Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 227

In most readily formed multicomponent oxide glasses, Al³⁺ is a network former predominantly present as AlO₄ tetrahedra. The latter are compositionally equivalent to AlO_4/2 if oxygen sharing is taken into account. Because the alumina chemical component (Al₂O₃ or AlO_3/2) has insufficient oxygen to form this species, one NBO, if present, will be converted to a BO for each added Al cation. Simple models of aluminosilicate melt structure have long assumed that, when alumina contents become large enough to balance all of modifier oxides (e.g. moles of Al₂O₃ = moles of Na₂O or CaO), NBO contents are reduced to zero and the glass or melt structure is comprised entirely of fully connected tetrahedra, by analogy with framework aluminosilicate crystals such as feldspars (e.g. NaAlSi₃O₈, CaAl₂Si₂O₈). This is a good approximation in some systems, especially those with alkali oxide modifiers only, and is supported by long‐known changes in properties with composition as well as diffraction and spectroscopic data. As alumina is added to alkali silicate melts and glasses, for example, the alkali cations are coordinated by fewer NBOs and more BOs, some of which will have partial negative formal charges, e.g. −1/4 for Si–O–Al and −1/2 for Al–O–Al. This change in role can be described as a transition from “network‐modifying” to “charge‐compensating” cation.

However, detailed spectroscopic studies, especially by ²⁷Al and ¹⁷O NMR and Raman, show that the structure can be more complex than indicated by this model, particularly in systems with modifier cations of high field strength. In Ca and Mg aluminosilicates, for example, significant concentrations of AlO₅ (typically 4–8% of Al cations) and even small amounts of AlO₆ groups are present throughout most of the glass‐forming regions [11]. Some NBOs also persist well into the peraluminous compositional range (e.g. with moles of Al₂O₃ > CaO). Trivalent modifier cations such as Y³⁺ and La³⁺ promote this shift in Al coordination, which increases even more obviously in peraluminous compositions and in aluminoborates and aluminophosphates. The mixing of these Al coordinations in the network must contribute to configurational entropy and related properties. As noted in Section 3, the distinction between “bridging” and “non‐bridging” oxygens becomes blurred as network cations increase in coordination number and their bonds to oxygen lengthen and weaken, complicating simple structure–property hypotheses. A few in‐situ X‐ray diffraction and Raman studies, and more detailed research on quenched, decompressed glasses, have clearly shown increases in Al coordination with pressure, which occurs more readily than for Si. NMR studies of glasses quenched from high‐pressure melts have shown that Al coordination increase is promoted by modifier cations with higher field strength [10].