Yoshiaki Kashiwaya1 and Kuniyoshi Ishii1
1Department of Metallurgical Engineering, Faculty of Engineering, Hokkaido University, Sapporo
The morphology and formation mechanism of carbon decomposed from CO gas on pure iron foil has been investigated at temperatures from 673 to 973 K. Carbon formed on iron foil consisted of two types of carbon, the particle-like carbon in the surface of the deposited layer and the fiberous one in the inner layer. On the other hand, the carbon deposited on powdery carbonyl iron was also fiberous. Thus the most of the precipitated carbon was fiberous. The carbon fiber had always a particle composed of C and Fe, so it appeared to grow from the particle acting as catalyst. The selected area diffraction (SAD) pattern of the particle was consistent with that of Fe-C (ASTM 3-0400), which has been known as the Fe-bearing graphite solid solution. The particle decomposed to the stable compound of Fe3C and ordinary graphite during annealing (773 K, 1.8×104 s) in He. A similar reaction proceeded under electron beam irradiation in an ultra-high voltage electron microscope (UHVEM), and it was found that the particle transformed to a stable phase through a Fe-C → Fe5C2→Fe3C process.
It was also observed by the UHVEM that each fiber was composed of a finer texture of graphite. The angle between the graphite < 002 > and the direction of fiber growth was about 20°ree and < 002 > (Fe-C)//< 002 > (graphite). The mechanism of growth of the fiber was considered from those experimental results as follows: (1) CO molecule adsorbs on Fe-C making the atmosphere around the Fe atom in catalyst. (2) The CO adsorbed decomposes to the carbon atom and CO2, and then the free carbon atom accumulates as intercalant between Fe-C(002) carbon planes. (3) As the intercalated carbon atoms increase, the atoms form a new A-stacking plane of Fe-C. (4) The old A-plane of Fe-C moves to the  direction, that is, the direction of fiber growth, and forms a new B-stacking plane of graphite. Then the growth axis and  of graphite make an angle of 20°ree, in good agreement with the value obtained by electron microscope observations.
carbon deposition, formation of carbon fiber, iron carbide, decomposition of carbon monoxide morphology, mechanism
Please do not copy without permission.