Flame Synthesis on Carbon Nanostructures of Liquid Fuels

Abstract

A great variety of carbon nanostructure (CNS) has already been synthesized in flames Unfortunately the complexity of combustion chemistry leads to less controlling of synthesized products In order to improve the understanding of the relation between flames and CNSs synthesized within experiments were conducted through flames in a stagnation-point liquid-pool system The operating parameters for the synthesis include fuel oxygen supply sampling position and sampling time In this study carbon nanotube (CNT) and carbon nano-onion (CNO) were synthesized using ethanol and heptane diffusion flames in a liquid-pool system which composed of an upper oxidizer duct and a lower liquid pool In the experiments a gaseous mixture of oxygen and nitrogen outflowed from the upper oxidizer duct and then impinged onto the vertically aligned pool to generate a planar and steady diffusion flame within a designated oxygen environment The effects on oxygen from the fuel side and high C-H ratio fuel can be investigated by ethanol and heptane respectively A nascent nickel mesh was used as the catalytic metal substrate to collect deposited materials The selection of fuel greatly influenced the structure of CNSs Only CNTs were synthesized while using ethanol which is an oxygenated fuel In contrast both CNTs and CNO were found in heptane and the determination of structure depends on other parameters The oxygen concentration influenced the flame environment and thus the synthesized carbon products In ethanol flames lowering the oxygen concentration increased the yield diameter and uniformity of CNTs The optimal operating conditions for CNT synthesis were an oxygen concentration in the range of 15–19% flame temperature of 460–870 °C and a sampling position of 0 5–1 mm below the upper edge of the blue flame front It is noteworthy that the concentration gradient of C2 species and CO governed the CNT growth directly CNTs were successfully fabricated in regions with uniform C2 species and CO distributions In heptane flames CNTs were synthesized in a weaker flame near extinction and CNOs were synthesized in a more sooty flame by adjusting oxygen concentration A transition from CNT to CNO was observed by variation of sampling position in a flame The structure of CNS is directly affected by the presence of soot layer due to the carbonaceous environment and the growth mechanisms of CNT and CNO The sampling time can alter the yield of CNSs depending on the temperature of sampling position but the structure of products is not affected

Date of Award	2016 Jul 28
Original language	English
Supervisor	Ta-Hui Lin (Supervisor)