Sintered NdFeB Permanent Magnets Research Articles

Role of vacuum diffusing temperature and time on improving hysteresis characteristics of grain boundary diffused NdFeB permanent magnets by Dy-metal vapor sorption

This study investigates the impact of vacuum grain boundary diffusion (GBD) conditions on sintered NdFeB permanent magnets using Dy-vapor sorption. The study analyzed variables such as vacuum diffusing treatment temperature (T), time (t), and diffusion depth (d). In a 4.5 mm-thick cube-shaped magnet, Dy diffusion increased intrinsic coercivity from 13.13 kOe to 20.56 kOe (56.6 %) with only a 1.5 % reduction in remanence for a modified two-stage grain boundary diffused magnet (940 °C/8 h + 950 °C/0.5 h), achieving commercial G52SH grade. This enhancement exceeds previous GBD studies on non-rare earth compounds or alloys. FE-EPMA analysis attributed the increased coercivity to a continuous Dy-rich region within Nd2Fe14B grains and a thicker core-shell structure forming the (Nd,Dy)2Fe14B phase, which suppresses reversed domain formation. Glow discharge optical emission spectrometer (GDOES) analysis revealed a 2.57 times increase in peak Dy content and a 1.9 times increase in deeper Dy content at 10 μm from the magnet surface compared to the original process. These findings indicate that Dy vapor sorption, combined with a modified two-stage vacuum heat treatment, enhances both the concentration and depth of Dy grain boundary diffusion. Furthermore, the resulting magnet has a clean surface without additional cleaning, beneficial for producing low-cost, high-performance NdFeB magnets.

Increase of coercivity and composition distribution in dysprosium-diffused NdFeB sintered magnets treated by vacuum evaporation method

This study examines the distribution of heavy rare-earth elements (HRE) and magnetic properties in sintered NdFeB permanent magnets treated with Dy vacuum evaporation grain boundary diffusion, considering variations in time and temperature. The HRE distribution and magnetic properties were analyzed using an electron probe microanalyzer (EPMA), glow discharge optical emission spectrometer (GDOES), and hysteresis loop meter. The optimal temperature gradient for achieving the best results was found to be between 850 °C and 950 °C for a duration of 10 h. This resulted in iHc of 1415 kJ/m and (BH)max of 396.2 kJ/m3, which is classified as N52H grade. In comparison, the untreated NdFeB magnets had iHc of 1105 kJ/m and (BH)max of 400.3 kJ/m3, classified as N52 M grade. The results suggest that 0.167 wt% consumption of vacuum evaporated Dy alloy can enhance the 28 % in iHc of the magnet and 5 % increase in (BH)max compared to the untreated ones. This innovative manufacturing method improves the utilization of rare heavy earth and enhances the overall magnetic properties. This material has the potential to be used in the development of low-weight rare earth-sintered NdFeB magnets with high intrinsic coercivity. This study's findings advance energy-efficient technologies such as electric vehicles and offshore wind turbines, aiding global energy conservation efforts.

Fabrication of Epoxy Composite Coatings with Micro-Nano Structure for Corrosion Resistance of Sintered NdFeB

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 (Zf = 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.

Preparation, microstructure and corrosion resistance of Al2O3 coatings by cathodic plasma electrolytic deposition on sintered NdFeB permanent magnet

The Al2O3 ceramic coatings were prepared on the sintered NdFeB magnets by cathodic plasma electrolytic deposition (CPED) method in aluminum nitrate alcohol solution. The surface and cross-sectional morphologies, element and phase composition, corrosion resistance and magnetic properties of the coatings were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermal shock, hydrothermal weight loss, electrochemical corrosion and physical property measurement system (PPMS), respectively. The results show that the coatings mainly consist of γ-Al2O3 and α-Al2O3 phases, and the volume of α-Al2O3 increases with increasing the voltages. The surface of the coatings shows a typical coral reef structure, and the thickness of the coatings increases from 29 µm to 53 µm with increasing the voltages. The Al2O3 coatings have excellent thermal shock resistance as well as the excellent corrosion resistance in hydrothermal conditions. The corrosion resistance of the coatings enhances with increasing the voltages, reaching the strongest at 470 V, and then decreases. This study provides a new method for forming ceramic coatings on the surface of sintered NdFeB magnets to improve its corrosion resistance.

