Regulation Of Grain Boundary And Magnetic Properties For Sintered NdFeB Magnets By Sputtering Dy-based Heavy Rare Earth Diffusion Layers

Sintered NdFeB magnets are widely used in the fields of new energy automobiles,aerospace industries and traditional electronics devices due to their excellent room-temperature magnetism.However,their magnetic properties will rapidly decrease or even fail when these sintered NdFeB magnets are subjected to high temperature enviroment.Therefore,the demand for the NdFeB magnets with high temperatures resistance is becoming stronger in industry.Based on this requirment,this work focuses on the regulation of the grain boundary and magnetism of sintered NdFeB by sputtering and diffusing Dy-based heavy rare earth layers.The main research contents and results are as follows:（1）Firstly,we used N50 magnets to explore the best conditions for the grain boundary diffusion of Dy element.A 3 um-thick Dy film was sputterred on the surface of the magnet.After heat treatment and diffusion,the best conditions for grain boundary diffusion were optimized to 900℃-5h+500℃-3h.The coercivity of the diffused magnet increases significantly from 12.1 k Oe of the original magnet to 16.0 k Oe,an increase of32.2%And the remanence decreases from 1.47 T to 1.39 T,only a decrease of 5.4%.Moreover,the influence of Dy thickness on the magnetic properties of different brands of magnets were studed and found that after the grain boundary diffusion（GBD）process the magnetic properties for N42 and N50 magnets were best,which can be used as original magnets for the subsequent grain boundary diffusion treatment.（2）Secondly,the effects of low-melting-point Dy Fe alloys with different proportions of Dy and Fe on the magnetic properties and microstructures of N50 original magnets were investigated.For the different alloys,the coercivity of the magnet diffused with Dy₈₀Fe₂₀alloy can reach to15.8 k Oe,while the remanence can not be significantly reduced due to the presence of Fe.Notably,both the temperature stability and anisotropy field of the diffused magnet are improved.The diffusion of Dy Fe alloy can effectively increase the diffusion depth and alleviate the accumulation of Dy on the surface of the magnet.The diffusion process can save the amount of heavy rare earth elements,reducing production cost at a certain degree.In order to improve the diffusion depth,the diffusion channels of the grain boundaries can be opened through acid etching treatment,by which the diffusion depth reaches to about 500μm,and the magnetic properties further are improved.（3）Finally,the effects of Dy/Al composite layers on the magnetic properties and microstructures of N50 magnets were studied.We determined the role of Al layers in grain boundary diffusion by designing different Dy/Al structures.The results indicate that the Al layer with appropriate thickness can drive or push the diffusion of Dy layer as the Dy layer thickness is fixed.The best magnetic property was obtained for the magnet diffused by the Al/Dy/Al composite layer.Compared with Dy element diffusion,the coercivity increases by 13.9%and the diffusion depth reaches 1000 um.The reason for the enhancement is that the addition of Al can increase the wettability of the grain boundaries and further help Dy diffusion along the grain boundaries.Therefore,the Dy/Al composite layers play a double role in grain boundary diffusion and regulation for the magnet.Furthermore,this design not only improves the temperature stability of the magnet,but also meets the demand for high-temperature magnets above 120℃.In short,we used the magnetron sputtering and grain boundary diffusion technologies to realize the effective diffusion of Dy-based heavy rare earth in NdFeB magnets and improve their magnetic properties,which provides technological guidance for the reduction of heavy rare earth in high-performance NdFeB permanent magnets.This achievement lays an important foundation for the research of cost-effective NdFeB magnets.