Quartz and Fused Silica

FIGURE 4.12. Etch rate of cathodically polarized thermal oxide in 0.2N HF solution and concentration of hydroxyls as a function of distance from the surface. After Schmidt and Ashner. (Reproduced by permission of The Electrochemical Society, Inc.) 

FIGURE 4.13. Etch rate of quartz as a function of pH in a sodium chloride solution. (Reprinted from Dove and Elston. 1992, with permission from Elsevier Science.) 

The etch rate of quartz depends on the crystallographic orientation of the surface. Table 4.3 shows the etch rate on the surface with different orientations. The difference among the four orientations can be as large as 1000-fold. The etched surface morphology is a function of solution composition and temperature, particularly the concentration of positive ions. An etching bath with added gives terraces but no pits, a 

FIGURE 4.14. Plot of dissolution rate versus etchant concentration for two etching temperatures. After Tellier.  

TABLE 4.3. Etch Rates (A/s) of Quartz of Different Crystallographic Orientations  

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Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

A commercial Cu0/Zn0/Al203 catalyst was coated on quartz and fused silica capillaries by Bravo et al. [29] for methanol steam reforming and compared with packed-bed catalysts. The coatings had a thickness of 25 pm and showed 97% conversion and 97% selectivity towards carbon dioxide at 230 °C reaction temperature, a water/methanol molar feed composition of 1.1 and a space velocity of 45 kgcat s moh1 (methanol). 

Bravo et al. [29] dealt with the coating of a commercial CuO/ZnO catalyst on quartz and fused-silica capillaries for future application in micro channels. The catalyst was mixed with boehmite as binder and water at a mass ratio of44 11 100. The boehmite was treated with hydrochloric or nitric acid before. The capillaries were pretreated with a hot sulfuric acid/solid oxidation step before coating. The capillaries were filled with the catalyst/binder suspension and then cleared with air. In this way, catalyst coatings up to 25 pm thick were obtained. The coatings were applied to methanol steam reforming (see Section 2.4.1). 

Skudelny D, Quartz and Fused silica Fillers for Epoxy Cast Resins. Quartzwerke GmbH, Frechen, Germany. 

A complication in doping experiments with silica is that the state of the impurity (A13+) may depend on the presence of other impurities. In quartz and fused silica, aluminum can be put in a substitutional position only if a charge-compensating cation such as Na+ is also present (86). If this is also true for silica gel, then the concentration of color that can be produced in a given sample will depend on the concentrations of both aluminum and some monovalent cation. 

The cells or cuvettes (also spelled cuvets) used in UV absorption or emission spectroscopy must be transparent to UV radiation. The most common materials used are quartz and fused silica. Quartz and fused silica are also chemically inert to most solvents, which make them sturdy and dependable in use. Note Solutions containing hydrofluoric acid or very strong bases, such as concentrated NaOH should never be used in these cells. Such solutions will etch the cell surfaces, making them useless for quantitative work.) Quartz and fused silica cells are also transparent in the visible and into the NIR region, so these could be used for all work in the UV and visible regions. These are also the most expensive cells, so if only the visible portion of the spectrum is to be used, there are cheaper cell materials available. 

Bravo et al. described a catalyst coating technique, which was applied for coating of commercial copper/zinc oxide catalyst onto quartz and fused silica capillaries [138]. The catalyst was milled with boehmite alumina and deionised water in a mass ratio of 44 11 100. The thickness of the coating, which was only 1 pm for this gel formulation, could be increased to some 25 pm by addition of hydrochloric acid. The catalyst was still active after the coating procedure. The development work was moving towards coating of ready-made microreactors in the future [138]. 

Silica Refractories. This type consists mainly of silica in three crystalline forms cristobalite [1446446-1]> tridymite [1546-32-3]> and quartz [14808-60-7]. Quartzite sands and silica gravels are the main raw materials, although lime and iron oxides are added to increase the mineralization of the tridymite and cristobalite. Uses include roof linings, refractories for coke ovens, coreless induction foundry furnaces, and fused-silica technical ceramic products. Consumption of silica refractories has declined dramatically since the 1960s as a result of the changes in the steel industry. 

Glass cuvettes are the least expensive but, because they absorb UV light, they can be used only above 340 nm. Quartz or fused silica cuvettes may be used throughout the UV and visible regions ( 200-800 nm). Disposable cuvettes are now commercially available in polymethacrylate (280-... 

Fig. 6.4.8. Hardness-temperature relationship for glass (1-3) and quartz (4). Glass processed from crystalline quartz (/), from fused silica (2) and Brasilian quartz (J). (After Westbrook, 1960)...

Heating the porous gel at high temperatures causes densification. The pores are eliminated and the density ultimately becomes equivalent to quartz or fused silica. 

The two most common substrates for thin film electrodes are various types of glass—soda-lime, Pyrex, and various forms of quartz or fused silica—and silicon wafers that have been treated to produce an insulating surface layer (typically a thermally grown oxide or nitride). Other possible substrates include mica, which can be readily cleaved to produce an ordered surface, and various ceramic materials. All of these materials can be produced in very flat, smooth... 

The glass fibers and fused-silica glass (Thermal American Fused Quartz Co.) were crushed and then dispersed in water. The pH of this near-neutral suspension was varied using KOH or HNO,. In some experiments, a hydrolyzed solution of y-APS was added to this suspension. Here, the initial pH was 10. The electrophoretic mobilities of glass fragments suspended in these solutions were measured without any further treatment except for the addition of electrolyte (10-3 M KNO,). These analyses were performed using a Rank Brothers Particle Micro-Electrophoresis Apparatus Mark II or a Pen Kem System 3000 Automated Electrokinetics Analyzer. 

A sample cell—normally parallelepiped in shape with a standard length of 1 cm and made of glass for the VIS region or quartz (or fused silica) for the UV region. The cell has an opening for inserting the sample and a stopper to prevent evaporation. 

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. 

Granular silica (crystalline quartz) and transparent silica plates (1 X 3 inches) were used for the sorption studies. The granular silica was supplied by the Ottawa Silica Co. and the Fisher Scientific Co. The silica was sized by dry-sieving with U. S. Standard sieves. In most cases, as narrow a range as possible was chosen, and the mean diameters, except where size was a variable, ranged from 275 to 387 microns. The transparent silica plates were fused silica with a smooth amorphous surface. 

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