• 한국자기학회지
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• 제8권3호
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• pp.125-130
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• 1998

13Cr-1.5Nb-Fe합금분말이 수분사법으로 제조되엇다. 제조된 합금뷴말을 압축기를 이용하여 ring현상으로 성형한후, 진공중에서 소결하였다. as-prepared 분말의 현상, 입도분포, 성분분석 및 포화자화물 조사하였으며, 소결된 시편의 투자율, 교류자기이력손실이 조사되었다. as-prepared 13Cr-1.5Nb-Fe합금분말의 포화자화는 160emu/g이었으며, 산소함유량은 약 600ppm이었다. 입도분포는 약 70$\mu$m에서 50%vol.을 나타낸다. 교류투자율은 소결온도 및 성형온도가 증가할수록 증가한다. 주파수 20KHz에서 교류자기이력손실은 12ton/cm2성령압력 및 120$0^{\circ}C$ 소결온도 조건에서 107W/cc로 가장우수하다.

• 천문학회보
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• 제37권2호
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• pp.110.1-110.1
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• 2012

We studied the source regions of 12 solar energetic particle (SEP) events seen between 2010 August and 2012 January at STEREO-A, B and ACE, when the two STEREO spacecraft were separated by about $180^{\circ}$. All events were associated with strong flares (C1 - X6) and fast coronal mass ejections (CMEs) accompanied by type II radio bursts. We have determined the arrival times of the SEP events at the three spacecraft. EUV waves observed in $195{\AA}$ and $193{\AA}$ channels of STEREO and SDO/AIA are tracked across the Sun and the arrival time of the EUV wave at the photospheric source of open field lines extending to the spacecraft connection points at 2.5 Rsun estimated. We found 7 events with flux enhancements in all spacecraft and 4 in two spacecraft. Most events came from a single source. The results show that magnetic field connections between source regions and the spacecraft play an important role in abrupt flux enhancements. In the most cases, EUV waves at the Sun are associated with a wide longitudinal spread of the SEPs.

• 한국세라믹학회지
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• 제32권5호
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• pp.543-548
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• 1995

Li-ferrite powders were prepared from mixture of Fe and Li salts using a hydrothermal method. Their crystal structure, microstructure and magnetic property were investigated with X-ray diffraction analysis, chemical analysis, SEM, and VSM. In the case of using FeCl3 as a precursor, Li-ferrite powders were synthesized. However, Fe3O4 was formed when the precursor was a divalented Fe2SO4 or FeCl2. The precipitation rate of Li-ferrite was increased as the reaction temperature increased. The optimum conditions of synthesis were the mole ratio of Fe+++/Li+=2, pH 13, the reaction temperature of 25$0^{\circ}C$ and the reaction time of 120min. With this condition, the spherical particles with good dispersion were obtained with average particle size of 0.4${\mu}{\textrm}{m}$ and saturation magnetization of 65 emu/g.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2006년도 하계학술발표대회 논문집
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• pp.162-167
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• 2006

Nanofluid is a kind of new engineering material consisting of nanoparticles dispersed in base fluid. Nanofluids could have various applications such as magnetic fluids, heat exchanger working fluids, lubricants, drug delivery and so on in present study, various nanoparticles, such as MWCNT (Multi-walled Carbon Nanotube), fullerene, copper oxide, and silicon dioxide are used to produce nanofluids. As base fluids, DI-water, ethylene glycol, oil, and silicon oil are used. To investigate the thermo-physical properties of nanofluids, thermal conductivity and kinematic viscosity are measured. Stability estimation of nanofluid is conducted with UV-vis spectrophoto-meter. In this study, the high pressure homogenizer is the most effective method to produce nanofluid with the prepared nanoparticle and base fluid. Excellently stable nanofluids are produced with the magnetron sputtering system. Thermal conductivity of nanofluid increases with increasing particle volume fraction except water-based fullerene nanofluid which has lower thermal conductivity than base fluid due to its lower thermal conductivity, 0.4 W/mK. The experimental results can't be predicted by Jang and Choi model.

• 대한금속재료학회지
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• 제49권1호
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• pp.46-51
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• 2011

Real-time formation of $L1_0$ phase of FePt nanoparticles in the gas phase during ultrasonic-spray pyrolysis is first discussed in the present study. Without any post heat treatment, $L1_0$ phase of FePt nanoparticles appeared at the temperature above $900^{\circ}C$ in the gas phase synthesis. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) studies revealed that FePt nanoparticles less than 10 nm in size contained small volume of $L1_0$ fct phase. However, in other samples obtained at the temperature below $900^{\circ}C$, iron oxide phase co-existed and no evidence of phase transformation was found. Thus, it is anticipated that the time of flight of particles required for crystallization and phase transformation was extended according to the increase of the collision rate. Finally, magnetic properties represented by coercivity and saturation magnetization and functional groups on the particle surface were discussed based on VSM and FT-IR results.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2003년도 춘계학술발표강연 및 논문개요집
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• pp.44-44
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• 2003

