• 비파괴검사학회지
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• 제21권3호
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• pp.269-276
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• 2001

복합재료의 초음파검사는 매직의 탄성이방성으로 인해 빔스큐(beam skew)현상 등이 발생하기 때문에 등방성 재료의 검사에 비해 훨씬 어렵다. 그 동안의 많은 연구의 결과로서 이방성 재료 네 탄성파의 전파 현상에 대한 해석해법이 정립되어 있으나, 그 해석해가 매우 복잡하여 실제 문제의 해를 구하기 위해서는 수치적인 방법에 의존하여야 하는 경우가 많다. 본 연구에서는 횡등방 이방성(transversely isotropic)으로 가정할 수 있는 단일방향 섬유 복합재료 내 점원에 의한 초음파 전파의 해석해를 먼저 구하고, 이러한 복합재료를 질량-스프링 모델로 모델링하여 해석해에 상응하는 수치해를 구하였다. 그 결과, 초음파의 전파에 대한 수치해와 해석해가 매우 잘 일치하는 것을 확인하였다. 동일한 수치해석법을 사용하여 초음파가 복합재료의 자유표면에서 반사되는 현상과 균일에 의해 산란되는 현상에 대한 수치해도 구하였으며 그 결과를 파동역학의 관점에서 고찰하였다. 본 연구에서 그 유효성이 확인된 수치 모델은 복합재료의 초음파검사를 시뮬레이션 함에 있어 매우 유용하게 사용될 수 있을 것이다.

• 지구물리와물리탐사
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• 제7권1호
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• pp.80-87
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• 2004

During excavation work for the construction of a highway bridge in a limestone area in Korea, several cavities were found, and construction work was stopped temporarily. Cavities under the bridge piers might seriously threaten the safety of the planned bridge, because they could lead to excessive subsidence and differential settlement of the pier foundations. In order to establish a method for reinforcement of the pier foundations, borehole radar reflection and tomography surveys were carried out, to locate cavities under the planned pier locations and to determine their sizes where they exist. Since travel time data from the crosshole radar survey showed anisotropy, we applied an anisotropic tomography inversion algorithm assuming heterogeneous elliptic anisotropy, in order to reconstruct three kinds of tomograms: tomograms of maximum and minimum velocities, and of the direction of the symmetry axis. The distribution of maximum velocity matched core logging results better than that of the minimum velocity. The degree of anisotropy, defined by the normalized difference between maximum and minimum velocities, was helpful in deciding whether an anomalous zone in a tomogram was a cavity or not. By careful examination of borehole radar reflection and tomography images, the spatial distributions of cavities were delineated, and most of them were interpreted as being filled with clay and/or water. All the interpretation results implied that two faults imaged clearly by a DC resistivity survey were among the most important factors controlling the groundwater movement in the survey area, and therefore were closely related to the development of cavities. The method of reinforcement of the pier foundations was based on the interpretation results, and the results were confirmed when construction work was resumed.

• 한국전기전자재료학회논문지
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• 제22권7호
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• pp.616-621
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• 2009

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating(ARC) and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si ARC layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated with SEM. The formation of a nanoporous Si layer about 100nm thick on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.

• 정보저장시스템학회논문집
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• 제1권2호
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• pp.182-187
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• 2005

A large area micro mirror is an optical element that functions as changing an optical path by reflection in integrated optical system. We fabricated the large area silicon mirror by anisotropic etching using MEMS for implementation of integrated optical pickup. In this work, we report the optimum conditions to better fabricate and design, greatly improve mirror surface quality. To obtain mirror surface of $45^{\circ},\;9.74^{\circ}$ off-axis silicon wafer from (100) plane was used in etching condition of $80^{\circ}C$ with 40wt.% KOH solution. After wet etching, polishing process by MR fluid was applied to mirror surface for reduction of roughness. In the next step, after polymer coating on the polished Si wafer, the Si mirror was fabricated by UV curing using a trapezoid bar-type way structure. Finally, we obtained peak to valley roughness about 50 nm in large area of $mm^2$ and it is applicable to optical pickup using blu-ray wavelength as well as infrared wavelength.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2007년도 추계학술대회 논문집
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• pp.70-71
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• 2007

Multicrystalline silicon(mc-Si) solar cells are steadily increasing their share of the PV market due to the lower material costs. However, commercial mc-Si solar cells have lower efficiency than singlecrystalline silicon solar cells. To improve efficiency of mc-Si solar cells, it is important to reduce optical losses from front surface reflection. Isotropic etching with acid solution based on hydrofluoric acid(HF) and nitric acid$(HNO_3)$ is one of the promising methods that can reduce surface reflectance for mc-Si solar cells. Anisotropic etching is not suitable for mc-Si because of its various grain orientations. In this paper, we isotropically etched mc-Si using acid solution. After that, etched surface was observed by Scanning Electron Microscope(SEM) and surface reflectance was measured. We obtained 29.29% surface reflectance by isotropic etching with acid solution in wavelength from 400nm to 1000nm for fabrication of mc-Si solar cells.

