High Maximum Energy Product Research Articles

Enhanced dipole-interaction in Nd-Dy-Fe-Co-B/Fe composite thick stacked trilayer

It is crucial to better understand the magnetization reversal process between soft and hard magnets and to achieve a high maximum energy product in thick composite multilayers. In this study, we find that the exchange interactions dominate in soft–hard-magnetic composite bilayers, while dipole interactions are predominant in soft–hard-magnetic composite trilayers. Based on the first-order reversal curve, magnetization reversal models are developed for both the thick composite bilayer and trilayer. Dipole interactions play an important role in the long range, resulting in higher coercivity and remanence in the thick trilayer. A multilayer in a stacked trilayer structure is achieved, which is composed of thick films with a perpendicular magnetic anisotropy at a thickness of up to 16 μm. The enhanced dipole interactions lead to a remanent polarization of 1 T and a maximum energy product of 22.5 MGOe. This work contributes to the preparation of thick films with a high maximum energy product for applications in magnetic microelectromechanical systems.

An experimental investigation and predictive modeling using machine learning technique for reclamation of metal values from scrap NdFeB magnets

Neodymium-Iron-Boron (NdFeB) magnets are widely used in various industries due to their exceptional magnetic properties, such as high coercivity, remanence, and maximum energy product. These magnets consist of rare earth elements (REEs) viz., neodymium (Nd), praseodymium (Pr), and Dysprosium (Dy), along with other metals. With the ever increasing demand for REEs and the need to bridge the supply gap, it is crucial to develop alternative methods for their extraction. Recycling metal from scrap magnets is a promising approach to address the pressure on the supply chain and work towards sustainable development. The primary goal of this study is to generate a predictive model based on machine learning to determine the optimal conditions for metal recovery from scrap NdFeB magnets through water leaching after chloridizing roasting. Bench-scale leaching experiments were carried out to generate a dataset for statistical process optimization and machine learning analysis. The leaching kinetics of neodymium was also explored, and mixed-controlled shrinking core model was found to be most suitable, with an activation energy of 58.11 kJ/mol in the temperature range of 25– 95 °C. This study is the first to utilize a machine learning approach to analyze the potential process variables and their impact on metal recovery from calcined NdFeB magnet powder. A comparative analysis between experimental and machine learning approaches is presented to predict the optimal conditions for selective recovery of metal values from scrap NdFeB magnets. The maximum efficiency of extraction of metal ions was found to occur at a temperature of 95 °C, solid to liquid ratio of 125 g/l, and leaching duration of 60 min.

Microstructure and magnetic properties of MnBi powders prepared by various ball milling processes

The MnBi intermetallic compound with high purity of low-temperature phase MnBi (LTP-MnBi) and high maximum energy product (BH)max were successfully prepared using various ball milling processes. The effects of ball milling speed and duration on the microstructures, phase compositions, and magnetic properties of MnBi alloys were systematically investigated. The particle size of MnBi and the purity of LTP-MnBi gradually decreased with increasing ball milling speed. An optimized ball milling speed of 175 rpm resulted in MnBi powder with the (BH)max of 9.22 MGOe and a squareness (M r/M s) of 93.7%. Furthermore, the ball milling duration was optimized to observe its impact on the MnBi alloy. As the duration increased, the average particle size of MnBi decreased from 27 μm at 0 h to 1.84 μm at 10 h. Simultaneously, the coercivity (H c) increased from 0.7 T at 0 h to 1.7 T at 10 h. Ultimately, the powder ball milled at 175 rpm for 5 h exhibited the highest (BH)max of 11.3 MGOe.

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.

