Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
Скачать книгу
Nb2O3, and Bi2O3. Because they have a geometrically well‐defined coordination shell, conditional glass formers give rise to structures that are well described as random networks. For instance, the networks formed by binary aluminate and gallate glasses are based on AlO4/2 and GaO4/2 tetrahedra (Figure 7c), respectively. On the other hand, in more complex systems involving a glass former, aluminum can occur with mixed four‐, five‐, and six‐coordination as clearly revealed by 27Al NMR spectra, which are very sensitive to Al speciation. Antimonite glasses form a network based on SbO3/2 trigonal pyramids (Figure 7b). Contrastingly, tellurite glasses form a network based on two different units, TeO4/2 pseudo‐trigonal bipyramids (or disphenoids, Figure 7e) and TeO3/2 trigonal pyramids (Figure 7b). Similarly, vanadate glasses involve a mixture of VO4 and VO5 units (Figure 7f and g). Also, binary niobate glasses seem to be dominated by five‐coordinated NbO5 units. In alkali titanate glasses, the network is formed from TiO4/2 tetrahedra, although five‐coordinated O=TiO4/2 units (cf. Figure 7g, but with a terminal apical oxygen) and octahedral TiO6 units (Figure 7h) can be found in more complex glasses.

      5.2 The Modified Random Network Model

      In the aforementioned depolymerization reaction, an NBO is conventionally depicted with a negative charge to balance the positive charge on the Na+ modifier cation, shown to indicate ionic bonding between the anions and cations. Although this reaction only shows two NBOs adjacent to the Na+, the coordination numbers of modifier cations are actually larger, typically greater than four. It is thus inevitable that the modifier cations are clustered in some way. According to Greaves' modified random network (MRN) model (Chapter 2.5), the modifiers coalesce into channels if their content exceeds a percolation limit. There are thus two interlacing sublattices: the network regions constructed from network formers and the inter‐network regions made up of modifiers (see Figure 8a in Chapter 2.5). Such a microstructure has important consequences for the physical and transport properties (e.g. ionic conductivity).

Glass formers (network formers) Intermediates/Conditional glass formers Network modifiers
B2O3 (3,4) SiO2 (4) GeO2 (4,5,6) P2O5 (4) As2O3 (3) Al2O3 (4,5,6) Ga2O3 (4,5,6) Sb2O3 (3,4) TiO2 (4,5,6) TeO2 (3,4) V2O5 (4,5) Nb2O5 (5) Bi2O3 (4–6?) ZnO (4) PbO (3,4,5) SnO (3,4,5) Tl2O (~3) Li2O (4,5) Na2O (4–7) K2O (5–9) Cs2O (6–12) MgO (4,5) CaO (~6) SrO (4–7) ZrO2 (~6) MnO (4–6) PbO (6–12) SnO (6–12) Tl2O (6–12)

      5.3 Network Connectivity and Q‐species

      5.4 Change of Coordination Number

Graph depicts the deviations of the relative abundance of Qn-species in lithium silicate glasses from an idealized binary distribution as a function of the Li2O/SiO2 ratio, J. Shading of the points indicating the original 29Si NMR experimental data: ▽ – Q4, ○ – Q3, ⋄ – Q2, □ – 


                  <div class= Скачать книгу