Effects of Sintering Temperature on Microstructure and Magnetic Properties of Nd–Fe–B Magnets

The sintered Nd–Fe–B (neodymium–iron–boron) magnet has been used for many applications in various fields such as acoustics, communications, and automation due to its excellent properties including high remanence, high coercivity, and large energy product. Especially high-coercivity sintered Nd–Fe–B magnets have been extensively applied in the field of permanent magnet motors. In the present work, the effects of sintering temperature on the structural and magnetic properties of a Nd₁₅Fe₇₇B₈-type magnet have been investigated. Sintered permanent magnets were produced from a Nd₁₅Fe₇₇B₈ commercial alloy. The magnetic properties were evaluated using an Automatic Magnet Tester. The magnets were successfully produced at different temperatures. It was seen that the best magnetic properties were obtained for the magnet produced at 1050 ^°C for 1 h. The structural evolution of the magnets has also been examined by means of X-ray diffraction (XRD) and polarized optical microscope. Nd₂Fe₁₄B, Fe₃B and some α-iron phases were observed by X-ray diffraction results.     

Grain boundary wetting in the NdFeB-based hard magnetic alloys

Since the end of 1980s, NdFeB-based hard magnetic alloys have been the materials with the highest available magnetic performance. NdFeB-based magnets are produced either by liquid-phase sintering or by melt spinning. In the present investigation, NdFeB alloys quenched after annealing in the semi-liquid state are used to study the wetting of Nd₂Fe₁₄B grain boundaries by a Nd-rich liquid phase. It is shown that a transition from partial wetting to complete wetting occurs with increasing temperature. The results are compared with the data in the literature for NdFeB-based alloys processed by liquid-phase sintering. The relation between wetting properties and magnetic performance of these alloys is also discussed.     

An Efficient Process for Recycling Nd–Fe–B Sludge as High-Performance Sintered Magnets

Given the increasing concern regarding the global decline in rare earth reserves and the environmental burden from current wet-process recycling techniques, it is urgent to develop an efficient recycling technique for leftover sludge from the manufacturing process of neodymium–iron–boron (Nd–Fe–B) sintered magnets. In the present study, centerless grinding sludge from the Nd–Fe–B sintered magnet machining process was selected as the starting material. The sludge was subjected to a reduction–diffusion (RD) process in order to synthesize recycled neodymium magnet (Nd₂Fe₁₄B) powder; during this process, most of the valuable elements, including neodymium (Nd), praseodymium (Pr), gadolinium (Gd), dysprosium (Dy), holmium (Ho), and cobalt (Co), were recovered simultaneously. Calcium chloride (CaCl₂) powder with a lower melting point was introduced into the RD process to reduce recycling cost and improve recycling efficiency. The mechanism of the reactions was investigated systematically by adjusting the reaction temperature and calcium/sludge weight ratio. It was found that single-phase Nd₂Fe₁₄B particles with good crystallinity were obtained when the calcium weight ratio (calcium/sludge) and reaction temperature were 40 wt% and 1050 °C, respectively. The recovered Nd₂Fe₁₄B particles were blended with 37.7 wt% Nd₄Fe₁₄B powder to fabricate Nd–Fe–B sintered magnets with a remanence of 12.1 kG (1 G = 1 × 10⁻⁴ T), and a coercivity of 14.6 kOe (1 Oe = 79.6 A·m⁻¹), resulting in an energy product of 34.5 MGOe. This recycling route promises a great advantage in recycling efficiency as well as in cost.     

Study of the formation of crystal texture in α-Fe/Nd2Fe14B nanocomposite magnets prepared by controlled melt-spinning

For further improving the magnetic properties of nanocomposite magnets, the study of the formation of crystal textures in the hard magnetic phase is of great significance. In the present study, a strong (00l) crystal texture with the c-axis perpendicular to the ribbon plane was obtained in Nd₂Fe₁₄B nanocrystals in the α-Fe/Nd₂Fe₁₄B nanocomposite magnets prepared by the controlled melt-spinning of NdPrFeCoB. The intensity of the texture weakens from the free surface layer of the ribbon to the layer attached with the wheel. The oriented Nd₂Fe₁₄B crystals have a fine equiaxed characteristic, d = 36 nm, in the layer attached with the wheel and a coarse columnar characteristic, d = 69 nm, in the layer near the free surface. The formation of the crystal texture in the Nd₂Fe₁₄B nanocrystals is attributed to a large temperature gradient normal to the ribbon plane during the melt-spinning process.     

Bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared by the phase transition of amorphous Nd9Fe81Co3Nb1B6 under high pressure

In the present study, the authors succeeded in preparing the bulk α-Fe/Nd₂Fe₁₄B nanocomposite magnets with a nearly full density by the phase transition of amorphous Nd₉Fe₈₁Co₃Nb₁B₆ powders under 1 GPa at a temperature of 750 °C. Compared to the magnets prepared from partly amorphous or nanocrystalline powders, the magnets show quite homogeneously distributed nanocrystals with a small grain size, 28 nm for the α-Fe phase and 35 nm for the Nd₂Fe₁₄B phase, which results in enhanced magnetic properties.     

各向异性Nd2Fe14B／α-Fe纳米晶复合磁体的制备和性能

采用声化学法、放电等离子烧结技术(SPS)和热变形工艺制备致密各向同性和各向异性Nd_2Fe_(14)B/αFe复合磁体,研究了软磁相包覆对磁体的结构和性能的影响.结果表明,软磁相α-Fe对各向同性Nd_2Fe_(14)B/α-Fe复合磁体的影响主要表现为增强两相间的交换耦合作用,从而提高剩磁.当α-Fe体积分数的数值适当(不超过2%)时,各向异性Nd_2Fe_(14)B/α-Fe磁体形成较好的c轴晶体织构,具有较高的磁性能.α-Fe体积分数为1%的磁体性能最高:B_r=1.367 T,H_(ci)=712 kA/m,(BH)_m=327 kJ/m~3.     

Effect of Heat Treatment on Microstructure and Magnetic Properties of Ce-Doped NdFeB Ribbons

The substitution for Nd by abundant element cerium (Ce) is a practical way for the comprehensive utilization of rare earth resources in NdFeB permanent magnets. In this letter, we have prepared the Ce-doped NdFeB ribbons and conventional NdFeB ribbons by melt quenching method and investigated the effects of heat treatment on the crystallization behavior, microstructure, and magnetic properties of the alloy. The results show that: (1) The crystallization behavior and the microstructural changes of the (Nd,Ce)FeB magnets are similar to the conventional NdFeB magnet when heat treatment. In addition, the Ce₂Fe₁₄B phase has a significant effect on the properties of the whole magnets. (2)The NdFeB phase and CeFeB phase are relatively close to each other after being precipitated from the amorphous phase. The coupling effect between the two phases is strong enough to weaken the effect of the addition of Ce and making the properties of the NdFeB magnets to not reduce too much after adding Ce.     

Switching of bistable magnetic states in (NdSmDy)(FeCo)B alloy in the vicinity of a spin-reorientation transition

Bistable magnetic states with two equiprobable orientations of the magnetization vector (corresponding to opposite polarities of a permanent magnet) exist in (NdSmDy)(FeCo)B magnetic alloy in the vicinity of a spin-reorientation transition. A critical value of the magnetic field strength ~1 kOe is determined, at which switching of these bistable magnetic states takes place. It is established that the polarity of polycrystalline sintered magnets of the Nd₂Fe₁₄B family in the vicinity of a spin-reorientation transition can be stabilized by a small external bias magnetic field, which opens up new possibilities for using these magnets in cryomagnetic systems.     

