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.     

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.     

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.     

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.     

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.     

A study of Nd2Fe14B and a neodymium-rich phase in sintered NdFeB magnets

The microstructure and properties of NdFeB sintered permanent magnets were analysed by different methods. Samples analysed were sintered and thermally treated. The hard magnetic Nd₂Fe₁₄B phase and amorphous neodymium-rich phase were observed by TEM. The neodymium-rich phase contained iron and boron, in elemental and in B₂O₃ form, which is known as a glass former. At the sintering temperature, Nd₂Fe₁₄B and the neodymium-rich phase are supersaturated with iron, which should be dissolved at the annealing temperature to react with neodymium and boron and form additional Nd₂Fe₁₄B phase. Iron precipitates of size up to 2 nm were detected in the Nd₂Fe₁₄B phase. These superparamagnetic precipitates of -Fe could affect the hard magnetic properties of NdFeB magnets.     

Molecule-based magnets

The conventional magnetic materials used in current technology, such as, Fe, Fe₂O₃, Cr₂O₃, SmCo₅, Nd₂Fe₁₄B etc are all atom-based, and their preparation/processing require high temperature routes. Employing self-assembly methods, it is possible to engineer a bulk molecular material with long-range magnetic order, mainly because one can play with the weak intermolecular interactions. Since the first successful synthesis of molecular magnets in 1986, a large variety of them have been synthesized, which can be categorized on the basis of the chemical nature of the magnetic units involved: organic-, metal-based systems, heterobimetallic assemblies, or mixed organic-inorganic systems. The design of molecule-based magnets has also been extended to the design of poly-functional molecular magnets, such as those exhibiting second-order optical nonlinearity, liquid crystallinity, or chirality simultaneously with long-range magnetic order. Solubility, low density and biocompatibility are attractive features of molecular magnets. Being weakly coloured, unlike their opaque classical magnet ‘cousins’ listed above, possibilities of photomagnetic switching exist. Persistent efforts also continue to design the ever-elusive polymer magnets towards applications in industry. While providing a brief overview of the field of molecular magnetism, this article highlights some recent developments in it, with emphasis on a few studies from the author’s own lab.     

Role of lanthanum in microstructure evolution and enhanced magnetic properties of cerium-containing sintered magnets

In this study, lanthanum was applied to strip cast Ce−Fe−B alloy to improve its phase composition and microstructure. The results reveal that lanthanum doping can significantly enhance the proportion of 2 : 14 : 1 phase and improve the microstructure of Ce−Fe−B alloy. Besides, the influence of the starting alloys structure on the microstructure and magnetic properties of final multi-phases cerium-containing magnets was also systematically investigated. Compared to the multi-phases magnet without lanthanum addition, a pronounced coercivity increment could be distinguished in lanthanum-doping multi-phases magnet, which could be attributed to the finer grain size together with ideal grain boundary. In this work, the superior performance of H_cj = 701.28 kA/m B_r = 1.30 T, and (BH)_max = 313.70 kJ/m³ were obtained by blending Nd−Fe−B alloy with (La_0.35Ce_0.65)-Fe−B alloy to meet 25.0 wt.% lanthanum-cerium utilization content, suggesting that the possibility to develop high abundant rare earth permanent magnetic materials.     

Glass forming ability of Fe–Co–Zr–Nd–B alloys and bulk permanent magnets derived from amorphous precursor

The glass forming ability (GFA) and magnetic properties for Fe_48−x Co₂₇Zr₃Nd _x B₂₂ (x = 0–6) alloys were investigated. It was found that the proper addition of Nd (4–5 at.%) was very effective in improving GFA. The as-cast Fe₄₄Co₂₇Zr₃Nd₄B₂₂ and Fe₄₃Co₂₇Zr₃Nd₅B₂₂ alloys exhibited good soft magnetic behavior, while showed hard magnetic property after annealing at 760 °C for 10 min. Bulk permanent magnets were obtained from crystallization of amorphous alloys, which could provide a promising way for the bulk magnet produced by the simple process of copper mold casting and subsequent heat treatment.     

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.     

Study on crystallization behavior and microstructure of melt-spun Nd2(Fe,Nb)14B/α-Fe alloys

The effects of Nb addition on the crystallization behavior, microstructure, and magnetic properties of melt-spun Nd₂Fe₁₄B/α-Fe nanocomposite magnets have been studied. The addition of Nb can significantly improve the thermal stability of amorphous phase in as-spun alloys, narrow the range between the onset crystallization temperature and the optimal annealing temperature, restrain the initial formation and growth of α-Fe and Nd₂Fe₁₄B. A finer and more homogeneous microstructure can be obtained in the Nb-doped alloy than in the Nb-free alloy. And the Nb addition makes the grains more equiaxed shape. The Nd₁₀Fe₈₃Nb₁B₆ alloy annealed at 715 °C for 10 min exhibits the improved magnetic properties, B _r = 0.90 T, _i H _c = 750 kA/m, (BH)_max = 120 kJ/m³, the intrinsic coercivity and the maximum energy product increase by 25% and 14%, compared with the Nd₁₀Fe₈₄B₆ alloy.     

