according to different aggregation states, substances can be divided into steady state, unsteady state and metastable state. Generally, bulk materials are stable; particles with a particle size of about 2nm are unstable, and their structure is constantly changing when observed under a high-power electron microscope; and powders with a particle size of about micrometers are in a metastable state. The increase in surface energy of ultrafine powder causes a series of changes in its properties, resulting in the "surface effect" of ultrafine powder; the single particle of ultrafine powder is small in size and has a small number of atoms, and its properties are infinite The bulk of the atom is different, which produces the "volume effect" of the ultrafine powder. These effects cause the unique properties of the ultrafine powder. at present, the characteristics of ultrafine powders are not yet fully understood, and the clearer characteristics can be summarized as follows.

(1) large specific surface area. as the particle size of the ultrafine powderis smaller, its specific surface area increases accordingly, and its surface energy also increases. For example, a powder with an average particle size of 0.01～0.1μm has a specific surface area of 10～70m2/g. The large specific surface area makes it have better dispersibility and adsorption performance.

(2) good activity. as the particle size becomes smaller, the number of surface atoms of the particles increases exponentially, which makes them have strong surface activity and catalytic properties, which can play a reinforcing role. participating in the reaction can significantly accelerate 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. If the melting point of au is 1063°C, the melting point of au powder with particle size of 2, 5, and 14nm will 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 0.5% ultrafine tungsten powder, it can be reduced to 1200～1300℃.

(4) strong magnetism. The volume of the ultrafine powder is smaller than the magnetic domain of the ferromagnetic substance. even if the particle is not magnetized, it is a permanent magnet and has a large coercivity. 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. The ultrafine powder with such high coercivity is an excellent raw material for manufacturing high-density recording tape.

(5) good light absorption and thermal conductivity. most ultra-fine powders have almost no thermal resistance at low or ultra-low temperatures, and 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, which is because the light is completely absorbed by the metal powder.

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