High coercivity (Nd8Y3)–(Fe62Nb3Cr1)–B23 magnets produced by injection casting

High coercivity (Nd₈Y₃)–(Fe₆₂Nb₃Cr₁)–B₂₃ magnets in rods have been produced by copper mold injection casting. Magnetic properties, microstructure, and phase evolution in the as-cast and annealed states have been presented and discussed. The (Nd₈Y₃)–(Fe₆₂Nb₃Cr₁)–B₂₃ alloys show hard magnetic properties in the as-cast state. Annealing induces ideal microstructure containing magnetically hard and soft phases with hard/soft volume ratio of 73/27. Exchange coupling between magnetically hard (Nd,Y)₂Fe₁₄B phase and soft α-Fe (Fe₃B) phase leads to excellent hard magnetic properties. Highest coercivity of 1230 kA/m in (Nd₈Y₃)–(Fe₆₂Nb₃Cr₁)–B₂₃ magnet originates from the large amount of hard phase with high magnetocrystalline anisotropy, formation of thin grain boundary phase and refinement of magnetic grains. Nb and Cr doping modify the magnetic phases. Annealed magnetic rods with a diameter of 2 mm and 36 mm in length demonstrated the maximum magnetic properties i.e., _iH_c of 1230 kA/m, B_r of 0.49 T, and (BH)_max of 45.6 kJ/m³.     

Magnetic Domain Evolution in Sintered Nd–Fe–B Magnet during Magnetization Process

In the present study, the magnetic domains and their evolution during magnetization process have been investigated for sintered Nd–Fe–B permanent magnets with Kerr microscopy. Observation of the magnetic domain evolution process during magnetization process shows that some domain walls were pinned at the grain boundary area under magnetic field up to 5 kOe. It is suggested that magnetic interaction between individual Nd₂Fe₁₄B grains contacting to each other leads to appearance of small closed domains near the grain boundary area, which are responsible for the pinning effect.     

Comparative Magnetic and Structural Properties Study of Micro- and Nanopowders of Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B Doped with Ni

Micropowders of melted and heat-treated Nd₁₆(Fe_76?x Ni _x )B₈ alloys system, with x = 0, 10, 20, and 25 (size distribution under 20 μm), were studied and compared with the study of nanopowders obtained, from the previous ones, by surfactant-assisted ball-milling process during 2 h. By XRD, a majority of Nd₂Fe₁₄B hard phase and a minority of α-Fe, Nd_1.1Fe₄B₄ and NdNi₂ phases were detected. The last one increases with Ni content. The crystallite size of the hard phase, in both types of samples, is not affected by the Ni content; however, the grains in micropowders are oblate, with a mean size of 37 nm, while those of the nanopowders are symmetric, with a mean size of 35 nm. Mössbauer spectra were fitted with seven sextets, which correspond to the six ferromagnetic sites of the hard phase and that of the α-Fe, and a doublet corresponding to the paramagnetic Nd_1.1Fe₄B₄ phase. The mean hyperfine magnetic field, for both types of samples, decreases with Ni content. The hysteresis loops of both types of samples show a hard magnetic character, however, the coercive field and the M _r/M _s values for nanopowders are greater than those obtained for micropowders for all the Ni contents. Values of H _c = 2 kOe and M _r/ M _s = 0.54 were obtained for nanopowders with 10 at.% Ni. From the hysteresis loops, which include the initial magnetization curve, Henkel plots for all the samples were obtained. These plots show that for micropowders, the predominant magnetic interaction is of dipolar type, while for nanopowders, the ferromagnetic exchange is the predominant one, which favored the magnetization.     

Hysteresis Modeling of Nanocrystalline NdFeB Magnets

It is discussed the modeling of hysteresis curves of nanocrystalline exchange coupled NdFeB magnets. The hysteresis was measured with an applied field of 9 T, using a vibrating sample magnetometer with superconducting coil. The demagnetizing curve of the first quadrant of the hysteresis was fitted using the Stoner-Wohlfarth (SW) model with the Callen-Liu-Cullen (CLC) modification. Interaction effects as exchange coupling of Nd₂(FeCo)₁₄B phase with the alpha iron phase were determined. The volume fraction of the soft phase can be accounted with the CLC model. Feasible methods for improving the maximum energy product (BH_max) of rare-earth transition metal magnets with the help of the SW-CLC model data are discussed.     

