• Journal of the Korean Magnetics Society
• /
• v.14 no.4
• /
• pp.131-137
• /
• 2004

For the aim of thin electromagnetic wave absorbers used in mobile telecommunication frequency band (0.8-2.0㎓), we investigate high-frequency magnetic, dielectric and microwave absorbing properties of iron particles dispersed in rubber matrix in this study. The major experimental variables are particle shape (sphere and flake) and initial particle size (in the range 5-70 $\mu\textrm{m}$) of iron powders. High value of magnetic permeability and dielectric permittivity can be obtained in the composites containing thin plate-shape (flake) iron particles (of which thickness is less than skin depth in ㎓frequency), which can be produced by mechanical forging of spherical iron powders using an attrition mill. This result is attributed to the reduction of eddy current loss (increase of permeability) and the increase of space charge polarization (increase of permeability). The optimum initial particle size is found to be about 10 $\mu\textrm{m}$ for the attainment of the material parameters (particularly, real part of complex permeability) satisfying the wave impedance matching. With the iron powders controlled in size and shape as absorbent fillers in rubber matrix, the thickness can be reduced to about 0.7mm with respect to -5㏈ reflection loss (70% power absorption) in mobile telecommunication frequency band.

• Proceedings of the Korean Magnestics Society Conference
• /
• 2013.05a
• /
• pp.27-27
• /
• 2013

• Journal of the Korean Magnetics Society
• /
• v.11 no.4
• /
• pp.173-178
• /
• 2001

The effects of heat-treatment with magnetic or non-magnetic field on magnetic properties of gas-atomized MPP dust cores subjected to various cooling processes after annealing were investigated. Upon magnetic-field annealing, ac permeability and core loss decreased with the increase of cooling rate, which were attributed to the generation of inhomogeneous internal stress and anomalous eddy current loss, respectively. It was not observed the formation of ordered phase and the related change in magnetic properties at the cooling stage for MPP dust cores. In MPP alloys, magnetic anisotropy was easily induced through the directional order, and permeability and core loss were changed under the conditions of low cooling rate and magnetic annealing.

• Korean Chemical Engineering Research
• /
• v.54 no.1
• /
• pp.44-51
• /
• 2016

The powder core, conventionally fabricated from iron particles coated with insulator, showed large eddy current loss under high frequency, because of small specific resistance. To overcome the eddy current loss, the increase in the specific resistance of powder cores was needed. In this study, copper oxide coating onto electrically conductive iron particles was performed using a planetary ball mill to increase the specific resistance. Coating factors were optimized by the Response surface methodology. The independent variables were the CuO mass fraction, mill revolution number, coating time, ball size, ball mass and sample mass. The response variable was the specific resistance. The optimization of six factors by the fractional factorial design indicated that CuO mass fraction, mill revolution number, and coating time were the key factors. The levels of these three factors were selected by the three-factors full factorial design and steepest ascent method. The steepest ascent method was used to approach the optimum range for maximum specific resistance. The Box-Behnken design was finally used to analyze the response surfaces of the screened factors for further optimization. The results of the Box-Behnken design showed that the CuO mass fraction and mill revolution number were the main factors affecting the efficiency of coating process. As the CuO mass fraction increased, the specific resistance increased. In contrast, the specific resistance increased with decreasing mill revolution number. The process optimization results revealed a high agreement between the experimental and the predicted data ($Adj-R^2=0.944$). The optimized CuO mass fraction, mill revolution number, and coating time were 0.4, 200 rpm, and 15 min, respectively. The measured value of the specific resistance of the coated pellet under the optimized conditions of the maximum specific resistance was $530k{\Omega}{\cdot}cm$.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
• /
• v.5 no.1
• /
• pp.53-64
• /
• 2007

The dynamic release behavior of Cs from high burn-up spent PWR fuel was experimentally performed under the conditions of a thermal treatment process such as voloxidation and sintering conditions. In voloxidation process, influence of the oxidation and reduction atmosphere on the Cs release characteristic using fragment type of spent fuel heated up to $1,500^{\circ}C$ was compared. In sintering process, temperature history effect on Cs release behavior was evaluated using green pellet under 4% $H_2/Ar$ environment. Temperature range for complete Cs release from spent fuel fragment under voloxidation condition was about $800^{\circ}C{\sim}1,200^{\circ}C$, but that of green pellet under the reduction atmosphere was $1,100^{\circ}C{\sim}1,400^{\circ}C$. Key parameters on Cs release behavior from spent fuel was powder formation as well as the diffusion rate of Cs compound to grain boundary and fuel surface.

