Alumino-silicate glasses

Oxynitride glasses are silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. As nitrogen increases, glass transition temperature, elastic modulus, viscosity and hardness increase while thermal expansion coefficient decreases. 

In alumino-silicate glasses, the structural disposition of Al is guided by the same considerations as that of Pb. In pure alumino-silicates, Al is found both in tetrahedral and in octahedral positions (Prabakar et al., 1991). The tetrahedral Al is in network positions, while octahedral Al is present as aP and is surrounded by NBOs as confirmed in NMR experiments. This is because if both Al atoms in AI2O3 have to occupy tetrahedral positions, it requires four oxygens, 2[A104/2] , which it does not possess and [Si04/2]° cannot provide it. Therefore, AI2O3 behaves as if it has become. 

Explain the process of dissolution of a sodium trisilicate glass in water. Explain the effect of pH on the process. Explain why a sodium alumino-silicate glass is more durable than a sodium silicate glass in water, but is less durable in a strong acid. 

The only ones that have so far found practical application in glass-ionomers for dentistry are alumino-silicate glasses, with fluoride and phosphate additions. These are typically calcium based, with some extra sodium, though successful commercial materials exist in which calcium has been wholly or partly replaced by strontium. 

R. Hill, D. Wood, M. Thomas, TrimethylsUylation analysis of the silicate structure of fluoro-alumino-silicate glasses and the structural role of fluorine, J. Mater. Sci. 34 (1999)1767-1774. 

The simulations strongly indicate, that, in alkali alumino-silicate glasses. Si is preferentially bonded to NBO, whilst A1 is preferentially bonded to BO and TBO. Indeed, this feature is quite clear when the structure is visualized. 

As seen in Fig. 15, in either the mixed-alkali silicate or mixed-alkali alumino-silicate glasses, clearly distinguishable Li-O and Na-O PDF peaks and coordination shells are observed, indicating that Li and Na ions adopt distinctly separate environments in there glasses, in accordance with the defect model proposed by LaCourse[83],... 

McKeown, D. A., Galeener, F. L., and Brown Jr., G. E. (1984) Raman Studies of A1 Coordination in Silica-rich Sodium Alumino-silicate Glasses and Some Related Minerals, J. Non-Cryst. Solids, 68, 361-378. 

McKeown, D. A. (1987) Radial Distribution Analysis of a Series of Silica-rich Sodium Alumino-silicate Glasses Using Energy Dispersive X-ray Diffiaction, Phys. Chem. Glasses, 28, 156-163. 

Alumino-silicate glasses have been found useful for work up to 750 C by Enderby and co-workers. These glasses combine a high softening point (800°C) with a medium coefficient of expansion (42.5 X 10 deg.C ) and it is therefore possible to use a lower melting point solder glass with the same expansivity as an adhesive. 

Fig. 8.46 Fracture toughness of a series of lithium-alumino-silicate glass ceramics plotted versus increasing growth time of the crystalline phase [20]. With kind permission of Elsevier...

The sodium alumino-silicate glass shown in Figure 17c alters glass structure and reduces Ne penetration. The A1 ions close the open structure by removing the NBO caused by the Na ions. The Na ions associate with the BO near the A1 ions. Thus, the open structure caused by the Na ions (and NBO) is removed by the A1 and yet the blocking effect of the Na ions remains. These two combine to inhibit Ne penetration (Kohler and Garofalini 1994). 

Webb EB, Garofalini SH (1994) Effect of adsorption of EAM metal atoms on the structure of a sodium alumino-silicate glass surface a molecular dynamics simulation. Surf Sci 319 381-393... 

The most common material is 96% alumina (AI2O3), with a typical composition of 96% AI2O3, 0.8% MgO, and 3.2% Si02. The magnesia and siHca form a magnesium-alumino-silicate glass with the alumina... 

Schiffer U., "Nucleation in Parent Glasses for Lithia Alumino-Silicate Glass-Ceramic" pp. 25-38 in Low Thermal Expansion Glass-Ceramics. Edited by H. Bach. Springer, Berlin, Heidelberg, Germany, 1995. 

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