Carbon nanotubes (NTC) have been discovered in 1991 by Iijima. The selective reduction of oxide solid solutions (Al2-2xFe2xO3, Mg1-yMyAl2O4 and Mg1-zMzO with M = Fe, Co, Ni or an alloy) in H2-CH4 mixtures leads to metal particles that are nanometric even at high temperatures, thereby allowing the formation of NTC by catalytic decomposition of CH4. Composite powders NTC-Metal-Oxide are thus obtained. The study of the synthesis parameters is based on a macroscopic method involving specific surface area measurements, correlated with electron microscopy observations. We have shown that the NTC grow during the temperature ramp, above 850°C. The NTC are mostly single or double-walled and their internal diameters range from 0.8 to 6 nm, showing that only catalyst particles that are smaller than 6 nm lead to NTC. The NTC are generally present as bundles of diameter lower than 50 nm and their length can exceed 100 μm. Isolated NTC are obtained by an acidic treatment of the NTC-Co-MgO powder which entirely dissolves the oxide matrix but some of the cobalt particles, encapsulated in graphene layers, are not dissolved. A mild oxidation of the composite powder, prior to the acidic treatment, allows to increase the carbon content up to 94 at.%. Dense composites have been prepared by hot-pressing, and by hot-extrusion in order to align the NTC. Hot-extrusion is made easier in the presence of metal, and is facilitated by the simultaneous presence of the metal and NTC. The electrical conductivity of composites containing NTC is of the order of 1 S.cm-1. An electrical conduction anisotropy is measured in the case of the extruded composites, verifying the lining-up effect. The mechanical characteristics (fracture strength and fracture toughness) of the NTC-containing composites are comparable with those of the composites without NTC, although their densification is up to 10% lower. Keywords : carbon nanotubes, nanocomposites, catalytic decomposition, oxides solid solutions, nanotubes extraction, hot extrusion.
