Study On Industrialization Technology Of Grain Boundary Diffusion Technology For Sintered NdFeB Magnets

Sintered NdFeB permanent magnet materials have outunderstanding overall magnetic performance,which are the fastest growing permanent magnet materials in the world.In recent years,the development of low-carbon economy such as hybrid electric vehicles and wind turbines have put forward higher requirements on the magnetic properties of sintered NdFeB magnets.However,the application of sintered NdFeB magnets is limited in these field due to the poor temperature stability of sintered NdFeB magnets,and the operating temperature is usually lower than 150?.In order to improve the thermal stability and coercivity of the sintered NdFeB magnets,the traditional method is adding heavy rare earth elements Dy or Tb into sintered NdFeB magnets.However,excessive addition of Dy or Tb causes a significant decrease in remanence and cost increase.The grain boundary diffusion?GBD?process can not only effectively improve the thermal stability of sintered NdFeB magnets,but also greatly improve the coercivity of magnets with few remanence reducing,while reducing the manufacturing cost and saving the heavy rare earth resources.Therefore,in this study,the spraying method for grain boundary diffusion process,and a high coercivity sintered NdFeB magnets are prepared by using diffusion sources of heavy rare earth element compounds,realizing industrial production.In this study,TbH₃ nanoparticles and TbF₃ microparticles were used as diffusion source to investigate the influence of different process parameters of spraying diffusion on GBD magnets,and obtained the optimal process parameters.Firstly,the influence of gun distance on spraying effect and the bonding force using different adhesive were studied.The results showed that when the gun distance was about 10 cm,the spraying effect was the best.0.02g adhesive C in 1 g diffusion source is the optimal parameters to the bonding force,which had no obvious effect on the performance of the magnet.Secondly,the magnetic properties,microstructure and magnetic hardening mechanism of GBD sintered NdFeB magnets with TbF₃ microparticles as diffusion source were studied.The results show that the increase of coercivity can be reach the requirement when the weight gain ratio is 0.58wt%⁰.85wt%,and the optimal coercivity of the magnet is reaches 23.28 kOe.The optimum heat treatment process is 925?-8 h-500?-3 h.By observing of the microstructure,it is found that the Tb element diffuses into the magnet through the Nd-rich phase,and forms a larger anisotropic?Nd,Tb?₂Fe₁₄B phase around the Nd₂Fe₁₄B main phase,which can improve the coercivity of sintered NdFeB magnets.Likewise,the magnetic properties,microstructure and magnetic hardening mechanism of GBD magnet with TbH₃ nanoparticles as diffusion source were studied.The results showed that the increase of coercivity can be reach the requirement when the weight gain ratio is 0.64 wt.%¹.23 wt.%,and the optimal coercivity of the magnet is 25.44 kOe.The optimum heat treatment process is 925?-8h-500?-3 h.By observing the microstructure,it can be found that due to the high activity of the TbH₃ nanoparticles,the Tb element of the 4 mm thick magnet diffuses to the center,and forms the obvious?Nd,Tb?₂Fe₁₄B shell structure to exceed 200?m.Due to the?Nd,Tb?₂Fe₁₄B shell formed is multiple and deep,its coercivity is greatly improved.In addition,in order to understand the influence of different diffusion sources on the thickness of different magnets,TbH₃ nanoparticles as diffusion source and GBD technique are combined to process NdFeB sintered magnets,and TbF₃ microparticles as diffusion source were prepared for comparision.For the magnets with thickness of 1mm,TbF₃ microparticles and TbH₃ nanoparticles as diffusion sources could enhance coercivity of 9 kOe and 12 kOe,respectively.However,the enhancement of coercivty of TbF₃ microparticles diffused magnets with the thickness of 10 mm is only 1.69 kOe.In contrast,we have noticed,most surprisingly,TbH₃ nanoparticles as diffusion source could heighten the coercivity of 6.9 kOe.To achieve a better understanding of the difference of TbF₃ microparticles and TbH₃ nanoparticles for GBD magnets,EPMA and MOKE of diffused magnets were performed.Note that EPMA shows Tb element is enriched on the surface of Nd₂Fe₁₄B grains,forming the core-shell structure leading to the enhancement of coercivity.More importantly,the diffusion distance of Tb in TbH₃ nanoparticles diffused magnets is much deeper than that of TbF₃ microparticles diffused magnets.It must also be mentioned that H elements would release in GBD process,while F element is still remaining in the magnets and impede the diffusion of Tb.MOKE images at thermal demagnetization and remanent states reveal that TbH₃nanoparticles diffused magnets demonstrate lager domain width d_D and less reversed domains compared to TbF₃ microparticles diffused magnets,indicating that TbH₃nanoparticles could form more core-shell structure and suppress the nucleation of reversed domains,thereby increase the coercivity conspicuously for thick magnets.This combination of TbH₃ nanoparticles and GBD technology presents a wide range of possibilities for the further development of NdFeB sintered magnets with thickness more than 6 mm.When the diffused TbH₃ nanoparticles magnet is polished off 2470?m on one side,the coercivity of the magnet can still reach 16.54kOe,indicating that the apparent core-shell structure is not decisive for the improvement of coercivity.