Flame synthesis of carbon nanostructures using mixed fuel in oxygen-enriched environment
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The effects of key parameters, namely oxygen concentration, mixed fuel, and sampling positions, on the formation of carbon nano-onions (CNOs) and carbon nanotubes (CNTs) were investigated in oxy-fuel inverse diffusion flames. Particular focus was put on the intermediate species in connection with the synthesis of CNOs and CNTs. Three patterns of carbon nanostructures were observed: CNTs only, CNOs only, and CNTs/CNOs cogeneration. An appropriate temperature range in the synthesis of CNTs was identified to lie between 400 and 1,000 °C, whereas the temperature range for the synthesis of CNOs was higher, within 800–1,250 °C. A threshold value of oxygen concentration, 30 %, existed for onset of CNO synthesis. Gas composition analysis indicated that no carbon nanomaterial was formed at low CO and C2H2 concentration as well as low substrate temperature (lower than 400 °C). Compared with the synthesis condition of CNTs only, the C2H2 concentration was higher for the onset of CNTs/CNOs cogeneration, whereas the CO concentration was maintained at the same level. Additionally, the critical C2H2 concentration for the onset of CNOs only was found to be 0.4 %. A large quantity of CNOs was observed for C2H2 concentration greater than 0.4 % and CO concentration greater than 4 %.
KeywordsCarbon nano-onions Carbon nanotubes Mixed fuel Oxy-fuel diffusion flames Combustion synthesis
The authors would like to thank the National Science Council, Taiwan, ROC, for their financial support under contract of NSC 100-2221-E-168-036-MY2.
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