• Korean Journal of Materials Research
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• v.29 no.9
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• pp.532-541
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• 2019

Information and communication technologies are developing rapidly as IC chip size becomes smaller and information processing becomes faster. With this development, digital circuit technology is being widely applied to mobile phones, wireless LANs, mobile terminals, and digital communications, in which high frequency range of GHz is used. In high-density electronic circuits, issues of noise and EMC(Electro-Magnetic Compatibility) arising from cross talk between interconnects or devices should be solved. In this study, sheet-type electromagnetic wave absorbers that cause electromagnetic wave attenuation are fabricated using composites based on soft magnetic metal powder and silicon rubber to solve the problem of electromagnetic waves generated in wireless communication products operating at the frequency range of 2.4 GHz. Sendust(Fe-Si-Al) and carbonyl iron(Fe-C) were used as soft magnetic metals, and their concentrations and sheet thicknesses were varied. Using soft magnetic metal powder, a sheet is fabricated to exhibit maximum electromagnetic attenuation in the target frequency band, and a value of 34.2dB(99.9 % absorption) is achieved at the target frequency.

• Korean Journal of Materials Research
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• v.29 no.4
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• pp.252-257
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• 2019

Fabrication of soft magnetic composite powders for the $Fe_2O_3-Ca$ system by mechanical alloying(MA) has been investigated at room temperature. It is found that soft magnetic composite powders in which CaO is dispersed in ${\alpha}-Fe$ matrix are obtained by MA of $Fe_2O_3$ with Ca for 5 hours. Changes in magnetization and coercivity also reflect the details of the solidstate reduction process of hematite by pure metal of Ca during MA. The saturation magnetization of MA powders increases with increasing MA time and reaches a maximum value of 65 emu/g after 7 hours of MA. The average grain size of ${\alpha}-Fe$ in MA powders, estimated by diffraction line-width, gradually decreases with increasing MA time and reaches 52 nm after 5 hours of MA. It can also be seen that the coercivity of the 5-hour MA sample is fairly high at 190 Oe, suggesting that the grain refinement of already-produced ${\alpha}-Fe$ tends to clearly occur during MA.

• Journal of Powder Materials
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• v.28 no.5
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• pp.429-434
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• 2021

Magnetic nanoparticles have a significant impact on the development of basic sciences and nanomedical, electronic, optical, and biotech industries. The development of magnetic structures with size homogeneity, magnetization, and particle dispersibility due to high-quality process development can broaden their utilization for separation analysis, structural color optics using surface modification, and energy/catalysts. In addition, magnetic nanoparticles simultaneously exhibit two properties: magnetic and plasmon resonance, which can be self-assembled and can improve signal sensitivity through plasmon resonance. This paper reports typical examples of the synthesis and properties of various magnetic nanoparticles, especially magnetoplasmonic nanoparticles developed in our laboratory over the past decade, and their optical, electrochemical, energy/catalytic, and bio-applications. In addition, the future value of magnetoplasmonic nanoparticles can be reevaluated by comparing them with that reported in the literature.

• Korean Journal of Metals and Materials
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• v.46 no.1
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• pp.44-51
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• 2008

The effects of magnetic powder size 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 composite sheet including small magnetic flakes with the size less than $26{\mu}m$ exhibited high power loss in the GHz frequency range as compared with the sheets having large alloy flakes of $45{\sim}75{\mu}m$. Moreover, both the complex permeability and the loss factor increased with the decrease in size of the alloy flakes. The large power loss of the sheets containing small magnetic flakes was attributed to the high complex permeability, especially their imaginary part. The high complex permeability of the sheets composed of small flakes was considered to be due to the highly thin shape of the flakes inducing low eddy-current loss.

• 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.

