Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
by observed increases in Al/Si disorder in glasses with higher fictive temperatures, making a larger contribution to configurational heat capacity. Subsequent extensive work on high‐pressure aluminosilicates has begun to elucidate the much more complex linkages among not only tetrahedral network species but five‐ and six‐coordinated Al and Si, where the mixing of all of these network species presumably contributes to increases in configurational entropy [16].
The same experimental approach can, in some borosilicate glass compositions, quantify the extent of mixing of boron and silicon network cations, which can be much greater than considered in early models based primarily on 11B NMR data (Figure 3). In compositions with modifier oxides, the structure is further complicated by the presence of both BO3 and BO4 groups. As for aluminosilicates, relatively highly charged B–O–B linkages between two of the latter seem to be at least partially “avoided.” When pairs of network cations are present that can have strong nuclear dipolar couplings, notably 11B and 27Al, or 27Al and 31P, double‐resonance NMR methods can reveal their relative proximities and even the correlations of species with multiple coordination environments, for example of BO3 groups with AlO4 [17]. These findings again can provide important constraints on mixing and contributions to order/disorder.
8 Perspectives
Technical advances continue to increase the quality, depth, and breadth of information that can be obtained about short‐range structure and order in oxide glasses, for example, microscale XAS, X‐ray Raman, improved neutron scattering facilities, and NMR at higher magnetic fields and with methods that allow better measurements of internuclear distances and connectivities. Combination of experimental data from different methods, to yield consistent models, is especially powerful, as illustrated in Figure 7 [18]. Some of these approaches are now feasible for in‐situ studies of high‐temperature liquids and even of liquids at high pressure and temperature. Advances in theory are beginning to allow accurate forward calculation of spectra (e.g. vibrational and NMR) from structural models [19]. This information can then be combined with experimental data to obtain much more complete views of structure, quantitative extent of disorder, and their links to thermodynamic properties that depend critically on configurational entropy, such as viscosity, heat capacity, and the bulk free energy. As researchers move beyond model‐dependent fitting and interpretation of spectra of glasses to more complete theoretical analyses, our understanding of the true complexities of these fascinating and useful materials will expand enormously.
Figure 7 Partial pair distribution functions gij(r) calculated from a “reverse Monte Carlo” model combining X‐ray and neutron diffraction data for various CaSiO3–MgSiO3 glasses.
Source: Reprinted with permission from [18].
References
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12 12 Brown, G.E. Jr., Farges, F., and Calas, G. (1995). X‐ray scattering and x‐ray spectroscopy studies of silicate melts. In: Structure, Dynamics, and Properties of Silicate Melts (eds. J.F. Stebbins, P.F. McMillan and D.B. Dingwell), 317–410. Washington, DC: Mineralogical Soc iety of America.
13 13 Eckert, H. (1994). Structural studies of non‐crystalline solids using solid state NMR. New experimental approaches and results. In: Solid‐State NMR IV. Methods and Applications of Solid‐State NMR (ed. B. Blümich), 127–202. Berlin: Springer‐Verlag.
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15 15 Davis, M.C., Sanders, K.J., Grandinetti, P.J. et al. (2011). Structural investigations of magnesium silicate glasses by 29Si 2D magic‐angle flipping NMR. J. Non‐Cryst. Solids 357: 2787–2795.
16 16 Lee, S.K., Fei, Y., Cody, G.D., and Mysen, B.O. (2003). Order and disorder in sodium silicate glasses and melts at 10 GPa. Geophys. Res. Lett. 30: 1845–1849.
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Note
1 Reviewers:G.H. Henderson, Department of Earth Sciences, University of Toronto, Toronto, Ontario, CanadaB.O. Mysen, Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
2.5 The Extended Structure of Glass
George Neville Greaves
Department of Metallurgy and Materials Science, University of Cambridge, Cambridge, UK
1 Introduction
The structure of glass stretches from atomic dimensions and local short-range order (SRO), which is often similar to that of the crystalline state, through intermediate-range order