Ultrafine powders are typically defined as particles with sizes in the micrometer or nanometer range. Compared with bulk conventional materials, they possess a much larger specific surface area, enhanced surface activity, and higher surface energy, thereby exhibiting superior optical, thermal, electrical, magnetic, and catalytic properties. As a class of functional materials, ultrafine powders have attracted extensive research attention in recent years and are increasingly being applied across diverse sectors of national economic development.

Introduction to Methods for Surface Coating of Ultrafine Powders:

1. Solid-phase reaction method:

Several metal salts or metal oxides are thoroughly mixed and ground according to the formulation, then calcined to directly obtain an ultrafine coated powder via a solid-phase reaction.

2. Hydrothermal method:

In a high-temperature, high-pressure sealed system using water as the reaction medium, a unique physicochemical environment is established that cannot be achieved under ambient conditions. This environment ensures complete dissolution of the reaction precursors and drives them to a certain degree of supersaturation, thereby generating growth units that subsequently undergo nucleation and crystallization to yield composite powders.

The hydrothermal method offers the following advantages: the synthesized core–shell nanomaterials exhibit high purity, narrow particle-size distribution, controllable crystalline composition and morphology, well-developed crystallites, and minimal agglomeration. The resulting products feature dense, uniform shell layers, and the prepared nanoparticles do not require subsequent calcination for crystallization.

3. Sol–gel method:

The modifier precursor is dissolved in water to form a homogeneous solution; the solute and solvent undergo hydrolysis or alcoholysis to yield a sol of the modifier (or its precursor). The pretreated particles to be coated are then uniformly mixed with this sol, ensuring uniform dispersion of the particles within the sol. The sol is subsequently processed into a gel, which is calcined at elevated temperatures to produce a powder whose external surface is coated with the modifier, thereby achieving surface modification of the powder.

Coated composite particles prepared by the sol–gel method exhibit advantages such as high purity, excellent chemical homogeneity, small particle size, and a narrow particle-size distribution. Moreover, this technique is easy to operate, requires simple equipment, and enables the synthesis of various functional materials at relatively low temperatures, finding promising applications in magnetic composites, luminescent composites, catalytic composites, and sensor fabrication.

4. Precipitation method:

A precipitant is added to a solution containing powder particles, or a substance that can trigger the formation of a precipitant within the reaction system is introduced, inducing a precipitation reaction that causes the modified ions to precipitate and deposit on the particle surface, thereby coating the particles.

Precipitation‑based coating typically involves depositing an inorganic oxide layer on the surface of nanoparticles. This approach allows for convenient control of the metal ion concentration in the system, as well as the release rate and dosage of the precipitant, making it particularly well suited for coating micron‑ and nanoscale powders with inorganic modifiers.

5. Microemulsion encapsulation method:

First, the ultrafine powder to be coated is prepared using the tiny water cores provided by a W/O (water-in-oil) microemulsion, and then the powder is coated and modified via microemulsion polymerization.

The microemulsion method for preparing nanomaterials has the following characteristics: (1) a narrow and easily controllable particle size distribution; (2) because the particle surfaces are coated with one or several layers of surfactant molecules, aggregation is suppressed, yielding an organic sol that exhibits excellent stability and can be stored for extended periods; (3) the reaction is carried out at atmospheric pressure under mild conditions, with simple equipment and easy scalability.

6. Non-uniform nucleation method:

According to crystallization‑process theory, a coating layer is formed through the heterogeneous nucleation and growth of modifier particles on the surface of the coated particle matrix. This approach enables precise control over both the thickness and the chemical composition of the coating.

In heterogeneous nucleation coating, the mass concentration of the modifier lies between the critical concentration for heterogeneous nucleation and the critical saturation concentration; thus, heterogeneous nucleation–based coating is a precipitation‑driven process that occurs within the range between the critical concentration for heterogeneous nucleation and the critical concentration for homogeneous nucleation.

7. Chemical plating:

This refers to a chemical process for depositing metals without the application of an external electric current, which includes displacement plating, contact plating, and reduction plating. Chemical plating is primarily used to coat ceramic powders with metal or to form composite coatings, achieving uniform dispersion of metal within the ceramic matrix and thereby enabling the fabrication of metal–ceramic composites.

In essence, this process involves the reduction of metal ions in the plating bath to metallic particles by a reducing agent under catalytic conditions, with deposition occurring on the powder surface. It is an autocatalytic redox reaction, enabling the formation of a metal coating of controlled thickness that exhibits uniform thickness and low porosity.

8. Supercritical Fluid Method:

Under supercritical conditions, reducing the pressure can induce supersaturation and achieve a high supersaturation rate, leading to the crystallization of solid solutes from the supercritical solution.

Because the crystallization process occurs in a quasi‑uniform medium, it can be more precisely controlled. Consequently, solid deposition from supercritical solutions is a promising new technique that can produce fine particles with very small average diameters and allows for precise control of their particle size distribution.

9. Chemical Vapor Deposition:

At relatively high temperatures, the mixed gas interacts with the substrate surface, causing certain components of the gas to decompose and form a metallic or compound coating on the substrate. This process typically involves three steps: the generation of volatile species; the transport of these volatiles to the deposition zone; and their chemical reaction with the substrate to produce a solid‑state product.

10. High-Energy Method:

Methods that employ infrared radiation, ultraviolet radiation, gamma rays, corona discharge, plasma, and other techniques to coat nanoparticles are collectively referred to as high-energy methods. These high-energy approaches typically rely on substances bearing reactive functional groups, which, under the influence of high-energy particles, achieve surface coating of the nanoparticles.

In summary, by applying a controlled surface coating to ultrafine powders, the particle surfaces acquire new physical, chemical, and other functional properties, thereby significantly enhancing particle dispersibility and compatibility with other materials. Surface coating technology effectively addresses the challenge of agglomeration in ultrafine powders.