Hydrothermal synthesis has a long history, with this method dating back to 1845. It refers to an effective method for inorganic synthesis and material processing, which uses an aqueous solution as the reaction system in a specially made closed reactor. By heating, pressurizing the reaction system or creating a relatively high – temperature and high – pressure reaction environment spontaneously, substances that are usually hardly soluble or insoluble can be dissolved and recrystallized. The equipment that provides a place for the hydrothermal synthesis reaction, that is, the closed reactor, is called a hydrothermal synthesis reactor. To sum up, the hydrothermal synthesis reactor, or hydrothermal reactor for short, is the equipment that provides a place (reactor) for hydrothermal synthesis (hydrothermal and solvothermal synthesis) reactions. It is generally a closed container or a medium – high pressure container. The temperature of a hydrothermal reactor is generally 100 – 1000 °C, and the pressure is 1 – 100 Mpa. The specific designed temperature and pressure can be different according to the synthesized products. The hydrothermal reactor is one of the commonly used reaction equipments in chemical synthesis. It can be used to study the reactivity of substances under hydrothermal conditions, the laws of synthesis reactions, and the structure and performance of synthesized products.
Due to the relatively high requirements for temperature and pressure in synthesis and the relatively violent reaction, hydrothermal reactors are generally small – sized reactors, and hydrothermal synthesis reactions are carried out through small – sized reactors. Hydrothermal reactions can be divided into hydrothermal oxidation, hydrothermal reduction, hydrothermal precipitation, hydrothermal synthesis, hydrothermal hydrolysis, hydrothermal crystallization, etc. according to different reaction types. Among them, hydrothermal crystallization is used relatively more frequently.
