Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 170

What is a glass from a structural standpoint? There are different answers dependent upon whether the emphasis is on structure, preparation methods, or thermodynamic properties. However, a simple structural definition is adopted here, according to which a material must be solid and have a noncrystalline structure to be called a glass.

Put simply, the description of a crystal involves a unit cell, containing a particular arrangement of atoms, which is then replicated periodically in three dimensions to build up the structure as illustrated by the 2‐D (two dimensional) representation of Figure 1. Crystal structures have translational symmetry because, if crystallite boundaries are neglected, the environment of a particular atom is the same as that of all equivalent atoms in all unit cells. Contrastingly, a glass lacks translational symmetry and a detailed understanding of its structure requires as much experimental information as possible. In addition to X‐ray (XRD) and neutron (ND) diffraction, inelastic neutron scattering and various other spectroscopies (X‐ray absorption, infrared, Brillouin, Mössbauer, and X‐ray photoelectron spectroscopies) may be used (Chapter 2.1), along with electron microscopy, physical property data (especially density), and theoretical modeling.

As is clear to any reader of this Encyclopedia, many types of materials can vitrify if they are solidified rapidly enough to avoid crystallization. Leaving aside metallic glasses (Chapter 7.10) or organic polymers (Chapters 8.7 and 8.8), however, the majority of useful glassy materials are formed by oxides or chalcogenides. This is the reason why this chapter is restricted to these materials and to the elementary concepts that are used to describe their structure.

These structures can be understood well in terms of the continuous random network (CRN) model, first propounded by Zachariasen [1], because the atomic bonds have some covalent (directional) character. The basic features of this model will thus be reviewed and related to fundamental structural information gathered for silica glass, the archetypal glass former, and the “mother” of all amorphous silicates. Because microcrystalline descriptions of glass structures in fact preceded Zachariasen's model, their basic limitations will also be summarized. The structural changes induced by the addition of so‐called network modifiers in oxide glasses will then be discussed at short‐ and medium‐length scales, along with the intermediate character of some oxides that may act as glass formers only when combined with some modifiers. Finally, the manner in which network glasses can depart from Zachariasen's model will be illustrated with chalcogenides.