Silica glassy state

Many materials can be made to exist as glasses. Hard candies, for example, consist primarily of sugar in the glassy state. What the term glass means to most people, however, is a product made from silica (Si02). The... 

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... 

In the present paper, we show that the quantum chemical (QC) simulation can be used as a simple and efficient tool for testing, whether known or theoretically predicted solid tends to form an amorphous phase. Two well-known substances, representing the extreme examples of crystalline and amorphous solids, namely lithium fluoride (LiF) and silica (Si02), are considered in order to illustrate the methodology. LiF is crystalline and hardly forms glassy state, while Si02 is known to form a number of non-crystalline isomorphes [4,5]. 

In conclusion, in this study the sol-gel synthesis of ZnO nanoclusters embedded in silica has been faced by the gel-derived binary system ZnO-Si02. As compared to oxide mixtures prepared by conventional solid-state reactions, in gel-derived systems the formation of ceramic silicate compounds is observed at lower temperatures than expected for traditional melting processing [173]. Through XPS it was possible to evidenee the importance of the choice of proper precursor compounds and proper annealing conditions to produce stable ZnO guest clusters dispersed into the host silica glassy matrix. 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

Although not as severe as fluorides, chlorides also reduce performance when they contact refractories in both the gaseous and liquid states. Chlorides also form soluble salts with most components of a refractory composition though they do not react with glassy materials. An excellent refractory material for use in chloride-containing service where temperatures do not exceed 1900°F is fused silica. This refractory performs well in exposures to chlorides because of its vitreous or glassy morphology. 

Figure 5.4 Transition State Theory for diffusion in condensed media, (a) General representation of the transition state theory, (b) Diffusive jump in glassy polymer [ 17j. Reprinted from journal of Membrane Science, 73, E. Smit, M. H. V. Mulder, C. A. Smolders, H. Karrenbeld, j. van Eerden and D. Eeil, Modelling of the diffusion of carbon dioxide in polyimide matrices by computer simulation, 247 257, Copyright (1992), with permission from Elsevier, (c) Diffusive jump in microporous silica, reprinted with permission from AlChE, Theory of gas diffusion and permeation in inorganic molecular-sieve membranes by A. B. Shelekhin, A. C. Dixon and Y. H. Ma, 41, 58 67, Copyright (1995) AlChE...

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