Glass network interstices

Acid resistance This property is best appreciated when the glass structure is understood. Most enamel frits are complex alkali metal borosilicates and can be visualised as a network of Si04 tetrahedra and BO, triangular configurations containing alkali metals such as lithium, sodium and potassium or alkaline earth metals, especially calcium and barium, in the network interstices. 

Ions, Atoms and Molecules in Interstices of a Glass Network... 

The glass formers SiC>2, B2O3 and AI2O3 provide a fixed random three-dimensional network in the interstices of which are located the modifier ions such as Li+, Na+, K+, Ca2+ and Mg2+. Some of these ions, particularly Li+ and Na+, are very mobile whereas others, such as Ca2+ and Mg2+, serve only to block the network. A little reflection leads to the following reasonable expectations which are borne out by observation ... 

While the simple lattice vibration model works well for close packed structures, additional processes can occur in less tightly packed network structures such as those found in glasses. Bond bending can alter the positions of atoms, as can rotation about an axis. These processes can counteract the expansion of the bond length due to the increased amplitude of vibration, resulting in very low expansion coefficients. Filling of interstices inhibits these processes and tends to cause an increase in thermal expansion coefficients. 

Addition of a divalent modifier such as Ca to a sodium silicate glass decreases the diffusivity of Na ions. The much less mobile divalent ions occupy interstices in the network and block the diffusion of the more mobile monovalent ions. This effect on diffusivity is at least partially responsible for the improvement in chemical durability of alkali silicate glasses which occurs when alkaline earth oxides are added to the composition. 

The density and refractive index of soda-lime-silica glasses are greater than those of vitreous silica due to the filling of interstices in the network by the sodium and calcium ions. As a result, the density increases to =2.5 gcm, while the refractive index increases to = 1.51. The small variations in these properties with typical variations in glass composition for commercial soda-lime-silica glasses is rarely of pragmatic importance. 

The substitution of aluminum for silicon in a silica covalent network leads to a charge unbalance, which must be compensated by extra-framework cations, mostly alkaline. This occurs in the cases of the so-called stuffed silicas these materials have structures strictly related to the crystalline forms of silica, but with cations in the interstices to counterbalance the presence of A1 ions substituting for Si. This is the case, for example, of Eucriptite (LaAlSi04, a stuffed /3-quartz) or nepheline (NaAlSiOa, a stuffed tridymite). A similar mechanism also occurs in the amorphous networks of glasses [175]. 

The free unoccupied volume in the interstices formed by the connected network former oxide tetrahedra determines the basic volume and, therefore, the density of the glass. Any network modifiers that will occupy the empty interstices will lead to an increase of the density of the glasses. The density increase depends of course on the atomic mass and the concentration of the modifier cations within the glass. However, it is not unlimited. The limit depends on the size of the interstices as well as the radius of the network modifier cations. If the network modifier cations are large it will cause the original network to expand, i.e. the volume increases. 

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