NdFeB Magnets Research Articles

Explaining Browns paradox in NdFeB magnets from micromagnetic simulations

We present the results of a systematic investigation into how the coercivity and maximum energy product of NdFeB permanent magnets are affected by magnetic and geometric microscopic properties. The results are based on numerical micromagnetic simulations carried out with the open source numerical framework MagTense. We considered artificially generated realistic microstructures for which we can control the number of crystal grains as well as the thickness of the intergrain region. Based on variations of the exchange constant, easy axis orientation, grain boundary width and intergrain material properties, the results indicate that while all of these can contribute to a reduction of coercivity, the easy axis orientation has the largest influence. For this, if the easy axis orientation is distributed within a cone with an opening angle of 15°, that is enough to reduce the coercivity by 1 T. Regarding the maximum energy product, the width of the grain boundary layer as well as the easy axis orientation were seen to have the largest influence, with the exchange constant only very weakly influencing the maximum energy product. Our analysis thus methodically clarifies the different factors contributing to the reduction of the value of the coercivity and maximum energy product when compared to the theoretical limit, a discrepancy known as Brown’s paradox.

Coercivity enhancement of hot-deformed NdFeB magnets by in situ two-end-diffusion with R70Cu30 alloy powders (R = NdPr and Ce)

We hereby present a simple process, called in situ two-end diffusion (TED), to reach coercivity enhancement with almost unchanged energy product for hot deformed (HD) NdFeB magnets. Coercivity of the HD magnets is increased from 15.1 to 18.7 kOe for the magnet TED with (Nd0.75Pr0.25)70Cu30 (NdPrCu), which is larger than 17.6 kOe for the magnet directly doped with 2 wt. % NdPrCu alloy. Besides, the magnet TED with NdPrCu exhibits good (00L) texture and therefore could support high (BH)max of 43.0 MGOe. In addition, the magnet TED with Ce70Cu30 (CeCu) exhibit superior magnetic properties of Br = 12.8 kG, iHc = 18.0 kOe and (BH)max = 40.7 MGOe to those of traditional doping with CeCu (Br = 11.7 kG, iHc = 16.4 kOe and (BH)max = 34.2 MGOe). For TED process, low-melting CeCu can infiltrate into the center of the magnet, and most Ce distribute at grain boundary. The distribution of Ce and Cu at grain boundary can reduce the magnetization of grain boundary phase, strengthen the decoupling effect between grains and contribute to the coercivity enhancement. This work demonstrates TED with low-melting HRE-free alloys is a simple method to effectively enhance coercivity and keep high Br and (BH)max for HD NdFeB magnets.

High-coercivity heavy-rare-earth-free NdFeB sintered magnets developed by doping Ce85Al15 and grain boundary diffusion with Pr70Cu15Al15

Traditional NdFeB sintered magnets with coercivity larger than 20 kOe, called super-high (SH) level, usually contains considerable amount of high-cost heavy rare earth (HRE), such as Dy or Tb. In this work, high coercivity of 21.4 kOe, which corresponds to SH level, is achieved for HRE-free NdFeB magnet by doping with Ce85Al15 (CeAl) alloy and grain boundary diffusion (GBD) with Pr70Cu15Al15 (PrCuAl) alloy powders, where Ce/RE ratio is about 15% within the magnet. After GBD treatment with PrCuAl on Ce-containing NdFeB magnets, the formation of Pr-containing shell at 2:14:1 grain surface can increase the local magnetic anisotropy filed and therefore enhance the coercivity. Most of Cu and Al, and parts of Ce and Pr appear at grain boundary and triple junction, which can reduce the magnetization of grain boundary phase, strengthen the decoupling effect between grains and thus contribute to high coercivity. This study provides a cost-effective way to obtain high coercivity for HRE-free NdFeB sintered magnet via doping Ce85Al15 and then GBD with Pr70Cu15Al15.

Effect of quenching rate on the structural and hard magnetic properties of Nd-Fe-B melt-spun ribbons

The phase structure, microstructure, magnetic and thermomagnetic properties of nanostructured Nd–Fe–B melt-spun ribbons were investigated. The melt-spun ribbons have been prepared at different wheel speeds varying from 17 to 25 m/s. The hard magnetic Nd2Fe14B phase with (00l) texture, indicating preferred crystallographic orientation, was observed in all the ribbons with some α-Fe(Co) as the minor phase. Nd2Fe14B grains are uniformly distributed with grain sizes ranging from 50 to 150 nm. A decrease in the average grain size of Nd2Fe14B and fading away of texture formation in the ribbons were found with the increase in the wheel speeds. The best combination of magnetic properties with a coercivity of 14.5 kOe, the saturation magnetization of 132 emu/g, and the energy product of 16 MGOe was achieved at 23 m/s and these ribbons are suitable for the fabrication of hot deformation Nd–Fe–B magnets.

