Hydrogen Decrepitation Process Research Articles

Enhanced anisotropy of Pr-Nd-Fe-B HDDR magnetic powders by regulating the hydrogen decrepitated process

The evolution of microstructures for different hydrogen decrepitated routes was investigated in hydrogenation-disproportionation-desorption-recombination (HDDR) magnetic powders, with particular focus on the fracture of the SC heat-treated alloy during two-step hydrogen decrepitation (HD) and the influence of the first step temperature on magnetic properties. As the temperature increases, the properties of the magnetic powders increase and then decrease, the optimized performance of HDDR magnetic powders was obtained with Br=1.45 T, (BH)max=393 kJ·m−3, DOA=0.79 when the first HD temperature is 65°C. The microscopic analysis shows that there are two problems in the one-step HD route: one is that the particle size is larger ∼150 μm and there are incomplete fractured cracks, the particle surface is more PrNd-rich phase, which leads to different reaction rates between the surface and center in HDDR particles, and the texture orientation of the fine grains at the edge of surface and crack is disordered. The second issue is that HD grains have significant angular difference with the c-axis orientation in incomplete fractured grain clusters, which is directly inherited by the HDDR magnetic powder, resulting in a misalignment texture. In comparison, the two-step HD process effectively addresses the issues derived from one-step HD process. Not only is the grain size of HDDR magnetic powders uniform and the width of the thin grain boundary phases (GBs) is less than the exchange coupling length, but also (Fe+Co) content exceeding 30 at% in GBs with weak magnetism. This is beneficial for enhancing exchange coupling effect between adjacent grains. Moreover, magnetic powder particle exhibits single-domain characteristics, which improves the grain texture orientation. This research provides a new insight for the development of high-orientation and high-performance HDDR magnetic powders.

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.

Effect of hydrogen pressure on hydrogenation and pulverization behavior of Sm(CoFeCuZr)z ingot and strip casting flake

Effect of hydrogen pressure on hydrogenation and pulverization behavior of Sm(CoFeCuZr)7.6 alloy prepared by plate mold (PM) and strip casting (SC) has been systematically investigated. Increasing hydrogen pressure can improve the hydrogenation ability. Both the PM and SC have the similar hydrogen absorption process, which can be divided into two stages. The first stage follows the Sieverts’ law (1–5 MPa) while the second one deviates from the Sieverts’ law (5–10 MPa). Rietveld refinement shows that the hydrogen absorption causes different lattice distortion of different phases without changing the phase structure. The internal stress caused by such lattice distortion is the primary reason that accelerates the pulverization of the alloys, and increasing hydrogen pressure can lead to the decrease of particle size of the PM and SC alloys. The hydrogen decrepitation (HD) process are studied in detail based on these results. The hydrogen absorption mechanism of Sm(CoFeCuZr)7.6 alloy can be regarded as a hydrogen dissolution process without phase transformation. In addition, the existence of a large amount of fine grains in the SC has an adverse effect on the alignment, which can be optimized by taking effective methods to reduce the overcooling rate. This work proves that the SC, HD and jet milling process can be a promising new approach to powder preparation for high-performance Sm(CoFeCuZr)z sintered magnets.

High Coercive RE-Fe-B Powders Recycled via HD and HDDR Process from Waste Magnets

In order to fabricate high-coercivity hard magnetic RE-Fe-B powders from the waste bulk magnets, the hydrogen decrepitation (HD) and hydrogenation-disproportionation-desorption-recombination (HDDR) processes were employed to the waste RE-Fe-B magnets. The optimum HD and HDDR conditions for the waste RE-Fe-B magnets were established through the systematic investigation on the effect of HD and HDDR treatment temperature on the magnetic and microstructural properties of final recycled powders. We found that the waste magnets were crushed into the flakes with a uniform size distribution when the HD treatment was carried at 200 ℃. By applying the HDDR process to the HD treated flakes at 920 ℃, high coercivity of 11.2 kOe could be achieved in the recycled RE-Fe-B HDDR powders. Based on the results from the microstructural characterization, the underlying reasons for the magnetic and microstructural changes of the recycled powders as a function of the HD and HDDR treatment temperature is discussed in detail.

