Search Results (38)

Nd–Fe–B permanent magnets are vital for numerous key technologies in strategic sectors such as renewable energy production, e-mobility, defense, and aerospace. Accordingly, the demand for rare earth elements (REEs) enormously increases in parallel to a significant uncertainty in their supply. Thus, research and innovative studies are focus on the investigation of sustainable solutions to the problem and a closed-loop value chain. The present study is based on two benign-by-design approaches aimed at decreasing the recycling loop span by preparing standardized batches of EoL Nd–Fe–B materials to be treated separately depending on their properties, as well as using mechanochemical method for waste processing. The previously reported benefits of both direct recycling and mechanochemistry include significant improvements in processing metrics, such as energy use, ecological impact, technology simplification, and cost reduction. Waste-sintered Nd–Fe–B magnets from motorbikes were collected, precisely sorted, selected, and pre-treated. The study presents a protocol of resource-efficient recycling through mechanochemical processing of non-oxidized sintered EoL magnets, involving the extraction of Nd₂Fe₁₄B magnetic grains and refining the material’s microstructure and particle size after 120 min of high-energy ball milling in a zirconia reactor. The recycled material preserves the main Nd₂Fe₁₄B magnetic phase, while an anisotropic particle shape and formation of a thin Nd/REE-rich layer on the grain surface were achieved. Full article

This work reports on the effect of the heavy rare earth element Er on Nd–Fe–B magnets by using a simple Er₆₉Fe₃₁ alloy additive, which is much less expensive than Dy and Tb elements. It was found that the corrosion resistance was improved with a minimal reduction in magnetic properties by rationally controlling the Er₆₉Fe₃₁ addition content. The main reason is that Er element partially replaces the Nd element at the edge of the main phase grain to form an (Er,Nd)₂Fe₁₄B shell with low H_A, which leads to a decrease in coercivity. However, the improvement in the corrosion resistance is mainly due to the Er₆₉Fe₃₁ alloy addition, which slows down the corrosion rate. Simultaneously, an investigation was conducted into the different advantages that target magnets when subjected to diverse heat treatment methodologies. The above findings may lead to the development of applications for other rare earth elements, thereby accelerating the development of low-cost permanent magnets comparable to commercially available sintered Nd-Fe-B magnets. Full article

Magnetocrystalline anisotropy has many advantages over shape anisotropy regarding coercivity in permanent magnets, making it a promising approach to enhance the coercivity of AlNiCo magnets. In this work, AlNiCo magnets with NdFeB-nanocrystalline phase were prepared by spark plasma sintering (SPS), and the effect of the NdFeB phase on coercivity was uncovered. AlNiCo powder with a spinodal structure and NdFeB powder with a nanocrystalline structure, which exhibit shape anisotropy and magnetocrystalline anisotropy, respectively, were sintered by SPS. With the advantages of low-temperature densification achieved by the SPS process, the spinodal and nanocrystalline structures were mostly retained. The microstructure analysis indicated that the SPS-ed magnet primarily consisted of AlNiCo regions with a spinodal structure, NdFeB regions with a nanocrystalline structure, and a transition region approximately 1~7 µm wide between them. A significant effect of the magnetic anisotropy of the NdFeB phase on magnetization behavior was found. The hysteresis loop of the SPS-ed magnets became single-phase magnetization, in contrast with the double-phase magnetization observed in the simple mixed powder. As the magnetocrystalline anisotropy of the NdFeB phase possesses higher coercivity, the coercivity of the SPS-ed magnet increased from 1250 Oe (of the AlNiCo raw powder) to 2490 Oe. This work provides valuable information for the coercivity enhancement of AlNiCo magnets. Full article

Two NdFeB magnets with distinct compositions were fabricated via an identical process. One magnet was doped with 2.5 wt.% of Ho, whereas the other remained undoped. Subsequently, grain boundary diffusion was performed on both magnets using metallic Tb, adopting the same set of technological parameters. A comprehensive analysis was conducted on the magnetic properties, phase compositions, microstructures, and elemental distributions of these two magnets. The findings indicate that the incorporation of Ho enhances the utilization efficiency of Tb. As a result, the magnets can achieve higher coercivity across different temperatures, with only a minor reduction in remanence. During the sintering process of the Ho-doped magnet, fine precipitated particles of Ho₂Fe₁₄B are generated inside the magnet. This phenomenon causes the refinement of the main grains of the magnet. The refined main grains facilitate the effective diffusion of Tb within the magnet, eliminating the formation of the anti-shell structure. Furthermore, when Ho substitutes for Nd, it leads to a more homogeneous distribution of the Nd-rich phase. Additionally, it increases the densification degree of the sintered NdFeB magnets. These effects contribute to a further enhancement of the magnets’ coercivity. Full article

