Grain Size Dependence of Exchange-Coupling Interaction between Magnetically Soft-Hard Grains and Effective Anisotropy

Owingtotheexchange couplinginteractionbe tweenmagneticalsoftandhardgrains ,nanocrystallinecompositepermanentmaterialscankeepboththehighsaturatemagnetizationofmagneticallysoftphaseandthehighanisotropyofhardphase ,andhenceobtainexcellenthardmagneticproperties .Besides ,contain inglowcontentofrare earthelements ,nanocrystallinecompositepermanentmaterialshavegoodpotentialap plicationinfuture .Skomskietal .[1] pointedoutthatthemagneticenergyproductofthealignednanocrys tallinecompositemagnetcouldbea…     

Coercivity enhancement of Nd2Fe14B/α-Fe nanocomposite magnets through neodymium diffusion under annealing

The coercivity enhancement of ball-milled Nd2Fe14B/α-Fe nanocomposite magnets was investigated. It was found that the coercivity could be enhanced through mixing a small amount of Nd powder with as-milled Fe-rich Nd-Fe-B powders. The annealed samples were investigated by means of X-ray diffraction, scanning electron microscopy and magnetic measurement systems. Under annealing, some of Nd powders promoted the formation of hard magnetic phase Nd2Fe14B. On the other hand, a few of Nd would diffuse into the interface of Nd2Fe14B/α-Fe nanocomposite to compensate for the loss of the interfacial magnetic anisotropy. These two features are all beneficial to the coercivity.     

Influence of Zr Additions on Microstructure and Magnetic Properties of Nanocomposite Nd10.5Fe78-xCo5ZrxB6.5 (x=0～5) Alloys

The influence of Zr addition on the microstructure and magnetic properties of nanocomposite Nd10.5Fe78-xCo5ZrxB6.5 (x=0～5) alloys was investigated. It was found that the intrinsic coercivity could be significantly improved by the addition of 2% (atom fraction) Zr. The presence of small amount of amorphous phase is responsible for the low intrinsic coercivity for Zr-free alloy. The small amount addition of Zr may suppress the growth of grains of α-Fe and Nd2Fe14B phases. The more homogeneous microstructure with an average grain size of 20 nm can be obtained for Nd10.5Fe76Co5Zr2B6.5 alloy.     

Computer Simulation of Effect of Intergrain Exchange Interaction on Magnetic Properties of Nanocomposite Magnets

Nanocompositemagnetsareoneofcandidatesformagnetsinthenextgeneration .Inananocompositemagnet,theexistenceofmagneticallysoftgrainswithahighsaturationmagnetizationenhancestherema nence,andconsequentlyahigh (BH ) max valueisachieved[1] .Asthemagnetizationinthesoftgrainsispreventedfromtheirreversalbytheintergrainex changeinteractionbetweensoftandhardgrains ,theintergrainexchangeinteractionplaysanimportantroleindeterminingmagneticproperties .Therefore ,itscontrolisconsideredtobeakeytechnologyforobt…     

双层复合永磁薄膜磁性能的模拟研究

利用微磁学理论模拟计算了Nd2Fe14B／α-Fe双层复合永磁薄膜的磁滞回线,并对双层膜体系的剩磁、矫顽力与软磁层厚度的关系进行了研究。结果显示,软磁层厚度小于临近尺寸时,磁滞回线为矩形,双层膜完全耦合;软磁层厚度与磁性能的关系表明,随着软磁层厚度的增加,剩磁先增大后减小,而矫顽力单调下降。     

Effect of Cobalt on HDDR Anisotropic NdFeB

The effect of cobalt on the magnetic properties and anisotropy of HDDR anisotropic NdFeB was studied.It is found that Co is helpful for preparing anisotropic NdFeB with high coercivity. The research on the initial microstructure for NdFeB alloy indicates that Co tends to enter the crystal lattice of Nd-rich phase and some of Co atoms also enter the crystal lattice of Nd2Fe14B. The dissolution of Co changes the stability of Nd-rich phase and Nd2Fe14 B in H2 atmosphere and affects the kinetics of HDDR phase transformation. As a result, the NdFeB powder exhibits a high coercive force and a strong anisotropy.     