The character and role of grain boundaries in NdFeB-type alloys and magnets

Abstract The character and role of grain boundaries in NdFeB alloys and magnets are described and discussed. The role of the grain boundaries in the hydrogen decrepitation process is considered and the influence of the character and mobility of grain boundaries on the properties of fully dense sintered magnets is described. Grain boundaries also play an important role in the hydrogenation disproportionation desorption and recombination process whereby the coarse grain, cast material is converted to very fine grain particles.

Experimental evidence for the suitability of the hydrogen decomposition process for the recycling of Nd-Fe-B sintered magnets

An effective and complete processing route for the recycling of sintered Nd-Fe-B scrap magnets was proposed. Sintered Nd-Fe-B magnets were subjected to the Hydrogen Decrepitation (HD) process at various temperatures in the range of 50–300 °C, at two different pressures, 50 kPa and 200 kPa, followed by vacuum dehydrogenation in the range of 720–820 °C. The structure refinement efficiency and magnetic properties of the powders obtained were characterized. Low hydrogen pressure (50 kPa) was found to increase the magnetic properties at each temperature for all powder fractions. It was also shown that particle refinement occurs at low (50 °C, 100 °C) temperatures for low (50 kPa) pressure and higher temperatures (200 °C, 300 °C) for high pressure (200 kPa), respectively. High pressure also accelerates the initialization of the HD process at each temperature. No correlation was found between hydrogenation temperature and magnetic properties of the powders, except for the small increase in coercivity increase with growing temperature. The coarse fraction (400–500 µm) was found to have the highest magnetic properties for almost each HD process. The best magnetic properties Hci = 497 kA/m, Br = 1.1 T, and (BH)max = 121 kJ/m3 were obtained for hydrogenation at a temperature of 50 °C, pressure 50 kPa and dehydrogenation at 780 °C.

Hydrogen Decrepitation Behaviors of Novel RE–Fe–B Strip-Casting Alloys Based on Misch Metal

To reduce the Nd-Fe-B material cost and balance the utilization of rare earth sources, the misch metal (MM) substitution for Nd in the sintered magnet attracts interest. Under the processing of fabricating Nd-Fe-B magnet, hydrogen decrepitation (HD) is widely used to obtain magnetic powder; however, the HD behaviors of RE-Fe-B strips based on MM may be complex. The HD behaviors of the [(Pr, Nd) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> MM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30.8</sub> FebalCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.4</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.97</sub> (x = 0-1) strips were studied under 0.2 MPa pressure. We find that it is similar to the Nd-Fe-B, 2:14:1 tetragonal structure of RE-Fe-B strips based on MM which is not broken during hydrogen decrepitating. But the REFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase in MM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30.8</sub> FebalCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.4</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.97</sub> disappears dramatically as a result of hydrogenation. For the samples with MM, the HD process is quicker and the particle size gets smaller than that for free MM, especially for samples with 35 wt.% MM, which may be attributed to La and Ce elements.

The use of thermal hydrogen decrepitation to recycle Nd-Fe-B magnets from electronic waste

Rare earth magnets based upon neodymium-iron-boron (NdFeB) are employed in many high tech applications, including hard disk drives (HDDs). The key elements in manufacturing NdFeB magnets are rare earth elements (REEs) such as neodymium. This element has been subject to significant supply shortfalls in the recent past. Recycling of NdFeB magnets contained within waste of electrical and electronic equipment (WEEE) could provide a secure and alternative supply of these materials. Various recycling approaches for the recovery of sintered NdFeB magnets have been widely explored. Hydrogen decrepitation (HD) can be used as a direct reuse approach and effective method of recycling process to turn solid sintered magnets into a demagnetised powder for further processing. In this work, sintered Nd-Fe-B magnets were processed without prior removal of the metallic protective layer using the thermal HD process as an alternative recycling method. The gas sorption analyzer was used to determine the quantity of the hydrogen absorbed by a samples of magnets, under controlled pressure (1, 2, 3, and 4 bar) and temperature (room, 100, 300, and 400?C) conditions, using Sieverts? volumetric method. The composition and morphology of the starting and the extracted/disintegrated materials were examined by ICP, XRD, and SEM-EDS analysis.

