Very Fine Spheres

Moving from a fixed to a fluidised-bed operation also required a new catalyst, and the one selected was a supported gold palladium system in the form of very fine spheres, prepared in collaboration with Johnson Matthey. Hence, gold-based catalysts are being used for this new fluidised-bed process, and are well established in fixed-bed processes for the large-scale manufacture of VAM. 

The Sorptometer appears to offer several ad, vantages for use in determining the surface area of fine powdered proplnt ingredients. By comparing the specific surface determined by N2 adsorption with the specific surface determined by other methods, an estimate of the deviation of the particles from a sphere can be made. For many very fine powders, N2 adsorption is one of the few methods available for describing the, size of the particles... 

Solids appear in one of two forms, either as crystals or powders. The difference is one of size, since many of the powders we use are in reality very fine crystals. This, of course, depends upon the manner in which the solid is prepared. Nevertheless, most solids that we encounter in the real world are in the form of powders. That is, they are in the form of discrete small particles of varying size. Each particle has its own unique diameter and size. Additionally, their physical proportions can vary in shape from spheres to needles. For a given powder, aU grains will be the same shape, but the particle shape and size can be eiltered by the method used to create them in the first place. Methods of particle formation include ... 

For very fine particles with intimate surface contact, these relatively weak secondary adhesive forces can be quite significant. For example, Rumpf [1] has estimated the contributions of van der Waals and electrostatic forces in agglomerates of fine-grained material. The calculations were made for quartz glass and yielded a binding force due to van der Waals forces between two spheres given by ... 

Particulates of fly ash are very fine. Some of the silica in the ash is found in the form of small silica spheres, called cenospheres or extendospheres, which make ash a very flowable material. This property not only makes ash miscible in a CBPC slurry, but it reduces the viscosity of the slurry and makes the slurry smooth, easily pumpable, and pourable. This property is a great advantage with CBPC-based drilling cements (Chapter 15). 

Deposition-precipitation of zirconium hydroxide on silica at a constant pH level of 4.5 leads to very finely divided zirconia. Fig. 1 shows an electron micrograph of the resulting catalyst precursor. Tiny zirconia particles have been deposited onto the non-porous silica spheres. 

With his very fine torsion balance, Coulomb was able to demonstrate that the repulsive force between two small spheres electrified with the same type of electricity is inversely proportional to the square of the distance between the centers of the two spheres. At the time, the electron had not yet been discovered, so the underlying reason for this remained a mystery but Coulomb was able to demonstrate that both repulsion and attraction followed this principle. He was not able to make the quantitative step to show that the force was also directly proportional to the product of the charges, but he did complete some experiments exploring this relationship. As a consequence, the law governing one of the four fundamental forces of nature is named Coulomb s law ... 

The terms fining and refining refer to the removal of gaseous inclusions, or bubbles, from the melt. Although the presence of bubbles in a glass sample is not necessarily detrimental for many scientific studies, bubbles are definitely undesirable in most commercial glasses. Bubbles in commercial products are almost always considered flaws and result in rejection of the product. These gas-filled inclusions may occur as very small spheres (< 0.4 mm diameter), which are frequently called seed... 

One of the issues is to provide a realistic theory for the summing of London forces over the geometry of a probe tip. This has been carried out by various groups, and found to differ from the standard Hamaker expressions for sphere approaching flat, especially when the probe was very fine, e.g. less than lOnm diameter at the tip. The results obtained for the force on the tip as a function of separation could then be explained, and the Hamaker constant determined. Alternatively, a small sphere could be glued to the cantilever, in which case the... 

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