Other Silicate Glasses

The surface conductivity of vitreous silica is also low compared to other silicate glasses. Because vitreous silica is not hydroscopic, water Alms containing exuded alkalies do not readily form on its surfaces. The surface conductivity, however, can increase signiAcantly with increasing relative humidity. A change in the relative humidity from 20 to 80% produces a millionfold increase in the surface conductivity (174). 

As with other silicate glasses, oxynitride glasses may be heat treated at appropriate temperatures to crystallise as glass-ceramics (Leng-Ward and Lewis, 1990 Thompson, 1992 Hampshire, 1993). The crystalline phases formed depend on both the composition of the parent glass and the heat-... 

Bioactive glasses can readily be made using the processes developed for other silicate glasses. The constituent oxides, or compounds that can be decomposed to oxides, are mixed in the right proportions and melted at high temperatures to produce a homogeneous melt. On... 

Chemical Reactivity - Reactivity with Water Reacts vigorously to form corrosive and toxic hydrofluoric acid Reactivity with Common Materials In the presence of moisture, is corrosive to glass, other siliceous materials, and most metals Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water, rinse with sodium bicarbonate or lime solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Hence it is concluded that in multi-component silicate glasses, the presence or absence of a MAE is a complicated function of composition and environment, and not necessarily entirely dependent on the ratio of one alkali to the other. 

Based on the discussion of the previous section, we would also expect that structural complexity and disorder present in glasses would tend to decrease the thermal conductivity. This is true, especially when alkalis are added to sodium silicate glasses. Figure 4.32 shows the effect of replacing SiOa in Na20-Si02 glass with other oxides. 

Refractive index values vary from 1.0003 for air to over 2.7 for some solid oxide ceramics. Silicate glasses have a much narrower range of valnes, from abont 1.5 to 1.9. The refractive indices (or indexes) of other materials can be fonnd in Appendix 9. 

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