Silica vitreous

Vitreous siUca has a wide range of commercial and scientific appHcations. Its unique combination of physical properties iacludes good chemical resistance, minimal thermal expansion, high refractotiness, and excellent optical transmission from the ultraviolet to the near-iafrared. 

Although vitreous siUca is a simple, single-component glass, its properties can vary significantly, depending on thermal history, the type and concentration of defects, and impurities. Vitreous siUca can, however, be one of the purest commercially available glassy materials. In synthetic vitreous sihcas, for example, total metal contamination is typically measured ia the 50—100 ppb range. Even at such a low level of impurities, differences ia properties, such as uv-transmission, are observed for various siUcas. 

Type Method OH Impurities, ppm Cl Cations Uv cutoff, nm Representative glasses 

II flame fusion of quart2 200-500 0 10-50 210 Homosil, OptosH, Vitreosil-055 

III flame hydrolysis 20 600-1200 50-100 1 170 Corning 7940, Dynasil 1000, Shinetsu P-10, SpectrosH, SuprasH, NSG-ES 

The structure of vitreous silica has regions of highly stressed bonds and defects such as oxygen vacancies, represented by Si-Si bonds, and peroxy defects, represented by Si-O-O-Si bonds. Additional defects occur at impurity sites, especially those associated with bound hydrogen species such as SiOH and SiH. 

The range of values for the thermal conductivity of glasses at room temperature extends from 1.38 W/mK (pure vitreous silica) to about 0.5 W/m K (high-lead-content glasses). The most commonly used silicate glasses have values between 0.9 and 1.2W/mK. All data in Table 3.4-16c are given for a temperature of 90 °C, with an accuracy of 5%. 

The mean isobaric specific heat capacities Cp (2Q°C 100 °C) listed in Table 3.4-16c were measured from the heat transfer from a hot glass sample at 100 °C into a liquid calorimeter at 20°C. The values of Cp 20°C 100 °C) and also of the true thermal capacity Cp 20 °C) for silicate glasses range from 0.42 to 0.84 J/gK. 

Vitreous silica has a unique set of properties. It is produced either from natural quartz by fusion or, if extreme purity is required, by chemical vapor deposition or via a sol-gel routes. Depending on the manufacturing process, variable quantities impurities are incorporated in the ppm or ppb range, such as Fe, Mg, Al, Mn, Ti, Ce, OH, Cl, and F. These impurities and radiation-induced defects, as well as complexes of impurities and defects, and also overtones, control the UV and IR transmittance. In the visible part of the spectrum, Rayleigh scattering from thermod)uiamically caused density fluctuations dominates. Defects are also responsible for the damage threshold under radiation load, and for fluorescence. The refractive index n and the absorption 

There are also many technical applications which make use of the chemical inertness, light weight, high temperature stability, thermal-shock resistance, and low thermal expansion of vitreous silica. A very low thermal 

The preeise data for materials from various suppliers differ slightly, depending on the thermal history and impurity concentration. The data listed in Table 3.4-16a-d and in Table 3.4-17, are for Lithosil Q0 (Schott Lithotec). The various quantities are defined in the same way as for optical glasses, as described in Sect. 3.4.5. 

The relatively open structure of vitreous silica provides space for the incorporation and diffusion of molecular species. The data in the literature are not very consistent Table 3.4-18 should serve as an orientation. 

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Figure Bl.26.17. (a) Observed and calculated ellipsometric [A(1), T(>l)] spectra for the Y2O3 film on vitreous silica. Angle of incidence 75°. (b) Best-fit model of the Y2O3 film on vitreous silica (Chindaudom P and Vedam K 1994 Physics of Thin Films vol 19, ed K Vedam (New York Academic) p 191).

THERMOELECTRICENERGYCONVERSION] (Vol23) -oxygen-generating system for [OXYGEN-GENERATION SYSTEMS] (Vol 17) -vitreous silica windows [SILICA - VITREOUS SILICA] (Vol 21)... 

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