Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 335

Glasses can be formed by various methods, including physical vapor deposition, solid‐state reactions, thermochemical and mechanochemical treatments, or liquid‐state reactions with sol–gel techniques (Chapter 8.1). Amorphous solids can also be prepared under the action of high pressure (Chapter 3.10) or by irradiation of crystals (Chapter 3.13). In industry or in Nature (Chapters 7.1 and 7.2), however, vitrification most frequently relies on the extremely strong viscosity increases when melts are cooled until the glass transition eventually takes place before nucleation and crystal growth have developed (Chapter 5.4). The topic dealt with in this chapter will thus be glass formation by melt cooling.

In a first approximation, the glass transition is conveniently characterized by a single parameter, the glass transition temperature T_g (Chapter 3.2). Under typical cooling rates of the order of 10 K/s, the standard T_g is the temperature at which the viscosity is about 10¹² Pa.s (10¹³ P) at the macroscopic observational timescales of 10²–10³ seconds that are relevant to actual glass formation. As defined in this way, T_g is always significantly lower than the melting (or liquidus) temperature T_m. It can be roughly estimated with the Kauzmann formula T_g ≈ 2T_m/3 [1].

In principle, any liquid vitrifies if the melt is cooled sufficiently fast to prevent crystallization from happening. This is by definition the case of the vast bulk of commercially used glasses, which are made up of oxides. In glass technology, SiO₂, GeO₂, B₂O₃, and P₂O₅ are archetypal glass formers in that they easily form glass networks by themselves or in combination with other oxides. But in practice it is not obvious to predict which materials readily vitrify and under what conditions they do so. As a matter of fact, the high viscosities that favor vitrification are related to structural factors whereas configurational complexity also contributes to frustrate crystallization. Here, particular attention will thus be paid not only to the kinetics of vitrification and its theoretical aspects but also to these factors.

In preamble, however, it is useful to examine the way in which glass is defined because of the possibly surprising fact that there is no generally accepted definition of this state of matter. Likewise, a few fundamental points will be summarized about relaxation, the process by which the structure and properties of an amorphous substance tend to reach their equilibrium values to vanish below the glass transition (Chapter 3.7).