Three-dimensional helical carbon materials: Microcoiled carbon fibers, carbon nanocoils, carbon nanotubes: Synthesis, properties and applications.

다국어 초록 (Multilingual Abstract)

Materials with a 3D-helical/spiral-structure in micron size have recently aroused a great deal of interests because of their helical morphology and unique properties. However, materials with a 3D helical structure are not commonly observed among industrially available materials. Researchers have been trying to synthesize various micro- and nano-sized 3D helical materials and are exploring the mechanisms, nature, and properties of these materials. Yet a systematic study on 3D helical carbon materials in micro- and nano-size has been missing. This research work is intended as a first step to fill this gap.
Among various 3D helical materials, carbon element has stimulated great interests. Micro coiled carbon fibers, carbon nanocoils, and carbon nanotubes are major types of 3D helical carbon materials ranging from micron to nano size. Synthesis of these 3D helical carbon materials by a catalytic chemical vapor deposition method is presented in this thesis. It involves a pyrolysis of hydrocarbon gas (e.g. acetylene) over transition metals, such as Ni, Fe, and Co, at high reaction temperature (500–1000°C). Besides the conventional thermal filament chemical vapor deposition method, a novel microwave chemical vapor deposition (MWCVD) method has been developed to synthesize micro- and nano-sized 3D helical carbon materials economically. The faster heating and cooling processes associated with microwave CVD have potential for large-scale production in the near future. Compared with previously reported microwave plasma enhanced chemical vapor deposition (MWPECVD) method, this method does not require high vacuum and much higher deposition rate is another major advantage. It has been found in this work that microwave plays an important role on coil morphology formation for micro coiled carbon fibers and carbon nanocoils. The large temperature gradient around the catalytic particles could be the reason. Different reaction factors have been checked to optimize the deposition.
Due to their extraordinary properties, carbon nanotubes have been expected to have wide applications. Efforts have been made on the synthesis of high quality carbon nanotubes economically in this work. A novel catalyst/catalyst support pair, iron/magnesium carbonate, has been developed for synthesis of multi-walled carbon nanotubes with high purity. The coil morphology is induced by insertion of pentagon-heptagon pairs into hexagonal network of nanotube wall periodically. Thorough purification of carbon nanotubes is always a concern before investigating their properties and potential applications. Impurities in raw carbon nanotube material have to be removed by chemical treatment. A couple of purification methods are presented in this work.
Various techniques have been used to characterize these micro- and nano-3D materials, such as scanning electron microscopy (SEM), energy dispersive spectrum (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer Emmett-Teller (BET), thermal gravimetric analysis (TGA), etc. Growth mechanisms are proposed based on the experimental and characterization results. It is verified that the nonuniform carbon deposition rate on catalyst particles leads to the bending of the carbon fiber/tubule, and hence results in the coil morphology.
To conclude, the research work reported here is a systematic study on synthesis, characterizations, and applications of micro- and nano-3D helical carbon materials, such as micro coiled carbon fibers, carbon nanocoils and carbon nanotubes. A few suggestions for future research directions are also listed.