Microstructure And Magnetic Properties Of Sintered Nd-Fe-B Magnets Optimized By Electrophoretic Deposition And Grain Boundary Diffusion

Sintered Nd-Fe-B magnets are widely used in high-precision fields such as aerospace,instrumentation,electronic communications,health care,and new energy due to their excellent comprehensive magnetic properties.However,the low coercivity and temperature stability of sintered Nd-Fe-B magnets restricted its application in the new field.Although the improved coercivity can be achieved by the traditional doping for heavy rare-earth elements,this method consumes a large amount of rare-earth resources and has a low cost-benefit ratio.In this paper,Al and Dy₂O₃ films were deposited on the surface of sintered Nd-Fe-B magnets by the electrophoretic deposition(EPD),and then the grain boundary diffusion(GBD)technique was performed to prepare the sintered NdFe-B magnets with excellent comprehensive magnetic properties.EPD was used to deposit the Al film with different particle sizes on the surface of sintered Nd-Fe-B magnets,and the optimal EPD and GBD processes of Al powders with different particle sizes were obtained.The results showed that the optimal process for EPD of 2 ?m Al powder was 90 V/60 s,and that of 500 nm and 50 nm Al powders were 90 V/30 s.Under the condition of optimal processes,three kinds of Al films all adhered well to the magnets,and the thicknesses were uniform and moderate.Due to the comprehensive effect of small size and interface effect,the obtained optimal GBD process was different for Al films with different particle sizes.Under the optimal process of 550? for 1 h,the 2 ?m and 500 nm Al film magnets obtained the largest coercivity of 926 kA·m-1 and 967 kA·m-1,respectively,enhancing by 26.5%and 32.1%.Meanwhile,the optimal process of 500? for 1 h were determined for the 50 nm Al magnet,and the coercivity,remanence and maximum energy products were 953 kA·m-1,1.41 T and 342 kJ·m-3,respectively,increasing by 30.2%,0.7%and 11.4%.The microstructure and composition research revealed that,after GBD,the morphology RE-rich phase inside the magnets changed from a large agglomerate to a more straight and smooth thin layer,which was beneficial in decreasing the demagnetizing field and enhancing the magnetic isolation effect,and finally contributed to the improvement of coercivity.The investigation for the temperature stability showed that,after Al film diffusion,the absolute values of remanence temperature coefficient a and coercivity temperature coefficient ? decrease to some extent,which indicated that the temperature stability of the magnet was improved.According to the EPD of Dy₂O₃ powders and its GBD experiment,it was found that,under the EPD optimal processes of 90 V/30 s,the Dy₂O₃ film was well adhered to the magnet,and the film was even and uniform.After annealed at 800? for 6 h and tempered at 500? for 1 h,the magnet achieved the best comprehensive magnetic performance.The coercivity,remanence,and maximum energy product were 934 kA·m-1,1.41 T and 349 kJ·m-3,increasing by 27.6%,0.7%and 13.7%,respectively.Microstructure and composition analysis founded that before and after the GBD,the RE-rich phase evolved from the large agglomerate to uniform distributed strip,meanwhile,the "core-shell"structure with a core of Nd2Fe14B and a shell of(Nd,Dy)2Fe14B phase was formed,which jointly contributed to the significant increase in coercivity.For the temperature stability,it was found that the temperature stability of the magnet was effectively improved after GBD.The electrochemical corrosion test demonstrated that after GBD,and the corrosion resistance of the magnet was effectively improved.