Grain Growth Behavior in Sintered Nd-Fe-B Magnets

The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyzed.Hence,possible grain growth mechanisms in these magnets were qualitatively discussed.The Nd2Fe14B grain growth proceeded at quite a high rate in the initial 0~1 h of sintering and from then onwards the grain growth rate decreased.A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth.On the basis of experimental results,two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets,that is,dissolution and re-precipitation of Nd2Fe14B particles,and Nd2Fe14B particle growth by coalescence.It was believed that Nd2Fe14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution,but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.     

Characterization of Microstructure and Magnetic Properties of NdFeB Magnet with Dy, Al and Cu Additions

A new alloy of Nd33.5Dy0.99Febal.Al0.52Cu0.1B1.15 (%, mass fraction) was fabricated by powder metallurgy. The effects of Dy, Al and Cu additions on the microstructure and magnetic properties of sintered NdFeB magnet were investigated. The additions of Dy, Al and Cu are effective to refine grains and improve coercivity. Moreover, suitable amounts of Dy, Al and Cu lead to a demagnetization curve with good rectangularity. It is found that the sintered NdFeB magnet has relatively high magnetic performance of Br=12.17kGs, jHc=13.52kOe and (BH)max=34.71MGOe. The sintered NdFeB sample was examined by magnetic force microscope which revealed the domain structures at the surface. It is revealed that the mean Nd2Fe14B grain size is significantly larger than the average scale of the magnetic contrast. An explanation about this is that most Nd2Fe14B grains in sintered NdFeB alloy are dominated with the muhidomain structures when the magnet is in thermally demagnetization state.     

Investigation of Coercivity Mechanism for Nd-Fe-B Magnets Prepared by Combination of Strip-Casting with Hydrogen Decrepitation Techniques

The coercivity mechanism of Nd-Fe-B magnets prepared by combination of strip-casting with hydrogen decrepi-tation techniques was investigated. The microstructure of (Nd0.935Dy0.065)14.5Fe79.4B6.1 magnet was observed. The average grain size is about 6 ～ 12 μm. The magnetizing field dependence of the hardmagnetic properties for the (Nd0.935Dy0.065)14.5 Fe79.4B6.1 and the temperature dependence of the coercivity for the Nd14.5Fe79.4B6.1 were investigated. Results show that the coereivity for magnets prepared by the combination of strip-casting with hydrogen decrepitation techniques is controlled by the nucleation mechanism.     

Investigation on mechanism of magnetization reversal for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element methods

Magnetization configurations were calculated under various magnetic fields for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element method. According to the configurations during demagnetization process, the mechanism of magnetization reversal was analyzed. For the Pr2Fe14B with 10 nm grains or its composite with 10vol.% α-Fe, the coercivity was determined by nucleation of reversed domain that took place at grain boundaries. However, for Pr2Fe14B with 30 nm grains, coercivity was controlled by pinning of the nucleated domain. For Pr2Fe14B/α-Fe with 30vol.% α-Fe, the demagnetization behavior was characterized by continuous reversal of α-Fe moment.     

晶界扩散中Tb对烧结NdFeB磁体微观组织和磁性能的影响

通过晶界扩散技术提升烧结钕铁硼(NdFeB)磁体矫顽力的方法已获得广泛应用,为了研究重稀土磁粉对磁体综合磁性能的影响,本文采用喷涂扩散的方法将重稀土Tb含量为6.0%(质量分数)的磁粉作为复合扩散源的一部分进行晶界扩散并制备了高性能烧结NdFeB磁体。结果表明,当主扩散源占比为60%(质量分数)时,Nd₄₀Tb₆₀对应扩散磁体的矫顽力最高达到21.52 kOe,矫顽力增幅明显。经过微观组织结构和XRD表征分析,重稀土元素Tb沿晶界相扩散进入磁体内部的同时发生了晶格取代反应,可在晶粒表层生成磁晶各向异性场更强的(Nd,Dy/Tb)₂Fe₁₄B硬磁相,显著增强了磁体矫顽力。当主扩散源占比为20%、40%和80%(质量分数)时,Nd₈₀Tb₂₀,Nd₆₀Tb₄₀和Nd₂₀Tb₈₀对应扩散磁体的矫顽力增幅较小,其中Nd₈₀Tb₂₀扩散...     

