• 한국초전도학회:학술대회논문집
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• 한국초전도학회 1999년도 High Temperature Superconductivity Vol.IX
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• pp.196-199
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• 1999

The coexistence of the superconductivity and magnetism in the Ho1-xDyxNi2B2C is studied by using Ginzburg-Landau theory. This material violates the standard do Gennes scaling showing the coexistence and complex interplay of superconducting and magnetic order yielding an anomalous phase diagram. We propose a phenomenological model which includes two magnetic and two superconducting order parameters accounting for the multi-band structure of this material. We describe the magnetic fluctuations and order and demonstrate that they lead to anomalous behavior of the upper critical field. The doping dependence of Tc in Ho1-xDyxNi2B2C showing a reentrance behavior are analyzed yielding a very good agreement with experimental data.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1994년도 하계학술대회 논문집 A
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• pp.79-81
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• 1994

A solution of magnetic fields for amorphous core transformer has been tried using homogenization technique. The technique, which is derived by applying asymptotic expansion to the standard finite element method, is helpful to analyse a joint part of amorphous core transformer microscopically. A butt-lap-step joint type of lamination method is modeled and its equivalent reluctivity is calculated to analyse various quantities of the magnetic fields. The algorithm is also applicable to other electric devices which have complicated material structure with repeated patterns.

• 한국시뮬레이션학회논문지
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• 제6권2호
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• pp.1-14
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• 1997

The characteristics of the flow and energy conversion in OMACON liquid-metal MHD system are investigated. Numerical simulation of two-phase flow in the OMACON system without magnetic field was carried out by the Phoenics code and the energy conversion characteristics are studied in association with the fact that the mechanical energy loss at the nozzle of the OMACON system are to be converted into electrical energy. In this system, working fluid (gas) is injected through the mixer located at the bottom of the riser, and is mixed with hot liquid metal. Therefore in the riser two-phase flow is developed under the influence of the gravity. In this study, the interaction between the gas and liquid is considered by the use of IPSA(InterPhase Slip Algorithm) where standard drag coefficient has been used. It has been assumed that in the flow regime the liquid is continuous and the gas is dispersed. For the liquid and gas, the continuity equations, momentum equations and energy equations are solved respectively in association with void fraction in the flow field. In order to calculate the energy conversion efficiency, firstly the ratio of the mechanical energy loss of liquid metal flow at the nozzle to the input thermal energy is considered. Secondly flow pattern of liquid metal in the generator has been analyzed, and the characteristics of the conversion of the mechanical energy into the electrical energy has been investigated. For an representative case where Hartmann number is 540 and magnetic field is 0.35 T, the present analysis shows that the energy conversion efficiency is 0.653. This result is considered to be reasonable in comparison with published experimental results.

• Journal of Magnetics
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• 제20권4호
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• pp.450-459
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• 2015

In this paper, the safety distances and maximum permissible power (MPP) of resonant wireless power transfer systems are defined and derived with regard to human exposure to electromagnetic field (EMF). The definition is based on the calculated induced current density and electric field in the standard human model located between the transmitting and receiving coil. In order to avoid the adverse health effects such as stimulation of nerve tissues, the induced current and electric field must not exceed the basic restriction values specified in EMF safety guidelines. The different combinations of diameters of the coils and the distance between the two coils are investigated and their effects are analyzed. Two versions of EMF safety guidelines (ICNIRP 1998 and ICNIRP 2010) are used as bases for safety distance calculation and the difference between the two guidelines are discussed.

• 예술인문사회 융합 멀티미디어 논문지
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• 제9권6호
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• pp.711-718
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• 2019

