Study On Improving The Magnetic Properties And Corrosion Resistance Of Sintered Nd-Fe-B Magnet By Electrophoretic Deposition

Sintered Nd-Fe-B magnets are widely used in new energy vehicles,wind power generation,high-performance motors and other fields requiring permanent magnets due to their superior comprehensive magnetic properties.At present,two important improvement directions for sintered Nd-Fe-B magnets include improving the coercivity and corrosion resistance of magnets.The electrophoretic deposition method(EPD)has the advantages of high production efficiency,low energy consumption,uniform film layer and controllable thickness.Its various advantages make it the most promising production process for large-scale production.In this paper,the diffusion source and epoxy resin film of grain boundary diffusion were deposited on the surface of magnets by electrophoretic deposition,which can improve the coercivity and corrosion resistance of sintered Nd-Fe-B magnets efficiently and at low cost.The traditional diffusion source used in grain boundary diffusion(GBDP)is heavy rare earth elements and their compounds.Improving the utilization rate of heavy rare earths can reduce resource consumption and improve product efficiency.In this paper,TbF3-Al mixed diffusion source was prepared by EPD for grain boundary diffusion.The influence of GBDP parameters on the properties of the magnet and structure and element distribution of the magnet before and after Al entering the magnet was studied.The coercivity enhancement mechanism of the mixed diffusion source was discussed.In addition,the effects of different EPD on the properties of epoxy resin films were discussed.The main results are as follows:(1)The feasibility of EPD of TbF3-Al mixed diffusion source on the surface of sintered Nd-Fe-B magnet was determined by experiments.The effect of electrophoresis time on the film quality and deposition rate,and the optimal deposition sequence of mixed diffusion sources were investigated.(2)The optimum heat treatment process of TbF3 diffusion source and TbF3-Al mixed diffusion source was determined.After the GBDP of the mixed diffusion source under the same mass of the diffusion source,the coercivity was reduced from the original magnet 976.78 kA/m to 1373.07 kA/m,an increase of 40.6%.After the grain boundary diffusion of the same mass TbF3 diffusion source,the coercivity is increased to 1263.48 kA/m,which is only 29.4%higher than that of the original magnet.Compared with the single diffusion source TbF3,the TbF3-Al mixed diffusion source has more obvious coercivity improvement effect and better temperature stability of magnetic properties.Compared with the pure Tb diffusion magnet,the temperature coefficient of coercivity of TbF3-Al diffusion magnet increases from-0.623%/℃ to-0.588%/℃,and is higher than-0.594%/℃ of the original magnet.Compared with pure TbF3 GBDP magnet,the heat treatment temperature required for TbF3-Al diffusion magnet to reach the highest coercivity decreases,and the heat treatment time also decreases.(3)Through morphology analysis,it was found that the continuous thin grain boundary phase increeased after A1 entered the magnet.Such structure is conducive to the improvement of coercivity.Through elemental analysis,it is found that the thin grain boundary phase is the main channel for Tb element to diffuse into the interior of the magnet,which indicates that the diffusion of Al is beneficial to Tb to improve the diffusion efficiency.(4)The main processes affecting the deposition of epoxy resin film on sintered Nd-Fe-B magnets include electrophoresis voltage,electrophoresis time and electrophoresis temperature.The best deposition process is voltage 100 V,time 120 s,temperature 30℃,and the polarization current density is 4.20×10-7 A/cm2.Which is far less than the polarization current density of the original magnet.Increasing the voltage,time and temperature of electrophoresis is conducive to obtaining a thicker film and improving corrosion resistance.However,excessive voltage,time and temperature will damage the uniformity of the film layer and offset the improvement of corrosion resistance caused by the thickening of the film layer.In addition,too thick film layer will lead to the decrease of magnetic properties of magnets.