• 한국진공학회지
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• 제6권3호
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• pp.220-226
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• 1997

실리콘 미세가공기술로 형성된 프레임 모양의 실리콘 기판에 의해 지지되는 -$Si_3N_4/300 nm-SiO_2/150 nm-Si_3N_4$ 광반사막을 제조하였으며, 이것을 광섬유와 결합하여 강도형 다모드 광섬유 압력센서를 제작하고 그 특성을 조사하였다. $Si_3N_4/SiO_2/Si_3N_4$다아아 프램을 광반사막으로 사용하기 위하여 이 다이아프램의 뒷면에 NiCr 및 Au 박막을 각각 진 공증착하여 광반사막에서의 광투과에 의한 광손실을 수%로 감소시킬 수 있었다. 유전체 다 이아프램의 상하에 각각 있는 $Si_3N_4$막은 KOH 수용액에 의한 실리콘 이방성 식각시 자동식 각 정지층 역할을 하여 다이아프램 두께의 재현성이 우수하였다. 다이아프램의 크기가 3$\times$ 3$\textrm{mm}^2$, 4$\times$4$\textrm{mm}^2$ 및 5$\times$5$\textrm{mm}^2$인 센서는 각각 0~126.64kPa, 0~79.98kPa 및 0~46.66kPa의 압력범위에서 선형적인 광출력-압력 특성을 나타내었으며, 이들 센서의 압력감도는 각각 약 20.69nW/kPa, 26.70nW/kPa 및 39.33nW/kPa로서, 다이아프램의 크기가 증가할수록 압력감 도도 증가하였다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 춘계학술회의 초록집
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• pp.16-16
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• 2010

GaN-based light-emitting diodes (LEDs) are attracting great interest as candidates for next-generation solid-state lighting, because of their long lifetime, small size, high efficacy, and low energy consumption. However, for general illumination applications, the external quantum efficiency of LEDs, determined by the internal quantum efficiency (IQE) and the light extraction efficiency, must be further increased. The IQE is determined by crystal quality and epitaxial layer structure and high value of IQE more than 70% for blue LEDs have been already reported. However, there is much room for improvement of light extraction efficiency because most of the generated photons from active layer remain inside LEDs by total internal reflection at the interface of semiconductor with air due to the high refractive index difference between LEDs epilayer (for GaN, n=2.5) and air (n=1). The light confining in LEDs will be reabsorbed by the metal electrode or active layer, reducing the efficacy of LEDs. Here, we present the first demonstration of enhanced light extraction by forming a MgO nano-pyramids structure on the surface of vertical-LEDs. The MgO nano-pyramids structure was successfully fabricated at room temperature using conventional electron-beam evaporation without any additional process. The nano-sized pyramids of MgO are formed on the surface during growth due to anisotropic characteristics between (111) and (200) plane of MgO. The ZnO layer with quarter-wavelength in thickness is inserted between GaN and MgO layers to increase the critical angle for total internal reflection, because the refractive index of ZnO (n=1.94) could be matched between GaN (n=2.5) and MgO (n=1.73). The MgO nano-pyramids structure and ZnO refractive-index modulation layer enhanced the light extraction efficiency ofV-LEDs with by 49%, comparing with the V-LEDs with a flat n-GaN surface. The angular-dependent emission intensity shows the enhanced light extraction through the side walls of V-LEDs as well as through the top surface of the n-GaN, because of the increase in critical angle for total internal reflection as well as light scattering at the MgO nano-pyramids surface.

