The cladding mechanism is still under study and is inconclusive. The main points of view are the following:
Coulomb electrostatic attraction mutual attraction mechanism. In this view, the coating agent has a charge opposite to the surface of the substrate, and the coating agent particles are adsorbed to the surface of the coated particles by Coulomb attraction.
Chemical bonding mechanism. A strong chemical bond is formed between the substrate and the coating through a chemical reaction, resulting in a uniform and dense coating layer.
supersaturation mechanism. This mechanism from the crystallographic point of view, that at a certain pH value, the presence of heterogeneous substances, such as the solution more than its supersaturation will have a large number of nuclei immediately generated, deposited on the surface of heterogeneous particles to form a coating layer.
Surface coating method of ultrafine powder
1. Mechanical mixing method.Mechanical forces such as extrusion, impact, shear and friction are used to evenly distribute the modifier on the outer surface of the powder particles, so that various components can penetrate and diffuse each other to form a coating. At present, the main applications are ball grinding method, stirring grinding method and high-speed airflow impact method. The advantages of this method are short processing time, easy control of the reaction process, continuous mass production, more conducive to the realization of a variety of resins, paraffin substances and liquidity modifiers to the coating of powder particles. However, this method is only used for the coating of micron-sized powders, and requires that the powders have a single dispersion.
Superfine powder material modified coating machine
2. Solid phase reaction method.Several metal salts or metal oxides are fully mixed according to the formula, ground, and then calcined, and the ultrafine coated powder is directly obtained by solid phase reaction.
3. Hydrothermal method.In the closed system of high temperature and high pressure, water is used as the medium to obtain the special physical and chemical environment that cannot be obtained under normal pressure, so that the reaction precursor is fully dissolved and reaches a certain degree of supersaturation, thus forming the growth element, and then nucleating and crystallizing to obtain the composite powder. The advantages of hydrothermal method are: the synthesis of nuclear-shell nano-powder has high purity, narrow particle size distribution, grain composition and morphology can be controlled, grain development is complete, the degree of agglomeration is light, the product shell is dense and uniform, the preparation of nano-powder does not require late crystallization heat treatment.
4Sol-gel method.Firstly, the modifier precursor is dissolved in water (or organic solvent) to form a uniform solution, and the solute and the solvent are hydrolyzed or alcoholysis to react to obtain the modifier (or its precursor) sol; then the pretreated coated particles are uniformly mixed with the sol, so that the particles are uniformly dispersed in the sol, the sol is converted into gel after treatment, and calcined at high temperature to obtain the powder coated with the modifier on the outer surface, so as to realize the surface modification of the powder. The coated composite particles prepared by sol-gel method have the advantages of high purity, good chemical uniformity, small particles and narrow particle size distribution, and the technology is easy to operate, simple equipment, and can be used to prepare various functional materials at low temperature. It has been well applied in the preparation of magnetic composite materials, luminescent composite materials, catalytic composite materials and sensors.
Rare earth doped nano-oxide coated with graphene
5. Precipitation method. A precipitant is added to the solution containing the powder particles, or a substance that can initiate the generation of the precipitant in the reaction system is added, so that the modified ions are precipitated on the surface of the particles, thereby coating the particles. Precipitation reaction coating is often coated with inorganic oxides on the surface of nanoparticles, which can easily control the concentration of metal ions in the system and the release rate and dose of the precipitant, and is particularly suitable for coating micro-nano powders with inorganic modifiers.
Surface-coated nano-magnesium hydroxide composite powder material of fly ash hollow microspheres
6, heterogeneous coagulation method (also known as "miscellaneous flocculation method").A method based on the principle that particles with opposite charges on the surface can attract each other and condense. If the diameter of one particle is much smaller than the diameter of the other charged particle, the small particles will adsorb on the outer surface of the large particle to form a coating during the agglomeration process. The key is to modify the surface of the particles, or directly adjust the pH value of the solution, thereby changing the surface charge of the particles.
