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
to those in the crystals (Figure 2) [4], as have many subsequent diffraction and spectroscopic studies. The short‐range disorder is most obvious in distributions of Si─O─Si bond angles, which have been determined from methods such as X‐ray and neutron scattering, vibrational spectroscopy, and 29Si and 17O NMR. This disorder is in turn related to distributions in the sizes and connections among the rings that can be mapped in structural models. The lack of other major sources of disorder is probably why the entropy difference between the liquid and crystal is the lowest known for any oxide [5].

Schematic illustration of the oxygen linkages in crystalline and glassy CaTiSiO5 as seen by 17O MAS NMR. Ti–O–Ti and Si–O–Ti oxygens are abundant in both, but peaks are much narrower in the crystal because of the long-range order, whereas the glass contains much greater local-scale disorder. The glass also contains abundant structural groups absent from this crystal, such as Si–O–Si and Si–O–Ca oxygens, requiring a more complex structure. Spinning sidebands are marked by black dots.

      Source: Modified from [3].

      Source: Reprinted with permission from [4].

Schematic illustration of the two-dimensional sketch of a mixed network oxide glass such as B2O3–SiO2. Boroxol groups are particularly abundant in pure B2O3 glass. Another aspect of the disorder is the degree of mixing of network cations, which can be determined by methods that count the number of different oxygen bridges, e.g. between BO3 groups (light color) and SiO4 units (darker color).

      Results from even the earliest X‐ray scattering studies of alkali silicate glasses (Figure 2) supported this basic concept [4]. Likewise, many vibrational spectroscopy studies have demonstrated increasing proportions of NBOs within silicate tetrahedra with increasing modifier contents. “Network‐modifier” cations such as Na+ and Ca2+ balance the negative charge on each of the NBOs: bond‐valence considerations generally require several such cations for each NBO as seen in the corresponding crystals. At low modifier concentrations, there are not enough NBOs to fill the coordination sphere of each modifier, with typically five to eight oxygens needed. This means that without cation clustering to allow nonrandom sharing of NBOs, some BOs must also serve this coordinating role. With higher field strengths of the modifier cations (defined as the formal charge divided by the square of the mean cation–oxygen distance), this arrangement is increasingly unstable and can lead to liquid–liquid phase separation over wider ranges of composition. With very high field‐strength modifiers (e.g. La3+, Zr4+), or for cations with relatively high electronegativities (e.g. Sn2+, Pb2+), the structural and chemical distinctions between BO and NBO may begin to blur, in that some “modifier” cations may take on low coordination numbers and Si–O–M linkages may become relatively strong. Such cations are often described as having “intermediate” character.

Schematic illustration of the two-dimensional representation of the modification of a glass network induced by addition of an alkali oxide M2O. The SiO4, BO3, GeO4 polyhedra are symbolized by the triangles. (top) Conversion of one bridging oxygen to two non-bridging oxygens (small, dark circles), which are charge balanced by the alkali cation (large circles). (bottom) Increase in the coordination numbers of two network cations (squares) and the formation of bridging oxygens with partial negative charges (small, light circles). Conversion between these two types of modified network, with either pressure or composition, is 


                  <div class= Скачать книгу