Silicon-rich surfaces

The synthesis of these materials is outlined in Scheme I. Transmission electron microscopy shows that the morphology of nearly equimolar compositions of the siloxane-chloromethylstyrene block copolymers is lamellar, and that the domain structure is in the order of 50-300 A. Microphase separation is confined to domains composed of similar segments and occurs on a scale comparable with the radius of gyration of the polymer chain. Auger electron spectroscopy indicates that the surface of these films is rich in silicon and is followed by a styrene-rich layer. This phenomenon arises from the difference in surface energy of the two phases. The siloxane moiety exhibits a lower surface energy and thus forms the silicon-rich surface layer. 

Si Al ratio of the outer layers (ca. 10 A) of the zeolite crystals. The extent to which the surface composition differs from the bulk composition appears to depend on preparation conditions, and all three possible situations (silicon rich surface, silicon deficient surface and surface composition equal to bulk composition) have been reported (refs. 12-14). Variations in aluminium distribution have also been probed by high resolution scanning electron microscopy (ref. 15) and energy dispersive X-ray analysis (ref. 16). 

Build-ups of coke in the coils of a pyrolysis furnace and in the transfer line exchanger necessitate shut-down of the unit for decoking purposes generally every one to six months. Many factors affect the rates of coke formation and collection on the surfaces of the coils and the transfer line exchangers. including the composition and roughness of the metal surfaces. For example. significantly less coke results on aluminized surfaces <1. 2) and on silica-rich or silicon-rich surfaces (3). Furthermore, more coke is normally formed on stainless steel surfaces that have been coked and decoked once as compared to new surfaces (2). 

This is a silicone modified analog of Biospan with a different stabilizer. It possesses a silicone-rich surface to enhance thromboresistance while maintaining the bulk properties of Biospan (Tables 4.3,4.12,4.13, and 4.14). 

A 1989 review of the literature by Clarke gave many examples which support the importance of copper-silicon rich phases near the surface during the MCS reaction. Clarke noted that CusSi (eta phase) forms above 880 °C but will form at 350 °C in the presence of chloride ion. Methyl chloride reacts with copper to form copper chloride which then serves as the chloride source needed for formation of the eta phase, thus explaining the shorter induction period obtained using copper chloride vs other copper catalysts. The mechanism of replacement of silicon from the surface is by diffusion of copper into the bulk silicon to reform a copper-silicon rich surface. Iron-silicon phases stabilize the eta phase and metal promoters catalyze chloride transfer, e.g. see equation 2. Silicon also reacts with ZnCl2 and AICI3. Excess zinc causes unproductive decomposition of MeCl to give methane. Finally, Clarke presented data that ruled out the importance of methyl radicals in the MCS reaction. 

One can readily note the close correlation between the observed variations of the catalytic activity and the evolution of surface nickel concentration (Figure 3A). However, the dramatic difference between the activity of nickel rich alloys [(Nl Sl2) and (Nl2Sl) ] and silicon rich Intermetalllcs [(Nl.Sl.) and (filSl.) ] tar exceeds... 

Successive burial of hydrogen-rich surface layers leads to the formation of the a-Si H material. The large amount of hydrogen at the surface is to saturate the surface dangling bonds. The much lower hydrogen content in the bulk is due to the solubility limit of hydrogen in silicon. 

More recent support has appeared for the importance of the copper-silicon rich phase on the silicon surface in the MCS reaction. Lieske and coworkers74 showed that redispersion of the eta phase can be an element of the induction period of the MCS reaction and seems to be brought about by the reaction itself. The Cu-Si surface species, perhaps Cu-Si surface compounds or extremely small Cu-Si particles, seem to be of similar importance as X-ray detectable Cu-Si phases. 

Blends of this copolymer with polystyrene were analysed by contact angle measurements and Rutherford backscattering (RBS) both of which showed that the surface was silicone rich. 

Depth profiles are usually presented as atomic concentrations versus sputter time, assuming we know the rate at which the sample sputters. A typical depth profile is shown in Figure 25. It is interesting to see that at the surface there is carbon, silicon dioxide and some molybdenum. As soon as the surface layer is sputtered off (300 A), the oxygen and carbon impurities drop to constant and small values. For this CVD film, the molybdenum silicide came out to be very silicon rich. We can also see that the stoichiometry of the silicide changed with position (depth) in the film. 

The Ru02 particles can not be reduced at room temperature, but reduce readily at 773 K. The ruthenium particles produced after this reduction procedure are estimated to be 16 nm in diameter from x-ray diffraction line width analysis. The reduction results in further loss of crystallinity, reflected by a drop in surface area and microporosity (Table 2). In addition, the position of the asymmetric T-O stretching vibration is at 1071 cm" 1, indicating a very silicon-rich material. 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

Unlike Voorhoeves model, in our concept XRD undetectable Cu-Si surface species with a very low total copper content steadily react with methyl chloride and can be steadily restored by copper dififtision from a copper source like q-CusSi onto the silicon surface. The plausibility of this idea was demonstrated in Fig. 1. Both Voorhoeve s and our model predict a break-down of the reaction, as soon as the silicon grain surface is essentially covered by high amounts of copper-rich species. As a matter of fact, the models only differ in the assumption about the relative mobility of the atoms involved. The high mobility of copper atoms in the Cu/Si system [19, 20], which has been shown only after Voorhoeves work, seems to favor our proposal. 

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