Investigation of metal-rich half-Heusler thermoelectrics : synthesis, structure and properties
Barczak, Sonia Agata
MetadataShow full item record
Thermoelectric materials that can directly convert heat into electricity offer a possible avenue to address the world’s increasing demand for energy. Metal-rich half-Heusler compounds are of interest due to their favourable electronic transport properties. Unfortunately, widespread application is limited by comparatively high thermal conductivity. The effect of processing and excess metal (Ni and Cu) on thermoelectric properties of MNiSn, M0.5M’0.5NiSn (M = Ti, Zr, Hf) and Ti0.5Zr0.25Hf0.25NiSn materials was investigated and is discussed in Chapters 3 and 4. This work revealed that Cu is effective n-type dopant, which improves electronic properties of half-Heusler materials. Detailed structure analysis, which included high-resolution synchrotron X-ray diffraction, neutron powder diffraction and electron microscopy revealed that most of the excess metals are randomly distributed on the interstitial sites, producing significant point defect scattering of phonons. In addition, Cu segregation leads to grain-by-grain compositional variations, with grains tending towards either half-Heusler or full-Heusler composition. In addition to the microstructural and properties studies, an in-situ neutron powder diffraction experiment was used to understand the formation of Ni-rich TiNi1+ySn (y = 0, 0.075 and 0.25) and multiphase M0.5M’0.5NiSn compositions during solid-state reaction. As described in Chapter 5, the half-Heusler formation occurs through a complex, multistep reaction, which involves many intermediates. ZrNiSn and Ti0.5Zr0.25Hf0.25NiSn underwent spontaneous self-propagating combustions, which is a new route to prepare impurity-free half-Heusler alloys. The last results Chapter describes the analysis of neutron total scattering data using Reverse Monte Carlo modelling. This study was performed to gain insight into spatial distribution of excess Ni within the half-Heusler structure. It confirmed random distribution of excess metal on the interstitial sites.