Experimental studies on the formation of soot and carbon nanotubes in hydrocarbon diffusion flames.

摘要

In this dissertation, formation of soot and carbon nanotubes in coflow laminar hydrocarbon-air diffusion flames is experimentally investigated. Soot particles are collected using two different techniques: impact flow sampling and thermophoretic sampling, and analyzed by SEM and TEM. The formation of precursor soot particles and the agglomeration of primary soot particles are studied. It is found that the coagulation dominates the early stage of soot growth, while the surface growth dominates the later stage of the soot growth. Effects of fuel structure, airflow rate and fuel flow rate on the soot formation are also investigated. The soot aggregates are characterized using Flow Field-Flow Fractionation method. The aggregate size distribution and mean diameters are obtained. The average aggregate diameter of the ethylene soot particles at the end of soot growth is about 142 nm, and the size range is about 25–350 nm.; Multi-walled carbon nanotubes with a diameter range of 20–60 nm are synthesized in methane and ethylene diffusion flames. The as-formed carbon nanotubes are entangled and curved with a length of several μm. In the methane flame, a Ni-Cr wire is used as the substrate for the nanotube growth. The catalyst particles are found to be nickel oxide and iron oxide. The effects of growth time, sampling height, nitrogen addition, nitric acid oxidation, fuel flow rate and substrate material on the nanotube growth are investigated.; In the ethylene flame, the catalyst particles are pre-formed by oxidation of a stainless steel grid by a premixed propane flame. When the flame is diluted by nitrogen, the nanotubes are straightened because of lower temperature. The effects of growth time and catalyst material on the nanotube growth are investigated. When a cobalt-coated grid is used as the substrate, well-aligned carbon nanotubes are formed.; Based on the internal structural studies conducted by a high-resolution TEM, the carbon nanotubes formed in the diffusion flames are found to have a root growth mechanism. Finally, a schematic model for the carbon nanotube growth in the diffusion flames is proposed.