Novel composition of Nd-Fe-B gas atomized powder to produce compression bonded magnets

New compositions of Nd-Fe-B powders were produced via gas atomization. Helium and argon were used as atomizing gases. For the same process parameters, helium resulted in a powder that is 6 times finer than the powder produced with argon. In both cases, the atomization conditions were set to achieve fine powders and, therefore, fine microstructures, since this is critical to obtain a suitable coercivity. The powders were sieved to separate the fraction below 20 μm (∼90 % of powder atomized with He). The microstructure is formed by a mix of elongated and equiaxial grains with random crystallographic orientation and an average grain size of ∼ 1 μm, which was measured by Electron Backscattered Diffraction. This kind of measurements and images were taken for the first time for fine helium atomized Nd-Fe-B powders. The embedding of the powder in copper was found to be useful, as the conventional mounting in metallographic conductive resin produced charging of the sample. Annealing up to 1000 °C did not cause any significant grain growth that could deteriorate the magnetic properties. Annealing at 700 °C for 15 min improved the magnetic properties due to the crystallization of residual amorphous phase. Warm compaction was used to produce magnets with a high volume fraction of magnetic powder (>70 %) and low real porosity (∼5 %). These magnets exhibit a high remanence (∼0.5 T) and maximum energy product (∼35 kJ/m3), combined with a suitable intrinsic coercivity (>500 kA/m). The new compositions and processing route could cover a market niche. The thoroughly analyzed powders could be also suitable for some additive manufacturing technologies that require spherical powders with very specific particle size distributions.

Effects of Ti–V pair substitution and grain boundary modification on physical and magnetic properties of Sm(Fe0.8Co0.2)12 bulk magnet

We successfully fabricated high-density Sm(Fe0.8Co0.2)12–2xTixVx (0.6 ≤ x ≤ 1.0) bulk magnets with high-purity ThMn12-type phase through the bulking of amorphous powder and subsequent annealing process. Depending on the Ti–V pair concentrations, the physical and magnetic properties including lattice constants, crystallized phases, magnetizations (M), and coercivities (Hc) of the fabricated samples were investigated. The highest maximum energy product, (BH)max, of 7.293 MGOe was obtained with x = 0.6; however, its Hc was limited to 3379 Oe due to the absence of a grain boundary phase, despite the average grain size being within 81 nm, which is close to the single domain size. Thus, an intergranular phase was developed by introducing Cu–Ga additive, i.e., Sm(Fe0.8Co0.2)10.8Ti0.6V0.6 + y wt. % Cu–Ga (1 ≤ y ≤ 3), to enhance the Hc. Uniform grain boundaries with Sm-rich phases were crystallized for y ≥ 2, and their thicknesses increased with higher y; the thickness of the grain boundary for the sample with y = 2 was 12 nm. The Hc of the samples with uniform grain boundaries were significantly enhanced from 3379 Oe to 4054 and 4317 Oe for y = 0 and y = 2 and 3, respectively. This paper presents the results of Ti–V pair substitution and grain boundary modification in an effort to improve magnetic properties by controlling crystalline phases, grain sizes, and grain boundaries.

Suppressing laves phase and overcoming magnetic properties tradeoff in nanostructured (Ce,La,Y)–Fe–B alloys via Ge substitution

Developing permanent magnets based on full high abundance rare-earth (RE) elements of Ce, La, and Y offers tremendous potential for the balanced utilization of RE resources, but the magnetic properties of these magnets are restricted due to the magnetic dilution caused by the existence of the paramagnetic REFe2 laves phase. Herein, the non-RE element Ge with high efficiency was introduced to enhance the magnetic performance of Ce-, La-, and Y-based RE–Fe–B nanocrystalline alloys, and the highest maximum energy product [(BH)max] of 65.6 kJ/m3 and an enhanced coercivity (Hcj) of 346 kA/m were achieved in the [(Ce0.8La0.2)0.5Y0.5]16Fe77.5B6Ge0.5 alloy. This improvement is attributed to the increased content of the hard magnetic RE2Fe14B phase with refined grain size, which is further confirmed by micromagnetic simulation. First-principles calculations and a microstructure analysis reveal that the laves phase is effectively suppressed by Ge addition due to the formation of the Ce5Ge3 phase with the lowest formation energy. This work clarifies the positive role of Ge in simultaneously enhancing the Hci and (BH)max of nanostructured (Ce,La,Y)–Fe–B alloys.

