Net Shape 3D Printed NdFeB Permanent Magnet Jaćimović, Jaćim; Binda, Federico; Herrmann, Lorenz G. ...
Advanced engineering materials,
August 2017, Letnik:
19, Številka:
8
Journal Article
Recenzirano
Three dimensional printing enables realization of complex shape rare earth permanent magnet that enable unlocking the full potential of electrical devices for energy consumption and renewable energy ...production.
In this review article, we focus on the relationship between permanent magnets and the electric motor, as this relationship has not been covered in a review paper before. With the increasing focus on ...battery research, other parts of the electric system have been neglected. To make electrification a smooth transition, as has been promised by governing bodies, we need to understand and improve the electric motor and its main component, the magnet. Today's review papers cover only the engineering perspective of the electric motor or the material-science perspective of the magnetic material, but not both together, which is a crucial part of understanding the needs of electric-motor design and the possibilities that a magnet can give them. We review the road that leads to today's state-of-the-art in electric motors and magnet design and give possible future roads to tackle the obstacles ahead and reach the goals of a fully electric transportation system. With new technologies now available, like additive manufacturing and artificial intelligence, electric motor designers have not yet exploited the possibilities the new freedom of design brings. New out-of-the-box designs will have to emerge to realize the full potential of the new technology. We also focus on the rare-earth crisis and how future price fluctuations can be avoided. Recycling plays a huge role in this, and developing a self-sustained circular economy will be critical, but the road to it is still very steep, as ongoing projects show.
The green transition initiatives and exploitation of renewable energy sources require the sustainable development of rare earth (RE)-based permanent magnets prominent technologies like wind turbine ...generators and electric vehicles. The recycling of RE-based permanent magnets is necessary for the future supply of critical rare-earth elements. The short-loop recycling strategies to directly reprocess Nd-Fe-B magnet waste are economically attractive and practical alternatives to conventional hydro- and pyrometallurgical processes. This study focuses on the development of a procedure to extract the (Nd, Pr)2Fe14B hard-magnetic phase from sintered Nd-Fe-B magnets. The extraction is achieved through preferential chemical leaching of the secondary, RE-rich phases using 1 M citric acid. Before the acid treatment, the magnets were pulverized through hydrogen decrepitation (HD) to increase the material’s surface-to-volume ratio. The as-pulverized Nd-Fe-B powder was subsequently exposed to a 1 M citric acid solution. The effect of leaching time (5–120 min) on the phase composition and magnetic properties was studied. The results of the microstructural (SEM) and compositional (ICP-MS) analyses and the study of thermal degassing profiles revealed that the RE-rich phase is preferentially leached within 5–15 min of reaction time. Leaching of the secondary phases from the magnet’s multi-phase microstructure is governed by the negative electrochemical potential of Nd and Pr. The extraction of (Nd, Pr)2Fe14B grains by the proposed acid leaching approach is compatible with the existing hydrogen processing of magnetic scrap (HPMS) technologies. The use of mild organic acid as a leaching medium makes the leaching process environmentally friendly, as the leaching medium can be easily neutralized after the reaction is completed.
Micro-nonuniform heating in the field-assisted sintering (FAST) of electrically conductive powders has been a topic of discussion in the materials science community. Microstructural specifics, such ...as neck formation at low consolidation temperatures and density variations, have previously been ascribed to local overheating at the particle-particle contacts due to the Joule effect. However, recent theoretical modelling studies suggest that the very fast diffusion of heat within the micron-sized particles prevents the overheating, thereby challenging the conventional understanding of FAST-related heating effects. To provide a new experimental perspective on the local overheating and underscore its pivotal role in controlling the microstructure formation, we have studied the phase transformations in a Nd-Fe-B-type multiphase metallic powder during FAST. The formation of the α-Fe phase, following the peritectic decomposition of the Nd2Fe14B matrix phase expected at ≈1180 °C (TPER), was observed for FAST temperatures (TFAST) below TPER. A correlation between the electric current and the final phase composition, which can only be explained by considering the local overheating effect, was established. We showed that the formation of the α-Fe phase at TFAST <TPER can be mitigated by (i) decreasing the electric current through the sample, which is achieved by lowering the heating rate from 100 to 10 °C/min or by using electrically highly conductive pressing tools (WC) and a non-conductive coating (BN), or by (ii) interparticle necking achieved through a thermal pre-treatment of the powder compact that decreases the overall resistance. Our findings emphasize the criticality of the electric current modulation to minimize any undesired phase transformation, paving the way for future developments in rapid, FAST-based strategies aimed at refining the microstructures and tailoring the properties of multiphase metallic materials.
