Glasses are a unique range of ceramic materials defined principally by their atomic structure.
Glasses do not exhibit the ordered crystalline structure of most other ceramics but instead have a highly disordered amorphous structure. This gives them very different properties to other crystalline ceramics.
The most widely used glasses are silicate glasses, formed from silica, SiO2. Silica consists of a 3D network of tetrahedra where every corner oxygen atom is shared with the adjacent tetrahedron. This SiO2 tetrahedral unit is also incorporated into chains and sheets (clays), forming different ceramics.
Pure silica can be made to exist as a glass, and is called fused silica. A glass is a material that has hardened and become rigid without crystallising, making it amorphous. Silicate glasses are the most widely used glasses.
All of the most important glasses are based on silica SiO2. Two glasses of special interest are soda-lime glass, used in windows and bottles, with composition 70SiO2.10CaO.15Na2O, and borosilicate glass, used in cooking and chemical glassware, with composition 80SiO2.15B2O3.5Na2O.
The ability of an oxide to form a glass upon cooling, i.e. an amorphous structure, depends on the structural relationship between the oxygen atoms and the cations of the oxide compound. In order to achieve a glass structure the oxide cations will bind with the oxygen atoms to form a tetrahedral network.
Oxides that form glasses are known as network formers. Common examples are SiO2, B2O3, GeO2, and P2O5.
Certain other oxides can be added to substitute for Si atoms in the tetrahedral structure. These oxides become part of the network and act as a stabiliser.
The oxides are known as intermediates and, in a similar manner to network modifiers, lower the melting point and viscosity of the glass thereby allowing it to be worked at lower temperatures.
Common examples of intermediates are TiO2, ZnO, PbO, and Al2O3.
Glass with added sodium as a network modifier.
Glass with added titanium as an intermediate.