Nd-rich Phase Research Articles

Micro-galvanic corrosion behaviour of Mg−(7,9)Al−1Fe−xNd alloys

The localized micro-galvanic corrosion process and the kinetic information of Mg−(7,9)Al−1Fe−xNd alloys were investigated by in situ observation under electrochemical control and in situ atomic force microscopy (AFM) in an electrolyte environment. The results revealed that the formation of the Nd-rich phase in alloys resulted in a decrease in the Volta potential difference from ~400 mV (AlFe3/α-Mg) to ~220 mV (Nd-rich/α-Mg), reducing the corrosion products around the cathodic phase and corrosion current density of the microscale area. The addition of Nd significantly improved the corrosion resistance, mainly due to the suppression of the micro-galvanic corrosion between the second phases and substrate. Finally, the corrosion mechanism of Mg−(7,9)Al−1Fe−xNd alloys was discussed based on in situ observations and electrochemical results.

Influence of dysprosium addition on corrosion behavior of NdFeB magnets

Purpose This paper aims to investigate the corrosion kinetics and corrosion behavior of NdFeB magnets with the addition of heavy rare earth dysprosium (Dy) for its inhibitory activity on poor corrosion resistance of NdFeB magnets. Design/methodology/approach To study the effect of dysprosium addition on corrosion behavior of NdFeB magnets and investigate its mechanism, potentiodynamic polarization, scanning electron microscopy (SEM), electrochemical impedance, energy dispersion spectrum (EDS) and scanning Kelvin probe force microscopy (SKPFM) were applied in the research. Besides, microstructures were observed by SEM equipped with EDS. Atomic force microscopy was introduced to analyze the morphology, potential image as well as the contact potential difference. The SKPFM mapping scan was applied to obtain the contact potential around Nd-rich phase at 0.1 Hz. The magnets were detected via X-ray diffraction. Findings Substitution of Nd with Dy led to improvement of corrosion resistance and reduced the potential difference between matrix and Nd-rich phase. Corrosion resistance is Nd-rich phase < the void < metal matrix; maximum potential difference between matrix and Nd-rich phase of Dy = 0, Dy = 3 and Dy = 6 Wt.% is 411.3, 279.4 and 255.8 mV, respectively. The corrosion rate of NdFeB magnet with 6 Wt.% Dy is about 67% of that without Dy at steady corrosion stage. The addition of Dy markedly enhanced the corrosion resistance of NdFeB magnets. Originality/value This research innovatively investigates the effect of adding heavy rare earth Dy to NdFeB permanent magnets on magnetic properties, as well as their effects on microstructure, phase structure and most importantly on corrosion resistance. Most scholars are studying the effect of element addition on magnetic properties but not on corrosion resistance. This paper creatively fills this research gap. NdFeB magnets are applied in smart cars, robotics, AI intelligence, etc. The in-depth research on corrosion resistance by adding heavy rare earths has made significant and outstanding contributions to promoting the rapid development of the rare earth industry.

Nitrogenation of Nd(Fe,Mo)12 powders for sintered magnets

The intermetallic Rare Earth-Fe12 (REFe12) compounds are considered as the strongest candidates for alternative high-performance permanent magnet material, thanks to the combination of high magnetocrystalline anisotropy, high Curie temperature and moderate saturation magnetization. However, the NdFe12 based compounds suffer from a weak uniaxial magnetocristalline anisotropy at room temperature. The insertion of light elements, like nitrogen, induces a strong uniaxial anisotropy and an increase of the Curie temperature as well. Nevertheless, the nitrogenation of these compounds have proved to be tricky, as the reaction kinetics is very slow at moderate temperatures, whereas the decomposition of the 1–12 phase occurs at higher temperatures. In the present study the aim is to develop nitrogenated powders suitable for the manufacture of Nd(Fe,Mo)12N-based sintered magnets, starting from Nd(Fe,Mo)12 precursors prepared by strip-casting. The nitrogenation of Nd(Fe,Mo)12 powders was undertaken to analyze the mechanisms of reaction and to determine the experimental conditions allowing to avoid both the formation of α-Fe and the nitrogenation of the Nd-rich intergranular phase. Structural and magnetic investigations are performed on the parent alloys and the nitrogenated powders, allowing to identify the mechanism of the solid-gas reaction and to determine, for the first time to our knowledge, the nitrogen thermodynamic equilibrium pressure between the 1–12 phase and its nitride.

