Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

Figure 11 Hydrolytic stability of different pure oxides in aqueous solution as a function of pH; the stability is given in terms of Gibbs energy of hydration ΔG_hydr; negative values of ΔG_hydr are plotted against pH to make the most stable cases appear at the bottom of the graphs.

      4.4 Chemical Durability

Chemical durability is a very complex property. Attempts at generating an oxide increment system for the prediction of this property are not recommended because the most basic information on chemical durability must reflect the stability of a glass in strong acids, neutral water, and strong caustic solutions. Irrespective of their role in the glass structure, even pure oxides exhibit quite a complicated stability pattern as a function of the solution pH. This is shown in Figure 11 (from [25], revised version). Clearly, a general statement like “alumina enhances the chemical durability of a glass” is erroneous. It is true, alumina enhances the chemical durability under moderately acid to fairly caustic conditions, but it destabilizes a glass exposed to strong acids. Anyway, Figure 11 provides a first guideline of the effect which may be expected from a specific oxide in a given pH range, and hence, to base a first step of development on this information. In Chapter 5.11, the approach is further elaborated.

5 Perspectives

      Challenges for future development mainly deal with the extension of both thermochemical and thermophysical databases for glass‐forming systems. The usefulness of phase diagrams and of thermochemical calculations for glass development has been demonstrated. Yet, when it comes to the databases on which the calculations of phase diagrams rest, a severe lack of results for multicomponent melts relevant to the glass industry is felt. This situation is due to the fact that the extension of databases is chiefly driven by the financially potent metallurgical industry whose compositional focus distinctly differs from the needs of the glass industry. A reliable approach to liquidus temperatures, even for the conventional container, float, or fiber glass branches, would open doors for significant process improvements, resulting in enhanced sand dissolution upon melting, higher pull rates, energy saving, enhanced glass quality, and reduced loss of expensive glass contact materials like platinum.

      Whereas thermochemical data sets (standard enthalpies and entropies, C_P polynomials) are available for about 6000 mineral substances, thermophysical standard data sets (including stiffness parameters, and their temperature coefficients) hardly exceed a number of a few hundreds only [26]. From such data, glass technologists might learn how the local atomic structure of a material in general influences the resulting mechanical properties. Thus, to date, the intense quest for stronger glasses rests on an extremely narrow scientific basis. The same is true for the adjustment of the thermal expansion coefficient of solder glasses and substrate glasses to contact materials with very high or very low thermal expansion coefficient (like copper, alumina, steel, or silica glass, low‐expansion glass ceramics, respectively).

      As stated above, as useful as the conventional oxide increment systems may be in the daily routine of industrial glass development, an approach to truly novel glass compositions with outstanding properties must be based on a deep understanding of the relation between chemical composition, structure, and properties. This is where the field of atomistic simulation should play a decisive role in a near future (cf. Chapters 2.7 and 2.8). The challenge for the coming decades thus consists in developing first‐principles tools suitable for industrial applications.

Beyond this, new glass‐forming systems with a high potential for application as functional materials are being developed. As described in Chapter 8.9, an important group with relevance at the industrial scale (not shown in the scheme of Figure 1) are hybrid glasses combining both inorganic and organic bonds in their structure. Such glasses have been synthesized via a sol–gel route for a long time (Chapter 8.2); recently, systems accessible by melting have been presented [27].

References