Protective surface layer

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

Besides pure chemical corrosion, solid products of corrosion in the system will give rise to erosive corrosion, in which the particles moving with the fluid will impact onto the surfaces and can remove protective surface layers. Such corrosion effects are most pronounced in regions of high fluid-stream velocity. 

At least four different explanations have been proposed to account for parabolic kinetics. The oldest and best established is the "protective-surface-layer" hypothesis. Correns and von Englehardt (6) proposed that diffusion of dissolved products through a surface layer which thickens with time explains the observed parabolic behavior. Garrels ( 12, 1 3) proposed that this protective surface consists of hydrogen feldspar, feldspar in which hydrogen had replaced alkali and alkaline earth cations. Wollast (j>) suggested that it consists of a secondary aluminous or alumino-silicate precipitate. In either case, a protective surface layer explains parabolic kinetics as follows If the concentration of any dissolved product at the boundary between the fresh feldspar... 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

The preparation of both, the particles themselves and the protective surface layer, has direct influence on their cytotoxicity. It is common belief that in the case of core/shell nanoparticles, properly prepared, close shell or multiple shells such as ZnS/Si02-shells prevents the leakage of toxic elements and thus makes cytotoxicity unlikely. Naturally, a better solution is to avoid cytotoxic materials in the first place. QDs, for example, can be synthesized without utilization of any class A or B elements InP/ZnS QDs have photophysical properties comparable to those of CdSe-based systems [43, 93]. Principally, whenever a new approach for QD synthesis or coating is used or if the QDs are applied in an extreme environment that could compromise their integrity, it is recommended to assess their cytotoxicity. 

Velbel, M.A. (1993) Formation of protective surface layers during silicate-mineral weathering under well-leached, oxidizing conditions. 

Various surface analysis techniques show that silicate glasses rapidly develop surface compositional profiles when exposed to water. When water is present as a vapor an alkali-rich layer (presumably a hydrated alkali carbonate) forms over the SiOj-rich layer. Water as a liquid dissolves the alkali and leaves the silica-rich film. As long as this SiC -rich film is stable the rate of corrosion due to diffusion is reduced with exposure time. Addition of multi-valent species to the glass or reactant results in formation of a complex protective surface layer in the glass which may be stable over a wide range of environmental conditions. 

Formation of protecting surface layer before or after ignition being in competition with the heat-induced decomposition of the polymer chains and oxidation of the formed fragments by radical process... 

Stabilization of the thickness pore size heat and gas barrier performance of the protecting surface layer, hindrance of the dipping of the melt phase... 

The efficiency of intumescent fire retardants could be enhanced by interlayers that deliver the active components to the surface (shown by two examples). The fire-retardant additives, delivered to the surface at early stage of combustion, accelerate the formation of protecting surface layer that hinders the degradation of the underlying material. This coating structure could be reinforced by an interlayer of ceramizing capability (e.g., polyborosiloxane). Phosphorus-free intumescent fire-retardant system could be formed by using such additive. 

With the exception of sputtering by C+ ions, the physical sputtering of Be and W appears to be well documented. Carbon bombardment, in general, leads to the deposition of protective surface layers (see Sect. 9.2.2). Only in conditions where carbon self-sputtering exceeds unity, i.e., at grazing incidence or at temperatures above 1800 K, could a few yield data points be obtained for clean W surfaces. 

Osawa and Doi (1992) described a surface layer of plasma-polymerized amorphous C over the transport layer to retard ozone interactions with the transport material along with increased transport material at the interface to compensate for any material that may be destroyed. The reasoning is that without a protective surface layer, the transport layer is destroyed by ozone but the surface is continually abraded so the cycling characteristics are relatively unaffected. However, in an overcoated photoreceptor the surface layer cannot be worn away and destruction of transport material by ozone results in a rising residual potential with cycling. This effect can be reduced by having a higher concentration of transport material at the interface. 

The major function of the oral epithelium is to provide a protective surface layer between the oral environment and the deeper tissues. The oral epithelium has a squamous epithelium of tightly packed cells that form distinct layers by a process of maturation from the deeper layers to the surface. The pattern of maturation differs in different regions of the oral mucosa... 

The implantation method will always be superior when very thin protective surface layers are needed or if one wants to form alloys for which low solubility limits forbid the use of conventional techniques. The work done until now has shown that implantation is a very good method to do basic research in the field of corrosion and oxidation. The fact of allowing every possible combination of surface alloys and a very low and controlled concentration of impurities as well as studying the influence of defects is unique. Therefore, this method should play a more and more important role in corrosion science in the future. 

Silicon is extremely important as a building block of unicellular algae -diatoms. The main building material of the frustule (the finely sculptured protective surface layer of diatoms) is a float-stone, an opal-like water-containing polymer of silica Diatoms are the only group of organisms whose development is totally dependent on the presence of soluble forms of silica in the environment. When silicon sources run out, DNA replication stops. 

X 10 torr seconds. Auger electron spectroscopy and 16) has shown that a layer which can be represented as Fe O OH protects iron from further attack. Direct evidence of hydrogen being present in the passive film formed in solution was provided by mass spectrometry (30). Electron diffraction (2, 29) has shown tHat the protective surface layers wHTch form on iron which has been air oxidized or passivated in solution are surprisingly similar. The structures are based on the cubic close packed oxygen lattice of y Fe O OH, or Fe304 ... 

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