• Journal of Magnetics
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• v.14 no.4
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• pp.155-160
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• 2009

This study examined the effects of a decrease in thickness of magnetic alloy flakes on the electromagnetic wave absorption characteristics of nanocrystalline $Fe_{73.5}Cu_1Nb_3Si_{15.5}B_7$ (at.%) alloy flakes/polymer composite sheets available for a quasi-microwave band. The thickness of FeCuNbSiB alloy flakes decreased to 1-2 $\mu$m with increasing milling time up to 24 h, and the composite sheet including alloy flakes milled for 24 h exhibited considerably enhanced power loss properties in the GHz range compared to the sheets having non-milled alloy powders. Although a considerable increase in loss factor upon milling was observed in the narrow frequency range of 4-6 GHz, there was no correlation between the complex permeability and flake thickness. However, the complex permittivity increased with increasing milling time, and there was good agreement between the milling time and the frequency dependences of the complex permittivity and power loss.

• Journal of electromagnetic engineering and science
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• v.8 no.2
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• pp.70-75
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• 2008

In this paper, the EM wave absorber was designed and fabricated for countermeasure against EMI from a ceiling of a tollgate in ETC system. We fabricated several samples in different composition ratios of MnZn-ferrite, Carbon, and CPE(Chlorinated Polyethylene). Absorption abilities were simulated in accordance with different thicknesses of the prepared absorbers and changed complex relative permittivity and permeability according to composition ratio. The optimized mixing ratio of MnZn-ferrite, Carbon, and CPE was found as 40:15:45 wt.% by experiments and simulation. Then the EM wave absorber was fabricated and tested using the simulated data. As a result, the developed EM wave absorber has the thickness of 3.3 mm and absorption ability was more than 20 dB in the case of normal incidence and more than 11 dB for the incident angle from 15 to 45 degrees at 5.8 GHz. Therefore, it was confirmed that the newly developed absorber can be used for ETC system.

• Journal of Navigation and Port Research
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• v.31 no.8
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• pp.671-674
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• 2007

In this paper, the EM wave absorber was designed and fabricated for ETC system, because ETC system has some problems including signal error and system-to-system interference. We fabricated some samples in different composition ratio of MnZn-ferrite, Carbon and CPE, confirmed that optimum composition ratio of Mn2n-ferrite, Carbon, CPE was 40 : 15 : 45 wt%. Complex relative permittivity and complex relative permeability was calculated by the measured data. And absorption abilities were simulated according to different thickness of the EM wave absorbers using complex relative permittivity and permeability. The EM wave absorber was fabricated based on simulated data Simulated and measured values agree well. As a result, the developed EM wave absorber has a thickness of 3.38 mm and absorption ability over 20 dB at 5.8 GHz.

• Journal of the Korean Magnetics Society
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• v.5 no.6
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• pp.947-951
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• 1995

The electromagnetic interference is prevented by the high magnetic loss of the ferrite. The absorbing property of electromagnetic wave could be improved by the ferrite that has a finer and more uniform microstructure. The thermal decomposition of organic acid salt provided the uniform composition and fine powder. The absorbing properties of electromagnetic wave were evaluated by the relative complex permeability, permittivity, and the attenuation which is calculated from the results of network analyzer. The permeability and permittivity were increased with increase of the density and with decrease of the grain size. The matching thickness could be reduced with increasing sintered temperature. The attenuation of the Cu-Ni-Zn ferrite showed over 20dB when the matching thickness and the matching frequency range were 6.75mm and from 160MHz to 640MHz, respectively.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.45 no.8
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• pp.60-68
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• 2008

The complex permittivity and permeability of various periodic metamaterial (MTM) cells are extracted by simulating a fictitious rectangular waveguide consisting of PEC and PMC walls. The shapes of the MTM cells include a thin wire (TW), a single split-ring resonator (SSRR), a double SRR (DSRR), a modified SRR, and a combined structure of the TW and the DSRR. The TW falls on a negative-$\varepsilon$/positive-$\mu$ region, the SRRs on a positive-$\varepsilon$/negative-$\mu$ region, and the combined structure on a negative-$\varepsilon$/negative-$\mu$ region. We also investigate how the permeability and permeability are affected by the dimension parameters of the MTM cells. Another extraction technique utilizing time domain signals is developed overcoming some limitations that the waveguide technique can not handle.

