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

Abstract

Oxides based on α-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–CH 4 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–Al 2O 3 nanocomposite powders may also contain Fe carbide 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 α-Al 1·8Fe 0·2O 3 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 Fe 2O 3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat% Fe (α-Al 1·9Fe 0·1O 3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from α-Al 1·8Fe 0·2O 3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH 4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality.