The direct energy conversion from heat to electricity without any moving part using the thermoelectric effects is attractive and has many promising applications in power generation and heat pumping (refrigeration) devices. However, the wide use of thermoelectric materials in daily life is still not practical due to its low efficiency. Presently, there has been renewed interest in thermoelectrics with new strategies for improving the efficiency, mostly by controlling the morphology and dimensionality, and manipulating the electronic structure of novel complex systems. There are new questions arise and several old questions have not yet been answered. My group at Michigan State University has been actively working on narrow band gap semiconductors and pseudo-gap systems including BiTe, BiSe, SbTe, AgPbmSbTem+2 (LAST), III-VI compounds, Mg2Si, Cu3SbSe4, Fe2VAl, ZrNiSn, etc., focusing on their thermoelectrics related properties. This dissertation tries to address some of the fundamental questions on the electronic structure and related thermoelectric properties of narrow band gap semiconductors and pseudo-gap systems, employing the state-of-art density functional theory (DFT) and Boltzmann's transport equation. I focuses on some well-known materials including Heusler compound, Fe2VAl, tetrahedrally bonded compounds (Cu3SbSe4 and related systems) and a newly reported novel nanocomposite system of Half-Heusler--Heusler (ZrNiSn--ZrNi2Sn). For the first two systems (Heusler and tetrahedrally coordinated systems), the effects of correlated electrons in the system containing d-electrons, valency of sp-elements, and the role of lone-pair electrons in band gap formation and effects on thermoelectric properties are fully investigated. For the third system, Half-Heusler--Heusler nanocomposite, I report a detailed energetics analysis of the nanostructure formation and its effects on the electronic structures of the interested systems.
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