Effect of NdCoGa alloy addition to waste wind turbine magnets to enhance the characteristics of recycled sintered NdFeB permanent magnets

Neodymium (NdFeB) permanent magnets play a key role in motors and other devices, and are important to both defense and civilian industrial uses; however, due to the limited rare earth resources, the issue of recycling waste NdFeB magnets to achieve sustainable development has attracted much attention. An innovative and valid method for the recycling of waste NdFeB magnets is demonstrated in this study. Recycled sintered NdFeB permanent magnets were prepared on a large scale up to 100 kg per batch by adding NdCoGa ternary rare earth alloy to waste wind turbine NdFeB magnets, and their magnetic properties, corrosion resistance and high temperature aging characteristics were evaluated. The recycled magnets were manufactured by mixing waste magnets A, B, and NdCoGa alloy according to weight percentage ratio of 90:7:3 respectively. Through BH Curve measurement and FE-EPMA qualitative element analysis, the differences in the magnetic properties and microstructure before and after adding the NdCoGa elements were observed. The results show that the magnetic properties of waste magnet A are Br = 12.60 kG, iHc = 18.92 kOe and (BH)max = 39.19 MGOe; the magnetic properties of waste magnet B are Br = 12.77 kG, iHc = 18.15 kOe and (BH)max = 40.25 MGOe. In contrast, the magnetic properties of the recycled magnets prepared in this study are Br = 12.63 kG, iHc = 18.72 kOe, and (BH)max = 39.95 MGOe, indicating that the proposed process restores the magnetic properties of the recycled magnets to the level of the original waste magnet. Also, a brand new commercial N40H magnet is further used as a control group, where the corrosion resistance and high temperature aging characteristics of bare materials were evaluated by Pressure Cooker Test (PCT), and oven/flux meter respectively. The results show that the average corrosion weights per unit area are 0.89 mg/cm2 and 1.23 mg/cm2, as well as the demagnetization rate of the high temperature aging is less than 0.4%. Compared with the new magnet, the recycled magnet has the same level of high temperature aging resistance, and 28% higher corrosion resistance. As a result, adding only 3% NdCoGa alloy to the waste magnet is beneficial to restore the magnetic properties and high temperature aging resistance, up to the level of new magnets, and also greatly enhances its corrosion resistance and reduces the cost of magnet remanufacturing. This result indicates that the recycled NdFeB permanent magnets developed in this research are more suitable for offshore wind turbine generators and electric vehicle motors operating at high temperatures and high humidity for a long period of time.

Brazing and magnetic properties control of sintered NdFeB permanent magnet with high coercivity

This article presents an application of vacuum brazing to join sintered NdFeB permanent magnet N33UH with high coercivity. The magnets were effectively brazed with BCu91AgP Cu-based brazing filler metal at 680°C. The shear strength of the joint was up to 113.19 MPa. Meanwhile, the principal magnetic properties decreased. Subsequently heat treatment for 3 hours at 460-550°C could notably improve magnetic properties. This increase in magnetic properties was mainly attributed to the formation of continuous thin Nd-rich grain boundary phases between the main phases. Heat treated at 520°C, Br, HcB, HcJ, and (BH)max reached 95.4%, 95.9%, 99.8% and 91.4% of as-received magnet, respectively.

Misorientations across boundary planes in a sintered NdFeB permanent magnet.

From the perspective of crystallography, there exist crystals as well as boundary planes in NdFeB permanent magnets and accordingly, there should exist orientation textures of Nd2Fe14B crystals and orientation texture of boundary planes. This work therefore aims to study the misorientations across boundary planes in a sintered NdFeB permanent magnet. As an interdisciplinary study, serial stereological methods are developed to extract more structural information from the material and as the result, spreading wetting and penetration of Nd-Rich phases to the Nd2Fe14B phase are quantificationally characterized. The entire boundaries are sorted into Nd2Fe14B/Nd2Fe14B grain boundaries (N/N boundaries) and Nd2Fe14B/(thin-layer-like)Nd-Rich phase boundaries (N/R boundaries). 31°/[0 0 1] and 60°/[0 0 1] twist boundaries are preferred among N/N boundaries, while 31°/[0 0 1] and 72°/[0 0 1] misorientations are preferred among N/R boundaries. The structural features of these misorientations are elaborated via the Five Parameter Analysis (FPA) method. The potential correlations between the grain boundary plane distributions (GBPDs) and magnetic properties are then discussed regarding how the anisotropic features at boundary locations impact coercivity, domain and remanence.

