Etching models

These authors suggested that the mechanism of etching is the formation of precious metal oxides (primarily of platinum) under reaction conditions and the subsequent volatilization of these oxides. Although it was noted that the same rate of metal loss and the same gross transformation did not take place in a pure oxygen environment, no attempt was made to reconcile these observations with the etching model. 

The final rectangular cavity bounded by four (111) planes independent of the initial opening shape can be explained with the (111) step etching model illustrated in Fig. 7.41. All of the (111) steps that intercept the etching (100) surface and are exposed within the opening defined by the mask are etched in a direction parallel to the (111) planes. The etching along these planes will continue until perfect (111) planes, which define the walls of the etched cavity, are reached and all of the exposed (111) steps vanish. 

FIGURE 7.47. Schematic illustration of comer undercut based on the (111) step etching model. 

KOH/propanol + H2O at 80°C (after Bean [131]). (b) Schematic illustration of corner undercutting based on the (111) step etching model shown in Figs 29 and 30. 

Etching of a corner defined by (111) planes can be explained by the (111) step etching model illustrated in Fig. 30. The etching of perfectly oriented (111) planes are determined by the generation of steps. 

The (100) surface tends to roughen quicker than (111) surface and the roughness tends to be permanent on (100) surface while it tends to be transient on (111) surface [47]. Such crystal orientation-dependent roughness could be explained by the anisotropic etching model illustrated in Fig. 30. The preferential etching at the (111) steps of the (111) terraces results in the removal of the terraces and reduction of the (111) steps and a reduction of microroughness. 

From the previous discussions, the residual oil was pulled and stripped from the rock surfaces. As shown in a 2D glass-etched model (see Figure 6.24), the residual oil after waterflood became isolated oil droplets. The polymer solution pulled the oil into oil columns. These oil columns became thinner and longer to form oil threads as they met the residual oil downstream. The oil upstream flowed along these oil threads to meet the residual oil downstream so that an oil bank was built. In the process of residual oil flowing along the oil threads, because of the cohesive force of the oil/water interfaces, it was also possible to form new oil droplets, which flowed downstream and coalesced with other oils. Now we are ready to discuss the role viscoelasticity plays. 

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. 

C2.18.3.5 MODEL STUDIES OF PHOTON- AND ELECTRON-ENHANCED ETCHING... 

Fig. 9. Corrosion model of silver development. As the haUde ion, X, is removed into solution at the etch pit, the silver ion,, travels interstitiaHy, Ag/ to the site of the latent image where it is converted to silver metal by reaction with the color developer, Dev. Dev represents oxidized developer.

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

---