• Korean Journal of Metals and Materials
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• v.47 no.12
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• pp.866-872
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• 2009

The effects of magnetic powder thickness on electromagnetic wave absorption characteristics in Fe-6.5Si-0.9Cr (wt%) alloy flakes/polymer composite sheets available for quasi-microwave band have been investigated. The atomized FeSiCr powders were milled by using attritor for 12, 24, and 36 h, powder thickness changed from $40{\mu}m$ to $3{\mu}m$ upon 36 h milling. The composite sheet, including thinned magnetic flakes, exhibited higher power loss in the GHz frequency range as compared with the sheets having thick flakes. Moreover, both the complex permeability and the loss factor increased with the decrease in thickness of the alloy flakes. Therefore, the enhanced power loss property of the sheets containing thin alloy flakes was attributed to the flakes of high complex permeability, especially their imaginary part. Additionally, the complex permittivity was also increased with the reduction of flake thickness, and this behavior was considered to be helpful for improvement of the electromagnetic wave absorption characteristics in the composite sheets, including thin alloy flakes.

• Journal of the Korean Magnetics Society
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• v.18 no.3
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• pp.115-119
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• 2008

The raw materials of permalloy were processed the ball-mill for 30 hours and the shape of permalloy particles was changed from sphere to flake type, which was observed using scanning electron microscope. The complex permittivity and permeability spectra and reflection loss of permalloy-rubber composite was measured using Network Analyzer in order to investigate the relationship between the microwave absorption and the material constants. The flake type permalloy-rubber composite showed high reflection loss, which was due to the high complex permittivity and permeability. Also, the matching frequency shifted toward lower frequency range with microwave absorber thickness, and the maximum reflection loss of -6.1 dB was observed in $1.65\;GHz{\sim}1.86\;GHz$ for a 1.3 mm thickness.

• Journal of the Korean Ceramic Society
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• v.29 no.3
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• pp.177-182
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• 1992

The effect of heat-treatment temperature on the microwave absorbing properties was investigated in Ni0.8Zn0.2Fe2O4 specimens. The composite specimens were prepared by modling and curing the mixture of prereacted ferrite powder and silicone rubber. The measurement of complex permeability and permittivity was made by the reflection/transmission method. The most sensitive material constants with heat-treatment temperature is the imaginary (loss) component of permeability. The higher the heat-treatment temperature, the greater the magnetic loss. The composite specimens with high magnetic loss exhibited superior microwave absorbing properties. The quantitative estimation of microwave absorbing properties were made by plotting the observed material constants on the calculated solution map of impedance-matching.

• Journal of the Korean Magnetics Society
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• v.24 no.2
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• pp.56-59
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• 2014

The samples of $Ba_2CoZnFe_{12}O_{22}$ was synthesized by the solid-state reaction method. The toroids of $Ba_2CoZnFe_{12}O_{22}$ were sintered with various sintering temperature at 1050, 1100, 1150, and $1200^{\circ}C$, and studied by x-ray diffractometer, vibrating sample magnetometer, network analyzer, and Mssbauer spectrometer. From the XRD patterns, the density of samples increased with increasing sintering temperature. From the magnetic hysteresis curves up to 10 kOe at 295 K, the saturation magnetization ($M_s$) of $Ba_2CoZnFe_{12}O_{22}$ samples in various sintered at 1050, 1100, 1150 ,and $1200^{\circ}C$ were showed around $M_s$= 33.0 emu/g. However, With increasing sintering temperature, the coercivity ($H_c$) of samples decrease. Complex permeability and permittivity of samples in various sintering temperatures were measured between 100MHz to 4 GHz. With increasing sintering temperature, the permeability of samples increase.

• Journal of Powder Materials
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• v.25 no.5
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• pp.375-378
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• 2018

Conductive and dielectric SiC are fabricated using electroless plating of Ni-Fe films on SiC chopped fibers to obtain lightweight and high-strength microwave absorbers. The electroless plating of Ni-Fe films is achieved using a two-step process of surface sensitizing and metal plating. The complex permeability and permittivity are measured for the composite specimens with the metalized SiC chopped fibers dispersed in a silicone rubber matrix. The original non-coated SiC fibers exhibit considerable dielectric losses. The complex permeability spectrum does not change significantly with the Ni-Fe coating. Moreover, dielectric constant is sensitively increased with Ni-Fe coating, owing to the increase of the space charge polarization. The improvements in absorption capability (lower reflection loss and small matching thickness) are evident with Ni-Fe coating on SiC fibers. For the composite SiC fibers coated with Ni-Fe thin films, a -35 dB reflection loss is predicted at 7.6 GHz with a matching thickness of 4 mm.