Abstract

Inorganic glasses are important ceramic materials and glasses share many of the desirable properties of crystalline ceramics even though they lack long-range order. The structure of glasses is not completely random, however. Glasses do have short-range order in the arrangement of anions around cations.

Glasses, less thermodynamically stable than crystals, must be “captured” in a metastable state, and this is typically accomplished by rapid cooling of a liquid.

The simplest inorganic glasses are composed of only one element: B, C, P, As, S, or Se. Glassy selenium in thin-film form has been used extensively in the photocopying industry. Inorganic glasses containing more than one element can be broadly categorized according to the type of anion. There are two main categories: chalcogenide glasses and halide glasses. Several criteria for predicting which compounds form glasses have been proposed. The best known guidelines are those originally presented for oxide glasses and known as Zachariasen Rules.

The most striking and useful characteristic of common silicate glass is its transparency to visible light. This transparency results from the absence of grain boundaries and delocalized electrons, which tend to scatter and absorb light. There are many applications in which glass is used as an electrical insulator. One example is glass-to-metal seals. Moreover, other glasses are useful as a result of ionic or electronic conductivity. Glasses are good thermal insulators, making them ideal materials for windows in buildings. The thermal conductivity of a glass increases markedly as it is crystallized to form a glass–ceramic. The thermal conductivity of an aerogel is exceptionally low.

Glass processing techniques include melt processing, sol–gel processing, vapor-phase processing, and thin-film techniques. Silicate glass is a familiar, ubiquitous material, used in beverage containers, window panes, and automobile windshields. Increasingly, however, unusual glasses are being employed in high technology applications, such as optical fibers for telecommunications, graded-refractive-index glasses, nonlinear optical glasses, acousto-optic glasses, fast-ion conducting glasses, glass–ceramics, and glass microspheres.