Effects of post-sinter annealing on microstructure and magnetic properties of Nd–Fe–B sintered magnets with Nd–Ga intergranular addition

We investigate the effects of post-sinter annealing on the microstructure and magnetic properties in B-lean Nd–Fe–B sintered magnets with different quantities of Nd–Ga intergranular additions. The magnet with fewer Nd–Ga additions can enhance 0.2 T in coercivity, with its remanences nearly unchanged after annealing. With the further increase of the Nd–Ga addition, the annealing process leads coercivity to increase 0.4 T, accompanied by a slight decrease of remanence. With the Nd–Ga addition further increasing and after annealing, however, the increase of coercivity is basically constant and the change of remanence is reduced. Microstructure observation indicates that the matrix grains are covered by continuous thin grain boundary phase in the magnets with an appropriate Nd–Ga concentration after the annealing process. However, the exceeding Nd–Ga addition brings out notable segregation of grain boundary phase, and prior formation of part RE6 Fe13 Ga phase in the sintered magnet. This prior formation results in a weaker change of remanence after the annealing process.Therefore, the diverse changes of magnetic properties with different Nd–Ga concentrations are based on the respective evolution of grain boundary after the annealing process.     

Effects of oxidation of DyH_3 in Nd–Fe–B sintered magnets

The effects of oxidation of Dy H3 with respect to dysprosium addition to Nd–Fe–B sintered magnets are examined.Samples sintered with the addition of freshly milled dysprosium hydride, dysprosium hydride exposed to air at room temperature for 15 min and dysprosium hydride exposed to air at 100°C for 3.5 hours are studied from the aspects of magnetic properties, microstructures, and their degradation, respectively. It is found that some oxidized dysprosium is distributed in the Nd-rich phase; hence, the decrease of remanence occurred. The degradation results indicate that preoxidised dysprosium can be a major factor in increasing the corrosion rate. The microstructures and corrosion acceleration test suggested that the oxidation is detrimental to remanence.     

Progress in recycling of Nd–Fe–B sintered magnet wastes

Significant efforts have been put into the recycling of bulk Nd–Fe–B sintered magnet wastes around the world in the past decade because bulk Nd–Fe–B sintered magnet wastes are valuable secondary rare-earth resources.There are two major facts behind the efforts.First, the waste magnets contain total rare-earth content as high as more than 30 wt.%, which is higher than most natural rare-earth mines.Second, the waste magnets maintain the physical and chemical properties of the original magnets even with deterioration of the properties on surfaces due to corrosion and contamination.In this review,various techniques for recycling bulk Nd–Fe–B sintered magnet wastes, the overall properties of the recycled Nd–Fe–B sintered magnets, and the mass production of recycled magnets from the wastes are reviewed.     

Coercivity mechanism of La–Nd–Fe–B films with Y spacer layer

The effect of the Y spacer layer on the phase composition, coercivity, and magnetization reversal processes of La–Nd–Fe–B films has been investigated. The addition of a 10 nm Y spacer layer increases the coercivity of the film to 1.36 T at 300 K and remains 0.938 T at 380 K. As the thickness of the Y spacer layer increases, Y participates in the formation of the main phase in the film, and further regulates the formation of La–B phases. The results of the first-order reversal curve(FORC) and mic...     