전자파 측정기와 스펙트럼 분석기를 통하여 가정에서 사용되고 있는 여러 종류의 가전제품에서 발생하는 전자파 발생 거동을 조사하였다. 전자레인지 동작 중에 30cm 떨어진 지점에서 측정된 전자파 전력은 8~11mW/㎡ 범위로 측정되었고 저주파 자기장의 강도는 60~80mG, 전기장 강도는 150~160V/m로 측정되었다. 스마트폰 무선 충전 패드의 경우 10cm 떨어진 지점에서 전자파 전력 0.4mW/㎡, 전기장 160V/m, 자기장 1mG로 측정되었다. 전자레인지 및 무선 충전 패드의 경우 10cm 이내로 근접하여 사용할 경우 전기장 크기가 인체 보호 기준을 초과하는 큰 값으로 측정되었으며 인체 유해성이 우려된다. 한편 TV, 헤어드라이어, 냉장고 등의 가전제품은 전자파, 전기장, 자기장 모두 매우 낮은 수준으로 나타났으며 인체 유해성은 없는 것으로 보인다. 전자파 차폐재의 경우 금속 재질의 Cu 패브릭과 메탈 포일은 높은 수준의 전자파 차단 성능을 나타내었으나, 폴리머 필름의 경우는 전자파 차단 특성이 낮은 수준으로 나타났다.

• 한국재료학회지
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• 제21권4호
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• pp.216-219
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• 2011

Perovskite manganites such as $RE_{1-x}A_xMnO_3$ (RE = rare earth, A = Ca, Sr, Ba) have been the subject of intense research in the last few years, ever since the discovery that these systems demonstrate colossal magnetoresistance (CMR). The CMR is usually explained with the double-exchange (DE) mechanism, and CMR materials have potential applications for magnetic switching, recording devices, and more. However, the intrinsic CMR effect is usually found under the conditions of a magnetic field of several Teslas and a narrow temperature range near the Curie temperature ($T_c$). This magnetic field and temperature range make practical applications impossible. Recently, another type of MR, called the low-field magnetoresistance(LFMR), has also been a research focus. This MR is typically found in polycrystalline half-metallic ferromagnets, and is associated with the spin-dependent charge transport across grain boundaries. Composites with compositions $La_{0.7}(Ca_{1-x}Sr_x)_{0.3}MnO_3)]_{0.99}/(BaTiO_3)_{0.01}$ $[(LCSMO)_{0.99}/(BTO)_{0.01}]$were prepared with different Sr doping levels x by a standard ceramic technique, and their electrical transport and magnetoresistance (MR) properties were investigated. The structure and morphology of the composites were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). BTO peaks could not be found in the XRD pattern because the amount of BTO in the composites was too small. As the content of x decreased, the crystal structure changed from orthorhombic to rhombohedral. This change can be explained by the fact that the crystal structure of pure LCMO is orthorhombic and the crystal structure of pure LSMO is rhombohedral. The SEM results indicate that LCSMO and BTO coexist in the composites and BTO mostly segregates at the grain boundaries of LCSMO, which are in accordance with the results of the magnetic measurements. The resistivity of all the composites was measured in the range of 90-400K at 0T, 0.5T magnetic field. The result indicates that the MR of the composites increases systematically as the Ca concentration increases, although the transition temperature $T_c$ shifts to a lower range.

• Journal of Advanced Marine Engineering and Technology
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• 제39권9호
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• pp.929-934
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• 2015

최근 선박의 건조는 고속화, 대형화에 따른 전자장비의 탑재 비율이 높아짐에 따라 선박의 사용주파수 대역의 확산이 전 세계적인 추세이다. 이로 인하여 전자소자 및 회로에서의 전자파 상호간의 간섭이 커다란 문제가 되고 있으며, 전자파 환경에서 인체에 대한 전자파 유해도가 주요 이슈상황이 되고 있다. 육상에서의 전자파 환경에 대한 지속적인 투자와 연구가 이루어지는데 반해 선박에서의 전자파 환경에 관한 시험 및 분석은 연구되지 않는 실정이다. 본 논문에서는 선내 추진 시스템에 대한 전자파 강도를 측정 분석 후 국내기준과 해외 기준에 측정 결과를 비교 분석하였다. 그 결과 측정된 결과가 국내 기준에는 만족하였지만 이탈리아 기준과 비교 시 배전반에서 기준치 초과와 다른 장비에서 측정값이 기준치와 근접함을 확인하였다. 따라서 충분한 실험을 통하여 선박의 추진시스템에 대한 전자파 인체보호기준지정과 정부차원의 인프라 구축이 필요하다.

