Research On High-coercivity Rare Earth Permanent Magnetic Materials With Low-cost

Rare earth permanent magnet materials have become one of the indispensable materials in modern defense industry and national economy due to their excellent comprehensive magnetic properties.The explosive growth of industrial fields such as artificial intelligence,5G communication,wind power generation,aerospace and new energy vehicles has increased the demand for sintered Nd-Fe-B magnets,while putting higher requirements on their performance.The addition of Dy and Tb heavy rare earths is the most effective way to improve the coercivity of sintered Nd-Fe-B magnets,but the heavy rare earth elements Dy and Tb are increasingly scarce and expensive,and the heavy rare earth elements have antiferromagnetic exchange coupling interaction with Fe,which will reduce the remanence and magnetic energy product of the magnets to some extent.Therefore,without the addition of Dy and Tb heavy rare earths,the study of high coercivity magnets and further enhancement of magnetic properties using grain boundary diffusion technology can further strengthen the theoretical foundation for high coercivity and high remanence sintered Nd-Fe-B permanent magnetic materials.The research work in this thesis is centered on high coercivity and low cost of sintered Nd-Fe-B permanent magnet materials,preparing sintered Nd-Fe-B magnets without the use of Dy and Tb elements to further reduce manufacturing costs,systematically studying and analyzing the coercivity growth mechanism;optimizing the preparation process by refining grain and grain boundary regulation techniques to improve the microstructure and grain boundary of Nd-Fe-B magnets.The magnetron sputtering deposition technique is used to deposit Tb film,Nd-Cu film and Tb-Nd-Cu composite film on the magnet surface,and the grain boundary diffusion technique is used to regulate the grain boundary structure of Nd-Fe-B magnets,which significantly improves the microstructure and magnetic properties of magnets and further increases the coercivity of magnets to achieve the extreme utilization of heavy rare earths.In this paper,we studied the magnet preparation process and analyzed the microscopic morphological structures of the magnet and the quick-set flakes at different copper roll speeds,and found that when the copper roll speed was 42 r/min,the columnar crystals of the quick-set flakes grew well and the morphological structures were uniform and slender without abnormal growth or uneven growth.At the same time,high coercivity sintered Nd-Fe-B magnets with good overall magnetic properties were prepared by using grain refinement and grain boundary control techniques at a sintering temperature of1060 ℃ and a primary and secondary tempering temperature of 900 ℃+500 ℃,with magnetic energy of Br=13.46 k Gs and Hc J=19.86 k Oe;and by co-adding Ga,Cu,and Zr and tempering process control.The remanent magnetization temperature coefficient is-0.1132 %/°C to-0.1263 %/°C and the coercivity temperature coefficient is-0.5127 %/°C to-0.7829 %/°C in the temperature range of 40-150 °C.When the HAST test time is 120 h,the weight loss of the magnet is only 0.519 mg/cm2.After first and second tempering,the microstructure of the magnets improved significantly,and the volume fraction of Nd-rich phase increased significantly from the sintered state to the second tempered state,and a thin and continuous Nd-rich grain boundary phase was formed under the joint action of gallium(Ga)and copper(Cu).In order to further improve the coercivity of heavy rare-earth-free sintered Nd-Fe-B magnets,Tb single film,Nd-Cu single film and Tb-Nd-Cu composite film were diffused by magnetron sputtering deposition technique,and the magnetic properties and microscopic morphology of the magnets were observed before and after grain boundary diffusion to investigate the effects of different diffusion processes and diffusion sources on the magnetic properties.The experimental results showed that the diffusion coercive force of Tb increased by 7.74 k Oe,or 44.13 %,at the optimum diffusion temperature of 900 ℃,tempering temperature of 500 ℃,and diffusion time of 6 h.The diffusion coercive force of Nd-Cu alloy increased by 0.91 k Oe,or 5.2 %,at the optimum diffusion temperature of880 ℃,tempering temperature of 500 ℃,and diffusion time of 6 h.The coercivity of the Tb-Nd-Cu composite film increased by 6.93 k Oe or 39.5 % under the optimal diffusion conditions for the bilayer-deposited diffusion magnets.In addition,comparing the microstructure of the magnets before and after diffusion,it is found that the coercivity growth mechanism differs under different diffusion methods.The diffusion coercivity increase of the composite film is less than that of the single film,and it enters the main phase grains along the grain boundaries to replace Nd,forming a Tb-rich shell nucleus structure,which is an important reference value for further study of the grain boundary diffusion coercivity growth mechanism.