Currently, the laboratory level of the maximum magnetic energy product of sintered NdFeB is very close to its theoretical limit value. Although the difference between the production level and the limit value is not large, the intrinsic coercive force of the sintered NdFeB is much lower than the theoretical limit value and can be largely improved. With continuous development of the application field of the NdFeB magnet, persons in the art are seeking to obtain higher coercive force. Therefore, the problem that how to make a full play of the inherent properties of the main phase of NdFeB and then improve the intrinsic coercive force Hcj of the sintered NdFeB becomes a hot issue to be studied at present.
Years of fundamental researches and production practices suggest that, it is a well-known effective method to add heavy rare earth elements, such as Dy (element dysprosium) and Tb (terbium), etc., in the production process of a magnet to substitute a part of Nd in the magnet, thereby improving the coercive force of the sintered NdFeB magnet.
The main reason is that Dy2Fe14B or Tb2Fe14B crystal has a higher magnetocrystalline anisotropy field than Nd2Fe14B crystal, that is, has higher theoretical intrinsic coercive force.
After a part of Nd in the main phase Nd2Fe14B is substituted by Dy and Tb, the magnetocrystalline anisotropy field of the generated solid-solution phase (Nd,Dy)2Fe14B or (Nd,Tb)2Fe14B is higher than that of Nd2Fe14B, thereby significantly improving the coercive force of the sintered magnet.
The methods for adding Dy and Tb generally include: a method of directly adding Dy and Tb in an alloy smelting process; or a dual-alloy method of a Dy/Tb-rich alloy and an NdFeB alloy. However, the defect of the two methods, especially the direct smelting method, is that the saturation magnetization of the magnet may significantly be reduced, thereby reducing the remanence and the maximum magnetic energy product of the magnet. The reason is that in the main phase Nd2Fe14B, the magnetic moments of Nd and Fe are arranged in parallel in the positive direction, and are superposed in the same direction; Dy/Tb and Fe are anti-ferromagnetically coupled, and the magnetic moment of Dy/Tb and that of Fe are superposed in opposite directions, thereby resulting in a reduction of the total magnetic moment.
Besides, as compared with Nd, the Dy and Tb-containing mineral reserves are rare and are mainly distributed in a few regions, and the prices of the metal Dy and Tb are much higher than that of the metal Nd, which results in a significant increase in the production cost of the magnet.
In recent years, the grain boundary heat diffusion process is used for effectively improving the intrinsic coercive force of the sintered NdFeB magnet, with rarely reducing the remanence and the magnetic energy product of the magnet. In this process, a substance layer containing heavy rare earth elements, such as metal powder of Dy or Tb or a compound containing Dy or Tb, is covered on the magnet by using methods such as coating, depositing, plating, sputtering, and adhering, and through heat treatment, the heavy rare earth elements are caused to diffuse into the interior of the magnet along an Nd-rich liquid grain boundary phase. In the heat treatment process, the diffusion speed of Dy/Tb in the grain boundary is much higher than that of Dy/Tb in the grain boundary diffusing into the interior of the main phase grains.
A thin and continuous shell layer containing heavy rare earth elements will be generated between the main phase of the sintered body and the rare earth-rich phase by adjusting the heat treatment temperature and time on the basis of the diffusion speed difference.
Because the coercive force of the sintered NdFeB magnet is determined by the anisotropy of main phase particles, the sintered NdFeB magnet with a high-concentration heavy rare earth element shell layer coated has a high coercive force outside main phase grains. The high-concentration regions are limited to the surface layer of each main phase grain, and the volume ratio of the high-concentration regions to the main phase grains is very low, so the remanence (Br) and the maximum magnetic energy product of the magnet basically remain the same.
For example, a diffusion coating technology on the surface of a magnet is disclosed in the patent publication CN1898757A applied by Shin-Etsu Chemical Co., Ltd. A sintered blank is processed into a thin magnet which is then dip-coated with the slurry formed by dispersing heavy rare earth micron-sized fine powder into water or an organic solvent, and a heat treatment is performed on the magnet in vacuum or in an inert gas atmosphere and at a temperature which is not higher than the sintering temperature. As a result, the coercive force is largely improved, and the remanence is substantially not reduced. This method not only saves the heavy rare earth, but also inhibits the reduction of the remanence.
The above methods can partly improve the Hcj and require a grain boundary heat diffusion process that is performed at about 900° C. and lasts for several hours, so that the heavy rare earth elements on the surface of the magnet move toward the interior of the magnet, and a high-content shell layer is formed on the surface of main phase grains of the magnet, and finally the coercive force of the magnet is improved.
However, as a normal heating manner (generally resistance heating) is adopted, the heating mechanism is mainly based on radiation and conduction, and the heating efficiency is low. Meanwhile, as the regions where grain boundary heat diffusion of heavy rare earth metal elements really occurs are merely centralized within a certain range on the surface layer of the magnet, it is a waste of energy to heat a part of the core portion of the magnet that does not participate in the diffusion process, and thus the production cost is increased.
If the heating efficiency can be effectively improved and localized heating can be selectively performed, the process is simplified, the time for heat treatment is reduced, the energy consumption is lowered, and the production cost of a magnet is reduced.