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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target="_blank" rel="nofollow" href="#ulink_d4b93a42-6e09-5fe8-ba80-0b2d4e7b0593">Figure 9 Deviations of the relative abundance of Qn ‐species in lithium silicate glasses (points) from an idealized binary distribution (continuous lines) as a function of the Li2O/SiO2 ratio, J [16]. Shading of the points indicating the original 29Si NMR experimental data (see key on the figure): ▽ – Q 4, ○ – Q 3, ⋄ – Q 2, □ – Q 1, △ – Q 0.

Schematic illustration of the two differing effects of the addition of a network modifier cation M+ on the borate network. (a) Fragment of the network of pure B2O3 glass. (b) Formation of non-bridging oxygens. (c) Formation of four-coordinated boron atoms. Graph depicts the boron-oxygen coordination number, nBO, for lithium borate glasses, Li2O–B2O3, as determined by 11B NMR, compared with the prediction from a charge-avoidance model. Graph depicts the Germanium-oxygen coordination number, nGeO, for cesium germanate glasses, Cs2O–GeO2, as determined by neutron diffraction, compared with the predictions of charge-avoidance models in which the higher GeOn coordination is either 5 or 6.

      Until this point, only SRO has been discussed, along with the structural characteristics that arise directly from interatomic bonding, namely bond lengths, coordination numbers, bond angles, and coordination polyhedra. Even though LRO is by definition lacking in glasses, some ordering exists at length scales between SRO and LRO. It is known as intermediate‐ or medium‐range order (IRO or MRO).

      It must be acknowledged that IRO is much harder to probe experimentally than SRO and is, therefore, much less well known. To investigate it, structural modeling may in fact be required to complement direct experimental evidence. This order can be variously characterized in terms of clustering (e.g. Greaves' MRN, cf. Figure 8a in Chapter 2.5) or free volume, for example, but the most widespread approach is to consider the rings that are defined by connected polyhedra in the CRN. Whereas a crystal contains only very few different rings (e.g. the 2‐D crystal of Figure 1 contains only six‐membered rings), the rings in a CRN have a wider and more varied distribution of sizes (Figures 2 and 4).