Mysterious Fine Particles Formed during the Rapid Expansion of Supercritical Water Solutions.

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Fine silica particles have been produced by the rapid expansion of supercritical water - SiO<SUB>2</SUB> solutions (RESS) at temperatures of 723 and 823 K and pressures from 50 to 100 MPa. New spherical particles sprouting whiskers have also been discovered in the autoclave after the RESS. The solubility of solids in supercritical fluids is a very sensitive function of temperature and pressure. Small changes of pressure result in large changes in density and solvent power, because supercritical fluids are highly compressible. Thus, the rapid expansion of supercritical solutions can give rise to very large supersaturation ratios. Nucleation rates are determined by the competition among solvent expansion, cooling due to depressurization, and high supersaturation. In order to control the product morphology, the effects of experimental parameters, such as preexpansion temperature and pressure, solute concentration, depressurization schemes, nozzle configuration, and sampling method on the product characteristics of materials, have been investigated by means of scanning electron microscopy and X-ray diffraction analysis. It is found that the control of particle size distribution is possible by regulating supersaturation ratio as well as suitable selection of preexpansion temperature and pressure. Unique features of the RESS process are discussed.
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