Silicon-containing layer, surface

As can be concluded from Fig. 4, the initial reactivity of the surface of Sited, is very high The relevant copper species present, possibly copper chlorides, which can easily form copper-silicidic phases by reaction with silicon, can easily attack the whole of the surface and react with surface silicon, resulting in Cu-Si species and finally metallic copper, e g, [26]. Due to this very fast formation of catalytically active Cu-Si species and of precursors thereof, the reaction becomes very fast already after a short time. But, on the other hand, this overall attack on the silicon surface gives copper species the possibility to be deposited practically over the whole of the silicon grain This means in terms of our model of catalytically active Cu-Si surface species, explained in the first section of this paper, that there is a lack of still free silicon surface area, which is needed in order to form the active "two-dimensional" Cu-Si species. The surface is simply blocked by thick copper-containing layers. As consequence, the reaction goes down after a short time and the contact mass reaches only low stationary activity. 

In contrast with the oxide ceramics, the nonoxide ceramics are not thermodynamically stable in oxidizing environments, although their stability in other chemical media is excellent. Materials of silicon-containing compounds can nevertheless be used under oxidizing conditions up to ca. 1600°C, because a passivation layer of Si02 is formed at the surface which strongly hinders further oxidation. 

Schlichting, J., Oxygen Transport Through SiUca Surface Layers on Silicon-Containing Ceramic Materials, High Temp.-High Press. [14] 717-724 (1982). 

Ciliberto and al. used XPS to study the penetration depth of the products introduced by Sanpaolesi s treatment in the marble matrix of the Donatello pulpit. The masterpieces of the famous Italian sculptor were in fact restored with a mixture of ZnSiFe and MgSiFe in 1941. The surface analysis showed that both the fluoride- and silicon-containing compoimds enrich the most external layers of the marble. Two small fragments, about 1.0-0.5cm wide and <0.5cm thick, were studied fi om a chemical point of view and replaced to their original position after the study in an attempt to minimize any damage or alterations to the work of art as awhole[48]. 

It is known that there are two modes of oxidation of the SiC surface (1) passive oxidation, in which Si02 layer is formed on SiC surface passivating the surface aud (2) active oxidation, in which the silicon-containing products are gaseous, such as Si or SiO [46-50]. Figure 4.17 shows a phase diagram for the interaction of O2 with SiC surface [50]. 

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