In order to improve the utilization ratio of HREE and avoid magnetic dilution effect, grain boundary modification process is proposed. Above all, antiferromagnetic coupling between Fe atoms and Dy atoms will generate serious magnetic dilution effect and substantially deteriorate Br and (BH)max. Coercivity mechanism of sintered Neodymium magnets indicates that reversed magnetic domain tends to nucleate at the boundary areas of the main phase, and uniform distribution of HREE will result in waste of resources and cost up. Alloying ProcessĪlloying process is referring to add a certain proportion of HREE Dy or Tb to the raw material of sintered Neodymium magnets, then all elements show homogenization of composition through the melting process. The frequently-used adding methods include traditional alloying process, grain boundary modification process, and grain boundary diffusion process. The H A of Dy 2Fe 14B and Tb 2Fe 14B are considerably higher than Nd 2Fe 14B, then adding small amounts of Dy or Tb element to replace Nd atom in main phase lattice will form (Nd, Dy) 2Fe 14B or (Nd, Tb) 2Fe 14B with higher H A which can effectively improve intrinsic coercivity. That is to say, the higher the magnetocrystalline anisotropy field of the main phase grain, the higher the coercivity of sintered Neodymium magnets. There exists a positive relationship between coercivity of sintered Neodymium magnets and magnetocrystalline anisotropy field of the main phase grain. Composition can be optimized through adding other elements to improve magnetocrystalline anisotropy field of the main phase grain. Optimization of microstructure focus on grain refinement and improve the distribution of Nd-rich phase. The intrinsic coercivity of sintered Neodymium magnets is mainly influenced by microstructure and composition. It always is a major issue to enhance Hcj while still maintaining high Br and (BH)max. Wind power generator, new energy vehicle, energy-saving household appliances, and latest mobile intelligent terminal are all require sintered Neodymium magnets not only have high (BH)max, but also have superior Hcj. Nd-rich phase is preferably in the form of layered structure and continuous distributed in grain boundary areas.Ĭoercivity Enhancement of Sintered Neodymium Magnets Its composition, structure, distribution, and morphology are highly sensitive to the process conditions. Nd-rich phase plays a key role in magnetic hardening of sintered Neodymium magnets. Nd 2Fe 14B main phase is the only hard magnetic phase in the sintered magnet and its volume fraction determines Br and (BH)max of Nd-Fe-B alloy. Many microstructure observations indicate that there are six phases exist in the sintered Neodymium magnets, then Nd 2Fe 14B main phase and Nd-rich phase is the best known due to their effects on the magnetic performance. Beyond that, reserve volume of Neodymium in Earth’s crust is relatively abundant which can maintain supply chain stability and cost advantage. Most of rare earth elements can form RE 2Fe 14B compound with Fe and B, and Nd 2Fe 14B compound has the highest saturation magnetization and functional magnetocrystalline anisotropy field among these RE 2Fe 14B compounds. It can be seen from the figure below, high (BH)max magnet can supply same magnetic field strength with less consumption, then development history of the permanent magnet is essentially a process of pursuing higher performance. The value of (BH)max represents permanent magnet’s ability to provide magnetostatic energy. Magnet with higher Br can offer stronger magnetic field strength, then higher Hcj can serve much better anti-interference ability. The practicability of the permanent magnet can be judged by stability of remanence Br, intrinsic coercivity Hcj, and maximum energy products (BH)max under external condition.
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