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
composition at least for oxide glass‐forming liquids [38, 39]. Compared with the tremendous variations of viscosity with temperature and composition, G thus is almost constant. If the viscosity is known, structural relaxation times can be readily estimated from Eq. (7).

      2.4 Configurational Properties

      2.4.1 Equivalence of Relaxation Kinetics

      It is usually more difficult to account for the kinetics of a reaction than for its thermodynamics. Relaxation in glass‐forming systems does not depart from this rule. Whereas a single‐order parameter such as the fictive temperature may be appropriate for characterizing the volume or enthalpy of a glass, relaxation kinetics requires models much too complex to be discussed here (see Chapter 3.7). One can nonetheless have a first look at the mechanisms involved in relaxation by examining whether their kinetics varies or not with the particular property considered.

      As done for viscosity, the kinetics of volume equilibration can, for instance, be measured by isothermal dilatometry experiments. If samples with the same thermal history are studied, comparisons between the relaxation kinetics of different properties can be made in terms of normalized variables

      (8)equation

Graph depicts the kinetics of equilibration for the viscosity and volume of E glass. Differences between the ascending branches mainly due to the uncertainties on the Y0 values caused by unrecorded relaxation during the initial thermal equilibration of the sample.

      Source: Data from [28].

Graphs depict the equivalence of the relaxation kinetics for the enthalpy, volume, and viscosity illustrated by 1 : 1 correlations between the relevant glass-transition temperatures determined by differential thermal analysis, dilatometry, and viscometry.

      Source: Data from [28].

      2.4.2 Vibrational vs. Configurational Relaxation

Graph depicts the relative importance of configurational and vibrational relaxation with increasing temperatures for a given property Y (a) after instantaneous temperature jumps ∆T (b).

      Source: Data from [40].

Graphs depict the vibrational and configurational contributions to the volume change of CaMgSi2O6 liquid after an abrupt temperature decrease from 982 to 972 K.

      Source: Data from [40], cf. Chapter 3.5.

      (9)equation

      where the subscripts l and g refer to the liquid and glass phases, respectively, and a further simplification arises from the fact that Cpg(Tg) may be considered to be the Dulong–Petit harmonic limit of 3 R/g atom (R = gas constant) the isochoric heat capacity [41].

      2.4.3 A Microscopic Picture