Formation and crystallization kinetics of Nd–Fe–B-based bulk amorphous alloy

In order to improve the glass-forming ability (GFA) of Nd–Fe–B ternary alloys to obtain fully amorphous bulk Nd–Fe–B-based alloy, the effects of Mo and Y doping on GFA of the alloys were investigated. It was found that the substitution of Mo for Fe and Y for Nd enhanced the GFA of the Nd–Y–Fe–Mo–B alloys. It was also revealed that the GFA of the samples was optimized by 4 at.% Mo doping and increased with the Y content. The fully amorphous structures were all formed in the Nd $_{6-{x}}$ Y $_{{x}}$ Fe $_{68}$ Mo $_{4}$ B $_{22}$ (x $=$ 1–5) alloy rods with 1.5 mm-diameter. After subsequent crystallization, the devitrified Nd $_{3}$ Y $_{3}$ Fe $_{68}$ Mo $_{4}$ B $_{22}$ alloy rod exhibited a uniform distribution of grains with a coercivity of 364.1 kA/m. The crystallization behavior of Nd $_{3}$ Y $_{3}$ Fe $_{68}$ Mo $_{4}$ B $_{22}$ BMG was investigated in isothermal situation. The Avrami exponent n determined by JAM plot is lower than 2.5, implying that the crystallization is mainly governed by a growth of particles with decreasing nucleation rate.     

Synthesis and Stabilization of Fe–Nd–B Nanoparticles for Biomedical Applications

Stable composition of Iron Neodymium Boron nanoparticles are formed by a chemical method. Conventional borohydride reduction method was used. The particles are in the size range of 30–100 nm. Silica coating was applied to stabilize and prepare the particles for in vitro applications such as cell separation and diagnostics. Morphology of particles has been studied along with the structure and magnetic properties.     

Effect of very small additions on the coercivity of Dy-free Nd–Fe–(Co)–B-sintered magnets

The effect of small substitutions on the coercivity of Nd–Fe–B and Nd–Fe–Co–B-sintered magnets has been investigated. Addition of 0.18 at% Ga was found to be the most effective for improving the coercivity in both Nd–Fe–B and Nd–Fe–Co–B without decreasing much the remanent magnetization. Scanning electron microscopy on Ga-free and Ga-added magnets did not reveal a noticeable difference in phase morphology. However, Fe and especially Co concentrations in the intergranular Nd-rich phase were found to be markedly increased after the small Ga addition. Combined addition of Co with a small amount of Cu did not increase the coercivity of the Dy-free magnets. However, none of the examined very small additives can yet be considered as an alternative to Dy for extending operating temperature range of the high-energy Nd–Fe–B magnets.     

Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets

The effect of Tb on the coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets has been investigated. The results showed that the addition of Tb enhanced the intrinsic coercivity, reduced the remanence and improved the impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.0 at% Tb was incorporated. The possible reasons for increasing the intrinsic coercivity and improving impact toughness of sintered magnets were analyzed, and the relations between the microstructure and impact toughness of sintered magnets were studied.     

Physical properties of magnetic alloy (Nd–Fe–B–C) nanoparticles generated in distilled water using Nd:YAG LASER

In this work, (Nd?CFe?CB?CC) magnetic nanoparticles are generated by pulses of Nd:YAG laser irradiation on the (Nd?CFe?CB?CC) magnetic target in distilled water. Exposure times were 1, 5, and 10 min. Percentages of elements in a bulk sample and nanoparticles are investigated by energy-dispersive X rays (EDX). Mean particle sizes of the nanosamples are analyzed by a transmission electron microscope (TEM). The average size of the nanoparticles is 6.23 nm. A typical selected-area electron-diffraction (SAED) ring pattern from the nanocrystals shows a tetragonal structure in (Nd?CFe?CB?CC) nanoparticles similar to the bulk sample. In order to investigate the nanoparticle stability, in two weeks after nanoparticle generation, the size distribution of nanoparticles is measured using dynamic light scattering (DLS). Using an atomic force microscope (AFM) and a magnetic force microscope (MFM), we show different aspects of generated nanoparticles.     

Improvement of microstructure and magnetic properties of Nd–Fe–B alloys by Nb and Co additions

In order to establish the role of niobium on the hydrogenation, disproportionation, desorption and recombination (HDDR) behavior of near-stoichiometric alloys, two alloys: NdI3Fe8OB7 and Nd13Fe78Nb1Co1B7 (at%) were investigated before, during and after the HDDR process. The microstructure of the as-cast Nb-free alloy before employing the HDDR process was found to consist of three phases, the matrix Nd₂Fe₁₄B (φ) phase, Nd-rich phase and a significant amount of free iron; whereas, the microstructure of the Nb-containing alloy consisted of only the first two phases.     

Effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets

The effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets has been investigated. The results showed that the addition of Co decreased the intrinsic coercivity and the temperature coefficient of remanence (α), and increased the temperature coefficient of coercivity (β) for sintered Nd–Fe–B magnets. The impact toughness of sintered Nd–Fe–B magnets with the addition of Co first decreases, reaches a minimum, and then starts to increase. The possible reasons for increasing the temperature coefficients of coercivity (β) for sintered Nd–Fe–B magnets were analyzed, and the relations between the microstructure and impact toughness of sintered Nd–Fe–B magnets were studied.     

Effect of annealing temperature on coercivity of Nd–Fe–B magnets with TbFeAl doping by process of hot-pressing

The Nd–Fe–B magnets are pre-sintered and then processed with hot-pressing, and the resulting magnets are called the hot-pressed pretreated(HPP) magnets. The coercivity of the HPP magnets increases as the annealed temperature increases.When the annealing temperature is 900℃, the coercivity of the magnet is only 17.6 kOe(1Oe = 79.5775 A·m~(-1)), but when the annealing temperature rises up to 1060℃, the coercivity of the magnet reaches 23.53 k Oe, which is remarkably increased by 33.7%. The microstructure analysis indicates that the grain surface of the HPP magnet becomes smoother as the annealed temperature increases. The microstructure factor α is changed according to the intrinsic coercivity model formula. The α of the magnet at 900℃ is only 0.578, but it is 0.825 at 1060℃. Microstructural optimization is due mainly to the increase of coercivity of the HPP magnet.     

Spectroscopic properties of Nd3+-doped tungsten–tellurite glasses

In this work, we investigate the spectroscopy properties of neodymium doped tungsten–tellurite glasses prepared in ambient and O₂-rich atmospheres. A conversion of TeO₄ to TeO₃ units was caused by the addition of Nd³⁺ into the glass, which was confirmed by absorption spectra and by Judd–Ofelt parameter behavior. The relaxation of the ⁴F_3/2 level is dominated by radiative decay and cross-relaxation between Nd³⁺ and Nd³⁺ ions. The energy transfer from Nd³⁺ to the hydroxyl group is negligible when compared to the cross-relaxation. The luminescence quantum efficiency values of the ⁴F_3/2 level decreases as the Nd³⁺ concentration increases, independently if determined by the Judd–Ofelt method or by the thermal lens technique. The observed reduction in the IR absorption associated to OH groups was not effective to improve the luminescence quantum efficiency.     

Effect of optimized aging processing on properties of the sintered Dy-doped Nd–Fe–B permanent magnet

We investigate the effect of the optimized aging processing on magnetism and mechanical property of the sintered Dydoped Nd–Fe–B permanent magnet. The experimental results show that the magnetism, especially intrinsic coercivity, of the optimized aged Dy-doped Nd–Fe–B magnet is more excellent than that of the sintered one, but the former's strength and hardness are lower than that of the latter. It was observed that the optimized aged Dy-doped Nd–Fe–B magnet have more uniform grain size, thinner(Nd, Dy)-rich boundary phase. By means of the EBSD technology, the number of larger angle grain boundaries in the optimized aged Dy-doped Nd–Fe–B magnet is more than that of the sintered one. The reasons for the increased intrinsic coercivity and decreased mechanical properties of the optimized aged Dy-doped Nd–Fe–B magnet are also discussed.     

