Agglomeration and dispersion of nano powders

At present, all kinds of nanometer powders can be easily prepared, but the agglomeration of nanometer powders seriously hinders the application of nanometer powders and the preparation of nanometer materials. For nanomaterials with self-assembled structure, the designed structure cannot be obtained due to the agglomeration problem. For nanocrystalline materials, the agglomeration will cause the abnormal growth of particles and lead to the degradation of properties. For the occasions where nanometer luminescence and nanometer catalysis are used directly, the agglomeration directly reduces the performance and efficiency of materials. Therefore, the development of nanotechnology to today, reunion has become the bottleneck of the continued development of nanotechnology. In this paper, the agglomeration mechanism of nano powders, especially the hard agglomeration mechanism, and the basic ways of dispersion in preparation and application of nano powders were reviewed. The so-called agglomeration of nano powder refers to the phenomenon that the original nano powder particles are connected with each other in the process of preparation, separation, treatment and storage, and form larger particle clusters from multiple particles. The agglomerations of nano powders are generally divided into soft agglomerations and hard agglomerations. For the soft agglomeration mechanism of nano powders, people have a consistent view, which is caused by van der Waals force and electrostatic attraction between molecules or atoms on the surface of nano powders. For hard agglomeration, different chemical compositions and preparation methods have different agglomeration mechanisms, which cannot be explained by a unified theory. The theory suggests that the hard agglomeration of nanometer powder materials in the separation and drying process of solvent is mainly caused by capillary action in the process of drainage. When the nanometer powder containing disperse medium is heated, the adsorbed liquid medium begins to evaporate. With the evaporation of water, the spacing between particles decreases, and the connecting capillary tubes are formed between the particles. With the further evaporation of the medium, part of the surface of the particles is exposed. However, water vapor goes out from both ends of the pore. In this way, due to the presence of capillary force, static tensile pressure is formed in the water, which can lead to the contraction of capillary pore wall. Therefore, it is considered that capillary force is the main reason for the formation of hard agglomeration of nano powders.

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