Spherical-shaped silica

Fluid cracking catalysts manufactured prior to 1960 were amorphous mixtures of silica and alumina, combined in such a manner that the mixture could be spray dried into a roughly spherical shape about 70 microns in diameter. Today s cracking catalyst in addition contains an inert filler and zeolite the principle active ingredient 0. today s cracking catalysts. 

Leaching of Sintered Granules. The initial studies in this work(J3) were done in granules consisting of 80% PW-4b spray-calcined simulated waste, 10% silica, and 10% borosilicate glass frit sintered at 1100 C. The granules were about 0.6 cm in diameter and weighed about 370 mg each. The surface area, based on the assumption of an impervious hard") spherical shape,... 

The confinement of a relatively large number of dye molecules in the small volume of a nanoparticle may trigger collective phenomena otherwise not observable in bulk solution. This has been demonstrated by Prasad and coworkers in the case of an ORMOSIL pH sensor.69 The PEBBLEs contain a naphthalenylvinylpyridine derivative (NVP) as pH-sensitive fluorescent dye which has been functionalized with a triethoxysilane anchor by reaction with an excess of (3-isocyanatopropyl)triethoxysi-lane (ICTES). The sol-gel polymerization in aqueous micellar solution of the NVP-ICTES derivative with VTES gives spherically shaped 33 nm silica nanoparticles in which the dye is covalently linked to the silica matrix and uniformly distributed in the nanoparticle volume. The NVP dye responds ratiometrically to protons, with a... 

The specific surface area (Sbet) °f silicas produced by burning of SiCl4 in an 02/H2/N2 flame can be varied over a large range from 50-500 m2/g (Table l).6,7 Features of the flame synthesis and the nature of amorphous nanosilicas cause certain generic characteristics (i) a roughly spherical shape of nonporous... 

On the assumption that a particular silica possesses a specific surface area of 400 m2 g-1, and consists of particles of the same size and regular spherical shape, simple calculations make it possible to show that each particle should have a diameter of about 7 nm and the ratio of surface silicon atoms to their total number in a particle should be equal to 0.31. 

Fig. 4 a, b. TEM micrographs of hollow silica spheres produced by calcining PS particles coated with (a) one and (b) three Si02 nanoparticle/PDADMAC layer pairs at 450°C. The wall thickness of the hollow capsules is approximately three times greater for those shown in (b) compared with those shown in (a), c, d Cross-sections of the hollow silica spheres of the same composition as those shown in (b). The hollow silica spheres retain the spherical shape of the original PS particle templates (see Fig. 3). (Adapted from [22,62] by permission of the American Association for the Advancement of Science and the American Chemical Society)...

Example 5.2 Compute the Cunningham correction factor for a silica dust particle (p = 2.65 g/cm3) having a diameter of 0.5 pm. Assume a spherical shape and 20°C. From Eq. 5.3... 

Risen and Wang developed a method and compositions for producing microlenses and optical filters. According to their method, carboxylated silicone or polysilicone precursor composition is applied to the surface of a substrate to form a precursor droplet, which is thermally oxidized to form a microlens. The substrates utilized were silica, silicates, borosilicate glasses, and silicones. The precursors, which are present in concentrated solutions, are viscous fluids which are used to form microdroplet precursors. A solvent such as ethanol or acetone is added to the precursors to modify and control their flow and surface tension properties, to facihtate the formation of spherical shape of the precursor on substrates. The precursor droplet volume is 4-600 picoliters and forms a droplet of 20 to 1000 micrometers in diameter. 

Effects of Solids Shape. The viscosities of emulsion-solids mixtures are compared when irregular-shaped silica sand and spherical glass beads are added separately to an oil emulsion. The results are shown in Figure 18 for different sizes of glass beads and silica sand for synthetic oil. 

High-performance liquid chromatography (HPLC) is a variant of the simple column technique, based on the discovery that chromatographic separations are vastly improved if the stationary phase is made up of very small, uniformly sized spherical particles. Small particle size ensures a large surface area for better adsorption, and a uniform spherical shape allows a tight, uniform packing. In practice, specially prepared and coated silica microspheres of 10-25 /j.m size are often used. Only 15 g of these microspheres have a surface area the size of a football field ... 

The coalescence of the two small droplets in contact is driven by the tendency for the doublet to approach a spherical shape corresponding to minimum surface free energy for the doublet volume. For liquid ptulicles including glassy materials like. silica, coalescence takes place by viscous flow. For a Newtonian liquid, during the initial stages of coale.scence of two liquid spheres of equal diameter, a neck forms at the contact point with a radius that grows as r(Frenkel, 1945). After a short initial period in which coalescence is very rapid, the rate of decrease in the surface area becomes linear in the deviation of the doublet surface area from the sphere of the same volume as the doublet (Koch and Friedlander, 1990) ... 

In common use are three principal materials of silica one natural (both crystalline and amorphous) and two so-called synthetic, products of thermal process (fumed silica grades) and wet process (precipitated silica). Particles of silica commonly have a spherical shape. 

When the sol-gel process is carried out in the microdomains of nanostruc-tured polymer matrices, this reaction leads to silica/polymer hybrid materials. By calcination the polymer matrix can be removed and the pure silica nanoparticles are obtained. In this way spherical, cylindrical, and pellet-shaped silica particles can be prepared in a controlled way [92,93] as shown in Fig. 14. 

The macroscopically different bulk structures of granulate and spray-dried silica gels are composed of small, almost spherically shaped particles, which can be seen in an enlarged transmission electron microscopic (TEM) micrograph (Figure 5). The latter... 

Porous Oxides Silica, Activated Alumina, Titania, Thoria, Zirconia Porous inorganic oxides are made through a sol-gel process. The sol is converted into a hydrogel that is subjected to dehydration to form a porous xerogel. Special techniques have been developed to combine the sol-gel transition vith an aim to develop spherical shaped particles (Figure 3.2). 

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