• 한국광학회지
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• 제3권3호
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• pp.183-190
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• 1992

$Al_x/Ga_{1-x}$ /As/AlAs GaAs 계로 이루어진 비대칭 이중 양자우물 구조의 광학적 특성을 photoluminescence, photoluminescence excitation, time-resolved photoluminescence를 통하여 조사하였다. 양자장벽 AlAs의 두께에 따른 특성 변화를 조사하기 위하여 두께를 15$\AA$., 150$\AA$로 제작하였다. 양자장벽이 15$\AA$인 경우 매우 빠른 전자의 관통 현상을 보여 주었으며, 이로 인해 $Al_x/Ga_{1-x}$As의 여기자 재결합에 해당하는 피크가 관찰되지 않았다. AlAs 양자장벽이 150$\AA$인 경우에는 $Al_x/Ga_{1-x}$As양자우물에서 여기자 재결합에 의한 피크가 50ps 이하로 빠른 decay시간을 보여 주었으며 이것은 양자장벽과의 $\Gamma$-X전이에 의한 것으로 사료되었다. GaAs양자우물에서의 luminescence decay는 두 시료 모두 1ns정도 이었으나, 15$\AA$인 경우에는 약 100ps의 rise시간이 존재하였으며 이것은 정공의 관통에 의한 시간으로 판명되었다.

• 한국콘텐츠학회논문지
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• 제9권3호
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• pp.108-117
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• 2009

현재 모바일콘텐츠 분류체계는 분류원칙에 위배되는 구조적 문제와 컨버전스에 기인한 서비스의 다양화를 수용하는데 한계를 드러낸다. 본 연구는 이러한 한계점의 극복방안으로 알리스(Al Ries)의 이론을 발전시킨 대칭형과 비대칭형의 분류기준을 적용한 새로운 모바일콘텐츠 분류체계를 제시 한다. 여기서 대칭형은 융합 과정에서 변이(變異)되어 새로운 형태의 모바일 콘텐츠가 생성되는 것을 의미하는 반면, 비대칭형은 독립적인 속성들이 유지된 채 융합된 조합(組合)을 의미한다. 제시된 분류체계를 적용하여 현(現) 모바일 콘텐츠를 분석하면 모바일 콘텐츠 개발 및 서비스가 대칭형을 지향하고 있다는 사실과 기술의 발전과 함께 과거의 비대칭형 모바일 콘텐츠 역시 대칭형으로 재개발 되고 있다는 콘텐츠 산업의 경향이 도출된다. 현재 모바일 콘텐츠는 대칭적 성향과 비대칭적 성향이 공존한다는 측면에서 모바일콘텐츠 분류에 있어 대칭형과 비대칭형의 개념적 분류는 당분간 요구된다고 볼 수 있다. 따라서 본 연구에서 도출된 모바일 분류체계는 모바일콘텐츠 관련 연구자에게 컨버전스 경향은 물론 속성을 비교 분석할 수 있는 근거를 제공하는 동시에 모바일콘텐츠 관련 개발전략과 포지셔닝(Positioning) 전략을 세울 수 있는 일정정도 방향성 및 토대를 제시한다고 볼 수 있다.

• Bulletin of the Korean Chemical Society
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• 제34권12호
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• pp.3690-3694
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• 2013

The $H_2Ge=Ge:$ and its derivatives ($X_2Ge=Ge:$, X = H, Me, F, Cl, Br, Ph, Ar${\ldots}{\ldots}$) is a new species. Its cycloaddition reactions is a new area for the study of germylene chemistry. The mechanism of the cycloaddition reaction between singlet state Cl2Ge=Ge: and formaldehyde has been investigated with CCSD(T)//MP2/$6-31G^*$ method. From the potential energy profile, it could be predicted that the reaction has only one dominant reaction pathway. The reaction rule presented is that the two reactants first form a fourmembered Ge-heterocyclic ring germylene through the [2+2] cycloaddition reaction. Because of the 4p unoccupied orbital of Ge: atom in the four-membered Ge-heterocyclic ring germylene and the ${\pi}$ orbital of formaldehyde forming a ${\pi}{\rightarrow}p$ donor-acceptor bond, the four-membered Ge-heterocyclic ring germylene further combines with formaldehyde to form an intermediate. Because the Ge: atom in intermediate hybridizes to an $sp^3$ hybrid orbital after transition state, then, intermediate isomerizes to a spiro-Ge-heterocyclic ring compound via a transition state. The research result indicates the laws of cycloaddition reaction between $H_2Ge=Ge:$ and formaldehyde, and laid the theory foundation of the cycloaddition reaction between $H_2Ge=Ge:$ and its derivatives ($X_2Ge=Ge:$, X = H, Me, F, Cl, Br, Ph, Ar${\ldots}{\ldots}$) and asymmetric ${\pi}$-bonded compounds, which is significant for the synthesis of small-ring and spiro-Ge-heterocyclic compounds. The study extends research area and enriches the research content of germylene chemistry.

