THE DUAL NATURE OF VAPOR-GROWN CARBON-FIBERS

Thin carbon filaments, grown from hydrocarbon-hydrogen mixtures at 700-1100-degrees-C (T(G)) on iron particles of different size (S(M)), and thicker fibres (resulting from depositing pyrolytic carbon on to filaments) have been studied by scanning and transmission electron microscopy (SEM and TEM). At T(G) > 1000-degrees-C filaments were hollow tubes with walls formed from hexagonal carbon layers, which, for S(M) < 50 nm, formed closed continuous co-axial cylinders parallel to the filament axis. These filaments, which are radially symmetric approximations of single crystals of graphite, have very good strength and electrical conductivity. For S(M) > 50 nm the layers were inclined at up to +/- 30-degrees to the filament axis, causing filaments to be very weak. At T(G) < 1000-degrees-C filaments were either segmented, consisting of hexagonal carbon layers conforming to the bottom shape of the iron particle, or were flat ribbons formed by stacking hexagonal carbon layers normal to the filament axis. The structure of the pyrolytic carbon was independent of that of the filament substrate and of the conditions of growth.