Synthesis And Characterization Of Carbon Nanotubes And Carbon Nanofibers

In this thesis, two kinds of 1D carbon nano-materials, carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been synthesized. Many potential applications have been proposed for CNTs, including field emission displays, modified electrodes, nanometer-sized semiconductor devices, probes, high-strength composites, energy storage and energy conversion devices and catalysts support, based on their good thermal conductivity similar to diamond, the special mechanical property, the high aspect ratio and hollow structure with nano-scale of carbon nanotubes. The diameter of carbon nanofibers (CNFs) is between carbon nanotubes and the carbon fibers prepared by CVD method, so they not only have all the characters of the carbon fibers prepared by CVD method, but also have the same structure, capability and application with carbon nanotubes. Many potential applications have been proposed for CNFs, including the anode of lithium ion battery, double lay capacitor electrode, high effective sorbent and structure strengthen materials, based on their high intensity and module, thermal capability stability, chemical active, electric conductivity. Because of their particular capability and potential applications, the CNTs and the CNFs are the two kinds of carbon nano-materials that have been study the most. However, the mass production of high quality CNTs and CNFs are the preconditions of their applications.In the thesis, the morphologies, structures, physical properties, preparation methods and growth mechanism of CNTs and CNFs were briefly reviewed. The Co/Mo/MgO catalysts are prepared by sol-gel method, after proper amount of element Co was added in MgMoO4. The catalysts have high activity and high efficiency. After reacted for 2 h, the maximum yield of synthesized MWNTs is over 70 times of the Co/Mo/MgO pristine catalysts. With a simple enlarged process, a single furnace can produce over 35 g MWNTs from 1 g Co/Mo/MgO catalyst in 30 min. The Co/Mo/MgO catalysts have better activity for carbon nanotubes synthesis than the MgMoO4 catalysts.Our research shows that the gas flow ratio has big influence on the activity ofCo/Mo/MgO catalysts, and there are phase transformations happened in the MWNTs synthesis process. Upon our electronic microscope observations and analyses, the mechanism of the MWNTs bundles is proposed. The absorption of hydrogen makes the cobalt-molybdenum nanoparticles broken from inside to outside of the catalyst, forming multiplayers cobalt-molybdenum nanoparticles on the surface of the support, which is the main cause to form nanotubes bundles. The synergism of cobalt-molybdenum causes the carbon atoms rapidly dissolved in the cobalt-molybdenum nanoparticles, which should be responsible for the high yield and quality of MWNTs.The catalysts for synthesize CNFs are prepared by a deposit method. Using C2H2 and H2 as a react gas, CNFs are obtained by a CVD method. Upon our analyses of the experiment result, we found that the catalysts have high activity and high efficiency, which can produce CNFs about 25 times of the catalysts. The CNFs synthesized are fish-bone graphite nanofibers, and the graphite platelets are not aligned well with low graphitization degree.The effect of gas flow ratio on the formation and the yield of graphite nanofibers were studied in details. It is found that the gas flow ratio only has effect on the yield of CNFs. The growth mechanism of fish-bone graphite nanofibers has also been discussed. We consider that the growth of CNFs is not a surface reaction but a process of carbon atom diffuse in the metal catalyst particles. The formation of the coil-like shape is probably because some surfaces of catalyst particles have fast carbon deposition rates than the other surfaces. Because of the enough carbon sources, CNFs can grow continuously until the cobalt particles encapsulated with the carbon layer.