Microstructure And Magnetic Properties In Sintered (Nd,Tb)-Fe-B Composite Magnets

Nd-Fe-B rare earth permanent magnets are widely used in automotive motors,wind power generation,medical devices,transportation and other fields because of their high performance and cost-effective.However,the coercivity of sintered Nd-Fe-B magnets produced at present is below from the theoretical value,The application of sintered Nd-Fe-B magnets in high temperature environment is severely restricted by their low practical coercive force and poor temperature stability.Because Tb-Fe-B has ultra-high magnetocrystalline anisotropy,the coercivity of magnets can be greatly improved.Therefore,the sintered（Nd,Tb）-Fe-B composite magnet was prepared by mixing Nd-Fe-B and Tb-Fe-B powers.Via adjusting the composition of Nd-Fe-B for regulating the atomic diffusion,it is espected to further improve the coercivity and other magnetic performances.In traditional sintered Nd-Fe-B magnets,with the decrease of Nd content,both the remanence and maximum magnetic energy product increase gradually,but the coercivity decreases gradually.The sintered composite magnet（Nd,Tb）-Fe-B was prepared by mixing Nd_13.2-xFe_80.8+xB₆（x=0,0.5,1,1.5）with Tb₁₇Fe₇₅B₈ powders at the mass ratio of 9:1,the content of Nd in Nd-Fe-B was adjusted and the content of Tb in Tb-Fe-B was increased properly.With the decrease of Nd content,the remanence and magnetic energy product of the magnet increase gradually,while the coercivity does not decrease,which is about 21.0kOe.Microstructure analysis shows that for decreasing Nd content of Nd-Fe-B,more Tb elements in Tb-Fe-B diffuse into the intergranular phase and the outer-layer Nd-Fe-B main phase.There not only exist Nd₂Fe₁₄B and Tb₂Fe₁₄B double main phases in magnet,but also exist（Nd,Tb）-Fe-B shell layer at the grain boundary of Nd-Fe-B phase.Magnetocrystalline anisotropy of Nd-Fe-B grain outer-layer increases,leading to the more increase of nucleation field of revered domain in magnetization reversal.So the coercivity of the magnet increases significantly in Nd_12.2Fe_81.8B₆/Tb₁₇Fe₇₅B₈ magnet.A high performance permanent magnet with coercivity of 21.0 kOe and maximum magnetic energy product of 39.1 MGOe was obtained in Nd_12.2Fe_81.8B₆/Tb₁₇Fe₇₅B₈ magnet.Sintered（Nd,Tb）-Fe-B magnets were prepared by mixing Nd_12.2Fe_81.8B₆ powders with different mass ratios of Tb₁₇Fe₇₅B₈ powders,in which the content of Tb₁₇Fe₇₅B₈ powder is 0wt.%,1 wt.%,2 wt.%,3 wt.%,5 wt.%,7 wt.%,10 wt.%,15 wt.%,20 wt.%.The coercivity of the magnet increases greatly and the product of remanence and magnetic energy decreases slightly after adding a small amount of Tb₁₇Fe₇₅B₈ powder.The coercivity increases to 15.8kOe as the addition of Tb₁₇Fe₇₅B₈ powder is 5 wt.%,and it is 2.2 times of that of Nd_12.2Fe_81.8B₆ magnet.With the addition of Tb₁₇Fe₇₅B₈ powder more than 3 wt.%,the coercivity increases nearly linearly,and the increase is larger than that of sintered magnets prepared by traditional methods.With the increase of Tb₁₇Fe₇₅B₈ addition,the remanence and magnetic energy product of the magnet decrease gradually.since the saturation magnetization of Tb-Fe-B is low compared to that of Nd-Fe-B.The magnets could be prepared by double-alloy method via mixing Nd-Fe-B with TbH_x powders.The coercivity of magnets can be greatly improved by adding TbH_x powders.However,as the TbH_x powder content exceeds 1 wt.%,the increase of coercivity begins to slow down,which is different from that of the sintered composite magnet of（Nd,Tb）-Fe-B with dual-main-phase prepared by maxing Nd-Fe-B with Tb-Fe-B.For exceeding 3 wt.%of Tb content,the increase trend of the coercivity of the dual-main-phase magnet increases nearly linearly,and the comprehensive magnetic properties of the magnets prepared by the double main phase method are better,In（Nd,Tb）-Fe-B composite magnet with dual-main-phase,Tb diffuses to the grain boundary of Nd-Fe-B to substitute for Nd,and therefor form（Nd,Tb）-Fe-B grain boundary phase with high magnetocrystalline anisotropy.In addition,there exists Tb-Fe-B main phase with ultra-high magnetocrystalline anisotropy.It is generally believed that the magnetization reversal occurs at Nd-Fe-B grain boundary phase,but Tb-Fe-B phase could prevents form the magnetization reversal of Nd-Fe-B phase,leading to the further improvement of the nucleation field and coercivity.So the sintered composite magnets with dual-main-phase magnet can bear higher coercivity and magnetic properties.