Commercial NdFeB magnets are often coated with different thin layers to increase corrosion resistance. Fast and reliable test methods are being developed, especially for the automotive industry. Since corrosion test methods can inadequately describe the operating conditions of the e-motor, magnets are usually only tested in the demagnetized state. Corrosion tests close to sintered NdFeB magnet e-motor application conditions have been applied. Corrosion tests for sintered NdFeB magnets are usually demagnetized and performed in aqueous solutions or vapor environments instead of organic substances such as oil. In this study, sintered NdFeB magnets were immersed in a pre-saturated water-based salt solution and placed in gearbox oil. The test conditions have been specially selected to test the suitability of the magnets for e-motor applications (e.g., in hybrid vehicles). The microstructural effect of magnetic properties and corrosion resistance on the NdFeB magnets have been systematically studied. The aim of the study is the realization of the coating on the sintered NdFeB magnet, which provides high corrosion resistance and significantly reduces the thickness of the coating and ensures maximum efficiency in the use of magnets. The results of these studies are thought to play an important role in determining and optimizing the usage strategy of coated NdFeB magnets. Full article

The grain boundary diffusion process employing a mixed diffusion source, comprising heavy rare-earth elements and low-melting metals, significantly enhances the coercivity (H_cj) of sintered Nd-Fe-B magnets. In the present study, Tb and Ga were deposited onto the surface of Nd-Fe-B magnets to serve as a diffusion source for improving hard magnetic properties. The effects of varying deposition sequences of Tb and Ga on the magnetic properties and microstructure of the magnets were analyzed. The findings demonstrate that TbF₃ grain boundary diffusion facilitated by Ga effectively increases the efficiency of Tb substitution, leading to enhanced coercivity. When Tb and Ga are deposited simultaneously, coercivity shows a notable improvement of 53.15% compared to the untreated magnet, with no reduction in remanence. Additionally, thermal stability is enhanced, resulting in superior overall magnetic properties. Microstructural analysis reveals that Ga promotes the diffusion of Tb into the magnet. In the magnet where Tb and Ga are co-deposited, the formation of a thinner and more uniform (Nd,Tb)₂Fe₁₄B shell–core structure, along with the greater infiltration depth of Tb, leads to a broader distribution of core–shell structures within the magnet. This effectively increases the anisotropy fields (H_A) of the main phase grains, preventing the nucleation of antiferromagnetic domains at the edges of main-phase grains, thereby enhancing coercivity. Furthermore, the corrosion resistance of the magnet subjected to mixed diffusion is improved. This study provides a foundation for producing highly efficient magnets with a lower content of heavy rare-earth elements. The simplicity and flexibility of the process make it highly suitable for industrial applications. Full article

Bonded Nd-Fe-B magnets have greater freedom of shape than sintered Nd-Fe-B magnets. The ring structure is one of the typical structures of bonded Nd-Fe-B materials. In this paper, we analyzed the generation and spread of demagnetization fault (DMF) and changes in motor performance. Meanwhile, a BLDC fitted with a bonded Nd-Fe-B magnet ring was analyzed for DMF under actual overload conditions. DMF occurred with obvious localization and variability, which was mainly concentrated on the side of each pole opposite to the direction of the motor’s operation, near the weak magnetic zones. The experimental results show that back electromotive force (EMF) and its harmonic had the same variation trends as the surface radial flux density of the magnet ring. The analysis with the EMF waveform and total harmonic distortion (THD) were proposed as a method for diagnosing the DMF. Finally, this paper presents a modified magnet ring. The anti-demagnetization capability of the modified magnet ring is effectively improved. This research can provide a reference for the design analysis of BLDCs using bonded Nd-Fe-B magnet rings. Full article

