Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 333

Figure 1 The influence of quench rate on the physical properties of a window glass: thermal strengthening visualized by the distribution of internal stresses in a Prince Rupert's drop (top). Determination made from an analysis of high‐precision polarimetry measurements (bottom) of the strain birefringence (Strain‐Matic©, Ilis GmbH, Germany).

Source: Photo by Henning Katte of a drop prepared by Armin Lenhart, courtesy Dominique de Ligny.

In most applications glass needs to be inert with respect to its environment. A particular glass is then selected for a given use on the basis of its physical properties. The range of glasses to be dealt in both industrial and natural contexts makes it necessary to pay attention to the strong dependences of physical properties on chemical composition, whose understanding is facilitated by the structural concepts presented in Section II. Because one produces most glasses by cooling melts, however, the fundamental issues to be discussed first are why vitrifiability varies so much with composition and how the glass transition varies with the cooling rate.

In the first chapter of this section both issues are discussed by M.J. Ojovan in the light of energetic, microscopic, and structural criteria (Chapter 3.1). Owing to the nonequilibrium nature of the glass transition, its thermodynamic treatment requires new concepts. From the notions of affinity and order parameter and simple relaxation models based on calorimetric measurements, insights are derived by J.‐L. Garden and H. Guillou on the entropy irreversibly created at the glass transition and on processes such as physical aging below T_g (Chapter 3.2). Within the framework of irreversible thermodynamics, the issue of entropy at the glass transition is then examined by P. Gujrati who shows how both communal entropy and free volume vanish at an ideal glass transition where the viscosity divergence would take place (Chapter 3.3).

Following these theoretical accounts, three chapters consider specific physical properties. In the first, B. Hehlen and B. Rufflé deal with the various modes of vibrations existing in glasses, paying particular attention to the low temperatures at which the boson peak represents an excess of vibrational modes with respect to those predicted by the Debye model for crystals (Chapter 3.4). Of direct practical interest, the densities of glasses and melts and their temperature and pressure derivatives are then reviewed by M. Toplis who also presents empirical models predicting these properties as a function of chemical composition (Chapter 3.5). A similar approach is followed by P. Richet and D. de Ligny in the next chapter, which is mainly devoted to heat capacity and entropy from near 0 K to superliquidus temperatures: at lower temperatures, the properties of glasses are exclusively vibrational and mainly determined by the oxygen coordination of cations, whereas the picture is markedly more complicated above the glass transition by configurational contributions to the properties of liquids, whose nature remains largely elusive (Chapter 3.6).

After these overviews of key physical properties, the ground is ready for a thorough discussion of relaxation processes. Relying mainly on calorimetric measurements, U. Fotheringham describes how the concept of fictive temperature can be incorporated into various relaxation models to predict accurately features of great practical interest such as thermal shrinkage and index of refraction changes as a function of time and temperature (Chapter 3.7). Because of extreme metastability, a special case of relaxation is that of glasses quenched at rates of the order of 10⁶ K/min. As revealed by calorimetric experiments examined by Y. Yue, these hyperquenched glasses do show unusual features related to structural heterogeneities and to the existence of fragile‐to‐strong rheological transitions in glass‐forming systems (Chapter 3.8).

The existence of polyamorphism, i.e. transitions from one amorphous phase to another, has recently been an unexpected discovery because the structure and properties of glasses were instead thought to vary continuously as a function of the quench temperature and pressure. As reviewed by P. McMillan and M. Wilding, these abrupt phase changes akin to first‐order transitions in crystals have been extensively investigated and their origin accounted for in terms of the topology of configurational‐energy landscapes (Chapter 3.9). Amorphous phases can also be prepared by high‐pressure compression of crystals that undergo structural collapse when their elastic limits are exceeded. As explained again in terms of configurational‐energy landscape by P. McMillan, D. Machon, and M. Wilding, the similarity of these phases with the dense polyamorphs formed at high pressures is not fortuitous (Chapter 3.10).

From the standpoint of mechanical stability, amorphous phases display a much greater variety of compression mechanisms than crystals, thanks to the lack of structural constraints imposed by the symmetry of a crystal lattice. In addition, the intrinsic strength of glasses should in principle be limited only by that of interatomic bonds since it is not reduced by grain boundaries. As explained by R. Hand, however, the actual strength is hundred times smaller because of the existence of surface flaws where the accumulation of stresses triggers breakage; whereas the abundance of flaws increases with time and their effects have to be treated statistically, their creation can be limited in various ways (Chapter 3.11). Strategies for strengthening glass are described in more detail by S. Karlsson and L. Wondraczek; in addition to elimination of flaws by mechanical polishing or flame finishing, these involve the creation of compressive stresses at the glass surfaces by either physical or chemical means, i.e. tempering by rapid, homogenous cooling (Figure 1) or exchange of smaller by bigger ions, e.g. replacement of Na⁺ by K⁺ (Chapter 3.12). Irradiation by energetic photons or subatomic particles is another source of defects reviewed by N. Ollier, S. Girard, and S. Peuget who describe their effects on optical and mechanical properties and the ways in which these can be reduced (Chapter 3.13).

The last chapter is devoted to the special case of amorphous ices, which have recently received much attention not so much because of their cosmochemical importance, but because they are prime illustrations of polyamorphism. Their formation and properties are thus reviewed by R. Tournier who goes into the details of a thermodynamic model of nucleation and growth originally designed for crystals – including numerical applications – to account for the formation of these amorphous ices and, in addition, to throw valuable light on the general problem of the glass transition in terms of transformations between supercooled liquids of different densities (Chapter 3.14).