Crystalline silicas particle size

Nucleants. Although nylons crystallize quickly, it is often an advantage, particularly for small parts, to accelerate this process to reduce cycle time and increase productivity. Nylon-6, which crystallizes more slowly than nylon-6,6, also benefits from nudeation in unreinforced formulations. Nucleants are generally fine-particle-size solids or materials that crystallize as fine particles before the nylon. The materials, eg, finely dispersed silicas or talc, seed the molten nylon and result in a higher density of small uniformly sized spherulites in nylon-6 the crystalline form is also changed. Nudeation increases tensile strength and stiffness but makes the material more britde. Mold shrinkage is lower for nudeated resins. 

Whereas the surface area of a crystalline silica is in fact the external surface area, the surface area and the pore size distribution of an amorphous silica are actually determined by the dimensions of the silica spheres (primary particles) that build up the network. For non-aggregated spherical particles, this relationship is very straightforward. In this silica type, the primary particles are not clustered and Sheinfain s3 globular theory can be applied. The globular theory predicts an inverse relationship between surface area and the primary particle size by the following equation ... 

Apart from the sorbents already mentioned in connection with HPTLC pre-coated layers, a micro-crystalline cellulose has also been produced in an average particle size and a narrow particle size distribution suitable for HPTLC (9.). HPTLC pre-coated plates cellulose P 254 s (E. Merck, Darmstadt), composed of this microcrystalline cellulose, were used to separate Trevespan 6058. This is a mixture of the substances ioxynil (3,5-diiodo-4-hydroxy-benzo-nitrile), flurenol (9-hydroxyfluorenecarboxylic acid) and MCPA (2-methyl-4-chlorophenoxyacetic acid). For comparison purposes Trevespan 6038 was also separated on HPTLC pre-coated plates silica gel 60 F 254 (E. Merck, Darmstadt). 

The endotherm that peaked at 651°C intensifies steadily with decreasing particle size, as is the case for the associated DTG peak. This endotherm represents increased formation of liquid phase with decreasing particle size. The enhanced weight loss, indicated by the DTG traces with decreasing particle size, does not coincide with the formation of detectable new crystalline phases in the coarse particle sizes, but does correspond to XRD detection of the formation of sodium disilicate in the fine particle size. Thus, decreasing particle size results in a significantly enhanced low temperature liquid phase attack on silica grains. 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

The Si02-X, SiOj-X, and SiOj-Y phases are hydroxyl-containing crystalline varieties of silica. The SiOj-X phase was obtained for the first time and is a well crystallized variety of Si02-X. The SiOj-Y phase, also obtained for the first time, is unlike any of the known silicate structures (Fig. 55). All these phases have a globular structure and particle size of the order of a few microns. According to infrared absorption spectra (Fig. 56), the structure of the intermediate phases is fairly disordered, with variable lattice parameters. 

Catalysts used in this process are of two types. Special acid-washed clays of particle diameters in the 200-400- xm range are used in fluidized bed versions of cracking [19]. In these units the catalyst is kept suspended or fluffed up on an upward moving stream of hot gas oil vapors, which ensures continuous exposure of all catalyst faces to the raw material and provides continuous turnover of catalyst. Synthetic catalysts are prepared from a mixture of 85-90% silica and 10-15% alumina, or from synthetic crystalline zeolites (molecular sieves) [19]. They are either used in a small particle size suitable for use in a fluidized bed, or can be formed into 3- to 4-mm diameter pellets appropriate for crackers, which use a moving bed for catalyst cycling. 

Fig.l shows the XRD patterns of the samples calcined at 500°C (The A1 diffraction peak comes from the sample frame made of aluminium). It can be seen that all samples show a diffraction pattern of Sn02 without any peak characteristic for silica, and that the longer the digestion time, the weaker and broader the Sn02 peaks, indicating a smaller particle size and lower crystallinity. 

Due to the particle size of some dusts, they are considered to be respirable and therefore represent a potential hazard to users (Golob, 1997). Diatomaceous earths may be of marine or freshwater origin, and usually contain about 90% SiC>2 (Golob, 1997). Marine diatoms contain high quantities of crystalline silica, which, if inhaled over a period of time, can result in silicosis and other respiratory diseases. However, dusts of larger particle sizes are relatively safe to use with minimum protective equipment (a simple dust mask). 

As previously pointed out, this book deals mostly with colloidal silicas, that is, disperse systems in which the disperse phase is silica in the colloidal state of subdivision. The colloidal state of subdivision comprises particles with a size sufficiently small (<1 fim) not to be affected by gravitational forces but sufficiently large (>1 nm) to show marked deviations from the properties of true solutions. In this particle size range, 1 nm (10 A) to 1 /xm (1000 nm), the interactions are dominated by short-range forces, such as van der Waals attraction and surface forces. On this basis the International Union of Pure and Applied Chemistry (IUPAC) suggested that a colloidal dispersion should be defined as a system in which particles of colloidal size (1-1000 nm) of any nature (solid, liquid, or gas) are dispersed in a continuous phase of a different composition or state (6). If the particles are solid they may be crystalline or amorphous. The disperse phase may also be small droplets of liquids, as in the case of emulsions, or gases, as for example in foams. 

A survey of the physical and chemical techniques to characterize the surface structure of amorphous and crystalline silica is presented by Unger in this book (Chapter 8). Methods to measure particle size and particle size distribution and surface area are discussed by Kirkland (Chapter 18) and by Allen and Davies (69). The use of some of these techniques by Morrow et al. (Chapter 9), Burneau et al. (Chapter 10), Vidal and Papier (Chapter 12), Kohler et al. (42), and Legrand et al. (17) to provide new insights into the silica surface structure was already mentioned in the section Silica Surface in this chapter. 

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. 

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