• Current Research on Agriculture and Life Sciences
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• 제31권2호
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• pp.75-82
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• 2013

Haploids are plants with a gametophytic number of chromosomes in their sporophytes. Androgenesis occurs from asymmetric division of pollen grains into generative cells and vegetative cells, followed by re-entry of the vegetative cell during S-phase, which causes microspores progress into G2/M transition in culture. One of the most interesting features of haploids is the possibility to produce doubled haploid (DH) individuals. Doubled haploidy is extremely useful to plant breeders because it enables shortened breeding periods and efficiency in selection of useful recessive agronomic traits. Doubled-haploid technology is not only applicable to breeding, but also to transformation programs of desired genes. In addition to practical breeding programs, DH lines provide useful materials of fundamental genetics including exploitation of QTLs and genes conferred with various agronomic traits by establishing DH populations. This paper provides historical overviews on androgenesis and describes several mechanisms associated with pollen embryogenesis, including mode of actions in pollen embryogenesis, mechanisms of chromosome doubling and factors affecting androgenesis. We also discuss recent progress in application of haploids to breeding, genes associated with in vitro response and drawbacks to anther culture for application of doubled haploids in crop breeding.

• 한국세라믹학회지
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• 제52권3호
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• pp.209-214
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• 2015

Silicon nitride films were deposited at $100^{\circ}C$ by using the catalytic chemical vapor deposition technique. The source gas mixing ratio, $R_N=[NH_3]/[SiH_4]$, was varied from 10 to 30, and the hydrogen dilution ratio, $R_H=[H_2]/[SiH_4]$, was varied from 20 to 100. The breakdown field strength reached a maximum value at $R_N=20$ and $R_H=20$, whereas the resistivity decreased in the same sample. The relative permittivity had a positive correlation with the breakdown field strength. The capacitance-voltage threshold curve showed an asymmetric hysteresis loop, which became more squared as $R_H$ increased. The width of the hysteresis window showed a negative correlation with the slope of the transition region, implying that the combined effect of $R_N$ and $R_H$ overides the interface defects while creating charge storage sites in the bulk region.

• Journal of Power Electronics
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• 제14권4호
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• pp.661-670
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• 2014

A new DC/DC converter with zero voltage switching is proposed for applications with high input voltage and high load current. The proposed converter has two circuit modules that share load current and power rating. Interleaved pulse-width modulation (PWM) is adopted to generate switch control signals. Thus, ripple currents are reduced at the input and output sides. For high-voltage applications, each circuit module includes two half-bridge legs that are connected in series to reduce switch voltage rating to $V_{in}/2$. These legs are controlled with the use of asymmetric PWM. To reduce the current rating of rectifier diodes and share load current for high-load-current applications, two center-tapped rectifiers are adopted in each circuit module. The primary windings of two transformers are connected in series at the high voltage side to balance output inductor currents. Two series capacitors are adopted at the AC terminals of the two half-bridge legs to balance the two input capacitor voltages. The resonant behavior of the inductance and capacitance at the transition interval enable MOSFETs to be switched on under zero voltage switching. The circuit configuration, system characteristics, and design are discussed in detail. Experiments based on a laboratory prototype are conducted to verify the effectiveness of the proposed converter.

