Rare earth permanent magnets constitute a mature technology, but the shock of the 2011 rare earth crisis led to the re-evaluation of many ideas from the 1980s and 1990s about possible new hard magnets containing little or no rare earth (or heavy rare earth). Nd–Fe–B magnets have been painstakingly and skillfully optimized for a wide range of applications in which high performance is required at reasonable cost. Sm–Co is the material of choice when high-temperature stability is required, and Sm–Fe–N magnets are making their way into some niche applications. The scope for improvement in these basic materials by substitution has been rather thoroughly explored, and the effects of processing techniques on the microstructure and hysteresis are largely understood. A big idea from a generation ago—which held real potential to raise the record energy product significantly—was the oriented exchange-spring hard/soft nanocomposite magnet; however, it has proved very difficult to realize. Nevertheless, the field has evolved, and innovation has flourished in other areas. For example, electrical personal transport has progressed from millions of electric bicycles to the point where cars and trucks with electrical drives are becoming mainstream, and looks ready to bring the dominance of the internal combustion engine to an end. As the limitations of particular permanent magnets become clearer, ingenuity and imagination are being used to design around them, and to exploit the available mix of rare earth resources most efficiently. Huge new markets in robotics beckon, and the opportunities offered by additive manufacturing are just beginning to be explored. New methods of increasing magnet stability at elevated temperature are being developed, and integrated multifunctionality of hard magnets with other useful properties is now envisaged. These themes are elaborated here, with various examples.
Nanoscale permanent magnetic materials, which possess excellent magnetic and mechanical properties, thermal stability, and corrosion resistance, have become a research hotspot for permanent magnets. In reality, however, the obtained maximum energy product, (BH)max, is not satisfactory in comparison with the theory limit, especially for exchange-coupled nanocomposite magnets. The construction of an ideal microstructure still remains a challenge in the synthesis and preparation of nanoscale permanent magnets. This work reported the impact of rapid thermal process (RTP) with electron-beam heating on the microstructures of Nd12.5-xFe80.8+xB6.2Nb0.2Ga0.3 (x = 0, 2.5) nanocomposites. It was found that the crystallization time was greatly reduced, from 15 min under the conventional annealing conditions to 0.1 s under the RTP. For Nd2Fe14B single-phase materials, the crystallization temperature of the RTP ribbons decreased by about 248 °C compared with that of the ribbons produced by the conventional annealing method. A synergetic crystallization of the Nd2Fe14B and α-Fe phases was observed under the RTP, which restrained not only the shape, size distribution, and compositions of the hard and the soft phases, but also the interface between them. This modification effect became more obvious as the fraction of Fe increased. Due to the improvement in the uniformity of the Nd2Fe14B and α-Fe phases, and their grain size distribution, better magnetic properties were achieved using RTP in comparison with the conventional annealing method.
Iron-rich compounds with the tetragonal ThMn12-type structure have the potential to meet current demands for rare-earth-lean permanent magnets with high energy density and operating temperatures of 150–200 ℃. However, while it is normal for magnet technology to lag behind the development of underlying magnetic material, this gap has always been unusually large for ThMn12-type magnets. The gap has widened further in recent years, as excellent combinations of intrinsic magnetic properties have been obtained in compounds synthesized with a smaller amount of structure-stabilizing elements (e.g., SmFe11V or Sm0.8Zr0.2Fe9.2Co2.3Ti0.5) or with no such elements (i.e., SmFe9.6Co2.4 thin films). The search for promising compounds continues—with increasing help coming from theoretical calculations. Unfortunately, progress in the development of magnets beyond polymer-bonded interstitially modified powders remains marginal. The introduction of lanthanum (La) was found to stabilize low-meltingtemperature minority phases in Sm(Fe,Ti)12 alloys, thus allowing for liquid-phase sintering for the first time. The high reactivity of La, however, has apparently undermined the development of coercivity (Hc). A controlled crystallization of the initially suppressed ThMn12-type phase makes ″bulk″ magnetic hardening possible, not only in Sm-Fe-V alloys (in which it has been known since the 1990s), but also is in La-added (Ce,Sm)(Fe,Ti)12 alloys. The properties of the bulk-hardened alloys, however, remain unsatisfactory. Mechanochemically synthesized (Sm,Zr)(Fe,Si)12 and (Sm,Zr)(Fe,Co,Ti)12 powders may become suitable for sintering into powerful fully dense magnets, although not before a higher degree of anisotropy in both alloys and a higher Hc in the latter alloy have been developed.
