Research on glass

Glasses and other amorphous materials -unlike crystals- have no long-range translational order. There is, however, some local order which is governed by the local potentials of the ions or atoms forming the glass network. While interatomic distances are fairly fixed given that the coordination number does not change, bond angles usually have a wider distribution in glasses. The coordination of network ions (or atoms) is determined by the local electron densities and potentials. Hence, coordination polyhedra (such as SiO4 tetrahedra or occasionally AlO6 octahedra) similar to those found in crystals occur also in amorphous materials. The network formed by the linkages between these polyhedra of network formers is interrupted by network modifiers such as alkali or alkaline earth ions which cause some oxygens at the corners of coordination polyhedra to be unconnected to neighbouring polyhedra ("non-bridging oxygens").

Fig: Bond angle. Fig: Coordination polyhedron: SiO<sub>4</sub> tetrahedron. Fig: Connectivity: Q<sup>3</sup> species. Bond angles, coordination polyhedra (such as this SiO4 tetrahedron) and their connectivity determine the structure of an amorphous network. A "Qn species" is an SiO4 tetrahedron with n bridging oxygens linking it to neighbouring SiO4 tetrahedra, hence leaving (4-n) oxygens at the corners non-bridging.

The ability of a melt to form a glass on cooling depends on its composition, particularly the amount of network modifiers present, since breaking oxygen bridges allows the network to become more flexible. A weakly glass-forming system may tend to crystallise, i.e. to form glass ceramics as a consequence of the precipitation of a stoichiometric crystalline component. In some cases, a phase separation into two amorphous phases, a modifier-rich one and a network former-rich phase, has been observed. We have observed this type of behaviour in potassium silicate glasses.
The technical process of batch melting, i.e. the formation of a homogeneous melt from raw material powders in bulk quantities, faces the problem that the different crystalline raw materials have different melting points. Hence, part of the chemical reactions taking place during batch melting are solid-solid or solid-liquid reactions at the (quartz) grain surface which are dominated by cation diffusion through the moving interface. An understanding of the local thermodynamics and kinetics will help balance melting temperatures and grain size to save energy and cost in the process.

Related pulications:
Batch melting kinetics / NMR
Batch melting of Na2O-SiO2 mixtures / NMR
Na2O-CaO-SiO2 glasses with variable modifier content / NMR
Phase separation in K2O-SiO2 glasses / WAXS and NMR

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