Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 259

The NBOs in glasses and melts are not equivalent energetically. Instead, the structure of metal oxide–SiO₂ glass and its precursor melt is described in terms of a small number of distinct coexisting silicate structural units commonly described as Q ⁿ ‐species with n = 0, 1, 2, 3, and 4 where n is the number of bridging oxygen (Chapter 2.4). The overall degree of polymerization, NBO/T, is related to Q ⁿ ‐species abundance:

(1)

where is the mol fraction of the Q ⁿ ‐species and n is the number of bridging oxygen in the individual Q ⁿ ‐species. The NBO/T‐parameter itself does not distinguish between different types of NBO.

The abundance of individual Q ⁿ ‐species changes with metal oxide/SiO₂ ratio of the material (Figure 7). In simple binary metal oxide–silica melts, the abundance of Q ³, Q ², and Q ¹ species reaches maximum values at stoichiometries near NBO/Si = 1, 2, and 3, respectively, and decreases on both sides of these maxima. Orthosilicate compositions comprise both Q ¹ and Q ⁰ species with free oxygen compensating for the presence of the polymerized Q ¹ structure. The Q ⁿ ‐species abundances also are affected by the ionization potential, Z/r ² (Z: formal electrical charge, r: ionic radius), of the metal cation (Figure 7) because steric hindrance near the NBO governs how individual metal cations will distribute themselves among the Q ⁿ ‐species. The ordering of alkalis and alkaline earths among energetically nonequivalent NBO in different Q ⁿ ‐species in multicomponent compositions also aids in the explanation of the mixed alkali effect. For comparison, in crystalline metal oxide–SiO₂ systems, analogous steric hindrance effect instead limits the minimum metal oxide/SiO₂ ratio of the crystalline materials below which crystalline compounds are not stable [9].

In the much more chemically complex natural magmatic liquids, Q ⁿ ‐species distributions resemble those observed for binary metal oxide glasses and melts [10]. The influence of individual network‐modifying cations is difficult to establish, however, because of wide ranges of compensating effects on structure from the large number of different network‐modifying cations.

Whether in simple binary metal oxide silicates or more complex systems, the equilibrium constant for the principal expression that describes the equilibria among the Q ⁿ ‐species, 2Q ⁿ ⇌ Q ^{n − 1} + Q ^{n + 1} (see Chapter 2.4), is positively correlated with Z/r ² at least for systems for which n = 3 in the aforementioned reaction to yield 2Q ³ ⇌ Q ² + Q ⁴ (Figure 8). The enthalpy change of the latter reaction at temperatures above the glass transition is also a positive function of the Z/r ² of the metal cation and is more sensitive to Z/r ² of the more polymerized (lower bulk NBO/Si‐value) silicate melt (Figure 8). This relationship obtains because steric hindrance near NBOs diminishes with decreasing average values of n of the Q ⁿ ‐species.

Figure 7 Abundance evolution (mol %) of Q ², Q ³, and Q ⁴ species in alkali silicate glasses as a function of their NBO/Si‐values of compositions as indicated in diagrams. For alkali silicate glasses, the metal/silicon ratio equals the NBO/Si, provided that all Si⁴⁺ is in tetrahedral coordination. The ionization potential, Z/r ², of K⁺ and Li⁺ is 0.46 and 1.49, respectively, assuming sixfold coordination of oxygen around the alkali metal. The curves for Na₂O─SiO₂ (Z/r ² of Na⁺: 0.8) fall in between those of Li₂O─SiO₂ and K₂O─SiO₂.

Figure 8 Enthalpy change, ∆H, for the disproportionation equilibrium, 2Q ⁿ ⇌ Q ^{n − 1} + Q ^{n + 1}, n = 3, as a function of ionization potential of alkali cation for two series of alkali metal (M) silicate compositions. In these systems, M/Si = NBO/Si assuming all Si⁴⁺ is in tetrahedral coordination in the glasses. The ∆H is derived from temperature‐dependent equilibrium constant at temperatures above the glass transition and assuming that mol fraction of Q ⁿ ‐species equals their activity. Note that the ∆H‐value increases with increasingly polymerized melts probably as a result of increasingly nonideal mixing of the Q ⁿ ‐species. (Source: Modified after [8].)