Graphite platelet deposit

The graphite platelet deposit is formed indirectly, at the expense of the other two deposit forms and also requires the participation of a metal catalyst. It has been found that if the temperature of a metal foil, coated with carbon deposit, is raised to about 1000°C, a significant fraction of the carbon is taken into solution by the metal. On subsequent cooling this carbon is precipitated as highly crystalline graphite, underlaying excess amorphous and filamentous material remaining on the surface. This platelet deposit is the most oxidation resistant of the three... 

The details of the structural characteristics of individual constituents in the various carbon deposits were obtained by examination of a number of specimens from each experiment in a JEOL 100 CX transmission electron microscope that was fitted with a high resolution pole piece, capable of 0.18 nm lattice resolution. Suitable transmission specimens were prepared by applying a drop of an ultrasonic dispersion of the deposit in iso-butanol to a carbon support film. In many cases the solid carbon product was found to consist entirely of filamentous structures. Variations in the width of the filaments as a function of both catalyst composition and growth conditions were determined from the measurements of over 300 such structures in each specimen. In certain samples evidence was found for the existence of another type of ca naceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures. Selected area electron diffraction studies were performed to ascertain the overall crystalline order of the carbon filaments and the shell-like materials produced from the various catalyst systems. 

Carbon deposits normally have a complex structure containing several different growth forms, which can be grouped under three headings amorphous, filamentous, and graphitic platelets... 

Metal granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

Figure 8. Schematic rendition of a possible growth mechanism of the shell-like graphite deposits, where the catalyst material undergoes a wetting and spreading action with the platelet structures and the particle is progressively depleted in size as the reaction proceeds.

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