Synthetic silica aspects

Aluminate cement, 2 415-416 Aluminate ions, silica sols and, 22 394s Aluminates, 2 273-279 analysis, 2 275-276 chemical reactions, 2 273-274 dispersants, 8 710t economic aspects, 2 275 health and safety factors, 2 276 manufacture, 2 274-275 physical properties of, 2 273-274 uses of, 2 276-277 Alumina trihydrate (ATH), 2 274 in synthetic fillers, 11 314-315 Alumina whisker reinforcement, 5 574t Alumina xerogels, X-ray diffraction of, 23 78... 

Toxicology. The toxicity of calcium silicate depends on particle size, aspect ratio, and amount of silica and respirable fiber. Synthetic nonfibrous calcium silicate is considered to be a nuisance dust. 

Solubility. An important aspect of silica chemistry concerns the silica— water system. The interaction of the various forms of silica with water has geological significance and is applied in steam-power engineering where the volatilization of silica and its deposition on turbine blades may occur (see Power GENERATION), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble silicates, colloidal silica, and silica gel. 

Bmno E, Pentinghaus H (1974) Substitution of cations in natural and synthetic feldspars. In MacKenzie WS, Zussmarm, J (eds) The Feldspars. Manchester Uitiv Press, Manchester, p 574-609 Buerger MJ (1951) Crystallographic aspects of phase transformations. In Smoluchowski R (ed) Phase Transformations in Solids. Wiley, New York, p 183-211 Buerger MJ (1954) The stuffed derivatives of the silica stiuctures. Am Mineral 39 600-614 Bulenda M, Schwabl F, Tauber UC (1996) Defect-induced condensation and the central peak at elastic phase transitions. Phys Rev B 54 6210-6221... 

The introduction to the section Silica Gels and Powders by W. Welsh constitutes an introduction to the study of silica powders. Detailed accounts of the synthetic processes and applications of fumed silicas, silica gels, and precipitated silicas are given by Ferch (Chapter 24) and Patterson (Chapter 32). For scientists, silica powders are of special interest because they offer the opportunity of working with very pure systems with well-controlled ultimate particle size and specific surface area. One of the most important aspects of silica powders is their adsorptive properties. These properties are the subject of the work by Kenny and Sing (Chapter 25), which includes the crystalline zeolitic silica known as silicalite. 

To fulfil these aspects, three synthetic pathways appear particularly promising, namely precipitation, sol-gel processing and electrochemical conversion. In fact, all three approaches have been successfully applied. The synthetic procedures, outcome and properties of the respective products are described in this chapter. The ceramic oxide most frequently synthesized in LLC phases is silica therefore the following section is going to be focused mainly on its preparation. 

Silica (silicon dioxide) particles, synthetic or natural based, have commonly been used in various polymer systems. Silica has a low coefficient of thermal expansion and high stiffness, translating into increased modulus of the compounded polymer. However, silica filler particles are not flake or plate like as talc or mica particles are, and typically have low aspect ratios. This means that unless the particles are very small, a silica filler addition provides a relatively low surface area for contacting the polymer, and thus it reinforces the resin system less than platy fillers. Researchers have also noted that mica-filled PP, for instance, contains fewer voids than silica-filled PP, which helps explain the higher strengths of mica compounds at 20% filler loadings. This tendency to form voids or cavities increases as silica filler content increases [7-20, 7-21). 

Jain, S., Goossens, H., Picchioni, R, Magusin, R, Mezari, B., and van Duin, M. 2005. Synthetic aspects and characterization of polypropylene-silica nanocomposites prepared via solid-state modification and sol-gel reactions. Polymer 46 6666-6681. 

In synthetic fillers it is sometimes difficult to separate fundamental from aggregate shape but, where there is sufficient incentive, then ways will usually be found to overcome such difficulties. Such incentives arose in carbon blacks and more recently in precipitated silica, where shapes are very complex, but an understanding is critical to their high value usage in the tyre industry. Much work has been done, especially by Medalia and Heckman [16], and by Hess and co-workers [17], to develop automatic image analysis procedures. Using such procedures, all the aspects described previously have been investigated. This work has much to teach us about other filler particles. 

A further advantage of PMMA relies on its availability. Uniformly sized PMMA spheres are prepared by polymerization of methyl methacrylate (MMA) in water. The product of the polymerization then takes the form of a colloidal suspension of solid particles that are so small that they tend not to settle. By centrifugation, the PMMA particles are forced to settle and pack into a solid, often called a colloidal crystal. In such colloidal crystals, the PMMA spheres are arranged in a close-packed fashion in the same manner as the silica spheres that make up natural opal [178]. Therefore, these materials can be referred to as synthetic opals. Several textbooks cover selective aspects of the physicochemical properties of PMMA [181,182]. 

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