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.
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•A single crystal of the Al5Fe2 compound is grown and characterized.•Its formation enthalpy and electronic structure are determined by DFT.•The (1 0 0) and (0 0 1) surfaces are ...investigated at the atomic scale.•The (0 0 1) surface exhibits a reconstruction due to the presence of Fe missing rows.•The (1 0 0) surface exhibits an incommensurate modulated structure.
The bulk structure of the η-Al5Fe2 intermetallic compound consists in an ordered framework of pentagonal antiprims with composition Al2Fe inside which nearly continuous chains of fractionally occupied Al sites exist. At low temperature, ordering of these Al channel atoms can occur, leading to various superstructures. Although the Al5Fe2 intermetallic phase is of technological importance being present in protective coating for steel parts, a detailed surface investigation of this compound has not been reported yet. Here we describe the growth of a single crystal of the Al5Fe2 compound by the Czochralski method. Its bulk structure is identified as the low temperature polymorph η″. Density Functional Theory calculations were performed to determine its formation enthalpy and its electronic structure. A deep pseudogap is noticeable at the Fermi energy, and this compound is found to be magnetic. Two samples have been extracted, presenting a surface oriented either perpendicular to the channel’s direction ((0 0 1) surface) or parallel to them ((1 0 0) surface). The two surfaces have been investigated by X-ray photoemission spectroscopy, low-energy electron diffraction and scanning tunneling microscopy. Both surfaces exhibit some superstructures of various complexities, whose origin can be explained in the light of density functional theory calculations performed on model surfaces.
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.
•Successful print of PPS filament mixed with Sr-ferrite in a commercially available material extrusion 3D printer.•Achieved full anisotropy printed magnetic material, just by using a permanent magnet ...for particle orientation.•3D printed magnetic materials using material extrusion can be used in harsh environments.
We describe the fabrication of SrFe12O19-based filaments, using polyphenylene sulphide (PPS) as the binder for the magnetic particles, and the subsequent printing of this filament with a 3D printer. PPS is an ideal polymer for applications in harsh environments, making it applicable for the automotive industry, where it is widely used with injection moulding. However, 3D printing this polymer introduces a major challenge. Because PPS is more difficult to extrude than polyamide, the filling factor in this study was set to 70 wt. %, which is lower than when used in injection moulding (close to 90 wt. %). The filament with a diameter of 2.75 mm was printed into a disk-shaped magnet with a diameter of 10 mm and a height of 4 mm using a HAGE 3D printer that uses a belt system for the filament extrusion. The magnets were printed onto a glass surface and onto a bulk Nd-Fe-B permanent magnet with an external magnetic field, parallel to the printer’s z-axis. Printing in the presence of a magnetic field was found to increase the magnet’s remanent magnetization by 61%, compared to an isotropic print. Without an external magnetic field we achieved a remanence of 23.9 emu/g for the 70 wt.% filling fraction, while when printing in a magnetic field, the value of the remanence improved to 39.7 emu/g because of the improved magnetic texture.
As the debate about TiO
2
food additive safety is still open, the present study focuses on the extraction and characterisation of TiO
2
(nano)particles added as a whitening agent to confectionary ...products, that is, chewing gum pellets. The aim was to (1) determine the colloidal properties of suspensions mutually containing TiO
2
and all other chewing gum ingredients in biologically relevant media (preingestion conditions); (2) characterise the TiO
2
(nano)particles extracted from the chewing gum coating (after ingestion); and (3) verify their potential photocatalysis. The particle size distribution, in agreement with the zeta potential results, indicated that a small but significant portion of the particle population retained mean dimensions close to the nanosize range, even in conditions of moderate stability, and in presence of all other ingredients. The dispersibility was enhanced by proteins (i.e., albumin), which acted as surfactants and reduced particle size. The particle extraction methods involved conventional techniques and no harmful chemicals. The presence of TiO
2
particles embedded in the sugar-based coating was confirmed, including 17–30% fraction in the nanorange (<100 nm). The decomposition of organics under UV irradiation proved the photocatalytic activity of the extracted (nano)particles. Surprisingly, photocatalysis occurred even in presence of an amorphous SiO
2
layer surrounding the TiO
2
particles.
The purpose of this study is to find new ternary intermetallic compounds within the Al-Cr-Sc system, which is typical of a push-pull system in which two constituents (Cr and Sc) are immiscible, ...whilst they form compounds, respectively, with the third constituent (Al). By arc-melting of the three components aluminium (Al), chromium (Cr) and scandium (Sc) under inert atmosphere, a range of different alloys was produced. The microstructure was observed using scanning electron microscopy (SEM) and the phase compositions were analysed by electron dispersive X-ray spectroscopy (EDS). The samples were measured with X-ray diffraction (XRD) and transmission electron microscopy. The results were confirmed by Rietveld refinement. Three different phases were found in the microstructure of the samples, the two known binary phases Al16Cr10 and Al3Sc and a new ternary phase coined ϕ-Al8Cr4Sc. ϕ-Al8Cr4Sc showed great similarities to other Al8Cr4RE compounds with tetragonal I4/mmm crystal structure. Starting from the structure of already known Al8Cr4RE crystals, ab initio calculations were performed to determine the crystallographic parameters of ϕ-Al8Cr4Sc and also to investigate its electronic structure, which identified the energy band factor as the key factor that determines the stability of this compound. The lattice parameters and atomic positions were found in good agreement with the ones obtained by Rietveld refinement. Experimental atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image analysis of the ϕ-Al8Cr4Sc phase confirmed the predicted atomic model.
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•For the first time, a ternary phase was uncovered in the Al-Cr-Sc push-pull alloy.•The crystalline structure is tetragonal, I4/mmm, with 26 atoms per unit cell.•DFT computation of the DOS gives insight in the stability of the compound.•The respective roles of hybridization and Hume-Rothery mechanisms are discussed.
Simple, low-cost methods for sensing volatile organic compounds that leave no trace and do not have a detrimental effect on the environment are able to protect communities from the impacts of ...contaminants in water supplies. This paper reports the development of a portable, autonomous, Internet of Things (IoT) electrochemical sensor for detecting formaldehyde in tap water. The sensor is assembled from electronics, i.e., a custom-designed sensor platform and developed HCHO detection system based on Ni(OH)sub.2-Ni nanowires (NWs) and synthetic-paper-based, screen-printed electrodes (pSPEs). The sensor platform, consisting of the IoT technology, a Wi-Fi communication system, and a miniaturized potentiostat can be easily connected to the Ni(OH)sub.2-Ni NWs and pSPEs via a three-terminal electrode. The custom-made sensor, which has a detection capability of 0.8 µM/24 ppb, was tested for an amperometric determination of the HCHO in deionized (DI) and tap-water-based alkaline electrolytes. This promising concept of an electrochemical IoT sensor that is easy to operate, rapid, and affordable (it is considerably cheaper than any lab-grade potentiostat) could lead to the straightforward detection of HCHO in tap water.
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