Joining of sintered NdFeB permanent magnet to steel using novel Zn-Sn-Bi-Nd alloy: Microstructure and soldering mechanism

The steel and sintered NdFeB permanent magnet were successfully connected by a novel Zn-Sn-Bi-Nd alloy. The effects of the soldering temperature (410–450 ° C) and holding time (20−40 s) on the microstructure and mechanical properties of soldered joints were investigated. The results showed that element Zn diffused to the surface of sintered NdFeB permanent magnet, with the formation of ξ -FeZn13/NdFe5.5Zn(B) interfacial layer. Meanwhile, the multilayered Γ-Fe3Zn10/δ-FeZn10/ ξ -FeZn13 phases were observed at the steel side. As prolonging holding time or increasing soldering temperature, the reaction layer grew thicker. The shear strength increased first and then decreased. Therefore, the maximum shear strength (110.7 MPa) was obtained at 430°C for 30 s. Proper reaction layer thickness is beneficial to the improvement of shear strength.

Improved microstructure and mechanical properties for sintered NdFeB permanent magnet/steel soldered joints by Nd addition

Abstract Sintered NdFeB permanent magnets with excellent magnetic properties are widely used in electromagnetic devices. The effect of Nd addition on microstructure and mechanical properties of sintered NdFeB permanent magnet/steel soldered joints was investigated by computational thermodynamics approach and coefficient of thermal expansion. The formation of NdSn3 after doping minor Nd into Zn-Sn-Bi solder improved the mechanical properties of soldered joints. On the one hand, NdSn3, as a low Gibbs free energy phase, promoted the heterogeneous nucleation of the β -Sn, resulting in the grain refinement. On the other hand, NdSn3 with lower coefficient of thermal expansion was beneficial for relaxing the residual stress.

Characteristics of a Mechanical Circuit Breaker with New Induction Needle and Magnets Type to Extinguish a DC Arc

Permanent magnets are currently attracting much attention as they are used in various application devices requiring high magnetic force. There are many types of permanent magnets with different properties, and they need to be tested for suitability for the applied devices. Sintered NdFeB permanent magnets have a large coercive force and are used in a variety of application systems. In particular, they are being used in circuit breakers to protect the DC transmission and distribution systems, which are actively being developed now. DC maintains a constant current value because it does not have a frequency. Thus, it is very difficult to interrupt large transient currents when an accident occurs. Therefore, a new type of induction needle and a permanent magnet near a mechanical break contact were combined. This induction needle is serially connected to a ground wire. This new breaking technique extinguishes the arch that occurs between the contacts by pushing it to the ground using an induction needle. The permanent magnet applied here, however, must be strong against a high-temperature arc that occurs quickly, and must generate a high magnetic force. Simulations were conducted with FEM (finite element method) and TDM (time difference method) using Maxwell, an electronic analysis program, and a permanent magnet that is appropriate for use as a component of a circuit breaker was selected. Consequently, it was found that the use conditions of permanent magnets varied by the arch generated in the mechanical circuit breaker.

Analytical Model and Analysis of Spiral-Coil EMAT Above a Ferromagnetic Plate

To further study the dominant mechanism, this article utilizes the analytical model to analyze the spiral-coil electromagnetic acoustic transducer (EMAT) above a ferromagnetic plate. The analytical model includes the spiral coil as well as the sintered NdFeB (S-NdFeB) permanent magnet (PM) whose analytical solutions inside the ferromagnetic material are rarely presented relative to the coil. Because of this point, this article presents the analytical model for S-NdFeB PM by the equivalent ampere current to compute the magnetic-flux density and compare the computational results with that by finite-element method (FEM) and measurement. The comparison shows that the analytical model is valid to analyze the spiral-coil EMATs. Based on the analytical model, both Lorentz force and magnetostrictive force are computed and compared, as well. Finally, it is found that the Lorentz force is a dominant mechanism in spiral-coil EMATs, which is verified by the experimental result.

