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
but could be expected in germanates and in high‐pressure silicates.

      7.1 Order and Disorder of Bridging and Non‐bridging Oxygens

      For systems with more than one type of modifier cation, large differences in size or charge might be expected to lead to some kind of ordering. This can be sampled by methods that “see” the coordination environments of oxygen species directly. For example, both one‐ and two‐2‐D 17O NMR spectra have shown that chemical shift distributions in mixed Na/K and mixed Ca/Mg silicate glasses are consistent with random distributions of the two cations around the NBOs, consistent with deductions from viscosity measurements and estimated configurational entropies. In contrast, the large field‐strength difference between K+ and Mg2+ leads to a concentration of the latter around the available NBO sites. This approach has shown the considerable complexity of order/disorder patterns that can occur in the distributions of modifier‐ and charge‐compensator cations around both NBO and BO sites in a number of other mixed‐cation silicates and aluminosilicates. Decreases in this type of ordering might be expected from entropic considerations at higher temperatures in the liquids.

      7.2 Qn Speciation

      In silicate and phosphate glasses, the best‐known aspect of network order/disorder, as affected by the modifier cations, involves the distributions of the NBOs. One can sample these distributions by counting the proportions of “Q n ” species, defined as tetrahedral groups with n BOs and 4‐n NBOs. Such measurements were originally done by Raman spectroscopy [2]. At least in some simple systems such as alkali silicates and phosphates, these proportions can often be readily quantified by 29Si or 31P NMR; such results can be used to evaluate cross sections for vibrational spectroscopy as well. In both approaches, peak fitting and associated assumptions about line shapes are usually required and can lead to some ambiguity. An early and important question was whether the number of Q n species present in a glass was the minimum derived from stoichiometry (in general two, but only one at special compositions), or whether entropy induced a greater variety. The clear detection of Q 2, Q 3, and Q 4 species in glasses such as Na2Si2O5, which could contain only Q 3 as in the crystal, confirmed the latter view. Simple equilibria among Q n species can be defined for n = 1, 2, 3:

      Apparent equilibrium constants kn for such reactions have been evaluated from Raman and NMR data on glasses, including 2‐D 29Si NMR on Ca and Mg silicates [15]. Higher field‐strength modifiers push these reactions to the right (increasing kn), presumably favoring the concentration of more NBOs around some Si sites and thus better local charge balance for small, highly charged modifiers. More Q 4 species are also generated, which correlates with higher thermodynamic activities of silica as deduced from phase diagrams [5]. In the range of observed speciation, greater kn values lead to greater contributions to the configurational entropy if models of random mixing of the Q n species are considered. But enhanced ordering of NBOs around higher field‐strength cations could counter this effect to some degree and, in the extreme, could lead to cation clustering and even incipient phase separation. Shifts of such equilibria have been measured with both in‐situ high‐temperature Raman spectroscopy and NMR on glasses with increasing fictive temperatures, the results from the two methods often agreeing well. Estimated enthalpies of reaction are usually positive, but are less so for higher field‐strength cations. If a random model is assumed (which can in some cases be tested by NMR methods yielding spatial correlations of different species), mixing of observed Q n populations can contribute a substantial fraction of the calorimetrically determined entropy differences between glasses and crystals.

      Complementary to these results for glasses in the normal, high‐silica, glass‐forming range are the recent findings for orthosilicate (e.g. Mg2SiO4) and even “sub‐orthosilicate” glasses formed by quenching in laser‐heated, gas‐levitated, containerless melting systems. Here, significant concentrations of Q 1 species can be observed by Raman and 29Si NMR, requiring as well the presence of nonstoichiometric “free oxide” ions. Direct evidence for the latter can be seen in 17O NMR spectra [6].

      7.3 Order/Disorder in Network Linkages

      The distribution of network cations (e.g. Si4+, Al3+, B3+, P5+) around BO in multicomponent oxide glasses and melts is in principle relatively simple to characterize, as each BO has only two such neighbors. This is a quite important problem, though, as it defines the extent of disorder among the various network components as well as the partitioning of partial charges on the BO, which in turn affects ordering of modifier or charge‐compensator cations. Multiple‐quantum 17O NMR has allowed such distributions to be directly quantified in some systems, through direct counting of proportions of different linkages among network species.

      Triple‐quantum 17O NMR spectra of alkali aluminosilicate glasses can fully resolve Si–O–Si, Si–O–Al, and Al–O–Al sites. This has enabled more precise formulations of the thermodynamics of equilibria such as:

      (4)equation