• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.105-105
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• 2018

스테인리스강은 내식성과 내구성이 우수하여 파이프 및 일반 구조용 고온재료에 널리 사용된다. 그러나 선박 및 해양플랜트 등의 고부가가치 산업에 사용될 경우 내피로성, 내구성 및 내식성이 더욱 요구되고 있다. 특히 해수 환경 하에서 스테인리스강은 재료 표면의 부동태 피막 파괴로 공식 또는 틈부식에 의한 국부부식을 초래하여 해양환경용 재료로 사용하는데 제한적이다. 플라즈마 이온질화는 저온에서 열처리가 가능하며 재료의 변형이 없어 스테인리스강에 널리 적용되는 열화학적 표면처리 기술이다. 플라즈마 이온질화는 일반적으로 고온에서 실시하여 스테인리스강의 기계적 특성을 향상시키는 목적으로 주로 적용하였으나, 저온-플라즈마 이온질화 처리 시 질소의 확산계수 증가로 표면에 S-phase 생성에 기인하여 부식 저항성이 향상된다고 알려져 있다[1-2]. 그러나 해수 펌프, 밸브, 스트레이너(Strainer) 등의 해양 환경용 기자재로 널리 사용되고 있는 주조 스테인리스강에 대한 플라즈마 이온질화 적용과 그 연구는 미비하다. 따라서 본 연구에서는 주조용 스테인리스강에 대하여 플라즈마 이온질화 기술을 적용하여 공정온도에 따른 해수 내 전기화학적 부식 특성을 규명하였다. 플라즈마 이온질화 공정은 $25%N_2$와 $75%H_2$ 비율로 $350^{\circ}C{\sim}500^{\circ}C$의 공정온도에서 10시간 동안 실시하였다. X-선 회절분석을 통해 공정온도 변수에 따른 표면에 형성된 질화층의 상변화를 분석하였다. 또한 비커스 경도계를 이용하여 표면경도를 측정하여 기계적 특성 향상을 확인하였다. 전기화학적 부식 실험 후 표면 손상 형상 관찰, 무게 감소량 및 손상 깊이 계측을 통해 공정 온도와 부식 저항 특성을 규명하였다. 또한 타펠 분석을 통해 모재와 플라즈마 이온질화 온도 변수에 따른 부식 속도를 비교 분석하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.2C
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• pp.181-187
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• 2012

Millimeter wave (MMW) readily penetrates fabrics, thus it can be used to detect objects concealed under clothing. A passive MMW imaging system can operate as a stand-off type sensor that scans people in both indoors and outdoors. However, because of the diffraction limit and low signal level, the imaging system often suffers from low image quality. Therefore, suitable statistical analysis and computational processing would be required for automatic analysis of the images. In this paper, a real-time concealed object detection is addressed by means of the multi-level segmentation. The histogram of the image is modeled with a Gaussian mixture distribution, and hidden object areas are segmented by a multi-level scheme involving $k$-means, the expectation-maximization algorithm, and a decision rule. The complete algorithm has been implemented in C++ environments on a standard computer for a real-time process. Experimental and simulation results confirm that the implemented system can achieve the real-time detection of concealed objects.

• Journal of the Korean Institute of Telematics and Electronics
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• v.12 no.3
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• pp.13-19
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• 1975

A detailed analysis has been carried out of a zigzag traveling wave electro-optic laser modulator usin3 CuCl, KDP Crystal. The inclusion of such practical factors as M/W and optical loss results in an optimum design in which modulator dimensions and bandwidth limitation are uniquely determined by optical and M/W dielectic properties of modulating crystal. The main conclusion of the analysis are: (1) CuCl, KDP may be used to produce 50% modulation over bandwidth of 10(GHz) with less than (watt) of modulating power. (2) The upper modulation frequency limit is set by a cutoff frequency which arises from finite width of optical beam.

• Proceedings of the Korea Water Resources Association Conference
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• 2016.05a
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• pp.174-174
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• 2016