• East Asian Economic Review
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• 제25권2호
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• pp.205-230
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• 2021

This study examines the impact of the number of coronavirus cases on regime-switching in stock return volatility. This study documents the empirical evidence that the COVID-19 cases had an asymmetric effect on the regime of stock return volatility. When the stock return is in the low volatility regime, the probability of switching to the high volatility regime in the next trading day increases as the number of cumulative cases increases. In contrast, in the high volatility regime, the effect of cumulative cases on the transition probability is not statistically significant. This study also documents the evidence that the government measures against the pandemic contribute to promoting the high volatility regime of the KOSPI during the pandemic. Besides, this study projects future stock prices through the Monte Carlo simulation based on the estimated parameters and the predicted number of the COVID-19 new cases. Under a scenario where the number of new cases rapidly increases, stock price indices in Korea are expected to be in a downward trend over the next three months. On the other hand, under the moderate scenario and the best scenario, the stock indices are likely to continue to rise.

• 한국막학회:학술대회논문집
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• 한국막학회 1999년도 The 7th Summer Workshop of the Membrane Society of Korea
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• pp.39-42
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• 1999

Perfluoropolymers represent the ultimate resistance to hostile chemical environments and high service temperature, attributed to the presence of fluorine in the polymer backbone, i.e. to the high bond energy of C-F and C-C bonds of fluorocarbons. Copolymers of Tetrafluoroethylene (TEE) and 2, 2, 4Trifluoro-5Trifluorometoxy- 1, 3Dioxole (TTD), commercially known as HYFLON AD, are amorphous perfluoropolymers with glass transition temperature (Tg)higher than room temperature, showing a thermal decomposition temperature exceeding 40$0^{\circ}C$. These polymer systems are highly soluble in fluorinated solvents, with low solution viscosities. This property allows the preparation of self-supported and composite membranes with desired membrane thickness. Symmetric and asymmetric perfluoropolymer membranes, made with HYFLON AD, have been prepared and evaluated. Porous and not porous symmetric membranes have been obtained by solvent evaporation with various processing conditions. Asymmetric membranes have been prepared by th wet phase inversion method. Measure of contact angle to distilled water have been carried out. Figure 1 compares experimental results with those of other commercial membranes. Contact angles of about 120$^{\circ}$for our amorphous perfluoropolymer membranes demonstrate that they posses a high hydrophobic character. Measure of contact angles to hexandecane have been also carried out to evaluate the organophobic character. Rsults are reported in Figure 2. The observed strong organophobicity leads to excellent fouling resistance and inertness. Porous membranes with pore size between 30 and 80 nanometers have shown no permeation to water at pressures as high as 10 bars. However high permeation to gases, such as O2, N2 and CO2, and no selectivities were observed. Considering the porous structure of the membrane, this behavior was expected. In consideration of the above properties, possible useful uses in th field of gas- liquid separations are envisaged for these membranes. A particularly promising application is in the field of membrane contactors, equipments in which membranes are used to improve mass transfer coefficients in respect to traditional extraction and absorption processes. Gas permeation properties have been evaluated for asymmetric membranes and composite symmetric ones. Experimental permselectivity values, obtained at different pressure differences, to various single gases are reported in Tab. 1, 2 and 3. Experimental data have been compared with literature data obtained with membranes made with different amorphous perfluoropolymer systems, such as copolymers of Perfluoro2, 2dimethyl dioxole (PDD) and Tetrafluorethylene, commercialized by the Du Pont Company with the trade name of Teflon AF. An interesting linear relationship between permeability and the glass transition temperature of the polymer constituting the membrane has been observed. Results are descussed in terms of polymer chain structure, which affects the presence of voids at molecular scale and their size distribution. Molecular Dyanmics studies are in progress in order to support the understanding of these results. A modified Theodoru- Suter method provided by the Amorphous Cell module of InsightII/Discover was used to determine the chain packing. A completely amorphous polymer box of about 3.5 nm was considered. Last but not least the use of amorphous perfluoropolymer membranes appears to be ideal when separation processes have to be performed in hostile environments, i.e. high temperatures and aggressive non-aqueous media, such as chemicals and solvents. In these cases Hyflon AD membranes can exploit the outstanding resistance of perfluoropolymers.