Multiscale simulation is a key research tool in the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Iron (Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms. The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure. Using machine learning techniques, the magnet's structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ·m−3)) were obtained for iron-tin-antimony (Fe3Sn0.75Sb0.25): (0.49, 290), L10-ordered iron-nickel (L10 FeNi): (1, 400), cobalt-iron-tantalum (CoFe6Ta): (0.87, 425), and manganese-aluminum (MnAl): (0.53, 80).
In this article, we report on the magnetic structure of DyFe11Ti and its thermal evolution as probed by neutron powder diffraction. A thermodiffraction technique was used to follow the temperature dependence of the magnetic moments, as well as their orientation. The Dy and Fe moments were coupled to each other in an antiparallel manner to form a ferrimagnet, where the easy magnetization direction at 2 K was the [
Amorphous Sm–Co films with uniaxial in-plane anisotropy have great potential for application in informationstorage media and spintronic materials. The most effective method to produce uniaxial in-plane anisotropy is to apply an in-plane magnetic field during deposition. However, this method inevitably requires more complex equipment. Here, we report a new way to produce uniaxial in-plane anisotropy by growing amorphous Sm–Co films onto (011)-cut single-crystal substrates in the absence of an external magnetic field. The tunable anisotropy constant, kA, is demonstrated with variation in the lattice parameter of the substrates. A kA value as high as about 3.3 × 104 J·m−3 was obtained in the amorphous Sm–Co film grown on a LaAlO3(011) substrate. Detailed analysis indicated that the preferential seeding and growth of ferromagnetic (FM) domains caused by the anisotropic strain of the substrates, along with the formed Sm–Co, Co–Co directional pair ordering, exert a substantial effect. This work provides a new way to obtain in-plane anisotropy in amorphous Sm–Co films.
Given the increasing concern regarding the global decline in rare earth reserves and the environmental burden from current wet-process recycling techniques, it is urgent to develop an efficient recycling technique for leftover sludge from the manufacturing process of neodymium-iron-boron (Nd–Fe–B) sintered magnets. In the present study, centerless grinding sludge from the Nd–Fe–B sintered magnet machining process was selected as the starting material. The sludge was subjected to a reduction-diffusion (RD) process in order to synthesize recycled neodymium magnet (Nd2Fe14B) powder; during this process, most of the valuable elements, including neodymium (Nd), praseodymium (Pr), gadolinium (Gd), dysprosium (Dy), holmium (Ho), and cobalt (Co), were recovered simultaneously. Calcium chloride (CaCl2) powder with a lower melting point was introduced into the RD process to reduce recycling cost and improve recycling efficiency. The mechanism of the reactions was investigated systematically by adjusting the reaction temperature and calcium/sludge weight ratio. It was found that single-phase Nd2Fe14B particles with good crystallinity were obtained when the calcium weight ratio (calcium/sludge) and reaction temperature were 40 wt% and 1050 °C, respectively. The recovered Nd2Fe14B particles were blended with 37.7 wt% Nd4Fe14B powder to fabricate Nd–Fe–B sintered magnets with a remanence of 12.1 kG, and a coercivity of 14.6 kOe, resulting in an energy product of 34.5 MGOe. This recycling route promises a great advantage in recycling efficiency as well as in cost.
