Carbon nucleation

Pytkowicz, R. M. Rates of inorganic calcium carbonate nucleation. J. Geol. 73, 196-199 (1965). 

Wheeler A. P., George J. W., and Evans C. R. (1981) Control of calcium carbonate nucleation and crystal growth by soluble matrix of oyster shell. Science 212, 1397-1398. 

Figure 13.3 shows the calculated potential energy surfaces for the oxidation of a C atom and the attachment of a C atom to a carbon nucleation center (C-C bond formation) on the (111) surface of Ni and the (111) surface of a Sn/Ni model alloy. The carbon nucleation center was modeled as a chain of carbon atoms on Ni(lll) organized in the geometry corresponding to the lowest energy. The DFT calculations indicated that on Ni(lll) the potential energy surfaces associated with the C-C and C-0 bond formation are very similar to each other. On the other hand. Fig. 13.3 also shows that on the Sn/Ni model alloy the overall activation barrier for the oxidation of carbon atoms is much lower than the overall activation barrier... 

DFT calculations have also demonstrated that in addition to the enhanced capacity to oxidize C atoms and CH fragments, the Sn/Ni surface alloy also lowers the thermodynamic driving force associated with the formation of carbon nucleation centers at low-coordinated Ni sites [16], DFT calculated adsorption energies for various carbon nucleation centers (a carbon atom, a cluster of 8 carbon atoms, and a graphene chain) on monometallic Ni and Sn/Ni surface alloy are shown in Fig. 13.4. The figure shows that for all carbon stractures, the under-coordinated sites on Ni (to model these sites, the Ni(211) surface was used) bind carbon more strongly than the under-coordinated sites on Sn/Ni(211) (in this model systan 1/3 of Ni edge atoms were displaced by Sn atoms). The reason for this is that Sn atoms break the ensembles of... 

For this reason, float-zoned silicon intended for the fabrication of semiconductor power devices is often specified -to contain low carbon concentration, e.g., less than 1 ppma. Carbon has been reported to play a role in the heterogeneous nucleation of oxide precipi-tates (37-40). In principle, carbon nucleation of oxide microprecipitates could be used to advan-tage in designing IC processes. In practice, however, the nonuniformity of the carbon distribution renders this an impractical approach. Thus in order to avoid uncontrolled variations in the oxide precipitation, low-carbon silicon is often specified for processes which use intrinsic gettering. 

Nickel carbide is not stable under SMR conditions. As a consequence, carbon nucleates in the form of whiskers after an induction period (tc), after which the carbon whisker grows at a constant rate ... 

Because of the very high supersaturation during the precipitation of most base metal hydroxides or carbonates, nucleation will be spontaneous, that is, homogeneous, which therefore explains why only limited deposition onto a preformed carrier occurs. Nucleation such as this by rapid mixing is generally much too fast for all the nuclei to diffuse into the pores and to deposit onto the internal surface of a preformed support. Therefore, significant amounts of precipitate may be formed separate from the support or on the external surface (Figure 7.2). 

In particular, time-resolved atomic resolution measurements of CNT growth show a conspicuous acceleration of the interfacial dynamics of the NP and a strong enhancement of carbon nucleation rate and CNT yield when gold interfacial impurities are introduced [92]. This is evidently a complex process to model by MD, but we can reasonably address the question of how the NP interfacial dynamics changes in the presence of Au impurities. 

The blockage of sites for carbon nucleation was the idea of the SPARG process (refer to Section 5.5), with chemisorbed sulphur passivating the ensembles for nucleation of carbon. 

We conclude from these observations that the carbon structures generated during pyrolysis cover part of the silica nanospheres, from which the aerogel skeleton is composed (Figure 1). A complete and even coverage of the inner surface by carbon cannot be expected, because part of the inner surface is not accessible and preferred growth around carbon nucleation sites is probable. Furthermore, an even distribution of carbon would lead to a very thin layer (at the obtained carbon content) which could not form graphitic structures. We therefore assume that only part of the inner surface (10-15%) is covered by a carbon layer of 1-2 nm thickness. 

Massot, L., Chamelot, R, Bouyer, F., and TaxU, P. (2003) Studies of carbon nucleation phenomena in molten alkaline fluoride media. Electrochim. Acta, 48, 465-471. 

Carbon Formation. Steam reforming involves the risk of carbon formation by the decomposition of methane and other hydrocarbons or by the Boudouard reaction (reactions (7) -(10)). Reactions (7) - (8) are catalyzed by nickel (Rostrup-Nielsen, 1984a). The carbon grows as a fibre (whisker) with a nickel crystal at the tip. The methane or carbon monoxide is adsorbed dissociatively on the nickel surface (Alstrup, 1988). Carbon atoms not reacting to gaseous molecules are dissolved in the nickel crystal, and solid carbon nucleates at the non-exposed side of the nickel crystal, preferably from Ae dense (111) surface planes. Reaction (10) results in pyrolytic carbon encapsulating the catalyst. 

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