Graphite formation

Fig. 1. Carbon-phase diagram where A, solvent-cataly2ed diamond growth B—G, diamond formation direcdy from graphite C, graphite formation from diamond, D, approximate region where formation of Lonsdaleite occurs from weU-ordered graphite crystals (7,8). To convert GPa to atm, multiply by...

F.J. Derbyshire, A.E.B. Presland, D.L. Trimm, Graphite formation by the dissolution-precipitation of carbon in cobait, nickel and iron, Carbon, 13 (1975) 111-113. 

The only difference between the two samples is the gold modification of the nickel nanoclusters. In these investigations, n-butane was used to test the activity, because it is known to cause the most severe graphite formation problems. Whereas the pure nickel catalyst was deactivated rapidly as a result of the formation of graphite, as confirmed by electron microscopy, for example, it was found that the conversion catalyzed by the gold/nickel sample was maintained almost constant. This comparison is consistent with the inference that the novel gold/nickel catalyst did not... 

After the work of Zhu et al. [166], oriented growth of diamond on Ni(lOO) was studied extensively by Glass and Sitar s groups at North Carolina State University (NCSU), and a three-step process was established to suppress graphite formation by HFCVD [170-172] ... 

Bengaard HS, Norskov JK, Sehested J, Clausen BS, Nielsen LP, Molenbroek AM, Rostrup-Nielsen JR (2002) Steam reforming and graphite formation on Ni catalysts. J Catal 209 365... 

Second, to investigate the effects of Na promotion on FTS with a view to understanding of the reaction mechanism, which remains controversial in regard to the role played by alkali promoters. Ensemble effects [89], electronic effects [90,91], and a decrease in the surface coverage and in the mobility of H ad-atoms [92,93] have been variously invoked, whereas Lahtinen and Somorjai [94] propose that the principal effect of K is to inhibit graphite formation. 

Graphitization processes are carried out at 2600 to 3000°C in inert atmospheres in direct or indirect processes. In direct processes (e.g. Acheson and Castner processes) the carbon articles between the electrodes are heated, either directly or indirectly with resistive materials in between, and the required temperature for graphite formation attained by resistive heating. In indirect processes there is no physical contact between the energy source and the carbon article. 

The increase in graphite formation correlates with a decrease in relative activity when looking at a series of Fe/K catalysts. 

This mechanism occurs in a continuous process allowing the growth of diamond crystals at low pressure (usually below atmospheric pressure) and high temperature (approximately 2000° C). Hydrogen also prevents the formation of aromatic species that lead to graphite formation through a selective etching process. 

The prevailing view on the role of graphite particles is that graphite formation competes with diamond formation and is to be avoided, However, enhancement of diamond nucleation on graphite powder, graphite fiber,graphite disk,l l and graphite has been... 

Metal dusting is to be expected if metallic materials are exposed to carbonaceous atmospheres at ac > 1, i.e. subject to possible graphite formation. One may define metal dusting as a graphite growth in or into metals or alloys... 

As noted before, metal dusting is to be expected if metallic materials are carburised at carbon activities ac > 1, i.e. under a strong driving force for graphite formation. The carbon from the gas molecules should react to graphite (and, in fact, that is the overall reaction which occurs in metal dusting) and destroy the materials. As yet, two different reaction paths have been observed. For iron and Fe-based alloys, the reaction sequence is as follows (see Fig. 1.7) ... 

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