Phase separation and chemistry evolution in Fe-rich Sm(CobalFe0.31Cu0.07Zr0.025)7.7 magnets: The effect of initial defect

The intrinsic coercivity of Fe-rich Sm2Co17-type permanent magnets is closely associated with the unique cellular microstructures. Usually, the continuous distribution of cell boundaries is destroyed in Fe-rich Sm2Co17-type magnets. Herein, a relatively high maximum energy product of about 32.7 MGOe and a nearly 300% increase in intrinsic coercivity are observed for Fe-rich Sm2Co17-type permanent magnets via regulating the initial defects to introduce numerous 1:5H precipitates nucleation sites. Synchrotron XRD and HR-TEM results reveal that the solution-treated Fe-rich Sm2Co17-type magnets consist of supersaturated 1:7H phase and a partial 2:17R phase. Furthermore, the solution-treated magnet with a lower fraction of 2:17R phase was found to have a higher density of initial defects through positron annihilation spectroscopy measurement. In the magnet with a higher density of initial defects, high density of defects-aggregated cell boundaries and high internal stress promote the precipitation of 1:5H phase. Moreover, the ordering transformation of 2:17R phase was facilized during aging treatment, and elements segregation was promoted. These results present a novel way in designing high-performance Fe-rich Sm2Co17-type magnets.

Enhancement mechanism of maximum energy product in hot-deformed Nd-Fe-B magnets by addition of ferromagnetic substances

It has been proved that the maximum magnetic energy product (BH)max of hot-deformed Nd-Fe-B magnets can be effectively improved by adding substances with either high melting point or high saturation magnetization. In this work, we selected a ferromagnetic nano Fe55Ni28Co17 alloy powders with both high melting point and high saturation magnetization as the dopant to improve the (BH)max. By the addition of 1 wt% FeNiCo, the remanence increases by over 5% (from 1.36 to 1.44 T), resulting in a significant enhancement of (BH)max from 355 to 396 kJ/m3 (from 44.6 to 49.7 MGOe). Microstructure observations reveal that the texture of grain alignment is improved, the concentrations of ferromagnetic elements (Fe, Ni, and Co) in the main phase and intergranular phase are increased so that the magnetization behavior of hot-deformed magnet changes (more easily shows reversible magnetization behaviors), which are the reasons why the high maximum energy product of hot-deformed magnet, is obtained.

Realization of high-performance HRE-free Pr-Fe-B sintered magnet through microstructure regulation

Currently, the continued high prices of the heavy rare earth (HRE) elements Dy and Tb are prompting an exploration of the development of HRE-free high-performance 2:14:1-type permanent magnets. In this paper, a high maximum energy product of 44.04 MGOe at room temperature (RT) was obtained while maintaining a coercivity greater than 20 kOe in HRE-free Pr-Fe-B sintered magnets through microstructural regulation, which reached an equivalent to the commercially 45SH level. The Pr-Fe-B sintered magnet possesses excellent magnetic properties at low temperatures, and its remanence and coercivity increased to 14.75 kG and 75.01 kOe at −170 °C, respectively. Microstructure analysis indicated that the fine and uniform columnar crystals in the strip-casting (SC) alloys provided conditions for the preparation of fine-grained jet milling (JM) powders and sintered magnets. After annealing, the continuous grain boundary (GB) layers and the fine grain size of 2.80 μm ensured high coercivity. Meanwhile, the reduction of microcracking at the GB helps densification and the improvement of c-axis alignment ensured high remanence. The above exploration of microstructural regulation throughout the metallurgical process can provide meaningful guidance for the development of HRE-free Pr-Fe-B sintered magnets with high performance.

