Thermodynamic Modeling of Hydrothermal Synthesis of Ceramic Powders

A thermodynamic method is proposed for analyzing the hydrothermal synthesis of ceramic materials. The method utilizes standard-state thermodynamic data for solid and aqueous species and a comprehensive activity coefficients model to represent solution nonideality. The method is used to generate phase stability diagrams for the species that predominate in the system. The stability diagrams can be used to predict the optimum suspension synthesis conditions (i.e., feedstock composition, pH and temperature) for hydrothermal synthesis of ceramic materials. The synthesis of barium titanate (BaTiO₃(s)) and lead titanate (PbTiO₃(s)) are discussed as examples. In the case of the synthesis of BaTiO₃(s), which can be obtained at temperatures as low as 363 K, it is important to use solutions of appropriate pH. Practical techniques have been suggested to maintain the required pH by using a correct molar ratio of feedstocks, such as barium hydroxide (Ba(OH)₂(s)) to titanium dioxide (TiO₂(s)) or use of a mineralizer such as sodium hydroxide (NaOH(s)). It has been shown that contact with atmospheric carbon dioxide (CO₂(g)) will always lead to the contamination of the product with barium carbonate (BaCO₃ (s)). Also, a low-temperature synthesis of PbTiO₃(s) has been proposed.