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Carbon nanofibres (CNF) have been formed by polyethylene pyrolysis, using a Ni catalyst and a hydrogen–argon gaseous mixture. The temperature interval of 500–700 °C was chosen, because small sized Ni catalyst particles are formed in this case. TEM and high resolution TEM studies revealed catalyst particles of spherical, oval, conical and faceted shapes, some of which have been twinned. The variety of shapes and sizes can be explained by the different values of the surface tension forces in the catalyst and in graphite in the various cases. Unusual ‘burst’ CNFs, distinguished by the alternation of dense and sparse regions, have been found in the deposit. A model of a periodic process, based on the decay of Ni carbide, is assumed for the explanation of the ‘burst’ CNF formation.

The paper reports the synthesis of carbon nanotubes from ethanol over group VIII (Fe, Co, Ni) catalysts derived from corresponding metallocenes. Several unexpected cooperative effects are reported, which are never observed in the case of individual metallocenes such as the commonly used ferrocene catalyst Fe(C5H5)2. The formation of very long (up to several µm) straight monocrystal metal kernels inside the carbon nanotubes was the most interesting effect. The use of trimetal catalysts (Fe1-x-yCoxNiy)(C5H5)2 resulted in the sharp increase in the yield of carbon nanotubes. The electrical conductivity of the produced nanotubes is determined by the nature of the catalyst. The variation of individual metals in the Ni-Co-Fe leads to a drop of the electrical resistivity of nanotube samples by the order of magnitude, i.e., from 1.0 × 10−3 to 1.1 × 10−5 Ω∙m. A controlled change in the electrophysical properties of the nanotubes can make it possible to expand their use as fillers in composites, photothermal and tunable magnetic nanomaterials with pre-designed electrical conductivity and other electromagnetic properties.

The effect of an electron beam on nanoparticles of two Fe carbide catalysts inside a carbon nanofiber was investigated in a transmission electron microscope. Electron beam exposure does not result in significant changes for cementite (θ-Fe 3 C). However, for Hägg carbide nanoparticles (χ-Fe 5 C 2 ), explosive decay is observed after exposure for 5-10 s. This produces small particles of cementite and γ -Fe, each covered with a multilayer carbon shell, and significantly modifies the carbon-fiber structure. It is considered that the decomposition of Hägg carbide is mostly due to the damage induced by high-energy electron collisions with the crystal lattice, accompanied by the heating of the particle and by mechanical stress provided by the carbon layers of the nanofiber.