Towards neodymium recycling: Analysis of the availability and recyclability of European waste flows

The world is facing a growing neodymium demand, creating the need for developing a recycling system to handle future waste flows. Recycling technologies are emerging, but the recycling system around them can only be established with knowledge about available end-of-life (EoL) products. Therefore, this study quantified neodymium waste in European countries using material flow analysis, and assessed the recyclability of major EoL products. For 2019, we find a waste flow of 7.7 kt Nd, consisting mostly of NdFeB magnets. HDDs represent a large current waste flow, while the demand for magnets in industrial applications is increasing. In the future, electric vehicle motors and wind turbines likely provide a source of neodymium with good recyclability. Consequently, there will be different product groups that determine the future waste volumes. To manage the changing waste flows, a neodymium recycling system should be developed with the product properties of future waste flows in mind. Meanwhile, the recyclability of products can be improved by addressing bottlenecks in the recycling chain.

An experimental study of magnetic compression technique for ureterovesical anastomosis in rabbits

This study aimed to explore the feasibility of the magnetic compression technique (MCT) for ureterovesical anastomosis in a rabbit model with ureteral obstruction. The distal ureteral obstruction model using female New Zealand rabbits was induced by ligating the distal end of the right ureter with silk thread for four weeks. A pair of cylindrical NdFeB magnets (daughter magnet and parent magnet) with a hole in the center was used for the ureterovesical anastomosis. The daughter magnet and the parent magnet were respectively placed close to the obstruction site through the dilated proximal ureter and urethra, and then the daughter-parent magnets pair was attracted together automatically. Postoperative X-rays were taken to confirm the position of the magnets. The anastomotic stoma specimens were obtained two weeks postoperatively, and the anastomotic stoma formation was observed by the naked eye and histological staining. The operation time and the anastomotic burst pressure were measured. The ureter was significantly dilated in the fourth week after ligation, which satisfied the placement of the daughter magnet. The ureterovesical magnet placements were successfully performed in ten experimental rabbits, with an operation time of 36.5 ± 6.09 min. The parent and daughter magnets attracted each other well and were subsequently removed through the urethra two weeks postoperatively, resulting in the establishment of ureterovesical anastomosis. The anastomotic burst pressure was 147.5 ± 14.59 mmHg. Gross specimens and histological examination of the anastomotic stoma showed that the anastomotic stoma healed well. MCT is feasible and simple for ureterovesical anastomosis.

Induction brazing of NdFeB magnet with PVD-deposited AgCu alloy

The NdFeB magnets service in high-power electric-mechanical conversion devices sometimes endure high stress and high-temperature impact, and adhesion of these magnets using glue cannot ensure the reliability. In this study, the NdFeB was induction brazed with physical vapor deposition (PVD)-deposited Ag72Cu28 alloy layer, then the microstructure, mechanical properties and magnetic properties of the brazed joints were characterized, and effects of the filler metal thickness and the induction heating time were analyzed. The results show that the PVD coating can effectively prevent the surface oxidation of the NdFeB. The AgCu filler metal penetrates into the NdFeB and is partly mixed with the liquefied Nd-rich intergranular phase of the NdFeB, forming a good metallurgical bonding. The thickness of the penetration layer increases obviously with increase of the heating time and filler metal thickness. The bonding strength of the NdFeB brazed joint increases firstly and then decreases with increasing heating time, and a peak bending strength of 196.94 MPa was obtained. Fracture of the brazed joint occurs inside the NdFeB substrate close to the penetration layer. After a further annealing, the coercivity of the brazed NdFeB sample is a little higher than that of the un-brazed sample, and the squareness decreases by only a little. Properties of the brazed joints might be further improved by optimizing the amount of filler metal and the induction heating parameters.