On the Magnetic and Structural Properties of Neodymium Iron Boron Nanoparticles

Neodymium iron boron nanoparticles were synthesized by means of sol–gel method. Correlation between magnetic properties and structural features were evaluated. The Nd–Fe–B gel was formed in hydrogen atmosphere. The gel was subsequently annealed under vacuum condition to obtain Nd–Fe–B oxide phases. The oxides powders were reduced at different temperatures of 750, 775, 800, 825, 850, and 875 ^°C for 3 h in a mixture of Ar and H₂ atmosphere to prepare Nd₂Fe₁₄B nanoparticles. The role of reduction temperature on phase, morphologies, microstructure, and magnetic properties of the final powders was investigated by employing X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), and vibrating sample magnetometer (VSM), respectively. The results show that Nd₂Fe₁₄B phase was formed successfully at temperatures of 750–875 ^°C. Maximum coercivity of 1757 Oe was obtained at 875 ^°C. The variation of coercivity was described by considering the particle size and magnetocrystalline anisotropy constant.     

Structural and Magnetic Properties of Nanocomposite Nd–Fe–B Prepared by Rapid Thermal Processing

Nanoscale permanent magnetic materials, which possess excellent magnetic and mechanical properties, thermal stability, and corrosion resistance, have become a research hotspot for permanent magnets. In reality, however, the obtained maximum energy product, (BH)_max, is not satisfactory in comparison with the theory limit, especially for exchange-coupled nanocomposite magnets. The construction of an ideal microstructure still remains a challenge in the synthesis and preparation of nanoscale permanent magnets. This work reported the impact of rapid thermal process (RTP) with electron-beam heating on the microstructures of Nd_12.5-xFe_80.8+xB_6.2Nb_0.2Ga_0.3 (x = 0, 2.5) nanocomposites. It was found that the crystallization time was greatly reduced, from 15 min under the conventional annealing conditions to 0.1 s under the RTP. For Nd₂Fe₁₄B single-phase materials, the crystallization temperature of the RTP ribbons decreased by about 248 °C compared with that of the ribbons produced by the conventional annealing method. A synergetic crystallization of the Nd₂Fe₁₄B and α-Fe phases was observed under the RTP, which restrained not only the shape, size distribution, and compositions of the hard and the soft phases, but also the interface between them. This modification effect became more obvious as the fraction of Fe increased. Due to the improvement in the uniformity of the Nd₂Fe₁₄B and α-Fe phases, and their grain size distribution, better magnetic properties were achieved using RTP in comparison with the conventional annealing method.     

Exchange coupled nanocomposite hard magnetic alloys

Abstract

The processing, structures and phase constitutions and the magnetic properties of nanocomposite hard magnetic alloys are reviewed. The emphasis is on rare earth (RE)–iron–boron alloys in which the hard magnetic phase RE₂Fe₁₄B is intermixed with one or more soft magnetic phases. Processing–structure–property relationships are the principal focus, in particular, the role of the hard and soft nanocrystallite dimensions in promoting intergrain ferromagnetic exchange coupling and the consequent enhancement of remanent magnetisation and the technologically important maximum energy density. The powder processing, chill block melt spinning, mechanical alloying and thin film deposition routes to develop nanocrystalline and nanocomposite structures are reviewed. The coercivity mechanism in ultrafine grained alloys and the influence of crystallite dimensions are discussed, as are the effects on intrinsic and extrinsic properties of RE substitutions, replacement of iron by other transition metals and enrichment of the boron content. Exchange enhancements in Sm–Co based nanocomposite bulk alloys and in nanoscale FePt/α-Fe composite thin films are briefly considered, together with thin film materials involving exchange coupling between ferromagnetic and antiferromagnetic phases, in core–shell type structures of transition metal compounds surrounded by oxides and in mechanically alloyed materials. The processing and magnetic properties of bonded magnets based on nanocrystalline/nanocomposite REFeB alloys are discussed. The possibility of producing anisotropic hard/soft composites with properties approaching the theoretical maximum is considered and the extent to which this goal has been realised for fully dense alloys identified.     

