Network glasses

Early theories relating to the structure of glass included the crystallite theory whereby glass is an aggregate of crystallites (extremely small crystals). However, structural determinations of vitreous SiO₂ and GeO₂ made by Warren and co-workers in the 1930s using x-ray diffraction showed the structure of glass to be typical of an amorphous solid In 1932 Zachariasen introduced the random network theory of glass in which the nature of bonding in the glass is the same as in the crystal but where the basic structural units in a glass are connected in a random manner in contrast to the periodic arrangement in a crystalline material. Despite the lack of long range order, the structure of glass does exhibit a high degree of ordering on short length scales due to the chemical bonding constraints in local atomic polyhedra. For example, the SiO₄ tetrahedra that form the fundamental structural units in silica glass represent a high degree of order, i.e. every silicon atom is coordinated by 4 oxygen atoms and the nearest neighbour Si-O bond length exhibits only a narrow distribution throughout the structure. The tetrahedra in silica also form a network of ring structures which leads to ordering on more intermediate length scales of up to approximately 10 Angstroms.

The structure of glasses differs from the structure of liquids just above the Tg which is revealed by the XRD analysis and high-precision measurements of third- and fifth-order non-linear dielectric susceptibilities . Glasses are generally characterised by a higher degree of connectivity compared liquids.

Alternative views of the structure of liquids and glasses include the interstitialcy model and the model of string-like correlated motion. Molecular dynamics computer simulations indicate these two models are closely connected

Oxide glass components can be classified as network formers, intermediates, or network modifiers. Traditional network formers (e.g. silicon, boron, germanium) form a highly cross-linked network of chemical bonds. Intermediates (e.g. titanium, aluminium, zirconium, beryllium, magnesium, zinc) can behave both as a network former or a network modifier, depending on the glass composition. The modifiers (calcium, lead, lithium, sodium, potassium) alter the network structure; they are usually present as ions, compensated by nearby non-bridging oxygen atoms, bound by one covalent bond to the glass network and holding one negative charge to compensate for the positive ion nearby. Some elements can play multiple roles; e.g. lead can act both as a network former (Pb4+ replacing Si4+), or as a modifier. The presence of non-bridging oxygens lowers the relative number of strong bonds in the material and disrupts the network, decreasing the viscosity of the melt and lowering the melting temperature.

The alkali metal ions are small and mobile; their presence in a glass allows a degree of electrical conductivity. Their mobility decreases the chemical resistance of the glass, allowing leaching by water and facilitating corrosion. Alkaline earth ions, with their two positive charges and requirement for two non-bridging oxygen ions to compensate for their charge, are much less mobile themselves and hinder diffusion of other ions, especially the alkali's. The most common commercial glass types contain both alkali and alkaline earth ions (usually sodium and calcium), for easier processing and satisfying corrosion resistance. Corrosion resistance of glass can be increased by dealkalization, removal of the alkali ions from the glass surface by reaction with sulphur or fluorine compounds. Presence of alkaline metal ions has also detrimental effect to the loss tangent of the glass, and to its electrical resistance; glass manufactured for electronics (sealing, vacuum tubes, lamps ...) have to take this in account.

Crystalline SiO₂edit

Silica (the chemical compound SiO₂) has a number of distinct crystalline forms: quartz, tridymite, cristobalite, and others (including the high pressure polymorphs Stishovite and Coesite). Nearly all of them involve tetrahedral SiO₄ units linked together by shared vertices in different arrangements. Si-O bond lengths vary between the different crystal forms. For example, in α-quartz the bond length is 161 pm, whereas in α-tridymite it ranges from 154–171 pm. The Si-O-Si bond angle also varies from 140° in α-tridymite to 144° in α-quartz to 180° in β-tridymite.

Glassy SiO₂edit

In amorphous silica (fused quartz), the SiO₄ tetrahedra form a network that does not exhibit any long-range order. However, the tetrahedra themselves represent a high degree of local ordering, i.e. every silicon atom is coordinated by 4 oxygen atoms and the nearest neighbour Si-O bond length exhibits only a narrow distribution throughout the structure. If one consider the atomic network of silica as a mechanical truss, this structure is isostatic, in the sense that the number of constraints acting between the atoms equals the number of degrees of freedom of the latter. According to the rigidity theory, this allows this material to show a great forming ability. Despite the lack of ordering on extended length scales, the tetrahedra also form a network of ring-like structures which lead to ordering on intermediate length scales (up to approximately 10 Angstroms or so). Under the application of high pressure (approximately 40 GPa) silica glass undergoes a continuous polyamorphic phase transition into an octahedral form, i.e. the Si atoms are surrounded by 6 oxygen atoms instead of four in the ambient pressure tetrahedral glass.