Research On The Preparation,Microstructure,and Magnetic Properties For Nd(Ce)-Fe-B Alloy

Rare earth permanent magnetic materials are widely used in various fields because of their excellent magnetic properties,but the consumption of rare earth is mainly concentrated in critical rare earth elements.As the highest abundance of rare earth elements,Cerium was used as a by-product of Nd and Pr chronically,which has not been effectively used in the permanent magnetic materials.The research of the application of high-abundance rare earth in permanent magnetic material has become the hotspot.In this dissertation,the effect of Ce doping on the magnetic properties of Nd-Ce-Fe-B-based permanent magnet materials is studied.The magnetic properties and microstructure of the spark plasma sintered magnets are investigated in details.The grain boundary diffusion process is used to improve microstructure and magnetic properties of sintered magnets.In this paper,the(Nd_1-xCe_x)₂Fe₁₄B ribbons with excellent magnetic properties were prepared by melt-spining technique.The melt-spining processes and heat treatment processes were optimized.It was found that under the optimum preparation conditions,the coercivity decreases as a whole,and the decrease of coercivity is attributed to the lower magnetocrystalline anisotropy field of Ce₂Fe₁₄B than that of Nd₂F₁₄B phase.However,the coercivity of 594 kA/m can be obtained when x = 0.2,which shows an anomalous increase phenomenon.It is attributed to the phase separation caused by the Ce mixed valence state.When x = 0.25,the maxmum magnetic energy product of NdCeFeB alloy can reach 95kJ/m³,which is only 3.1% lower than that of pure Nd2Fe14 B.Subsequently,the ribbons with a nominal composition of(Nd_1-xCe_x)_13.5Fe₈₁B_5.5 were prepared.It was found that the addition of total rare earth content did not completely suppress the precipitation of the ?-Fe phase.When x = 0.2,the coercivity of the ribbons also exhibits an abnormally increasing phenomenon,and the coercivity is up to 937 kA/m,which is only 0.43% lower than that of the Ce-free sample.The Henkel plots shows that the intergranular exchange coupling is the strongest when x = 0.2.It is indicated that high abundance rare earth Ce has a great potential in rare earth permanent magnetic materials.Based on Nd_13.7Co_6.7Ga_0.5Fe_73.5B_5.6 melt-spining powder,the nanocrystalline sintered magnets were prepared by SPS technology.It is shown that SPS magnets consist of grains with different aspect ratios and different crystal growth directions.The anisotropic nanocrystalline hot deformed magnets were successfully prepared by hot deformation technique.The remanence increases from 0.76 T to 1.34 T,however,the coercivity decreases obviously from 1636 kA/m to 226 kA/m.By analyzing recoil loops,a significant opening phenomenon is found.TEM results show that the accumulation of Nd-rich phase in the hot deformed magnet may be the one of the main reasons of the decreased coercivity.And it also causes the recoil loops to exhibit a large degree of open state.Finally,the sintered magnets with nominal composition of Ce_13.5Fe₈₁B_5.5 were prepared by spark plasma sintering.It was found that the addition of Pr₆₈Cu₃₂ to the precursor powder could effectively improve magnetic properties of magnets,and the coercivity increases from 68 kA/m to 357 kA/m.Then,the coercivity of the sintered magnets is further increased by the grain boundary diffusion process.The maxmum coercivity value of 558 kA/m can be obtained by the grain boundary diffusion of Pr₆₈Cu₃₂ alloy.The microstructural analysis shows that the increase of coercivity is attributed to the diffusion of PrCu into Ce₂Fe₁₄B phase and the increased grain boundary phase thickness.