Microstructural Investigation of Discarded NdFeB Magnets After Low-Temperature Hydrogenation

Due to continuously increasing demand and limited resources of rare-earth elements (REEs), new solutions are being sought to overcome the supply risk of REEs. To mitigate the supply risk of REEs, much attention has recently been paid to recycling. Despite the more common recycling methods, including hydrometallurgical and pyrometallurgical processes, hydrogen processing of magnetic scrap (HPMS) is still in the development stage. Magnet-to-magnet recycling via hydrogenation of discarded NdFeB magnets provides a fine powder suitable for the production of new magnets from secondary sources. One of the crucial aspects of HPMS is the degree of recovery of the magnetic properties, as the yield efficiency can easily reach over 95%. The amount, morphology, and distribution of the Nd-rich phase are the key parameters to achieve the excellent performance of the magnet by isolating the matrix grain. Therefore, a better insight into the microstructure of the matrix grains and the Nd-rich phase before and after hydrogenation is essential. In this study, a low-temperature hydrogenation process in the range of room temperature to 400 °C was conducted as the first step to recycle NdFeB magnets from discarded hard disk drives (HDDs), and the hydrogenated powder was characterized by electron microscopy and X-ray diffraction. The results show that there are three different morphologies of the Nd-rich phase, which undergo two different transformations through oxidation and hydride formation. While at lower temperatures (below 250 °C) the degree of pulverization is higher and the experimental evidence of hydride formation is less clear, at higher temperatures the degree of pulverization decreases. The formation of neodymium hydride at higher temperatures prevents further oxidation of the Nd-rich phase due to its high stability.Graphical

Exploring the influence of anisotropy and microstructure of the sintered Nd-Fe-B magnet using nanoindentation and scratching tests

This study evaluated the influence of magnetic anisotropy and microstructure on the mechanical properties and material removal mechanism of anisotropic sintered Nd-Fe-B magnet. Nanoindentation tests were performed to measure the hardness (H), elastic (Es) and shear (G) modulus on two faces and main phases of the Nd-Fe-B material. To understand the contribution of the Nd2Fe14B1 and Nd-rich phases on material removal mechanism, scratching tests were also employed in the present study. Results showed that the Nd-Fe-B exhibits magnetic anisotropy with a high degree of uniaxial texture. Fracture analysis on the parallel and perpendicular faces related to the c-axis revealed no difference in fracture mechanisms. The load-displacement curves indicated that there is similar elastoplastic deformations induced by the penetration of indenter on the analyzed faces. Although Nd-Fe-B exhibits high magnetic anisotropy, the mechanical properties obtained from the two faces studied had not statistic difference. Nd-Fe-B is composed of hard and soft phases, where the Nd2Fe14B1 grain exhibited H = 12.5 ± 0.5 GPa, Es = 191.6 ± 12.6 and G = 77.3 ± 5.1 GPa, whereas the Nd-rich phase had values of H = 6.5 ± 0.7 GPa, Es = 124.0 ± 12.3 GPa and G = 50.0 ± 4.8 GPa. The Nd2Fe14B1 and Nd-rich phases strongly influenced scratch groove formation, being observed intergranular microcrack, replacement of Nd2Fe14B1 grain and bulging of Nd-rich grain boundaries. The mechanical properties of the Nd-Fe-B magnet were not affected by magnetic anisotropy or the high degree of uniaxial texture. However, the Nd2Fe14B1 grain presents higher hardness, elastic and shear modulus than the Nd-rich phase. Such characteristics can affect the machined surface formation when employing abrasive machining processes.

