Study On Nd-Fe-B Thermal Demagnetization Behaviour And Grain Boundary Diffusion Method To Modified Magnetic Property

Sintered Nd-Fe-B magnets are widely used in the fields of motors and electric vehicles because of their excellent magnetic properties.As a result of various factors,Nd-Fe-B magnets are easily demagnetized in the process of employ.Among them,temperature is one of the most important factors.This article adopted two kinds of Nd-Fe-B magnets with different size and properties as the research object to study thermal failure behaviors at high temperatures and use grain boundary diffusion method to improve the magnetic properties of the magnets with poor performance.The main findings of this paper are as follows:By studying the magnetic properties,microstructure and phase composition of N35 and38SH magnets with different sizes at different temperatures,it is shown that the stability of magnetic properties increases with the increase of Pc and magnetic properties.The results of SEM images show that the decreasing of magnetic flux recovery rate after high temperature treatment is mainly due to the discontinuous distribution of the grain boundary phase and some grains begin to precipitate cerium-rich phases.And the XRD patterns indicated that oxygen is more easily enter into the lattice of the main phase at high temperatures,thereby affecting the recovery of magnetic properties.A uniform layer of LiF and TbF₃ mixed powder was attached to the surface of the magnetic block by ultrasonic coating.The optimum diffusion temperature,diffusion time,and optimum ratio of the mixed system for the grain boundary diffusion were determined.When the composition of the binary system is 50 wt%LiF+50 wt%TbF₃,the overall performance of N35 magnet is the best.By changing the diffusion temperature and time,the coercive force of the 6 mm thick N35 magnet was raised from 13.19 KOe to 21.49 KOe,and the coercive force of the 10 mm N35 thick magnet was raised to 19.2 KOe.This shows that the use of LiF and TbF₃ mixed binary system as diffusion source not only can significantly improve the magnetic properties of N35 magnets,but also show superior processing capabilities for larger size N35 magnets.Through the analysis of microstructure,the grain boundary phase transition in the magnet is wide and continuous after diffusion under the optimum condition,which is benefit to reduce coupling effect of magnetic exchange.According to the results of line scanning,it is found that the concentration of heavy rare earth elements in the grains did not decrease with the increase of diffusion depth.,which means the process of diffusion is not only affected by the grain boundary diffusion,but also by the surface diffusion factors.Heavy rare-earth elements are integrated into main phase and microstructure optimization,which makes the large size magnets also gain higher coercivity.