Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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cation for tetrahedral Al3+, whereas in rock wool with a composition more akin to natural basalt (Table 1; see also [1], chapter 18), alkali metals and Ca2+ serve to charge‐balance Al3+ together. Somewhat similar structural features can be found in the glass containment of incinerated household waste.

Graph depicts the energetics of Al,Si substitution along meta-aluminosilicate joins as a function of ionization potential, Z/r2, of metal cation that serves to charge-balance Al3+ in tetrahedral coordination. Heat of solution of glasses in lead molten lead borate solution is used as a measure of the substitution energetics. Simple and systematic relations with Z/r2 are evident, but with distinct separation of relationships for cations with different charge-balancing cations. This difference stems from different substitution mechanisms of Al3+ for Si4+ depending on whether the charge-balance is accomplished with monovalent or divalent cations.

      3.1 General Remarks

      In order to characterize the structure of depolymerized, chemically complex aluminosilicate glasses and melts (Figure 1), it is first necessary to describe the structure of simple binary metal oxide–silica compositions. With this information, one can then consider multicomponent metal oxide silicate and aluminosilicate glasses and melts.

Graph depicts the summary of distribution of charge-balancing cations in natural magmatic liquids of basalt and rhyolite melt compositions as a function of the NBO/T of the melts.

      The properties and behavior of SiO2 in metal oxide silicate melts and glasses differ somewhat from those of pure silica glass and melt. The partial molar volume of this component is slightly smaller (26.8 cm3/mol) than the volume of pure SiO2 (27.3 cm3/mol) because some of the oxygen in these glasses and melts are nonbridging (NBO) and the partial molar volume of NBO is slightly less than that of bridging oxygen. In metal oxide silicate, the partial molar volume of SiO2 is independent of composition, however, over wide composition range [8]. Systematic relations between metal/silicon ratio can also be seen in other physical and chemical properties such as viscosity, conductivity, thermal expansion, and compressibility of glasses and melts [1].

      In ternary and more complex metal oxide silica melts, the values of most properties cannot be described as linear combinations of the endmembers (mixed alkali effect). For example, window glass, which is essentially a mixture of Ca‐ and Na‐silicate components, is in this category. This behavior is related to the steric effects that govern metal cation ordering among different NBO in ternary and more complex metal oxide–SiO2 glasses and melts. Ordering affects configurational and mixing properties and, therefore, rheological and thermodynamic properties. The greater the contrast in electronic properties such as their electrical charge and ionic radius of the network‐modifying cations, the greater the effect of mixing on melt and glass properties. This ultimately leads to liquid immiscibility in SiO2‐rich metal oxide–SiO2 melts. In fact, at given temperature the width of the immiscibility gap is a positive function, Z/r 2 (Z = formal electrical charge, r = ionic radius), of the metal cation.

      3.2 Structure

      Structural characterization of simple and complex metal oxide silicate glasses and melts can be expressed in terms of nonbridging oxygen, NBO, per tetrahedrally coordinated cation, T (Chapter 2.4). The NBO/T‐values of commercial glasses range from about 0.2–0.3 (for Pyrex glass, for example) to values greater than 3.0 for some slags (Chapter 7.4). The NBO/T of typical window glass is about 0.8, which is similar to those of rock wool. In nature, the NBO/T‐values of melts from individual rock types fall within relatively broad ranges (Figure 5). In general, there is a negative correlation between the NBO/T‐value and the SiO2 concentration.