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

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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rel="nofollow" href="#fb3_img_img_a2235d2c-77e9-5dbc-9a0d-c84c056e4697.gif" alt="Photos depict the visualization of MD simulations of two tetrahedral glasses with vastly different atomic volumes but both conforming to the CRN prescription. (a) SiO2 glass (11.1 Å3). (b) Hybrid glass ZIF-8 (Zn(C4H5N2)2) (73.4 Å3)."/>

      Source: Images courtesy of J. Du (a) and W. Chen (b).

      As for metallic glasses (Chapter 7.10), these also exhibit soft collective vibrations whose origins are similar to oxide glasses boson peaks [3, 20]. If these are accessed from low‐temperature Cp experiments, then the enthalpy captured at supercooled temperatures can also be recorded. A direct link exists between IBP and the glass enthalpy, which can be reduced by annealing Figure 5f [20]. As annealing increases, the glass density, νBP, also increases while IBP decreases (compare Figure 5e and f).

      Whereas inelastic scattering S(ω) measures the VDOS integrated over Q, and the structure factor S(Q) time‐averaged atomic distributions, both derive from the dynamic structure factor S(Q,ω), which through comparisons with experiment affords a global view of the structure and dynamics of glassy systems and melts over extended regions of space and time. Related to S(Q,ω) is the intermediate scattering function F(Q,t), which registers structural relaxation from liquid to glass as a function of time [1]. In the limit t → ∞ F(Q,t)/S(Q) yields the non‐ergodicity factor f(Q,T), which is particularly relevant in the present context as it records the degree to which a liquid departs from thermodynamic equilibrium as it is supercooled (Section 4.1). It is readily measured using inelastic X‐ray scattering (IXS). Structural relaxation is dominated by fast β processes at high temperatures, with slow α processes emerging through the supercooled region, only to be frozen out at the glass transition Tg.

      3.1 Network Glasses

Graphs depict the collective atomic vibrations involved in the boson peak observed either dynamically in the reduced density of states g(E)/E2 (a, b) or thermodynamically in the non-Debye excess low-temperature specific heat Cp/T3 (c, d) for silica (left) and densified silica (right). Similar features occur in crystalline SiO2 isomorphs of similar density. (e) INS spectra of the collapse of zeolite Y, the cage subunits merging into a single peak of lower intensity IBP as a glass is formed while νBP increases – dashed arrow. (f) Boson peak in the metallic glass Zr50Cu40Al10 where annealing increases the density, but decreases the trapped enthalpy and the Cp/T3 intensity IBP falls as νBP increases – dashed arrow.

      Source: (a–d) Reproduced from [18] © (2014) APS; (e) reproduced from [16] © (2005) AAAS; (f) reproduced from [20] © AIP.

      In network glasses LRO begins at around 6 Å – the width of a typical sixfold ring (Figures 1 and 4) – and continues as far as out as features in the RDF can be discerned (Figure 2). Providing a direct link with a multiplicity of rings of corner‐sharing polyhedra with different sizes, LRO is perpetuated through modest variations in bond angles, as illustrated in Figure 6 with the two‐dimensional (2‐D) distributions directly observed for silica [1, 22]. Combinations of experimental RDFs with computer simulations afford 3‐D models of network topology where rings are often puckered in conformations foreign to crystalline geometries through variation and twisting of dihedral angles (Figures 1 and 4). The network statistics in SiO2 glass include five‐, six‐, and sevenfold rings, as illustrated schematically in Figure 7, in contrast to the sixfold ring topology of crystalline silicates. In addition, three‐ and fourfold rings are also found, but in much smaller proportions [1, 6]. They have been identified with the oxygen “defects” that give rise to breathing modes in Raman spectra [1]. These miniature rings increase in number when pressure is applied, for example, in indentation experiments. The converse applies in B2O3