Nucleation oxygen dissociation

The nature of the nucleation site as well as the kinetics of formation and dissociation of the H-induced and stabilized platelets have yet to be experimentally studied in detail. However, their generation in float-zone (Johnson and Herring, 1988b) as well as Czochralski-grown (Fig. 8) silicon argues against the involvement of oxygen or carbon in platelet nucleation. 

The mechanism of sulphate formation is still under debate. Recently, a kinetic model has been proposed for alumina sulphation [13]. Two different types of alumina surface sites could be involved in the elementary stages. The first one would be an oxygen atom on the surface that would favor SO2 adsorption and then sulphation. The second one would allow the dissociation of oxygen. After the formation of superficial sulphates on the surface of the alumina, a nucleation of core sulphates, via a slow process, causes formation of irreversible chemical species. 

The formation of whisker carbon cannot be tolerated in a tubular reformer. The important question is whether or not carbon is formed, and not the rate at which it may be formed. In terms of the growth mechanism, it means to extend the induction period (to in Equation 5.1) to infinity. This is achieved by keeping the steady-state activity of carbon smaller than one (refer to Section 5.2.4). The carbon formation depends on the kinetic balance between the surface reaction of the adsorbed hydrocarbon with oxygen species and the further dissociation of the hydrocarbon into adsorbed carbon atoms, which can nucleate to whisker carbon. However, this approach is complex and there is a need for simple guidelines using simple thermodynamic calculations. 

In addition to adsorption, dissociation or ionization of the oxygen (i.e. charge transfer from zinc to oxygen) it is also possible for phase formation to be rate-determining (nucleation, early stages of layer formation) [489]. Whatever the decisive elementary step is, its rate will after a very short transient, to a good approximation, not be explicitly dependent on the layer thickness. If the surface reaction is stationary then L is time independent and we obtain a linear growth law... 

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