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Preparation and Thermoelectric Properties of β -FeSi2 and ZnO and Their Composite Ceramics

Author: QuXiuRong
Tutor: JiaDeChang
School: Harbin Institute of Technology
Course: Materials Science
Keywords: ZnO β-FeSi2 First-principle theory Si nanowires coated thermoelectric properties
CLC: TQ174.13
Type: PhD thesis
Year: 2010
Downloads: 147
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Abstract


In this thesis, varieties of electronic structures ofβ-FeSi2-ZnO due to doping in detail by using firsy principles pseudo-potential methods based on the density function theory. The thermoelectronic properties are estimated on theory. Al-doped and Al/Sn co-doped ZnO bulks are prepared by chemical co-deposition method. The effects of the dopant on the structure and properties of theβ-FeSi2-ZnO sample are investigated. Severial ZnO nanomaterials are also synthesized, and the formation mechanisms are analysized.β-FeSi2 powders can be obtained by high-energy ball-milling and heat-treating. The effects of milling time and treated temperature on the microstructures ofβ-FeSi2 powders are discussed, and the phase-transition mechanism of Fe-Si alloys is discussed. Well-proportioned ZnO nanoparticles-coatedβ-FeSi2 sample are obtained by sol-gel method.The caculated results show that the electronic structures of Al and Al/Sn co-doped ZnO samples change obviously. With increase of the dopants concentration, Fermi energy levels move from top of the valence band into conduction band. The gap nature inβ-FeSi2 turn from indirect to direct when Co atom is induced in the structure, and the band gap is narrower compared with undopedβ-FeSi2.Thus, it is estimated that the doping can improve the thermolelectronic properties ofβ-FeSi2-ZnO sample.With the concentration of Al increasing, the thermoelectronic properties will improve. The pores will increase with Sn concentration, and steady porous structures can be formed when Sn single doping. Thermal conductivity deduced distinctly with the increase of Sn concentration.Octahedral and belt-like ZnO precursors can by synthesized by controlling reaction conditions. Octahedral and belt-like ZnO superstructures can be obtained by decomposition the corresponding precursors. ZnO rods and flowers can be obtained by exposed the belt-like ZnO precursor into humid atmosphere for a period of time. On the other hands, Al-doped ZnO nanoplates can be obtained by appropriate concentration.α-Fe2Si5 andε-FeSi powders are first obtained by high-energy ball milling, which starting from elemental powders. The particle sizes decrease with the increase of milling time from 20 h to 30 h, and the particle sizes can reach 500 nm after 30h.α-Fe2Si5 andε-FeSi phases can completely transform intoβ-FeSi2 during annealing for 5h at 800 oC. When Si is extensive, Si nanowires can be observed in bothβ-FeSi2 powders and bulks sinterred by SPS. Due to the existence of Si nanowires, the thermal conductivity is reduced to about 4W·m-1K-1.In order to obtain well-proportioned ZnO nanoparticles-coatedβ-FeSi2 sample,β-FeSi2 particles were treated by surfactant. The diffusion of Si atoms will be held back due to the coating of ZnO nanoparticles, thus Si nanowires will appear even though Si is not excessive. The thermoelectronic performances of the ZnO nanoparticles-dopedβ-FeSi2 materials were improved distinctly. Especially, For 8 wt.% ZnO-doped samples, the total thermal conductivity decreases with increasing temperature, even though above 600oC, due to the increament of phone scattering on the grain boundaries.

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