Optimization of flame synthesis for carbon nanotubes via supported catalysts

The possibilities of flame synthesized growth of carbon nanotube (CNT) directly on a supporting substrate to be used in the final application are presented. Direct growth precludes detrimental, time-intensive processing steps and thus has great commercial viability. This approach was demonstrated using catalysts deposited upon SS meshes. Through choice of fuel based on C/H ratio and flame fuel/air equivalence ratio, a range of CO and H₂ post-flame concentrations were realized. Within the boundaries established by ethane and acetylene fuels, a region was identified as conducive to CNT synthesis. Within this H₂-CO concentration map, considerable variations in CNT yield and quality were observed. The base growth mechanism appears to dominate at low CO concentrations as very few catalyst particles are evident at CNT tips as observed in the SEM images for CO mole fractions < 0.1. In contrast, the higher CO concentrations result in tip growth where deposited catalyst particles and surface fragments become detached from the surface through breakup induced by surface carbide formation. The former mechanism has better potential for synthesis of CNT with uniform size and graphitic structure. With these criteria, using SEM and TEM imaging, CO and H₂ mole fractions near 0.105 proved optimal. Such conditions are realized by using a premixed ethylene-air mixture of φ = 1.62. This is an abstract of a paper presented at the 30th International Symposium on Combustion (Chicago, IL 7/25-30/2004).