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Flaky structures can effectively enhance the microwave absorbing properties of metallic magnetic particles, but the interaction between the electromagnetic waves and particles in different directions is not well understood. Here, the oriented FeSiAl/paraffin composites were prepared under a rotational magnetic field and made into co-axial rings for electromagnetic measurements. The FeSiAl flakes in the rings are oriented along different angles with respect to the electric and magnetic fields of the microwave. The effects of the angles on the electric and magnetic properties as well as the microwave absorbing properties of the composite are investigated. It is found that the electric and magnetic properties can be modified independently by adjusting the angles between the flakes and the electric field and between the flakes and the magnetic field, respectively. The underlying mechanisms can be understood using electromagnetic and micromagnetic simulations. The anisotropic permittivity originates from the induced electric field of the flaky particles, while the anisotropic permeability results from the effect of the demagnetization field. An optimal orientation structure can reduce the permittivity by suppressing the induced electric field while maintaining high permeability by reducing the demagnetization energy in the magnetization process, which is beneficial to the impedance matching of the composite. Thus, the orientation structure is extremely efficient in regulating the microwave absorption performance of the flaky particles. This work proposed a general strategy for microwave absorbers to modulate microwave absorption performances between high-frequency and low-frequency by simply operating the orientation structures of the flaky particles. [ABSTRACT FROM AUTHOR]

Flake-shaped FeSiCr (FFSC) material is expected to be a promising microwave absorbent due to its excellent magnetic properties and environmental resistance. By introducing carbon-based materials through suitable coatings, the electromagnetic parameters and energy loss can be tuned to improve the performance of FFSC. A facile solution-blending method was deployed to prepare graphite- and epoxy resin-encapsulated FFSC (FFSC@G/E) powders with a core–shell structure. FFSC@G2000/E showed excellent performance in the X band (8–12 GHz), a minimum reflection loss (RLmin) of −42.77 dB at a thickness of 3 mm and a maximum effective absorption bandwidth (EABmax, RL < −10 dB) that reached 4.55 GHz at a thickness of 2.7 mm. This work provides a route for the production of novel high-performance microwave absorbers. [ABSTRACT FROM AUTHOR]

Domestic induction heating shows advantages over traditional open fire heating, but the uneven temperature distribution in the pan is still one of the problems to be solved. In this work, to improve the heating uniformity of the induction cooker, the finite element analysis (FEA) is employed to investigate the relationship between the structure of induction coil and the temperature distribution in the heating pan. The results indicate that the structure of single-coil can be optimized by varying the winding turns in different positions. More uniform temperature distribution in the pan has been obtained by increasing the number of turns at the center and edge of the coil. The advance of this design is to achieve uniform induction heating without redesigning the power source or adding an additional coil. Based on the simulation results, the refined induction coil has been fabricated and assembled into a commercial domestic induction cooker with little modification of the power supply system. A specific parameter named relative standard deviation (RSD) is proposed to define the temperature uniformity. By real-time temperature measurement, the improved heating uniformity has been demonstrated for the refined system. The present work provides a cost-effective and efficient solution to improve heating performance of domestic induction cooker. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Back extrusion is an important process to prepare radially oriented NdFeB ring magnets. In this work, we fabricate the ring magnets using amorphous magnetic powders as the raw material. The microstructure, magnetic properties, corrosion resistance, and mechanical properties of the backward extruded magnet at different positions along the axial direction have been investigated, and the inhomogeneity of the magnet is clarified. The results showed that the grains in the middle region of the ring magnet exhibit a strong c-axis orientation, whereas the grains at the bottom and top regions are disordered with random orientation. The microstructure variation is related to the distribution of the grain boundary phase and the degree of grain deformation. Due to the microstructure difference, the magnetic properties, temperature stability, corrosion resistance, and mechanical properties in the middle region of the magnet are higher than those in the top and bottom regions. The exchange coupling between grains also varies in different regions, which is related to the grain size and grain boundary thickness. In addition, different Co element segregations were observed in different regions, which has a crucial effect on the Curie temperature and thermal stability of the magnet. The microstructure difference also leads to the variation of corrosion resistance and mechanical properties for the samples from different regions of the magnet. This work suggests that the amorphous powder can be used to directly prepare radially oriented ring magnets, and the inhomogeneity of the magnet should be fully understood. [ABSTRACT FROM AUTHOR]

The preparation of nanocrystalline anisotropic NdFeB permanent magnets generally includes two steps, hot pressing (HP) and hot deformation (HD). Herein, one‐step HD process is proposed to fabricate NdFeB magnets using amorphous powders. The effects of deformation ratio on the microstructure, magnetic properties, and corrosion resistance are investigated. Compared to traditional HP and HD process, one‐step HD can significantly reduce the volume fraction of coarse‐grain zone in the magnet. The increase of deformation ratio can promote the formation of the textured platelet‐shaped grains and uniform grain boundary phase. The magnetic properties, temperature stability, and corrosion resistance of the magnets increase with increasing deformation ratio until 80%. The best magnetic properties with intrinsic coercivity Hcj = 930 kA m−1, remanence Jr = 1.17 T, and maximum energy product (BH)max = 263 kJ m−3 have been achieved for the magnet prepared by one‐step HD, which are comparable to those obtained by conventional HP/HD. In addition, different Co element segregations are observed in the magnets with different deformation ratios, which have important effects on the Curie temperature and thermal stability of the magnets. The results suggest that, by replacing HP/HD process and using amorphous powder, one‐step HD can simplify the process while maintaining good properties. [ABSTRACT FROM AUTHOR]

