Vitreous silica continued

The structure of vitreous silica is a continuous random network of Si04 tetrahedra, linked through the sharing of comers. It differs from crystalline silica in having a broader distribution of Si—O—Si bond angles and a more random distribution of one tetrahedron with respect to another (44). The density is 2.2 g/cm3. 

The market for fused silica started in 1906 with the sale of silica muffles and pipes. That same year resulted in the incorporation of the Thermal Syndicate Ltd. Since that time, worldwide sale of vitreous silica material and fabricated products has continued to grow. The sales of vitreous silica ingots, tubes, rods, plates, fabricated products, photomask blanks, cmcibles, and optics was estimated to be between 800 million to 1 billion in 1995. These figures do not, however, take into account the optical waveguide market based on fused silica technology. 

The behaviour of elastic moduli of vitreous silica is also anomalous. The Young s modulus seems to continuously increase in the region of 100 to 1000 K. This has been understood on the basis of the experimentally observed extremely low values of thermal expansivity of vitreous silica. The modulus, M, can be treated as a function of any two thermodynamic variables M = M V, T) ox M = M(P, T). The temperature dependence of the modulus can therefore be written as,... 

The general feature of vitreous silica as a continuously connected random network of Si04 tetrahedra was first defined by Zachariasen [15], This nature of vitreous silica was verified by Warren et al. [16] within the limits of the X-ray diffraction techniques of that day. Several, subsequent studies have investigated the structure of vitreous silica and generally confirmed the open structure proposed by Zachariasen. Mozzi and Warren [17] substantially refined the X-ray work done by Warren et al. [16]... 

D 240 (1992) Test method for heat of combustion of liquid hydrocarbon fuels by bomb calorimetry D 696 (1998) Standard Test Method for Coefficient of Linear Expansion of Plastics between — 30 °C and 30°C with a Vitreous Silica Dilatometer D 1519 (2000) Test method for rubber chemicals - melting range D 1826 (1994) Test method for calorific (heating) value of gases in natural gas range by continuous recording calorimeter... 

The MCVD modified chemical vapor deposition) method also relies on the production of glass from halide vapors. The deposition process occurs inside a vitreous silica tube, which is heated from the outside and which serves as the cladding for the fiber. The reaction of the vapors now occurs without contamination by gases from the flames, which never contact the deposited material. Consolidation of the soot occurs simultaneously with deposition. The process continues until the desired layer thickness is reached, after which the entire tube is collapsed by increasing the external temperature to complete the preform. 

The structure of vitreous silica consists of a continuous, random network of corner-sharing Si04 tetrahedra. Extending this model to alkali silicate glasses requires that the alkali cations be regarded as network modifiers, as shown in Figure 11 The addition of each alkali oxide unit results in the replacement of... 

As for the effect of hydroxyl ion, it is not possible to see how it could catalyze the dissolution of stishovite in which silicon has already reached its maximum coordination number. No data seem to be available on the effect of pH on the dissolution rate of stishovite, but it is interesting that at pH 8.4 it dissolves about as fast as vitreous silica when compared on the basis of equal areas of surface being exposed to the solution. Furthermore, it continues to dissolve past the saturation level for vitreous or amorphous silica. The concentration of soluble silica can reach as high as 190 ppm, at which point colloidal particles are nucleated (139). It is likely that stishovite is hydrolytically unstable and would eventually decompose completely to amorphous silica. Whether or not" pH has an effect on the rate of hydrolysis is not known. 

Temperature Limit Like any other ceramic material, many factors affect the maximum use temperature of high purity silica products. In general, 2000°F is the highest temperature limit for cyclic service. When the temperature goes above 2000°F, the vitreous/fused silica grains will crystallize to cristobalite and quartz. If the operating temperature is then cycled, the various silica inversions can take place which will tear the brick apart. When operation is restricted to continuous service only, then the maximum use temperature is approximately 3000°F. 

---