Microstructure And Properties Of Sintered Nd-Fe-B Permanent Magnetic Materials Prepared By Grain Boundary Diffusion Of Thin Films

Because its excellent internal magnetic properties,sintered Nd-Fe-B magnets are widely used in civil and high-tech industries such as electronics,medical,automotive and aerospace,etc.However,at present,Nd-Fe-B magnets still have the disadvantages of poor high temperature stability and weak corrosion resistance,which limits their further application in various fields.Higher coercivity is a prerequisite for the application of magnets in high temperature working environments.Traditional methods,such as doping with heavy rare earth,despite of its workable effect,pose huge pressure upon the shortage using of heavy rare earth.In this paper,an environmentally friendly technique,named magnetron sputtering technology,was carried out to deposit the elemental and composite films on the surface of sintered Nd-Fe-B magnets,after a suitable grain boundary diffusion process,the diffused magnets with excellent comprehensive properties were successfully prepared.In this paper,magnetron sputtering technology was used to deposit Dy elemental thin films on the surface of sintered Nd-Fe-B magnets,and the magnetron sputtering and grain boundary diffusion processes were optimized.The results show that the film is dense and uniform when the sputtering power is 125 W and the deposition time is 10 min.And the magnets produced by the 800℃/5 h grain boundary diffusion process have the highest coercivity,up to Hci=1189 k A/m,reach an increase of 22.3%.Microstructure studies have found that higher and lower temperature diffusions are likely to cause agglomeration of the RE-rich phase,which adversely affects coercivity.When the grain boundary diffusion time is increased to 7 h,the comprehensive magnetic properties of the magnet are the best,among which Hci=1174 k A/m,Jr=1.205 T,and(BH)max=246 k J/m3.Meanwhile,it is found that after the grain boundary diffusion,the recoil loops of the magnet changed from the open state to the closed state,which is derived from the microstructure optimization of the magnet after diffusion.Corrosion resistance test results show that the corrosion resistance of Dy film diffusion magnets is improved compared with the original magnets.When the grain boundary diffusion was carried out using the Mn elemental film layer as a diffusion source,it was found that the coercivity of the magnet did not increase but decreased.Based on the co-sputtering technique,a Dy Mn composite film layer was prepared.Which was consisted of crystalline and amorphous Dy and Mn phases.After optimized the magnetron sputtering process,it was found that the coercivity reach the highest value at 1298 k A/m when the sputtering time was 30 min,which was 33.5% higher than that of the original magnet.The diffusion process shows that the optimal heat treatment condition of Dy Mn composite film coated sample is 750℃/5 h.and the magnetic properties of the magnet can achieve: Hci=1310 k A/m,Jr=1.208 T,(BH)max=259 k J/m3.The annealing temperature and time required are lower than that of the single Dy film diffusion,and the comprehensive magnetic properties of the magnet are further improved.Corrosion resistance test results show that the corrosion resistance of Dy Mn diffusion magnets is significantly improved.The grain boundary diffusion process of non-rare earth Al film was studied.It was found that the magnetic properties of the magnets prepared at 550℃/1 h were the best.Microstructural studies show that in the intergranular phase region,a weak ferromagnetic phase Al-rich phase(Nd26.7Pr10.5Fe53.9Al8.9)is found in the intergranular phase region,which can increase intergranular exchange coupling and reduce the nucleation field of the demagnetized domain,is not conducive to the increase of the coercivity of the magnet.The thermal stability study found that the coercivity temperature coefficient |β| and the remanence temperature coefficient |α| of the Al film diffusion magnet were all decreased,which indicates that the temperature stability of the diffusion magnet has been improved.The grain boundary diffusion magnet with excellent magnetic properties is prepared by step-sputtering and diffusion of Dy and Al.The optimal comprehensive magnetic properties can reach: Hci=1383 k A/m,Jr=1.179 T,(BH)max=250 k J/m3,the coercivity increased by 42.3% compared to the as-prepared magnet.Moreover,the temperature stability is also significantly improved compared to the magnets produced by the diffusion of the single film.