Surfaces phase segregation

In the particular case of nickel-copper alloys their hydrogen pretreatment may result in phase segregation (48), at least at the surface. The desegregated rich in nickel alloy can display its relatively high catalytic quality and even keep it down to a certain temperature (lower than in the case of nickel itself), which would be the critical temperature of a given Ni-Cu-H system. 

J.C. Shelton, H.R. Patil, J.M. Blakely, Equilibrium segregation of carbon to a nickel (111) surface A surface phase transition, Surface Science, 43 (1974) 493-520. 

In principle, the molar ratio of different thiols in a mixed SAM is the same as their original molar ratio in the solution which was used for the formation. In other words, for a mixture of two thiol compounds which does not show demixing tendencies (phase segregation), a random attachment of both compounds onto the surface can be assumed [16]. This observation offers the potential to mix a cw-substituted alkane thiol with short-chain nonsubstituted thiols. As the result, anchor molecules are available for which steric hindrance is minimized (cf. Fig. 3). 

The observed enhancement in oxygen index could be attributed to phase segregation in these block copolymers, which leads to domination of siloxane on the polymer surface. Siloxanes have solid-phase activity rather than vapor-phase activity and reduce flammability through increased formation of pyrolytic char. 

In a study of the application of Pd-Si amorphous alloys as selective hydrogenation catalysts [3] it was found that in situ activation provides a route to active and selective catalysts, whereas ex situ activation caused the crystallization of the system into the thermodynamically stable Pd + SiC>2 system, which is indistinguishable in its activity and poor selectivity from conventional catalysts of the same composition. In this study it was possible to show conclusively that all amorphous alloys are not amorphous on their surfaces as they undergo, in reaction gas atmospheres, chemically-induced phase segregation which starts the crystallization process according to Figure 2 (pathway 2). 

Furthermore, the friction forces acting in the flow field can induce phase segregation at the mould surface [189]. As pointed out by Cakmak and Cronin [191], in PP/EP blends with a high content of EP particles even shear amplification phenomena may occur due to the presence of the small rubber particles. The shear amplification results from considerable shear fields occurring in small gaps between rubber particles which in turn are subjected to the macroscopic shear field extended over the whole width of the sample. 

Catalytic molecular surface species may undergo drastic changes in their structure in the presence of reactants. For example, polymeric clusters may transform into highly distorted monomeric species. A crystalline phase may become mobile at its Tammann temperature, as shown by Raman spectroscopy, and it may spread over oxide supports driven by the reduction of the overall surface free energy. Reactive environments trigger many structural transformations, exemplified by particle sintering, dispersion of bulk phases, segregation of surface species into bulk phases, and solid-state reactions between supported oxides and supports. 

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