Magnetic Microstructure Of (Nd,Ce)-Fe-B Sintered Magnets

In the past decade,the development and utilization of high-abundance rare earths have attracted widespread attention.Among them,the use of dual-main-phase（DMP）magnets and grain boundary diffusion process are effective methods to improve the performance of（Nd,Ce）-Fe-B magnets.However,the heterogeneity of the structure and composition of DMP magnets inevitably affect the microstructure and magnetism of the magnets;How does the high abundance element Ce and grain boundary diffusion affect the phase composition of magnets;What are the effects of element substitution with different contents on the microstructure and magnetism of magnets;What changes will the structure caused by grain boundary diffusion affect the coercivity mechanism and temperature demagnetization mechanism in（Nd,Ce）-Fe-B magnets and heterogeneous systems with dual main phases;These scientific issues still require further exploration.The relationship between the microcrystalline structure,magnetic microstructure,and macroscopic magnetic properties of the magnet is closely related.Exploring the relationship between the microstructure and macroscopic magnetism of high abundance rare earth permanent magnets with DMP from the perspective of magnetic domain structure has important theoretical significance and application value.In response to the above scientific questions,this thesis investigated the magnetic microstructure,grain orientation,microstructure,and composition distribution of（Nd,Ce）-Fe-B sintered magnets prepared by different compositions and single and double main phase processes using magnetic force microscopy（MFM）,electron probe microanalyzer（EPMA）,and transmission electron microscopy（TEM）techniques,and deeply analyzed the correlation and influence mechanisms.By in-situ characterization of the magnetic domain structure of（Nd,Ce）-Fe-B sintered magnets in the demagnetized and remanent states at different temperatures,the evolution law of magnetic domain structure is revealed,and the temperature demagnetization mechanism and coercivity mechanism are studied.The main research content and conclusions are as follows:（1）The magnetic domains and microstructure of（Nd,Ce）-Fe-B sintered magnets with different process compositions were investigated.The magnetic microstructure of(Nd_1-xCe_x)₂Fe₁₄B（x=0,0.3,0.8,1）sintered magnets with different Ce contents exhibits typical magnetic domain characteristics of uniaxial anisotropic magnets macroscopically.As the Ce content increases,（Nd,Ce）-Fe-B magnets exhibit more complex fine domain structures.The results show that with the increase of Ce content,the grain boundary area becomes larger,the REFe₂ phase increases,and the magnetic orientation deteriorates.The magnetic domains of DMP（Nd,Ce）-Fe-B sintered magnets exhibit a distribution of coarse and fine domains compared to single-main-phase magnets.By in-situ characterization of magnetic domain structures at different temperatures on remanent state samples of（Nd,Ce）-Fe-B sintered magnets using single-main-phase（SMP）and DMP processes,it was revealed that the nucleation position of reverse domains was mainly concentrated at grain boundaries during the high-temperature demagnetization process.Through in-situ EPMA/MFM analysis,it was found that most of the Ce-rich grains in the DMP（Nd,Ce）-Fe-B sintered magnets exhibit reverse domain nucleation first,and are more prone to further expansion and growth.This is due to the lower anisotropic field and poorer thermal stability of Ce-rich grains compared to Nd-rich grains in DMP magnets,which leads to the tendency of Ce-rich grains to nucleate and expand reverse magnetic domains during thermal demagnetization.（2）The exploration was conducted on the special heterogeneous system of（Nd,Ce）-Fe-B sintered magnets prepared by DMP.We investigated the effect of different Ce contents and different alloy strip components on the magnetic microstructure of DMP（Nd,Ce）-Fe-B sintered magnet alloys.The results indicate that there is no significant difference in the magnetic domain structure of DMP（Nd,Ce）-Fe-B sintered magnets with different Ce contents in the thermally demagnetized state,which is the distribution of finer and coarser domain structures.As the Ce content increases,the average magnetic domain width decreases.When the Ce content is the same,the DMP（Nd,Ce）-Fe-B magnet with different alloy strip components exhibits significant differences in performance due to changes in phase composition and microstructure.There are two different grain boundary regions with different phase compositions at the triangular grain boundaries in the sintered dual main phase（Nd,Ce）-Fe-B magnets,corresponding to different magnetic domain structure patterns.Specifically,it is manifested as non-magnetic rare earth oxides without magnetic domain signals and weakly magnetic phases at room temperature dominated by the aggregation of Ce and Fe elements with small stripe domains.At the same time,there is a large accumulation of stray fields near the Ce-rich grains and large grain boundaries,resulting in more branching and small magnetic domains in this area.The composition distribution and microstructure,which are not conducive to improving the coercivity of the magnet,affect the magnetic properties of the magnet.The difference in coercivity performance of DMP（Nd,Ce）-Fe-B sintered magnets is due to the difference in the effective anisotropy constant K value between the two phases and the different distribution patterns of the anisotropic field caused by the core-shell structure.（3）The study was conducted on the relationship between the micro-magnetic structure,magnetic properties,and phase composition of the Dy grain boundary diffusion process in（Nd,Ce）-Fe-B sintered magnets.A comparative study was conducted on the differences in diffusion mechanisms between SMP and DMP magnets,revealing the in-situ magnetic domain evolution during the temperature demagnetization process of the magnets.The results show that the average magnetic domain width of the diffusion magnet increases in the thermally demagnetized state,while the non-magnetic phase area at the grain boundaries in the magnet significantly decreases.As the concentration of diffusion elements decreases and the diffusion depth increases,the domain width of the grains,and the proportion of single-domain grains also decreases.The inhomogeneous structure and composition of the magnet after diffusion will change the local magnetism inside the magnet,which will be reflected in the magnetic domain structure.There are differences in the diffusion mechanism of SMP and DMP（Nd,Ce）-Fe-B sintered magnets in grain boundary diffusion.Grain boundary diffusion leads to different anisotropic fields and magnetic hardness in the inner and outer layers of the grain,enhancing the exchange coupling effect between the inner and outer layers of the grain.For diffusion dual main phase magnets,Dy elements tend to enter the interior of the Nd-rich main phase along grain boundaries and are relatively less likely to diffuse into the Ce-rich grain main phase.This synergistic effect results in heavy rare earth diffusion sources not accumulating too much on the surface,but tend to diffuse along the grain boundary phase to deeper inside the magnet.Through the evolution process of the in-situ magnetic domain structure of the diffusion DMP magnet during temperature demagnetization,it can be inferred that the reversal domain nucleation sites are located at the edge of the grain.In non-diffused DMP magnets,Ce-rich grains tend to undergo reversal domain nucleation and expansion first,while in diffusion magnets,both Ce-rich and Nd-rich grains may exhibit reversal domain nucleation sites due to the enhanced anisotropy caused by grain boundary diffusion.The new structure brought about by this diffusion affects the demagnetization of DMP（Nd,Ce）-Fe-B magnets,proving that diffusion effectively compensates for the poor anisotropy of Ce-rich grains and temperature stability in DMP（Nd,Ce）-Fe-B magnets.This provides a good theoretical basis and guidance for the optimization of the preparation process of high-performance（Nd,Ce）-Fe-B sintered magnets.