Mechanism Of Sintered NdFeB Doped By Dy-(Al)-Cu Alloy From Molten Salt Electrolysis And Its Influence On Magnetic Properties

Among the existing magnetic materials,sintered Nd Fe B magnets show the highest magnetic energy product,which arises from the excellent intrinsic magnetism of the Nd₂Fe₁₄B unit cell.The magnetic properties of sintered Nd Fe B comply with their composition and microstructure.Additives such as Dy and Tb are preferred to improve the surface anisotropy of hard magnetic grains from the perspective of the composition.Adding heavy rare earth elements or rare earth alloys into Nd Fe B magnets by grain boundary diffusion technology can not only effectively improve the comprehensive magnetic properties of magnets but also greatly reduce the usage number of heavy rare earth elements and improve the cost performance.However,the effect of grain boundary diffusion will decrease with the increasing magnet thickness because the limited diffusion depth of heavy rare earth along the thickness direction of the magnet will lead to an insufficient increase in the coercivity of the thick magnet.Compared with the alloy obtained by doping smelting,the low melting point Dy alloy prepared by molten salt electrolysis is mainly a single-phase Dy-M master alloy,which can effectively reduce the melting point and self-diffusion activation energy of the diffusion agent and provide smoother diffusion channels to improve the diffusion rate.In this paper,the first principle calculation of(Nd_1-xDy_x)₂Fe₁₄B,Nd₂（Fe Al）₁₄B and Nd₂Fe₁₄BC（O）_0.25cell stability,electronic and bonding characteristics and their magnetic property parameters are studied.On the basis of the calculation,different Dy-M master alloys were used as grain boundary diffusion sources to prepare high-performance Nd-Fe-B.The diffusion rate and diffusion depth of molten salt electrolytic Dy-M alloy and doped molten Dy-M alloy were compared,the application effect of molten salt electrolytic alloy in sintered Nd-Fe-B was evaluated.The results show that the Dy rich shell phase was also a 2:14:1 tetragonal phase structure,and its stability increased with the increase of Dy content.Its magnetism mainly arises from the spin of the Fe-3d electron and the ferromagnetic coupling between Fe-3d and RE-4f,in which Fe and Nd are ferromagnetic coupling meanwhile Fe and Dy are antiferromagnetic coupling.The structure of Al atoms substituted for Fe atoms in 8j2 crystal position is the most stable,and Al has little effect on the intrinsic magnetic properties of the Nd Fe B main phase.The incorporation of C and O into the main phase causes lattice distortion,and the magnetic properties of Nd Fe B deteriorate.The magnetic calculation results of the introduction of Dy atoms into the main phase unit cell show that the increase in the Dy atomic weight,the saturation magnetization,the saturation magnetic polarization,and the theoretical magnetic energy product gradually decreases,and the value of the magnetocrystalline anisotropy constant increases and tends to be stable,the magnetocrystalline anisotropy field increases.The grain boundary diffusion experiments of molten salt electrolysis of Dy-M master alloy and counter-doped smelting alloy show that with the increase of Dy content,the remanence of the diffusion-treated magnet decreases continuously,and the coercive force increases accordingly.The analysis of the diffusion direction shows that the diffusion rate of the Dy-M alloy in molten salt electrolysis is faster,which can effectively increase the diffusion depth of Dy toward the thickness of the magnet.