An unprecedentedly short milling time of 30 s was applied to gas-atomized MnAl powder in order to develop permanent magnet properties and, in particular, coercivity. It is shown that such a short processing time followed by annealing results in efficient nanostructuring and controlled phase transformation. The defects resulting from the microstrain induced during milling, together with the creation of the β-phase during post-annealing, act as pinning centers resulting in an enhanced coercivity. This study shows the importance of finding a balance between the formation of the ferromagnetic τ-MnAl phase and the β-phase in order to establish a compromise between magnetization and coercivity. A coercivity as high as 4.2 kOe (1 Oe = 79.6 A·m-1 ) was obtained after milling (30 s) and annealing, which is comparable to values previously reported in the literature for milling times exceeding 20 h. This reduction of the postannealing temperature by 75 ℃ for the as-milled powder and a 2.5-fold increase in coercivity, while maintaining practically unchanged the remanence of the annealed gas-atomized material, opens a new path for the synthesis of isotropic MnAl-based powder.
Ferroelastic ABO4 type RETaO4 and RENbO4 ceramics (where RE stands for rare earth) are being investigated as promising thermal barrier coatings (TBCs), and the mechanical properties of RETaO4 have been found to be better than those of RENbO4. In this work, B-site substitution of tantalum (Ta) is used to optimize the thermal and mechanical properties of EuNbO4 fabricated through a solid-state reaction (SSR). The crystal structure is clarified by means of X-ray diffraction (XRD) and Raman spectroscopy; and the surface microstructure is surveyed via scanning electronic microscope (SEM). The Young's modulus and the thermal expansion coefficient (TEC) of EuNbO4 are effectively increased; with respective maximum values of 169 GPa and 11.2×10–6 K–1 (at 1200 °C). The thermal conductivity is reduced to 1.52 W·K−1·m−1 (at 700 °C), and the thermal radiation resistance is improved. The relationship between the phonon thermal diffusivity and temperature was established in order to determine the intrinsic phonon thermal conductivity by eliminating the thermal radiation effects. The results indicate that the thermal and mechanical properties of EuNbO4 can be effectually optimized via the B-site substitution of Ta, and that
this proposed material can be applied as a high-temperature structural ceramic in future.
On-aim control of protein adsorption onto a solid surface remains challenging due to the complex interactions involved in this process. Through computational simulation, it is possible to gain molecular-level mechanistic insight into the movement of proteins at the water-solid interface, which allows better prediction of protein behaviors in adsorption and fouling systems. In this work, a mesoscale coarse-grained simulation method was used to investigate the aggregation and adsorption processes of multiple 12-Ala hydrophobic peptides onto a gold surface. It was observed that around half (46.6%) of the 12-Ala peptide chains could form aggregates. 30.0% of the individual peptides were rapidly adsorbed onto the solid surface; after a crawling process on the surface, some of these (51.0%) merged into each other or merged with floating peptides to form adsorbed aggregates. The change in the solid-liquid interface due to peptide deposition has a potential influence on the further adsorption of single peptide chains and aggregates in the bulk water. Overall, the findings from this work help to reveal the mechanism of multi-peptide adsorption, and consequentially build a basis for the understanding of multi-protein adsorption onto a solid surface.
In this research, two novel folded lattice-core sandwich cylinders were designed, manufactured, and tested. The lattice core has periodic zigzag corrugations, whose ridges and valleys are directed axially or circumferentially. Free vibration and axial compression experiments were performed to reveal the fundamental frequency, free vibration modes, bearing capacity, and failure mode of the cylinder. A folded lattice core effectively restricts local buckling by reducing the dimension of the local skin periodic cell, and improves the global buckling resistance by enhancing the shear stiffness of the sandwich core. The cylinders fail at the mode of material failure and possess excellent load-carrying capacity. An axially directed folded sandwich cylinder has greater load-carrying capacity, while a circumferentially directed folded sandwich cylinder has higher fundamental frequencies. These two types of folded lattices provide a selection for engineers when designing a sandwich cylinder requiring strength or vibration. This research also presents a feasible way to fabricate a large-dimensional folded structure and promote its engineering application.