Ultrafine powder material
Any solid material has a certain shape and occupies a corresponding space, that is, has a certain size. We usually refer to powders or fine particles, generally referred to as solid materials having a size of 1 mm or less. When the particle diameter of the solid particles is between 0.1 μm and 10 μm, it is called fine particles, or sub-superfine particles, and dust floating in the air mostly falls within this range. When the particle diameter reaches 0.1 μm or less, it is called ultrafine particles. Ultrafine particles can be further divided into three grades: large, medium and small ultrafine particles. Large ultrafine particles with a particle size between 100 a and 1000 a; ultrafine particles with a particle size between 20 and 100 a; small ultrafine particles with a particle size below 20 a. At present, the preparation of small and medium-sized ultrafine particles is still difficult, so the ultrafine powder material described in this section refers to solid particles having a particle diameter of between 0.1 μm and 0.01 μm. It can be seen that the ultrafine particles we describe are intermediate materials between bulk materials and atoms or molecules. They are composed of a small number of atoms or molecules obtained by artificial means. They maintain the chemistry of the original materials. Properties, while atoms or groups of molecules in metastable state are thermodynamically unstable. So their research and development is the key to understanding how the micro world transitions to the macro world. With the development of electron microscopy, the existence, size and shape of ultrafine particles have been observed very clearly.
Compared with its general powder, ultrafine particles have found a series of peculiar properties, such as low melting point, high chemical activity, strong magnetic properties, good heat conduction, and abnormal absorption of electromagnetic waves. These changes in properties are mainly due to "surface effects" and "volume effects." Although some characteristics and applications of ultrafine particles have yet to be further researched and developed, the above-mentioned odd properties have opened up a bright future for its wide application.
The finer the particle size of the ultrafine particles, the more pronounced the decrease in the melting point. The melting point of the silver block is 900 ° C, and the melting point of the ultrafine particles can be reduced to below 100 ° C, and it can be dissolved in hot water. The melting point of the gold nugget is 1064 ° C, and the melting point of the ultrafine particles having a particle diameter of 20 a is only 327 ° C. Since the melting point is lowered, the powder of the metal, alloy or compound can be sintered at a lower temperature to obtain various mechanical parts, which not only saves energy, reduces the difficulty of the manufacturing process, but more importantly, can obtain excellent performance. part. When high-melting-point materials such as WC, SiC, BN, Si3N4, etc. are used as structural materials, the manufacturing process requires high-temperature sintering, and when ultra-fine particles are used, it can be carried out at a very low temperature and high-density sintering without additives. body. This has very good practical significance for opening up a wider range of applications for high-performance inorganic structural materials. The smaller the diameter of the ultrafine particles, the larger the total specific surface area, the corresponding increase in surface energy, and the higher chemical activity. It can be used as a high-efficiency catalyst for chemical reactions and as a combustion-supporting additive for rocket solid fuels. The existing practice shows that the ultrafine particle Ni and Cu-Zn alloy are the main components of the catalyst, the efficiency of organic hydrogenation is 10 times that of the conventional catalyst; in the solid rocket fuel, less than 1% by weight Ultrafine particles of aluminum powder or nickel powder can double the amount of heat burned per gram of fuel.
The most advanced application of magnetic properties is for magnetic materials. Ultra-high-density magnetic tapes and video tapes with better performance have been developed using γ-Fe203, Cr02 and metal ultrafine particles. The recording density is 10 times that of the past and has good stability. More applications are now being developed, such as new liquid colloidal magnetic fluid materials, mechanical seals, and loudspeakers.
The metals and powders we usually see reflect light and exhibit a metallic luster. However, the ultrafine particles of the metal are completely tarnished, and the finer the particles, the deeper the black. This may be due to the fact that the light waves are completely absorbed. In addition to being used as a light absorbing material in solar energy utilization, this property can also be utilized as a coating for a hot wire type detector by utilizing its absorption of infrared rays. For example, the ultrafine particles of ferric oxide and the zinc stearate dispersant are added to the polystyrene resin to form a film, which has good permeability to visible light and good absorption to ultraviolet light. If it is added to plastic, it can be made into a transparent plastic container that is UV-resistant and has a much better transparency than brown glass. If added to a food packaging bag, it can protect food from ultraviolet light and effectively extend the shelf life.
Ultrafine particles are being used in catalysis, low temperature sintering, composites, new functional materials, tunnel engineering, medicine and bioengineering, and have achieved very exciting results. The research history of ultrafine particles is generally considered to have been calculated from 1962. In fact, more comprehensive development research began in the 1980s. On the whole, it should be said that this work is still at the initial stage of research, and there are still many technical and theoretical issues to be further explored. No wonder some scientists predict that ultrafine particles will be the new functional materials of the 21st century.
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