Recovery of Rare Earth Elements through Spent NdFeB Magnet Oxidation (First Part)

Due to their remarkable magnetic properties, such as a high maximum energy product, high remanence, and high coercivity, NdFeB magnets are used in a variety of technological applications. Because of their very limited recycling, high numbers of spent NdFeB magnets are widely available in the market. In addition to China’s monopoly on the supply of most rare earth elements, there is a need for the recovery of these critical metals, as their high import price poses an economic and environmental challenge for manufacturers. This paper proposes a pyrometallurgical recycling method for end-of-life NdFeB magnets by oxidizing them in air as first required step. The main goal of this method is to oxidize rare earth elements from NdFeB magnets in order to prepare them for the carbothermic reduction. The experimental conditions, such as the oxidation temperature and time, were studied in order to establish the phase transformation during oxidation using the Factsage Database and experimental conditions. Our thermogravimetric analysis TGA analysis revealed an increased sample mass by 35% between room temperature and 1100 °C, which is very close to the total calculated theoretical value of oxygen (36.8% for all elements, and only 3.6% for rare earth elements REE), confirming the complete oxidation of the material. The obtained quantitative analysis of the oxidation product, in (%), demonstrated values of 53.41 Fe2O3, 10.37 Fe3O4; 16.45 NdFeO3; 0.45 Nd2O3, 1.28 Dy2O3, 1.07 Pr2O3, and 5.22 α-Fe.

Tripling magnetic energy product in magnetic hard/soft nanocomposite permanent magnets

Nanocomposite materials comprising two or more nanoscale functional components attracted more attention due to their high performance than that of the corresponding single counterpart, which have wide potential applications in many fields. The manipulation of the component phases, fraction/distribution of each phase, and grain size in the nanocomposite is of priority to obtain a high performance. Here, in this work, a two-step process of optimal high-energy ball milling followed by optimal low temperature annealing had been adopted to fabricate Sm–Co/FeCo nanocomposite magnets with triple magnetic energy product in triple-phase nanocomposites. An evolution of microstructure including morphology, phase constitution and grain size towards a high performance magnet had been investigated, which resulted in a record high maximum energy product of 29.1 MGOe. The microstructure of well distributed nanoscale SmCo3 and SmCo5 duplex hard phases and soft phase with almost equal percent is the key factor to get this high performance. The fabrication procedure in controlling the microstructure with uniformly distributed triple phases and nanoscale grain size will shed light on synthesizing other types of nanocomposite materials with high performance.

Comparison and process study of hot-pressed and hot-deformed Nd-Fe-B magnets prepared by amorphous and nanocrystalline powders

Hot-pressed and hot-deformed Nd-Fe-B magnets are generally made from rapidly quenched powder after crystallization treatment. In this work, the amorphous melt-spun ribbons are also employed directly as the precursor. The amorphous and crystallized powders with the composition of Nd28.47Pr0.56FebalB0.97Co3.63Al0.13Nb0.56Zr0.44 were employed for fabricating hot-pressed and hot-deformed magnets. The influences of processing parameters on the microstructure and magnetic properties have been investigated. For the hot-pressed magnets produced from both crystallized and amorphous powders, the grain pre-orientation and magnetic properties including coercivity, remanence and maximum energy product increase with the increasing temperature, pressure and holding time in the ranges of 650–800 °C, 300–600 MPa and 30–150 min, respectively. The hard magnetic properties of the hot-pressed magnets obtained from amorphous powder are slightly higher than those from crystalline powder due to the smaller grain size of the former. The magnetic properties of hot-deformed magnets are mainly determined by the deformation temperature and not influenced by the state of the powder and hot-pressed precursors. The hot-pressed magnets and hot deformed magnets with different microstructures exhibit different magnetization mechanisms. The optimal hot-deformation temperature is found to be 750 °C for obtaining a high maximum energy product of 39 MGOe. The present results suggest that the rapidly quenched amorphous powder can be used to not only directly prepare hot-pressed magnets with higher magnetic properties, but also to reduce the process step for hot-deformed Nd-Fe-B magnets.

