Investigation into Reduced Thermal Conductivity for Half-Heusler Alloys and Identification of Novel Multinary Chalcogenides Possessing Intrinsically Low Thermal Conductivity

The current climate makes the need for sustainable energy production a priority, leading to keen interest towards the advancement of “green” energy sources and technology. This necessity has led to a multidisciplinary scientific approach with collaborations across various fields that are looking to solve this extremely important problem. One such technology is thermoelectric devices, which are attractive due to the fact that they provide direct solid-state conversion of heat to electrical power, and vice versa. This in turn allows for both waste heat recovery and sustainable refrigeration. One key aspect that is preventing the wide scale commercial use of thermoelectric devices is the thermoelectric properties of the materials, in particular the thermoelectric performance of p-type materials, as compared to n-type. A requirement for a good thermoelectric material is low thermal conductivity. Therefore, the identification of new materials that possess low thermal conductivity is vital in the enhancement of thermoelectric performance. However, building on the current understanding of known materials is also extremely necessary and a combination of these two approaches is explored in this dissertation. The first material studied is the half-Heusler, NbFeSb, which has been shown to possess promising thermoelectric properties, although it has relatively large thermal conductivity (\u3e 6 W/m-K at room temperature). Ternary and quaternary chalcogenides often possess intrinsically low thermal conductivity due to their structural complexity and are of interest in other energy related fields, such as for photovoltaic applications, leading to the investigation into some multinary chalcogenides. NbFeSb has been reported to have good p-type thermoelectric properties when doped, whereas the un-doped material has low resistivity, a very small and negative Seebeck coefficient and a large thermal conductivity. However our high-quality induction melted NbFeSb displays markedly different behavior in the Seebeck coefficient, leading a comprehensive structural and chemical analysis resulting in the interesting role of anti-phase boundaries on the electrical transport. This work builds on the current understanding of this material and a further systematic investigation was employed in order to identify a more consistent and reproducible synthesis method for NbFeSb. A new reproducible method was achieved and was tested with the successful synthesis of both a doped and a small-grain specimen that led to a significantly reduced thermal conductivity. The quaternary chalcogenide, Cu1+xMn2-xInTe4 (x = 0, 0.2, 0.3) was synthesized by direct reaction, and a Cu-excess approach was chosen for doping due to its success in other quaternary compositions. Structural and stoichiometric compositions were analyzed by a combination of Xray diffraction and Rietveld refinement. The temperature-dependent transport properties were measured and their potential for thermoelectric applications is investigated. In addition, the ternary chalcogenides, Cu4Bi4X9 (X = S, Se), were synthesized by direct reaction. Their temperature-dependent thermal properties were studied and the mechanisms of their intrinsically low thermal conductivity values discussed, including the reasons for the lower values observed in the Se containing compound. The electrical properties of the S containing compound were also measured. This investigation helps assess the suitability for these new ternary chalcogenides for various applications