Particles Crystallization, Sintering

Suppose that a body made up of fine particles can sinter by either the crystal diffusion mechanism BS XL or the grain-boundary diffusion mechanism BS B as illustrated in Fig. 16.7. How will the relative sintering rates due to these two mechanisms vary as ... 

Refractory Fibers Recently, zirconia-based insulating material with a low density and a low thermal conductivity has been developed in the form of fibers, paper, felt, board and shaped articles. The material is a cubic zirconia soHd solution stabilized with yttria, and has a maximum usable temperature of >2100 C. The innovative fabrication technique involves the use of an organic precursor fiber as a structural template, impregnated with an aqueous solution of zirconium chloride and yttrium chloride. The metallic salts are deposited within the organic fiber, which can subsequently be burned off by a controlled oxidation. The hollow remainder is then fired at a sufficiently high temperature (800-1300 °C) so as to induce crystallization, after which the oxide particles are sintered to develop a ceramic bond. Other techniques to produce refractory fibers involve phase inver-... 

The concept of pressure on a surface is a macroscopic concept. At the microscopic scale, atomic diffusion in a crystallized phase occurs primarily due to the movements of the vacancies. However, the equilibrium concentration of the vacancies is less under a convex surface than under a plane surface, and it is higher under a concave surface than under a plane surface. Thus, the vacancies migrate from the high concentration areas to the low concentration areas, an action which implies a movement contrary to that of the atoms. The effects are all the more obvious according to how marked the curvature (1/r) is and therefore the smaller particles are sintering is facilitated by the use of fine powders (diameters of about a micrometer). However, the pressure variations and the energies brought into play by the interfacial effects still remain very low. 

Testing. Chemical analyses are done on all manufactured abrasives, as well as physical tests such as sieve analyses, specific gravity, impact strength, and loose poured density (a rough measure of particle shape). Special abrasives such as sintered sol—gel aluminas require more sophisticated tests such as electron microscope measurement of a-alumina crystal si2e, and indentation microhardness. 

If the phases present can be unambiguously identified, microscopy can be used to determine the geometry of interface initiation and advance, and to provide information about particle sizes of components of mixed reactants in a powder. Problems of interpretation arise where materials are poorly crystallized and where crystallites are small, opaque, porous or form solid solutions. With the hot-stage microscope, the progress of reactions can be followed in some instances and the occurrence of sintering and/or melting detected. 

Besides, without addictive AICI3 as a crystal conversion agent, phase composition of most neogenic Ti02 particles was anatase in our experiment. Conversions active energy finm anatase to rutile was 460 kJ/mol [5], with temperature arose, crystal conversion rate as well as mass fraction of rutile would increase [6,7]. Hence, after a lot of heat accumulated, phase composition of particle-sintered layer was rutile. 

PET flakes have different crystallinities. The wall particles are oriented and crystallized, while the flakes from the neck and bottom of non-heat-set bottles are amorphous and require crystallization to prevent sintering before they can be subjected to SSP. Separating the thick amorphous PET flakes before SSP to circumvent the sticking risk and to improve the uniformity of the product has also been suggested [122], However, this may only be commercially acceptable if the separated flakes can be used in a final application. 

We observe that for the bonds depicted in Figs, lb and le, an atom M or an atom R, to which the chemisorbed particle C is attached, are more weakly bound to the lattice than the normal ions M+ or, respectively, Rr. As a result, in some cases we can expect that the molecule CM or CR may evaporate that is, the particle C upon desorption may carry off with it an atom of the lattice, thereby violating the stoichiometric composition of the crystal. In all cases such adsorption should facilitate surface creep which plays such an important role in the sintering, recrystallization, and disintegration of solids in reaction. This may also explain the well-known influence of adsorption on the surface mobility of the adsorbent atoms. 

Hematite obtained at low temperatures retains the acicular morphology of the goethite precursor crystals, but at temperatures >600 °C, a sintering process leads to irregular particles of hematite. 

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