• 한국자기학회지
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• 제22권1호
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• pp.15-18
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• 2012

Olivine 구조인 $LiFe_{0.9}Mn_{0.1}PO_4$ 분말 시료를 직접합성법(solid state method) 으로 제조하였으며, 결정학적 및 자기적 특성을 x-선 회절(x-ray diffractometer), 초전도 양자 간섭계(superconducting quantum interference devices) 및 뫼스바우어 분광(M$\ddot{o}$ssbauer spectroscopy) 실험을 이용하여 연구하였다. $LiFe_{0.9}Mn_{0.1}PO_4$ 시료의 결정구조는 공간그룹이 Pnma인 orthorhombic 구조임을 Rietveld 정련법으로 분석하였다. $LiFe_{0.9}Mn_{0.1}PO_4$ 시료의 닐온도 (N$\acute{e}$el temperature; $T_N$)는 50 K으로 나타내었고 닐온도에서 자기 상전이가 일어나는 것을 초전도 양자 간섭계 실험을 통하여 확인하였다. Fe(Mn)-O 이온간 거리를 분석하여 $FeO_6(MnO_6)$ 팔면체 구조가 비대칭임을 확인하였고 그 구조로 인하여 강한 결정장에 영향을 받으며, 닐온도 이상에서 자기 2중극자 상호작용은 사라지고, 강한 결정장에 의한 전기 4중극자 작용만이 존재하여 두 개의 흡수선이 나타나는 것을 뫼스바우어 분광 실험을 통하여 분석하였다.

• Strategy21
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• 통권31호
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• pp.57-84
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• 2013

North Korea's continuous threats and provocative behaviors have aggravated tension on the Korean peninsula particularly with the recent nuclear weapons test. South Korea's best way to cope with this situation is to maintain the balance among three policy directions: dialogue, sanctions, and deterrence. Among the three, I argue that deterrence should be prioritized. There are different sources of deterrence such as military power, economic power, and diplomatic clouts. States can build deterrence capability independently. Alternatively, they may do so through relations with other states including alliances, bilateral relations, or multilateral relations in the international community. What South Korea needs most urgently is to maintain deterrence against North Korea's local provocations through the enhancement of independent military capability particularly by addressing the asymmetric vulnerability between militaries of the South and the North. Most of all, the South Korean government should recognize the seriousness of the negative consequences that North Korea's 'Nuclear shadow strategy' would bring about for the inter-Korea relations and security situations in Northeast Asia. Based on this understanding, it should develop an 'assertive deterrence strategy' that emphasizes 'multi-purpose, multi-stage, and tailored deterrence whose main idea lies in punitive retaliation.' This deterrence strategy requires a flexible targeting policy and a variety of retaliatory measures capable of taking out all targets in North Korea. At the same time, the force structures of the army, the air force, and the navy should be improved in a way that maximizes their deterrence capability. For example, the army should work on expanding the guided missile command and the special forces command and reforming the reserve forces. The navy and the air force should increase striking capabilities including air-to-ground, ship-to-ground, and submarine-to-ground strikes to a great extent. The marine corps can enhance its deterrence capability by changing the force structure from the stationary defense-oriented one that would have to suffer some degree of troop attrition at the early stage of hostilities to the one that focuses on 'counteroffensive landing operations.' The government should continue efforts for defense reform in order to obtain these capabilities while building the 'Korean-style triad system' that consists of advanced air, ground, and surface/ subsurface weapon systems. Besides these measures, South Korea should start to acquire a minimum level of nuclear potential within the legal boundary that the international law defines. For this, South Korea should withdraw from the Nuclear Non-proliferation Treaty. Moreover, it should obtain the right to process and enrich uranium through changing the U.S.-South Korea nuclear cooperation treaty. Whether or not we should be armed with nuclear weapons should not be understood in terms of "all or nothing." We should consider an 'in-between' option as the Japanese case proves. With regard to the wartime OPCON transition, we need to re-consider the timing of the transition as an effort to demonstrate the costliness of North Korea's provocative behaviors. If impossible, South Korea should take measures to make the Strategic Alliance 2015 serve as a persisting deterrence system against North Korea. As the last point, all the following governments of South Korea should keep in mind that continuing reconciliatory efforts should always be pursued along with other security policies toward North Korea.