Effect of Bi addition on the corrosion resistance and mechanical properties of sintered NdFeB permanent magnet/steel soldered joints

In this study, novel Zn-Sn-xBi high-temperature solders were designed to join sintered NdFeB permanent magnets and steel. The effect of Bi addition on the corrosion resistance and mechanical properties of the soldered joints was systematically investigated. The results indicated that introducing Bi with specific content obviously decreased the undercooling of the solder. Reducing the undercooling by promoting the solidification process and the formation of Bi precipitates as nucleation sites could refine the microstructure. The improvement of corrosion resistance was attributed to the finer microstructure and the Bi precipitates serving as anodic barriers. Moreover, the sound soldered joints were obtained with finer β-Sn dendrites and a more uniform distribution of Bi precipitates when appropriate Bi was added. The shear strength of the soldered joint with 5 wt% Bi addition was 36.6% higher than the shear strength of the ZS soldered joint.

Microstructure and Interfacial Evolution of Sintered NdFeB Permanent Magnet/Steel Joint Soldered with Zn-Sn Alloy

Sintered NdFeB permanent magnets and steel were soldered successfully using Zn-Sn alloy. The effects of Sn content on the microstructure and mechanical properties of soldered joints were investigated. The results showed that the typical interfacial microstructure represented NdFeB/NdFe5.5Zn(B) + ξ-FeZn13/Zn + β-Sn/ξ-FeZn13 + δ-FeZn10 + Γ-Fe3Zn10/steel. With the addition of Sn, the thickness of the NdFe5.5Zn(B) phase increased, while more β-Sn was observed in the soldering seam. The maximum shear strength reached 45.3 MPa with a Sn proportion of 6 wt.%, and excessive growth of the NdFe5.5Zn(B) phase in the joints deteriorated the mechanical properties. The fracture morphology exhibited a ductile-to-brittle transition with the occurrence of interfacial fractures due to thick intermetallic compounds.

Calculation of the Remnant Magnetization and Magnetic Saturation Characteristics for Sintered NdFeB Permanent Magnets Utilizing Finite Element Transient Simulations

This article presents the numerical calculation of the remnant magnetization and magnetic saturation characteristics for sintered neodymium-iron-boron (NdFeB) permanent magnets (PMs) utilizing 3-D finite element (FE) transient simulations. The nonlinear anisotropic properties of the sintered NdFeB PMs are employed to model a sintered NdFeB cylinder magnet. In addition, a magnetizing solenoid is coupled with an external equivalent current pulse circuit to analyze the magnetization process of the sintered NdFeB magnet. The remnant magnetization flux of the sintered NdFeB PM is compared and validated with numerical results and special laboratory magnet measurements published in the literature. On the other hand, a magnet saturation study is carried out; applying different saturation flux densities to the PM and the remnant magnetization flux is computed for each case. The saturation curve is computed, and the minimum saturation flux density is obtained to fully saturate the sintered NdFeB PMs. Finally, some virgin and demagnetized sintered NdFeB PMs are magnetized and saturated in the laboratory, and the minimum saturation flux density and the saturation curve are measured and compared with the FE simulation results. The results presented in this article show that the sintered NdFeB PMs are fully magnetized if a minimum saturation flux density of 3 T is applied during the magnetization process. The FE methodology presented in this article can be utilized by magnetizing fixture manufacturers and PM manufacturers to design efficient magnetizing fixtures and to predict the correct magnetization and saturation of different commercial grades of the sintered NdFeB PMs or other PM materials utilized in the magnetic industry and in special applications as PM motors.

