Rearrangement processes 1058 INDEX

Extensive theoretical work in support of both models of thermal etching was produced before 1970. The theoretical basis for the thermodynamic model was the concept of reduction in total surface energy by the preferential formation of low-energy, low-index planes. The true equilibrium shape of a crystal is the shape with the lowest surface energy, as noted by Curie (36) and Gibbs (37). The thermodynamic models provide no information regarding the process of surface rearrangement. 

Figure 4.19 A schematic diagram showing the process offaceting, (a) the initial surface, with surface energy yo at an angle to the low-index planes of the crystal (b) the crystal after it has rearranged to two sets of planes with surface energies y 1 and yz (c) a schematic Wulff plot for this system and (d) polar plot of 1/y for this...

The same condensation process is also activated upon UV irradiation of sol-gel films that could be done under different conditions, such as in vacuum, in controlled atmosphere, and in air. In general, it is observed, for instance, by in situ FTIR, that photoirradiation produces a decrease in the silanols and, in turn, an increase in the intensity of the sihca (or other oxides such as titania) absorption bands. The condensation of the films is also reflected by change in several properties, such as refractive index, thickness, hardness, and energy sur-fece. On the other hand, it has also been observed that the structural changes induced in the silica network are not limited to condensation, and even more intimate rearrangements of the silica tetrahedra from unstable folded linear Si02 structures to stable linear Si—O—Si bonds can be activated [23]. 

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