• Elastomers and Composites
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• 제54권2호
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• pp.110-117
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• 2019

The characterization of the ozone oxidation for raw natural rubber (NR) was investigated under controlled conditions through image and fourier transform infrared (FT-IR) analysis. The ozone oxidation was performed on a transparent thin film of raw NR coated on a KBr window in a dark chamber at $40^{\circ}C$ under low humidity conditions to completely exclude thermal, moisture, or light oxidation. The ozone concentration was set at 40 parts per hundred million (pphm). Before or after exposure to ozone, the image of the thin film for raw NR was observed at a right or tilted angle. FT-IR absorption spectra were measured in the transmission mode according to ozone exposure time. The ozone oxidation of NR was determined by the changes in the absorption peaks at 1736, 1715, 1697, and $833cm^{-1}$, which were assigned to an aldehyde group (-CHO), a ketone group (-COR), an inter-hydrogen bond between carbonyl group (-C=O) from an aldehyde or a ketone and an amide group (-CONH-) of protein, and a cis-methine group ($is-CCH_3=CH-$, respectively. During ozone exposure period, the results indicated that the formation of the carbonyl group of aldehyde or ketone was directly related to the decrement of the double bond of cis-1,4-polyisoprene. Only carbonyl compounds such as aldehydes or ketones seemed to be formed through chain scission by ozone. Long thin cracks with one orientation at regular intervals, which resulted in consecutive chain scission, were observed by image analysis. Therefore, one possible two-step mechanism for the formation of aldehyde and ketone was suggested.

• 한국재료학회지
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• 제29권2호
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• pp.116-120
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• 2019

We prepared $Y_3Al_5O_{12};Ce^{3+},Pr3^{+}$ transparent ceramic phosphor using a solid state reaction method. By XRD pattern analysis and SEM measurement, our phosphors reveal an Ia-3d(230) space group of cubic structure, and the transparent ceramic phosphor has a polycrystal state with some internal cracks and pores. In the Raman scattering measurement with an increasing temperature, lattice vibrations of the transparent ceramic phosphor decrease due to its more perfect crystal structure and symmetry. Thus, low phonon generation is possible at high temperature. Optical properties of the transparent ceramic phosphor have broader excitation spectra due to a large internal reflection. There is a wide emission band from the green to yellow region, and the red color emission between 610 nm and 640 nm is also observed. The red-yellow phosphor optical characteristics enable a high Color Rendering Index (CRI) in combination with blue emitting LED or LD. Due to its good thermal properties of low phonon generation at high temperature and a wide emission range for high CRI characteristics, the transparent ceramic phosphor is shown to be a good candidate for high power solid state white lighting.

• 대한조선학회논문집
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• 제56권4호
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• pp.335-342
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• 2019

The objective of this study is to analyze the mechanical properties of plywood used as a thermal insulating material for LNG CCS (Liquefied Natural Gas, Cargo Containment System). It is created by bonding an odd number of parallel and perpendicular direction for preventing contraction and expansion of wood. Also plywood is widely used as LNG CCS insulating material because of its durability, light weight and high stiffness. Since LNG CCS is loaded with liquid cargo, the impact load by sloshing during operation and the wide temperature range (room temperature, low temperature, cryogenic temperature) exposed during loading, unloading should be considered. The thickness of the plywood which is used for the membrane type MARKIII was selected as the thickness of the test specimen. In this present study, plywood is analyzed by the fracture behavior and mechanical properties of plywood by temperature and grain direction. In addition, it is necessary to analyze the fracture shape and predict the fracture strain by using regression model because the critical load may cause cracks inside the tank, which may affect the leakage of cryogenic liquid.

• 대한금속재료학회지
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• 제50권4호
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• pp.271-279
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• 2012

9-12% Cr steels have been used in thermal power plants which repeat start and stop operations. Major factors of fatigue life are temperature, frequency, stress ratio, holding time, microstructure, and environment. Normally, fatigue life decreases at high temperature, low frequency, high stress ratio, and long holding time conditions. A Mod.9Cr-1Mo steel, called G91, was developed at ORNL (Oak Ridge National Laboratory, USA) and was adopted as a high-temperature structural material in the ASME Code in 2004. However, its low-cycle fatigue and fatigue crack growth characteristics have been rarely studied. In this work, we have investigated the low-cycle fatigue crack growth behaviors of G91 steel under various test conditions in terms of temperature and stress ratio. As temperature and stress ratio increase, the crack growth rate becomes faster and striation distance also increases. On the other hand, the number of branch cracks decreases.

