Glass is a versatile and sometimes enigmatic substance. It functions equally well as a carrier of light, a protector of man and his inventions, an object of art, or an indispensable tool for the exploration of science. Depending upon composition, it can be made stronger than steel1 or soluble in water; a detector of nuclear radiations2 or the source of a powerful laser beam.3,4 Certain compositions are known to possess either negative, zero or positive expansion coefficients. Still others are colorless or black, or even variable in their light transmissive properties. 5,6,7 Electrical conductivities can vary from 10-3 − 10-18 ohm-1 cm-1 with both electronic and ionic type mechanisms being clearly substantiated. Paramagnetic or diamagnetic glasses can also be made, depending again upon choice of composition. Given this enormous diversity of properties, it is clear as to why the use of glass might transcend many industries and fields of science. It is also found in nature. Terrestrial (obsidian), celestial (tektites) and more recently, lunar varieties have been identified.
The composition of glasses may vary over wide limits and includes 60 of the chemical elements.
The properties of glass vary continuously with composition in a predictable manner. Among other things this means that unlike many crystalline substances, stoichiometry of a cooled glass is never a problem. In fact, that term is of little value in describing multicomponent glasses.
The internal energy of a glass is almost always greater than that corresponding to a crystalline phase of the same composition.
Glass is isotropic in its properties; it can be made extremely pure and homogeneous. It can also be fabricated into a variety of shapes and sizes.
Even without a complete and detailed knowledge of the structure of a cooled glass, the exploitation of the above factors is readily achieved by simply viewing glass as a substance somewhat continuous with its liquid state. This state, like all liquids, possesses a finite vapor pressure,8,9 a boiling point, a temperature dependent viscosity, and can be regarded as a solution of oxides or elements.
While at this liquid state, or at somewhat lower temperatures where conversion to a solid may ensue, many chemical reactions such as decomposition, oxidation-reduction, precipitation and/or dissolution, are readily achieved.10,11 For silicate systems, which are particularly viscous in their molten state, these reactions are very sensitive to time and temperature. Because these parameters are easily controlled, it is comparatively straight forward to start, halt, advance, or slow down these reactions as desired. Coupling this with a natural tendency for all glasses to revert to a lower energy crystalline state, gives rise to a highly desirable materials design capability. That is, as glassy solids convert to a crystalline phase(s), the properties of these solids will vary continously in a controllable manner from those associated with the parent glass to those characteristic of the crystalline phase.
All of these considerations constitute a powerful control feature for the fabrication of materials with specific properties within certain limits. In fact, the potential use of glass as a precursor for derivative materials represents an additional diversity unqiue in materials science.
KeywordsVitreous State Temperature Dependent Viscosity Expansion Curve Vitreous Silica Fictive Temperature
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