Development of new half-Heusler thermoelectric materials using composite, doping techniques, and density functional theory (DFT)

Thermoelectric materials are expected to play a significant role in future renewable energy sources. However, the current low efficiency of particularly half-Heusler compounds renders these technologies less competitive. This thesis addresses the limitations by improving material performance to enhance device efficiency with experimental and theoretical studies aimed at industrializing thermoelectric technology. In this context, the half-Heusler compound Ti0.5Zr0.25Hf0.25NiSn0.98Sb0.02, which exhibits one of the highest thermoelectric performances in the 600°C to 1000°C range, has shown a performance increase of 10% to 25% upon the addition of tantalum and molybdenum at micro and nano scales using a composite approach. Furthermore, new, double half- Heusler compounds Ta2FeNiSn2, Nb2FeNiSn2, and V2FeNiSn2 have been theoretically calculated using DFT in detail and subsequently synthesized and characterized experimentally for the first time in the literature. In the las part, the double half-Heusler compound Ti2FeNiSb2 has been theoretically and experimentally developed, with a comparative analysis of electronic transport properties. Based on the obtained results, the importance of determining the correct relaxation time in the calculation of electronic transport properties is emphasized. Subsequently, by partially substituting titanium with zirconium, niobium, and vanadium in the structure experimentally, a remarkable 250% performance improvement was achieved. The findings contribute significantly to the discovery and performance enhancement of new thermoelectric materials, supporting the development of more efficient thermoelectric devices in the future.