Silica Glass and Related Glasses

There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

Water occurs in glass-ionomer and related cements in at least two different states (Wilson McLean, 1988 Prosser Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson Crisp, 1975). The alternative descriptions loosely bound and tightly bound have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. 

It is very difficult to obtain values for the intrinsic hardnesses of silicate and related types of glass. Therefore, no attempts at quantitative analyses will be made here. A semi-empirical method has been proposed by Yamane and Mackenzie (1974) based on the geometric mean of bond strength relative to silica, shear modulus, and bulk modulus. For 50 silicate glasses it yields estimates within ten percent of measured values, and for a few non-silicate glasses it is quite successful, as Figure 14.2 indicates. 

Ab initio calculations have been made of H, O, A1 and Si NMR parameters for sodium aluminosilicate glasses and hydrous silica.331 Two-dimensional 170 3Q MAS-NMR experiments on sodium aluminosilicate glass gave evidence for A1 O Al links.332 27A1 NMR spectra were used to study structural localisation of Al3+ in aluminosilicate clays.333 170 MAS-NMR and MQ MAS-NMR experiments have been carried out on hybrid aluminosilicate xerogels, providing evidence for Si-O-Al and Si O Si sites.334 Several other NMR studies have been made on aluminosilicate glasses and related systems.335 338... 

The potential capabilities of microemulsion synthesis of colloidal silica and related materials are yet to be realized. For example, the compartmen-talization of reagents at the molecular level in these media may allow close control of chemical homogeneity in the synthesis of glass (e.g., Si-Al) particles. In situ synthesis of silica-supported catalysts and synthesis protocols involving seeding techniques, coating techniques, or infiltration are also possible. 

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