Silica layer

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

When using the thin silica spacer layer, however, it was found that the results from the above-mentioned methods did not agree with the direct measurements from the Taly-surf profilemeter, as shown in Fig. 4(a). This was tentatively ascribed to the effect of penetration of the reflecting beam into the substrate. With a very thin silica layer, the depth of penetration and thus the phase change would depend upon the thickness of the silica spacer layer and also upon that of any oil film present. 

It is very clear that silicon is one of the most important materials in modern technologies, especially in electronics. Silicon is also one of most common element on the earth. Silicon surface is readily oxidized under ambient condition. A silicon substrate is covered by a silica (SiO c) layer. This silica layer can be controlled easily by chemical reagents, heating, electrochemical treatment, and so on. 

The separations of some nonionic tensides having biological activity and consisting of ethyleneoxide oligomer mixtures were performed in many different TEC systems (silica and alumina as the stationary phase and single solvent or binary mixtures as the mobile phase). Selectivity was higher on alumina than on the silica layer. Both... 

FIGURE 11.8 Correlation of R, values of selected furanocoumarins meassured on Florisil and silica layers developed with 5% diisopropyl ether in CH2CI2 + H (7 3) solid circles and with 15% AcOEt in B (open circles). For abbreviations see Figure 11.5. (For details, see Waksmundzka-Hajnos, M. and Wawrzynowicz, T., 7. Planar Chromatogr., 5, 169-174, 1992.)... 

Also, different selectivity systems of were nsed for the separation of the alkaloid fraction from Corydalis solida herb. The extract was fractionated by the nse of a sihca layer elnted with 10% propanol-2 in dichloromethane (see Fignre 11.15a). Fraction 1 elnted dynamically from the adsorbent was rechromatographed by the nse of silica layer and eluent of higher strength containing acetonitrille + propanol-2 + acetic acid + dichloromethane. It enables the separation of the six zones of alkaloids from fraction I (see densitogram in Figure 11.15b). 

A considerable number of systems have been used to separate chlorophylls on thin layers [30,31]. The most readily applicable layers are prepared from cellulose, silica, or sucrose and use hydrocarbon carriers with a polar modifier, usually acetone, in the developing solvent. However, silica layers cause a level of decomposition that is unacceptable for preparative work. Sucrose layers offer no particular advantages in separation and are neither commercially available nor recommended. 

Anthraquinone glycosides and aglycones can be readily separated on silica layers rising moderately polar developing solvents [41 3]. The best such solvents eonsist of ethyl acetate modified to increase polarity by the addition of alcohols or water for the glycosides or changed to decrease polarity by inclusion of hydrocarbon components. 

Aluminum was quantitatively determined by spectrophotometry (Amax 540 nm) after extracting from silica layer with double-distilled water and developing the color with 0.1% aqueous aluminon. 

The latest innovation is the introduction of ultra-thin silica layers. These layers are only 10 xm thick (compared to 200-250 pm in conventional plates) and are not based on granular adsorbents but consist of monolithic silica. Ultra-thin layer chromatography (UTLC) plates offer a unique combination of short migration distances, fast development times and extremely low solvent consumption. The absence of silica particles allows UTLC silica gel layers to be manufactured without any sort of binders, that are normally needed to stabilise silica particles at the glass support surface. UTLC plates will significantly reduce analysis time, solvent consumption and increase sensitivity in both qualitative and quantitative applications (Table 4.35). Miniaturised planar chromatography will rival other microanalytical techniques. 

Klotz, M., Ayral, A., Guizard, C. and Cot, L. (2000) Synthesis conditions for hexagonal mesoporous silica layers. Journal of Materials Chemistry, 10 (3), 663-669. 

Migration of Ca2+ and P043- groups to the surface through the silica layer. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

Rayss J., Sudolski G., Ion absorption in the porous sol-gel silica layer in the fibre optic pH sensor, Sens. Actuat B 2002 87 397-405. 

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