Surface migration, high-temperature

The migration of titania from the support onto a Pt surface during high temperature reduction is clearly demonstrated and correlated with chemisorption... 

The thermal stability of the titania layer was found to vary with the metal substrate. On Pt, this oxide layer was found to diffuse into the bulk at high temperatures and segregate back to the surface at low temperatures.(19) On Rh, the titania layer did not disappear from the surface at high temperatures however, we did observe evidence for the migration of oxide... 

The surface is often described as a weak surface layer and in the case of plastics one can include the surface stresses, general contamination, the presence of abhesive ingredients, i.e., process aids which have migrated to the surface. Some high temperature moulding processes may lead to variable and unwanted oxidation and/or reversion (crosslink... 

Experiments on vapor-deposited iron films also point to an extent of adsorption of carbon monoxide of about 1.0 x 10 molecules per cm. For example, the effect of carbon monoxide on the electrical conductance of an iron film is represented in Fig. 5.6. When the level of adsorption reaches 0.7 to 1.0 x 10 molecules per cm, the conductance is unaffected by further addition of carbon monoxide. It appears that the smaller ratios measured on iron catalysts after more severe reduction treatments arise from segregation of oxides to the iron surface. The oxides are present initially within the iron and migrate to the iron surface at high temperatures. 

Additives such as antioxidants and photostabilizers of low-molecular weight face two major problems (1) they may evaporate during high temperature moulding and extrusion process or (2) they may migrate to the surface of the plastic and get extracted. There are, in general, three ways of overcoming these problems. 

Such a hindered mobility would prevent Zr02 nanoparticles from self-aggregating in the state of sol and allow for migration from the core to the surface of condensing (shrinking) silica at high temperatures. 

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