Monitoring of the hydrogen decrepitation process by acoustic emission

Acoustic Emission Testing (AT) was applied to monitor the Hydrogen Decrepitation (HD) process for two different NdFeB-type materials: sintered NdFeB magnet and annealed NdFeB ingot alloy. Acoustic signals of clearly different characteristics were acquired for these two materials, indicating different course of the HD process. Acoustic waves of high signal strength and high peak amplitude were generated for the sintered NdFeB magnet whereas acoustic signals of low energy and low intensity were registered for the annealed NdFeB alloy. These findings were supported by real-time visual observation of the HD process. The sintered NdFeB magnet decrepitates rapidly and its particles spatter for a longer distance comparing to the ingot NdFeB alloy. Different behavior of the investigated materials can be explained in relation to their structural properties and manufacturing history. Fine microstructure, homogenously distributed thin layer of the Nd-rich grain boundary phase and stress accumulated after fabrication process can be responsible for more intense characteristic of the HD process in case of the sintered NdFeB magnets.

Effect of hydrogen pressure on hydrogen absorption of waste Nd-Fe-B sintered magnets

Abstract Hydrogen treatment technology is developed to recycle the waste Nd-Fe-B sintered magnets in the way of short route, low cost, and high efficiency. In present study, effect of hydrogen pressure on hydrogen decrepitation (HD) process of sintered Nd-Fe-B strip casting flakes (SC) and waste sintered magnets (SM) was systematically studied. Both SC and SM show the approximate HD process, which accelerates with increasing pressure. The surface activation process accelerates with increasing pressure, and almost disappears when the initial hydrogen pressure reaches to 6 MPa. The hydrogen content of SM is lower than SC, and increases with increasing pressure. For the SM, blasting power of HD process decreases with the increase of the initial hydrogen pressure. It provides beneficial reference for recycling waste Nd-Fe-B sintered magnets.

Recycling of SmCo5 magnets by HD process

Hydrogen decrepitation process has been applied for the first time for the direct recycling of SmCo5 magnets. Industrially produced sintered SmCo5 magnets were decrepitated by hydrogen gas at a pressure of 1 bar to 9.5 bar at room temperature in a planetary rotating jar. After decrepitation, the starting sintered magnets were reduced to a powder with a particle size of less than 200 µm. The produced powder was used for the preparation of recycled SmCo5 magnets. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction studies and magnetic measurements were used to follow the decrepitation and the sintering processes. The measured remanence and maximum energy product of the recycled magnet are 0.94 T and 171.1 kJ/m3, respectively, in comparison with 0.91 T and 156.8 kJ/m3, respectively for the original magnet before recycling. It was also observed that, there is refinement in the microstructure after recycling in comparison to the original magnet.

Novel hydrogen decrepitation behaviors of (La, Ce)-Fe-B strips

La and Ce substitution for Nd in the 2:14:1-type sintered magnet is of commercial interest to reduce the material cost and to balance the utilization of rare earth (RE) sources. As hydrogen decrepitation (HD) is widely utilized to prepare the magnetic powders during magnets fabrication, incorporating La and Ce into the Nd-Fe-B permanent magnets, however, may exert complex influences on the decrepitation behavior. In the present work, through a comparative study of the HD behaviors between the (La, Ce)-Fe-B strips and the conventional Nd-Fe-B ones, we find that similar to the Nd-Fe-B system, increasing hydrogen pressures from 2.5 to 5.5 MPa do not break the 2:14:1 tetragonal structure of (La, Ce)-Fe-B strips. The enhanced hydrogen absorption behaviors are observed with increasing pressure, which are still inferior to that of the Nd-Fe-B strips. This should be ascribed to the higher oxygen affinity of La and Ce than that of Nd, leading to the decreased amount of active RE-rich phase and limited hydrogen diffusion channel. As a result, the hydrogen absorption of 2:14:1 matrix phase is significantly suppressed, dramatically weakening the exothermic effect. This finding suggests that La and Ce with stable 2:14:1 tetragonal structure upon HD process are promising alternatives for Nd, despite that more precise oxygen control is necessary for the microstructure modification and magnetic performance enhancement of (La, Ce)-Fe-B sintered magnets.

