RESISTANCE OF REFRACTORY PRODUCTS TO CARBON MONOXIDE: KINETICS OF CARBON DEPOSITION IN CONSIDERATION OF APPARENT CARBON MODIFICATIONS

摘要

Recent studies performed at the Research Association for Refractories (FGF) in Bonn emphasise the importance of the carbon modification which is deposited in the refractory material: High resolution transmission electron microscopy clearly show beside minor amounts of microcrystalline graphite the appearance of at least two macromolecule like carbon structures. In an other scientific context they are very well described in the literature and known as "bucky onions" and "carbon nanotubes". Both can be deduced from the fullerenes with their most prominent species, C60. The growing rate of carbon in refractory products applied in reducing atmosphere is significantly controlled by the predominant carbon modification: While carbon nanotubes will expedite the carbon deposition, bucky onions on the other side will decrease the total growing rate. This is due to the fact that the latter effectively entraps and inactivates the catalytic iron particles without the formation of destructive amounts of carbon. On that basis of knowledge and under consideration of the appropriate literature, the formation process of destructive carbon emplacements into ceramic structure is discussed: The emplacement of carbon in principle requires the formation of liquid iron nano droplets which subsequently absorb carbon till the saturation limit. Thereafter hexahedral carbon structures are segregated on the surface of the droplet. This kind of formation is technically known as VLS (vapour-liquid-solid) process. At present it is still in discussion which parameters are responsible for what carbon modification, nano tube or bucky onion. First investigations imply that the ambient conditions (temperature and gas composition) are of basic importance.