Vitreous structure

Development of the organosilicon chemistry allows the access to very different monomers for this approach. In all cases, the objective is to combine the qualities of the Si-O-Si network (vitreous structure, transparency, thermal and chemical stability) with those of the organic entities [7]. 

In the low-pressure region there are also metastable states associated with Ice I. Normal Ice I is hexagonal, as we have seen, and may therefore be designated Ice Ih. Below about —120 °C it is possible to produce a cubic modification. Ice I, from the vapour or from some of the high-pressure ices. This slowly reverts to Ice Ih at higher temperatures. Ice condensed slowly from the vapour below —160 °C has an amorphous or vitreous structure which again is only metastable and reverts to 1 and then to Ih as the temperature is raised. 

Finally, the role of minor constituents, impurities, and defects must be included to completely characterize a vitreous structure. In many cases. 

In the temperature range from -20°C to -80°C glycerin controls the crystallization process and makes it reversible, thus eliminating eutectic solutions caused by the presence of salt solutions. It also shifts the coordinates of eutectic borders and contributes to the formation of vitreous structures. 

Sebag, J. (1989) The Vitreous. Structure, Function, and Pathology, Springer-Verlag, New York. 

A. Takada, C. R. A. Catlow, and G. D. Price,/. Phys. Condens. Matter, 7, 8693 (1995). Computer Modelling of B2O3 II. Molecular Dynamics Simulations of Vitreous Structures. 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Because all of these structures share the same short-range bonding scheme, the density differences iadicate that vitreous siUca has a substantial iaterstitial volume and can be compacted. 

Many of the high-pressure forms of ice are also based on silica structures (Table 14.9) and in ice II, VIII and IX the protons are ordered, the last 2 being low-temperature forms of ice VII and III respectively in which the protons are disordered. Note also that the high-pressure polymorphs VI and VII can exist at temperatures as high as 80°C and that, as expected, the high-pressure forms have substantially greater densities than that for ice I. A vitreous form of ice can be obtained by condensing water vapour at temperatures of — 160°C or below. 

A similar, but highly porous, vitreous carbon material—reticulated vitreous carbon (RVC)—has found widespread application for flow analysis and spectro-electrochemistry (25). As shown in Figure 4-10, RVC is an open-pore ( spongelike ) material such a network combines the electrochemical properties of glassy carbon with many structural and hydrodynamic advantages. These include a very high surface area ( 66 cm2 cm-3 for the 100-ppi grade), 90-97% void volume, and a low resistance to fluid flow. 

FIGURE 4-10 The open-pore structure of reticulated vitreous carbon. 

Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

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