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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/119059
Integration of Si/Si-Ge nanostructures in micro-thermoelectric generators
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[eng] Silicon and silicon-germanium nanostructures were grown, integrated, optimized and characterized for their application in thermoelectric generation. Specifically two kinds of nanostructures were worked: silicon and silicon-germanium nanowire arrays (Si/Si-Ge NW) and polycrystalline silicon nanotube fabrics (pSi NT).
The results are dived in four chapters. Chapters 3, 4 and 5 deal with Si/Si-Ge NWs, while chapter 6 presents the pSi NT fabrics.
In Chapter 3 the growth and integration of Si/Si-Ge NWs was studied, in order to optimize their properties for thermoelectric application in micro-thermoelectric generators (µTEG). First, the methods for depositing gold nanoparticles prior to NW growth were studied. Second, the growth of NWs from the gold nanoparticles in a Chemical Vapour Deposition (CVD) process was comprehensively studied and optimized for subsequent integration of NWs in µTEGs, both of Si and Si-Ge. All important properties – NW length, diameter, density, doping and alignment
– could be controlled by tuning the seeding gold nanoparticles and the process conditions, namely temperature, pressure, flows of reactants and growth time. Finally, integration was demonstrated in micro-structures for thermoelectric generation and characterization. The optimization process yielded to fully integrated thermoelectric Si/Si-Ge NW arrays with diameters and densities of ~100 nm and 5 NW/µm2 respectively.
In Chapter 4 the Si NWs were thermoelectrically characterized. The Seebeck coefficient, electrical conductivity and thermal conductivity of arrays and single Si-NWs were measured in microstructures devoted to characterization comprising NWs integrated as in final µTEG application. Additionally a novel atomic force microscope based method for determination of thermal conductivity was explored. Then the results were discussed comparing them with existing literature. A ZT of 0.022 was found at room temperature, revealing an improvement of factor 2-3 with respect to bulk.
In Chapter 5 The harvesting capabilities of µTEGs with integrated Si/Si-Ge NWs was assessed. The thermal gradient and the power of the µTEGs was assessed for two generation of devices and for two thermoelectric materials, namely Si and Si-Ge NWs, which were integrated for the first time in functional generators. Also a study on heat sinking and convection effects was conducted adding insight towards further device improvement. Finally, the results were discussed and compared with literature. The maximum power densities attained were 4.5
µW/cm2 for the Si NWs and 4.9 µW/cm2 for the Si-Ge NWs while harvesting over surfaces at
350 ºC.
Chapter 6 deals with pSi NT fibers. First this new material concept and the growth route are presented, showing the fabrication steps and the control of the resulting properties by CVD method. Then the material is thermoelectrically characterized, by measuring its Seebeck coefficient and electrical and thermal conductivities up to 450 ºC. A ZT of 0.12 was found, doubling the optimally doped bulk at this temperature. Finally a proof of concept was demonstrated by assessing the thermal harvesting capabilities of the material on top of hot surfaces. A maximum of 3.5 mW/cm2 was attained at 650 ºC.
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GADEA, Gerard. Integration of Si/Si-Ge nanostructures in micro-thermoelectric generators. [consulta: 9 de desembre de 2025]. [Disponible a: https://hdl.handle.net/2445/119059]