Effects of Magnetic Annealing on Magnetic Properties in Nd4Fe76Co3(Hf1-xGax)B16 Alloy

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纳米晶Fe_3B/Nd_2Fe_(14)B粘结永磁体的磁性能研究

本文研究了粘结剂含量和温度－时间变化对纳米晶Ｆｅ３Ｂ／Ｎｄ２Ｆｅ１４Ｂ粘结永磁材料的磁性能影响。结果表明降低粘结剂含量可显著地提高纳米晶Ｆｅ３Ｂ／Ｎｄ２Ｆｅ１４Ｂ粘结永磁的磁性能；环境温度对纳米晶Ｆｅ３Ｂ／Ｎｄ２Ｆｅ１４Ｂ粘结永磁的磁性能具有重要影响，在较高的温度下其磁性能将显著下降，因此纳米晶Ｆｅ３Ｂ／Ｎｄ２Ｆｅ１４Ｂ粘结永磁只能在较低的温度环境下使用     

Microstructures and Magnetic Properties of As-Cast RE（Dy）-Fe-C（B） Alloys

RE2 Fe14 Ccompoundswithtetragonalcrystalstructureareformedforalmostallrareearthelements(RE ) .Theirmagneticpropertiesareveryclosetothoseofthecorrespondingborides[1～ 5] .Formostofthelightrare earthelements ,La ,Ce ,ProrNd ,itisdifficulttoproducetheRE2 Fe14 Cphase[5～ 7] .Thereasonisthefairlylowtemperature (Tt)ofthesolidstatephasetransformationinthesecompounds .TheydecomposeeasilyintoRE2 Fe17Cxcompoundsathighertemperatures[8,9] .Highcoercivitiescanbeobtainedthroughacontrolledtransformat…     

Coercivity enhancement of Ce-Fe-B sintered magnets by low-melting point intergranular additive

Ce-Fe-B sintered magnets with enhanced coercivity were prepared by the powder metallurgy method. The mechanism of the coercivity enhancement in Ce-Fe-B sintered magnets with the low-melting point intergranular additive was discussed in details. It was speculated that the low coercivity of Ce-Fe-B sintered magnet was related to the irregular sharps and relatively low magneto-anisotropy field of the matrix phase. After introducing a 20 wt.% Nd-based intergranular additive, the coercivity markedly increased from 108 Oe to 2560 Oe due to the formation of thin and continuous grain boundary layers and the surface modification of the matrix phase grains. Additionally, the formation of the high anisotropy field(Nd,Ce)_2Fe_(14)B shell was beneficial to the increase of the coercivity as well. This work suggested that adding low-melting point intergranular additives was effective to fabricate the practical Ce-Fe-B sintered magnets.     

Influence of gadolinium on microstructure and magnetic properties of sintered NdGdFeB magnets

The effect of Gd content on microstructure and magnetic properties of sintered Nd33.03-xGdxFe65.65B1.32 (x=0-2) was studied in this paper to improve the thermal stability of NdFeB and to reduce the raw material cost. The results showed that better magnet performance could be obtained by adding Gd (0-1.5 wt.%) with partial substitution of Nd in Nd33.03-xGdxFe65.65B1.32. It was also found that the Nd33.03-xGdxFe65.65B1.32 magnets showed the best performance when Gd addition increased to 1.0 wt.%. The temperature coefficient Br (α) could be improved from -0.15%/oC to -0.063%/oC (maximum work temperature 120 oC) and the Curie temperature could be improved from 315 oC to 323 oC because the Gd2Fe14B had positive temperature coefficient Br (α) and higher Curie temperature than that of Nd2Fe14B. The coercivity could be improved from 10.2 to 11.48 kOe and the microstructure was close to ideal microstructure. The magnetic performance decreased sharply by adding Gd (above 2 wt.%) with partial substitution of Nd in Nd33.03-xGdxFe65.65B1.32 because the Gd element concen-trated in the grain boundaries.     

