Amorphous silicate glasses

Silicates Amorphous silicates Glass, cobalt doped... 

Silicate glasses have amorphous structures produced by addition of salts that disrupt the crystalline structure. They can be attacked by strong base and hydrofluoric acid. 

Diffusion coefficients in amorphous solids such as oxide glasses and glasslike amorphous metals can be measured using any of the methods applicable to crystals. In this way it is possible to obtain the diffusion coefficients of, say, alkah and alkaline earth metals in silicate glasses or the diffusion of metal impurities in amorphous alloys. Unlike diffusion in crystals, diffusion coefficients in amorphous solids tend to alter over time, due to relaxation of the amorphous state at the temperature of the diffusion experiment. 

Besides the multiplicity of defects that can be envisaged, there is also a wide range of solid phases within which such defects can reside. The differences between an alloy, a metallic sulfide, a crystalline fluoride, a silicate glass, or an amorphous polymer are significant. Moreover, developments in crystal growth and the production of nanoparticles have changed the perspective of earlier studies, which were usually made on polycrystalline solids, sometimes with uncertain degrees of impurity present. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

The chemistries of phosphates and silicates are similar, but the morphology of the crystals of the sparingly soluble phosphates are unsuited for fiber applications. Amorphous phosphate glasses can be easily spun into fibers in a process similar to the manufacture of fiberglass. Unfortunately, amorphous phosphates lack both strength and hydrolytic stability. 

From a mineralogy viewpoint, IDPs are aggregates of mostly sub-micron-sized crystalline silicates (olivine and pyroxene), amorphous silicates, sulfides, and minor refractory minerals, held together by an organic-rich, carbonaceous matrix. Large fractions, 30-60 wt%, of these IDPs are amorphous silicates, known as glass with... 

Other amorphous silicates are also found in IDPs. The compositions range from ferromagnesian silica, with variable Mg/Fe ratios, containing Ca and Al (Bradley 1988) to Fe-Mg-bearing aluminosilica(Klock Stadermann 1994). Bradley (1988) reported pyroxene glass in one IDP. 

One of the most powerful applications of these correlations between NMR parameters and stmcture is to provide a better understanding of the stmcture of amorphous materials which are very difficult to study by other techniques. Silicate glasses have been studied by relating the 8iso.cs value of Si to the bond angle distribution (Dupree and Pettifer 1984, Pettifer et al. 1988). In the case of O, the parameter has been used to determine the Si-O-Si bond angle distribution (Faman et al. 1992) using the relationship ... 

Figure 30. Broad overview of oxygen diffusion data in crystalline and amorphous silicon dioxide as well as silicate glasses and liquids over a wide temperature range, redrawn from Lamkin et al. (1992). Note how permeation rates, denoted by heavy lines and D02, ate faster than diffusion rates involving interaction with network oxygen, D o, in any given type of medium.

With multicomponent amorphous polymeric materials, prediction of the occurrence of a surface enrichment effect is very difficult because of many uncertain parameters. With worked multicomponent silicate glasses, the wet grinding and polishing process, and the cleaning with liquids, leaches out the soluble constituents,... 

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