Carbon nanotubes Fe alumina nanocomposites. Part I: Influence of the Fe content on the synthesis of powders

Oxides based on a-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H-2-CH4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe cap bide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i.e. a smaller average tube diameter and/or, more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using alpha-Al1.8Fe0.2O3 as starting compound, i.e. the maximum Fe concentration (10 cat%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe2O3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat% Fe (alpha-Al1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from alpha-Al1.8Fe0.2O3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (less than or equal to 10 cat%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality. (C) 1998 Elsevier Science Limited. All rights reserved.