Enhanced Nd-Rich Phase and Oxygen Control in Reduction-Diffusion Powders via Tailored Washing Process for Additive-Free Sintering of Fine Nd-Fe-B Particles

Enhanced Nd-Rich Phase and Oxygen Control in Reduction-Diffusion Powders via Tailored Washing Process for Additive-Free Sintering of Fine Nd-Fe-B Particles

Micromagnetic simulations of Nd-Fe-B hot-deformed magnets subjected to eutectic grain boundary diffusion process

The grain boundary diffusion process (GBDP) is one of the most efficient treatments for increasing the coercivity (Hc) of Nd-Fe-B magnets. However, this enhancement typically occurs at the expense of remanence (Mr). In this study, micromagnetic simulations were performed to quantify this tradeoff in hot-deformed Nd-Fe-B magnets subjected to the GBDP using a Nd-based eutectic alloy. The GBDP was imitated in a series of models with a gradually increasing volume fraction of the infiltrated Nd-rich nonmagnetic phase. This infiltration reduced the remanence and grain connectivity via the remaining thin magnetic intergranular phase (IGP), which in turn increased the coercivity. We distinguished between the roles of exchange and magnetostatic interactions in this coercivity enhancement. Furthermore, the simulated Mr vs. Hc curves defined realistic limits for coercivity that depended on the IGP magnetization, which was estimated to be 0.9 ± 0.1 T by reproducing experimental Mr vs. Hc data from the literature.

Utility and influence mechanism of densification modulation on grain boundary diffusion in NdFeB magnets

Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets. This study investigates the factors that affect the efficiency of grain boundary diffusion, starting from the properties of the diffusion matrix. Through the adjustment of the sintering process, we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbHx diffusion. The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets. According to the experimental results, the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets. The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets. Additionally, grain boundary diffusion notably enhances the density of initial low-density magnets, addressing the issue of low density during the sintering stage. Before the diffusion treatment, the Nd-rich phases primarily concentrate at the triangular grain boundaries, resulting in an increased number of cavity defects in the magnets. These cavity defects contain atoms in a higher energy state, making them more prone to transition. Consequently, the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy, accelerating atom diffusion. The presence of larger cavities also provides more space for atom migration, thereby promoting the diffusion process. After the diffusion treatment, the proportion of bulk Nd-rich phases significantly decreases, and they infiltrate between the grains to fill the cavity defects, forming continuous fine grain boundaries. Based on these observations, the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.

Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd–Fe–B HDDR magnets

Hydrogen–disproportionation–desorption–recombination (HDDR) Nd–Fe–B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m−3 was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m−1 could be obtained followed by a grain boundary diffusion of Pr40Tb30Al20Cu10. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd–Fe–B HDDR magnets with optimized performance.

Coercivity saturation in Nd-Fe-B sintered magnets treated by grain boundary diffusion process of Tb-Ni-Al alloy

Microstructures of Nd-Fe-B sintered magnets treated by a grain boundary diffusion (GBD) process with different Tb coating amounts are investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Tb-substituted shell structures are formed in the outer portion of all Nd2Fe14B grains in the magnets. However, thick Tb-rich shells are formed on a micrometer scale on Nd2Fe14B grains in the region up to about 0.5 mm from the coating surface of the magnet, and thin Tb-rich shells several tens of nanometers in size are also formed in the inner region. As the Tb coating amount increases, the Tb concentration and thickness of the thick Tb-rich shells increase. At the same time, the Nd replaced by Tb forms Nd-rich grain boundary (GB) phases, and a large amount of those Nd-rich GB phases are discharged to the coating surface. The coercivity enhancement in GBD is mostly due to the formation of the thin Tb-rich shells, whereas the formation of thick Tb-rich shells wastes a large amount of Tb, causing coercivity saturation. Therefore, it is important to suppress the formation of thick Tb-rich shells and form thin Tb-rich shells with a high Tb concentration uniformly throughout the magnets.

Microstructure of Nd2Fe17N3 Magnetic Powder Surface With Nano-α-Fe Phases Separated Surface Layer

We investigated the application of Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> magnetic powder with planar magnetic anisotropy to magnetic field amplification materials at several tens of MHz. Consequently, we succeeded in developing a magnetic property with real permeability ~ 8 and imaginary permeability below 0.8 at 100 MHz using a phosphoric acid coating and a simple heat treatment process in air. In this study, microstructural analysis conducted using transmission electron microscopy and X-ray diffraction revealed that the coating layer became thicker, and the Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> surface was phase-separated into nano-sized α-Fe phases and Nd-rich phases, either oxide, oxynitride, or nitride. Additionally, the nano-sized α-Fe phases and the nano-sized Nd-rich phases were oriented over a wide area spanning several hundred nanometers. We concluded that these characteristic structures have led to improved properties.

