Since its introduction in the 1980s, NdFeB permanent magnet materials have been widely used in various fields such as automobiles, wind power, aerospace, and military industries with excellent magnetic properties. In recent years, the demand for wind power generation, new energy vehicles, etc. has continued to increase. This puts forward higher requirements for the coercive force and temperature stability of NdFeB permanent magnet materials.
Because the magnetic crystal anisotropy field of the Dy2Fe14B phase is much stronger than that of the Nd2Fe14B phase and the Curie temperature is relatively high, the coercivity and temperature stability of the material can be greatly improved. In sintered NdFeB materials, the content of rhenium is very high, and some can reach more than 10%. Everyone knows that the price of heavy rare-earth element europium is high, and a large amount of addition will increase the production cost of neodymium iron boron. Therefore, how to reduce the amount of europium under the premise of ensuring high coercivity and temperature stability has become an important issue.
The traditional element addition method is to add in the melting process, that is, Dy, Tb and Nd, Fe, B and other elements are smelted together, Dy distribution in the grain boundary and the main phase of the grain in the magnet. However, research shows that Dy at the grain boundary has the most significant effect on improving the coercive force, and the traditional method of adding elements is a “waste of resources”.
Japanese researchers first proposed the concept of “grain boundary diffusion”. They used a special process to make Dy exist only at the grain boundary and not enter the crystal by 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 milling process, and Dy atoms diffused along the grain boundaries during subsequent sintering. Dy and Fe located at the grain boundary are antiferromagnetically coupled, and the coercive force of the material increases from 800 kA / m to 1800 kA / m with almost no reduction in remanence.
Damage to the surface of the magnet after machining will weaken the magnetic properties, especially for small-size samples, the coercivity is significantly reduced, and grain boundary diffusion technology can be used to repair and increase the magnetic surface magnetic properties. At present, the grain boundary diffusion technology has received widespread attention, and its preparation processes mainly include evaporation diffusion, magnetron sputtering, surface coating, and the like.
The Dy / Tb evaporation process on the surface of the neodymium-iron-boron magnet is to place the heavy rare earth element or its compound and the original sample to be processed in a steaming furnace, and use high temperature heating to evaporate the heavy rare earth element at high temperature. It is deposited on the surface of the original magnet and diffuses into the magnet along the grain boundary.
The evaporation diffusion method can simultaneously perform sublimation of Dy evaporation source, deposition on the surface of neodymium-iron-boron, and diffusion in the magnet under the condition of high temperature heating. The advantage of using the evaporation diffusion method is to diffuse heavy rare-earth elements. It is more sufficient to reduce the amount of heavy rare-earth elements and reduce the cost, thereby successfully preparing a high coercivity and low rare-earth content neodymium-iron-boron magnet.
Unlike the evaporation diffusion method described above, magnetron sputtering separates the Dy deposition process from the diffusion process. It deposits Dy on the surface of the original magnet by physical sputtering, and then performs high temperature diffusion. Magnetron sputtering has the advantages of uniform film thickness and obvious coercive force enhancement effect.
Some research experiments have shown that after sintered and tempered N35 magnets are treated with sputtering Dy, the coercive force is greatly improved when the remanence is reduced by only 0.009T and 0.03T, which are increased by 708.44kA / m and 665.46, respectively. kA / m, the increase rate is as high as 73.5% and 64.8%, and the average mass fraction of Dy element of the magnet after Dy infiltration does not increase by more than 0.4%. After infiltrating the Dy-treated magnet, Dy is enriched in a continuous band at the Nd-rich phase, which makes the Nd-rich phase more continuous and smooth. The improvement of the Nd-rich phase structure and morphology is one of the reasons for the increase in coercive force. The (Nd, Dy) 2Fe14B epitaxial layer formed has a large magnetic crystal 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 coercive force.
The surface coating method refers to coating a rare earth compound directly on the surface of the original magnet sample, and performing a high-temperature heat treatment diffusion in a rare gas atmosphere after drying treatment. Using this method can significantly improve 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.