Coercivity And Service Performance Improvement Of Nd-Fe-B Permanent Magnets By Grain Boundary Diffusion And Underlying Mechanisms

Grain boundary diffusion（GBD）process is one of the significant progresses in the field of Nd-Fe-B permanent magnets in the 21st century.GBD diffuses the diffusion source into Nd-Fe-B magnets along the grain boundary（GB）.By improving the intrinsic magnetic property or optimizing the GB structure,it can reduce the content of heavy rare earth（HRE）in the magnets and simultaneously improve the coercivity.With the increasing demand for high-performance magnets in renewable energy and other fields,further reducing the cost and improving the efficiency of GBD is required.In addition,the renewable energy applications require the Nd-Fe-B magnets to not only exhibit high coercivity,but also have enough service performance,such as high electrical resistivity and good corrosion resistance.However,at present,the investigation on service performance enhancement of Nd-Fe-B magnets by GBD is rare.Furthermore,in the current production of high-performance Nd-Fe-B magnets,the GBD process is added to the existing sintering process as an additional step,which increases the process cost to a certain extent.Therefore,simplifying the fabrication process of GBD treated magnets is also an effective way to improve the performance-cost ratio of the magnets.This work proposes several approaches to improve the magnetic properties and other service properties of Nd-Fe-B magnets by GBD process.The substitution of light rare earth elements（LRE:Pr,Ce and La）and non-rare earth elements（non-RE:Al,Ni,Cu and Cr）are used to reduce the content of HRE in the diffusion source and improve the diffusion efficiency of HRE.The GB regulation is developed to enhance the resistivity of the magnet in order to reduce the eddy current heating under an alternating electric field.Al,Ni and Cr are used in diffusion source to improve the corrosion resistance of magnets,and the GBD and surface protection processes are combined to reduce the processing cost of the magnets.The related physical mechanism in detail in this thesis discusses.The main research contents are as follows:A Pr-containing Pr-Tb-Al-Cu alloy diffusion source is developed.Pr and Al were added into the Tb-Cu alloy to reduce the melting point of the Tb-Cu diffusion source and improve the diffusion efficiency of Tb.It is found that the coercivity and resistivity of sintered magnets can be simultaneously improved.By the diffusion of Pr₃₅Tb₃₅Cu₁₀Al₂₀,the coercivity and resistivity of the magnet are improved from 1101 k A/m and 2.13×10^-6Ω·m to 1917 k A/m and 2.60×10^-6Ω·m,respectively.The formation of Tb-rich shell with high anisotropic field is the main reason for the coercivity enhancement.The formation of continuous GB layer is beneficial to isolating the hard magnetic grains for magnetic decoupling.In addition,both rare earth（RE）and non-RE elements in the diffusion source play important roles in coercivity enhancement.Combined with the experimental and simulation results,Pr has a stronger tendency of than Tb in GB segregation,leading to the formation of a large number of RE-rich oxides at GBs during the diffusion of Pr₃₅Tb₃₅Cu₁₀Al₂₀.The isolation of main phase grains by RE rich oxides with high resistance is the main reason for the increase of resistivity.Enhancing the resistivity of rotor magnets is beneficial to reducing the eddy current density of the magnets under an alternating electric field,and thus reduce the temperature increase of the magnet.This has similar effect as improving the high-temperature stability of magnetic properties.By modifying the RE composition in Tb-Cu diffusion source,a mixed RE diffusion source of Ce-La-Tb-Cu alloy with high abundance REs of Ce and La is proposed to further improve the performance/cost ratio of magnets.It is found that the sufficient of Tb can be also obtained by modifying the ratio of Ce and La.The optimized magnet exhibits a coercivity of 1822 k A/m and coercivity temperature coefficient of-0.47%/K for by the diffusion of(Ce₂₀La₈₀)_17.5Tb_52.5Cu₃₀.Compared with the Tb₇₀Cu₃₀ diffused magnet,the(Ce₂₀La₈₀)_17.5Tb_52.5Cu₃₀ diffused one has a similar coercivity enhancement,and an even smaller absolute value of coercivity coefficient.Meanwhile,the material cost of diffusion source is reduced by 23%.The results from electron probe microanalysis and first-principles calculation confirm that the addition of Ce and La can reduce the melting point of the Tb₇₀Cu₃₀ diffusion source and thus increases the diffusion depth of Tb,resulting in uniform distribution of Tb in2:14:1 grains.In comparison,Ce can promote the GBD of Tb,while La can inhibit the formation of Ce-rich grains.The proposed relationship between properties and microstructure can be confirmed by micromagnetic simulation.By modifying the non-RE composition in diffusion source,Pr-Tb-Cu-Al and Pr-Tb-Cu-Ni GBD sources containing Al and Ni are developed.Pr-Tb-Cu-（Al/Ni）diffusion sources exhibit high diffusion efficiency and good oxidation resistance,which can effectively improve the coercivity and chemical stability of magnet.The Pr₃₅Tb₃₅Cu₁₀Al₂₀ and Pr₃₅Tb₃₅Cu₁₀Ni₂₀alloys still perform greatly in coercivity enhancement after an oxidation at 200℃for 8 h because the passivation of Al and Ni inhibits the oxidation of RE elements in the diffusion source,resulting in a deep diffusion of RE elements.After the diffusion of Pr₃₅Tb₃₅Cu₃₀,Pr₃₅Tb₃₅Cu₁₀Al₂₀ and Pr₃₅Tb₃₅Cu₁₀Ni₂₀ alloys,the coercivity of the magnet increases from 1034k A/m to 1603,1838 and 1670 k A/m,respectively.The added Al performs better than Ni in coercivity enhancement mainly because Al can form eutectic alloy with Cu and further lower the melting point of diffusion source.In addition,Al can effectively“dilute”the ferromagnetism of GB phase,which is beneficial to the magnetically decoupling.The corrosion resistance of the Pr₃₅Tb₃₅Cu₁₀（Al/Ni）₂₀ diffused magnets is distinctly enhanced,and the corrosion current density decreases from 14.92μA/cm² to 7.58μA/cm² and 6.21μA/cm²,respectively.The main reason is that the addition of Al and Ni can increase the corrosion potential of GB phase and reduce the potential difference between the main phase and GB phase,which slows down the galvanic corrosion of the magnet.Based on the fact that non-RE elements can increase the corrosion resistance of magnets,a non-RE alloy diffusion process is developed,which can slightly improve the coercivity of magnets and realize the process combination of GBD and surface protection.Al-Cr alloy films are used as diffusion source and protection coating.After the diffusion heat treatment,the two key processing steps of high-performance Nd-Fe-B magnets are combined into one,which shortens the process step and saves the process cost.It is found that,during the diffusion heat treatment,Al atoms in the Al-Cr coating diffuse into the magnet through the GB,which improves the wettability between the intergranular phase and the main phase.This is beneficial to the formation of the thin GB layers for magnetic decoupling,and thus improves the coercivity of the magnet.By diffusion of Al₈₀Cr₂₀ and Al₇₀Cr₃₀ alloys,the coercivity is increased by 95k A/m and 89 k A/m,respectively.Cr plays an important role in improving the hardness of the coating and modifying the diffusion behavior of Al,in order to obtain a dense surface of coating after the annealing.After the diffusion heat treatment,the Al₇₀Cr₃₀ coating exhibits high hardness of 784±10 HV,high scratch resistance,and good corrosion resistance.The results of this work provided theoretical and experimental basis for further understanding the GBD behavior of various elements,developing efficient GBD sources,improving the service performance of GBD treated magnets,and further improving the performance-cost ratio of magnets.