Plastic deformation modeling of backward extrusion process for Nd–Fe–B ring magnets

3D finite element-based software (3D DEFORM) was used to simulate the thermal extrusion process of nanocrystalline magnetic ring. The effective stresses and effective strains for a ring magnet at different stages of the extrusion process were determined by simulation. The effective strains at different stages are displayed. The effective stresses on the cross section are determined by simulation. The test results of magnetic properties were of good validation of the three-dimensional finite element analysis for nanocrystalline backward extruded ring. 3D finite element-based plastic deformation simulation is proved to be an effective way to analyze the hot extrusion process of nanocrystalline magnetic ring, and to provide guiding for the mold design of thermal extrusion.     

Coercivity enhancement of anisotropic Dy-free Nd–Fe–B powders by conventional HDDR process

Coercivity enhancement of Dy-free Nd–Fe–Co–B–Ga–Zr powders was studied using the conventional hydrogenation–decomposition–desorption–recombination (HDDR) process. It was found that the addition of Al together with the proper Nd content and the slow hydrogen desorption of the HDDR treatment can induce high coercivity in the powder. For example, the 14.0 at% Nd–2.0 at% Al powder exhibits H_cJ of 1560 kA/m, B_r of 1.22 T, and (BH)_max of 257 kJ/m³. The high coercivity inducement of the powder is thought to be attributed to the formation of Nd-rich phase, which continuously surrounds fine Nd₂Fe₁₄B grains.     

Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography

Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd₂Fe₁₄B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning.     

Understanding the high temperature oxidation and ignition behaviour of two-phase Mg–Nd alloys and a comparison to single phase Mg–Nd

The oxidation kinetics and the mechanism of two-phase Mg–Nd alloys were investigated via isothermal heating experiments conducted in dry air at 500 °C for 12 h. The oxidation kinetic curves reveal improved oxidation resistance on neodymium (Nd)-containing alloys compared to pure Mg. A lower mass gain was detected at 2.5-%Nd than at 6-Nd%, which was related to the lower amount of intermetallic phase on the alloy surface. The intermetallic phase has a significant effect on the oxide growth stage. Nd₂O₃ formation on the intermetallic phases creates diffusion paths for oxygen to the metal/oxide interface, affecting both the oxidation kinetics and the oxidation resistance of the alloys. The formation of a Nd-depleted region at the subsurface due to extensive Nd oxidation at the oxide/intermetallic interface lowers the protective ability of the oxide scale. As increasing the Nd content of binary Mg–Nd alloys above 0.5 wt% shifts the alloys from single-phase region to two-phase region, it adversely affects the ignition resistance.     

Regulating element distribution to improve magnetic properties of sintered Nd–Fe–B/Tb–Fe–B composite magnets

Nd content was varied in Nd_(13.2-x)Fe_(80.8+x)B_6(x = 0, 0.5, 1, and 1.5) to optimize the magnetic properties of sintered Nd–Fe–B/Tb–Fe–B composite magnets, which were prepared by mixing 9 g of Nd–Fe–B with 1 g of Tb_(17)Fe_(75)B_8 powder.In conventional magnets, by reducing Nd content, the coercivity of 10.4 kOe in Nd_(13.2)Fe_(80.8)B_6 decreases to 7.2 kOe in Nd_(12.2)Fe_(81.8)B_6; meanwhile, in Nd–Fe–B/Tb–Fe–B magnets the coercivity does not decrease when reducing Nd content.In the intergranular phase, the Tb content increases owing to the reducing Nd content of the Nd–Fe–B alloy in the sintered composite magnets.Therefore, the excess Tb in Tb_(17)Fe_(75)B_8 enters the intergranular phase, and more Tb atoms can substitute for Nd at the grain boundary of the Nd–Fe–B phase, leading to a more significant increase in coercivity.The remanence increases with reducing Nd content, and the energy product of 39.1 MGOe with a high coercivity of 21.0 kOe is obtained in Nd_(12.2)Fe_(81.8)B_6/Tb_(17)Fe_(75)B_8 magnets.These investigations show that magnetic properties can be further improved by regulating the element distribution in sintered composite magnets.