Structural inhomogeneity of carbon fibres grown from carbonaceous gases ona substrate

In the present work the structural inhomogeneity of carbon fibres grown fro m the vapour phase, vapour grown carbon fibres (VGCF), on a substrate has b een characterised. Two kinds of VGCF and three experimental techniques have been used for this purpose. Electrical resistivity measurements at room te mperature have been performed on single fibres before and after a thinning process by means of controlled oxidation in air. Moreover, electrical resis tivity and transversal magnetoresistance measurements have been accomplishe d on both sorts of VGCF. It has been demonstrated the presence of an inner part or core in this type of carbon fibres, which is characterised by a turbostratic structure, and of an outer or cortical layer with a lower degree of crystalline perfection . The turbostratic structure of the core has been evidenced by its negative magnetoresistance. The cortical layer is made up of concentric layers. Thi s fact is reflected in an increase of electrical resistivity with fibre dia meter, as well as in the way they get thinner by controlled oxidation in ai r at high temperatures (gasification in sublayers). The presence of outer t hick concentric sublayers in VGCF, and the change in the deposition rate of pyrolytic carbon is evidenced by a decrease in resistivity, and by a chang e in behaviour of the electrical resistivity with fibre thinning.