An oxide such as Na₂O, K₂O or CaO that breaks bridges in a glass network to create non-bridging oxygens, with its cations occupying interstices in the network. It lowers the viscosity.
Japanese name: 網目修飾酸化物
Network modifier appears in 1 practice question across 1 topic, including Glasses and amorphous materials.
Network modifiers are oxides that do not form a glass network by themselves but enter the network of network formers and change its structure and properties. Typical examples are the alkali metal oxides Li₂O, Na₂O and K₂O and the alkaline earth oxides MgO, CaO and BaO. The oxygen brought in by a modifier breaks bridges in the network, and the cations occupy spaces in the network so as to neutralize the negative charge of nearby non-bridging oxygens. The cations are often surrounded by about five to eight oxygens and do not form strongly directional bonds like the network-forming cations. These weak, nondirectional bonds change the properties of glass greatly.
Whether an oxide forms or modifies the network is sometimes organized by the field strength Z/a², obtained from the cation charge Z and the cation–oxygen distance a (Å) (Dietzel's classification). As a rough guide, the network-forming Si⁴⁺ and B³⁺ have large values of about 1.3 to 2, the modifiers Na⁺ and K⁺ about 0.1–0.2 and Ca²⁺ about 0.3, while intermediate oxides such as Al³⁺ fall in between at about 0.5–1.0. Even among modifiers, divalent cations with stronger fields attract the surrounding oxygens strongly and hold the breaks in the network together, so they reduce chemical durability less than monovalent alkalis.
Adding modifiers weakens the connectivity of the network, lowering viscosity and the glass transition temperature so that the glass can be melted and formed at lower temperatures. In exchange, thermal expansion becomes larger and chemical durability lower. Alkali ions can also move through spaces in the network, giving glass a slight electrical conductivity. When two kinds of alkali are mixed, the mixed alkali effect is known, in which the electrical conductivity and ion diffusion coefficients fall greatly compared with a single alkali, even at the same total alkali content. The choice of type and amount of modifiers sets the balance between ease of melting, durability and electrical properties.
Soda-lime glass is a composition in which about 13% Na₂O is added to SiO₂ to make it easier to melt and about 10% CaO is added to restore durability, with a little MgO as well. K₂O acts much like Na₂O, and the two are often used together. PbO acts both as a modifier and as part of the network and raises the refractive index greatly, so it has been used in crystal glass. In chemical strengthening, the modifier cation Na⁺ in the surface layer is exchanged for the larger K⁺ to introduce compressive stress. Modifiers are the main means of adjusting the ease of melting, strength and optical properties of glass.
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.
Bridges in the network decrease and non-bridging oxygens increase, so viscosity and the glass transition temperature fall and the glass becomes easier to melt and form. On the other hand, thermal expansion grows and components leach more easily into water and acids, so chemical durability falls. Too much modifier makes the melt crystallize rather than form glass.
Because Na₂O and SiO₂ alone give a glass that melts easily but leaches readily into water and has low durability. Adding divalent Ca²⁺, which holds the breaks in the network together strongly, raises durability. Too much CaO makes crystallization easier, however, so the amount is adjusted.
It is not fully understood, but it is thought that alkali ions of different sizes create differently shaped sites in the network, and ions of the other kind move into those sites less readily, so the pathways for ions become blocked. It is used in glasses where electrical insulation or chemical durability is to be improved.
Na₂O, K₂O and CaO are network modifiers: they break the network and occupy its interstices, and they do not form glass by themselves.
Which set correctly lists network-forming oxides, which for…Network former Amorphous silicon Chemically strengthened glass Metallic glass Non-bridging oxygen Reduced glass transition temperature Tempered glass