are 0.80 for Zr₀.₅Hf₀.₅CoSb₀.₈Sn₀.₂ and 1.50 for Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₀.₉₉₈Sb₀.₀₀₂, respectively.

Undoped MNiSn (M = Ti, Zr, Hf) materials were synthesized first in this project. TiNiSn and HfNiSn ternaries displayed ZT values of 0.33 and 0.28 correspondingly. Alloying the M site improved physical properties, as ZT was 0.40 for Ti₀.₅Hf₀.₅NiSn and 0.55 for Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn. Doping was optimized next to make further improvements to the figure of merit. Both p-type Ti₀.₅Zr₀.₂₅Hf₀.₂₅CoₓNi₁₋ₓSn and n-type Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₁₋ₓSbₓ materials were synthesized. The highest ZT was 0.18 for Ti₀.₅Zr₀.₂₅Hf₀.₂₅Co₀.₁₅Ni₀.₈₅Sn and 0.82 for Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₀.₉₉Sb₀.₀₁. ZrO2 nano-particles were added to Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₀.₉₉Sb₀.₀₁ samples in an attempt to increase ZT through reductions in thermal conductivity. However, power factor losses resulted in no improvement.