• Journal of the Optical Society of Korea
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• 제16권2호
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• pp.181-184
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• 2012

We report, for the first time, operation frequency dependence of current density-voltage ($J_{OLED}-V_{OLED}$) shift for multi-layer organic light-emitting diodes (OLEDs). When the OLEDs were electrically stressed for 21 hours with 50% duty voltage pulses at 60, 120, 240, and 360 Hz, the JOLED-VOLED shifts were suppressed by half for 360 Hz operation compared with 60 Hz operation, but with little change in emission efficiencies. This frequency dependent $J_{OLED}-V_{OLED}$ shift is believed to be commonly observed for typical multi-layer OLEDs and can be used to further improve lifetime of digitally-driven active-matrix OLED displays.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 1998년도 추계종합학술대회 논문집
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• pp.589-592
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• 1998

The performance of inductively coupled plasma (ICP) is enhanced by axial magnetic field driven by alternating current Helmholtz coils in this work. Langmuir pobe is used to characterize the plasma, and the etching performance is demonstrated with phororesist stripping process. It is shown that its density and uniformity depends on the frequency of driving current to the magnetic field.

• Journal of Magnetics
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• 제14권3호
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• pp.101-103
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• 2009

The spin transfer torque efficiency was determined experimentally by observing the current-driven domainwall depinning of Pt/CoFe/Pt nanowires with perpendicular magnetic anisotropy. The depinning time was exponentially proportional to the applied magnetic field, and was well explained by the Neel-Brown formula. The depinning time and threshold magnetic field were varied considerably by injecting current into the nanowire. The spin transfer torque efficiency was estimated to be $(7.2{\pm}0.9){\times}10^{-15}Tm^2$/A from the linear dependence of the threshold current density with respect to the applied magnetic field.

• 한국항만학회지
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• 제13권2호
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• pp.409-418
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• 1999

Gamcheon Bay has complicated and unique flow pattern which is affected by topography characteristics of a semi-closed bay and warm water discharge from the power plant located at the bay. It is very important to consider warm water discharge with tidal period and wind effects, which are mainly acting on the surface flow patterns in Gamcheon Bay. The results of observation show as follows; i) Because Gamcheon bay has much elongated shape to the north-south direction and narrow entrance with two separated breakwaters, the flow is very weak inside the bay. ii) The main flow path is at the west side of the bay. The direction of flood current is northward along the west side of the bay and the direction of ebb current is southward along the west side of the bay. The southward direction of warm water discharge has curved to the west side of the bay. iii)At the period of flood current for neap tide, the flow direction is southward in the bay, which is thought by the effects of warm water discharge. But at the bottom layer, the effects of tidal current reached to the middle of the bay, and showed features like eddy. iv) The wind effect is very strong, especially, prominent in the west and east side of the bay.

• KIEE International Transaction on Electrical Machinery and Energy Conversion Systems
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• 제4B권3호
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• pp.108-113
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• 2004

This paper presents methods to improve performance of the power supply system in a NASA deep space explorer. In the Stirling engine driven reciprocating Brushless DC (BLDC) generator, the accurate position information of the prime mover is important to diagnose the performance of the engine and prevent distortion of the output power. Since sensors to detect the position are fragile and unreliable, and conventional sensorless techniques have drawbacks in the low speed region, a novel sensorless position detection technique for the prime mover has been proposed and verified. Another major issue of the generator for the spacecraft is power density maximization. The mass of the power system is important to the mass of the satellite. Therefore, the components of the spacecraft should be lightweight. Conventional rectification methods cannot achieve the maximum power possible due to non-optimal current waveforms. The optimal current waveform for maximizing power density and minimizing machine size and weight in a nonsinusoidal power supply system has been derived, incorporated in a control system, and verified by simulation work.

• 해양환경안전학회지
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• 제26권3호
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• pp.242-253
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• 2020

This study examined the ef ects of freshwater discharge by artificial dikes from the Kanwol and Bunam lakes on the dynamics in the Chunsu Bay, Yellow Sea, Korea, during the summer season based on three-dimensional numerical modeling experiments. Model performances were evaluated in terms of skill scores for tidal elevation, velocity, temperature, and salinity and these scores mostly exceeded 90 %. The variability in residual currents before and after the freshwater discharge was examined. The large amount of lake water discharge through artificial dikes may result in a dramatically changed density field in the Chunsu Bay, leading to an estuarine circulation system. The density-driven current formed as a result of the freshwater inflow through the artificial dikes (Kanwol/Bunam) caused a partial change in the tidal circulation and a change in the scale and location of paired residual eddies. The stratification formed by strengthened static stability following the freshwater discharge led to a dramatic increase in the Richardson number and lasted for a few weeks. The strong stratification suppressed the vertical flux and inhibited surface aerated water mixing with bottom water. This phenomenon would have direct and indirect impacts on the marine environment such as hypoxia/anoxia formation at the bottom.

