Amorphous Materials: Glasses

Some liquids become extremely viscous as the temperatureA physical property that indicates whether one object can transfer thermal energy to another object. falls toward their freezing points, often because they consist of macromolecules. An example is quartz, SiO₂.

When quartz melts (at 1610°C), a few Si—O bonds break, but most remain intact. The liquid contains large covalently bonded fragments of the original structure and is highly viscous. When the liquid is cooled, the macromolecular fragments cannot readily slide past one another to attain the regular solid structure of quartz. Instead, a collection of interconnected, randomly oriented tetrahedrons of oxygen atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. surrounding silicon atoms is formed, as shown in the figure below. The material having this structure is known as fused silica.

A feasible structure for fused silica. Although each Si atom is connected to its regular tetrahedron of O atoms and each O atom to two Si atoms, there is no overall regular arrangement as in a crystal of quartz. This situation is sometimes described as short-range order and long-range disorder and is typical of all glassy materials, including those which are not compounds of Si.

Fused silica is an example of an amorphous material or glass. It is highly rigid at room temperature, but it does not have the long-range microscopic regularity of a solid crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point.. Consequently it cannot be made to cleave along a plane. Instead, like ordinary window glass, it shatters into irregular fragments when struck sharply. (Window glass is primarily silica, but oxides of sodium and calcium are added to lower the melting pointThe temperature at which a solid becomes a liquid. Also called freezing point..) Since the microscopic structure of a glass is random, like that of a liquid, scientific purists describe glasses as highly viscous liquids, not as solids.