Flame Synthesis of Single-Walled Carbon Nanotubes

Height, Murray J., Jack B. Howard, Jefferson W. Tester, John B. Vander Sande. Flame Synthesis of Single-Walled Carbon Nanotubes. Carbon. 42 2295-2307(2004).

Purpose of the Study

The purpose of the study was to synthesize and analyze single-walled carbon nanotubes using a combustion flame methodology. A premixed flame synthesis of CNTs is much lower in cost and higher in yield than other production methods. Conditions for nanotube formation were examined and the carbon distribution characteristics were analyzed after synthesis.

Methods

A fuel rich flame was used as the requisite fuel-rich, high temperature environment necessary for nanotube formation. Although both diffusion flames and premixed flames are observed to produce CNTs, a premixed 1-dimensional flame was chosen to further understand nanotube formation zone and nanotube structure over a range of operating conditions.

A premixed acetylene/oxygen/argon flame with argon dilution at a pressure of 6.7 kPa was used. Iron pentacarbonate was chosen as the metallic precursor. The burner consisted of a copper plate with uniformly spaced holes filled with steel wool to facilitate uniform flow of the reactant gases. The burner was mounted on a vertical translation stage that allowed various heights above burner (HAB) to be observed. Thermophoretic sampling onto TEM grids allowed the characterization of nanotubes at different heights and lengths of time in flame.

Key Findings

1.     A zone of less than 40 mm burner height was dominated by discrete metal particles with a distribution of particle sizes.

2.     Nanotubes formed between 30 mm and 50 mm above the burner at a rate of 10 to 100 microns per second.

3.     The upper region of the flame (50 to 70 mm HAB) was dominated by tangled webs of nanotubes formed by the coalescence of nanotubes formed earlier in the flame.

4.     The nanotube formation window lies in a fuel equivalence ratio region between 1.5 and 1.9, with lower values dominated by discrete particles and higher values dominated by soot-like structures.

5.     Operating at a fuel equivalence ratio of 1.6 with a higher proportion of iron-oxide particles in flame produces the highest quality nanotube material relative to condensed material.

Definitions

Fuel Equivalence Ratio: ratio of the fuel to oxidizer present to the fuel to oxidizer in the stoichiometric case