Precipitation from solution powder synthesis

Precipitation from solution under hydrothermal conditions has been widely used for the synthesis of fine crystalline particles of various oxides [178-180]. The process involves heating reactants, such as metal salts, oxide, hydroxide, or even metal powder, in the form of solution or suspension, at certain temperatures. Water is the most widely used solvent. In this case, the precipitation temperatures are set between the boiling and critical points of water, i.e., 100-374 °C, while the pressures are up to 22.1 MPa, which is the vapor pressure of water at the critical point. Due to the presence of high pressures, hardened steel autoclaves are usually used to carry out hydrothermal reactions. The autoclaves have inner surfaces of which are lined with a plastic, such as Teflon, to prevent corrosion of the vessels. Similar to chemical precipitation method, hydrothermal synthesis also offers almost unlimited flexibility in combination of types and concentrations of starting reactants, additives, pH levels, temperatures, time durations, and so on. 

Ultra-flne-grained highly reactive yttria powders, suitable especially for the preparation of transparent ceramics, are prepared by various methods including combustion synthesis [293], precipitation [294, 295], hydrothermal synthesis [296], electrospray pyrolysis [297], and sol-gel [298]. In order to improve the dispersion and sinterability of yttria powders, seed crystals are often added [296]. A significant refinement of yttria powders prepared by precipitation from solution may be achieved by the addition of sulfate ions to the reaction mixture [299] (Figure 1.25). 

Tor powder synthesis a large variety of methods is available, for example mixed-oxide solution precipitation or emulsion precipitation. Figure 8.11 shows as an example a (Y,Gd)202 powder prepared by three different precipitation techniques. The powders obtained ate different in particle morphology as well as in particle size. Powders prepared by the citrate and the oxalate precipitation techniques are strongly agglomerated [14], From powders derived in this way, compacts are formed. These powder compacts demonstrate total luminescent properties, but have still low light output due to the high porosity of about 50 vol.%. 

Application of powder X-ray diffraction for the analysis of polymorphic purity of A/-(4-nitrophenyl)-A/-morpholinothiourea (37) samples prepared by conventional solution synthesis in dichloromethane and mechanochemical liquid-assisted grinding approach. At low concentration, polycrystalline powder of form I precipitates out, while of form II precipitates from more concentrated soiution and mechanochemical synthesis. 

CaP powders synthesized in the laboratory with a chemical solution precipitation method with different synthesis p>arameters (temperature, final suspension pH, maturation time) as-synthesized and then thermally treated at various temperatures from 200°C up to 1400<>C ... 

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