Abstract
Issues to be addressed for improving the properties of carbon nanotubes include the control of their shape and innermost and outermost diameters. In this study, carbon nanotubes were synthesized from methane as a raw material using nickel catalyst with different particle sizes supported by alumina (Ni/Al2O3 catalyst) using a microwave plasma technique. The effects of irradiation time and nickel particle size on the formation of carbon nanotubes were investigated, focusing on the composition of decomposed gases and carbon yield. The particle size of the catalyst (10 and 30 nm) was controlled using thermal growth. The results of a synthesis experiment using nickel catalyst with a particle size of 10 nm indicated that carbon yield tended to increase with increasing microwave irradiation time. In this case, the methane conversion was constant at 95% and acetylene selectivity decreased with longer irradiation time. SEM observation of deposited carbon confirmed that filamentous and granular forms of carbon were formed and that the filamentous carbon consisted of straight and helical carbon structures. The proportion of granular carbon decreased with increasing irradiation time. TEM observation confirmed that multi-wall carbon nanotubes with 10 graphite walls were formed, regardless of irradiation time. The carbon nanotubes formed using nickel catalyst with a particle size of 30 nm had a larger outermost diameter and a larger number of graphite walls than those formed with a particle size of 10 nm, suggesting that difference in nickel particle size contributes to the radial growth of carbon nanotubes.