NdFeB Magnets Recycling Process: An Alternative Method to Produce Mixed Rare Earth Oxide from Scrap NdFeB Magnets

Neodymium iron boron magnets (NdFeB) play a critical role in various technological applications due to their outstanding magnetic properties, such as high maximum energy product, high remanence and high coercivity. Production of NdFeB is expected to rise significantly in the coming years, for this reason, demand for the rare earth elements (REE) will not only remain high but it also will increase even more. The recovery of rare earth elements has become essential to satisfy this demand in recent years. In the present study rare earth elements recovery from NdFeB magnets as new promising process flowsheet is proposed as follows; (1) acid baking process is performed to decompose the NdFeB magnet to increase in the extraction efficiency for Nd, Pr, and Dy. (2) Iron was removed from the leach liquor during hydrolysis. (3) The production of REE-oxide from leach liquor using ultrasonic spray pyrolysis method. Recovery of mixed REE-oxide from NdFeB magnets via ultrasonic spray pyrolysis method between 700 °C and 1000 °C is a new innovative step in comparison to traditional combination of precipitation with sodium carbonate and thermal decomposition of rare earth carbonate at 850 °C. The synthesized mixed REE- oxide powders were characterized by X-ray diffraction analysis (XRD). Morphological properties and phase content of mixed REE- oxide were revealed by scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX) analysis. To obtain the size and particle size distribution of REE-oxide, a search algorithm based on an image-processing technique was executed in MATLAB. The obtained particles are spherical with sizes between 362 and 540 nm. The experimental values of the particle sizes of REE- oxide were compared with theoretically predicted ones.

Influence of Additional Micro-Sized Particles of Dy–Nd–Cu–Al on Magnetic Properties of Sintered Nd–Fe–B Magnets

In this work, we investigated the influence of concentration of the additional micro-sized particles of Dy40Nd30Al30 and Nd40Cu30Al30 on magnetic properties of the sintered Nd16.5Fe77B6.5 magnets. The additional particles with size in the range of 1-3 μm were prepared by ball milling method and then mixed into micrometer Nd16.5Fe77B6.5 master powder with different weight fractions before magnetic anisotropic pressing, vacuum sintering and annealing. The results show that the coercivity of the sintered Nd-Fe-B magnets can be improved considerably by introducing additional particles to the grain boundaries. The improvement of the coercivity Hc of the magnets is clearly dependent on the composition and concentration of the additional microparticles. The Hc increases linearly from 8.5 kOe to 17 kOe with increasing the weight fraction of the Dy40Nd30Al30 microparticles from 0 to 5%. Meanwhile, the coercivity of the magnet reaches a maximum value of 11.7 kOe with 4 wt% addition of Nd40Cu30Al30. The quite high maximum energy products, (BH)max > 30 MGOe, were also obtained for the magnets added with the microparticles. The obtained hard magnetic parameters of the magnets can be applied in practice.

Growth of quasi-texture in nanostructured magnets with ultra-high coercivity

Controlling the phases and structure of nanostructured rare-earth-based permanent magnets is essential to develop magnets with high performance in clean energy technologies. Importantly, simultaneously controlling the texture and morphology of nano-grains can lead to both high coercivity and high maximum energy product in the nanostructured magnets. Here, we demonstrated the use of electron-beam exposure (EBE) as a rapid heating technique to control the crystallization of the Pr-Fe-B magnet. Due to the effect of the extremely high heating rate, EBE can be adapted to control the phase, grain size, and morphology of the amorphous precursor as well as the texture of Pr-Fe-B nano-grains. A quasi-texture nanostructure with optimal magnetic exchange interactions is thus achieved, leading to a record-high coercivity of 29.1 kOe at room temperature. We further applied the Landau model and Coble-Creep model to explain the positive impact of the EBE on the crystallization and microstructure of the Pr-Fe-B magnet.