Effect of Nb on microstructure and magnetic decay of sintered NdFeB magnets in NaCl solution

Herein, the influence of Nb on the microstructure and magnetic decay of sintered NdFeB magnets was investigated. The NbFeB and Fe2Nb phases were determined through field emission scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, and selected area electron diffraction. The effects of Nb on the crystal structure of the magnet were studied via electron backscatter diffraction analysis. Laser scanning confocal microscopy, a pulsed field magnetometer, and a fluxmeter were used to measure the changes in the surface roughness and magnetic properties of the magnet during corrosion. Based on the experimental result, Nb can optimize the distribution of the Nd-rich phase inside the magnet, reduce the generation of crystal defects, refine the grains, and promote the crystal orientation along the [001] direction. During magnet corrosion, the Nb compound captured hydrogen atoms, causing internal lattice distortion and stress concentration in the matrix phase thereby leading to crush. The broken matrix phase adhered to the magnet surface, isolating the magnet from contact with the corrosive solution and improving the corrosion resistance of the magnet. Moreover, Nb can reduce the impact of the surface structure damage on the demagnetization of the magnet during corrosion, thereby lowering magnetic decay.

Understanding the role of Cu element in nanocrystalline hot deformed Nd-Fe-B magnets

Alloying with Cu element has long been established as a means of improving the performance of sintered NdFeB magnets, but its effect on nanocrystalline hot-deformed magnets has not been thoroughly discussed yet. In this study, we conducted a systematic comparison of the microstructure and magnetic properties of nanocrystalline Nd14Fe80B6 and Nd14Fe79B6Cu magnets. The results showed that the introduction of Cu significantly enhances the coercivity of both melt-spun and hot-deformed Nd-Fe-B magnets, owing to the improved magnetically decoupling between neighboring Nd2Fe14B matrix grains by the enrichment of Cu at Nd-rich grain boundary (GB) layer. Despite the magnetic dilution resulting from the introduction of non-magnetic Cu, a significant improvement of remanence from 11.0 kGs to 12.2 kGs is observed in the hot-deformed Nd-Fe-B magnet with 1 at.% Cu addition. The enhancement is attributed to the improved orientation of the Cu-added magnet, as evidenced by the preferential grain alignment from microstructural characterization. The addition of trace Cu changes the properties of the Nd-rich GB phase, lowering its melting point and improving the hot-workability of the Nd-Fe-B magnet during the hot-deformation process. Overall, this study clarifies the positive role of Cu in nanocrystalline Nd-Fe-B magnets and highlights its importance for grain alignment during hot-deformation processing.

Effect of non-rare-earth element enrichment on in-the-column secondary electron image contrast of Nd-rich phases in Nd-Fe-B sintered magnets

In-the-column secondary electron (SE) imaging has demonstrated an important role in identifying various Nd-rich phases in Nd-Fe-B sintered magnets at a microscale field of view. It is well known that modifying the microstructure of Nd-Fe-B sintered magnets by adding non-rear-earth (NRE) elements dissolved into the intergranular Nd-rich phases during post-sinter annealing can effectively increase the magnet coercivity. However, the effect of NRE element enrichment on in-the-column SE image contrast of Nd-rich phases has not received much attention. In this study, we have combined a focused ion/electron dual beam system (FIB/SEM) and a scanning/transmission electron microscope (S/TEM) to investigate the in-the-column SE image contrast, structure, and element distribution of intergranular Nd-rich phases in commercial heavy-rare-earth (HRE) free Nd-Fe-B sintered magnets. In-lens SE imaging revealed that Cu, Co, or Al-enriched neodymium oxides with fcc, Ia3¯-type, or amorphous structures are brighter than the Nd2Fe14B matrix at an accelerating voltage of 2 kV and a working distance of 4 mm. Bright stripes inside the intergranular fcc NdOx were also visible in high-resolution through-the-column SE imaging, reflecting the nanoscale composition variation with Co or Cu enrichment. Contrarily, at the same imaging conditions, the nearly pure fcc NdOx and Ia3¯-Nd2O3 exhibit darker than the Nd2Fe14B matrix in the in-lens SE image. It has been demonstrated that the in-lens SE image contrast of Nd-rich phases with respect to the Nd2Fe14B matrix remains constant by systematically adjusting the accelerating voltage from 2 kV to 5 kV and the working distance from 4 mm to 6 mm. Our research results suggest that low-voltage in-the-column SE imaging could be used to track element diffusion in triple junction and grain boundary Nd-rich phases of Nd-Fe-B sintered magnets during post-sinter annealing by in-situ MEMS heating in SEM.

