Surface etching

In practical applications, gas-surface etching reactions are carried out in plasma reactors over the approximate pressure range 10 -1 Torr, and deposition reactions are carried out by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV below 10 Torr) or by chemical vapour deposition (CVD) in the approximate range 10 -10 Torr. These applied processes can be quite complex, and key individual reaction rate constants are needed as input for modelling and simulation studies—and ultimately for optimization—of the overall processes. 

The SEM can also be used to provide crystallographic information. Surfaces that to exhibit grain structure (fracture surfaces, etched, or decorated surfaces) can obviously be characterized as to grain size and shape. Electrons also can be channeled through a crystal lattice and when channeling occurs, fewer backscattered electrons can exit the surface. The channeling patterns so generated can be used to determine lattice parameters and strain. 

Figure 7. Example of space-resolved photoinduced microwave conductivity mapping of semiconductor interface distribution of photoconductivity in natural pyrite (from Murgul, Turkey, surface etched in acid solution). The overflow was adjusted to show patterns of low photoactivity. For color version please see color plates opposite p. 452.

After the cleaning process, other techniques are used to prepare the surface of the substrate for coating. Some techniques include drying, surface etching, and chemical surface preparation. Examples of chemical surface preparation include the formation of an oxide layer or the monolayer assembly of an adhesion promoter on the surface. These processes modify the surface of the substrates so as to facilitate the subsequent deposition process. In surface preparation, frequently, the hydrophilic/hydrophobic character of the surface is controlled to match the coating solution properties. For example, Van Driessche et al.19 reported on improving the wettability of Ni-4at%W tapes... 

For all runs, there is a clear decrease in measured pit density with increasing C/CQ ratio. In the closed experiments, pit densities reach background levels (<3 x 10 cm ) at C/C = 0.75. In the flow experiments, samples from crystal R5 (etched for 6.5 hours) and R5SE (crystal R5 after etching 6.5 hours cleaned and re-etched for 25 more hours) show background levels (1 x 10 cm"" ) above C/CQ 0.8. Figure 3 shows R5SE surfaces etched above and below 0.8 CQ. 

Figure 2. The effect of dissolved Si concentration on etch pit density on quartz surfaces etched a) in sealed autoclaves for 6.5 hours, b) in a flow reactor for 6.5 hours (R5), 31.5 hours (R5SK), and 25-28 hours (R9). Reproduced with permission from Ref. 16. Copyright 1986 Pergamon Press.

Low temperature etching. Our data suggests that, under hydrothermal conditions the rate of pit formation is dramatically reduced, although perhaps not completely stopped, at C = Ccrjt. Etch pits on a natural, hydrothermally-etched quartz surface therefore indicate extended dissolution times, but not necessarily etching at C < Ccrit This is because the rate of etch pit formation even above Ccr t can be significant at elevated temperatures (as shown by crystal R9). However, at low temperatures, formation of etch pits when C > C would be less likely, and natural surfaces etched at low temperature should record the saturation state of the etching fluid. 

Fig. 11A-C. Scanning electron micrographs of fused silica capillary surfaces etched with methanolic ammonium hydrogen difluoride solution. (Reprinted with permission from [78], Copyright 2000 Elsevier). Etching process was carried out for A 3 h at 300 °C B, 2 h at 300 °C and 2 h at 400 °C C 2 h at 300 °C and 1 h at 400 °C...

FIGURE 1.3 Schematic representation of the silanization procedure of borosilicate or fused silica capillary column inner walls, (a) Surface etching under alkaline conditions, (b) attachment of reactive groups by condensation with silanol, (c) chemical linkage of polymer (PS/DVB considered as example) by free radical polymerization. 

M.J. Larsen, O. Fejerskov, Surface etching and subsurface demineralization of dental enamel induced by a strong acid, Scand. J. Dent. Res. 85 (1977) 320-326. 

Fig.1 The scheme of anisotropic surface etching and isotropic deposition of Au—TMTU complexes at step edges of the Au(ni) surface in TMTU-containing electrolytes [23].

In the final chapter, Wei and Phillips tie together old and new results characterizing the processes of surface etching. They summarize evidence that chemical etching takes place by reactions of gas-phase free radicals. This subject pertains to catalyst redispersion and regeneration, and the chapter links the catalysis literature and literature less often consulted by catalytic scientists and engineers. 

Solubility of Ionomers. Ionic bonding with metal ions decreases solubility in organic solvents (6,11). Commercial ionomers can generally be swollen by certain solvents such as aromatic hydrocarbons at elevated temperatures, but do not dissolve completely to give viscous solutions. Resistance to surface etching by oiganic solvents is high in most cases. 

Surface etching can be isotropic or anisotropic. For isotropic etching the etching rate is the same in any direction, for anisotropic it is higher in one direction. 

The most stable structures of initial oxidation may be the backbond and dimer bridge structures. Both structures contain di-coordinated oxygen. These two configurations seem to be the initial building blocks of oxide film growth and the key species for surface etching. 

---