• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.425.1-425.1
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• 2014

Thermal transport in nanomaterials is not only scientifically interesting but also technological important for various future electronic, bio, and energy device applications. Among the various computation approaches to investigate lattice thermal transport phenomena in nanoscale, the atomistic nonequilibrium Green's function approach based on first-principles density functional theory calculations appeared as a promising method given the continued miniaturization of devices and the difficulty of developing classical force constants for novel nanoscale interfaces. Among the nanometerials, carbon atomic chains, namely the cumulene (all-doulble bonds, ${\cdots}C=C=C=C{\cdots}$) and polyyne (alternation of single and triple bonds, ${\cdots}C{\equiv}C-C{\equiv}C{\cdots}$) can be considered as the extream cases of interconnction materials for nanodevices. After the discovery and realization of carbon atomic chains, their electronic transport properties have been widely studied. For the thermal transport properties, however, there have been few literatures for this simple linear chain system. In this work, we first report on the development of a non-equilibrium Green's function theory-based computational tool for atomistic thermal transport calculations of nanojunctions. Using the developed tool, we investigated phonon dispersion and transmission properties of polyethylene (${\cdots}CH2-CH2-CH2-CH2{\cdots}$) and polyene (${\cdots}CH-CH-CH-CH{\cdots}$) structures as well as the cumulene and polyyne. The resulting phonon dispersion from polyethylene and polyene showed agreement with previous results. Compared to the cumulene, the gap was found near the ${\Gamma}$ point of the phonon dispersion of polyyne as the prediction of Peierls distortion, and this feature was reflected in the phonon transmission of polyyne. We also investigated the range of interatomic force interactions with increase in the size of the simulation system to check the convergence criteria. Compared to polyethylene and polyene, polyyne and cumulene showed spatially long-ranged force interactions. This is reflected on the differences in phonon transport caused by the delicate differences in electronic structure.

• Publications of The Korean Astronomical Society
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• v.8 no.1
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• pp.163-168
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• 1993

At intermediate mass transfer rates, accretion disks in binary star systems undergo a thermally-driven limit cycle instability. This instability leads to outburst episodes when the disk is bright and the flow through the disk is rapid separated by long intervals when the disk is dim and the flow through it is low. This intrinsic outburst mechanism can help to understand a wide range of astrophysical phenomena from dwarf novae to soft X -ray transients involving white dwarf, neutron star, and black holes. and to a deeper understanding of the mechanism of angular transport and viscosity in the accretion disk.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.2
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• pp.198-209
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• 1997

This paper is concerned about the performance of alternative refrigerants of R-502 which has been widely used in transport and low temperature refrigeration. A breadboard heat pump was constructed with counterflow heat exchangers and tested with R-502 and transitional alternatives of R-402A and R-402B and long-term alternatives of R-404A and R-507 to compare the performance of each refrigerant. Measurements were conducted at two different condensing temperatures of $43.3^{\circ}C$ and $52.0^{\circ}C$ and the evaporating temperature was varied over a range from $-25^{\circ}C$ to $-5^{\circ}C$. The evaporator superheat and condenser subcooling were maintained constant at about $5^{\circ}C$ for all tests. Test results showed that all alternative fluids tested in this research work can be used as 'drop-in' fluids to replace R-502 without any major problem. It is also found that in the long run HFC alternatives are to be used due to their favorable environmental characteristics and better performance.

• Proceedings of the Korea Air Pollution Research Association Conference
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• 1999.10a
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• pp.338-345
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• 1999

동북아시아 지역은 최근의 급속한 경제성장으로 오염물질의 배출량도 급격한 증가추세에 있다. 중국의 배출량이 동북아지역 배출량의 대부분을 차지하고 있으며, 산출자료에 따라 다르기는 하지만, 그 중 아황산가스는 80％이상을 중국이 배출하고 있는 것으로 나타나고 있다(김용표, 1999). 강수나 에어로졸 생성에 중요한 역할을 하는 주도적인 물질인 황화합물은 대류권 하부에 주로 2일에서 1주일정도 체류하면서 보통 수백에서 수천 km까지 이동이 가능하다.(중략)

• Proceedings of the Korea Air Pollution Research Association Conference
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• 2000.04a
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• pp.187-190
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• 2000

대기오염물질의 장거리 수송과 관련해서는 다음의 두 가지 과정이 지배적으로 나타난다. 하나는 종관바람장에 의해 먼 거리를 이동하는 과정을 들 수가 있고(Cox et at. 1975, Apling et al. 1977) 다른 한가지는 열적으로 형성된 국지순환계에 의해 수송되는 과정을 들 수 있다. (Kurita and Ueda 1990) 이러한 과정 중에서 대기오염물의 고농도에 의한 피해가 부각되는 것은 후자라고 할 수 있다 이는 고농도의 대기오염문제는 종관장의 영향이 적어서 오염물의 확산이 부진한 날에 나타나기 때문이다. (중략)

• Proceedings of the Korea Air Pollution Research Association Conference
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• 2003.11a
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• pp.47-48
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• 2003

