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【分享】两篇关于==纳米材料==的国外博士论文
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NANOSTRUCTURED MATERIALS FOR ENERGY CONVERSION CASE WESTERN RESERVE UNIVERSITY，2008 链接http://www.namipan.com/d/1.pdf/1 ... e305ee36f3f7ccf6200 PROCESSING, MICROSTRUCTURE EVOLUTION AND PROPERTIES OF NANOSCALE ALUMINUM ALLOYS UNIVERSITY OF CINCINNATI，2005 链接 http://www.namipan.com/d/0.pdf/e ... bcb120fbb5a3c636b01 |
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补充说明 PROCESSING, MICROSTRUCTURE EVOLUTION AND PROPERTIES OF NANOSCALE ALUMINUM ALLOYS ABSTRACT In this project, phase transformations and precipitation behavior in age-hardenable nanoscale materials systems, using Al-Cu alloys as model materials, were first studied. The Al-Cu nanoparticles were synthesized by a Plasma Ablation process and found to contain a 2~5 nm thick adherent aluminum oxide scale, which prevented further oxidation. On aging of the particles, a precipitation sequence consisting of, nearly pure Cu precipitates to the metastable θ′ to equilibrium θ was observed, with all three forming along the oxide-particle interface. The structure of θ′ and its interface with the Al matrix has been characterized in detail. Ultrafine Al-Cu nanoparticles (5~25 nm) were also synthesized by inert gas condensation (IGC) and their aging behavior was studied. These particles were found to be quite stable against precipitation. Secondly, pure Al nanoparticles were prepared by the Exploding Wire process and their sintering and consolidation behavior were studied. It was found that nanopowders of Al could be processed to bulk structures with high hardness and density. Sintering temperature was found to have a dominant effect on density, hardness and microstructure. Sintering at temperatures >600 °C led to breakup of the oxide scale, leading to an interesting nanocomposite composed of 100~200 nm Al oxide dispersed in a bimodal nanometer-micrometer size Al matrix grains. Although there was some grain growth, the randomly dispersed oxide fragments were quite effective in pinning the Al grain boundaries, preventing excessive grain growth and retaining high hardness. Cold rolling and hot rolling were effective methods for attaining full densification and high hardness. Thirdly, the microstructure evolution and mechanical behavior of Al-Al2O3 nanocomposites were studied. The composites can retain high strength at elevated temperature and thermal soaking has practically no detrimental effect on strength. Although the ductility of the composite remains quite low, there was substantial evidence for high localized plasticity. The strengthening mechanisms of the composite include: Orowan strengthening, grain size strengthening and Forest strengthening. Finally, the microstructure evolution and mechanical behavior of 2024Al-Al2O3 nanocomposites were studied. This 2024Al-Al2O3 composite exhibits similar thermal stability and high strength at elevated temperature as Al-Al2O3. On ageing, the matrix of 2024Al-Al2O3 composites revealed a precipitation sequence of: αAl → GP/GPB → θ′/S′ → θ/S. The strengthening mechanisms of the 2024Al-Al2O3 composites include precipitation strengthening, Orowan strengthening, grain size strengthening and Forest strengthening. Nanostructured Materials for Energy Conversion ABSTRACT This thesis focuses on the synthesis and characterization of chalcogenide nanomaterials for thermoelectric energy conversion and N-doped nanostructured metal oxides for photocatalysis. A sonoelectrochemical method was applied to synthesize Bi2Se3 heterostructured nanowires and PbTe nanorods. Experimental parameters that affect the growth of these nanoparticles are discussed. Furthermore, nanostructured ntype Bi2Se3 and p-type PbSe thin films were fabricated by a chemical bath deposition method. Thermoelectric transport measurements showed large Seebeck coefficients for both Bi2Se3 and PbSe thin films at room temperature. The crystal orientation and transport properties of nanostructured PbSe thin films are pH sensitive. The results of transport measurements on nanostructured PbSe thin films suggest that nanostructuring can enhance the Seebeck coefficient but lower the electrical conductivity by restricting the mobility. On the other hand, a chemical approach was introduced to incorporate N into TiO2, ZrO2, HfO2, CeO2 and SnO2 nanoparticles. XPS results indicate that N could be doped into these materials and N-doping levels could be controlled. Reflectance spectra show that visible-light absorptions of the nanoparticles are enhanced through N-doping. Furthermore, an enhanced visible-light photocatalytic activity was observed for N-doped metal oxide nanoparticles. Both the optical and photocatalytic properties of N-doped nanoparticles are dependent on N-doping levels. |
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