Amorphous silicon dioxide 3.38

Silicic acid silica gel amorphous silicon dioxide... 

In addition to these, there are many more polymers like phosphonitrichloride, stibnates, amorphous silicon dioxide, natural and synthetic silicates, etc. 

SiNW (made of silicon, silicon-covered with silica, or silica) can be either crystalline or amorphous. The interaction of silicon and metal catalysts, the evolution of silicon precipitating out of the catalysts, and the reactions followed the precipitation determine the morphology and crystallinity of SiNW. In many cases, SiNW have been observed with a silicon core covered with an amorphous silicon dioxide sheath. 

HRTEM results showed that SiNW were made of amorphous materials. EDX of the nanowires showed both Si and O peaks, with the ratio ranged from SiOi s to Si02. The observation that SiNW were amorphous materials was further supported by the Raman measurements, in which very small amounts of crystalline Si was detected. Even the amorphous Si peak at 480 cm was undetectable due to the presence of high PL signal. On the basis of the information presented here, we concluded that the majority of the SiNW are made of amorphous silicon dioxide, with a certain amount of oxygen deficiencies. 

Some inorganic impact modifiers are also marketed, such as amorphous silicon dioxide. 

FIGURE 12.3 Segment of amorphous silicon dioxide (Si02) structure. 

The fact that silanol persistence can be favored by equilibrium conditions rather than control of condensation kinetics by steric or electronic factors is usually not considered. The phase separation which results from highly condensed systems continuously removes material from deposition solutions, depleting soluble silane species. While condensed silanols or siloxanes are typically not regarded as participating in a reversible reaction with water or alcohol, they do indeed participate in an equilibrium reaction. Iler [16] has shown that even hydrated amorphous silicon dioxide has an equilibrium solubility in methanol, which implies the formation of soluble low molecular... 

Transmission electron microscopy (tern) is used to analyze the structure of crystals, such as distinguishing between amorphous silicon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain structures. Tern requires samples that are very thin (10—50 nm) sections, and is a destructive as well as time-consuming method of analysis. 

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

The first mention of a plasma for low-temperature deposition of glassy oxides seems to have been made by Sterling and Swann1S), who reported the deposition of amorphous silicon dioxide layers with the aid of a 1 MHz radio frequency discharge at... 

D. L. Griscom, The Natures of Point Defects in Amorphous Silicon Dioxide, in G. Pacchioni, L. Skuja, D. L. Griscom (Eds.), Defects in... 

Fumed Silica. Today colloidal silica (fumed silica) is the most common thixotropic agent in epoxy resins. Fumed silica, an amorphous silicon dioxide, is a versatile, efficient additive used for flow control and thixotropy. Fumed silica has long been the dominant thixotrope employed in the adhesive and sealant industry. 

Fumed silica appears as a fluffy white powder characterized by an extremly low bulk density down to the range of about 20-50 g f. In contrast, the submicron fumed silica particle consists of amorphous silicon dioxide and, hence, its true density is about 2200 g 1 ... 

The model of two-level systems was suggested for amorphous silicon dioxide (fused quartz) where the increase of the distance R between Si atoms compared to that of the crystal was found [191]. Due to the extention of bonds, the O atom in the chain Si-O-Si has several equilibrium positions. 

Silica. Amorphous silicon dioxide (glass). It is a structural component in many organisms, such as diatoms and horsetails. 

Yang KY, Glemza R, Jarowski Cl. Effects of amorphous silicon dioxides on drug dissolution. / Pharm Sci 1979 68 560-565. 

Boehm, H.-P. and Schneider, M. (1959). The hydroxyl groups on the surface of the amorphous silicon dioxide Aerosil and their reactions (in German). Z. Anorg. Allg. Chem., 301, 326-35. 

Figure 12.33 Crystalline and amorphous silicon dioxide. A, The atomic arrangement of cristo-balite, one of the many crystalline forms of silica (SI02), shows the regularity of cubic closest packing. B, The atomic arrangement of a quartz glass is amorphous with a generally disordered structure.

Figure 30. Broad overview of oxygen diffusion data in crystalline and amorphous silicon dioxide as well as silicate glasses and liquids over a wide temperature range, redrawn from Lamkin et al. (1992). Note how permeation rates, denoted by heavy lines and D02, ate faster than diffusion rates involving interaction with network oxygen, D o, in any given type of medium.

Nowadays, amorphous silicon dioxide may be used as corrosion-resistant coatings in various types of cans and as moisture-impermeable films, as well as for rheology control in materials such as paints, silicone rubbers, polyester compounds, dental formulations, emulsions, cosmetics, and excipients in pharmaceuticals. 

ViLLOTA R and Hawkes JG (1985) Food applications and the toxicological and nutritional implications of amorphous silicon dioxide. Crit Rev Food Sci Nutr 23 289-321. 

Spectrophotometric determination. In the sample of the dust fall-out, silicon dioxide is transformed into a solution by melting with a mixture NaHC03 -f NaCl. The silicic acid formed yields Molybdenum Blue by reaction with ammonium molybdate and the concentration is determined by spectrophotometric measurements. The portion of amorphous silicon dioxide is dissolved by boiling with NaOH solution and the determination is performed in the same way as mentioned above [18]. 

An especially interesting form of silicon dioxide is silica gel, a powdery form of amorphous silicon dioxide that is highly adsorbent. An adsorbent material (in contrast to an absorbent material) is one that is capable of removing a material, such as water, ammonia, alcohol, or other gases, out of the air. The second material bonds weakly to the outer surface of silica gel particles. Silica gel is able to adsorb anywhere from 30 to 50 percent of its own weight in water... 

Intrinsic Defect Formation in Fluorine-Doped Amorphous Silicon Dioxide.105... 

INTRINSIC DEFECT FORMATION IN FLUORINE-DOPED AMORPHOUS SILICON DIOXIDE... 

Griscom, D. L., Self-trapped holes in amorphous silicon dioxide, Phys. Rev. B 40, 4224 (1989). 

Skuia, L., Optically active oxygen-deficiency-related centers in amorphous silicon dioxide, J. Non-Cryst. Solids 239, 16 (1998a). 

In The Chemistry of Silica [1], Her used the term silica powders as a broad category encompassing silica gels, precipitated silicas, and fumed or pyrogenic silicas. These are all forms of synthetic amorphous silicon dioxide, a broad category that also includes another form — silica sols or coUoidal silica — that is not a powder, but rather a dispersion of discrete silica particles in a liquid medium. 

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