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How are glass, ceramics and glass-ceramics defined?

   

Frequently asked questions

Most of us think of glass as the transparent materials found as windows in buildings and cars. However, strictly speaking the term glass describes a state of matter where the atoms/molecules are randomly arranged, as in the diagram below.

An ASTM definition of glass describes the inorganic, amorphous, product of a rapidly cooled melt. Using this definition, rapidly cooled metals can also be said to be glassy. Terms such as glassy, amorphous and vitreous all describe the same thing; a material with a randomly arranged atomic structure.

Contrary to popular belief, glasses cannot be thought of as very viscous liquids. Glasses exhibit a glass transition temperature, below which they are true solids and above which they flow albeit as a very viscous liquid.

The diagrams below demonstrate some of the structural differences between a glass and a ceramic on the atomic scale, even though they can have exactly the same composition. For example, silica glass has the same composition as quartz (crystallised silica). However, in glass, the building blocks (SiO4 tetrahedra) are arranged randomly; whereas silica crystals have a very ordered structure.

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It is possible to turn a glass into a ceramic; by heating it up. This allows rearrangement from a random to an ordered structure and an ordered structure is more stable than a disordered one. Materials that are initially fabricated as glasses (and perhaps shaped using glass moulding techniques) and converted to a ceramic to enhance their properties are called glass-ceramics. A well known example of a glass-ceramic is the 'ceramic' cooker hob, shown in the photograph below, which has been developed to have a thermal expansion coefficient close to zero. This allows it to be rapidly heated and cooled without generating stresses in the hob material.

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