Popis: |
The structure of carbon deposited on metal substrates has been studied by thermally decomposing methane over iron, cobalt and nickel surfaces. Methane decomposition temperatures were 650°C and 750°C in the case of iron and 750°C for cobalt and nickel. Transmission electron microscopy established the presence of two distinct types of carbon deposit—termed as “flake” and “polycrystalline”. Electron and X-ray diffraction properties of the former afforded evidence for its perfectly crystalline graphite nature. Surface perfection and reactivity studies also revealed the equivalence of “flake” carbon to natural or synthetic graphite. There was also some evidence of the metal substrate within specimens of this type of deposit. “Polycrystalline” carbons were characterised by their electron diffraction properties, which were indicative of a non-graphitic structure. Fibrous specimens of “polycrystalline” carbon displayed preferred orientation of the component carbon crystallite basal planes parallel to the fibre axis. Evidence of the metal substrate within this material was given by electron microprobe analysis and oxidation techniques. |