High Performance Permanent Magnets Based On Cheap And Abundant Rare Earths

Nd-Fe-B sintered magnet has been widely used in various fields due to its outstanding magnetic properties.The increasing demands of Nd-Fe-B magnets have consumed amounts of the rare earth especially those closely-relied Pr,Nd,Tb and Dy,which made an unbalanced ultilization of natural resources.However,those abundant rare earth elements of Ce and La with much lower price are overstocked and are rarely used in permanent magnets.Thus,fabricating high-performance magnets with much lower material cost by reducing the usage of heavy rare earth elements of Tb and Dy and making using of the abundant rare earth elements Ce and La has become the subject.The strong magnetic properties of Nd-Fe-B magnets originate from the Nd2Fe14B tetragonal phase.On one hand,to meet the requirement of high temperature conditions,the magnets should have high room temperatures coercivity.The most common way to enhance the coercivity of Nd-Fe-B sintered magnets is to substitute Nd by heavy rare earth elements of Tb and Dy in the 2:14:1 lattice.High coercivity can be obtained due to the increased magnetic anisotropy,but the remanence and maximum energy product will be reduced due to the antiferromagnetic coupling of Tb/Dy atoms with Fe.On the other,Ce/La₂Fe₁₄B has a much lower intrinsic magnetic properties to that of Nd/Pr2Fe14B,which will certainly induce a magnetic dilution effect by incorporating Ce/La in the magnets.Therefore,fabricating the magnets high coercivity by reducing Tb/Dy and suppressing the magnetic dilution effects caused by Ce/La pose us a big challenge.To deal with the main concerns mentioned above,our work of high performance permanent magnets with cheap and abundant rare earth were carried on.One is to design a Dy-containing grain boundary phase to improve the local-magnetic anisotropy in outer regions of the main phase gains by the grain boundary diffusion of Dy during the sintering,thus contributing a high coercivity with low Dy content.The other is to construct a multi-main-phase?MMP?structure by the inhomogeneous substitution of Ce for Nd in the magnets,thus obtaining a high-performance and high-Ce-content magnets with suppressed the magnetic dilution effects by the magnetic interactions among the chemically-different regions.The main results of the work are as follows:(Pr₃₇Dy₃₀Cu₃₃)-H_x hydride powders were prepared and incorporated into Nd-Fe-B sintered magnets as the intergranular additive by the grain boundary restructuring process.The formed Nd2Fe14B/?Nd,Dy?2Fe14B core-shell structure effectively improves the coercivity and minimize the remanence loss simultaneously.Forming Nd2Fe14B/?Nd,Dy?2Fe14B core-shell structure by intergranular adding Dy-containing sources into Nd-Fe-B sintered magnets is effective to improve coercivity and to minimize remanence loss simultaneously.However,the excessive Dy located in the intergranular regions has nearly no hard magnetic contribution,causing its low utilization efficiency.In this work,diluted Dy powders(Pr₃₇Dy₃₀Cu₃₃)-H_x were prepared and incorporated into Nd-Fe-B sintered magnets via a dual-alloy approach.The coercivity increases rapidly from 15.0 to 18.2 kOe by 21.3%with 2.0 wt%?Pr,Dy,Cu?-H_x addition?the equivalent Dy is only 0.32 at%?.The deduced coercivity incremental ratio is 10.0 kOe per unit Dy at%.Dehydrogenation reaction of?Pr,Dy,Cu?-H_x occurs during sintering,which favors Dy diffusion towards the 2:14:1 phase grains as well as smoothing the grain boundaries?GBs?.The enhanced local anisotropic field and the well decoupled 2:14:1 phase grains contribute to such rapid coercivity enhancement.This work suggests that adding diluted Dy hydrides is promising for fabricating high coercivity Nd-Fe-B sintered magnets with less heavy rare-earth consumption.Based on the knowledge of the magnetic properties for different Ce-substituted MMP magnets,high performance and high Ce content sintered magnets were successfully fabricated.Low Ce-content MMP magnet shows nonlinear decreasing of the magnetic properties,and 18 wt.%Ce substituted magnet has a slow decreasing of coercivity.Moreover,the squareness factor of Hk/Hcj is recovered of Ce-18 magnets.Increasing the Ce content can effectively reduce the melting point of the grain boundary RE-rich phase,promoting the wettability between the main phase grains and liquid RE-rich phase during high temperature.Thus,the optimized microstructure can suppress the interface defects and reduce the possible sites of nucleation and local stray field,which helps the uniform demagnetization process.In the high Ce-substituted?Ce-45?MMP magnet,Ce tends to concentrate in the regions of grain surface and the interspace between the grains by the high temperature annealing.The coercivity shows first increasing at the low temperature and then decreasing even below the as-sintered one at a relatively high temperature.The complex coercivity behavior of Ce-45 magnets can be attributed to the competition between the optimized microstructure and reduced HA on the grain surface.By the MMP approach,we obtained the sintered magnet with magnetic properties of Hcj= 9.0 kOe,Br= 12.4 kG,?BH?max=36.7 MGOe even when 45 wt.%Ce substituting for Nd.The relations between magnetic properties and microstructure evolutions have been fully understood.The magnetic interactions among the chemical different regions in the MMP magnets effectively suppress the magnetic dilution effects by incorporating Ce,which guarantee the superior magnetic performance of the MMP magnets to the SMP?single main phase?ones.Besides,the microstructure also plays an important role on the final magnetic performance of the MMP magnet.REFe₂ phase that forms in the high Ce-containing Nd-Ce-Fe-B magnets has been considered to be harmful to magnetic performance due to its soft magnetism.In this work,we found that REFe₂ phase with lower melting point than the 2:14:1 phase plays positive role on optimizing the microstructure and retaining magnetic performance of the Nd-Ce-Fe-B BMP magnets.The wettability of 2:14:1 phase can be improved by sintering above the melting point of REFe₂ phase,which promotes densification of the magnet and the formation of continuous and smooth grain boundary phases.This contributes to the weakened short-range exchange coupling between adjacent grains,hence ensures superior magnetic performance of BMP magnets to the single main phase?SMP?ones with the same average composition.