Origin of the quasi-periodic coarse grain regions in hot-deformed Nd-Fe-B magnets

Quasi-periodic coarse grain regions (CGRs) are inevitable in hot-deformed Nd-Fe-B magnets, which are extremely harmful to magnetic properties, especially coercivity. However, the reasons for their formation are not yet fully understood. In this work, we systematically study grain growth behaviors in different regions of nanocrystalline and amorphous ribbons during the heat treatment and try to figure out the relationship between the grain growth and the CGRs of hot-deformed (HDed) magnets. The results indicates that coarse grains (CGs) strongly depend on the temperature and tend to form on the wheel-contacted side (WS) in the two kinds of melt-spun ribbons at the high temperature. However, the formation mechanisms of CGs share obvious differences. The CGs in nanocrystalline ribbons originate from the abnormal growth of fine grains on WS due to the high surface energy and abundant Nd-rich phases. As comparison, the CGs in amorphous ribbons result from the high temperature growth of large-sized grains caused by the low nucleation rate and high growth rate on the WS. Careful microstructural characterization of the HDed magnets reveal that the CGs are also located at the WS of the initial ribbons and the formation mechanisms of the CGs in HDed magnets follows the nanocrystalline and amorphous ribbons. This work provides theoretical guidance for the suppression of CGRs in HDed magnets.

Formation and magnetic enhancement mechanism of sandwich-structure grain boundary phase in Ce magnets

In this study, the formation mechanism of sandwich-structure grain boundary phase in sintered Ce magnets with various Pr-Nd-Al alloy additions is investigated in detail. The sandwich-structure grain boundary between the main phases is composed of RE6(Fe, TM)11Al3 phase at the center and symmetrical Nd-rich phase at both sides. Based on the analysis of phase diagrams, the formation mechanism is revealed from the following aspects: i) The amorphous formation ability of Nd-rich liquid phase in Ce magnet during sintering can be enhanced by introducing Pr-Nd-Al alloy; ii) The RE6(Fe, TM)11Al3 phase preferentially nucleates and grows during the process of cooling; iii) The interfacial tension among Nd-rich liquid phase, main phase and RE6(Fe, TM)11Al3 phase contributes to the formation of the sandwich-structure grain boundary. With the addition of Pr-Nd-Al alloy, the activity of Al entering the main phase is inhibited, resulting in almost no reduction of remanence and Curie temperature. The structural stability and decoupling effect of the sandwich-structure grain boundary phase are more conducive to the improvement of the coercivity and thermal stability of the Ce magnets. The coercivity of Ce magnet changes from 12.7 to 14.93 kOe (the increase extent is 17.6%). In the temperature range 20–100 ℃, the coercivity temperature coefficient varies from -0.767 to -0.680 %/℃ and the temperature stability obviously ameliorates about 11.3%.

Grain boundary diffusion efficiency of sintered Nd-Fe-B magnets by Al, Sn and Zn addition

The composition and structure of sintered Nd-Fe-B magnets influence the depth and efficiency of grain boundary diffusion. In this paper, the submicron Al, Sn, Zn metal powders are introduced in the precursor powder to improve the infiltration effect. The transformation temperature of the Nd-rich phase decreases from 471 °C for the standard alloy to 436 °C, 442 °C and 443 °C for Al, Sn and Zn elements addition, respectively. The addition of Al, Sn and Zn improves the microstructure and promotes the diffusion of Tb into the 2:14:1 phase. The addition of these low melting point elements effectively promotes the diffusion depth and the total amount of Tb element at the early stage of the diffusion. The intrinsic coercivity of Al-, Sn- and Zn-added magnets increased by 8.86 kOe, 9.70 kOe and 8.98 kOe, respectively. These results provide a basis for the preparation of high coercivity Nd-Fe-B magnets using less heavy rare earth elements.