7. Microemulsion coating method.First, the ultrafine powder to be coated is prepared by the tiny water core provided by W/O (water in oil) microemulsion, and then the powder is coated and modified by microemulsion polymerization. Compared with other preparation methods of nanomaterials, the preparation of nanomaterials by microemulsion method has the following characteristics:(1) the particle size distribution is narrow and easier to control;(2) because the particle surface is coated with one layer (or several layers) of surfactant molecules, it is not easy to coalesce, the obtained organosol has good stability and can be placed for a long time;(3) the reaction is carried out under normal pressure, the reaction temperature is mild, the device is simple and easy to realize.
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. The method can precisely control the thickness and chemical composition of the coating layer. In the non-uniform nucleation coating, the mass concentration of the modifier is between the critical concentration of non-uniform nucleation and the critical saturation concentration, so the non-uniform nucleation coating is a precipitation coating that occurs between the critical concentration of non-uniform nucleation and the critical concentration of homogeneous nucleation.
9, chemical plating method.It refers to the process of metal precipitation by chemical method without external current. There are three methods: replacement method, contact plating method and reduction method. The electroless plating method is mainly used for coating metal or composite coating on the surface of ceramic powder to achieve uniform mixing of ceramic and metal, so as to prepare metal-ceramic composite materials. Its essence is that the metal ions in the plating solution are reduced by the reducing agent to metal particles deposited on the surface of the powder under the catalytic action, which is an automatic catalytic oxidation-reduction reaction process, so a certain thickness of the metal coating can be obtained, and the coating thickness is uniform, low porosity.
10. Supercritical fluid method.It is a new technology that is still being studied. In the supercritical case, reducing the pressure can result in supersaturation, and high supersaturation rates can be achieved, allowing the solid solute to crystallize out of the supercritical solution. Since the crystallization process is carried out in a quasi-homogeneous medium, more accurate control can be obtained. Thus, solid deposition from supercritical solutions is a promising new technique for producing fine particles with a very small average particle size and for controlling the particle size distribution.
11. Chemical vapor deposition.At relatively high temperatures, the mixed gas interacts with the surface of the substrate, causing certain components of the mixed gas to decompose and form a coating of a metal or compound on the substrate. It generally includes three steps: generating volatile substances; transporting volatile substances to the precipitation zone; and chemically reacting with the matrix to generate solid products.
12, high energy method.Methods for coating nanoparticles with infrared rays, ultraviolet rays, gamma rays, corona discharge, plasma, etc., are collectively referred to as high energy methods. High energy method is often the use of some substances with active functional groups in the role of high-energy particles in the surface coating of nanoparticles.
13, Spray thermal decomposition.The process principle is to spray a mixed solution of several salts containing the required positive ions into a mist, and send it into a reaction chamber heated to a set temperature, and generate fine composite powder particles through the reaction. In this process, from raw materials to product powder, including solution, spray, reaction and collection of four basic links.
Silica-coated zirconium diboride-silicon carbide composite powder
14, microencapsulation method.A surface modification method in which a homogeneous film with a certain thickness is covered on the surface of a powder. The particle size of microcapsules is usually 2~1000 μm, and the thickness of wall material is 0.2~10 μm. Microcapsules can change the appearance of the core material without changing its nature, but also can control the release conditions of the core material; the material in the phase reaction can play a role in isolation, for long-term preservation; the toxic substances can play a hidden role. Microcapsule technology has been widely used in pharmaceutical, food, coating, adhesive, printing, catalyst and other industries.
The selection of surface coating technology should be considered according to the characteristics of core powder and coating material and the application of modified composite powder. With the development of science and technology, ultrafine powder coating technology will be further improved, and it is expected to prepare ultrafine composite particles with multi-function, multi-component and stronger stability, which will open up a broader application prospect for composite particles. At present, the surface coating mechanism of ultrafine powder and
Preparation of Superfine Powders with Better Properties by Combining Various Coating Methods
It will be the future direction of research in this field.