• 한국결정성장학회지
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• 제32권3호
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• pp.115-120
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• 2022

반도체 웨이퍼용 실리콘 잉곳 제조과정에서 사용되는 석영유리 도가니 내측표면의brownish ring (BR)에 대해 연구하였다. BR의 크기는 20~30 ㎛이고 비대칭 갈색 고리형태이다. 도가니 위치에 따라 BR의 크기와 분포가 상이하며, Si 잉곳 성장시 도가니 온도가 가장 높은 round 부가 가장 크고 많았다. BR은 석영유리보다 열팽창계수가 큰 cristobalite를 함유하고 있어 표면 crack이 나타나는 것으로 판단된다. BR의 발색 현상과 p in hole은 산소 결손에 의한 것으로 생각된다.

• 한국초전도ㆍ저온공학회논문지
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• 제24권3호
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• pp.47-51
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• 2022

With the innovative development of bio, pharmaceutical, and semiconductor technologies, it is essential to demand a next-generation transfer system that minimizes dust and vibrations generated during the manufacturing process. In order to develop dust-free and non-contact transfer systems, the high temperature superconductor (HTS) bulks have been applied as a magnet for levitation. However, sintered HTS bulk magnets are limited in their applications due to their relatively low critical current density (J_c) of several kA/cm² and low mechanical properties as a ceramic material. In addition, during cooling to cryogenic temperatures repeatedly, cracks and damage may occur by thermal shock. On the other hand, the bulk magnets made by stacked HTS tapes have various advantages, such as relatively high mechanical properties by alternate stacking of the metal and ceramic layer, high magnetic levitation performance by using coated conductors with high J_c of several MA/cm², consistent superconducting properties, miniaturization, light-weight, etc. In this study, we tried to fabricate HTS tapes stacked bulk magnets with 60 mm × 60 mm area and various numbers of HTS tape stacked layers for magnetic levitation. In order to examine the levitation forces of bulk magnets stacked with HTS tapes from 1 to 16 layers, specialized force measurement apparatus was made and adapted to measure the levitation force. By increasing the number of HTS tapes stacked layers, the levitation force of bulk magnet become larger. 16 HTS tapes stacked bulk magnets show promising levitation force of about 23.5 N, 6.538 kPa at 10 mm of levitated distance from NdFeB permanent magnet.

• 한국전기전자재료학회논문지
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• 제36권1호
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• pp.10-15
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• 2023

The carbonaceous materials have attracted much attention for utilization of anode materials for lithium-ion batteries. Among them, hollow carbon spheres have great advantages (high specific capacity and good rate capability) to replace currently used graphite anode materials, due to their unique features such as high surface areas, high electrical conductivities, and outstanding chemical and thermal stability. Herein, we have synthesized various sizes of hollow carbon spheres by a facile hardtemplate method and investigated the anode properties for lithium-ion batteries. The obtained hollow carbon spheres have uniform diameters of 350 ~ 600 nm by varying the template condition, and they do not have any cracks after the optimization of the process. Increasing the diameter of hollow carbon spheres decreases their specific capacities, since the larger hollow carbon spheres have more useless spaces inside that could have a disadvantage for lithium storage. The hollow carbon spheres have outstanding rate and cyclic performance, which is originated from the high surface area and high electrical properties of the hollow carbon spheres. Therefore, hollow carbon spheres with smaller diameters are expected to have higher specific capacities, and the noble channel structures through various doping approaches can give the great possibility of high lithium storage properties.

