Joonsik Lee

Synthesis and Characterizations of Surface-Coated Superparamagentic Magnetite Nanoparticles

The uncoated magnetite and the 2, 3-meso-dimercaptosuccinic acid (DMSA)-coated Fe₃O₄ nanoparticles were synthesized by a coprecipitation method, respectively. The DMSA surface modification on magnetite nanoparticles was observed by FT-IR. The average particle sizes of the uncoated magnetite and the DMSA coated magnetite were exhibited approximately 10.4 and 12.3 nm by TEM results. DMSA surface-coated Fe₃O₄ can introduce a dense and thin outer hydroxyl group (-OH) shell. In order to characterize the magnetic behaviors, the magnetization processes of the magnetic nanoparticles were measured with the temperature range of 5 K to 300 K. The mean blocking temperatures of the uncoated Fe₃O₄ and the DMSA surface-coated Fe₃O₄ exhibited about 172 ±28 K and 215 ±30 K, respectively. The value of magnetization of the DMSA-coated Fe₃O₄ nanoparticles was not so different with that of the uncoated Fe₃O₄.

Comparison of the Magnetic Properties for the Surface-Modified Magnetite Nanoparticles

The Fe₃O₄ nanoparticles were synthesized by a coprecipitation method, and their particles were capsulated by 3-thiopheneacetic acid (3TA), 2, 3-meso-dimercaptosuccinic acid (DMSA) and Polyethylene glycol (PEG), respectively. The 3TA-, PEG-, and DMSA-coated Fe₃O₄ nanoparticles are well dispersed in aqueous solution. The average particle sizes of the Fe₃O₄ and the 3TA-, PEG-, and DMSA-coated magnetite nanoparticles were exhibited approximately 10.4, 12, 11, and 12 nm by TEM results. The mean blocking temperatures of the uncoated Fe₃O₄ and the 3TA, PEG, DMSA surface-coated Fe₃O₄ nanoparticles exhibited about 245±10 K, 220±10 K, 142±12 K, and 200±10 K, respectively. The values of g factor of the uncoated Fe₃O₄ and the 3TA-, PEG-, and DMSA-coated Fe₃O₄ nanoparticles were obtained 2.22, 2.21, 2.22, and 2.19, respectively.

Microwave power absorption of magnetic composites filled with surface coated Fe-Al-Si flakes

The Al2O3 coated Fe-Al-Si flakes were employed as a magnetic filler in 150 μm-thick dielectric matrix. The saturation magnetizations of the coated Fe-Al-Si flakes were decreased down to 98 emu/g with the increment of the coating thickness (170 nm, 250 nm, 350 nm) in comparison with that of uncoated Fe-Al-Si flakes (130 emu/g). The power absorptions for Al2O3 coated Fe-Al-Si flakes composites were decreased from 62.7% to 57.4% (at 5 GHz) with the increment of coating thickness in comparison with that of uncoated Fe-Al-Si flakes composite (80% at 5 GHz). The thermal conductivities of Al2O3 coated composites were increased up to 0.249 W/m K in comparison with that of uncoated composite of 0.096 W/m K.

High electrical resistivity Nd-Fe-B die-upset magnet doped with eutectic DyF3–LiF salt mixture

Nd-Fe-B-type die-upset magnet with high electrical resistivity was prepared by doping of eutectic DyF3–LiF salt mixture. Mixture of melt-spun Nd-Fe-B flakes (MQU-F: Nd13.6Fe73.6Co6.6Ga0.6B5.6) and eutectic binary (DyF3–LiF) salt (25 mol% DyF3 – 75 mol% LiF) was hot-pressed and then die-upset. By adding the eutectic salt mixture (> 4 wt%), electrical resistivity of the die-upset magnet was enhanced to over 400 μΩ.cm compared to 190 μΩ.cm of the un-doped magnet. Remarkable enhancement of the electrical resistivity was attributed to homogeneous and continuous coverage of the interface between flakes by the easily melted eutectic salt dielectric mixture. It was revealed that active substitution of the Nd atoms in neighboring flakes by the Dy atoms from the added (DyF3–LiF) salt mixture had occurred during such a quick thermal processing of hot-pressing and die-upsetting. This Dy substitution led to coercivity enhancement in the die-upset magnet doped with the eutectic (DyF3–LiF) salt mixture. Coercivity and r...

High-frequency behavior of FeN thin films fabricated by using reactive sputtering

We used ferromagnetic resonance (FMR) and its relationship with the static magnetic properties to investigate the high-frequency behavior of FeN thin films prepared by using reactive sputtering. The FMR was observed in the frequency range from 2 to 18 GHz in the FeN films fabricated at a proper nitrogen flow rate (NFR). In those FeN thin films, a decrease in the saturation magnetization and a corresponding decrease of the FMR frequency were observed as the NFR was increased during the deposition. The external field dependences of the FMR frequencies fit the Kittel formula well, and the Land´e g-factors determined from the fit were found to be very close to the free electron value. The high-field damping parameters were almost insensitive to the NFR. However, the lowfield damping parameters exhibited a high sensitivity to the NFR very similar to the dependence of the hard-axis coercivity on the NFR, suggesting that extrinsic material properties, such as impurities and defect structures, could be important in deciding the low-field damping behavior.

