Since the advent of NdFeB permanent magnet materials in the 1980s, they have been widely used in various fields such as automobiles, wind power, aerospace, and military industries due to their excellent magnetic properties. In recent years, the demand for wind power generation and new energy vehicles has continued to increase. This puts forward higher requirements on the coercive force and temperature stability of NdFeB permanent magnet materials.
Since the magneto crystalline anisotropy field of the Dy2Fe14B phase is much stronger than that of the Nd2Fe14B phase, and the Curie temperature is relatively high, the coercive force and temperature stability of the material can be greatly improved. In sintered NdFeB materials, the content of dysprosium element is very high, and some can reach more than 10%. We all know that the price of heavy rare earth element dysprosium and terbium is relatively high, adding a large amount will increase the production cost of NdFeB, so how to reduce the amount of dysprosium and terbium under the premise of ensuring high coercive force and temperature stability has become an important issue.
The traditional method of adding elements is to add them during the smelting process, that is, to smelt Dy, Tb and Nd, Fe, B and other elements together. In the magnet made, there are Dy distributions in the grain boundary and the main phase in the grain. However, studies have shown that Dy at the grain boundary has the most significant effect on improving the coercive force, and the traditional element addition method is a bit "wasting resources".
Japanese researchers first proposed the concept of "grain boundary diffusion". They used a special process to make Dy only exist in the grain boundary and not enter the grain through diffusion. This not only improved the performance of NdFeB materials, but also greatly reduced Dy. The total amount of elements reduces the cost of materials. They deposited Dy vapor on the surface of the particles during the pulverization process, and the diffusion of Dy atoms along the grain boundaries occurred during the subsequent sintering process. Dy and Fe located at the grain boundary are antiferromagnetically coupled, and the coercive force of the material increases from 800kA/m to 1800kA/m with almost no reduction in remanence.
The damage to the surface of the magnet after machining will lead to the weakening of the magnetic properties, especially for small-sized samples, the coercive force is significantly reduced, and the grain boundary diffusion technology can be used to repair and increase the magnetic properties of the magnet surface. At present, grain boundary diffusion technology has received widespread attention, and its preparation processes mainly include evaporation diffusion, magnetron sputtering, and surface coating.
Evaporation diffusion
The process of evaporating Dy/Tb on the surface of NdFeB magnets is to place the heavy rare earth elements or their compounds and the original sample to be treated in the evaporation furnace, use high-temperature heating to evaporate the heavy rare earth elements at high temperature, and induce them under the induction of external rare gases Deposits on the surface of the original magnet and diffuses along the grain boundaries into the magnet.
The evaporation diffusion method can be used in the state of high temperature heating, the sublimation of the Dy evaporation source, the deposition on the surface of NdFeB, and the diffusion process in the magnet can be carried out simultaneously. The advantage of using the evaporation diffusion method is that the heavy rare earth elements can be diffused. The NdFeB magnets with high coercive force and low rare earth content were successfully prepared by reducing the usage of heavy rare earth elements and reducing the cost.
Magnetron sputtering
Different from the evaporation diffusion method mentioned above, magnetron sputtering separates the Dy deposition process from the diffusion process. It deposits Dy on the surface of the original magnet through physical sputtering, and then diffuses at high temperature. Magnetron sputtering has the advantages of uniform film layer and obvious coercivity improvement effect. Some research experiments have shown that after the N35 sintered and tempered magnets are treated with Dy by sputtering, the coercive force is greatly improved, and the remanence is only reduced by 0.009T and 0.03T, respectively increased by 708.44kA/m and 665.46 kA/m, the increases were as high as 73.5% and 64.8%, respectively, and the average mass fraction of the Dy element in the magnet after the Dy infiltration treatment did not increase by more than 0.4%. In the magnet after infiltration of Dy, Dy is enriched in the Nd-rich phase in the form of continuous bands, making the Nd-rich phase more continuous and smoother. The improvement of the morphology of the Nd-rich phase is one of the reasons for the increase in coercive force. The formed (Nd,Dy)2Fe14B epitaxial layer has a large magneto crystalline anisotropy field, and the hardening of the grain epitaxial layer can better suppress the reverse domain nucleation, which is also the main reason for the increase of the coercive force.
Surface coating
The surface coating method refers to the fact that the rare earth compound is directly coated on the surface of the original magnet sample, and after drying, it is diffused by high temperature heat treatment in a rare gas atmosphere. Using this method can significantly increase the coercive force of the magnet. The advantage is that the process is simple and convenient. The disadvantage is that it is easy to cause uneven coating and insufficient diffusion.