• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.320-320
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• 2010

The dye-sensitized solar cells (DSSCs) have achieved so far the highest validated efficiency over 11%. However, the cells with the best performance utilize volatile solvent as a electrolyte, which can cause some practical limitations for the long-term operation. This is one of the most substantial problems to be resolved for the commercialization of DSSCs. In order to improve the long-term stability, many research groups have reported new electrolyte system, to replace the liquid type electrolyte by non-volatile ones. In this work, we studied long-term stability of the DSSCs with various types of electrolytes such as (PVDF HFP) based polymer, eutectic melts of ionic liquids, and liquid based solvent. The cells with various electrolytes have been exposed to the condition under thermal stress and illumination over 1000 hours. We will report the change of photovoltaic properties with time and investigate the degradation mechanism with the impedance spectroscopic analysis.

• 대한수학회지
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• 제53권6호
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• pp.1331-1345
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• 2016

A K-g-frame is a generalization of a g-frame. It can be used to reconstruct elements from the range of a bounded linear operator K in Hilbert spaces. K-g-frames have a certain advantage compared with g-frames in practical applications. In this paper, the interchangeability of two g-Bessel sequences with respect to a K-g-frame, which is different from a g-frame, is discussed. Several construction methods of K-g-frames are also proposed. Finally, by means of the methods and techniques in frame theory, several results of the stability of K-g-frames are obtained.

• Journal of Mechanical Science and Technology
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• 제19권11호
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• pp.1975-1987
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• 2005

An input time delay always exists in practical systems. Analysis of the delay phenomenon in a continuous-time domain is sophisticated. It is appropriate to obtain its corresponding discrete-time model for implementation via digital computers. In this paper a new scheme for the discretization of nonlinear systems using Taylor series expansion and the zero-order hold assumption is proposed. The mathematical structure of the new discretization method is analyzed. On the basis of this structure the sampled-data representation of nonlinear systems with time-delayed multi-input is presented. The delayed multi-input general equation has been derived. In particular, the effect of the time-discretization method on key properties of nonlinear control systems, such as equilibrium properties and asymptotic stability, is examined. Additionally, hybrid discretization schemes that result from a combination of the scaling and squaring technique (SST) with the Taylor series expansion are also proposed, especially under conditions of very low sampling rates. Practical issues associated with the selection of the method's parameters to meet CPU time and accuracy requirements, are examined as well. A performance of the proposed method is evaluated using a nonlinear system with time delay maneuvering an automobile.

• 한국환경과학회지
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• 제1권1호
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• pp.35-43
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• 1992

Handling the emergency problems such as Chemobyl accident require real time prediction of pollutants dispersion. One-point real time sounding at pollutant source and simple model including turbulent-radiation process are very important to predict dispersion at real time. The stability categories obtained by one-dimensional numerical model (including PBL dynamics and radiative process) are good agreement with observational data (Golder, 1972). Therefore, the meteorological parameters (thermal, moisture and momentum fluxes; sensible and latent heat; Monin-Obukhov length and bulk Richardson number; vertical diffusion coefficient and TKE; mixing height) calculated by this model will be useful to understand the structure of stable boundary layer and to handling the emergency problems such as dangerous gasses accident. Especially, this simple model has strong merit for practical dispersion models which require turbulence process but does not takes long time to real predictions. According to the results of this model, the urban area has stronger vertical dispersion and weaker horizontal dispersion than rural area during daytime in summer season. The maximum stability class of urban area and rural area are "A" and "B" at 14 LST, respectively. After 20 LST, both urban and rural area have weak vertical dispersion, but they have strong horizontal dispersion. Generally, the urban area have larger radius of horizontal dispersion than rural area. Considering the resolution and time consuming problems of three dimensional grid model, one-dimensional model with one-point real sounding have strong merit for practical dispersion model.al dispersion model.

• Wind and Structures
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• 제38권5호
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• pp.399-410
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• 2024

Long-span suspension bridges located in typhoon-prone regions face significant risks of flutter instability, particularly in girder erection. Despite the implementation of aerodynamic countermeasures designed for the service stage, the flutter stability of bridge in girder erection may not meet the required standards. Nowadays, the double-deck truss girder is increasingly common in practical engineering which exhibits different performance from the single-deck truss girder. To gain insights into the flutter performance of this girder type and determine temporary aerodynamic countermeasures for flutter suppression in girder erection, wind tunnel tests were conducted. The effects of affiliated members on the flutter performance were first examined. Subsequently, different aerodynamic countermeasures were designed and their effectiveness was tested. The results indicate that the stabilizers above and below the upper and lower decks are the most effective for the flutter stability of bridge at positive and negative angles of attack, respectively. The higher the stabilizers are, the better the effect on flutter suppression achieves. Considering the feasibility in practical engineering, a temporary stabilizer above the upper deck was considered. It is expected that the results could provide references for the aerodynamic design of double-deck truss girder during erection.