Injection moldable, self-healable, and recyclable rubber-bonded NdFeB magnets with the magnetic particulates content up to 90 wt%

To apply the facile injection molding technique commonly used in thermoplastics processing to rubber-bonded magnets, typical highly filled thermosetting composites, an approach based on Diels–Alder (DA) chemistry is proposed by the authors. In the proof-of-concept experiments, the oligomers of hydroxyl-terminated polybutadiene rubber are successively functionalized by furyl and trifluoromethyl, and the magnetic NdFeB particles are modified by maleimide. By taking advantage of the reversible DA reaction, cross-linkages are established among the treated polybutadiene oligomers and NdFeB particles. The composite exhibits very low viscosity due to de-cross-linking when heated to retro-DA reaction temperature, which provides the highly filled system with necessary thermal processibility. In addition, the fluorine groups of the rubber phase are aggregated surrounding the particulate fillers allowing for well separation of the latter in the melt state. During the subsequent cooling, the DA bonds are re-built up in the rubber and at the particles/rubber interface, respectively, offering the robustness of traditional thermosets. As a result, the polybutadiene-bonded magnets with NdFeB content up to 90 wt% can be injection molded and possess rather high magnetic properties, mechanical properties, self-healability, and recyclability. The present work combines the merits of thermoplastics and thermosets and satisfies the contradictory requirements of high filling, efficient manufacturing, and balanced performance of functional polymer composites.

Elevated corrosion resistance and improved magnetic properties of sintered Nd-Fe-B magnets via the infiltration of DyCr film

The low coercivity at high temperature and poor corrosion resistance limit the application field of Nd-Fe-B magnets. In this work, a DyCr film was prepared on the surface of Nd-Fe-B magnet through magnetron sputtering, and then subjected to grain boundary diffusion (GBD) to improve magnetic properties and enhance corrosion resistance. The results show that the residual Cr-rich film exists on the surface of NdFeB magnets, acting as a protective coating to enhance the corrosion resistance, which is evidenced by the weight loss experiment and scanning electron microscopy (SEM) image. Moreover, Cr element in the grain boundary phase increase the electrochemical potential from − 0.985 to − 0.733 V, which is also beneficial in improving corrosion resistance. Transmission electron microscope (TEM) analysis confirmed that the crystal structure of intergranular phase remained unchanged after diffusion, i.e. BCC-RE2O3 (RE=Rare Earth) phase. The coercivity was improved significantly, obtaining an increment of 19.4% comparing to that of initial magnet, which is attributed to the formed Dy-rich shell with high magnetocrystalline anisotropy field and the improved distribution of grain boundary phase. This suggests that it is feasible to fabricate high coercivity magnets and strong corrosion resistance only through grain boundary diffusion, which is expected to further shorten the processing flow of Nd-Fe-B magnets.

Comparison of the corrosion resistances of chromium-passivated and cerium-passivated Zn/NdFeB magnets

Abstract Chromium-free passivation of Zn coating on NdFeB magnets becomes a research hotspot due to the serious harm of chromium ions to the human body. Chromium-based and cerium-based passivation technologies are conducted on electroplating Zn/NdFeB respectively. Morphologies, elemental compositions and phase structures of the two passivated coatings are characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The corrosion resistances of the two passivated specimens are compared by neutral salt spray test, accelerated aging test and electrochemical measurements. A complete and smooth passivation film can be obtained on the surface of Zn/NdFeB, filling the gaps and pores in Zn coating. Compared with un-passivated Zn/NdFeB, Zn(Ce)/NdFeB and Zn(Cr)/NdFeB possess excellent corrosion resistance. In comparison, Zn(Ce)/NdFeB possesses excellent anti-corrosion performance, increasing the red-rust appearing time from 288 to 432 h, which is still lower than that of Zn(Cr)/NdFeB (528 h). Therefore, the self-repair effect of cerium passivation technology during the corrosion process should be further studied to achieve the purpose of replacing chromium passivation technology.

Recycling of NdFeB magnets employing oxidation, selective leaching, and iron precipitation in an autoclave.

The increasing production of neodymium-iron-boron (NdFeB) magnets for technological applications results in disposal problems. NdFeB magnets contain a significant quantity of rare earth elements (REEs). China is the largest REEs producer, but it applies quotas and increases the export prices of REEs. To address this issue, this study aims at investigating the recovery process of REEs from scrap NdFeB magnets. After oxidation of NdFeB magnet powders, selective leaching with nitric acid was carried out to achieve high-purity REE-rich leaching liquor. First, the oxidation kinetics of NdFeB powders was studied in detail to determine the oxidation temperature and duration. Afterwards, the effects of selective leaching parameters, including acid concentration, leaching temperature, stirring speed and solid/liquid ratio, were examined by analysis of variance (ANOVA) analysis based on Taguchi method. The most substantial parameters were assigned to be the temperature and solid/liquid ratio. Eventually, the dissolution kinetics were studied to propose a model for REEs. Several universal equations for dissolution kinetics were tested, and (1 - (1 - x) = k × tn) gives the best results for REEs. The findings show that the leaching process follows the shrinking core model. Activation energy was calculated to be 40.375 kJ mol-1 for REEs. As the last step, the iron dissolved during leaching was precipitated as hematite in the autoclave. The hematite precipitation experiments were performed based on the Box-Behnken design. The effect of precipitation parameters was investigated by ANOVA analysis, and the precipitation process was optimized using response surface methodology (RSM), which resulted in the minimum iron and maximum REEs content in the leach liquor.

