The initiation and growth of filamentous carbon from α-iron in H₂, CH₄, H₂O, CO₂, and CO gas mixtures

The initiation and growth mechanisms of filamentous carbon over iron foils were studied at 900 K and 1 bar pressure. Various gas mixtures of CO, CO₂, CH₄, H₂, and H₂O were used to fix the solid phase compositions based on nonequilibrium phase diagrams. Solid phase compositions were verified using X-ray and electron diffraction. Gravimetric analysis indicated that in dry gas mixtures the initial rate of fractional weight gain was a direct function of the P_CO P_H2 product; for water containing experiments it was related to the P_H2O P _H2P_CO ratio. X-Ray diffraction analysis of the solid suggested that the maximum rate of fractional weight gain coincided with complete carbiding of the "surface" layers. Examination of the foils in an electron microscope indicated the surface breaks up into a nodular morphology, and these nodules are comprised of filamentous carbon. An initiation mechanism is proposed which assumes that Fe₃C acts to increase over all surface area through surface breakup and also acts as a catalyst for carbon deposition and subsequent filament growth.