Hydrothermal synthesis is an important synthesis route and effective method for solid compounds and new materials with special structure and functional properties. The hydrothermal synthesis method refers to a closed system, with water as the solvent, the reaction mixture is carried out in a corrosion-resistant stainless steel autoclave under the water's own pressure and a certain temperature.
According to the reaction temperature, the hydrothermal synthesis method can be divided into three categories:
① Low-temperature hydrothermal synthesis method. In the industry or in the laboratory, the temperature range that is convenient for operation is below 100℃, and the hydrothermal reaction usually carried out below 100℃ is called low-temperature hydrothermal synthesis.
②Medium temperature hydrothermal synthesis method. Usually the hydrothermal synthesis at 100~300℃ is called mid-temperature hydrothermal synthesis. Most of the artificial synthesis of molecular sieves is carried out in this temperature range.
③High temperature and high pressure hydrothermal synthesis method. At present, the high temperature and high pressure hydrothermal synthesis temperature has reached 1000 ℃ and the pressure is as high as 0.3 GPa. High temperature and high pressure hydrothermal synthesis is an important inorganic synthesis and crystal preparation method. It uses water as the reaction medium. The properties of the supercritical state and the special properties of the reactants at high temperature and pressure are used for synthesis reactions.
In the high temperature and high pressure hydrothermal system, the properties of water will undergo the following changes: the vapor pressure rises, the density becomes lower, the surface tension becomes lower, the viscosity becomes lower, and the ion product becomes higher. The hydrothermal reaction under high temperature and high pressure has three characteristics: the first is the acceleration of the complex ion reaction; the second is to intensify the hydrolysis reaction; the third is to make the redox potential change significantly. Hydrothermal synthesis
It has a wide range of uses, and can be used for pore-making synthesis of inorganic substances, crystal cultivation, synthesis of ultra-fine powder, synthesis of artificial minerals, etc.
The application of hydrothermal synthesis to the synthesis of nanomaterials has occurred in the past 20 years. Domestic reports on the synthesis of nanometer absorbing materials by hydrothermal synthesis are mostly in the review category, and there are few specific experimental reports. Compared with other methods of preparing nanomaterials, the hydrothermal synthesis method has the following characteristics: ①The hydrothermal method can directly obtain a good-finished powder without high temperature burning treatment and ball milling, thereby avoiding the powder that may be formed in the process. Hard agglomeration, impurities and structural defects of the powder, the powder exhibits strong activity during the sintering process; ② It is easy to get a suitable stoichiometric ratio and grain shape; ③ Cheaper raw materials can be used, and the process is relatively simple. , Hydrothermal synthesis of ferrite has made great progress.