Kwang-Won Jeon

Synthesis and Magnetic Properties of BaFe

The effect of Al contents on the magnetic properties of Ba hexaferrite powders has been investigated in order to increase the coercivity. In this study, a self-propagating combustion method has been developed to form Ba hexaferrite nanopowders. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis showed that nanopowders with a size of 40-60 nm were successfully produced. With increasing Al contents, the coercivity of the nanopowders increased until the powders had a magnetoplumbite structure. We obtained a coercivity of 7559 Oe in BaFe10Al2O19 powders calcined at 850deg for 1 h.

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We report the growth of porous magnetite microspheres via a template-free hydrothermal route at relatively low temperatures. The growth parameters including the concentration of reactant, reaction temperature and reaction time were examined for the formation of nanopores in the microspheres. Ascorbic acid played a crucial role in the formation of nanoporous microspheres, and also acts as a reducing agent, resulting from Fe3+ conversion into Fe2+ iron during the hydrothermal process. The resulting samples were used to remove acid orange 7 (AO7) and Cr(VI) from water at room temperature via adsorption. The maximum AO7 adsorption capacity calculated from the Langmuir–Freundlich model was found to be 69.13 mg g−1 for an optimum porous structure, which is significantly greater than that of structures formed without ascorbic acid (i.e., 4.61 mg g−1). Simultaneously, the Cr(VI) removal performance of the porous structure (i.e., 96.98 mg g−1) was found to be superior than that of pristine structure (i.e., 10.67 mg g−1). The enhanced removal activity was attributed to the existence of a large number of nanopores, resulting in a large specific surface area of 87.36 m2 g−1. Recycling tests were carried using a magnetic field to separate the powder samples from water, and the samples were found to be stable for up to three cycles. These results show that these porous microspheres have potential applications in cost-effective wastewater treatment.

Al

MnBi magnets were prepared by melt-spinning, annealing, ball-milling, magnetic alignment, and subsequent hot compaction process. Mn-Bi melt-spun ribbons contained mainly unreacted Mn and Bi phases, including the low-temperature phase of MnBi. To increase the amount of this MnBi phase, these melt-spun ribbons were annealed under an oxygen-controlled atmosphere. As a result, a maximum saturation magnetization value of 57.90 emu/g was obtained when the melt-spun ribbons were annealed at 300 °C for 12 h. Bulk magnets fabricated by magnetic alignment and hot compaction processes showed a density of 8.71 g/cm3, a Mr/Ms ratio of 0.79, and a (BH)max value of 6.7 MGOe at room temperature. With the temperature increasing, the coercivity changed from 6387 Oe at room temperature to 18540 Oe at 150 °C and a (BH)max measured 5.5 MGOe at 150 °C.

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Magnetic materials has been applied to various fields due to their energy convertible properties between electrical and mechanical energy. Particularly, permanent magnets have been currently attracted much attention because they produce external magnetic field without any electrical current. For high efficiency, a demand for permanent magnets containing rare earth elements has been continuously increased, which abruptly raises the price and causes the supply difficulty of rare earth materials. Therefore, the development of permanent magnets with less or without rare earth elements become a urgent issue. In this report, the current trend and major issues on high efficiency permanent magnets, particularly exchange-coupling magnets, are discussed.

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The coercivity and the remanence of sintered M-type ferrites are highly dependent on the microstructure of the ferrite particles, particularly the size, morphology, and crystal orientation of the particles. To control the microstructures of ferrite particles, it is strongly recommended to control the nucleation and grain growth of them. In this paper, molten NaCl was used to control the crystal orientation and shape of Sr ferrite (SrFe12O19) particles. By changing the calcination conditions, Sr ferrite particles were obtained which had a hexagonal plate shape, and whose c-axis was aligned to the normal direction of the plate. Without any sophisticated magnetic alignment, Sr ferrite particles calcined at 1050 °C showed a saturation magnetization of 73.5 emu/g and a coercivity of 3267 G. This method was shown to be effective for developing high-performance hard ferrites.

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Magnetic nanoparticles with a core/shell structure were fabricated by a sol-gel method and following hydrogen reduction process. The reduction process changed the crystal structure of synthesized nanoparticles from iron-oxide single phase to iron phase covered with an iron-aluminum oxide shell layer. These nanoparticles were dispersed by homogenizing and ultrasonication in order to form ferromagnetic ink. Using synthesized ink, patterns with soft magnetic properties were successfully printed on Si substrates by a direct printing method.

Nanopowders

In this paper, the anisotropic consolidation behavior of isotropic Nd–Fe–B HDDR powders has been studied as a function of the strain, strain rate, and processing temperature. The compacts, produced by hot-pressing at 700 °C under 400 MPa in vacuum, were subjected to die-upsetting at 700 °C and 800 °C with different deformation conditions of strain rate 0.01–0.001 s⁻¹ and strain 0.5–1.5. After die-upset, the coercivity and remanence decreased and increased with increasing strain, respectively, with the change of grain morphology from the sphere to platelet. And the high deformation temperature and slow deformation rate reduce the coercivity further. The coercivity of die-upset magnet, produced at 700 °C with a strain of 1.4 and a strain rate of 0.001 s⁻¹, was largely increased about 2 kOe after post-annealing at 800 °C. The maximum coercivity, remanence, and (BH)_max were 10.4 kOe, 12.4 kG, and 35.6 MGOe, respectively.

Stable and magnetically reusable nanoporous magnetite micro/nanospheres for rapid extraction of carcinogenic contaminants from water

The microstructures and magnetic properties of Zr2Co11 phase prepared by a rapid solidification method and subsequent annealing at 500 °C were studied using X-ray diffraction and electron diffraction. At a wheel surface velocity of 20 m/s, both orthorhombic and rhombohedral structures were observed in as-spun ribbons. At a wheel surface velocity of 40 m/s, the amount of orthorhombic structure decreased, and the saturation magnetization increased. However, at a wheel surface velocity of 60 m/s, the saturation magnetization sharply decreased, and the phase identification was limited by peak broadening. The subsequent annealing of this ribbon resulted in the rhombohedral structure, a saturation magnetization of 83.47 emu/g, and a coercivity of 2091 Oe.