Multi-parameter optimisation of carbon nanotube synthesis in fluidised-beds

Multi-walled carbon nanotubes (MWCNTs) were synthesised on alumina supported Fe catalysts contained within a fluidised-bed, using ethylene gas as a carbon source. The influence of temperature, deposition time, feedstock concentration, fluidisation ratio, and their interactions was investigated using a statistical design methodology to give a static global optimisation of the multi-dimensional nanotube formation space. The optimum synthesis conditions, determined from these experiments, were highly variable. The maximum MWCNT yield for 10% ethylene was found to occur at a flowrate of 6 SLPM and temperature of 850 °C. For 25% ethylene it was between 3 and 4.5 SLPM at 650 °C. However, CNTs with a higher degree of graphitisation were favoured at higher temperatures and shorter deposition times. The fluidisation ratio played a significant role in determining the overall conversion rate.