Study On Mechanism And Process Optimization Of Grain Boundary Diffusion For Sintered Nd-Fe-B Magnets

In order to meet the demand for high-performance sintered Nd-Fe-B magnets in new energy fields such as electric vehicles and wind power generation,which have been rapidly developed in recent years,the development of high comprehensive magnetic performance magnets with high remanence and high coercivity has become the current focus of research in Nd-Fe-B permanent magnet materials.Among various methods to improve the comprehensive magnetic properties of sintered Nd-Fe-B magnets,grain boundary diffusion technology is recognized as one of the most promising methods for preparing high-performance sintered NdFeB magnets.However,heavy rare earth elements are the key to the increase of coercivity in the grain boundary diffusion process,and the distribution law inside the magnet is still unclear.In order to further exploit the technical potential of the grain boundary diffusion process and make full use of the ability of heavy rare earth elements to increase the coercivity,this paper systematically studied the physical model of grain boundary diffusion and the distribution law of heavy rare earth elements.And on this basis,the effect of grain size on the distribution of heavy rare earth elements and the magnetic properties of the diffused magnet during grain boundary diffusion are studied.On the other hand,heavy rare earth fluorides have been widely used in industrial production as a diffusion source.However,the influence of the introduction of the F element on the grain boundary diffusion process has not been fully elucidated.Therefore,in this thesis,a series of physicochemical changes during the diffusion process of heavy rare earth fluoride are systematically studied,and the whole diffusion process of heavy rare earth fluoride is clarified.And the diffusion process of heavy rare earth fluorides is optimized according to above conclusion.The main research contents and results are as follows:The physical model of the grain boundary diffusion process of sintered Nd-Fe-B magnets and the distribution function of heavy rare earth elements in the process of grain boundary diffusion are constructed.Combined with the results of microstructure characterization,the grain boundary diffusion process of sintered Nd-Fe-B magnets involves two processes:diffusion of heavy rare earth elements along grain boundaries and from grain boundaries to main phases.The distribution function of heavy rare earth elements in the process of grain boundary diffusion is gived by appropriate approximation of the diffusion model.After the diffusion of the magnets with different grain sizes,it is found that the increase of the coercivity of the magnet decreases with the decrease of the grain size after the grain boundary diffusion.The microstructure analysis shows that the rare earth elements in the magnets with larger grain size have deeper diffusion depth,and the core-shell structure formed on the magnet surface is also more obvious,which is beneficial to the improvement of coercivity.The larger the grain size,the wider and more continuous the grain boundary phase is,which facilitates the atoms exchange between the grain boundary phase and the diffusion source,resulting in an increase in the concentration of heavy rare earth in the grain boundary phase of the magnet surface.According to the fitting results of the distribution function of heavy rare earth elements,the higher initial grain concentration in the grain boundary phase of the magnet surface is the root cause of the difference in the distribution of heavy rare earth and the difference in magnetic properties.In order to overcome the non-uniformity of the distribution of heavy rare earth elements inside the magnet,?PrNd?H_x is introduced as a secondary phase in the diffusion source.The Pr and Nd elements can diffuse into the surface region of the magnet in a small amount,dilute the concentration of heavy rare earth elements in the surface region of the magnet,and weaken the diffusion of heavy rare earth elements into the main phase grains in the surface region of the magnet,so the distribution of heavy rare earth elements inside the magnet is more uniform.The grain boundary diffusion process and microstructural changes of heavy rare earth fluorides were studied.During the diffusion process,the heavy rare earth fluoride reacts with the Nd atom in the magnet to form the NdF₃ phase and the free Dy atom.The NdF₃ phase reacts with the trace oxygen in the sintering furnace to form the NdOF phase and the free F atom.The diffusion of the F atoms into the magnet results in the formation of an F-rich phase and a Cu-rich phase in the grain boundary phase.The F-rich phase causes damage to the microstructure of the magnet surface and migration of the Cu-rich phase into the interior of the magnet,resulting in the absence of Cu on the surface of the magnet.On the other hand,the diffusion of heavy rare earth elements into the magnet causes an increase in the total amount of rare earth on the surface of the magnet.The reduction of the Cu element and the increase in the total amount of the rare earth are favorable for the growth of the main phase,thereby forming an abnormally grown region in the magnet surface layer.The region of abnormal growth is extremely low in coercivity,which causes deterioration of the demagnetization curve and a decrease in remanence.After removing this area,the demagnetization curve and remanence of the magnet are restored.In addition,the high temperature and high vacuum environment in the diffusion process is beneficial to the volatilization of rare earth elements,which causes the waste of heavy rare earth elements and the depletion of the Nd-rich phase on the surface of the base magnet,which is not conducive to the improvement of the coercivity of the magnet.In order to reduce the influence of F element on the microstructure and inhibit the volatilization of heavy rare earth elements in the diffusion process,a new diffusion method,stack diffusion process,was designed.The local closed environment formed by the close packing of the magnets is used to reduce the contact of NdF₃ with O₂ to inhibit the oxidation of NdF3,thereby weakening the diffusion of F element into the magnet and the resultant destruction of the microstructure and the induction of abnormal growth of the magnet surface;on the other hand,it can inhibit the volatilization of heavy rare earth elements,and improve the efficiency of heavy rare earth elements.