An acid-free process for selective REE recovery from spent NdFeB magnets by room-temperature electrolysis

An acid-free electrochemical process has been developed for the selective recovery of REEs from EOL NdFeB magnets at room temperature. Fe2+ produced by NdFeB was oxidized at and hydrolyzed to Fe(OH)3in situ to separate from REEs. Subsequently, the recycling of ZnCl2 electrolyte was realized.

Corrosion Behavior of NdFeB Magnets in Different Aqueous Solutions

Sintered NdFeB magnets possess excellent magnetic properties, but behave passively in alkaline and saline media and are susceptible to corrosion in acidic environments. The corrosion behavior of NdFeB magnets in industrial environments: potassium hydroxide (KOH), perchloric acid (HClO4) and sodium chloride (NaCl), was investigated by immersion and electrochemical tests. The immersion test concluded that the most corrosive electrolyte of the three studied was HClO4. Electrochemical tests showed that the NdFeB sample without magnetic order had better corrosion resistance. Metallographic microscopy and scanning electron microscopy support the obtained corrosion data. X-ray photoelectron spectroscopy indicates that the main elements on the surface of the analyzed sample (Nd, Fe, B) are in the oxidized state after corrosion.

Kinetic study of rare earth elements extraction from decrepitated magnet powder using liquid magnesium.

This study presents a comprehensive investigation of neodymium extraction from decrepitated magnet powder using liquid magnesium. Neodymium extraction from the decrepitated magnet into the liquid magnesium was assessed between 700 and 900 °C by measuring the average length of the diffusion zone in sintered samples of 3 mm-thickness. Experiments were conducted in a reactor which a design allows a homogeneous distribution of magnesium with efficient agitation. An empirical model was used to model the growth kinetics of the diffusion zone by using the Rosin-Rammler equation and estimate particle size distribution. The results were extrapolated to decrepitated magnet powder particles to simulate the neodymium extraction performances. Remarkably, the treatment time required was relatively short, not exceeding 22 minutes, and varied depending on the extraction target and temperature. Both temperature and the setpoint for the volume rate to be treated emerged as critical factors. Their impact on the process was thoroughly examined and discussed. These findings offer promising insights into the industrial feasibility of the use of liquid magnesium for rare-earth extraction from spent permanent magnet.

Magnetic Properties Enhancement of Hot Deformed Ndfeb Magnets by Eliminating Coarse Grains

Magnetic Properties Enhancement of Hot Deformed Ndfeb Magnets by Eliminating Coarse Grains

Bone Regeneration Guided by a Magnetized Scaffold in an Ovine Defect Model

The reconstruction of large segmental defects still represents a critical issue in the orthopedic field. The use of functionalized scaffolds able to create a magnetic environment is a fascinating option to guide the onset of regenerative processes. In the present study, a porous hydroxyapatite scaffold, incorporating superparamagnetic Fe3O4 nanoparticles (MNPs), was implanted in a critical bone defect realized in sheep metatarsus. Superparamagnetic nanoparticles functionalized with hyperbranched poly(epsilon-Lysine) peptides and physically complexed with vascular endothelial growth factor (VEGF) where injected in situ to penetrate the magnetic scaffold. The scaffold was fixed with cylindrical permanent NdFeB magnets implanted proximally, and the magnetic forces generated by the magnets enabled the capture of the injected nanoparticles forming a VEGF gradient in its porosity. After 16 weeks, histomorphometric measurements were performed to quantify bone growth and bone-to-implant contact, while the mechanical properties of regenerated bone via an atomic force microscopy (AFM) analysis were investigated. The results showed increased bone regeneration at the magnetized interface; this regeneration was higher in the VEGF-MNP-treated group, while the nanomechanical behavior of the tissue was similar to the pattern of the magnetic field distribution. This new approach provides insights into the ability of magnetic technologies to stimulate bone formation, improving bone/scaffold interaction.