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•Micro-nonuniform heating during FAST was experimentally verified for Nd-Fe-B system.•Local overheating due to the Joule effect leads to formation of α-Fe phase.•Matrix phase decomposition can be prevented by decreasing the current in the sample.•Nonequilibrium microstructure formation influences the densification kinetics.
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.
•TRE-lean jet-milled Nd-Fe-B powders can be consolidated to full density with SPS.•The rapid non-equilibrium SPS hinders the grain growth during the sintering step.•The electrical effects associated ...with the SPS govern the microstructure formation.•A post-SPS thermal treatment is necessary for the final hard-magnetic properties.
Sintered Nd-Fe-B permanent magnets are the first choice for electro-mechanical devices that rely on hard-magnetic materials to provide strong magnetic fields in a variety of operating conditions. However, the limitations of conventional powder-metallurgy methods regarding the complexity of the magnet’s geometry restrict the design freedom for electrical motors. Here, we propose a spark-plasma-sintering (SPS) approach to the waste-free net-shape manufacture of anisotropic Nd-Fe-B magnets. We investigated the effect of the SPS parameters on the density, magnetic properties and microstructure of disc-shaped samples prepared from a rare-earth-lean jet-milled powder. The absence of a grain-boundary phase and the presence of α-Fe in the as-sintered samples were identified as the main factors hindering the development of the intrinsic coercivity. The observed microstructural features were correlated with electrical effects specific to the rapid, non-equilibrium SPS. A post-SPS thermal treatment was found to be necessary for achieving the hard-magnetic properties. Our findings pave the way towards developing an SPS-based sintering procedure with great potential for the manufacture of complex- and net-shape permanent magnets for high-performance electrical devices.
•Nd-Fe-B magnets with locally tailored magnetic properties can be prepared with SPS.•Consolidation temperature of ≈650 °C is favorable for co-sintering multiple melt-spun powders.•Nd-Dy-Fe-B powder’s ...coercivity (2075 kA/m) is preserved in a multicomponent magnet.•Interdiffusion between different Nd-Fe-B materials during sintering is hindered.•Reliable performance over a wide range of operating temperatures is ensured.
The magnetic properties of an Nd-Fe-B-type permanent magnet depend on the microstructure and the chemistry of the material. To compensate for unfavorable microstructural features, such as micron-sized grains of the hard-magnetic phase, the intrinsic coercivity is often enhanced by the addition of heavy-rare-earth elements, but these additions also reduce the magnetization. In some applications, like electro-mechanical devices, a high intrinsic coercivity is only required in certain regions of a magnet. In such cases, using magnets with locally tailored magnetic properties, i.e., multicomponent magnets, would enhance the magnet-containing device’s performance. Here, we propose a spark-plasma-sintering (SPS) approach to the manufacture of multicomponent Nd-Fe-B magnets. By exploiting the SPS-specific processing conditions, namely fast heating rates (100 °C/min) and low consolidation temperatures (≈670 °C), such magnets can be prepared directly from nanostructured melt-spun powders (the one-step SPS approach) or SPS-processed precursor magnets (the two-step SPS approach). Optimizing the SPS processing conditions prevents the grain coarsening related to the pre-existing microstructural inhomogeneities of the powders. The magnetic characterization reveals reliable performance over a wide range of operating temperatures. The high remanent magnetization of a heavy-rare-earth-free powder (Br = 0.82 T) and the high intrinsic coercivity of a powder containing 1.5 at. % Dy (Hci = 2075 kA/m) are preserved in a multicomponent magnet characterized by an abrupt change in magnetic properties close to the interface between the respective magnet parts.