• 문화기술의 융합
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• 제9권5호
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• pp.529-534
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• 2023

고속 및 대면적 인쇄를 위한 라인 프린팅 기술은 늘어난 헤드 길이만큼 헤드 내부로 잉크를 공급하는 유로를 확보해야 하는 구조적 취약점과 제조 과정에서 발생하는 잔류응력에 의한 피드홀 변형으로 인해 노즐층이 파손되거나 잉크가 누출되는 결함이 있다. 따라서 본 논문에서는 견고하고 신뢰할 수 있으며 라인 프린팅 방식에 보다 적합한 열전사 방식의 잉크젯 프린트 헤드 형상을 제안하고자 한다. 먼저 실험을 통해 초기 라인 프린팅 헤드의 변형량을 측정한 후 이를 등가의 하중량으로 변환하였으며 FEA 해석을 통해 하중 추정 방법의 타당성을 검증하였다. 또한 헤드 크기를 증가시키지 않으면서 변형을 최소화할 수 있도록 기둥이나 지지벽으로 단위 노즐을 보강하거나 지지빔이나 건/습식각된 브릿지를 추가하여 내부 강성을 증가시킨 헤드 구조를 설계하였으며, 피드홀 변형이 최대 90% 감소하는 것을 확인하였다. 제안된 형상 중 공정 편의성과 제작비용을 고려하여 건식각된 피드홀 브릿지 형태의 헤드를 선정하였으며 실제 제작을 통해 노즐층 변형이나 잉크 누출 없이 정상 작동하는 것을 확인하였다.

• 한국강구조학회 논문집
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• 제25권5호
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• pp.569-578
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• 2013

초고층 건축물의 RC 코아벽체 선행공법에서는 일반적으로 벽체와 철골보사이의 접합을 위해 강판을 벽체에 매입설치한다. 코어벽체에 설치한 매입강판에 철골보를 접합하기 위해서는 거셋플레이트(또는 단일판)을 사용하여 매입강판에 T-형으로 용접한다. 이에 용접입열에 의해서 매입강판은 열팽창과 용접변형이 발생하고 매입강판 주변 콘크리트 온도상승을 주어 구조적 안정성 평가가 필요하다. 이에, 본 연구에서는 매입강판과 거셋플레이트 사이의 용접자세(수평 및 수직자세), 콘크리트 타설후 용접시점 및 매입강판의 연단거리에 따른 매입강판 배면온도를 계측 하였다. 또한, 비정상 온도해석을 통하여 실험결과와 비교하였다. 다음으로 매입강판에 스터드앵커 접합한 후, Push-out 실험을 통한 구조성능을 조사하였다. 전단실험 결과 매입강판 용접열영향에 대해서 용접에 따른 최대하중은 14~19% 이내로 감소하였으며, 콘크리트 타설 후 용접시점에 따른 영향으로 타설 후 재령이 길수록 최대하중은 상승함을 알 수 있었다.

• 한국세라믹학회지
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• 제52권6호
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• pp.410-416
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• 2015

Recently, a new $Y_2O_3$ coating deposited using the EB-PVD method has been developed for erosion resistant applications in fluorocarbon plasma environments. In this study, surface crack formation in the $Y_2O_3$ coating has been analyzed in terms of residual stress and elastic modulus. The coating, deposited on silicon substrate at temperatures higher than $600^{\circ}C$, showed itself to be sound, without surface cracks. When the residual stress of the coating was measured using the Stoney formula, it was found to be considerably lower than the value calculated using the elastic modulus and thermal expansion coefficient of bulk $Y_2O_3$. In addition, amorphous $SiO_2$ and crystalline $Al_2O_3$ coatings were similarly prepared and their residual stresses were compared to the calculated values. From nano-indentation measurement, the elastic modulus of the $Y_2O_3$ coating in the direction parallel to the coating surface was found to be lower than that in the normal direction. The lower modulus in the parallel direction was confirmed independently using the load-deflection curves of a micro-cantilever made of $Y_2O_3$ coating and from the average residual stress-temperature curve of the coated sample. The elastic modulus in these experiments was around 33 ~ 35 GPa, which is much lower than that of a sintered bulk sample. Thus, this low elastic modulus, which may come from the columnar feather-like structure of the coating, contributed to decreasing the average residual tensile stress. Finally, in terms of toughness and thermal cycling stability, the implications of the lowered elastic modulus are discussed.