Molten Salt Synthesis of Ceramic Powders

Molten salt synthesis, one of the methods of preparing ceramic powders, involves the use of a molten salt as the medium for preparing complex oxides from their constituent materials (oxides and carbonates). Ceramic powders are prepared from solid, liquid, and gas phases by various methods (Rahaman, 2003). For large-scale commercial production, ceramic powders are fabricated mainly from the solid phase by a conventional powder metallurgical method. Molten salt synthesis is a modification of the powder metallurgical method. Salt with a low melting point is added to the reactants and heated above the melting point of the salt. The molten salt acts as the solvent. Molten salts have been used as additives to enhance the rates of solid state reactions for a long time. The amount of salt is small, typically a few percent of the total weight. In contrast, in molten salt synthesis, a large amount of salt is used as the solvent to control powder characteristics (size, shape, etc.). In this sense, molten salt synthesis is different from the flux method, which uses the salt as an additive to enhance the reaction rate. Typical examples of salts used in molten salt synthesis are chlorides and sulfates. In many cases, eutectic mixtures of salts are used to lower the liquid formation temperature. The melting points of NaCl and KCl are 801°C and 770°C, respectively, and that of 0.5NaCl– 0.5KCl (eutectic composition) is 650°C. For example, 0.635Li2SO4–0.365Na2SO4 is the most commonly used salt among sulfates because of its low melting temperature, which is 594°C, whereas that of Na2SO4–K2SO4 is 823°C. The solubilities of oxides in molten salts vary greatly from less than 1 x 10–10 mole fraction to more than 0.5 mole fraction, typically 1×10–3 1×10–7 mole fraction (Arendt et al., 1979). In many cases, the formation reaction occurs in the presence of solid reactant particles. In this sense, molten salt is somewhat different from ordinary solvents, which dissolve all reactant particles and the product particles precipitate from a homogeneous liquid phase. Generally, a complex oxide powder is prepared from reactants by the following procedure. A mixture of the reactants and salt is heated above the melting temperature of the salt. At the heating temperature, the salt melts and the product particles form. The characteristics of the product powder are controlled by selecting the temperature and duration of the heating. Then, the reacted mass is cooled to room temperature and washed with an appropriate solvent (typically, water) to remove the salt. The complex oxide powder is obtained after drying. The procedure is the same as that of a conventional powder metallurgical method and is easily scaled up for the fabrication of large quantities of materials. The use of molten salt is a common method to grow single crystals from solution (Elwell & Scheel, 1975). In this method, the reactant materials are completely dissolved in molten salt