Composite Powder Modified Sintered Nd-Fe-B Magnets For Large Coercivity Via Grain Boundary Diffusion

High-coercivity sintered Nd-Fe-B magnets strongly depend on the expensive heavy rare earth（HRE）Dy/Tb elements.However,environmental and economic concerns limit the application of scarce HRE resources.The grain boundary diffusion（GBD）process,as one of the technologies to effectively utilize HRE elements,has been widely studied.Constructing HRE-rich shell with high magnetic anisotropy field（H_A）and Nd₂Fe₁₄B core with high saturation magnetization（M_s）is the guiding direction to regulate the 2:14:1 main phase.However,HRE elements are easy to accumulate on the surface of magnets,which limits their diffusion efficiency.In this study,we designed a new preparation technology of composite diffusion sources and a new GBD technique to improve the diffusion efficiency of HRE elements by synergistic diffusion of various elements.Pr₈₀Al₂₀,Al and Cu powders were co-diffused with HRE powders,respectively.Composite diffusion sources can improve the utilization rate of HRE by flexibly adjusting the proportion of HRE and light/non rare earth,thus realizing the high-quality utilization of HRE.The method reported in this work for preparing high-coercivity sintered Nd-Fe-B magnets by GBD with composite diffusion sources is a feasible and superior method with huge application perspectives.The main results are as follows:（1）Al and Cu with a particle size of 800 nm are co-diffused with Dy with a particle size of400 nm,respectively,which are marked as D-A and D-C.For D-C diffused magnet,the coercivity increases from the original 12.76 k Oe to 20.14 k Oe,which shows an increase of 7.38 k Oe.A thinner Dy-rich shell and uniformly distributed RE-rich GB phase were formed.However,a stable F-rich diffusion layer was formed on the surface of the D-A diffused magnet.The formation of the diffusion layer leads to the accumulation of Dy on the surface of the magnet,which consumes a large amount of Dy and seriously hinders the diffusion of Dy.Therefore,there is only a small increase in coercivity for D-A diffused magnets.Considering the criticality of Dy,the co-diffusion of Cu and Dy F₃is more conducive to improving the utilization rate of Dy and preparing high coercivity sintered Nd-Fe-B magnets.（2）Based on surface coating technology,a novel composite Pr₈₀Al₂₀particle surface-coated with Dy F₃was designed as diffusion source.The ratio of Pr₈₀Al₂₀to Dy F₃powders was optimized,which breaks the limitation of low melting point properties on the alloy composition.We systematically studied the influence of composite diffusion sources(Pr₈₀Al₂₀)_X（Dy F₃）_1-X（X=0,0.1,0.3,0.5,0.7,0.9,1,marked as PA-X）on the microstructure and magnetic properties of magnets.The coercivity of magnets is almost proportional to Pr₈₀Al₂₀content in composite powders.The coercivity increases from the original 12.76 k Oe to 24.39 k Oe and 23.28 k Oe（X=0.7 and 0.9）,which shows an increase of 91.15%and 82.45%,respectively.Huge coercivity increments of11.63 k Oe and 10.52 k Oe reported in this work exceed the maximum values in literature for GBD with Dy-containing sources in sintered Nd-Fe-B magnets.Microstructural features show that the combined strengthening effect of magnetic isolation by the continuous Pr-Al-rich GB phases and magnetic hardening by Dy-Pr-rich shells significantly improve the coercivity of the PA-0.7 and PA-0.9 diffused magnets.Especially,no accumulation of Dy element occurs in the PA-0.9 diffused magnet.The composite diffusion sources provide a new method to improve the diffusion efficiency of Dy.