Amorphous solid

An amorphous solid is a solid in which the atoms do not have an ordered atomic structure. Solids which are not amorphous are called crystalline solids. Most classes of materials can be found in the amorphous form, common window glass is a ceramic, many plastic materials are amorphous, and metals can be made amorphous by rapid solidification or ion implantation

Amorphous solids are also known as glasses. However, the term glass traditionally refers to amorphous oxides, and especially silicate based materials. Amorphous solids which are not oxides can also be called glass, but are often referred to with special terminology, for example amorphous metals could be called 'metallic glasses' .

There is no fine line between the completely amorphous solid and the crystalline solid with very small grain size. X-ray diffraction techniques can be used to probe the crystal structure of a material, and when some amorphous solids begin to crystallize, there is not necessarily a sudden transition to crystallinity. Thus, some materials cannot be classified as purely amorphous. Scientists use the phrase 'short range order' to speak of crystalline ordering over small lengths scales, i.e. on the order of 1 nm.

There is, however, a very pronounced transition between a very viscous liquid and an amorphous solid. This is the glass transition, which occurs at a temperature significantly below the melting temperature. The transition temperature depends on cooling rate, with the glass transition occuring at higher temperatures when faster cooling rates are applied. The glass transition is a second order transition, in that there is no enthalpy associated with it. Compare this with melting ( a first order transition ) where the enthalpy of melting ( also called heat of melting ) is the energy absorbed by the solid in order to melt it.