Influence Of Mm Content On Magnetic Properties Of Sintered R-Fe-B Magnets

Due to the higher price and less abundance of Nd,with low-cost and higher abundance of La,Ce partial substitution of Nd in Nd-Fe-B has become a hot topic in the study of rare earth permanent magnet in recent years.MM?Mischmetal?is a mixture of La,Ce,Pr and Nd,and it is the intermediate product of rare earth separation process.Using MM to subsitute partial substitution of Nd in Nd-Fe-B can not only add La,Ce to the magnet but also further reduce the cost of the magnet.This is conducive to the balance and comprehensive utilization of rare earth.In this paper,the MM-Fe-B magnets and MM-Fe-B/Nd-Fe-B multiple-main-phase magnets are prepared and studied,the basic magnetic properties and microstructure of the magnets are analyzed.The main conclusions are as follows:With nominal compositions of MM₁₄Fe_79.9B_6.1(MM=La_28.0Ce_52.0Pr_5.1Nd_14.7Sm_0.20)alloy were prepared by arc-melting in argon atmosphere.The lattice constant and the theoretical density of MM2Fe14 B are a = 8.780 ?,c = 12.185 ? and ? = 7.589 g/cm3,respectively by means of XRD refinement,which are smaller than that of Nd₂Fe₁₄ B.MM₁₄Fe_79.9B_6.1 magnets were prepared by conventional sintering method.The Curie temperature of the sintered MM2Fe14 B magnet is about 210 oC,which is lower than that of Nd₂Fe₁₄ B.When the sintering temperature increased to 1030 oC,the density of the magnet is 7.52 g/cm3.After the first stage tempering at 900 oC,the?BH?max and Hcj had a slight increase.The maximum value of?BH?max = 7.72 MGOe and Hcj = 1.08 kOe was obtained when sintered at 1010 oC and tempering at 900 oC,respectively.The grain size grew very large when the sintering temperature increased to 1050 oC,and the magnetic properties deteriorated rapidly.La enters the main phase grain by EDS analysis,the proportion of La in mischmetal is about 28 at.%,while the proportion of La in rare earths of the main grain is about 20.5 at.%,it reduced by 7.5 at.% in grains.That is mainly because La-Fe-B is very difficult to form the 2: 14: 1 phase.MM₁₄Fe_79.9B_6.1/Nd_13.5Fe_80.5B₆ magnets were fabricated by dual alloy method?MM,mischmetal?.When the sintering temperature is 1030 °C,the density of the magnet is 7.55 g / cm3.When the proportion of mixed rare earth MM?MM/R,R = total rare reath?is equal or greater than 30.3 at.%,magnets have two Curie temperatures Tc1 and Tc2.Curie temperatures Tc1 corresponds to the main phase who contains more LaCe,Curie temperatures Tc2 corresponds to the main phase who contains more Nd.Tc1 decreases from 276.5 oC to 256.6 oC with the content of MM increasing from 30.3 at.% to 50.6 at.%.When the MM/R = 30.3 at.%,the magnetic properties can reach the level of the intrinsic coercivity Hcj = 7.11 kOe and the maximum energy product?BH?max = 41 MGOe.With the increase of MM content,the proportion of La,Ce,Pr and Nd in grains is inhomogeneity,this inhomogeneity can enhance the exchange coupling effect in the grains,and improve the remanence Br.For multiple-main-phase magnet,the coercive force is decided domain wall pinning mechanism.It is found that the Ce2Fe14B/Nd₂Fe₁₄ B magnet with a "core-shell structure" has a higher coercive force than that which have no this structure by micromagnetic simulation.The coercive force does not increase with the thickness of the "shell",because the thickness of the "shell" increases,the inhomogeneity of the magnet will decrease.It will weaken the long-range magnetostatic interaction in the magnet,and resulting in a decrease of coercivity.Different proportions of MM₁₄Fe_79.9B_6.1/N45?approximately mass compositions:?Pr,Nd,Dy?30.5Febal?Al,Cu?0.7B1?multiple-main-phase magnets were prepared by double alloy method.In the case of direct second stage tempering,the coercivity of the magnet decreases slightly as the tempering temperature increases.The magnetic properties of the magnets after first stage tempering and second stage tempering are substantially equivalent to those of the magnets subjected to direct two stage tempering.Therefore,in the preparation of MMFeB and NdFeB multiple-main-phase magnets,the first stage tempering treatment is not required.The best magnetic properties of magnets with 24.1 at.% content of LaCe are Br = 13.22 kGs;Hcj = 9.55 kOe;?BH?max = 40.39 MGOe.