Solvothermal method is a general term for chemical reactions carried out in a fluid such as solvent or steam at high temperature and high pressure. By using the solution as the reaction medium in a special closed reaction vessel (autoclave), the vessel is heated to create a high-temperature and high-pressure reaction environment, so that normally insoluble or insoluble substances are dissolved and recrystallized. Solvothermal method is one of the effective methods for directly synthesizing single crystals by wet chemical method. It provides a special physical and chemical environment for the reaction and crystallization of precursors that cannot be obtained under normal pressure conditions. Its advantages are: low reaction temperature, mild reaction conditions; controllable components, high purity; no need for ball milling and calcination. In addition, the phase, linearity and morphology of the crystal grains can be controlled by controlling the reaction conditions, which greatly simplifies the preparation process.
Hydrothermal reaction is a general term for chemical reactions that occur in fluids such as water (aqueous solution) or steam at high temperature and pressure. The hydrothermal synthesis reaction is carried out under high temperature and high pressure, so special technical requirements for the hydrothermal and solvothermal synthesis chemical reaction system are generated, such as reactors resistant to high temperature, high pressure and chemical corrosion. Hydrothermal synthesis chemistry focuses on the study of the reactivity of substances, synthesis rules, and the structure and properties of synthesized products under hydrothermal synthesis conditions.
The hydrothermal reaction is mainly characterized by liquid phase reaction. Obviously, different reaction mechanisms may firstly lead to the formation of different structures. In addition, even if the same structure is generated, different "groups" may be introduced into the synthetic material due to the difference in the initial generation mechanism, such as the formation of perfect crystals under liquid phase conditions. We already know that the microstructure and properties of the material are related to the source of the material, so different synthesis systems and methods may introduce different "groups" into the final material. The hydrothermal reaction focuses on the preparation, synthesis and assembly of special compounds and powder materials under solvothermal conditions. What is important is that phases or species that cannot be produced by solid-phase reactions can be produced through hydrothermal reaction.
Hydrothermal synthesis refers to a synthesis method in which the original mixture is reacted in a sealed system such as an autoclave, with water as the solvent, at a certain temperature and the autogenous pressure of water. Because under high temperature and high pressure hydrothermal conditions, it can provide a special physical and chemical environment that cannot be obtained under normal pressure conditions, so that the precursor can be fully dissolved in the reaction system and reach a certain degree of supersaturation, thereby forming atoms Or molecular growth element, nucleation and crystallization to generate powder or nanocrystal. The powder prepared by the hydrothermal method has complete crystal grain development, uniform particle size distribution, and less agglomeration between particles, and a material with an ideal stoichiometric composition can be obtained. The particle size is controllable, the raw materials are cheaper, and the production cost is low. Moreover, the powder does not need to be calcined and can be directly used for processing and shaping, which can avoid the agglomeration, growth of crystal grains and easy mixing of impurities during the calcining process.
Compared with other methods, hydrothermal crystal growth has the following characteristics: ①Hydrothermal crystals are grown under relatively low thermal stress conditions, so their dislocation density is much lower than that of crystals grown in high-temperature melts; ②Water Thermal crystal growth uses a relatively low temperature, so low-temperature isomers that are difficult to obtain by other methods can be obtained; ③The hydrothermal method of crystal growth is carried out in a closed system, and the reaction atmosphere can be controlled to form oxidation or reduction reaction conditions. Realize the formation of certain phases of other difficult-to-obtain substances; ④In the hydrothermal reaction system, the convection of the solution is fast, and the diffusion of solute is very effective, so the hydrothermal crystal has a faster growth rate.
The hydrothermal method is one of the main methods for preparing positive electrode material powders with good crystallization and no agglomeration. Compared with other wet chemical methods, the hydrothermal method has the following characteristics: ①The hydrothermal method can directly obtain well-crystallized powder without high-temperature sintering treatment, which avoids powder agglomeration that may be formed during the sintering process; ② The phase and morphology of the powder crystal grains are related to the hydrothermal reaction conditions; ③The crystal grain size is adjustable, and the powder crystal grain size prepared by the hydrothermal method is related to the reaction conditions (reaction temperature, reaction time, precursor formation, etc.) ④The preparation process is relatively simple. The current technologies for preparing powder by hydrothermal method mainly include hydrothermal oxidation, hydrothermal precipitation, hydrothermal crystallization, and hydrothermal synthesis.
Hydrothermal oxidation refers to the use of elemental metal as a precursor to obtain the corresponding metal oxide powder through a hydrothermal reaction.
Hydrothermal precipitation is based on the mixed aqueous solution of the reactants as the precursor, and the powder product is obtained through hydrothermal treatment and reaction.
Hydrothermal crystallization refers to the process of using amorphous precursors to form crystal grains with intact crystallization properties after hydrothermal reaction. Hydrothermal synthesis can be understood as the reaction to synthesize binary or even multi-element compounds under hydrothermal conditions with monobasic metal oxides or salts as precursors.
The hydrothermal method is currently mainly used to prepare polycrystalline thin films because it does not require high-temperature firing treatment to achieve the transformation from amorphous to crystalline state. The use of other wet chemical methods such as Sol-gel to prepare polycrystalline thin films requires a burning process. In this process, it is easy to cause macroscopic defects such as film cracking and shedding. There are two types of technology for preparing polycrystalline thin films by hydrothermal method: one is ordinary hydrothermal reaction, and the other is hydrothermal technology with direct current electric field, the so-called hydrothermal electrochemical method, which is currently more applied. There are many main methods, but I won’t introduce them in detail here.