Hyunkyung Choi’s research while affiliated with Korea Atomic Energy Research Institute and other places

This study aims to enhance the amorphous formation ability and magnetic properties that are crucial for the production of high-quality nanocrystalline alloys. The structural, thermal, and magnetic characteristics of the alloy ribbons were analyzed through a systematic adjustment of Nb content, and, including Nb, significantly improved the amorphous formation ability and thermal stability of the alloy, which is vital for nanocrystalline production. By varying the Nb content within Fe85-xSi2B8P4Cu1Nbx (x = 0.0, 0.5, 1.0, and 1.5), we explored finer adjustments to achieve homogeneous amorphousness during the melt spinning process. Careful control over the Nb content facilitated the production of amorphous ribbons with consistent homogeneity, which was critical for the subsequent fabrication of nanocrystalline structures through heat treatment. As a result, the amorphous ribbon of Fe85.5Si2B8P4Cu1Nb0.5 showed a low coercivity of 7 A/m. The heat treatment showed a remarkably high saturation magnetic flux density of 1.94 T. Additionally, the grain size (D) decreased as the Nb content increased, with D values ranging from 25.09 nm to 24.29 nm, as calculated by the Scherrer formula. Mössbauer spectroscopy confirmed the formation of nanocrystalline and residual amorphous phases. The hyperfine magnetic field values (Beff) decreased from 25.7 T to 24.7 T in the amorphous samples and reached 33.0 T in the nanocrystalline phases. This study highlights Nb’s positive impact on thermal stability and amorphous formation capacity in Fe-Si-B-P-Cu alloys, culminating in the successful fabrication of nanocrystalline ribbons with superior structural and magnetic properties.

Among various iron carbide phases, χ-Fe5C2, a highly active phase in Fischer–Tropsch synthesis, was directly synthesized using a wet-chemical route, which makes a pre-activation step unnecessary. In addition, χ-Fe5C2 nanoparticles were encapsulated with mesoporous silica for protection from deactivation. Further structural analysis showed that the protective silica shell had a partially ordered mesoporous structure with a short range. According to the XRD result, the sintering of χ-Fe5C2 crystals did not seem to be significant, which was believed to be the beneficial effect of the protective shell providing restrictive geometrical space for nanoparticles. More interestingly, the protective silica shell was also found to be effective in maintaining the phase of χ-Fe5C2 against re-oxidation and transformation to other iron carbide phases. Fischer–Tropsch activity of χ-Fe5C2 in this study was comparable to or higher than those from previous reports. In addition, CO2 selectivity was found to be very low after stabilization.

Hyperthermia is used to combat and reduce the effects of tumors. This study relates to Co1−xMnxFe2O4 magnetic nanoparticles and their potential in hyperthermia treatment applications. Co1−xMnxFe2O4 nanoparticles were prepared using the high-temperature thermal decomposition method, and the cubic spinel structure with the Fd-3m space group was confirmed through X-ray diffraction analysis. The self-heating temperature was measured using a magneTherm device, and the Mössbauer spectrum covers six lines of sites A and B. Consequently, the composition, particle size, and frequency conditions of magnetic nanoparticles capable of generating self-heating temperatures near the cancer cell death temperature of 42–43 °C were established.