• 한국정밀공학회지
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• 제31권3호
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• pp.189-197
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• 2014

Diffractive optical elements (DOEs) - in general a complex pattern of micro- and nano-scale structures - can modulate and transform light in a predetermined way. Their importance is being increased nowadays because they can be designed to handle a number of simultaneous tasks. In view point of machining DOEs, it is a big challenge to fabricate micro- and nano-scale structures on a free-form surfaces. In this paper, the state-of-the-art of the ultra-precision machine tools is reviewed. Also some technical issues which determine the machine tool accuracy are discussed.

• 한국의상디자인학회지
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• 제19권1호
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• pp.55-70
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• 2017

This study researched transition process of fur costume from ancient times to 20th century, extracted esthetic characteristics, and then understood how the esthetic characteristics appear in the recent fur collection. The result is as follows. First, magical meaning was revealed on leopard fur that ancient chief priests wore on top of shenti to symbolize immorality and fur tunics to which christian symbol was added in Byzantine. During middle age and the Renaissance, extravagance appeared on sable and amin jackets that the noble decorated to show off their high status. During the rococo age and mid-20th century, sensuality and elegance appeared on the fur dresses that the female body was emphasized. Second, in recent collection, extravagance appeared on over-sized and voluminous fox-coat, sexsuality and elegance were revealed through mink-dresses and jackets with soft touch and gloss that curved-silhouette was emphasized. In addition, practicality and functionality were shown through short length and light, soft material, and activeness was in the mink jacket to which functional elements such as zippers and rubber were added. To pursue uniqueness and fun, fox jackets were made with over-sized vivid green color. Besides, abnormality and vulgarness appeared through the fashion with abnormal and distracted images with ambiguous forms and new processing methods applied.

• 한국세라믹학회지
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• 제45권1호
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• pp.36-42
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• 2008

Carbon/$TiO_2$ composite photocatalysts were thermally synthesized with different mixing ratios of anatase to phenol resin through an ethanol solvent dissolving method. The XRD patterns revealed that only anatase phase can be identified for Carbon/$TiO_2$ composites. The diffraction peaks of carbon were not observed, however, due to the low carbon content on the $TiO_2$ surfaces and the low crystallinity of amorphous carbon. The results of chemical elemental analyses of the Carbon/$TiO_2$ composites showed that most of the spectra for these samples gave stronger peaks for carbon and Ti metal than that of any other elements. The BET surface area increases to the maximum value of $488\;m^2/g$ with the area depending on the amount of phenol resin. From the SEM images, small $TiO_2$ particles were homogeneously distributed to a composite cluster with the porosity of phenol resin-based carbon. From the photocatalytic results, the MB degradation should be attributed to the three kinds of synergetic effects, such as photocatalysis, adsorptivity, and electron transfer by light absorption between supporter $TiO_2$ and carbon.

• 한국세라믹학회지
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• 제45권4호
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• pp.196-203
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• 2008

Metal-carbon/$TiO_2$ composite photocatalysts were thermally synthesized through the mixing of anatase to metal(Cu, Zn) containing phenol resin in an ethanol solvent coagulation method. The BET surface area increases, with the increase depending on the amount of metal salt used. From SEM images, metal components and carbon derived from phenol resin that contains metal was homogeneously distributed to composite particles with porosity. XRD patterns revealed that metal and titanium dioxide phase can be identified for metal-carbon/$TiO_2$ composites, however, the diffraction peaks of carbon were not observed due to the low carbon content on the $TiO_2$ surfaces and due to the low crystallinity of the amorphous carbon. The results of a chemical elemental analysis of the metal-carbon/$TiO_2$ composites showed that most of the spectra for these samples gave stronger peaks for C, O, treated metal components and Ti metal compared to that of any other elements. According to photocatalytic results, the MB degradation can be attributed to the three types of synergetic effect: photocatalysis, adsorptivity and electron transfer, according to the light absorption between the supporter $TiO_2$, metal species, and carbon layers.

• Journal of Power Electronics
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• 제10권2호
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• pp.115-124
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• 2010

In this paper, a simple yet efficient auto mode-hop ripple control structure for buck converters with light load operation enhancement is proposed. The converter, which operates under a wide range of input and output voltages, makes use of a state-dependent hysteretic comparator. Depending on the output current, the converter automatically changes the operating mode. This improves the efficiency and reduces the output voltage ripple for a wide range of output currents for given input and output voltages. The sensitivity of the output voltage to the circuit elements is less than 14%, which is seven times lower than that for conventional converters. To assess the efficiency of the proposed converter, it is designed and implemented with commercially available components. The converter provides an output voltage in the range of 0.9V to 31V for load currents of up to 3A when the input voltage is in the range of 5V to 32V. Analytical design expressions which model the operation of the converter are also presented. This circuit can be implemented easily in a single chip with an external inductor and capacitor for both fixed and variable output voltage applications.