• 대한자기공명의과학회:학술대회논문집
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• 대한자기공명의과학회 2003년도 제8차 학술대회 초록집
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• pp.103-105
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• 2003

After fifteen years of development, Magnetic Resonance (MR) technology for human imaging and spectroscopy is reaching a refined state with FDA approved 3T clinical products from Siemens, GE, and Philips. Broker has cleared CE approval with a 4T system. Varian supports a 4T system platform as well. Shielded magnets are standard at 3T from GE, Oxford, Magnex, and IGC. A shielded 4T whole body magnet is available from Oxford. Stronger switched gradients and dynamic shim coils, desired at any field, areespecially useful at higher static magnetic fields B0. In addition to the higher currents required for higher resolution slice or volume selection afforded by higher SNR, whole body gradient coils will be driven at increasing slew rates to meet the needs of new cardiac applications and other requirements. For example 3T and 4T systems are now being equipped with 2kV, 500A gradient coils and amplifiers capable of generating 4G/cm in 200msec, over a 67+/-cm bore diameter. High field EPI applications require oscillation rates at 1 kHz and higher. To achieve a benchmark 0.2 ppm shim over a 30cm sphere in a high field magnet, at least four stages of shimming need to be considered. 1) A good high field magnet will be built to a homogeneity spec. falling in the range of 100 to 150 ppm over this 30cm spherical "sweet spot" 2) Most modern high field magnets will also have superconducting shim coils capable of finding 1.5 ppm by their adjustment during system installation. 3) Passive ferro-magnetic shimming combined with 4) active, high order room temperature shim coils (as many as five orders are now being recommended) will accomplish 0.2 ppm over the 30cm sphere, and 0.1 ppm over a human brain in even the highest field magnets for human studies. Safety concerns for strong, fast gradients at any B0 field include acoustic noise and peripheral nerve stimulation. One or more of the mechanical decoupling methods may lead to quieter gradients. Patient positioning relative to asymmetric or short gradient coils may limit peripheral nerve stimulation at higher slew rates. Gradient designs combining a short coil for local speed and strength with a longer coil for coverage are being developed for 3T systems. Local gradients give another approach to maximizing performance over a limited region while keeping within the physiologically imposed dB0/dt performance limits.

• 대한의용생체공학회:의공학회지
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• 제15권2호
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• pp.175-182
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• 1994

The international standard for digital compression and coding of continuous-tone still image known as JPEG (Joint Photographic Experts Group) standard is investigated for medical image archiving and communication. The JPEG standard has widely been accepted in the areas of electronic image communication, computer graphics, and multimedia applications, however, due to the lossy character of the JPEG compression its application to the field of medical imaging has been limited. In this paper, the JPEG standard is investigated for medical image compression with a series of head sections of magnetic resonance (MR) images (256 and 4096 graylevels, $256 {\times}256$size). Two types of Huffman codes are employed, i. e., one is optimized to the image statistics to be encoded and the other is a predetermined code, and their coding efficiencies are examined. From experiments, compression ratios of higher than 15 were obtained for the MR images without noticeable distortion. Error signal in the reconstructed images by the JPEG standard appears close to random noise. Compared to existing full-frame bit-allocation technique used for radiological image compression, the JPEG standard achieves higher compression with less Gibb's artifact. Feature of the progressive image build-up of the JPEG progressive coding may be useful in remote diognosis when data is transmitted through slow public communication channel.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2011년도 제42회 하계학술대회
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• pp.1538-1539
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• 2011

In this study, we have investigated the dielectric breakdown by measuring AC (60Hz) breakdown strength of the fluids in accordance with IEC 156 standard and have compared the results with references. It was found that the dielectric breakdown voltage of pure transformer oil is around 12 [kV] with the gap distance of 1.5mm between electrodes. In case of our transformer oil based magnetic fluids with 0.1% < ${\Phi}$(volume concentration of magnetic particles) <0.6%, the dielectric breakdown voltage shows above 30 [kV], which is 2.5 times higher than that of pure transformer oil. It can be explained by the changed ionization process by adding nanoparticles in pure transformer oil, which is due to trapped fast electrons and slow negative nanoparticles. Moreover, in case of the fluid with applied magnetic field, the dielectric breakdown voltage increases above 40 [kV], which is 3.3 times higher than that of pure transformer oil.