• 디자인학연구
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• 제20권3호
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• pp.5-14
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• 2007

본 연구의 목적은 성격과 신체 만족도에 따라 선호하는 패션 제품의 이미지에 관해 알아보고자 한다. 연구 방법은 선행 문헌 자료를 분석한 후 이를 토대로 설문지를 작성, 배포, 회수한 후 통계처리 하였다. 연구 대상은 만 35$\sim$59세의 우리나라 중년 여성 352명이다. 자료 분석은 SAS와 SPSS를 사용하여 인자분석, 군집분석, ANOVA, Duncan's Multiple Range Test를 실시하였다. 연구 결과 자신의 신체에 대한 만족도가 높을수록 정서적 안정성과 자신감이 높았으며, 보다 대담하고 여성스러운 이미지의 패션 제품을 선호하였다. 성격 특성과 선호하는 패션 제품 이미지와의 관계를 살펴본 결과 지배성이 높은 사람일수록 대담한 이미지의 패션 제품을 선호하였으며, 책임성이 높은 사람일수록 패션 제품에서 실용성을 추구하였으나 대담성에 관해서는 선호도가 낮았다. 정서적 안정성이 높은 사람일수록 품위 있고 실용적인 패션 제품을 선호하였으나 대담스러운 이미지에 관해서는 선호도가 낮았다. 한편 사교성이 높은 사람일수록 품위있고 남성스러운 이미지를 선호하였다. 그리고 자신감이 높은 사람일수록 대담하고 남성스러운 이미지의 패션 제품을 선호하였다.

• 무역상무연구
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• 제39권
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• pp.87-103
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• 2008

Two kinds of security devices such as independent guarantees and standby letters of credit have been widely used in the international transactions. These devices design to protect one of the parties from a breach by its counter-party. Main uses of these guarantees and standby letters of credit are as follows : bid guarantee, performance guarantee, advance payment guarantee, payment guarantee, retention guarantee, etc. The standby letters of credit were first invented in the U.S.A. and have been widely used in the international and domestic contracts in the U.S.A. But the practical use of these credits is very unsatisfactory in Korea. The purpose of this study is to serve the increase of practical use of the standby letters of credit in Korea through the comparison study on the practical use of the credits between Korea and the U.S.A. Both devices are very similar in function, but they are very different in forms. The one has the form of letter of credits but the other has the form of guarantee. The letter of credit has the stability of governing rule, the legal certainty, and the preference in the field of the trade community comparing to the guarantee. I recommend to use standby letter of credit instead of bank guarantee in international transactions because of the merits of the credit aforesaid.

• 전력전자학회논문지
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• 제22권2호
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• pp.102-108
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• 2017

Proper design guides are proposed for a practical dual-active bridge (DAB) converter based on the mathematical model on the steady state. The DAB converter is popular in bidirectional application due to its zero-voltage capability and easy bidirectional operation for seamless control, high efficiency, and performance. Some design considerations are taken to overcome the limitation of the DAB converter. The practical design methodology of power stage is discussed to minimize the conduction and switching losses of the DAB converter. Small-signal model and frequency response are derived and analyzed based on the generalized average method, which considers equivalent series resistance, to improve the dynamics, stability, and reliability with voltage regulation of the practical DAB converter. The design of closed-loop control is discussed by the derived small-signal model to obtain the pertinent gain and phase margin in steady-state operation. Experimental results of a 3.3 kW prototype of DAB converter demonstrate the validity and effectiveness of the proposed methods.

• 조명전기설비학회논문지
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• 제16권5호
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• pp.83-89
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• 2002

고도의 산업화에 따른 물류량의 증가와 인구 도시집중 현상은 교통난을 더욱 더 심화시키고 있으며 수송효율성 향상 대책으로서 전기철도가 새롭게 부각되고 있다. 특히 전기철도는 수송력이 월등히 높고 안전성 및 신속성과 친환경적인 요소로 인해 세계 각국에서 미래 교통문제 해결의 최선의 대안으로 제시되고 있다. 우리나라 역시 경부고속전철사업이 추진중이며, 300［km/h］급의 서비스가 개시될 예정이다. 일반적으로 송.배전 계통은 3상 평형부하인 반면에 전기철도 부하는 단상 대용량 집중부하로서 이동과 정지가 빈번하고 부하변동이 극심한 전기적 특성을 가지고 있기 때문에, 우리나라 154［㎸］계통은 단락용량이 큼에도 불구하고 시스템의 안정도 평가는 필요하다 하겠다. 본 논문에서는 수식적으로 유도된 안정도 지수를 통한 시스템 안정도를 평가하고 시스템 안정도와 전력손실의 관계를 제시하였다. 또한 사례연구에서는 PSS/E를 활용하여 병렬 무효전력보상기기를 우리나라 154［㎸］계통에 적용하여 손실 감소를 통한 시스템 안정도 향상을 입증하였다.

• Geomechanics and Engineering
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• 제34권5호
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• pp.491-505
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• 2023

The practical usage of underground space and demand for vehicular tunnels necessitate the construction of non-circular wide rectangular tunnels. However, constructing large tunnels in soft clayey soil conditions with no ground improvement can lead to excessive ground deformations and collapse. In recent years, in situ ground improvement techniques such as jet grouting and deep cement mixing are often utilized to perform cement-stabilisation around the tunnel boundary to prevent large deformations and failure. This paper discusses the stability characteristics and failure behaviour of a wide rectangular tunnel in cement-treated soft clays. First, the plane strain finite element model is developed and validated with the results of centrifuge model tests available in the past literature. The critical tunnel support pressures computed from the numerical study are found to be in good agreement with those of centrifuge model tests. The influence of varying strength and thickness of improved soil surround, and cover depth are studied on the stability and failure modes of a rectangular tunnel. It is observed that the failure behaviour of the tunnel in improved soil surround depends on the ratio of the strength of improved soil surround to the strength of surrounding soil, i.e., q_ui/q_us, rather than just q_ui. For low q_ui/q_us ratios,the stability increases with the cover; however, for the high strength improved soil surrounds with q_ui >> q_us, the stability decreases with the cover. The failure chart, modified stability equation, and stability chart are also proposed as preliminary design guidelines for constructing rectangular tunnels in the improved soil surrounded by soft clays.