Environmentally Friendly Approach for Nd2Fe14B Magnetic Phase Extraction by Selective Chemical Leaching: A Proof-of-Concept Study.

The green transition initiative has exposed the importance of effective recycling of Nd-Fe-B magnets for achieving sustainability and foreign independence. In this study, we considered strip-cast, hydrogenated, jet-milled Nd-Fe-B powder as a case study to explore the potential for selective chemical leaching of the Nd-rich phase, aiming to extract the Nd2Fe14B matrix phase. Diluted citric and nitric acids at concentrations of 0.01, 0.1, and 1 M were considered potential leaching mediums, and the leaching time was 15 min. Microstructural investigation, magnetic characterization, and elemental compositional analysis were performed to investigate leaching efficiency and selectivity. Based on SEM analysis, Nd/Fe ratio monitoring via ICP-MS, and the high moment/mass value at 160 emu/g for the sample leached with 1 M citric acid, 1 M citric acid proved highly selective toward the Nd-rich phase. Exposure to nitric acid resulted in a structurally damaged Nd2Fe14B matrix phase and severely diminished moment/mass value at 96.2 emu/g, thus making the nitric acid unsuitable for selective leaching. The presence of hydrogen introduced into the material via the hydrogen decrepitation process did not notably influence the leaching dynamics. The proposed leaching process based on mild organic acids is environmentally friendly and can be scaled up and adopted for reprocessing industrial scrap or end-of-life Nd-Fe-B magnets to obtain single-phase Nd-Fe-B powders that can be used for novel magnet-making.

Simultaneous enhancement of magnetic properties and electric resistance of hot-deformed Nd-Fe-B magnets by doping insulating nano-diamonds

The combination of high coercivity and resistivity is a strong requirement for permanent magnets to resist thermal demagnetization under complex operating conditions. However, this still remains a challenge for conventional strategies, which always sacrifice the maximum energy product as the coercivity or resistivity increases. In this work, we report a unique strategy of introducing only 0.1 wt% of insulating nano-diamonds (NDs) into the hot-deformed Nd-Fe-B magnets to significantly and simultaneously improve both magnetic properties (Hci, Br, (BH)max and Hk/Hcj) and resistivity. In particular, the coercivity increased by 60% from 746 to 1193 kA/m. Further microstructure observations indicate that this exceptional effect stems from NDs can effectively inhibit the formation of coarse grains and optimize the distribution of Nd-rich phases, ensuring a strong pinning effect to enhance hard magnetic performance. Meanwhile, the presence of additional grain boundaries and NDs can also lead to higher resistivity by preventing the movement of free electrons. Our work provides a distinctive strategy for interface modification with excellent comprehensive performance in permanent magnets.

Grain size and coercivity tuning in Nd2Fe14B-based magnets prepared by high pressure hydrogen milling

Nd2Fe14B-based permanent magnets play important role in the emerging green energy technologies due to their superb energy product (BH)max. High coercivity at elevated operating temperatures is crucial for device performance and as an extrinsic property it is largely determined by the microstructure of the magnet. In this work, we use high pressure hydrogen milling to reduce the average grain size in Nd2Fe14B powders towards the critical single domain regime to study the influence on the resultant coercivity. In addition, Nd content has been varied in a broad range to investigate how it affects the coercivity. Milling in 100 bar hydrogen enables complete decomposition of the Nd2Fe14B phase into α-Fe, NdH2 and Fe2B at nominally room temperature. A subsequent hydrogen desorption heat treatment leads to the recombination to the parent phase, now with nearly two orders of magnitude reduction in the grain size. The results show that indeed Hc peaks around the critical single domain grain size of ≈200 nm of Nd2Fe14B. Higher contents of Nd-rich grain boundary phase lead to a continuous increase in coercivity up to μ0Hc = 1.5 T, likely due to the suppression of long-range magnetostatic interactions between the nanocrystalline Nd2Fe14B grains.