Different grades of Nd-Fe-B permanent magnets are available on the market today and their magnetic properties highly depend on the microstructure, which is controlled by the chemical composition of ...the alloy and the magnets' manufacturing route. Gas atomization is a rapid solidification technique that is seldom used for the production of Nd-Fe-B magnetic powders due to the cooling rates that are several orders of magnitude lower than those achieved with the melt spinning. In addition, rare-earth-rich powders experience significant loss of hard magnetic properties at high temperatures in air. We prepared a novel type of bulk Nd-Fe-B permanent magnet from gas-atomized powders using the Spark Plasma Sintering (SPS) technique. Samples manufactured from rare-earth-poor material were porous, while high density was achieved with a neodymium-rich powder. The high-temperature instability was effectively overcome with compaction of the atomized material into a dense magnet. The intrinsic coercivity of the latter surpassed the value of the optimally heat-treated spherical powder. The samples' magnetic, as well as the mechanical properties, reflected the unfavourable microstructural characteristics of the initial atomized material. Refinement of the microstructure achieved with sieving increased the coercivity of the heavy-rare-earth-free bulk magnet to ∼1000 kA/m.
•Dense bulk Nd-Fe-B magnets can be prepared from the TRE-rich gas-atomized powders.•SPS processing significantly improves the intrinsic coercivity of a Nd-rich powder.•Raw powder's high-temperature instability is overcome with the SPS consolidation.•Mechanical properties of bulk magnets reflect the inhomogeneous microstructure.
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•High coercivity is required only at the magnet's edges in wind turbine generators.•Several NdFeB materials with distinct properties can be arranged in a single magnet.•Conventional ...and SPS sintering are viable for preparation of multicomponent magnets.•Nonuniform distribution of magnetization is favorable for the target applications.
Magnetic remanence (Br) and coercivity (Hc) of hard magnets play a crucial role in electro-mechanical devices like electrical motors or generators. Elevated remanence ensures high efficiency of the device, and large coercivity protects the magnet from demagnetization. Usually, the increase of one comes at the cost of the other, such that the material optimization has a trade-off character. In this work, we demonstrate that in many electrical machines which use permanent magnets, high coercivity is required mainly at the edges of the magnet. This enables us to design a novel form of a permanent magnet, called a multicomponent magnet, where the sides of the magnet have high coercivity, while the central part is characterized by high remanence. Such multicomponent magnets are realized by two independent methods, namely, conventional high-temperature sintering and Spark Plasma Sintering (SPS). The magnetic and mechanical performances are suitable for applications, and simulation results using the magnetic characteristics of the multicomponent geometry predict its benefit on the device functionality. Further tuning of the geometry of the multicomponent permanent magnets opens avenues for promising applications, particularly in electrical motors for electrical vehicles.
•PE depolymerization was investigated in the presence of natural aluminosilicates.•Acid leaching and Al grafting significantly increased concentration of acid sites.•Mesoporosity and Brønsted acidity ...enable high PE depolymerization activity.•53% liquid, 0.4% coke and 46.6% gas yields were achieved using MAl catalyst.•Alkanes constitute the liquid phase, whereas alkenes are only present in the gas phase.
Different chemical modifications were performed to the natural aluminosilicate Montanit300® in order to improve its catalytic activity in PE depolymerization. Performance of such prepared catalysts was compared to established solid acid catalysts, such as HZSM-5, sulfonated and fluorinated γ-Al2O3 and amorphous silica–alumina. Pyridine TG and DRIFTS characterization revealed mild acid treatment and aluminum grafting as successful in increasing acid site density through impurity removal and specific surface area increase. Mesoporous catalyst structure that allows facile diffusion through its pore network, together with high-density Brønsted acid sites, was found to be crucial to obtain high catalytic activity. The T50 value for PE depolymerization was lowered by 162°C with sulfonated γ-Al2O3 solid, compared to non-catalyzed reaction, whereas with aluminum-grafted Montanit300® catalyst this value was lowered by 65°C. PE depolymerization products present in the condensed liquid phase using aluminum-grafted Montanit300® catalyst were exclusively alkanes with chain length up to 21 carbon atoms. Liquid, coke and gas yields were found to be 53, 0.4 and 46.6%, respectively, the latter consisting of linear and branched C2–C4 alkenes and alkanes.
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