Anisotropic Distribution of Phase Boundaries and its Potential Correlation with Magnetic Properties in a Sintered NdFeB Permanent Magnet

This work reports the grain boundary character distribution (GBCD) on a statistical meaning of phase boundaries that are between Nd2Fe14B and thin‐layer‐like Nd‐rich phases in a sintered NdFeB magnet. In the low‐textured sample, the phase boundaries are divided into preferred phase boundaries (with a preferred misorientation of 31°/[001] across the boundary plane) and random phase boundaries. By the five parameter analysis (FPA) method, the orientations of phase boundaries as well as the subsets are textured. The correlation between the GBCDs and magnetic properties is suggested via the illustrated anisotropic features at phase boundary positions.

Grain boundary diffusion treatment of sintered NdFeB magnets by low cost La-Al-Cu alloys with various Al/Cu ratios

Abstract Different from many previous reports, La-Al-Cu alloys with different Al/Cu ratios containing only high abundant rare earth element La were employed for grain boundary diffusion treatment for sintered NdFeB permanent magnets. It is found that Al and Cu contents should be optimized to achieve coercivity enhancement. The coercivity of the magnet was enhanced from 1000 to 1006, 1077 and 1156 kA/m after La70Al10Cu20, La70Al15Cu15 and La70Al20Cu10 alloys diffusion, at 800 °C for 1 h followed by 500 °C for 3 h. The coercivity stability was also improved, indicated by the enhanced temperature coefficient of β from −0.63 to −0.61, −0.61 and −0.60%/K. Microstructure investigation indicates that the formation of continuous grain boundary phase after diffusion reduces the magnetic exchange interaction and contributes to the enhanced coercivity. After diffusion, the La2O3 phase was observed in both the triple junction and strip grain boundary. La, Al and Cu elements showed different distributions in the magnets. The diffusion coefficients obtained from Fick’s second law indicate that La diffuses faster than Al and Cu at 800 °C, which explains the various effects of the diffusion sources and different coercivity enhancements by the alloys with different Al/Cu ratios. Although the process should be further optimized, the current results demonstrate that the grain boundary diffusion using low cost La-Al-Cu alloys can improve the microstructure and enhance the coercivity of NdFeB magnets.

Effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets

Abstract This paper investigates the effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets. The microstructure of the NdFeB permanent magnets was characterised using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis. 0.1 wt% of Ag was added to the overall NdFeB composition to study the effects of Ag on the microstructural changes. The NdFeB and NdFeB + Ag magnets exhibit a two-phase structure with the main Nd2Fe14B phase and the Nd-rich grain boundary phase. It is found that Ag is enriched in the Nd-rich grain boundary phase and tends to alloy with the minor element, Cu. An enhanced coercivity (Hcj) is obtained in the NdFeB + Ag whilst the remanence (Br) and maximum magnetic energy product ((BH)max) decrease due to the addition of 0.1 wt% Ag. Impact resistance tests were carried out to examine the impact toughness of NdFeB and NdFeB + Ag magnets. It is shown that the magnets still exhibit a brittle fracture behaviour and no significant increase in the impact resistance of the NdFeB + Ag magnet is presented.

Fabrication and Properties of Micro-arc Oxidation Coatings on Sintered NdFeB Permanent Magnets

Abstract In order to improve the corrosion resistance of the sintered NdFeB permanent magnets, Al2O3 ceramic coatings were prepared by a two-step micro-arc oxidation process in an aluminate solution. During the micro-arc oxidation process, the voltage-time curve could be divided into four regions consistent with the valve metals. Results show that the coating exhibits a typical micro-arc oxidation porous surface with the thickness of ∼5 μm, and is only composed of Al2O3 crystalline phase with some Fe elements and a few Nd and P elements. The surface roughness of coated NdFeB magnets is higher than that of uncoated sample, while the corrosion resistance is enhanced by one order of magnitude. Unfortunately, the magnetic properties (remanence and maximum energy product) of coated NdFeB sample decrease in different degrees.

Progress of sintered NdFeB permanent magnets by the diffusion of non-rare earth elements and their alloy compounds

Progress of sintered NdFeB permanent magnets by the diffusion of non-rare earth elements and their alloy compounds

Retracted: Morphology and electrochemical comparison of ceramic and composite anti‐corrosion barriers for sintered NdFeB permanent magnets

AbstractThis article has been retracted due to a substantial overlap with a previously published article in another journal.