• 방송공학회논문지
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• 제27권2호
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• pp.228-239
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• 2022

본 논문에서는 카셰어링 서비스(car sharing service)에서 차량 상태 무인 검수를 위한 흠집 탐지 딥 러닝 모델을 제안한다. 기존의 차량 상태 검수 시스템은 대여 전, 후 사진에서 각각 흠집을 탐지하는 딥 러닝 모델과 탐지된 두 흠집 영상을 수작업으로 대조하여 새롭게 발생한 흠집을 탐색하는 두 단계로 구성되어 있다. 따라서 수동작업이 필요한 두 단계 모델을 한 단계로 줄이는 무인 흠집 탐지 모델을 위성영상에서 변화를 탐지하는 딥 러닝 모델에 전이 학습을 적용하여 구축한다. 그리고 광택 처리된 자동차 표면의 휘도가 비등방성이고 비전문가인 이용자가 일반 카메라로 촬영하기 때문에 정반사(specular reflection)가 흠집 탐지 성능에 크게 영향을 미친다. 따라서 정반사광으로 발생하는 오탐지를 감소시키기 위하여 정반사광 성분을 제거하는 전처리 과정을 적용한다. 이용자가 휴대폰 카메라로 촬영한 데이터에 대해 제안하는 시스템은 주관적인 측면과 정밀도(precision), 재현율(recall), F1, Kappa 척도면에서 각각 67.90%, 74.56%, 71.08%, 70.18%로서 높은 일치도를 보인다.

• 폴리머
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• 제26권4호
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• pp.523-534
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• 2002

약 60-$130^{\circ}C$의 온도범위에서 cholesteric 반사색깔을 나타내는 새로운 히드록시프로필셀룰로오스 (HPC)와 에스테르화도 (DE)가 1-3 범위의 값을 갖는 HPC의 아크릴산 에스테르들 (ESs)을 제조하였다. 또한 DE가 2 이상인 ESs가 $50^{\circ}C$에서 나타내는 열방성 cholesteric 상에 UV광을 조사시킴에 의해 광학 pitch ($\lambda_m$)가 전 가시광 파장영역에 존재하는 ES 가교필름들을 제조하였다. 미가교와 가교시료들에 대한 열 및 광학 특성과 아세톤 중에서의 가교필름의 팽윤거동을 검토 하였다. HPC 자체와 동일하게 ESs의 $\lambda_m$은 온도가 증가함에 따라 증가하였다. 그러나 동일한 온도에서 나타내는 $\lambda_m$은 ES가 HPC에 비해 크며 ES의 $\lambda_m$은 DE가 증가함에 따라 감소하였다. 가교시료들의 $\lambda_m$의 온도의존성은 ESs에 비해 대단히 약하였다. 또한, ESs는 DE가 증가함에 따라 낮은 액정상에서 등방상의 액체로의 전이온도를 나타내는 것과는 판이하게, 가교물질들은 액체상으로의 전이를 일으키지 않으며 약 $210^{\circ}C$에서 분해가 일어나는 것으로 나타났다. 가교필름들은 이방성 팽윤을 나타내었으며, 이러한 사실은 ES분자들간에 2차원적인 가교가 우선적으로 일어남을 시사한다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.454-454
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• 2014

Silicon microwire array is one of the promising platforms as a means for developing highly efficient solar cells thanks to the enhanced light trapping efficiency. Among the various fabrication methods of microstructures, deep reactive ion etching (DRIE) process has been extensively used in fabrication of high aspect ratio microwire arrays. In this presentation, we show precisely controlled Si microwire arrays by tuning the DRIE process conditions. A periodic microdisk arrays were patterned on 4-inch Si wafer (p-type, $1{\sim}10{\Omega}cm$) using photolithography. After developing the pattern, 150-nm-thick Al was deposited and lifted-off to leave Al microdisk arrays on the starting Si wafer. Periodic Al microdisk arrays (diameter of $2{\mu}m$ and periodic distance of $2{\mu}m$) were used as an etch mask. A DRIE process (Tegal 200) is used for anisotropic deep silicon etching at room temperature. During the process, $SF_6$ and $C_4F_8$ gases were used for the etching and surface passivation, respectively. The length and shape of microwire arrays were controlled by etching time and $SF_6/C_4F_8$ ratio. By adjusting $SF_6/C_4F_8$ gas ratio, the shape of Si microwire can be controlled, resulting in the formation of tapered or vertical microwires. After DRIE process, the residual polymer and etching damage on the surface of the microwires were removed using piranha solution ($H_2SO_4:H_2O_2=4:1$) followed by thermal oxidation ($900^{\circ}C$, 40 min). The oxide layer formed through the thermal oxidation was etched by diluted hydrofluoric acid (1 wt% HF). The surface morphology of a Si microwire arrays was characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Optical reflection measurements were performed over 300~1100 nm wavelengths using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent) in which a 60 mm integrating sphere (Labsphere) is equipped to account for total light (diffuse and specular) reflected from the samples. The total reflection by the microwire arrays sample was reduced from 20 % to 10 % of the incident light over the visible region when the length of the microwire was increased from $10{\mu}m$ to $30{\mu}m$.