The new powder formed by coating the surface of the inorganic ultrafine powder is a composite powder of a shell-core structure. The coating mechanism mainly has the following viewpoints:

1) Coulomb electrostatic attraction mutual attraction mechanism.

In this view, the coating agent has an opposite charge to the surface of the substrate, and the Coulomb attraction causes the coating particles to adsorb to the surface of the coated particles.

2) Chemical bond mechanism.

A chemical bond is used to form a strong chemical bond between the substrate and the cladding to form a uniform and dense coating. The coating layer is firmly bonded to the substrate and is not easily peeled off, but it is required to have a certain functional group on the surface of the substrate.

3) Mechanism of supersaturation.

From the perspective of crystallography, it is believed that at a certain pH value, when a heterogeneous substance is present, if a solution exceeds its supersaturation, a large number of crystal nuclei will be formed immediately, and deposited on the surface of the heterogeneous particles to form a package. The coating, at which time the concentration of crystals precipitated is lower than the concentration of the non-heterogeneous material in the solution. This is because during the crystal nucleation and growth of the heterogeneous system, the new phase nucleates or grows on the existing solid phase, and the increase of the surface free energy of the system is less than its own nucleation (homogeneous nucleation). The increase in surface free energy, so the nucleation and growth of molecules at the heterophase interface takes precedence over the homogeneous nucleation in the system.

First, solid phase coating

The solid phase coating method refers to a nano-coated powder prepared from a solid phase raw material, and can be classified into a mechanical mixing method and a solid phase reaction method according to the characteristics of the preparation process.

1) Mechanical mixing method

The mechanical mixing method utilizes mechanical forces such as extrusion, impact, shearing, friction, etc. to uniformly distribute the modifier on the outer surface of the powder particles, so that the various components infiltrate and diffuse each other to form a coating. The method has the advantages of short processing time, easy control of the reaction process, continuous batch production, and is advantageous for realizing coating of powder particles by various resins, paraffin substances and fluidity modifiers. However, this method is only used for the coating of micron-sized powders, and the powder is required to have a single dispersibility.

2) Solid phase reaction method

The solid phase reaction method is to mix and grind several metal salts or metal oxides according to the formula, and then calcination, and directly obtain the ultrafine coating powder through the solid phase reaction.

Second, liquid coating

The liquid phase chemical method utilizes a chemical reaction in a wet environment to form a modifying additive to surface coat the particles. Compared with other methods, this method is easy to form a core-shell structure. Commonly used liquid coating methods include hydrothermal method, precipitation method, sol-gel method, and electroless plating method.

Oxygen Generator

ETR PSA Oxygen Generator