우리나라에서 장거리 이동 연구가 본격화되기 시작한 것은 1990년대 이후이다. 1980년대에도 황사시기에 오염물질의 변화를 관찰하는 등 (이민희 등, 1988) 장거리 이동에 관한 연구가 없었던 것은 아니나 국지배출에 의한 1차 오염이 지배적인 상황에서 주목받기가 쉽지 않았다. 그러나 1990년대 청정연료 보급 등 정부 시책에 따라 세계 1, 2위를 다투던 1차 오염이 해결의 실마리가 잡히고, G-7 환경공학기술개발사업의 일환으로 산성비 연구가, NASA의 PEM-West (Pacific Exploratory Mission in the western Pacific)사업의 일환으로 제주도 고산에서 측정이 진행되면서(Chen et al., 1997) 우리나라 장거리 이동연구는 틀을 갖추게 되었다. (중략)

• Ceramist
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• v.21 no.1
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• pp.98-111
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• 2018

The lead-based perovskite (CH3NH3PbI3) material has a high molar coefficient, high crystallinity at low temperature, and long range of balanced electron-hole transport length. In addition, PCE of perovskite solar cells (PSCs) has been dramatically improved by over 22% by amending the electronic quality of perovskite and by using state-of-the-art hole transporting materials (HTMs) such as tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) due to enhanced charge transport toward the electrode via properly aligned energy levels with respect to the perovskite. Replacing the spiro-OMeTAD with new HTMs with the desired properties of appropriate energy levels, high hole mobility in its pristine form, low cost, and easy processable materials is necessary for attaining highly efficient and stable PSCs, which are anticipated to be truly compatible for practical application. Furthermore, Recently Pb-free perovskite materials much attention as an alternative light-harvesting active layer material instead of lead based perovskite in photovoltaic cells. In this work, we demonstrate a Pb-free perovskite material for the light harvesting and emitter as optoelectronic devices.

• Progress in Superconductivity and Cryogenics
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• v.24 no.2
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• pp.23-26
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• 2022

A lossless transport of an arbitrary waveform in a frequency range of 10⁶-10⁹ Hz through a conduction channel in a cryogenic temperature is of importance for a high-speed operation of quantum device. However, it is hard to use a commercial oscilloscope to directly detect the waveform travelling in a device located in a cryogenic system. Here, we developed a cryogenic voltage sampling technique by using a Schottky barrier gate prepared on a surface of a GaAs/AlGaAs device, which revealed that an incident rectangle waveform can transport through a 1 mm long two-dimensional conduction channel without waveform deformation up to 20 MHz, while further study is needed to increase the detection frequency.

• Journal of Korean Society for Atmospheric Environment
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• v.8 no.3
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• pp.162-168
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• 1992

Numerical solutions to the advection equations used for long-range transport air pollution models are calculated using three numerical methods; Antidiffusion correction method(Smolarkiewicz, 1983), Positive definite advecton scheme obtained by nonlinear renormalization of the advective fluxes(Bott, 1989), and Positive definite pseudospectral method(Bartnicki, 1989). Accuracy, numerical diffusion and computational time requirement are compared for two-dimensional transport calculations in a uniform rotational flow field. The solutions from three methods are positive definite. Bartnicki(1989)'s method is most conservative but requires approximately 10 times as much computational time as Smolarkiewicz(1983)'s method of which numerical diffusion is the largest. All three methods are more conservative for a cone shape initial condition than for a rectangular block initial condition with a steep gradient.

• Atmosphere
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• v.32 no.2
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• pp.119-133
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• 2022

From 2015 to 2021, high-concentration fine dust episodes with a daily average PM_2.5 concentration of 50 ㎍ m^-3 or higher were selected and classified into 3 types [long range transport (LRT), mixed (MIX) and Local emission and stagnant (LES)] using synoptic chart and backward trajectory analysis. And relationships between the fine particle data (PM_2.5 and PM₁₀ concentration and PM_2.5/PM₁₀ ratio) and meteorological data (PBLH, Ta, WS, U-wind, and Rainfall) were analyzed using hourly observation for the classification episodes on the Korean Peninsula and the Seoul metropolitan area (SMA). In LRT, relatively large particles such as dust are usually included, and in LES, fine particle is abundant. In the Korean peninsula, the rainfall was relatively increased centered on the middle and western coasts in MIX and LES. In the SMA, wind speed was rather strong in LRT and weak in LES. In LRT, rainfall was centered in Seoul, and in MIX and LES, rainfall appeared around Seoul. However, when the dust cases were excluded, the difference between the LRT and other types of air quality was decreased, but the meteorological variables (Ta, RH, Pa, PBLH, etc.) were further strengthened. In the case of the Korean Peninsula, it is difficult to find a clear relationship because regional influences (topographical elevation, cities and coasts, etc.) are complexly included in a rather wide area. In the SMA, it is analyzed that the effects of urbanization such as the urban heat island centered on Seoul coincide with the sea and land winds, resulting in a combination of high concentrations and meteorological phenomena.