Growth mechanism of carbon nanotubes(CNTs) synthesized by chemical vapor deposition is abided by two growth modes. These growth modes are classified by the position of activated catalytic metal particle in the CNTs. Growth mode can be also affected by interaction between substrate and catalytic metal and induced energy such as thermal and plasma. We studied the reaction of catalytic metal to the substrate and growth mode of CNTs. Various substrates such as Si(100), graphite plate, coming glass, sapphire and AAO membrane are used to study the relation between catalytic metal and substrate in the synthesis of CNTs. For catalytic metal, thin film was deposited on various substrate via sputtering technique with a thickness of ∼20nm and magnetic fluids with none-sized particles were dispersed on AAO membrane. After laying process on AAO membrane, it was dried at 80$^{\circ}C$ for 8 hour. Synthesizing of CNTs was carried out at 900$^{\circ}C$ in NH3/C2H2 mixture gases flow for 10minutes.

• 천문학회보
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• 제46권1호
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• pp.44.1-44.1
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• 2021

한기로 나누어 보면, 흑점의 관측 빈도는 두 기간에서 비슷하지만, 오로라는 냉한기에 집중적으로 관측된다. 특이하게도, 크기가 큰 흑점의 경우는 냉한기보다 온난기에서 관측 빈도가 세 배 이상 높다. 또한, 흑점과 관련된 오로라의 강도를 분석해보면 크기가 큰 흑점은 작은 흑점보다 2~3배 이상 지구영향성이 높다는 것을 알 수 있다.

• 천문학회보
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• 제45권1호
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• pp.52.2-52.2
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• 2020

At quasi-perpendicular shocks in the high-�� (��=Pgas/Pmag~100) intracluster medium (ICM), various microinstabilities occur by the temperature anisotropies and/or drift motions of plasma. In the downstream, the Alfvén ion cyclotron instability (AIC) due to the ion temperature anisotropy (Ti⊥>Ti║) is triggered by shock-reflected ions, the whistler instability (WI) is driven by the electron temperature anisotropy (Te⊥>Te║) as a consequence of the shock compression of magnetic fields, and the mirror instability is generated due to the ion and/or electron temperature anisotropy. At the shock foot, the modified two stream instability (MTSI) is possibly excited by the cross-field drift between ions and electrons. In the upstream, electron firehose instability (EFI) is driven by the electron temperature anisotropy or the relative drift between incoming and reflected electrons. These microinstabilities play important roles in the particle acceleration in ICM shocks, so understanding of the microinstabilities and the resultant plasma waves is essential. In this study, based on a linear stability analysis, the basic properties of the microinstabilities in ICM shocks and the ion/electron scale fluctuations are described. We then discuss the implication of our work on the electron pre-acceleration in ICM shocks.

• 한국초전도ㆍ저온공학회논문지
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• 제25권3호
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• pp.18-21
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• 2023

Pollution of the environment by micro-plastics is now a worldwide problem. Plastics are difficult to decompose and put a great load on the marine environment. Especially a plastic with a size of 5 mm or less is defined as micro-plastic and are carried by ocean currents over long distances, causing global pollution. These are not easily decomposed in the natural environment. In this paper, we aimed to experimentally demonstrate that micro-plastics in seawater can be continuously separated by electromagnetic Archimedes force. Using polyethylene particles of 3 mm in diameter as the separation target, a flow channel was fabricated and separation conditions were investigated by particle trajectory calculations for separation experiments. Based on the calculation results, a solenoid-type superconducting magnet was used as a source of magnetic field to conduct separation experiments of micro-plastics in seawater. Although a high separation rate was assumed in the simulation results, the experimental results did not show any significant improvement in the separation rate due to the electromagnetic Archimedes force. It was found that the gas generated by the electrolytic reaction may have inhibited the migration of the particles.

• Archives of Metallurgy and Materials
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• 제64권2호
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• pp.623-626
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• 2019

Rare earth Nd-Fe-B, a widely used magnet composition, was synthesized in a shape of powders using gas atomization, a rapid solidification based process. The microstructure and properties were investigated in accordance with solidification rate and densification. Detailed microstructural characterization was performed by using scanning electron microscope (SEM) and the structural properties were measured by using X-ray diffraction. Iron in the form of α-Fe phase was observed in powder of about 30 ㎛. It was expected that fraction of Nd₂Fe₁₄B phase increased rapidly with decrease in powder size, on the other hand that of α-Fe phase was decreased. Nd-rich phase diffused from grain boundary to particle boundary after hot deformation due to capillary action. The coercivity of the alloy decreased with increase in powder size. After hot deformation, Nd₂Fe₁₄B phase tend to align to c-axis.