Hydrogen Decrepitation and Spark Plasma Sintering to Produce Recycled SmCo5 Magnets With High Coercivity

Spark plasma sintering (SPS) technique is applied in combination with hydrogen decrepitation process for the recycling of SmCo5 magnets. The SmCo5 magnets for recycling were first decrepitated by hydrogen gas of a pressure of 4 bar for 3 hours to produce decrepitated powder. This powder was then used to prepare isotropic sintered magnets using the SPS technique, by sintering at 800-1000 oC for 1 minute. Full densification of the SPS-ed magnets was possible at a temperature of 1000 oC. The sample sintered at 900 oC showed the best internal coercivity (jHc) of higher than 1500 kA/m with high remanence (Br) value of 0.47 T and energy product (BH(max)) of 43.4 kJ/m3. The properties of the SPS-ed sample sintered at 900 oC were compared with conventionally sintered (CVS-ed) sample prepared by using fresh SmCo5 powder. The results showed the improvement of the magnetic properties of the SPS-ed sample in comparison to the CVS-ed sample at room temperature, and the possibility to use the SPS-ed sample at high temperature of 180 oC, where the sample showed good magnetic properties of jHc of 1502 kA/m, Br of 0.44 T and BH(max) of 36.4 kJ/m3. The microstructure and X-ray diffraction patterns of the SPS-ed and the CVS-ed samples were studied; where the samples showed to basically consist of SmCo5 matrix phase with Sm2Co7 and Sm-oxides.

Hydrocyclone Separation of Hydrogen Decrepitated NdFeB

Hydrogen decrepitation (HD) is an effective and environmentally friendly technique for recycling of neodymium-iron-boron (NdFeB) magnets. During the HD process, the NdFeB breaks down into a matrix phase (Nd2Fe14BHx) and RE-rich grain boundary phase. The grain boundary phase in the HD powder is &lt;2 μm in size. Recycled NdFeB material has a higher oxygen content compared to the primary source material. This additional oxygen mainly occurs at the Rare Earth (RE) rich grain boundary phase (GBP), because rare earth elements oxidise rapidly when exposed to air. This higher oxygen level in the material results in a drop in density, coercivity, and remanence of sintered NdFeB magnets. The particle size of the GBP is too small to separate by sieving or conventional screening technology. In this work, an attempt has been made to separate the GBP from the matrix phase using a hydrocyclone, and to optimise the separation process. HD powder, obtained from hard disk drive (HDD) scrap NdFeB sintered magnets, was used as a starting material and passed through a hydrocyclone a total number of six times. The X-ray fluorescence (XRF) analysis and sieve analysis of overflows showed the matrix phase had been directed to the underflow while the GBP was directed to the overflow. The optimum separation was achieved with three passes. Underflow and overflow samples were further analysed using an optical microscope and MagScan and matrix phase particles were found to be magnetic.

Microstructural and Texture Evolution of Strip Cast Nd–Fe–B Flake

In this work, the microstructure and texture evolution of the strip cast Nd–Fe–B flake has been systematically investigated by correlating multiple state-of-the-art characterization techniques. We found that (i) besides the existence of random ultrafine equiaxed grains at the wheel side of the flake, elongated (001) textured grains were formed into a V-shape zone between neighboring nucleation sites, which possibly resulted from the in-plane growth of the low energy preferred growth direction of grains (a axis). Both ultrafine random equiaxed grains and elongated (001) textured grains are detrimental to achieving a high-performance Nd–Fe–B magnet, due to the inhomogeneous grain shape and nonuniform distribution of rare earth-rich phase within these grains or along the grain boundaries, which deteriorate the alignment of the Nd–Fe–B powders in the subsequent hydrogen decrepitation process and jet milling procedure. To overcome the issues mentioned above, two potential approaches are proposed, which are inc...