• Journal of ILASS-Korea
• /
• v.8 no.1
• /
• pp.29-36
• /
• 2003

Spray characteristics on diesel- water emulsion are analyzed in high pressure injection for several variables such as water content, injection pressure. Spray Patterns were visualized under various water content and injection pressures. Spray tip penetration was increased and spray angle decreased in accordance with increasing of water content. But these characteristics were enhanced with increase of injection pressure to high pressure.

• Resources Recycling
• /
• v.11 no.4
• /
• pp.3-10
• /
• 2002

The characteristics of dry and wet milled powder prepared by 1 cycle OREOX (oxidation and reduction of oxide fuels) treatment were investigated using the simulated spent fuel pellet. Sintered pellets simulating spent nuclear fuel burned in reactor were fabricated from $UO_2$ powder using as a starting material in fabrication of nuclear fuel. The 1 cycle OREOX-treated powder was prepared by only one path of oxidation md reduction of the simulated pellet. Powder having average particle size of less than 1 $\mu\textrm{m}$ could be easily obtained by dry milling, but not be achieved by wet milling. And, specific surface area of dry milled pow-der was higher than that of wet milled powder. Dry milled powder formed loose agglomerate, while wet milled powder showed the shape of irregular and angular particles. Dry milled powder provided higher green density, resulting in higher sintered density of higher than 95% TD and average grain size of larger than 8 $\mu\textrm{m}$ satisfying the standard specification of sintered pellets.

• Journal of the Korean Magnetics Society
• /
• v.17 no.6
• /
• pp.242-245
• /
• 2007

Conduction noise attenuation by composite sheets of high magnetic and dielectric loss has been analyzed by using electromagnetic field simulator which employs finite element method. The simulation model consists of microstrip line with planar input/output ports and noise absorbers (magnetic composite sheets containing iron flake particles as absorbent fillers). Reflection and transmission parameters $(S_{11}\;and\;S_{21})$ and power loss are calculated as a function of frequency with variation of sheet size and thickness. The simulated value is in good agreement with measured one and it is demonstrated that the proposed simulation technique can be effectively used in the design and characterization of noise absorbing materials in the RF transmission lines.

• Korean Journal of Materials Research
• /
• v.3 no.3
• /
• pp.245-252
• /
• 1993

We investigated the changes of (U, Ce)$O_2$ powder characteristics with $CeO_2$ contents and ball-milling time and then studied on the sintering properties with those (U, Ce)$O_2$ powder characteristics. From the results of this study, it was concluded that the longer ball-milling time of (U, Ce)$O_2$ powder was, the finer its particle size was. Green and sintered densities were decreased with $CeO_2$ contensts increase. And also $CeO_2$ was recongized deteriorating oxide on the $UO_2$ sintering. In case of the lOwt. % $CeO_2$ contents, (U, Ce)$O_2$ sintered pellet which was made of ball-milled powder for 4 hours had few pores and its pores got near to the sphere. And its sintered density had the highest. Because its powder had higher surface area and its packing ratio was appropriated much better than others.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.13 no.3
• /
• pp.131-137
• /
• 2005

Ultra high pressure injection equipment was developed to estimate and analyze the spray characteristics in ultra high pressure injection. Spray patterns were visualized by schlieren method and analyzed in ultra high pressure. Spray tip penetration, spray thickness, spray volume, and entrained air mass increased with the increase of the injection pressure. But over 2,800 bars of the injection pressure region, it was shown that the rate of improvement was not increased remarkably ,and the spray characteristics such as spray penetration, volume, and entrained air mass were reversed and got worse at 4,140 bars.