There is no completely consistent statement about the particle size boundaries of ultrafine powders. Due to the difference in the use, preparation method and technical level of ultrafine powders in various countries and industries, the particle size of ultrafine powders is divided. For example, the particle size of ultrafine powders in Japan is set to be 0.1 μm or less. Recently, some foreign scholars have divided the particle size of 100μm~1μm into ultrafine powder, and divided into first to third ultrafine powders according to the equipment used. For mineral processing, Chinese scholars usually refer to powder materials with a particle size of less than 10 μm as “ultrafine powder”.

1.2 Characteristics of ultrafine powder
At present, the characteristics of ultrafine powders are not fully understood, and the characteristics that have been relatively clear can be summarized as follows:
(1) The specific surface area is large. Since the ultrafine powder has a small particle size, its specific surface area is correspondingly increased, and the surface energy is also increased. The specific surface area is large, so that it has good dispersibility and adsorption properties.
(2) Good activity. As the particle size becomes smaller, the number of atoms on the surface of the particle increases exponentially, which makes it have strong surface activity and catalytic properties, can play a reinforcing role, and has good chemical reactivity.
(3) The melting point is low. Many studies have shown that the smaller the particle size of a substance, the lower its melting point.
(4) Strong magnetic properties. The volume of the ultrafine powder is smaller than that of the ferromagnetic substance. Such a particle is a permanent magnet even if it is not magnetized, and has a large coercive force, and is an excellent raw material for manufacturing a high-density recording tape.
(5) Good light absorption and thermal conductivity. The ultrafine powder, especially the ultrafine metal powder, when the particle size is less than 100 nm, is mostly black, and the finer the particle size, the darker the color, which is completely absorbed by the metal powder.
2 ultra-fine powder application
Ultrafine powder is not only a functional material itself, but also shows a broad application prospect for the compounding and development of new functional materials. The ultrafine powder has a large specific surface area, a high surface activity, a high chemical reaction rate, a fast dissolution rate, a large sintered body strength, and unique electrical, magnetic, optical properties, etc., due to its fine particle size, narrow distribution, and uniform mass. Widely used in many technical fields.
2.1 Chemical industry, light industry
Ultra-fine powder can be used as a filler to fill plastics such as PP and PVC, reducing raw material costs and improving product performance. Processing graphite into GRT energy-saving anti-wear additive can improve mechanical lubricity, save automobile fuel and reduce overhaul times; ultra-fine kaolin as paper filler can improve paper whiteness and improve product grade; in addition, many ultrafine powders can be added. The body is made into a highly efficient catalyst for catalytic cracking in the petroleum industry. At present, combined with low temperature, freezing and embrittlement technology, organic polymer materials such as rubber, plastic and synthetic resin are processed into ultrafine powder of organic matter.
2.2 Microelectronics Industry
Typical examples of ultrafine powders used in the microelectronics industry are electronic pastes (TiO2, BaTiO3, Cu), magnetic recording materials (γ--Fe2O3), and electronic ceramic powders (BaTiO3). There are also sensors (SnO2) and light, radio wave absorbing materials and infrared radiation materials.
2.3 Medicine, pesticide industry
After processing the pesticide into ultra-fine powder, the dosage can be reduced by more than 20%, while the crop yield is increased by about 20%. Some products can replace the imported; because the blood cells in the blood are larger than 0.01μm, ultra-fine particles of <0.01μm can be prepared. Injecting blood vessels for effective treatment or health check; making the drug into ultrafine powder (or microcapsule), not only convenient to take, but also can improve the utilization of active ingredients and reduce drug consumption.
2.4 Material Industry
The application of ultrafine powders in the modern materials industry is also highly valued. Modern industrial materials have very specific requirements for the raw materials used in order to meet the processing needs and meet the application requirements. At present, foreign fine chemicals and new materials account for more than 80% of ultrafine powders as the basic raw materials, 95% of Switzerland, and 30% to 60% of the cost of powder raw materials. To a certain extent, ultrafine powders have laid a good foundation for the research of these countries in related fields at the world's leading level.

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