Vitreous silica applications

Astrakanite, 5 785t 22 846, 863 Astringents, 7 847-848 Astronomy, vitreous silica applications in, 22 443-444... 

Although vitreous silica is nominally a homogeneous isotropic amorphous material, and should normally remain so during its service life, it is in fact in a metastable condition. The tendency to revert to crystalline forms with attendant deterioration in mechanical durability places severe limitations on the range of applications. Figure 18.2 illustrates the polymorphic forms of silica, and the dimensional changes accompanying each transition. 

Neutron irradiation of silicon, 22 487 of vitreous silica, 22 435 Neutron moderators, deuterium application, 8 460... 

Optical absorption, of hydrogenated and hydrogen- free films, 17 206 Optical amplifiers, 11 145-146 Optical applications U.S. patents in, 12 614t of vitreous silica, 22 440-441 Optical cavities, 14 849 Optical cells, for high pressure measurements, 13 417-419 Optical coatings, cerium application,... 

Optical elements, liquid crystalline materials in, 15 116—117 Optical emission spectra, 14 833-837 plutonium, 19 671—673 Optical emission spectroscopy (OES), archaeological materials, 5 742 Optical fiber(s), 13 391-392 24 618 defects in, 11 145 drawing of, 11 141-145 fabrication of, 11 135-141 health care applications for, 13 397 overcladding of, 11 144 remote measurements using, 14 234 in sensors, 22 270-271 sol-gel processing of, 11 144-145 strength of, 11 141-145 vitreous silica in, 22 444 Optical fiber sensors, 12 614-616 Optical germanium, 12 556... 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

Transparent vitreous silica chemical durability of, 22 417 density of, 22 422 devitrification of, 22 421 manufacture of, 22 412-415 viscosity of, 22 424t Transpeptidases, 3 27 Transport. See also Transportation of ascorbic acid, 25 771 of hydrated lime, 15 56-57 of quicklime, 15 56 of radioactive waste, 25 855-856 in waste collection, 25 869-870 Transportation, 25 322-348. See also Shipping Transport aluminum applications, 2 340-341 cost of, 25 323... 

The diversity of silica forms and their properties leads to a broad range of applications. Reviews on usage include those on crystalline and vitreous silicas (94), on silica sols (95), on silica gels and precipitated silicas (96), and on pyrogenic silicas, precipitated silicas, and silica gels (97). 

Vitreous silica has a wide range of commercial and scientific applications. Its unique combination of physical properties includes good chemical resistance, minimal thermal expansion, high refractoriness, and excellent optical transmission from the ultraviolet to the near-infrared. 

The diffusion of metal ions in vitreous silica has not been studied as extensively as that of the gaseous species. The alkali metals have received the most attention because their behavior is important in electrical applications. The diffusion coefficients for various metal ions are listed in Table 5. The general trend is for the diffusion coefficient to increase with larger ionic sizes and higher valences. 

Thermal Applications. The protection of precious-metal thermocouples in high temperature pyrometry is an important application of vitreous silica. Although satin tubing is usually employed, transparent tubes are superior for protecting couples when used in a reducing atmosphere (220). 

Optical Applications. Vitreous silica is ideal for many optical applications because of its excellent ultraviolet transmission, resistance to radiation darkening, optical polishing properties, and physical and chemical stability. It is used for prisms, lenses, cells, windows, and other optical components where ultraviolet transmission is critical. Cuvettes used in scatter and spectrophotometer cells are manufactured from fused silica and fused quartz because of the transmissive properties and high purity (222). 

Mechanical Applications. The volume of vitreous silica used for fibers is a very small part of the total consumption. However, some interesting and significant applications have been developed in the laboratory, particulady in the area of measurements. 

Antireflective (AR) coatings are required on optics to reduce the reflective surface losses. Vitreous silica coatings in the form of porous or multilayer films are used extensively in this application. Antireflective coatings have been developed which employ colloidal fused silica sol—gel particles made from organometallic materials (253). 

Historically, fused quartz referred to transparent products produced from quartz crystal rock, and fused silica referred to opaque products produced from sand. With the advent of new manufacturing techniques, transparent products can now be produced from sand, so the old distinction is no longer applicable. Currently, the term fused quartz is used whenever the raw product is either quartz rock or sand. The term fused silica is used whenever the raw product is synthetically derived (from SiCl4). Generically, the term quartz glass or, better yet, vitreous silica can be used to cover the whole range of materials. 

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