Growth-rate And Morphology Controlled Of Carbon Nanofibers

Carbon nanofibers (CNFs) are novel structural carbon materials with good application potentials in many fields. A key factor to the successful application of this material relies on its large scale production with well defined structrures and low cost. In this thesis, the carbon nanofibers sytheiszed by CO disproportionation on iron catalyst are investigated. The influences of synthetic conditions and catalyst properties on the growth rate are explored. TEM, SEM, XRD and nitrogen physical adsorption are used to characterize the structural properties of the produced CNFs and a relationship between its kinetic characteristics and the morphology of CNFs is established. The underlying causes of the relationship are also discussed by characterizing the changes of the catalyst particles'structural properties. Finally, cold mold experiments and conceptual design are carried out on a rotary kiln reactor, which is suitable for continous growth of CNFs. The main results of this work are summarized as follows:(1) H2 concentration is a prominent factor that influences the syntheis of carbon nanofibers by CO disproportionation on iron catalyst. TPD-MS characterization and theoretical analysis results indicate that the presence of H2 in the feed gas may change the reaction shechme and can accelerate CO disproportionation on iron catalyst. The CNFs preparation experiment results show that the maximal growth rate first increases and then decreases with the increase of H2 concentration and the induction period and life time of the catalyst are also shortened at elevated H2 concentration. The morphology, texture and graphitization degree of as-synthesized CNFs are also found to be influenced by H2 concentration.(2) The morphology of CNFs is strongly related to the maximal growth rate in case of adoption of identical reduction conditions. As the maximal growth rate increases, the morphology changed from twist to helical, then to straight and tight helical and finally to amorphous. The maximal growth rate of CNFs can be controlled by adjusting the H2 concentration, CO concentration and temperature. And so is the morphology of CNFs.(3) During the process of CNFs synthesis, Fe particles first would be fragmentated to small particles and transformed to iron carbide. The morphology of the particles are determined by the synthetic conditions, which can affect the induction period, maximal reaction rate, deactivation rate and finally the morphology of the CNFs.(4) The catalyst particles have different particle sizes and composition after reduction under various conditions. As a result, CNFs have different morphologies and structural properties even though the maximal growth rates are identical. This result also reveals that the morphology and structure of CNFs are determined by the properties of catalyst particles. To preparation of CNFs with well defined structures, a proper reduction of the catalyst is very important.(5) The reaction scheme of CO disproportionation on iron catalyst is studied by using BOC and transition state theory. A kinetic model is developed using microkinetic method. The modeling results are found to fit the experiment data well and can be used for controlling and monitoring the morphology of produced CNFs. The modeling results also show the energy barriers for the formation of CH4 and H2O are very high and it is hard for CH4 and H2O to form in the reaction system, which is consistent with the experimental results.(6) The average residence time of catalyst in the rotary kiln decreases linearly with the increase of inclination height at both high and low rotating rates. The average residence time decreases faster at high rotating rate then slower at a low rotaing rate. At different inclination heights, the average residence time of catalyst is inversely proportional to the rotating rate until it reaches 9 r/min, above which the effect of rotating rate on the average residence time of catalyst can be neglected. The distribution density of residence time becomes more concentrated as the rotate speed and the inclination height increase.