Structure of Silica Glass

Fig. 6.6. Structure of silica glass (a) the silicon tetrahedron and (b) noncrystalline network of silicon tetrahedra.

Next to materials of the glass-ceramics type, many varieties of chalcedony, such as agate, carneol, onyx, sardonyx, heliotrope and jasper, exhibit similar changes in hardness resulting from different consolidation of the cryptocrystalline structure of silica among mineral individuals. 

Very recently J.D. Mills et al. [19] have written a nano-structured Bragg reflection grating within the volume of silica glass using focused infrared laser light. Such photo-induced structures have potential for a variety of photonics applications, including information storage. 

Hydrogen can be forced at high pressure into the structure of silica-germania glasses. It has been proposed that the photosensitivity of these materials is related to the formation of Si-OH, so that in defect-free glasses the photoactivity depends on the presence of hydrogen. The H NMR characteristics of the different hydrogen centres... 

We argued earlier that some of the Maxwell constraints can be removed by the action of symmetry, and this allows for the possibility of a structure having some RUM flexibility. This, however, is not the whole story. A network glass, such as silica, has no internal symmetry, and might therefore be thought incapable of supporting RUMs. Recent calculations of the RUM density of states of silica glass have shown that this is not the case. 

The so called porous silica is made by the acid etching of annealed sodium borosilicate glass. The bulk structure is glass mainly of silica and has a uniform pore size distribution in the mesopore region. These are used as a source of silica glass. 

These are methods for the simulation of a flat amorphous surface. To simulate the atomic structure of a porous oxide adsorbent like silica gel, one may first simulate the bulk amorphous silica. Then cut out of it globules and arrange them in space to model the pore structure of silica gel. Other applications of this idea include the creation of pores such as those found in porous glass by deleting atoms from a simulated block of solid in such a way as to leave a cylindrical pore. 

Blanc obtained a porous structure of silica by leaching leucite (KA1 Si2 O5) with strong acid. Ultrafilters of sintered glass have been reported which have a pore diameter of about 1.5p. Zeolite crystals have been suggested as molecular sieves. Manning plated nickel on wire gauze of nickel and bronze to get pore size of 50 mp to 300 mp. 

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