Application of combined load for obtaining ultra-fine grained structure in magnetic alloys of the Fe-Cr-Co system

Fe-Cr-Co system magnets are known for their high remanence and maximum energy product along with high mechanical properties. However, since the thermomagnetic treatment of the alloy implies the spinodal decomposition, which in turn drops the ductility of the material, finding a balance of magnetic and mechanical properties is in focus of many scientist due to its relevance. One of possible paths for finding this balance is application of hot deformation approach. The processes of dynamic recrystallization during hot deformation by means of compression accompanied torsion of magnetic alloys Fe-25%Cr-15%Co and Fe-30%Cr-8%Co of Fe-Cr-Co ternary system were studied. It is shown that the chosen method of deformation can be effectively applied to receive ultrafine-grained structure in the vicinity of the deformation zones and obtaining the gradient type structure in considered alloys. Founding on the analysis of results obtained, basic principles for enhancement of the alloy properties during thermomechanical treatment were figured out. Specific values of strain temperature and velocity for both considered alloys were proposed.

Preparation of Sm−Fe−N Bulk Magnets with High Maximum Energy Products

Preparation of Sm−Fe−N Bulk Magnets with High Maximum Energy Products

Effect of Al/Cu on the magnetic properties and microstructure of Nd-Fe-B sintered magnet by diffusing Pr-Tb-(Cu, Al) alloys

Grain boundary diffusion process (GBDP) using Pr60Tb10Cu30−xAlx (x = 0, 5, 15, 25, 30, atomic fraction) alloy ribbons, named as A0–A30 alloys, was applied to high maximum energy product (BH)max Nd-Fe-B sintered magnet. After diffusion, coercivity (Hci) of A0 magnet is significantly enhanced from the original 11.4 kOe to 19.5 kOe with remanence (Br) decreasing from 14.65 kGs to14.32 kGs, which is mainly attributed to magnetic hardening effect by Tb-rich shells. With substituting 5 at% Al for Cu, the Hci and Br of A5 magnet are simultaneously enhanced by the homogenized distribution of Tb-rich shells and the reduced interface energy of Nd-rich phase and Nd2Fe14B phase. The highest coercivity of 22.4 kOe is obtained for A25 magnet by further strengthening magnetic isolation of Nd2Fe14B phase, however, which results in a drastic decrease in Br due to the increased fraction of Nd-rich phase and too much substitution of Al for Fe in Nd2Fe14B phase. Therefore, the Hci (kOe) + (BH)max (MGOe) reaches to the peak value for A5 magnet. Also, it is proven that Cu/Al co-addition can effectively improve Tb diffusion efficiency by modifying intergranular Nd-rich phase.

Influence mechanism of Fe content on the magnetic properties of Sm2Co17-type sintered magnets: Microstructure and Microchemistry

The effects of the Fe content on the magnetic properties and the microstructures of the Sm(Co0.925-xFexCu0.05Zr0.025)7.74 (x = 0.21–0.31) sintered magnets were investigated. The influence mechanism of the Fe content on the remanence (Br) and intrinsic coercivity (Hcj) of the magnets were studied depending on the comprehensive analysis of the cellular/lamellar structures and the element distribution of the cell and cell wall phases. The incomplete cellular structure, lower Cu content and higher Fe content of the cell wall phases were considered as the main reasons for the low Hcj of the magnet with x = 0.31. The slope of linear increase of the Br of the magnet with x range from 0.27 to 0.31 is lower than that of the magnet with x range from 0.21 to 0.27 indicating a lower Br enhancement efficiency which is mainly caused by the high Fe content of the cell wall phase of the magnet with x > 0.27. A high maximum energy product ((BH)max) of 32.36 MGOe with a Hcj of 34.62 kOe was obtained for the Sm(Co0.655Fe0.27Cu0.05Zr0.025)7.74 magnet.