Intelligent synthesis of smart ceramic materials

Hydrothermal synthesis is a process that utilizes single or heterogeneous phase reactions in aqueous media at elevated temperature (T > 25°C) and pressure (P > 100 kPa) to crystallize ceramic materials directly from solution [1,6]. Reactants used in hydrothermal synthesis are usually called precursors, which are administered in forms of solutions, gels, or suspensions. However, hydrothermal growth of single crystals requires in most cases use of solid nutrient, which recrystallizes during the growth process. Mineralizers are organic or inorganic additives that are used to control pH or enhance solubility. Other additives, also organic or inorganic, are used to serve other functions, such as controlling crystal morphology, chemical composition, particle dispersion, etc. Syntheses are usually conducted at autogeneous pressure, which corresponds to the saturated vapor pressure of the solution at the speci? ed temperature and composition of the hydrothermal solution. Higher pressures up to over 500 MPa and temperatures over 1000°C [5] may be necessary to facilitate reactant dissolution and growth of certain types of ceramic materials, and are usually applied in single crystal growth. Nevertheless, mild conditions are preferred for commercial processes where temperatures are less than 350°C and pressures less than approximately 50 MPa. Intensive research has led to a better understanding of hydrothermal chemistry, which has signi? - cantly reduced the reaction time, temperature, and pressure for hydrothermal crystallization of ceramic materials, predominantly powders and coatings (T < 200°C, P < 1.5 MPa) [2,4,5,7]. Th is breakthrough has made hydrothermal synthesis more economical since processes can be engineered using cost-e? ective and proven pressure reactor technology and methodologies already established by the chemical process industry.