• Composites Research
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• v.31 no.3
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• pp.111-116
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• 2018

In this paper, magnetic FeCoNi particles have been grown through electroless plating on the surface of graphene, and then this hybrid material has been dispersed by various surfactants to prepare films. The pyridine surfactant shows the highest dispersability and low surface resistance value (351 Ohm/sq) and the electromagnetic shielding ability at the frequency of 10 GHz. Specially, the evaporation of the pyridine during the drying process could be able to form the internal conductive network and high dispersion of FeCoNi on the surface of graphene.

• Resources Recycling
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• v.9 no.5
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• pp.28-32
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• 2000

In this study, the new processing method to produce magnetite for mono-component black toner was studied using spray roaster. The produced magnetite is compared with TMB 125 made by Magnox, USA. When the weight ratio of the ferrous chloride and the ferric chloride is varied from 3:7~7:3 under $N_2$ atmosphere, the 100% major phase of magnetite is obtained. However, when the content of he ferrous chloride is higher than 70wt%, the content of FeO is increased. The phase of $\Gamma-Fe_2O_3$ is shown in th range of over 70wt% of the ferric chloride. The magnetite produced by spray roasting has a saturation magnetization of 80~85 emu/g and a coercivity of 110~130 Oe. The shape of the particle is irregular sphere and hte size of the magnetite is the range of $0.3~0.5\mu\textrm{m}$. It is shown that the magnetite made by spary roasting is enough to apply to the magnetic material of toner.

• Composites Research
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• v.35 no.3
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• pp.175-181
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• 2022

Due to the increasing number of wireless communication devices in mmWave frequency bands, there is a high demand for electromagnetic interference (EMI) shielding and heat dissipating materials to avoid device malfunctions. This paper proposes an EMI shielding composite film with a high heat dissipation characteristic. To achieve this, a conductive grid is integrated with a polymer-based composite layer including magnetic and heat dissipating filler materials. A high shielding effectiveness (>40 dB), low reflection shielding effectiveness (<3 dB), high thermal conductivity (>10 W/m·K), thin thickness (<500 ㎛) are simultaneously achieved with a tailored design of composite layer compositions and grid geometries in 5G communication band of 26.5 GHz.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2012.05a
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• pp.263-265
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• 2012

잠수함이 가장 취약한 시기는 잠수함이 스노클이나 잠망경 운용을 위하여 잠망경 심도로 항해할 경우이며, 이때에는 비음향 탐지센서인 레이더와 광학, 사람에 의한 시각에 탐지될 확률이 매우 높다. 따라서 본 논문에서는 이러한 상황에서 탐지되는 취약성을 극복하고자 잠수함 마스트 및 잠망경 부분에 비음향 스텔스를 적용하고자 한다. 먼저 비음향 탐지센서에 대해서 조사하고, 그에 따른 스텔스 기법을 분야별 분석하여 최적화한 결과 다층형 구조로 선체표면부터 RAM layer, IR layer, Camouflage layer 구조로 각각 RAM layer는 자성재료인 페라이트계열로 3~5mm, IR layer는 Ceramic 또는 Nickel 계열로 1~2mm, Camouflage layer는 군청색 계열 페인팅을 제시하였다.

• Proceedings of the KIEE Conference
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• 2006.10d
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• pp.40-42
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• 2006

연자성 복합재료는 기존의 철판 대비 기계적, 자기적 특성 떨어지는 것이 사실 이였다. 본 연구에서는 이러한 문제점 개선을 위해서 저점도 윤활제 적용을 통한 빼기 압력 저감 방안을 도출하였다. 연자성체의 고강도화, 우수한 자기적 특성을 얻기 위해서 높은 압력의 성형이 필수적인데 높은 압력으로 성형시 빼기 압력이 높아져서 금형이 파손되는 등 문제점으로 인해 우수한 특성을 얻을 수 없었다. 본 연구에서는 이러한 문제점을 해결하기 위해서 저 점도액체 윤활제, 고점도 액체 윤활제, 고체 윤활제 3가지의 윤활제 특성을 검토 하였다. 또한 저점도 액체 윤활제 적용을 통해서 기존 대비 강도 10%, 철손 10%, 빼기 압력 30%를 저감 하였다.