An oxide such as SiO₂, B₂O₃ or P₂O₅ whose cations form oxygen polyhedra that share corners, building a glass network even on its own.
Japanese name: 網目形成酸化物
Network former appears in 1 practice question across 1 topic, including Glasses and amorphous materials.
In 1932 Zachariasen proposed that an oxide forms glass readily when it can build a continuous network without long-range periodicity while keeping the same short-range bonding as the crystal. As conditions he named a small cation coordination number of 3 or 4, oxygen bonded to no more than two cations, polyhedra sharing only corners rather than edges or faces, and each polyhedron sharing at least three corners. SiO₂, B₂O₃, P₂O₅ and GeO₂ meet these conditions and form glass on their own.
Alkali and alkaline-earth oxides such as Na₂O, K₂O and CaO do not form glass by themselves; when they join the network, they break Si-O-Si bridges and create non-bridging oxygens. Their cations occupy interstices in the network, and these oxides are called network modifiers. Al₂O₃, ZnO, TiO₂ and the like are called intermediate oxides; they hardly form glass alone, but depending on composition they either join the network or modify it. For example, Al³⁺ whose charge is compensated by Na⁺ and the like enters the network as AlO₄ tetrahedra and reduces the number of non-bridging oxygens.
The more network formers and bridges a glass has, the higher its viscosity and glass transition temperature, the smaller its thermal expansion and the higher its chemical durability. Silica glass is the most fully connected glass and has excellent heat resistance, but shaping it requires very high temperatures. Soda-lime glass is a composition that lowers the forming temperature with Na₂O and restores durability with CaO. Borosilicate glass adds B₂O₃ to reduce thermal expansion and is used for laboratory ware and heat-resistant tableware. The proportions of formers and modifiers are chosen to suit the application.
Answer: C. SiO₂, B₂O₃, P₂O₅
In SiO₂, B₂O₃, P₂O₅ and GeO₂, the cations form polyhedra with three- or fourfold oxygen coordination that link by sharing their corner oxygens, so each forms a glass network on its own. Zachariasen named conditions such as a small cation coordination number and polyhedra sharing only corners, not edges or faces, as favoring glass formation. Na₂O, K₂O and CaO are network modifiers: they break the network and occupy its interstices, and they do not form glass by themselves. Al₂O₃ and ZnO are called intermediate oxides; they hardly form glass alone, but when their charge is compensated by alkalis and the like they join the network, for example as AlO₄ tetrahedra. MgO usually acts as a modifier.
In B₂O₃ glass, planar triangular BO₃ units, in which boron is bonded to three oxygens, share corners to form a network. Adding alkali converts some boron to fourfold-coordinated BO₄ tetrahedra, strengthening the network, so up to a certain composition property changes are not simple. This is called the boron anomaly.
Yes. Silica glass (fused silica), made of SiO₂ alone, is the classic example; its thermal expansion is very small and it transmits ultraviolet light well. Its viscosity is high, however, and melting and shaping it requires temperatures of about 2000 °C, so it is costly to make.
Metallic glasses are held together by metallic bonding and have no directional network like oxide glasses. They become glass through a densely packed, disordered arrangement of atoms, and how readily they form depends on differences in atomic size and on the heat of mixing.
Network modifier Amorphous silicon Chemically strengthened glass Metallic glass Non-bridging oxygen Reduced glass transition temperature Tempered glass