Oxidation model silicon, discussion

This paper deals with the activity and para-selectivity of small crystals of H-ZSM-5 zeolites modified by various metal cations located at different sites. The effect of Fe, Mn (which can be expected to enhance coke removal) and Al cations, modelling an electron acceptor site without redox properties, located mostly in the zeolite channels or on the zeolite outer surface, has been investigated. Attention has been paid to the alkylation of toluene with ethylene, including coke formation and its removal by oxidation. The surface deposition of silicon is discussed to explain some effects of the metal cation location in the zeolite on its para-selectivity. 

The conclusion that can be drawn from the experiments just discussed is that, except for the very small particles, the photoluminescence of our Si nanocrystals, which are produced by CO2 laser-assisted pyrolysis of silane and which are gently oxidized in air under normal conditions, can be perfectly explained on the basis of the quantum confinement model, that is, by the radiative recombination of electron-hole pairs confined in the nanocrystals [15]. In order to obtain high quantum yields, the nanoparticles must be defect-free in particular, they must be perfectly monocrystalline and all dangling bonds must be passivated, for example by a silicon oxide layer. Indeed, high-resolution electron microscopy (HREM) studies have shown that our Si nanoparticles are composed of a perfect diamond-phase crystalline core and a surrounding layer of SiO [19]. 

It has been speculated that there is a common origin of the reduced chemical etch rate for (111) oriented silicon substrates and for highly p-type doped substrates. But the electrochemical investigations discussed above indicate that the passivation of highly doped p-type Si can be ascribed to an oxide film already present at OCP, while no such oxide film is observed on (111) silicon below PP. This supports models that ascribe the reduced chemical etch rate on (111) planes to a retarded kinetic for Si surface atoms with three backbonds, present at (111) interfaces [Gil, A12], as discussed in Section 4.1. 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

A quantitative description of the diverse morphological features of PS requires the integration of the aspects discussed above as well as the fundamental reaction processes involved in silicon/electrolyte interface structure, anodic dissolution, and anodic oxide formation and dissolution as detailed in Chapters 2-5. Any mathematical formulation for the mechanisms of PS formation without such a global integration would be limited in the scope of its validity and in the power to explain details. In addition, a globally and microscopically accurate model would also require the full characterization of all of the morphological features of PS in relation to all of the... 

Multi-component ceramics allow the optimization of various physical properties. These include ceramics which form multi-component oxides as well as fiher-rein-forced ceramic matrix composites. However, the oxidation behavior of these materials is complex compared with the pure materials. The leading fiber-reinforced composites are silicon-based and contain continuous SiC fibers with coatings of graphitic carbon or hexagonal boron nitride. The oxidation of the fiber coating at intermediate temperatures is a major issue and models of this process are discussed for both carbon and boron nitride coatings. 

To test the ideas discussed in the previous section, we considered the model system most widely used for the investigation of the effects of confinement in thin polymer films - polystyrene on wafers of silicon covered by a native oxide layer [23]. 

---