Optical spectroscopy as a tool for studying the solution chemistry of neodymium(III).

In nature, the elements of the inorganic part of the periodic table are found in three forms: metals, ions in salts & minerals, and ions in solution. The ions may be coordinated to simple or complicated ligands. They may form purely electrostatic or partially covalent bonds. A common trend is that the more covalent bonds an element form, the more we know of its physicochemical properties. The rare earths form purely electrostatic bonds, thus, our understanding of the solution chemistry of these elements is limited-yet important. Most rare earth elements used today pass through hydrometallurgical processes that rely on the solution chemistry of these elements, even through the critical applications are in alloys and functional materials. Through developments in optical spectroscopy, total X-ray scattering, and quantum chemical methods we are posed to remedy this situation: we are ready to create predictive structure-property relationships in the field of lanthanide solution chemistry. The scope of this review is to summarise the state-of-the-art for neodymium(III), to go through the structure-property relationships that are in use. In the form of NdFeB magnets, neodymium plays a crucial role in green energy production and electric propulsion. As a 4f3 ion in solution it is also one of the simpler rare earth ions, and the Nd(III) ion has characteristic optical properties that can be exploited as a handle in physicochemical studies. Here, we start with a critical review of the current concepts used to relate structure and electronic energy levels. We follow with our suggested approach of using the methodology from molecular photophysics to relate optical properties and structure, and conclude with selected literature examples.

The influence of Holmium substitution on the magnetic and mechanical properties of Nd2Fe14B: a first principle study

In this study, the ab initio density functional theory (DFT) approach is used to investigate the effect of Holmium (Ho) on the magnetic and mechanical properties of Nd2Fe14B magnets. Neodymium-based permanent magnets (Nd2Fe14B) are the potential permanent magnets to be used in various applications such as wind turbines and electric vehicles. These magnets possess high magnetic field strength and resistance to demagnetization. However, they suffer from low operating temperatures below 585 K. Substitution of Nd with Ho enhances the magnetic strength of the Nd2Fe14B permanent magnet. NdHoFe14B magnets are found to be thermodynamically stable due to negative heats of formation, which is in good agreement with the calculated density of states. Partial substitution of Nd with Ho improves the stability and the magnetic strength of Nd2Fe14B permanent magnets. The findings will provide insight into the future development of permanent magnetic compounds with good magnetic strength.

Microstructure optimization and magnetic properties enhancement of hot pressed Nd-Fe-B magnets via grain boundary diffusion of NdCu powder

Microstructure modification of Nd-Fe-B magnets plays a key role in regulating magnetic properties. In this work, low melting point NdCu alloy was added to improve the coercivity of NdFeB ribbons, and then high coercivity nanocrystalline NdFeB magnets were prepared by hot pressing sintering (HPS). The result shows that the coercivity of NdFeB powders blending with 15 wt% Nd-Cu powders obtained a maximum increment, as high as 12.69 kOe. After HPS, the fully dense magnets with excellent magnetic properties are obtained. Microstructure analysis confirmed that the Nd-Cu alloy has diffused into the interior of hard magnetic grains along the grain boundary during heat treatment. The enrichment of Nd, Pr, and Cu in the grain boundary phase weakens the magnetism of the grain boundary phase and reduces the intergranular exchange coupling. This is the main reason for the coercivity enhancement. In addition, the diffusion of Nd70Cu30 can significantly improve the thermal stability of the magnets.

New Magnetically Assembled Electrode Consisting of Magnetic Activated Carbon Particles and Ti/Sb-SnO2 for a More Flexible and Cost-Effective Electrochemical Oxidation Wastewater Treatment

Magnetic activated carbon particles (Fe3O4/active carbon composites) as auxiliary electrodes (AEs) were fixed on the surface of Ti/Sb-SnO2 foil by a NdFeB magnet to form a new magnetically assembled electrode (MAE). Characterizations including cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy were carried out. The electrochemical oxidation performances of the new MAE towards different simulated wastewaters (azo dye acid red G, phenol, and lignosulfonate) were also studied. Series of the electrochemical properties of MAE were found to be varied with the loading amounts of AEs. The electrochemical area as well as the number of active sites increased significantly with the AEs loading, and the charge transfer was also facilitated by these AEs. Target pollutants’ removal of all simulated wastewaters were found to be enhanced when loading appropriate amounts of AEs. The accumulation of intermediate products was also determined by the AEs loading amount. This new MAE may provide a landscape of a more cost-effective and flexible electrochemical oxidation wastewater treatment (EOWT).