Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 255

The second most important network‐forming component in complex aluminosilicate glasses and melts is Al₂O₃ (Table 1). Its concentration range in most natural magma and commercial applications (5–20 wt % Al₂O₃) can have profound influence on glass and melt properties compared with pure SiO₂. These include better glass‐forming ability of melts, improved durability, lower viscosity, and lower thermal expansion.

The type of metal cations serving to charge‐balance tetrahedrally coordinated Al³⁺ is central to understanding the structural roles of Al³⁺ in silicate melts and glasses and, therefore, their physicochemical properties. Charge‐balance commonly is achieved with alkali metals and alkaline earths (as in feldspar structures, for example). With an alkali metal, M⁺, one Al³⁺ can be charge‐balanced provided that X_M+ ≥ X_Al3+, whereas for alkaline earths, the requirement is 0.5 X_M2+ ≥ X_Al3+, where X_M+, X_Al3+, and X_M2+ are atomic fractions of the respective cations.

The structural environment near alkalis and alkaline earths depends on whether these ions play a charge‐balancing or a network‐modifying role [5]. The type and proportion of charge‐balancing cations also are important because of their different effect on the energetics of the O─Al bonds and, therefore, on glass and melt properties. This is seen, for example, in enthalpy of solution (Figure 3), viscosity, and also in melt and glass density, compressibility, and thermal expansion.

Figure 2 Distribution of intertetrahedral angle, ∠(Si–O–Si)^o, in SiO₂ glass from fitting of ²⁹Si MAS NMR spectra to an angle distribution function. Note that the maximum corresponds to that of cristobalite at its liquidus temperature (1723 °C), and is also similar to that obtained from X‐ray diffraction of SiO₂ glass. A recent ¹⁷O NMR two‐dimensional dynamic angle study resulted in 147° [3]. These angle distributions are consistent with a SiO₂ glass structure comprising predominantly six‐membered rings of three‐dimensionally interconnected SiO₄ tetrahedra.

The glass structure along SiO₂–MAlO₂ joins (M = alkali metal as charge‐balancing cation – meta‐aluminosilicate; see Figure 1) is a continuous evolution of the SiO₂ glass structure with substitution of Al³⁺ for Si⁴⁺ in tetrahedral coordination and with only a very small percentage or fraction of a percent of Al³⁺ in different structural roles. There is marginally more Al³⁺ in such roles in glasses along the SiO₂–CaAl₂O₄ join [7].

The K⁺, Na⁺, and Ca²⁺ are the dominant charge‐balancers in natural magmatic liquids (Figure 4). For melts and glasses with multiple potential cations for Al³⁺ charge‐balance, thermodynamic data can be used to establish relative stability of aluminate complexes. There is near equal stability of (KAl)⁴⁺ and (NaAl)⁴⁺ charge‐balance followed by (Ca_0.5Al)⁴⁺. In natural rocks, the proportion of Ca²⁺ relative to (Na⁺ + K⁺) decreases with increasing SiO₂ concentration so that in rhyolite melt, for example, alkalis dominate over Ca⁺ for charge‐balancing of Al³⁺. For less silica‐rich melts, the main charge‐balancing cation is Ca²⁺. For commercial glass such as glass wool, Na⁺ is the principal charge‐balancing 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.

Figure 3 Energetics of Al,Si substitution along meta‐aluminosilicate joins as a function of ionization potential, Z/r ², of metal cation that serves to charge‐balance Al³⁺ in tetrahedral coordination (ionic radius, r, assumed that of six‐coordinated metal cations – data from [6]). Heat of solution of glasses in molten lead borate solution is used as a measure of the substitution energetics. Simple and systematic relations with Z/r ² 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.