Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 175

The only oxides that can readily form glasses on their own are SiO₂, GeO₂, B₂O₃, P₂O₅, and As₂O₃. They are known as glass or network formers. Their structures are well described as random networks (Figures 2 and 4), involving well‐defined oxygen coordination polyhedra, namely SiO_4/2 tetrahedra (Figure 7c), GeO_4/2 tetrahedra (Figure 7c), O=PO_3/2 tetrahedra (Figure 7d), BO_3/2 triangles (Figure 7a), and AsO_3/2 trigonal pyramids (Figure 7b). The ability to vitrify readily arises as a consequence of the network structure.

If a second oxide with weaker, ionic bonding is added, then it can lead to some depolymerization of the network, as shown schematically for Na₂O by the following reaction.

The additional oxygen introduced as Na₂O is accommodated in the network by the breaking of bridges; a single BO, bonded to two silicon atoms, is replaced by two non‐bridging oxygens (NBOs), each bonded to one silicon atom. In this depolymerization reaction, the NBOs are shown with a negative charge, balancing the positive charges on the Na⁺ cations. As sketched in a 2‐D representation (Figure 4 of Chapter 2.4), such second oxides modify the network structure of the glass former, but do not become part of the network itself, hence their name of modifiers. Alkali and alkaline earth oxides are the most common examples. They cannot form a glass by themselves, but only in combination with a glass former.

Figure 7 Structural units in network glasses: (a) AO_3/2 triangle; (b) AO_3/2 trigonal pyramid; (c) AO_4/2 tetrahedron; (d) O=PO_3/2 tetrahedron (double bond P=O shown dashed); (e) AO_4/2 pseudo‐trigonal bipyramid (disphenoid); (f) AO_5/2 trigonal bipyramid; (g) AO_5/2 square pyramid‐based unit; (h) AO_6/2 octahedron.

Figure 8 Neutron correlation function for lithium disilicate glass [14]. The Li─O shaded peak is negative, making it readily identifiable in comparison with the positive Si─O and O─O peaks (Figure 5b).

This structural view emerged from early XRD studies where the modifier cations M were regarded as being stuffed into available holes in the network. However, subsequent studies have shown that modifiers actually have a well‐defined coordination shell with a fairly narrow distribution of M─O bond lengths, as exemplified in Figure 8 by the neutron correlation function for lithium disilicate glass where the Li─O bond peak is clearly apparent [14]. In contrast, however, there is little evidence that the oxygen coordination polyhedra of modifier cations generally have a well‐defined geometry, which would involve a narrow distribution of O──O bond angles.

Glass formers have strong bonds and a low coordination number, the tetrahedral value of four being the most common, whereas modifiers have weak bonds and coordination numbers typically greater than four. These high values, combined with a lower cation charge, mean that M─O bonds are much weaker and, hence, that all glass properties are profoundly altered by the introduction of modifiers. For example, the addition of Na₂O generally reduces viscosity, glass transition temperatures, and melting conditions.

There are other oxides known as conditional glass formers, because their structural role is intermediate between those of formers and modifiers. They do not readily form a glass on their own, but can readily do so in combination with a modifier as, for example, has been known for over a century for Al₂O₃ in CaO–Al₂O₃ melts. In addition to Al₂O₃, other notable conditional glass formers are Ga₂O₃, Sb₂O₃, TiO₂, TeO₂, V₂O₅, 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 AlO_4/2 and GaO_4/2 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 SbO_3/2 trigonal pyramids (Figure 7b). Contrastingly, tellurite glasses form a network based on two different units, TeO_4/2 pseudo‐trigonal bipyramids (or disphenoids, Figure 7e) and TeO_3/2 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 TiO_4/2 tetrahedra, although five‐coordinated O=TiO_4/2 units (cf. Figure 7g, but with a terminal apical oxygen) and octahedral TiO₆ units (Figure 7h) can be found in more complex glasses.

As listed in Table 1, the most commonly studied oxides can thus be classified structurally according to their role in the formation of glass networks [15]. This classification is not exact, but nonetheless represents a useful guide; for example, TeO₂ and Sb₂O₃ have been classified as conditional glass formers, but actually both vitrify if quenched rapidly enough. Besides, some oxides with a lone‐pair cation, such as PbO, are listed as both a modifier and a conditional glass former. Depending on whether or not the lone‐pair of electrons is stereochemically active, the oxide acts as a network former (with a low coordination number) or as a modifier (with a high coordination number).