Effect of the Ce Content on the Magnetic Properties and Microstructure of CeCo<Subscript>5</Subscript>-based Sintered Bulk Magnets

CeCo₅-based magnets have recently attracted much attention due to their moderate magnetic performance and low cost. Nevertheless, there have been few studies on the effects of Ce content on the magnetic properties and microstructures of CeCo₅-based magnets. In response to this, the magnetic properties of sintered bulk magnets with nominal compositions of Ce(Co_0.73Cu_0.135Fe_0.135) _z (z =?4.95, 5.15, 5.35, and 5.55), prepared by the conventional powder metallurgy method, were investigated here. Based on experimental findings, it was shown that Ce(Co_0.73Cu_0.135Fe_0.135)_5.15 sintered bulk magnets had comprehensive magnetic properties—maximum energy product of 80 kJ m^??3 (10 MGOe) and intrinsic coercivity (H _cj) of 452 kA m^??1 (5.69 kOe)—superior to those previously reported by us. For z =?5.35 and 5.55, due to the presence of the minor Ce₂Co₁₇ phase (which has a Curie temperature (T _c ) <?20 °C), magnets had low H _cj values. Based on x-ray diffraction and scanning electron microscopy observations, it was suggested that the volume fraction of the 1:5 matrix phase was the main factor determining the H _cj of CeCo₅-based sintered bulk magnets obtained with different Ce contents. Furthermore, the importance of the dispersion characteristics of the Ce₂O₃ phase within the matrix was emphasized. Uniform dispersion of the Ce₂O₃ phase can significantly improve the overall magnetic performance of CeCo₅-based magnets.     

Phase relations and hydrogen absorption of neodymium-iron-(boron) alloys

Based on an investigation of the phase correlations in the system Nd-Fe-B, a complete study of the hydrogen absorption of Nd-Fe-B alloys is presented, mainly concerning the permanent magnet material Nd₂Fe₁₄ B. Absorption isotherms of the compound Nd₂Fe₁₄ B and of the master alloy for permanent magnet production, Nd₁₅Fe₇₇B₈, have been recorded and their hydrogenation behaviour is discussed. Thermal desorption spectra support the conclusion that neodymium hydride is the second hydride phase in the hydrogenated master alloy. In the Nd-Fe binary system, Nd₂Fe₁₇ was confirmed as the only equilibrium phase at 870 and 1170 K. The properties of a new ferromagnetic ternary hydride Nd₂Fe₁₇H_x,x = 0 to 5, with the Th₂Zn₁₇ type structure, space group , are reported.     

Hydrogen as a working atmosphere for manufacturing permanent magnets based on rare-earth metals

The phase compositions and magnetic properties of permanents magnets of the systems Sm – Co and Nd – Fe – B are analyzed. Features of the hydrogenation–disproportionation–desorption–recombination (HDDR) process in the Nd₂ Fe₁₄ B intermetallic are considered. Using the Dd – Fe – B system as an example, we assess stages of manufacture of commercial permanent magnets and show the prospect of using hydrogen as a working atmosphere for the manufacture of magnetic powder. It is established that the HDDR of Dd – Fe – B alloys leads to their homogenization, grain refinement to a grain size of 0.2 to 0.5 μm, and an increase in the volume content of the main ferromagnetic phase Dd₂ Fe₁₄ B. By optimizing such a treatment, we managed to increase the magnetic energy (by 10%) and the lift force (by 25 – 27%) of Dd – Fe – B commercial permanent magnets.     

Mechanosynthesis of Nd-Fe-B alloys

A mechanical alloying technique has been applied for Nd-Fe-B alloy synthesis from the mixture of neodymium, iron and Fe-B powders. The direct formation of Nd₂Fe₁₄B phase ( phase) was not observed, but an Nd-Fe multilayer structure was formed during the milling process. Annealing of milled powders at 1023 K for 1 h resulted in magnet formation. The dependence of the magnetic properties on milling time was observed. For the applied milling device and parameters, the optimum milling time proved to be 4 h and the coercive force reached a value of about 1000 kAm^–1.