• Journal of Astronomy and Space Sciences
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• 제32권1호
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• pp.1-11
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• 2015

Earth's magnetopause separating the fast and often turbulent magnetosheath and the relatively stagnant magnetosphere provides various forms of free energy that generate low-frequency surface waves. The source mechanism of this energy includes current-driven kinetic physical processes such as magnetic reconnection on the dayside magnetopause and flux transfer events drifting along the magnetopause, and velocity shear-driven (Kelvin-Helmholtz instability) or density/pressure gradient-driven (Rayleigh-Taylor instability) magnetohydro-dynamics (MHD) instabilities. The solar wind external perturbations (impulsive transient pressure pulses or quasi-periodic dynamic pressure variations) act as seed fluctuations for the magnetopause waves and trigger ULF pulsations inside the magnetosphere via global modes or mode conversion at the magnetopause. The magnetopause waves thus play an important role in the solar wind-magnetosphere coupling, which is the key to space weather. This paper presents recent findings regarding the generation of surface waves (e.g., Kelvin-Helmholtz waves) at the Earth's magnetopause and analytic and observational studies accountable for the linking of the magnetopause waves and inner magnetospheric ULF pulsations, and the impacts of magnetopause waves on the dynamics of the magnetopause and on the inner magnetosphere.

• 한국수자원학회논문집
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• 제38권1호
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• pp.83-96
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• 2005

해안매립이 해양의 동수력학, 환경 및 생태계에 미치는 영향을 분석하기 위하여 3차원 동수력학 수치해석을 실시하였다. 이 연구에서는 방조제 건설에 따른 조석, 바람 및 밀도변화 성분을 포함한 조류와 잔차류의 변화를 수치모의 하였다. $\sigma$-좌표로 변환된 지배방정식은 음해유한차분법을 이용하여 해석하였다. 수치모형은 조석의 4대 주요 구성성분인 M$_2$, S$_2$, $K_1$ 및 $O_1$의 조석표를 이용하여 검증하였다. 수치해석결과, 주로 조석 및 바람에 의한 잔차류의 변화가 큰 것으로 나타났다.

• 한국표면공학회지
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• 제52권2호
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• pp.72-77
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• 2019

To study the characteristics of AC driven OLED, we fabricated the fluorescent OLEDs and analyzed the electroluminescence characteristics of OLEDs with AC negative voltage. The luminance and the current density of the OLED decreased, and the number and size of the dark spots increased in proportion to the duration time and level of the applied AC negative voltage. The current efficiency of the OLED was improved when high AC negative voltage was applied within a short time. When the AC negative voltage of 10 V was applied for 1 minute, the efficiency was improved by 12.4%. Also, the degradation of luminance and current efficiency due to the duration of light emission was improved in the case of OLED applied for 1 minute with 10 V AC negative voltage. These are expected as a result of the improvement of the leakage current characteristics by eliminating the short-circuit region formed by the defect of the OLED at the AC negative high voltage. As a result, the continuous application of AC negative voltage reduced the luminance and the current density of OLED, but the temporary application of AC negative voltage with the proper time and voltage could improve the efficiency and lifetime of OLED.

• Nuclear Engineering and Technology
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• 제37권5호
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• pp.447-456
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• 2005

Laser driven accelerators promise to provide an alternative to conventional accelerator technology. They rely on the excitation of large amplitude density waves in a plasma by the photon pressure of an intense laser. The density oscillations in which electrons and ions are separated, result in extremely large longitudinal electric fields that can be several orders of magnitude larger than those that are used in today's radio-frequency accelerators. Whereas this principle had been demonstrated experimentally for nearly two decades, it was not until 2004 that the production of high quality electron beams around 100 MeV was demonstrated. Analysis, aided by particle-in-cell simulations, as well as experiments with various plasma lengths and densities, indicate that tailoring the length of the accelerator, together with loading of the accelerating structure with beam, are the keys to production of mono-energetic electron beams. Increasing the energy towards a GeV and beyond will require reducing the plasma density and design criteria are discussed for an optimized accelerator module. The current progress and future directions are summarized through comparison with conventional accelerators, highlighting the unique short and long term prospects for intense radiation sources and high energy accelerators based on laser-drivenplasma accelerators.