Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 177

In pure SiO₂ glass, all silicon atoms are bonded to four BOs. However, as a modifier is added, the average number of BOs bonded to a silicon decreases, and there is a corresponding increase in the number of NBOs bonded to a silicon. Experimental evidence indicates very strongly that all of the silicon atoms remain tetrahedrally coordinated in almost all silicate glass systems. Thus, when a modifier is added, there is a mixture of Q ⁿ tetrahedra, where n and 4 − n are the numbers of BOs and NBOs bonded to the silicon, respectively (cf. Chapter 2.4). Hence, Q ⁴ represents a silicon atom bonded to four BOs (as in pure SiO₂ glass), Q ³ a silicon atom bonded to three BOs and one NBO, and so on. The abundances of these species can be quantitatively determined by ²⁹Si NMR measurements as illustrated in Figure 9 for lithium silicate glasses [16]. The distribution of Si sites between the different Q ⁿ ‐species is not statistically random, but, to first approximation, follows instead a binary rule: the addition of small amounts of modifier to the glass leads to the conversion of Q ⁴ to only Q ³ until composition J = Li₂O/SiO₂ = 0.5, equivalent to 33.3 mol % Li₂O, is reached, at which all silicons are on Q ³ sites; then follows a region of composition 0.5 < J < 1.0, where the addition of more modifier leads to the conversion of Q ³ sites to only Q ², and so on. This evolution has important consequences for the connectivity of the silicate network. For example, a glass with a majority of Q ² sites is dominated by chains (or isolated rings) of silicon tetrahedra. When J > 1.0 (i.e. for less than 50 mol % SiO₂), the 3‐D connectivity of the structure breaks down. These materials are known as invert glasses since their structure is dominated by the bonds to the modifier cations, thus inverting the roles of these cations.