Vitreous silica described

The production of vitreous silica from chemical precursors was first described in patents filed in 1934, including a fabrication method in which fine, high purity powders were produced by decomposing silanes (39). Forms were then cast from aqueous slips. More importandy, a flame hydrolysis process which used SiCl4 as the chemical precursor was described (40). This latter approach led to a marked improvement in glass purity and served as the basis for the processes used in the 1990s to make synthetic vitreous silica. 

Preliminary tests with samples of vitreous silica, quartz, cristobalite, and tridymite not treated with NaOH showed an effect which for quartz was previously described (7, 13) repeated tests on the same sample did not reproduce the dissolution pattern but gave reduced values. The example in Figure 8 shows three of four repeated runs on the same cristobalite sample. Although the surface area remained constant, the concentration increase with time was always slower than in the preceding... 

According to Malyj and Griffiths (1983), determining the equilibrium rotational or vibrational temperature by the Stokes/anti-Stokes ratio is not as simple and straightforward as the equations imply. The authors discuss the problems which evolve as a result of using standard lamps and show how to meet these difficulties by using reference materials to measure the temperature as well as to determine the instrumental spectral response function. The list of suitable materials includes vitreous silica and liquid cyclohexane, which are easy to handle and available in most laboratories. The publication includes a detailed statistical analysis of systematic errors and also describes tests with a number of transparent materials. 

The central element of the dilatometer is the vitreous silica tube. Vitreous silica is quartz having the coefficient of thermal expansion 100 times less than that of most plastics. The sample is placed into the tube with one end of the sample against a spring loaded push-rod, slide into a cylindrical furnace, which is placed in the -30°C controlled environment, such as liquid bath. The ASTM test describes details of the test. As the sample contracts, the motion is transmitted and detected by a... 

Vessal et al. (1989, 1993) undertook a simulation of vitreous silica at constant volume starting from a molten silica configuration with the experimental density of vitreous silica at 7000 K. A cubic simulation box with periodic boundary conditions was used containing 216 Si4+ and 432 O2- ions. The interatomic potentials included both two- and three-body terms and were of the type described in the previous section. The total time for annealing of vitreous silica was 36 ps. A time-step of 1 fs was employed. The RDFs and bond angle distributions (BADs) that are obtained for the vitreous system are shown in Figs. 12.1 and 12.2, respectively. 

The processes and compositions of this invention are concerned with amorphous silica, in contradistinction to crystalline silica, hereinafter sometimes referred to as cris-tobalite. Such terms as fused silica, vitreous silica, vitrified silica, and fused quartz have often been used interchangeably to describe amorphous silica bodies. The presence of crystallinity in a silica body is readily ascertainable by such standard techniques as studying an x-ray diffraction pattern of the silica body. 

Kennedy and associates developed data against which subsequent work was often compared (25, 130, 131). In 1954, extensive studies were made by Wyart and Sabatier (132), who measured the solubility of quartz, tridymite, cristobalite, and vitreous silica in both water and steam phases at up to 480 bars and 470 C. Increased solubility of quartz with pressure had earlier been examined up to 600 C by Morey (133), and more recently by Heitmann (30) Anderson and Burnham (134) examined solubility in water and salt and alkali solutions up to 900 C and 6 kilo-bars. Solubility was only slightly reduced by the presence of salt, but increased in direct proportion to the base added. The solubility of quartz under hydrothermal conditions is described in three papers by Kitahara (135) with special attention to supercritical conditions up to 500°C and 900 bars. The heat of solution of quartz calculated from solubility data was 7.8 kcal moIe. In 1965, Heitmann (30) surveyed the solubility of silica in water and stream and assembled data based on more than 1000 experiments up to 650 C and 300 kg cm on the basis of which a complete solubility diagram was constructed. Verifying earlier work, the solubility of silica is shown to increase with increasing density of steam or water, and reach a maximum near the critical point of water. The thermodynamic properties and solubility of quartz up to 600°C and 5 kilobars pressure are being summarized by Hegel-son(136). 

An investigation was made of the diffusivity of Ar, in Ar-saturated silica fibres, by using a modified version of Sievert s method. This furnished results which were in good agreement with previous data. At 500 to 905C, the diffusivity of Ar in vitreous silica was described by ... 

The entire spectrum of inorganic fibers can be divided into two classes, based on differences in the crystallinity of the solids (Ray, 1978). Synthetic fibers have been known as man-made mineral fibers (MMMF) and manmade vitreous fibers (MMVF). But fibrous materials can be approached or divided in other ways. For example, in the Concise Encyclopedia of Chemical Technology (1985) the entry for chemical fibers includes both manmade and natural polymers, with the discussion centering on carbon-based compounds such as acetates, acrylics, and cellulose. Fibers of other inorganic compounds were not mentioned in the encyclopedia under this entry, but silica glass fibers were described under the heading Optical Fibers. ... 

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