Thermal conductivity prediction of magnetic composite sheet for near-field electromagnetic absorption

The magnetic composite sheets were designed by using core-shell structured magnetic fillers instead of uncoated magnetic fillers to resolve concurrently the electromagnetic interference and thermal radiation problems. To predict the thermal conductivity of composite sheet, we calculated the thermal conductivity of the uncoated magnetic fillers and core-shell structured fillers. And then, the thermal conductivity of the magnetic composites sheet filled with core-shell structured magnetic fillers was calculated and compared with that of the uncoated magnetic fillers filled in composite sheet. The magnetic core and shell material are employed the typical Fe-Al-Si flake (60 μm × 60 μm × 1 μm) and 250 nm-thick AlN with high thermal conductivity, respectively. The longitudinal thermal conductivity of the core-shell structured magnetic composite sheet (2.45 W/m·K) enhanced about 33.4% in comparison with that of uncoated magnetic fillers (1.83 W/m·K) for the 50 vol. % magnetic filler in polymer matrix.

Metallic Crack Detections by Planar Inductive Coil Sensor Under AC and DC Magnetic Fields

To detect the surface and the opposite side cracks on iron specimen under AC and DC magnetic fields, the planar inductive coil sensors were employed. When the induced signals were measured, the planar inductive coil sensor and the magnetic field source were lifted off about 2 mm from the top surface of the specimen. AC magnetic fields and DC magnetic fields were applied to the specimens by single straight Cu coil and NdFeB permanent magnet, respectively. The detected signals at crack positions were good coincidence with those of the simulation results.

Feasibility study of electrophoresis deposition of DyF3 on Nd-Fe-B particles for coercivity enhancement

Feasibility of the electrophoresis deposition (EPD) technique for homogeneous and adhesive deposition of DyF3 particles on the Nd-Fe-B-type particles was studied, and coercivity enhancement in the diffusion-treated Nd-Fe-B-type particles deposited with DyF3 by EPD was investigated. HDDR-treated Nd12.5Fe80.6B6.4Ga0.3Nb0.2 particles were deposited with DyF3 particles by EPD. More homogeneous and adhesive deposition of DyF3 particles on the surface of Nd-Fe-B particles was made by the EPD with respect to conventional dip-coating, and this led to more active and homogeneous diffusion of Dy. More profound coercivity enhancement was achieved in the diffusion-treated Nd-Fe-B-type particles deposited with DyF3 by EPD compared to dip-coated particles.

Ceramics-bonded Nd-Fe-B-type magnet with high electrical resistivity

Ceramics-bonded magnet with remarkably high electrical resistivity was fabricated by hot-pressing the mixture of Nd13.6Fe73.6Co6.6Ga0.6B5.6 alloy melt-spun flakes and dielectric Bi2O3-SiO2-B2O3 ceramics powder with low melting point. Coercivity of the ceramics-bonded magnet decreased with increasing the addition of ceramics binder, and this was attributed to the increased demagnetizing factor. Thin oxidized layer on the flake surface formed by reaction between the flake and oxide binder also contributed to reducing coercivity in the ceramics-bonded magnet. Highly resistive ceramics-bonded magnet containing 30 vol% ceramics binder still had good magnetic performance and high mechanical strength at 175 oC: iHc = 5 kOe, Mr = 4.8 kG, (BH)max = 4.3 MGOe, and over 900 MPa.Ceramics-bonded magnet with remarkably high electrical resistivity was fabricated by hot-pressing the mixture of Nd13.6Fe73.6Co6.6Ga0.6B5.6 alloy melt-spun flakes and dielectric Bi2O3-SiO2-B2O3 ceramics powder with low melting point. Coercivity of the ceramics-bonded magnet decreased with increasing the addition of ceramics binder, and this was attributed to the increased demagnetizing factor. Thin oxidized layer on the flake surface formed by reaction between the flake and oxide binder also contributed to reducing coercivity in the ceramics-bonded magnet. Highly resistive ceramics-bonded magnet containing 30 vol% ceramics binder still had good magnetic performance and high mechanical strength at 175 oC: iHc = 5 kOe, Mr = 4.8 kG, (BH)max = 4.3 MGOe, and over 900 MPa.

Synthesis and magnetic properties of FeCo/edge-oxidized graphene nanocomposites

The graphene oxide composites with magnetic nanoparticles have attracted in energy storage, and electromagnetic shield and absorption (ESA) applications [1].