Glasses and Partial Crystallinity

Abstract

Glasses are non-crystalline or amorphous elastic solids formed by cooling from the fully molten state at a rate sufficiently rapid that crystallisation is prevented. The glass, or vitreous, state can be achieved in all classes of materials. Inorganic glasses have compositions which are the same as or similar to those of ceramics. Many polymeric materials exist in the glass state and, recently, metallic glasses have been developed. A glass is not necessarily structureless. For example, in vitreous silica the bonding between atoms is the same as in the crystalline forms of silica with each silicon atom bonded to four oxygen atoms forming an SiO₄ tetrahedron. The difference is that the tetrahedra are linked together into a large symmetrical array in the crystalline forms whereas, in vitreous silica, there is no long range order (see Figure 6.2).