하천은 침식작용과 퇴적작용에 의해 유사를 하류방향으로 이동시킨다. 하천의 침식작용은 하상을 변화시키고 교량, 보 등의 하천 구조물의 안전성을 저해하여 국민의 재산과 생명에 막대한 피해를 입힌다. 또한 퇴적작용은 상류로부터 이동한 유사를 하도 내에 퇴적시켜 하천의 형상을 변화시키고, 통수면적을 감소시켜 홍수기에 범람 빈도와 규모를 증가시킨다. 이처럼 부유사 자료는 하천 구조물의 설계, 수자원 개발 및 관리를 위한 하천계획의 전반에 있어 매우 중요한 자료이지만, 국내의 경우 유사량 측정방법에 대한 연구가 미비하여 대부분 유사량 채집기를 활용한 직접 측정이 이루어지고 있다. 하지만 유사량 채집기를 활용한 관측은 매우 제한된 지역에서 간헐적으로 실시되고 있어 측정 자료가 부족한 실정이다. 이러한 한계를 극복하고자 간접적인 측정 방식을 개발하였으나 우리나라의 강우 특성상 홍수기를 거치면서 하천의 수리학적 특성이 변화하여 관계식의 신뢰도가 떨어지며 자주 갱신해야 한다는 어려움이 있다. 본 연구에서는 횡방향 유속과 신호대잡음비(Signal to Noise Ratio, SNR)를 측정하는 H-ADCP(SonTek, SL-3000)와 레이저 회절을 이용하여 지점의 입도분포와 부유사 농도를 측정하는 레이저부유사측정기(LISST : Laser In-Situ Scattering and Transmissometry)를 이용하여 자료를 취득하였다. 그리고 취득된 신호대잡음비, 부유사 농도간의 관계분석을 통해 회귀식을 구축한 후, 초음파 산란도로 정의되는 신호대잡음비를 활용하여 실내 실험수로의 부유사량을 산정하였고 실측 부유사량과의 비교를 통해 오차 분석을 실시하였다. 오차 분석 결과 실측 부유사량은 138.15g/s, 추정 부유사량은 165.372g/s로 신호대잡음비를 이용하여 추정한 부유사량이 약 19% 과다산정 하는 것으로 나타났다. 본 연구의 결과는 현재 우리나라에 다수 설치되어 있는 수평초음파도플러유속계(H-ADCP)를 활용한 지속적인 부유사량 관측의 토대를 마련할 것으로 사료된다.

• Journal of the Korean GEO-environmental Society
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• v.24 no.3
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• pp.23-27
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• 2023

Graphitic carbon nitride (g-C₃N₄) has been used as effective photocatalyst for degradation of antibiotics under visible light irradiation. However, the fast recombination of hole-electron pair may limit their photocatalytic efficiency. In our study, Ag was grafted on g-C₃N₄/g-C₃N₄ isotype heterojunction by a microwave-assisted decomposition method. The structure and physical properties of heterojunction photocatalyst were characterized through X-ray diffraction, UV-DRS, FT-IR, and Photoluminescence analyses. Ag decorated g-C₃N₄/g-C₃N₄ isotype heterojunction exhibited excellent photocatalytic activity for degradation of sulfamethoxazole under irradiation under visible light irradiation within 210 min, which is higher than g-C₃N₄/g-C₃N₄ isotype heterojunction and bulk g-C₃N₄. The addition of Ag may broaden the visible light absorption and restrict the recombination of hole-electron pair because of the surface plasmons resonance, resulting in the improving the photocatalytic activity.

• Journal of the Korean Ceramic Society
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• v.39 no.9
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• pp.863-870
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• 2002

The Single phase $Pb(Fe_{1/2}Ta_{1/2})O_3$ (x=0.0∼1.0) solid solutions were successfully synthesized and their ordering structures as well as dielectric properties were investigated ${{r(Nb^{5+})=r(Ta^{5+})=0.78 {\AA},\;AW(Nb^{5+})=92.91,\;AW(Ta^{5+})=180.95}}$. While $Pb(Fe_{1/2}Ta_{1/2})O_3$ showed typical relaxor ferroelectric characteristics such as dielectric relaxation and diffuse phase transition, the sharpeness of the phase transition increased as $Ta^{5+}$ was replaced by $Nb^{5+}$ and finally $Pb(Fe_{1/2}Nb_{1/2})O_3$ showed normal ferroelectric characteristics with no dielectric relaxation. By using Raman spectroscopy, it was revealed that the $Fe^{3+}\;and\;Ta^{5+}\;of\;Pb(Fe_{1/2}Ta_{1/2})O_3$ were stoichiometrically 1:1 ordered within the short range which can be hardly probed even by TEM. Also, The degree of ordering in $Pb(Fe_{1/2}Ta_{1/2})O_3$ decreased as $Ta^{5+}$ was replaced by $Nb^{5+}$ and finally $Fe^{3+}\;and\;Nb^{5+}\;of\;Pb(Fe_{1/2}Nb_{1/2})O_3$ were completely disordered. The relaxor ferroelectric characteristics of $Pb(Fe_{1/2}Ta_{1/2})O_3$ could be correlated with the stoichiometric 1:1 ordering of B-site cations within the short range which can be hardly probed even by TEM. Also, the decrease of the relaxor ferroelectric characteristics with the replacement of $Ta^{5+}\;by\;Nb^{5+}$ could be correlated with the weakening of the ordering and the normal ferroelectric characteristics of $Pb(Fe_{1/2}Nb_{1/2})O_3$ could be correlated with the complete disordering of B-site cations.

