Glassy silica

Fig. 5.11. (a) Atom packing in amorphous (glassy) silica, (b) How the addition of soda breaks up the bonding in amorphous, silica, giving soda glass. 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

This is, even to a greater extent, true of oxide glasses with the coordination number Z > 6. It is obvious that with another increase in pressure to P 1 Mbar (the stability region of an a-PbC>2 structure type), the coordination number in glassy silica should slightly rise [98] and approach eight at P 2-3 Mbar (the stability region of a pyrite-like structure of crystalline silica). This state of the... 

There are a variety of other silicas. These include the metastable silica-W, which has a very low density (1.97 gcm ), and whose structure consists of edge-sharing chains of Si04 tetrahedra, the microcrystaUine diatomaceous earths, various forms of noncry stalline silica inclnding vitreous silica (glassy silica), and the finely divided amorphous silicas such as dry silica gel, fumed silicas, and colloidal silicas. 

The result is a rigid heterogeneous mass of small interlocking crystals bound together by glassy silica. 

The reliability of other statements concerning the structure of the glassy silica surface depends on the reliability of the interatomic potential model and the adequacy of the simulation procedure. We have already mentioned that if the two-body potential of Eq. (2)... 

Both ionic and covalent natures of the Si-0 bond contribute to the preference for (Si04)4- tetrahedron formation in both crystalline and glassy silicas. In addition, each O anion is coordinated by two Si cations, corresponding to comer sharing of the oxide tetrahedra, preventing the close-packing of anion layers and resulting in relatively open structures [5],... 

Crystalline silicas contain ordered arrangements of anion tetrahedra, whereas glassy silica has a high degree of randomness. Comparisons of these networks indicate that both have the basic tetrahedral unit, the same O-Si-O bond angle (109.5°), an O/Si ratio of two, and full connectivity of tetrahedra. An equivalent short-range order has been found in both crystalline and glassy silica, as shown schematically in Fig. 7. 

FIGURE 8,4 Thermal conductivity of quartz and glassy silica as a function of temperature [7]. The quartz thermal conductivity exhibits a T behavior at low temperature, a peak at about 10 K, then reduction at higher temperatures. This is typical of a crystalline solid. For amorphous glass the thermal conductivity increases as T2 plateaus between 1 to 10 K and then increases monotonically with temperature. Also plotted are the predictions of the Cahill-Pohl and Einstein models. The Cahill-Pohl model provides accurate predictions for temperatures higher than 50 K but cannot predict the low temperature behavior. The Einstein model predictions are much lower than the measured values. 

In the example above, the adhesion forces are small, so the paint gel is soft. Paint gel is much softer than a silica gel made by drying the particulate precipitate made from acidified sodium silicate solution. In this case the silica particles are extremely adherent and can resist large stresses. However, even such a hard, glassy silica gel can be moulded by the application of large pressures, of the order of 1000 atm (100 MPa). 

Slightly pre-oxidized Si-C-0 fibers have a uniform glassy silica coating. Their decomposition occurs at higher temperatures. This suggests that the silica coating remains gas tight and prevents the evolution of CO and SiO up to at least 1400 C [69]. 

O Q Travertin or siliceous tuff Calcedoine, jasper Amorphous Glassy Silica Rocks made from 50 wt.% sihca precipitaed from saturated aqueous solutions or metasomatic reaction. Diatomite are made from accumulation of siliceous skeletons of microorganisms. 

Class F fly ash The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 20% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime or hydrated lime, with the presence of water in order to react and produce cementitious compounds. Alternatively, the addition of a chemical activator such as sodium silicate (water glass) to a Class F ash leads to the formation of a geopolymer. 

Other possible processing routes include the calcia processing route (reaction of zirconium silicate with CaO at 1600 °C, and leaching the calcium silicate formed by treatment with HCl), and the plasma route (thermal dissociation of zirconium silicate in a plasma reactor above 2100 °C, and leaching the glassy silica formed with NaOH). 

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