Characterization of carbon nanoclusters produced by arc-discharge
The research covered in this dissertation presents a systematic study for growth phenomena of carbon nanoclusters prepared by a arc discharge technique. Through a series of experiments, it has been found that morphology and yield of nanotubes strongly depend on the processing parameters. A low current density of 190 A/cm², a discharge voltage of 27 V, a high helium pressure of 550 torr, and a minimum electrode gap offered the best condition for synthesis of carbon nanotubes. According to a detailed study of carbon nanoclusters by TEM, the rich variety of growth phenomena, in particular non-concentric and complex branching phenomena are reported. These phenomena have demonstrated that the growth process of nanotubes is apparently more complex than what the existing growth models suggest. Further refinement and expansions of these models are important for technological exploitation of nanotubes and may be identified and guided by the present results. The reaction of foreign materials with carbon vapor in the arc results in their encapsulation into carbon cages, as well as the formation of other novel forms of carbon nanoclusters. Among more than eighteen elements introduced into the discharge in this study, B, Y, Zr, Nb, and Mo are most easily encapsulated in the form of their carbides into carbon nanoclusters. Through the detailed study, it has been found that the encapsulation occurs most easily in the materials with the incompletely-filled 4d electron shell for the refractory materials. Furthermore, it is reported that Fe, Co, Ni, or YC₂ working as catalysts stimulate the formation of single-walled nanotubes (SWTs), and mixtures of these catalysts greatly enhance the yield of SWTs. Based on morphologies of the star patterns of SWTs produced by an anode containing YC₂, their step-by-step growth mechanism is proposed. In addition to the SWTs, it has been found that Fe or Ni also stimulates the formation of strings of carbon nanobeads. A growth model for this phenomenon is presented. Finally, the conversion of carbon nanotubes into SiC whiskers is reported. In contrast to the conventional process, starting with nanotubes offers a high purity SiC whisker without any metal impurity.
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