This study investigates the effect of Ce on the diffusion behavior of Dy-Cu alloys. The addition of Ce reduces the diffusion source melting point and promotes the formation of low-melting alloy phases, benefiting the diffusion behavior. The diffusion source with 10 wt.% of Ce shows the best magnetic performance, the coercivity of the magnet increases from 18.47 kOe to 23.60 kOe, and the incremental coercivity reaches 5.13 kOe. Ce diffusion improves the utilization of Dy, enhances diffusion uniformity, and promotes coercivity improvement. Ce also optimizes and regulates grain boundary phase structure distribution, consistent with magnetic property changes. Dy₇₀Cu₃₀ with an excessive thick shell layer wastes Dy and reduces utilization, while Dy₆₀Ce₁₀Cu₃₀ has a relatively thin and uniform shell layer. Ce mainly distributes in the intergranular phase region, promoting Dy diffusion from the intergranular phase to form a shell layer. Excessive Ce can distort the magnet’s crystal structure, hindering magnetic property improvement. This study provides insights into optimizing the diffusion process and improving the Dy-Cu alloy. Full article

This study investigates the influence of a magnetic field on the microstructure and properties of Al₂O₃–Ni composites fabricated via centrifugal slip casting at 1500 rpm. Al₂O₃ and Ni powders were combined with water and deflocculants, homogenized, and then cast into a porous plaster mold surrounded by Nd-Fe-B magnets. The resulting composites, sintered in a reducing atmosphere, exhibited a three-zone structure with varying Ni content due to the combined effects of the magnetic field and centrifugal force. SEM, EDX, and XRD analyses confirmed the distribution and composition of the phases. Hardness tests revealed the highest values at the outermost zone, with a gradual decrease toward the inner zones. Compression tests employing digital image correlation revealed high internal stresses and a significant improvement in compressive strength compared to non-magnetic field methods. This study confirms that magnetic field-assisted centrifugal slip casting significantly enhances the structural, hardness, and compressive strength properties of Al₂O₃–Ni composites, indicating promising potential for advanced applications. Full article

Rare earth resource recycling is an important endeavor for environmental protection and resource utilization. This study explores the method of preparing regenerated magnets using waste magnets as raw materials based on existing processes. By utilizing existing Nd-Fe-B production equipment, various waste magnets are transformed into recycled powder. Next, nascent Nd-Fe-B powders with slightly higher rare earth content are selected as the repairing agent. The regenerated magnets are prepared by incorporating the nascent powder into the recycled powder. The focus lies in investigating the repairing effect of the nascent powder repairing agent on the microstructure of regenerated magnets and exploring the influence of sintering temperature and powder addition on the magnetic properties and microstructure of the regenerated magnets. The results showed that the nascent powder increased the proportion of grain boundary phases and effectively repaired the grain boundary structure of the regenerated magnets. In addition, the Pr element in the nascent powder replaces the Ce element in the recycled powder, which ultimately improves the magnetic properties of the regenerated magnet in a comprehensive manner. This study provides valuable insights and guidance for rare earth resource recycling and the preparation of regenerated magnets. Full article

The more effective use of readily available Ce in FeNdB sintered magnets is an important step towards more resource-efficient, sustainable, and cost-effective permanent magnets. These magnets have the potential to bridge the gap between high-performance FeNdB and hard ferrite magnets. However, for higher degrees of cerium substitution (>25%), the magnetic properties deteriorate due to the lower intrinsic magnetic properties of Fe₁₄Ce₂B and the formation of the Laves phase Fe₂Ce in the grain boundaries. In this paper, sintered magnets with the composition Fe70.9-(Ce_xNd_1-x)18.8-B5.8-M4.5 (M = Co, Ti, Al, Ga, and Cu; with Ti, Al, Ga, and Cu less than 2.0 at% in total and Co_bal; x = 0.5 and 0.75) were fabricated and analyzed. It was possible to obtain coercive fields for higher degrees of Ce substitution, which previous commercially available magnets have only shown for significantly lower degrees of Ce substitution. For x = 0.5, coercivity, remanence, and maximum energy product of µ₀H_c = 1.29 T (H_c = 1026 kA/m), J_r = 1.02 T, and (BH)_max = 176.5 kJ/m³ were achieved at room temperature for x = 0.75 µ₀H_c = 0.72 T (H_c = 573 kA/m), J_r = 0.80 T, and (BH)_max = 114.5 kJ/m³, respectively. Full article

In the last fifteen years, several groups have investigated metal injection moulding (MIM) of NdFeB powder to produce isotropic or anisotropic rare earth magnets of greater geometric complexity than that achieved by the conventional pressing and sintering approach. However, due to the powder’s high affinity for oxygen and carbon uptake, sufficient remanence and coercivity remains difficult. This article presents a novel approach to producing NdFeB magnets from recycled material using Powder Extrusion Moulding (PEM) in a continuous process. The process route uses powder obtained from recycling rare earth magnets through Hydrogen Processing of Magnetic Scrap (HPMS). This article presents the results of tailored powder processing, the production of mouldable feedstock based on a special binder system, and moulding with PEM to produce green and sintered parts. The magnetic properties and microstructures of debinded and sintered samples are presented and discussed, focusing on the influence of filling ratio and challenging processing conditions on interstitial content as well as density and magnetic properties. Full article

