When the size of the magnetic material is reduced to the nanometer scale, there will be some special physical and chemical properties different from the bulk material. Permanent magnet nanoparticles or flakes have important applications in many fields. For example, rare earth permanent magnet particle film can obtain excellent permanent magnet performance without high temperature annealing, and obtain special use in magnetic MEMS; rare earth permanent magnet nanoparticle has high anisotropy and ferromagnetic-superparamagnetic critical dimension is low. The performance of the ferrofluid can be improved; the rare earth permanent magnet nanoparticle ferromagnetic-superparamagnetic critical dimension is lower than the critical dimension of the existing magnetic recording material, and can be used for ultra-high density magnetic recording, breaking the storage density limit of the existing magnetic recording material; Wait.
However, the preparation of anisotropic magnetic nano permanent magnet particles is a big challenge. Although soft magnetic nanoparticles can be synthesized by physical and chemical methods, the preparation of rare earth hard magnetic nanoparticles has been a problem. The main reason is that rare earths are easily oxidized and the composition is complicated. Although rare earth permanent magnet nanoparticles can be prepared by magnetron sputtering or evaporation, the experimental conditions are high and it is difficult to achieve industrialization. Powders with sub-micron particle sizes are generally available using conventional high-energy ball milling techniques, but it is difficult to further reduce the particle size by adjusting ball milling process parameters (such as extended ball milling time).
Recent studies have found that high performance rare earth permanent magnet nanoparticles can be successfully prepared by surfactant-assisted ball milling. In conventional dry or wet milling, the broken particles are again cold welded due to the high surface energy, so the particles are difficult to further refine. Surfactant-assisted ball milling is the addition of a surfactant to the wet grinding. The surfactant adheres to the surface of the particles, reducing the surface energy and preventing the cold welding from occurring, so that the particles can be refined to the nanometer scale. Surfactants play a number of important roles in the surfactant-assisted ball milling process: in addition to preventing cold soldering, the particles are further refined, and because they adhere to the surface of the nanoparticles, they are suspended in the solvent for a longer period of time. Better separation of nanoparticles of different sizes; as a surface lubricant, it will lead to different dissociation and fragmentation processes of the particles in the ball milling, and obtain nanomaterials with different morphologies, and also have protective effects on the nanoparticles. A large number of studies and experiments have shown that ball milling with appropriate surfactants and organic solvents as the medium is the best method for preparing rare earth permanent magnet nanoparticles. This method prevents the occurrence of powder agglomeration and cold welding during ball milling, which is very effective. The particle size is reduced, and the friction between the particles can be reduced to prevent oxidation of fine particles in the ball milling. The method has been successfully used to prepare nano particles of less than 10 nanometers and nanosheets of several tens of nanometers, and directly obtain Sm-Co and Nd-Fe-B anisotropic nanosheet materials with high coercivity. Importantly, the surfactant-assisted ball milling process is relatively simple and more promising for industrialization. Moreover, the process is completely carried out at room temperature or even at a low temperature, so it is particularly suitable for the preparation of chemically active rare earth nanomaterials.
However, although the surfactant-assisted ball milling technique can obtain permanent magnet particles having a size as low as several nanometers, the ball-milled product tends to have a poor particle size distribution, resulting in low coercivity. Particle grading techniques such as ultrasonic vibration, static and centrifugal separation can be used. The size of the ball mill product can be controlled by the settling time and centrifugal separation of the particle solution to prepare rare earth nanoparticles with narrow particle size distribution, so that the coercive force is obvious. Raise.
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