Effect of Sc substitution on hydrogen storage properties of Ti–V–Cr–Mn alloys

The microstructure and hydrogen absorption/desorption properties of (40-x)Ti–40V–10Cr–10Mn-xSc (x = 0, 5, 10, 15) were investigated. The result shows that the Sc-free sample has a V-based solid solution phase with body-center cubic structure (BCC phase), while the Sc-contained samples consist of a BCC phase, a C14-type Laves phase, a Sc-based hydride phase with face-center cubic (FCC) structure (derived from hydrogen decrepitation process). The hydrogen absorption kinetic process can be described by chemical reaction mechanism when x = 0, 10, 15 and 3-D diffusion mechanism when x = 5. As the Sc content increases, the hydrogen absorption capacity at 298 K first decreases and then increases, getting the minimum when x = 5. However, the hydrogen desorption capacity and hysteresis at 298 K are gradually improved with Sc content increasing.

Recycling of scrap sintered Nd–Fe–B magnets as anisotropic bonded magnets via hydrogen decrepitation process

The scrap sintered Nd–Fe–B magnets were recycled as the raw materials for bonded magnets using the hydrogen decrepitation (HD) process. The HD powders have the lowest oxygen and hydrogen content by hydrogenation at 150 °C with 1 bar H2 pressure and dehydrogenation at 600 °C. The powders with the largest particle size (>380 μm) bear the best magnetic properties (Br = 110.59 emu/g, Hcj = 9005.80 Oe). The magnetic properties of the resulting bonded magnet prepared from the above powders were Br = 7.02 kGs, Hcj = 4.56 kOe, (BH)max = 7.13 MGOe.

Research on Hydrogen Absorption Mechanism Modeling of the NdFeB Hydrogen Decrepitation Process

The behavior of hydrogen absorption in the NdFeB Hydrogen Decrepitation process is affected by the shape of the NdFeB alloy, pressure and temperature curves of the reaction process, which makes the reaction process with characteristics of nonlinear, time-varying parameters and coupling. In this study, we proposed a state space modeling method to describe the dynamic mechanism model of the hydrogen decrepitation process, and determine the main parameters of the model according to the mass balance, energy balance and kinetic equations. Then implemented the simulation model of the hydrogen absorption dynamic mechanism model using the matlab language. The simulation results were compared with the measured data, which verified the correctness of the model.

Properties of nanoparticles prepared from NdFeB-based compound for magnetic hyperthermia application

Nanoparticles were prepared from a NdFeB-based alloy using the hydrogen decrepitation process together with high-energy ball milling and tested as heating agent for magnetic hyperthermia. In the milling time range evaluated (up to 10 h), the magnetic moment per mass at H = 1.59 MA m−1 is superior than 70 A m2 kg−1; however, the intrinsic coercivity might be inferior than 20 kA m−1. The material presents both ferromagnetic and superparamagnetic particles constituted by a mixture of phases due to the incomplete disproportionation reaction of Nd2Fe14BHx during milling. Solutions prepared with deionized water and magnetic particles exposed to an AC magnetic field (Hmax ∼ 3.7 kA m−1 and f = 228 kHz) exhibited 26 K ≤ ΔTmax ≤ 44 K with a maximum estimated specific absorption rate (SAR) of 225 W kg−1. For the pure magnetic material milled for the longest period of time (10 h), the SAR was estimated as ∼2500 W kg−1. In vitro tests indicated that the powders have acceptable cytotoxicity over a wide range of concentration (0.1–100 µg ml−1) due to the coating applied during milling.

Preparation and Properties of Anisotropic Nano-crystalline NdFeB Powders Made by Hydrogen Decrepitation of Die Upsetting Magnets

Anisotropic nanocrystalline NdFeB powders were prepared by hydrogen decrepitation (HD) of die upsetting magnets. The effects of varying temperatures of HD on the microstructure and magnetic properties of the anisotropic NdFeB particles were studied. It shows that the powders which obtained by HD process at higher temperature were larger than that at lower temperature, and the HD powders show a well anisotropy at 723 K, the remanence (Br) was more than 12.46 kG, the maximum energy product ((BH)max) was 19.06 MGOe, and the coercivity (Hcj) was 7.2 kOe. The microstructure of the anisotropic powders revealed that with a reasonable HD temperature, the platelet grains were not destroyed. They were nearly 150-300 nm long and 30-50 nm wide. The results indicate that HD process was an effective way to prepare the anisotropic NdFeB powders.