• Journal of the Korean Electrochemical Society
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• v.15 no.2
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• pp.95-100
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• 2012

$LiFePO_4$ is an attractive cathode material due to its low cost, good cyclability and safety. But it has low ionic conductivity and working voltage impose a limitation on its application for commercial products. In order to solve these problems, the iron($Fe^{2+}$)site in $LiFePO_4$ can be substituted with other transition metal ions such as $Mn^{2+}$ in pursuance of increase the working voltage. Also, reducing the size of electrode materials to nanometer scale can improve the power density because of a larger electrode-electrolyte contact area and shorter diffusion lengths for Li ions in crystals. Therefore, we chose electrospinning as a general method to prepare $Li[Fe_{0.9}Mn_{0.1}]PO_4$ to increase the surface area. Also, there have been very a few reports on the synthesis of cathode materials by electrospinning method for Lithium ion batteries. The morphology and nanostructure of the obtained $Li[Fe_{0.9}Mn_{0.1}]PO_4$ nanofibers were characterized using scanning electron microscopy(SEM). X-ray diffraction(XRD) measurements were also carried out in order to determine the structure of $Li[Fe_{0.9}Mn_{0.1}]PO_4$ nanofibers. Electrochemical properties of $Li[Fe_{0.9}Mn_{0.1}]PO_4$ were investigated with charge/discharge measurements, electrochemical impedance spectroscopy measurements(EIS).

• Journal of the Korean Society of Marine Environment & Safety
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• v.26 no.6
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• pp.742-750
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• 2020

The flow noise generated by hull appendages is directly related to the performance of the sonar in terms of self-noise and induces a secondary noise source through interaction with the propeller and rudder. Thus, the noise in the near field should be analyzed accurately. However, the acoustic analogy method is an indirect method that is not used to simulate the propagation of an acoustic signal directly; therefore, diffraction, reflection, and scattering characteristics cannot be considered, and near-field analysis is limited. In this study, the propagation process of flow noise in water was directly simulated by using the lattice Boltzmann method. The lattice Boltzmann method could be used to analyze flow noise by simulating the collision and streaming processes of molecules, and it is suitable for noise analysis because of its compressibility, low dissipation rate, and low dispersion rate characteristics. The flow noise source was derived using Reynolds-averaged Navier-Stokes equations for the hull appendages, and the propagation process of the flow noise was directly simulated using the lattice Boltzmann method by applying the developed flow-acoustic boundary conditions. The derived results were compared with Ffowcs Williams-Hawkings results and hydrodynamic pressure results based on the receiver location to verify the usefulness of the lattice Boltzmann method within the near-field range in comparison with other techniques.

• Journal of the Korea Concrete Institute
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• v.26 no.3
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• pp.369-375
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• 2014

This study examined the changes of microstructural compositions in cement matrix according to the depth from the surface of a reinforced concrete (RC) column exposed to fire. The RC column was exposed to a standard fire for 180 minutes. After the fire test, core samples passing through the column section were obtained. Using the core samples, the remaining fractions of calcium-silicate-hydrates (C-S-H) and calcium hydroxide in cement matrix at the surface, the depth of 40 mm and 80 mm and the center (175 mm) were examined using thermal gravimetric analysis (TGA) and X-ray diffraction analysis (XRDA). Using nuclear magnetic resonance (NMR) technique, the silicate polymerization of C-S-H in cement matrix was also evaluated. The experimental results indicated that the amount of C-S-H loss at the center of column experiencing the transferred fire temperature of $236^{\circ}C$ has been underestimated as the TGA results showed the highest C-S-H contents are located at the depth of 80 mm, where the transferred fire temperature is $419^{\circ}C$. Moreover, the destruction of silicate connections at the center was observed as similar as that at the depth of 40 mm, where the transferred fire temperature was $618^{\circ}C$. This might be attributed to the temperature changes during cooling time after the fire test was neglected. Due to the relatively low thermal conductivity of concrete, the high temperature, which can affect the change of microstructure in cements, will hold longer at the center of the column than other depth.

• Journal of the Korean Applied Science and Technology
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• v.31 no.2
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• pp.231-237
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• 2014

The supercritical fluids (SCFs) have been widely used for material synthesis and processing due to their remarkable properties including low viscosity, high diffusivity and low surface tension. Carbon dioxide is one of the suitable solvents in SCFs processes in terms of its advantages such as easy processibility (with low critical temperature and pressure), inexpensive, nonflammable, nontoxic, and readily available. However, it has generally low solubility for high molecular weight polymers with the exception of fluoropolymers and siloxane polymers. Therefore, hydrocarbon solvents and hydrochlorofluorocarbons have been used for various SCFs process by its high solubility for high molecular weight polymers. In this report, a PMMA/clay nanocomposites were fabricated by using supercritical fluid process. The $Na^+$-MMT(montmorillonites)was modified by a fluorinated surfactant which is able to enhance compatibility with the chlorodifluoromethane(HCFC-22) and thus, improve dispersability of the clay in the polymer matrix. The PMMA/fluorinated surfactant modified clay nanocomposite shows enhanced mechanical and thermal properties which characterized by X-raydiffraction(XRD), Thermo gravimetric analysis(TGA), Dynamic mechanical analysis (DMA) and Transmission electron microscopy (TEM).