Study On Structure And Magnetic Properties Of Spark Plasma Sintering-Thermal Deformation Nd-Fe-B Magnet

Nd-Fe-B permanent magnet materials are widely used in biomedical,aerospace,electronic communication and other fields because of their high coercivity and maximum magnetic energy product.With the continuous innovation and change of application fields,the pursuit of highquality Nd-Fe-B permanent magnets has become the current trend of the times.Therefore,how to obtain Nd-Fc-B permanent magnets with higher performance,strong texture and good uniformity is one of the important problems to be solved urgently.In the current industrial production and research,although the traditional sintering method combined with hot pressing/hot deformation process can obtain high-performance magnets,it has long sintering time,high sintering temperature and poor internal uniformity of magnets.The research shows that the shortcomings can be made up by using spark plasma sintering（SPS）and doping and grain boundary diffusion.Therefore,in this paper,the anisotropic Nd-Fe-B bulk magnets with full density and high performance are prepared by technology combined with hot pressing/hot deformation process.The comprehensive magnetic properties and uniformity of permanent magnets are further improved by doping rare earth oxide Dy₂O₃ and strong reducing Ca element by grain boundary diffusion technology.The structural evolution law and the influence mechanism of magnetic properties in the magnets are systematically studied.Firstly,under the conditions of pressure of 40 MPa and holding time of 20 min,the changes of microstructure and magnetic properties or hot-pressed magnets at different temperatures and different parts（upper/middle/lower）were investigated.The results show that the magnetic properties of the same part generally increase first and then decrease with the increase of hot pressing temperature.The comprehensive magnetic properties of the middle part are the best when the hot pressing temperature is 700℃.The distribution of thin Nd-rich phase is more uniform,and the demagnetization exchange coupling effect is stronger.In addition,under the conditions of pressure of 30 MPa and holding time of 10 min,the comprehensive magnetic properties of Nd-Fe-B magnets are the best when the thermal deformation temperature is 800℃,which are B_r=12.2 kGs,H_cj=9.5 kOe and（BH）_max=30.7 MGOe.At this time,the grain is partially refined and the internal texture is enhanced.Secondly,the influence of Dy₂O₃ doping on the structure and magnetic properties of hotpressed/hot-deformed Nd-Fe-B magnets was investigated.In the hot pressing process,the doping amount of Dy₂O₃ is 3 wt.%and the comprehensive magnetic properties of the middle part are the best.Dy produced by partial reduction at grain boundaries enters the main phase and forms（Nd,Dy）2F214B phase,and the anisotropic field inside the main phase is enhanced.With the increase of the doping content of Dy₂O₃,the Curie temperature of hot-pressed magnets first increases and then decreases,while the resistivity continues to increase.At the same time,doping appropriate amount of Dy₂O₃ can improve the uniformity of magnetic properties in different parts.During the hot deformation process,the doping amount of Dy₂O₃ with the best comprehensive magnetic properties is 3 wt.%,which are B_r=12.8 kGs,H_cj=13.3 kOe and（BH）_max=32.4 MGOe.The internal grain shape of the magnet changed from strip-like shape before thermal deformation to ellipse shape,and the grain was refined and the orientation degree was improved.Finally,on the basis of the optimal doping amount of Dy₂O₃,the influence of Ca doping on the structure and magnetic properties of hot-pressed/hot-deformed Nd-Fe-B magnets was investigated.With the increase of Ca doping content,the resistivity of hot-pressed magnets continues to increase,and the Curie temperature first increases and then decreases during the hotpressing process.The volume fraction of nonmagnetic phase at the grain boundary increases,and the lattice parameters of hard magnetic phase decrease.When the Ca doping content is 1 wt.%,the comprehensive magnetic properties of the middle part are the best.During the hot deformation process,the optimum Ca doping amount with comprehensive magnetic properties is 1 wt.%,which are B_r=12.0 kGs,H_cj=16.2 kOe and（BH）_max=33.5 MGOe.The internal grain size decreases,more Dy is replaced and diffused into the main phase,which improves the anisotropic field inside the hot deformed magnet,so the coercivity is improved.