Silica and Silicate Glasses

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

Silicon tetrachloride can be used to produce a smoke like powder known commercially as silica fume fumed silica, pyrogenic silica) by reaction in an oxygen-hydrogen flame  

The fluffy product is extremely finely divided (particle size typically 7-40 nm, powder density 0.03-0.06 g cm ) but nonporous (surface area ca. 200-400 m g )- Like fused silica, silica fume is amorphous, and this, together with the small particle size, gives it relatively high solubility. It is used as a thixotropic thickening agent for epoxy resins, in special high strength 

Similarly for the case of oxygen atom also there is a partial charge on it which is given by. 

Since there is one Si atom and 2.5 oxygen atoms in [Si03/20], the net 

There are three equations (12.02 to 12.04) and three unknowns (A, A and jjfeff), which are readily solved. The partial charges work out to be A5,= +0.116 A = -0.446. The net charge on Q3 is 1.0 A, . + 2.5A =-l. We will be making reference to this method of calculating partial charges in the discussion of several glasses discussed in this and later chapters. 

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The susceptibility of hardness measurements of silica and silicate glasses to environmental factors is consistent with the effects of water on the deformation of quartz. The load effect and indentation size effect appear to be a result of the frictional forces at the indenter-specimen interfaces. 

A good example of the role of such calculations is provided by the recent work of Vessal et al. (amplified in Chapter 12) who have developed by MD quenching procedures, models for both silica and silicate glasses. Plate III shows their model of vitreous Na2Si2C>5, an intriguing feature of which is the clear indication of a microsegregation of the framework modifying Na+ cations into loosely defined channels in the silicate matrix. Such behaviour is in accordance with the Modified Random Network (MRN) model of silicate systems... 

Computer simulation proved to be an excellent tool to study a large variety of problems. Stmctore of solids and interfacial processes were largely studied by numerical simulations of the corresponding real systems [49-52]. The specific cases of bulk silica and silicate glasses were reviewed quite recently [53]. Amorphous silica was the material chosen to study by computer simulation in several cases [41,54-56]. 

Bunker R C (1994) Molecular mechanisms for corrosion of silica and silicate glasses. J Non-Cryst Solids 179 300-308... 

E.A. Leed, C.G. Pantano, Computer modeling of water adsorption on silica and silicate glass fracture surfaces. J. Non-Cryst. Solids 325(1-3), 48-60 (2003)... 

Aqueous hydrogen fluoride is a weak acid (see above) and dissolves silica and silicates to form hexafluorosilicic acid hence glass is etched by the acid, which must be kept in polythene bottles. 

NH4HF2 is used to solubilize silica and silicates in siliceous rocks of oil wells, thus to regenerate oil flow as a neutralizer for alkalies in textile plants and commercial laundries for removing stains from fabrics for treating, polishing and rapid frosting of glass plates, window panes, picture frames, ampoules and optical lenses to produce pure salts of metal fluorides in treat-... 

Silicon is the most important constituent of igneous and many sedimentary rocks, occurring in combination with oxygen in feldspars, micas, quartz, sands and shales. The element is used in electronic devices, while silicon in combination with oxygen as silica and silicates finds application in concrete, bricks, pottery, enamels, glasses, optical fibers for telecommunications, and refractory (high-temperature resistant) materials. 

Although hydrofluoric acid is a relatively weak acid, it is unusually reactive in certain other respects. For example, it has an extremely corrosive action on the skin and produces burns which are very painful and slow to heal. Hydrofluoric acid also attacks the silica and silicates present in glass ... 

A survey of the literature [28, 58-65, 71, 72, 74-80] indicates that the most frequent exposure to silicon involves exposure to silica and silicates mainly in their crystalline forms. Health risks associated with the exposure to other sihcon containing compounds were reported in the mortality study of 16.661 manmade mineral fiber workers employed during 1945 to 1963 at one of 17 U.S. manufacturing plants [75]. Fiber exposure in the plants producing fibrous glass or mineral wool, or both, was associated with increased... 

The refractive index values of solid oxides lie in the range of 1.3 to 2.7 with silicate glasses they vary between 1.5 and 1.9, while extreme values exhibited by special glasses are 1.35 and 2.25. The Abbe value lies in the range of 20 to 100. Current soda-Iime--silica and borosilicate glasses have 1.48 — 1.53 and v = 58 —70. The refractive index is increased most by PbO and BaO. The dispersion usually increases simultaneously with refractive index. 

The Earth s crust is largely composed of silica and silicate minerals, which form the principal constituents of all rocks and of the sands, clays and soils that result from degradation of rocks. Most inorganic building materials are based on silicate minerals and include natural silicates such as sandstone, granite and slate, as well as manufactured materials such as cement, concrete (see Box 14.10) and ordinary glass (see above). Clays are used in the ceramics industry and mica is used as an electrical insulator. 

Thus, fibers are best formed from strong silica and silicate melts by down-drawing them from preforms or from conventional bushings. Glass fibers can be formed from fragile melts, but they must be super-cooled to a viscosity in the log 2.5 and log 3.0 poise range. 

An entire chapter, Chapter 6, is devoted to general and special purpose glass fibers derived from strong silica and silicate melts. Two excellent reviews of commercial general-purpose fibers [30-31] are also available. 

Introduction of additional species into silica creates silicate glasses with properties quite different from those of the parent pure silica system. Structurally, the addition of alkali metals to silica alters the structure, creates non-bridging oxygen (NBO), lowers the liquidus, and decreases durability in aqueous solution (Weyl and Marboe 1964 Doremus 1979 De Jong and Brown 1980 De Jong et al. 1981 ... 

Precipitated silicas and silicates have widespread applications in rubber. They have two distinct functions in rubber (1) as a reinforcing filler and (2) as an integral part of proprietary bonding systems. The products are produced by precipitation from sodium silicate (water glass) by reaction with acids. Replacement of the acid, in part or in total, by metal salts (of calcium or aluminium) results in the production of metal silicates. Because of the purity of the water glass used, the products are free from contaminants. The precipitate is filtered and dried using a variety of techniques. 

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