Structure and Magnetic Properties of Nd-Fe-B/a-Fe Nanocomposite Magnets by Co, Nb, Dy Substitutions

Structure and magnetic properties of the nanocomposite magnets prepared by mechanical alloying procedure with composition 55 wt pct Nd (Fe0.92B0.08)5.5+45 wt pct a-Fe, 55 wt pct Nd(Fe0.8-xCo0.12Nbx B0.08)5.5+45 wt pct a-Fe (x=0.00, 0.01, 0.03) and 55 wt pct (Nd0.9Dy0.1) (Fe0.77Co0.12Nb0.03B0.08)5.5+45 wt pct a-Fe were studied. It was found that substitution of Co for Fe could significantly improve the permanent magnetic properties of the nanocomposite magnets and typically, the maximum magnetic energy product was increased from 104.8 kJ/m3 (13.1 MGOe) to 141.6 kJ/m3 (17.7 MGOe). In contrast to the case of conventional nominally single-phase magnets, the addition of Nb results in promoting the growth of a-Fe grain and is thus unfavorable for the improvement of permanent magnetic properties of the nanocomposites. Although the addition of Dy can increase the coercivity of the magnets, the increase of magnetic anisotropy of hard phase leads to decrease of the critical grain size of soft phase. Additionally it causes the difficulty of preparing the nanocomposites because it is more difficult to control the grain size of soft phase to meet the requirement of appropriate exchange coupling between hard and soft grains.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

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.     

Electrical and Magnetic Properties of Zr4+ and Ni2+ Ions Substituted Mn–Cu Spinel Ferrites

The Mn–Cu nanoferrites of composition Mn_0.5Cu_0.5Fe_2−2x Ni _x Zr _x O₄ (0.00≤x≤0.80) were synthesized by incorporating dopant Zr⁴⁺ and Ni²⁺ cations via a chemical coprecipitation technique. The single phase of the prepared spinel nanoferrites with particle sizes 15–31 nm was confirmed by XRD. Both electrical and magnetic properties are found to depend on the x contents and explained by considering the Zr⁴⁺ and Ni²⁺ cations distribution, displacement of the Fe³⁺ ions by dopants, changes in magnetic moment at tetrahedral A site and octahedral B site, Fe³⁺(A)–O₂–Fe³⁺ [B] linkages and extent of the primary superexchange interactions (A)–O–[B] of the Fe³⁺ ions on the A and B-sites. DC resistivity, saturation magnetization (M _s), remanence (M _r) and Bohr magneton (n _B) are observed to increase up to x≤0.40, while these parameters fall at relatively higher value of x≥0.60.     

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.     

Comparison on the magnetic properties and microstructure of directly casted Nd-Fe-Ti-B and Nd-Fe-Ti-Zr-Cr-B-C rod magnets with various diameters

Magnetic properties and microstructure of directly quenched Nd_9.5Fe_72.5Ti₃B₁₅ and Nd_9.5Fe_71.5Ti_2.5Zr_0.5Cr₁B_14.5C_0.5 rod magnets with various diameters of 0.7-1.5 mm have been investigated. TMA shows that all the studied alloy rods mainly consist of a large amount of a 2:14:1 phase. Although the magnetic properties of these two alloys are decreased with the increase of diameter, the latter alloy, consisting of more elements, can preserve attractive magnetic properties up to a larger diameter of 1.3 mm, due to the grain refinement effect induced by multi-component substitutions.     

Production of amorphous Fe–B alloy and α-Fe by chemical reduction of hematite using sodium borohydride

Natural and well-crystallized hematite was suspended in water and treated at room-temperature (RT) with sodium borohydride in acid medium. The product of the reaction is a highly magnetic black powder, which is stable at RT. The NaBH₄ treatment converts about half of the hematite to an amorphous Fe–B alloy and to a small fraction of sub-micron sized, amorphous metallic-Fe nodules. Heating at 400 °C of this composite has resulted in the crystallization and/or oxidation of more than half of the amorphous Fe–B phase to α-Fe and Fe₃O₄ and B₂O₃, respectively. Further, the already present superficial, amorphous metallic Fe is converted to α-Fe and the original, plate like morphology of the hematite has changed to a mix of nodular and acicular particles. After treatment at 800 °C, the metallic Fe and the amorphous Fe–B have completely vanished, and the resulting product consists of hematite and FeBO₃ embedded in a matrix of α-Fe₂O₃.     

Computational Design of Rare-Earth Reduced Permanent Magnets

Multiscale simulation is a key research tool in the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Iron (Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms. The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure. Using machine learning techniques, the magnet’s structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ·m⁻³)) were obtained for iron-tin-antimony (Fe₃Sn_0.75Sb_0.25): (0.49, 290), L1₀-ordered iron-nickel (L1₀ FeNi): (1, 400), cobalt-iron-tantalum (CoFe₆Ta): (0.87, 425), and manganese-aluminum (MnAl): (0.53, 80).     

Hydrothermal synthesis and magnetic properties of CoxFe1−x/CoyLazFe3−y−zO4 composites

A series of iron–cobalt alloy and cobalt–ferrite composites doped with La³⁺ (Co_xFe_1−x/Co_yLa_zFe_3−y−zO₄) in which the Fe–Co alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe (OH)₂ and the reduction of Co (II) by Fe⁰ in a concentrated and hot KOH solution. when x ≤ 0.1, the structures of the Fe_xCo_1−x alloy and cobalt–ferrite are fcc structure; and when x ≥ 0.25, the structures of the Fe_xCo_1−x alloy is bcc structure. The fcc structure of alloy is favored for [KOH] close to 9 N, Co(II)/Fe(II) ratios between 0.5 and 0.9 and short reaction time of synthesis. And the bcc structure of the alloy is favored for [KOH] close to 1 N, Co(II)/Fe(II) ratios between 0.1 and 0.5 and long reaction time of synthesis. A low [KOH] favors nucleation leading to octahedral of 1 μm. And [KOH] of 9–12 N favors particle growth. The metal occurs in square particles of 100–150 nm included within the spinel. Powder X-ray diffraction (XRD), thermal gravity analysis (TGA) and different thermal analysis (DTA), scanning electron microscope (SEM), transmission electron micrograph (TEM) and vibrating sample magnetometer (VSM) were employed characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe (II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by La³⁺ ions on structure, magnetic properties of the composites were investigated.