According to different aggregation states, substances can be divided into stable, unstable and metastable states. Generally, bulk materials are stable; particles with a particle size of about 2 nm are unstable, and their structures are constantly changing when observed under a high-power electron microscope; and powders with a particle size of about micron are in a metastable state. The increase in the surface energy of ultrafine powder causes a series of changes in its properties, resulting in the "surface effect" of ultrafine powder; the individual particles of ultrafine powder are small in size and have a small number of atoms, and their properties are similar to those containing "infinite" multiple The different lump materials of atoms produce the "volume effect" of ultrafine powders. These effects cause the unique properties of ultrafine powders. At present, the characteristics of ultrafine powder are not fully understood. The relatively clear characteristics can be summarized as follows.

(1) Large specific surface area. Since the particle size of ultrafine powder is smaller, its specific surface area increases accordingly, and its surface energy also increases. For example, for powders with an average particle size of 0.01 to 0.1 μm, the specific surface area can generally reach 10 to 70 m2/g. The large specific surface area gives it good dispersion and adsorption properties.

(2) Good activity. As the particle size becomes smaller, the number of atoms on the surface of the particles increases exponentially, making them have strong surface activity and catalytic properties, which can play a reinforcing role. Participating in the reaction can significantly speed up the reaction speed, and has good chemical reactivity. The properties of fine powder are mainly manifested in surface properties.

(3) Low melting point. Many studies have shown that the smaller the particle size of a substance, the lower its melting point. For example, the melting point of Au is 1063°C, and the melting points of Au powder with particle sizes of 2, 5, and 14 nm drop to 330, 830, and 956°C respectively; the sintering temperature of ordinary tungsten powder is as high as 3000°C, and 0.1% ~ After adding 0.5% ultra-fine tungsten powder, the temperature can be reduced to 1200~1300℃.

(4) Strong magnetism. The volume of ultrafine powder is smaller than the magnetic domain of a strong magnetic substance. Even if it is not magnetized, this particle is a permanent magnet and has a large coercive force. For example, the coercivity of γ--Fe2O3 and CrO2 ultrafine powder with a particle size of 0.3μm is (4.0~5.6)×104A/m; while the coercivity of pure iron powder with a length of 0.3~0.7μm and a width of 0.05μm is ( 8.0～11.9)×104A/m. Ultrafine powder with such high coercivity is an excellent raw material for manufacturing high-density recording tapes.

(5) Good light absorption and thermal conductivity. Most ultrafine powders have almost no thermal resistance at low or ultra-low temperatures. Silver powder has good thermal conductivity at ultra-low temperatures, which is of great significance in ultra-low temperature engineering research. Ultrafine powder, especially ultrafine metal powder, when the particle size is less than 100nm, most of it is black, and the finer the particle size, the darker the color. This is because light is completely absorbed by the metal powder.