Carbon-rich surface layer

The SiC fiber monofilaments consist of a SiC sheath with an outer diameter of 142 j-m surrounding a pyrolitic graphite-coated carbon core with a diameter of 37 jim. On the surface of the SiC fiber contained two layers of carbon-rich surface coating. Each layer is a mixture of amorphous carbon and SiC. 

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. 

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. 

Of course, oxygen is not the only impurity that will react with beryllium. Another material that is important in forming mixed-material layers with beryllium is carbon. The saturated value of retention that has been found in beryllium surfaces exposed to a large deuterium ion fluence could easily be overshadowed if a carbon rich layer forms on the beryllium surface due to impurity carbon ions in the incident plasma flux. The hydrogen retention properties of plasma deposited carbon films has been shown to dominate the total retention in beryllium samples exposed to the plasma at lower temperature. Once the sample temperature during exposure approaches 500°C there is little difference between the retention in Be/C mixed-material layers compared to clean beryllium samples [48]. The temperature dependence of the retention of carbon containing mixed material layers, as well as that of clean beryllium surfaces is shown in Fig. 14.10. There are two possible explanations for the reduced retention in the mixed-material layers formed at elevated temperature. The first is that beryllium carbide forms more readily at elevated temperature and less retention is expected in beryllium carbide [11]. The second is that carbon films deposited at elevated temperature also tend to retain less hydrogen isotopes [49]. 

Examples of the application of these equations to the activities of and 2 °Th in marine sediments are presented in Figs. 7.4 and 7.5. with a half life of 5730 y is appropriate for dating sediments younger than 4-5 half lives or up to c.30 Iq BP. This includes the late Quaternary through the Holocene. Profiles of versus depth for the carbonate-rich sediment cores shown in Fig. 7.4 reveal a surface layer of constant age in the top 5-10 cm followed by linear decreases with depth that reflect sedimentation rates of 0.25 and 1 cm ky The uniform top section is mixed by bioturbation (mixing... 

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