Synthesis, structure and properties of TiCoSb-based half-Heusler thermoelectrics
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Half-Heusler compounds have gained significant attention due to their potential as thermoelectric materials for high temperature waste heat recovery. They are characterised by a favourable combination of large Seebeck coefficients and low electrical resistivities, leading to large power factors but also have relatively large thermal conductivities, which limit the thermoelectric figure of merit ZT to values near one. TiCoSb-based half Heusler compounds are considered as promising p-type thermoelectric materials, however, studies on these compounds are fewer in number as compared to other half Heusler systems. For this reason, three novel series of TiCoSb-based half Heuslers were prepared and studied in this thesis. A detailed investigation of the Ti1-xVxCoSb1-xSnx half Heusler alloys is provided in Chapter 3. This study revealed that V and Sn co-substitution improves the high-temperature thermal stability of these materials. Thereafter, in Chapter 4, attempts were made to p-type dope these materials by substitution of Fe on the Co site. Surprisingly, electrical property measurements revealed n-type conduction for a number of the investigated samples. Thus, this study provides the first experimental observation of an unexpected change in carrier type in half Heusler compounds. The last results chapter of this thesis explores TiCoSb-based nanocomposites. Five series of compositions (TiCoSb)1-x(TiM2Sn)x (M2 = Fe2, FeCo, Co2, CoNi, Ni2) were prepared. In which, these compounds are expected to segregate into half Heusler and full Heusler phases. All samples were prepared using solid-state reactions. The synthesised materials were structurally characterised using X-ray and neutron powder diffraction. Complementary scanning and transmission electron microscopy was performed to probe the microstructure and compositional homogeneity of the investigated samples. This has allowed, coupled with measuring the thermoelectric properties, the structure-property relations to be established.