High coercivity Nd-Ce-Fe-B nanostructured ribbons prepared from melt spinning technique

The Ce-substituted(Nd_(1-x)Ce_x)_(12.2) Fe_(81.6) B_(6.2)(x=0.0, 0.2, 0.4, 0.6) nanocrystalline ribbons were prepared by annealing amorphous ribbons from melt spinning. It is found that all ribbons are in a multiphase state consisting of a-Fe phase, Nd(Ce)-rich phases and RE_2 Fe_(14) B(RE = Nd, Ce) phases. However, the coercivity of all annealed ribbons can reach a considerably high value without doping any heavy rare earth or other coercivity enhanced elements. A strong intergranular exchange coupling appears in these nanocrystalline ribbons. The Nd_(12.2) Fe_(81.6) B_(6.2) ribbons with multiphase have a coercivity of about 11.3 k Oe, and the coercivity decreases slightly with increasing Ce content. A coercivity of 7.5 kOe can be obtained when60 at% of Nd is replaced by Ce(x = 0.6) due to the grain refinement and the strong intergranular exchange coupling. This provides a practical approach of fabricating high coercivity Ce-substituted Nd-Fe-B materials.     

Effect of high pressure on microstructure of crystallizing amorphous Nd9Fe85B6 alloy

The effect of high pressure on the microstructure of annealed amorphous Nd9Fe85B6 alloy was studied. It was found that application of high pressure made the microstructure of the crystallized alloy much more homogeneous. The average grain size of the Nd2Fe14B phase decreased with the increase of pressure, whereas, the size of the α-Fe first increased when a pressure of 1 GPa was applied and then decreased with further increase of pressure. Pressure-induced （410） texture of the Nd2Fe14B phase was also observed. The present study suggested an effective route for controlling the microstructure in a nanoscale solid.     

Texture of Nanocomposite Nd2Fe14B/α-Fe Melt Spun Ribbons

Highly textured nanocomposite Nd9Fe84.5Nb0.5B6 ribbons were obtained by rapid solidification at wheel speeds from 15 to 20 m·s-1.X-ray diffraction(XRD)shows that these ribbons are preferentially orientated with the c axis of Nd2Fe14B phase perpendicular to the surface of the ribbons.The texture of Nd2Fe14B phase on the free surface is stronger than that on the contact surface and decreases with increase of wheel speed.The texture of Nd2Fe14B phase seems to be caused by the preferential growth of the crystal nuclei favorable orientation with their c-axes along the temperature gradient during rapid solidification,which can be explained by a growth circular cone(GCC).The intensity of the anisotropy can be expressed by the magnitude of the cone generating angle.The supercooling velocity and wheel speed will influence the cone generating angle,which in turn influences the anisotropy.The remanence and maximum energy product in the direction perpendicular to the ribbon surface is much higher than that in the direction parallel with the ribbon surface.High remanence and maximum energy product can be obtained in the direction perpendicular to the ribbon surface.The texture of Nd2Fe14B phase decreases gradually after annealing above 600 ℃,which is mainly attributed to the precipitation of new α-Fe from Nd2Fe14B phase with super-saturated Fe.Decreasing the annealing temperature or reducing the annealing time is beneficial for preserving the anisotropy of Nd2Fe14B phase after annealing.     

Improvement in magnetic properties,corrosion resistance and microstructure of Nd–Fe–B sintered magnets through intergranular addition of Tb68Ni32

New energy vehicles and offshore wind power industries have a high demand for sintered Nd–Fe–B magnets with high intrinsic coercivity and high corrosion resistance. In this study, the magnetic properties, anticorrosion properties, and microstructure of Nd–Fe–B sintered magnets with the intergranular addition of low-melting-point eutectic Tb₆₈Ni₃₂ alloy powders were investigated. The aim is to determine if the addition of Tb₆₈Ni₃₂ can improve these properties. A low melting-point eutectic alloy Tb₆₈Ni₃₂ powders was prepared as a grain boundary additive and blended with the master alloy powders prior to sintering. The coercivity of the resultant magnets gradually increases from 1468 to 2151 kA/m by adding increasing amounts of Tb₆₈Ni₃₂. At the same time, the remanence first increases and then slightly decreases. After studying the microstructure and elemental composition of the Tb₆₈Ni₃₂ added magnets, it is found that the significant increase in coercivity and the negligible reduction in remanence is due to densification, improved grain orientation, a uniform and continuous boundary phase distribution, as well as the generation of a (Nd,Pr,Tb)₂Fe₁₄B “core–shell” structure surrounding the main-phase grain. Moreover, the corrosion resistance of the magnet is greatly improved owing to the enhancement of electrochemical stability, as well as the optimization of the distribution and morphology of the intergranular phase.