Exchange Coupling And Magnetic Properties In (Nd,Dy)-Fe-B Sintered Composite Magnets

Nd-Fe-B permanent magnet has been widely used in various fields because of its excellent magnetic properties since it was invented,and it has become one of the most widely used magnetic materials at present.However,the sintered Nd-Fe-B has low coercivity and poor thermal stability,which limits the application range of sintered Nd-Fe-B in the field at high temperature.In order to solve this problem,researchers added heavy rare earth element Dy/Tb,which can effectively improve the coercivity of Nd-Fe-B.But the content of Dy/Tb elements in rare earth resources is only about 1/10 of that of Nd elements,so Dy/Tb is scarce strategic resources.The magnetic moments of Dy/Tb atoms are anti-ferromagnetic coupled with those of Fe atoms,and so the remanence and the maximum magnetic energy product would decrease significantly in magnets.Therefore,use a small amount of Dy/Tb elements to prepare the sintered magnets with high coercive force and high magnetic energy product is the industry’s research difficulties.In this thesis,（Nd,Dy）-Fe-B two-phase composite permanent magnets were prepared by mixing Nd-Fe-B and Dy-Fe-B powders.With the increase of Dy-Fe-B powder addition,the anisotropy field of sintered magnets increases,the orientation of magnets becomes easier in the pressing process,and the orientation degree is improved in the sintered magnets.Meanwhile,the coercivity increases,but the remanence and magnetic energy product decrease in magnets.The microstructure characterization shows that there exists Dy-Fe-B phase in the magnet.Dy atoms on the surface of Nd-Fe-B phase grain form core-shell structure,which is caused by the atomic diffusion during sintering process.As the mixture ratio of Nd-Fe-B and Dy-Fe-B is9.5:0.5,the composite magnets have the best comprehensive magnetic properties,and the coercivity is 17.89 k Oe and magnetic energy product is 49.76 MGOe.Since the uneven distribution of Dy element is regulated in the two-phase composite magnet and the Dy content is lower than that of the traditional magnet,the decrease degree of remanence and magnetic energy product is lower,and the magnets with high coercivity and high magnetic energy product can be prepared in the（Nd,Dy）-Fe-B composite permanent magnets.In this thesis,the reverse magnetization process was studied by means of thermal activation measurement,and the mechanism of coercivity enhancement was explored in the dual-phase composite magnets.The square degree of the reverse magnetization curve is well in（Nd,Dy）-Fe-B composite permanent magnet,indicating that the reverse magnetization process of grains is basically uniform,and the coupling between grains would make the reverse magnetization process tend to be consistent in the magnets.Dy elements distributed on the surface of Nd-Fe-B phase grains form core-shell structure,which could improve the nucleation field of magnetization reversal,and so the coercivity is improved and the domain wall nucleation size is reduced in magnetization reversal.However,thermal activation measurements show that the size of the domain wall does not decrease in magnetization reversal nucleation,indicating that there is coupling effect between grains.The coupling effect between Dy-Fe-B and Nd-Fe-B grains increases the size of the domain wall in magnetization reversal,and thus counterbalances the effect of core-shell structure on the reduction of the domain wall size.Owing the core-shell structure and exchange coupling,the coercivity increases significantly in（Nd,Dy）-Fe-B two-phase composite permanent magnet.Employing grain boundary diffusion could further improve the coercivity of sintered magnets with a small amount of heavy rare earth elements.Via coating Tb F₃for heat treatment diffusion on the surface of the conventional magnet,the coercivity increases by 5 k Oe.The study shows that Tb element diffuses to the surface of the main phase phase grain form core-shell structure which could improve the nucleation field of magnetization reversal,and so the coercivity is improved.However,there are few studies on dual-phase composite（Nd,Dy）-Fe-B magnets in grain boundary diffusion.In this thesis,the grain boundary diffusion of（Nd,Dy）-Fe-B double main phase magnets was studied.Tb F₃was selected as the diffusion source,The coercivity of the dual-phase（Nd,Dy）-Fe-B magnet was further improved by optimizing the sintering process through heat treatment.After diffusion,the coercivity of（Nd,Dy）-Fe-B composite magnet increases by 5 k Oe.The degree of coercivity increase is the same as that of the traditional single-phase Nd-Fe-B grain boundary diffusion magnet.This fact shows that the diffusion and the increase of coercivity in composite magnet with high coercivity and high energy product are consistent with those of single main phase magnet,and the grain boundary diffusion is effective way to improve the coercivity in（Nd,Dy）-Fe-B two-phase composite magnet.