• Journal of Electrical Engineering and Technology
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• 제13권3호
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• pp.1265-1274
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• 2018

This paper proposes a virtual flux (VF) and positive-sequence power based control strategy to improve the performance of grid-interfaced three-phase voltage source converters against unbalanced and distorted grid conditions. By using a second-order generalized integrator (SOGI) based VF observer, the proposed strategy achieves an AC voltage sensorless and grid frequency adaptive control. Aiming to realize a balanced sinusoidal line current operation, the fundamental positive-sequence component based instantaneous power is utilized as the control variable. Moreover, the fundamental negative-sequence VF feedforward and the harmonic attenuation ability of a sequence component generator are employed to further enhance the unbalance regulation ability and the harmonic tolerance of line currents, respectively. Finally, the proposed scheme is completed by combining the foregoing two elements with a predictive direct power control (PDPC). In order to verify the feasibility and validity of the proposed SOGI-VFPDPC, the scenarios of unbalanced voltage dip, higher harmonic distortion and grid frequency deviation are investigated in simulation and experimental studies. The corresponding results demonstrate that the proposed strategy ensures a balanced sinusoidal line current operation with excellent steady-state and transient behaviors under general grid conditions.

• 한국산학기술학회논문지
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• 제14권3호
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• pp.1027-1032
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• 2013

LED는 저탄소 그린에너지 시대의 등기구로서 각광을 받고 있다. 다른 조명용 광원에 비해 친환경적이고 높은 에너지 효율을 가지고 있고 수명이 길다는 장점을 가지고 있지만, 공급전력 중 80%이상이 열에너지로 전환되며, 이에 따른 온도상승이 불가피 하여 높은 온도가 단점으로 꼽히고 있다. 온도상승은 LED소자의 수명에 영향을 미치기 때문에 방열시스템이 중요하게 자리 잡고 있다. 따라서 본 논문에서는 방열성능 향상을 위하여 LED 전구의 heat sink의 형상에 대한 열해석을 통하여 방열 시스템의 효용성을 분석하였다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.53-53
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• 2010

Car industry has required light-weight steels, but still with strong mechanical strength. To meet this requirement, a variety of researches on Fe-Al alloys have been performed. As Al is being added in a disordered manner, alloys become more ductile and show higher yield stress. At a certain concentration of Al, however, the Fe-Al alloy system falls in a second phase whose mechanical strength is worsened. To understand the microscopic role of Al, we investigate the stability and the elastic properties of various Fe-Al alloys using ab initio density functional theory. At agiven Al concentration, the equilibrium geometry is obtained among several disordered Fe-Al alloy structures by performing the geometry relaxation. The formation energies and elastic properties such as bulk moduli of the equilibrium structures are also computed as a function of Al concentration. We also investigate the effects of different elements such as Si and Mn. Fe-Si alloy systems exhibit unusual mechanical behaviors requiring further investigation to understand their physical origin. Especially, the microscopic role of Mn is investigated to find its physical origin of preventing the Fe-Al alloy system from forming an unfavorable second phase. The effect of manganese on mechanical properties of Fe-based alloys is also explored.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제45회 하계 정기학술대회 초록집
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• pp.207.2-207.2
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• 2013

Oxide-based thin film transistors have been attempted as powerful candidates for driving circuits for active-matrix organic light-emitting diodes and transparent electronics. The oxide TFTs are based on the amorphous multi-component oxides involving zinc, indium, and/or tin elements as main cation sources. The current work employed RF sputtering in order to deposit zinc-tin oxide thin films applicable to transparent oxide thin film transistors. The deposited thin film was characterized and probed in terms of materials and devices. The physical/chemical characterizations were performed using X-ray diffraction, Atomic Force Microscopy, Spectroscopic Ellipsometry, and X-ray Photoelectron Spectroscopy. The thin film transistors were fabricated using a bottom-gated structure where thermally-grown silicon oxide layers were applied as gate-dielectric materials. The inherent properties of oxide thin films are combined with the corresponding device performances with the aim to fabricating the multi-component oxide thin films being optimized towards transparent electronics.

• 소성∙가공
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• 제12권5호
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• pp.493-497
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• 2003

The present investigation aims at visualizing the crack tip stress field using a mechanoluminescence material. The well known compound $SrAl_2O_4$:$Eu^{2+}$ was adopted as a mechanolurninescence material. Two more trivalent rare-earth elements such as Dy and Nd were taken into consideration as codopants to provide the appropriate trap levels. Samples of a variety of compositions were prepared by varing $Eu^{2+}$, $Dy^{3+}$, and $Nd^{3+}$ doping contents, for which the combinatorial chemistry method was used. In order to search for the optimum composition for the highest mechanoluminescence, the luminescence induced by a compressive device including a CCD camera. In parallel, a compact tension specimen was prepared by mixing the luminescence powders of optimum composition and epoxy resin. Crack initiation from the mechanically machined sharp note tip and its growth during loading were found to be associated with the extent of light emission from $SrAl_2O_4$.