14 methods of surface coating of ultrafine powder

Ultrafine powders usually refer to particles with a particle size of micrometers or nanometers. Compared with bulk conventional materials, they have larger specific surface area, surface activity and higher surface energy, thus showing excellent optical, thermal, electrical, magnetic, catalytic and other properties. Ultrafine powders have been widely studied as a functional material in recent years and have been increasingly widely used in various fields of national economic development.

However, due to the unique agglomeration and dispersion problems of ultrafine powders, they have lost many excellent properties, which seriously restricts the industrial application of ultrafine powders.

Methods for coating the surface of ultrafine powders

1. Mechanical mixing method. Use mechanical forces such as extrusion, impact, shearing, and friction to evenly distribute the modifier on the outer surface of the powder particles, so that various components can penetrate and diffuse into each other to form a coating. The main methods currently used are ball grinding, stirring grinding, and high-speed airflow impact.

2. Solid-phase reaction method. Mix and grind several metal salts or metal oxides according to the formula, and then calcine them to directly obtain ultrafine coated powders through solid-phase reaction.

3. Hydrothermal method. In a closed system of high temperature and high pressure, water is used as a medium to obtain a special physical and chemical environment that cannot be obtained under normal pressure conditions, so that the reaction precursor is fully dissolved and reaches a certain degree of supersaturation, thereby forming a growth unit, and then nucleating and crystallizing to obtain a composite powder.

4. Sol-gel method. First, the modifier precursor is dissolved in water (or an organic solvent) to form a uniform solution, and the solute and the solvent are hydrolyzed or alcoholyzed to obtain a modifier (or its precursor) sol; then the pretreated coated particles are uniformly mixed with the sol to make the particles uniformly dispersed in the sol, and the sol is treated to be converted into a gel, and calcined at a high temperature to obtain a powder coated with a modifier on the surface, thereby achieving surface modification of the powder.

5. Precipitation method. Add a precipitant to a solution containing powder particles, or add a substance that can trigger the generation of a precipitant in the reaction system, so that the modified ions undergo a precipitation reaction and precipitate on the surface of the particles, thereby coating the particles.

6. Heterogeneous coagulation method (also known as “heteroflocculation method”). A method proposed based on the principle that particles with opposite charges on the surface can attract each other and coagulate.

7. Microemulsion coating method. First, the ultrafine powder to be coated is prepared by the tiny water core provided by the W/O (water-in-oil) type microemulsion, and then the powder is coated and modified by microemulsion polymerization.

8. Non-uniform nucleation method. According to the LAMER crystallization process theory, the coating layer is formed by the non-uniform nucleation and growth of the modifier particles on the coated particle matrix.

9. Chemical plating method. It refers to the process of metal precipitation by chemical method without applying external current. There are three methods: replacement method, contact plating method and reduction method.

10. Supercritical fluid method. It is a new technology still under research. In supercritical conditions, reducing the pressure can lead to supersaturation, and can achieve a high supersaturation rate, so that the solid solute crystallizes from the supercritical solution.

11. Chemical vapor deposition. At a relatively high temperature, the mixed gas interacts with the surface of the substrate, causing some components in the mixed gas to decompose and form a coating of metal or compound on the substrate.

12. High energy method. The method of coating nanoparticles using infrared, ultraviolet, gamma rays, corona discharge, plasma, etc. is collectively referred to as high energy method. The high energy method often uses some substances with active functional groups to achieve surface coating of nanoparticles under the action of high energy particles.

13. Spray thermal decomposition method. The process principle is to spray a mixed solution of several salts containing the required positive ions into a mist, send it into a reaction chamber heated to a set temperature, and generate fine composite powder particles through reaction.

14. Microencapsulation method. A surface modification method that covers a uniform film with a certain thickness on the surface of the powder. The particle size of the microcapsules usually prepared is 2 to 1000 μm, and the wall material thickness is 0.2 to 10 μm.