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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22 4.5 FeO(T) 0.8 0.2 10.6 2.90 4.73 7.08 10.94 4.02 MnO 0.10 0.82 0.05 0.03 1.05 MgO 3.6 0.3 2.5 9.1 0.48 1.03 1.82 2.5 1.86 CaO 8.7 1.1 8 10 1.53 1.47 1.85 1.38 2.28 Na2O 14.3 1.5 16.5 5.6 4.03 0.81 0.77 0.55 1.57 K2O 0.2 0.7 1.4 3.76 0.96 0.86 0.38 1.01 NBO/T 0.79 0.00 0.62 0.99 0.08 0.18 0.36 0.72 0.22

      The coexistence of distinct structural units has important consequences because it has been invoked to account for the unusual properties of SiO2 glass such as a room‐temperature density maximum for glass quenched from temperatures near 1505 °C. Besides, a density minimum is observed near 950 °C for structurally relaxed glass. The anomalous pressure‐ and temperature‐dependence of SiO2 glass compressibility, with maxima near 3 GPa and 100 K, respectively, can also be modeled with two coexisting three‐dimensional structures in SiO2 glass.

      2.2 Al2O3

      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 XM+ ≥ XAl3+, whereas for alkaline earths, the requirement is 0.5 XM2+ ≥ XAl3+, where XM+, XAl3+, and XM2+ are atomic fractions of the respective cations.

Graph depicts the 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, 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 degree. 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].