LaFe11.6Si1.4/10wt%PrxCo7 (x = 1, 2, 3, 5) magnetocaloric composites were prepared by spark plasma sintering (SPS) and diffusion annealing. The phase composition, microstructure, magnetic, mechanical and thermal properties were studied. The addition of PrxCo7 binder can be used to tune the phase fraction and magnetic properties. Pr2Co7 binder promoted the formation of 1:13 phase. LaFe11.6Si1.4/10wt%Pr2Co7 composites possess the highest 1:13 content (~ 89 wt%). The Curie temperature (TC) can be tuned in the technologically useful range of 280–347 K. Relatively large maximum magnetic entropy change (− ∆SM)max (2 T) of 1.45−3.16 J/kg K and refrigeration capacity (RC) (2 T) of 117–137 J/kg were obtained. These composites exhibit superior compressive strength of 1005–1250 MPa and excellent thermal conductivity of 17.68–27.77 W/m K. Thus, LaFe11.6Si1.4/10wt%PrxCo7 magnetocaloric composites possess excellent mechanical and thermal properties, with adjustable magnetic properties and have the potential application for near room temperature magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) of multiple rare earths has proven effective in enhancing the performance of permanent magnets, whereas the diffusion behaviors are still unclear due to their intricate interactions. In this work, the synergistic effects of three rare earths (Tb, Ce, and La) on the grain boundaries diffusion process are investigated, achieving a high‐performance diffused Nd–Fe–B magnet with the high coercivity of 1822 kA m−1 and thermal stability of −0.447% K−1. This promotion originates from the improved diffusion depth of Tb at specific regions, induced by the coordination effects of Ce and La, as evidenced via electron probe microanalysis and the first‐principles calculations. It thus results in the uniform formation of Tb‐diffused magnetic harden grains, associated with the strongly pinned domain walls, which is further observed directly by in situ Lorentz transmission electron microscopy and reconstructed by micromagnetic simulations. The study provides insight into the role of element GBD mechanism and has promising potential to explore other element systems as diffusion sources for permanent magnets. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) efficiency of heavy rare-earth (HRE) elements for Nd–Fe–B magnets is closely related to the chemical composition of the initial magnets but this has not been well studied yet. Here, we demonstrated the significant difference in the diffusion efficiency of Tb for Pr–Nd–Fe–B and Nd–Fe–B magnets. After the GBD treatment with Tb70Cu30 alloy, the coercivity of the Pr–Nd–Fe–B magnet increases from 1324 to 1837 kA m−1, while that of the Nd–Fe–B magnet increases from 1269 to 1952 kA m−1. The Nd–Fe–B magnet achieves a significantly higher coercivity enhancement of 54% than Pr–Nd–Fe–B (39%) mainly due to the deeper infiltration depth of Tb in the Nd–Fe–B magnet. Since Pr is more active than Nd, more oxygen is introduced into the Pr–Nd–Fe–B magnet during powder metallurgy. A great amount of rare-earth oxides existing at the grain boundaries block the channels for Tb diffusion and result in the accumulation of Tb at the diffusion surface, which prevents the deep diffusion of Tb in the Pr–Nd–Fe–B magnet. In addition, both experiment and thermodynamic calculations demonstrate that HREs of Tb and Dy prefer to react with the existing (Pr,Nd)2O3 at the triple-junction regions, which causes unnecessary consumption of HRE at triple-junction phase. Hence, to achieve high GBD efficiency, the Pr-free Nd–Fe–B initial magnets with low content of O are preferable. The present results indicate that the rational selection of initial magnets is important for performance improvement by GBD treatment and also for the efficient use of rare-earth resources. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) is an effective approach for improving the coercivity of Nd-Fe-B magnets with less heavy rare earth consumption. However, it generally only works well for thin magnets since the infiltration depth of diffusion source is limited. Here, a GBD approach that mainly diffuses the source from the edge and corner areas of the magnet, named selected area GBD (SAGBD), is proposed for treating thick magnets. After diffusing Pr-Tb-Al-Cu alloy by SAGBD, the coercivity of a 12-mm thick magnet was increased from 1070 to 1675 kA/m. By this approach, the coercivity enhancement with 1 wt. % Tb was 465 kA·m−1, 1.6 times as much as that by diffusion from two easy magnetization planes. For achieving a coercivity enhancement of ∼600 kA/m, the thickness of treated magnet was increased by 4 mm. Although the diffusion from six planes can further increase the coercivity, SAGBD leads to high efficient use of Tb and less reduction of remanence. The microstructure characterizations and micromagnetic simulation demonstrated that the edges and corners are magnetic "weak" areas of the magnet, and SAGBD can effectively hinder the nucleation of reversed domains in these places and the rapid reversal propagation in whole magnet. The present approach is important for the high-efficient use of the diffusion source and the fabrication of large-size Nd-Fe-B products with high stability. [ABSTRACT FROM AUTHOR]