A novel micro-/nano-structured coating for corrosion resistance of sintered NdFeB was constructed based on the synergetic effect between the “bridge” structure of nanoparticles and the “labyrinth effect” of micro-/nanoflakes. Iron-titanium nanopaste (ITNP) and micro-/nanoflake silver powder (MNFS) were added into epoxy resin to prepare an epoxy micro-/nano-composite coating material, and then the prepared composite coating material was coated on sintered NdFeB permanent magnets by air spraying, obtaining an epoxy composite coating with a micro-/nanostructure. The effect of the micro-/nanomaterials on the microstructure and corrosion resistance of the composite coatings was investigated, and the mechanisms of the enhancement in corrosion resistance were proposed. The results show that when immersed in 3.5 wt.% NaCl solution for 32 days, the ranking of the capacitive time constant radius and impedance modulus of the coatings at a lower frequency (Z_f = 0.01 Hz) is pure EP < EP/ITNP < EP/MNFS < EP/ITNP/MNFS, respectively, and the salt spray test time of the EP/ITNP/MNFS coating is more than 720 h, which is more than twice that of the pure EP coating. Full article

High-performance sintered Nd-Pr-Fe-B magnets were successfully prepared by depositing Dy/Tb films on the surface using magnetron sputtering, which resulted in superior grain boundary diffusion (GBD) under heat treatments. The course of the diffusion was assessed using an electron probe microanalyzer (EPMA) and inductively coupled plasma (ICP). The magnetic properties and thermal stability of the magnets before and after diffusion were investigated. The results show that, mainly due to the increased and optimized Nd-Pr-rich phases and the formation of the (Nd,Pr,Dy/Tb)₂Fe₁₄B shell structure surrounding the (Nd,Pr)₂Fe₁₄B grains, the coercivity of the Dy- and Tb-diffused magnets was enhanced from 16.7 kOe to 24.8 kOe and 28.4 kOe, respectively, while the corresponding maximum energy product (BHmax) was 48.1 MGOe and 48.5 MGOe, respectively. The consumption of Dy/Tb in this work (0.35 wt% Dy in the Dy-diffused magnet and 0.42 wt% Tb in the Tb-diffused magnet) is much lower than that of previously reported magnets with comparable coercivity. Furthermore, Dy- or Tb-diffused magnets exhibit better thermal stability than that of the original magnet, owing to the better resistance to thermal disturbances of the magnets with optimized microstructure. This work can provide useful guidance for preparing Nd-Fe-B magnets with low cost and high performance. Full article

The green transition initiatives and exploitation of renewable energy sources require the sustainable development of rare earth (RE)-based permanent magnets prominent technologies like wind turbine generators and electric vehicles. The recycling of RE-based permanent magnets is necessary for the future supply of critical rare-earth elements. The short-loop recycling strategies to directly reprocess Nd-Fe-B magnet waste are economically attractive and practical alternatives to conventional hydro- and pyrometallurgical processes. This study focuses on the development of a procedure to extract the (Nd, Pr)₂Fe₁₄B hard-magnetic phase from sintered Nd-Fe-B magnets. The extraction is achieved through preferential chemical leaching of the secondary, RE-rich phases using 1 M citric acid. Before the acid treatment, the magnets were pulverized through hydrogen decrepitation (HD) to increase the material’s surface-to-volume ratio. The as-pulverized Nd-Fe-B powder was subsequently exposed to a 1 M citric acid solution. The effect of leaching time (5–120 min) on the phase composition and magnetic properties was studied. The results of the microstructural (SEM) and compositional (ICP-MS) analyses and the study of thermal degassing profiles revealed that the RE-rich phase is preferentially leached within 5–15 min of reaction time. Leaching of the secondary phases from the magnet’s multi-phase microstructure is governed by the negative electrochemical potential of Nd and Pr. The extraction of (Nd, Pr)₂Fe₁₄B grains by the proposed acid leaching approach is compatible with the existing hydrogen processing of magnetic scrap (HPMS) technologies. The use of mild organic acid as a leaching medium makes the leaching process environmentally friendly, as the leaching medium can be easily neutralized after the reaction is completed. Full article