Jinu Kim

Characterization of as-synthesized FeCo magnetic nanoparticles by coprecipitation method

FeCo nanoparticles were synthesized by a coprecipitation method without any post treatments. The structure, morphology, and magnetic properties of the FeCo nanoparticles formed at different sodium borohydride reaction times (5, 10, 20, and 30 min) were examined. X-ray diffraction revealed the representative α-FeCo peaks in all samples. The mean crystalline sizes slightly decreased from 31 nm to 21 nm with increasing reducing agent reaction time. A maximum saturation magnetization of 204 emu/g at 300 K was obtained in the sample with a reaction time of 5 min.

RF Power Absorption Enhancement of Magnetic Composites with Conductive Grid

To evaluate the electromagnetic power absorption in near field, the magnetic composites with the conductive grids were simulated using the typical permeability frequency profiles. The transmission power absorptions of the magnetic composites on microstrip line were extracted by the 3D FEM simulation program of HFSS. The magnitudes of power absorptions were greatly enhanced up to 98% and broadened the absorbing frequency band over 5 ㎓ by the insertion of a conductive grid in magnetic composite. The initial frequency of the power absorption can controlled by the change of the ferromagnetic resonance frequencies of the magnetic composite.

Alternating magnetic field heat behaviors of PVDF fibrous mats filled with iron oxide nanoparticles

To study the magnetic heat behaviors, iron oxide nanoparticles (IONPs) and the polyvinylidene fluoride (PVDF) fibrous mats filled with IONPs were prepared by using coprecipitaion method and the electrospinning technique. The synthesized IONPs exhibited a magnetization of about 72 emu/g with average diameter of about 10 nm. The magnetizations of PVDF fibrous mats filled with IONPs showed 2.6 emu/g, 5.5 emu/g and 9.9 emu/g for 5 wt.%, 10 wt.% and 20 wt.% IONPs concentration, respectively. The heat of the magnetic fibrous mats were measured under various alternating magnetic fields (90, 128, and 167 Oe), frequencies (190, 250 and 355 kHz). The maximum saturated temperature showed up to 62 °C for 20 wt.% IONPs filled in PVDF fibrous mat under 167 Oe and 355 kHz.

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].

The effects of shape anisotropy of the magnetic particles in a composite on RF power absorption

The shape anisotropy of the magnetic particles in a composite film for EMI applications plays a key role in their performance. For a few types of the particles such as wires and hollow fibers, permeabilities of the particles and in-line attenuation of the composite were evaluated by simulations. With the change of the shape anisotropy by aspect ratio or hollow ratio, power absorption could be controlled in frequency. In experiments, Ni/FeCo magnetic hollow fibbers were fabricated by electroless plating and power loss of the composite filled with the fibbers was evaluated.

Thermo-sensitive Electrospun Fibrous Magnetic Composite Sheets

The PVDF fibrous composite filled with iron oxide nanoparticles were prepared by using the electrospinning technique. The electrospun composite have the thickness in the range of 60-80 μm with the average fibrous diameters of 500-900 nm. The magnetizations of PVDF fibrous composite filled with iron oxide nanoparticles showed 4.5 emu/g, 3.1 emu/g and 1.6 emu/g at 1.5 T of external magnetic field for 20 wt.%, 10 wt.% and 5 wt.% iron oxide nanoparticles, respectively. The heat elevation of the magnetic composite were measured under various AC magnetic fields, frequency and the ambient temperatures. The temperature reached up to 46.3℃ from 36oC at 128 Oe and 355 kHz for 20 wt.% iron oxide nanoparticles filled in PVDF fibrous composite sheet. The specific absorption rate of theses sheets increased from 0.041 W/g to 0.236 W/g with the increment of AC magnetic field from 90 Oe to 167 Oe at 190 kHz, respectively.

RF Power Absorption Enhancement by Optimization of the Conductive Grid in a Magnetic Composite

Electromagnetic noise suppression was studied by near-field power absorption of a magnetic composite film composed of magnetic fillers and metallic mesh grid in nonmagnetic matrix, which were simulated using a commercial finite-element electromagnetic solver. The geometrical characteristics of the grid on the power absorption were evaluated using the transmission line method with a microstrip line. For conductive grid spacing and the period of lines composing a grid, maximum power absorption was obtained when the width of microstrip signal line was comparable with the grid spacing. The vertical position of the grid in magnetic composite was suggested to be at the center or top position of the composite for higher power absorption. The power absorptions and bandwidth of frequency were enhanced using thin metallic grid on magnetic composite in comparison with that of thick metallic grid.