Improving thermoelectric properties of half-Heuslers through vacancy and interstitial engineering
Ferluccio, Daniella Anna
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Thermoelectrics can be used to produce electricity from waste heat. This offers a potential solution for a greener future and sustainable energy. Materials with the ability to optimise electrical and thermal properties from manipulating their structure, such as half-Heuslers are interesting candidates for thermoelectric waste heat recovery. The solid solution between NbCoSn and NbCoSb was investigated and analysed in Chapter 3. This research discovered that vacancies are created on the Nb site as Sb is substituted for Sn, enabling the valence electron count to be maintained near 18. Structural analysis was conducted by neutron powder diffraction which confirmed the presence of vacancies. Modelling of the thermal conductivity demonstrated that these vacancies act as point defect scattering centres which helps lower the lattice thermal conductivity. The findings in Chapter 3 prompted a further investigation into nominally 19 valence electron compounds (XxCoSb, Xx = V0.87, Nb0.85, Ta0.81). Chapter 4 details these findings through analysis of neutron powder diffraction data and electron microscopy. Inherent vacancies are confirmed in compositions thought to have 19 valence electrons. Unusually low lattice thermal conductivity was observed in V0.87CoSb. Two new half-Heusler compositions with inherent vacancies were identified from this work, yielding new insights into the thermoelectric properties of these interesting materials. The effectiveness of direct substitution, interstitial metals and a combination of both effects was investigated in the TiNiSn half-Heuslers in Chapter 5. This work revealed that Ni and Cu tends to accommodate on the interstitial site which was effective in the reducing thermal conductivity. Neutron powder diffraction and electron microscopy was carried out to gain insight into site occupancies and grain sizes. Furthermore, an attempt at p-type substitution using Co was carried out but found to be ineffective, leading to poor thermoelectric efficiencies despite low thermal conductivities. Finally, Chapter 6 highlights a much-needed study into the thermal stability and thermal expansion of well know n- and p-type half-Heusler materials. This was undertaken using variable temperature Synchrotron X-ray powder diffraction and allowed the determination of decomposition temperatures and thermal expansion coefficient. Two candidates were identified as potential compositions for thermoelectric device legs due to their similar expansions and stability.