• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.12
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• pp.1663-1668
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• 2012

A spacer grid, which is one of the structural components in a PWR fuel, is an interconnected array of slotted grid straps that are welded at the intersections to form an egg-crate structure. The spacer grid is required to have sufficient lateral crush strength to enable nuclear reactor shut-down during abnormal operating environments. Previous studies on the lateral crush strength analysis of the spacer grid were performed using only the base material properties. In this study, to investigate the effect of the lateral crush strength of the spacer grid when using the mechanical properties in the weld zone instead of the base material properties, lateral crush strength analysis by considering the mechanical properties in the weld zone as obtained from the instrumented indentation technique was performed, and the results were compared with those of previous studies.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.825-833
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• 2006

To investigate the moderator coolability for CANDU-6 reactors, a test facility (HU-KINS) has been manufactured as a 1/8 scaled-down of a calandria tank. In the design of the test facility, a scaling law was developed in such a way to consider the thermal-hydraulic characteristics of a CANDU-6 moderator. The proposed scaling law takes into consideration of the energy conservation, the dynamic similitude such as dimensionless numbers, Archimedes number (Ar) and Reynolds number (Re), and thermal-hydraulic properties similitude. Using this proposed scaling law, the thermal-hydraulic scaling analyses of similar test facilities such as the SPEL (1/10 scale) and the STERN (1/4 scale), have been identified. As a result, in the case of the SPEL, while the energy conservation is well defined, the similarities of Ar and the heat density are not well considered. As for the similarity of the STERN, while both the energy conservation and the characteristics of Ar are well defined, the heat density is not. In the meanwhile, the HU-KINS test facility with 1/8 length scaled-down is well similitude in compliance with all similarities of the energy conservation, the fluid dynamics and thermal-hydraulic properties. To verify the adequacy of the similarities in terms of thermal-hydraulics, a computational fluid dynamic (CFD) analysis has been conducted using the CFX-5 code. As the results of the CFD analyses, the predicted flow patterns and variation of axial properties inside the calandria tank are well consistant with those of previous studies performed with FLUENT and this implies that the present scaling method is acceptable.

• Membrane Journal
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• v.19 no.2
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• pp.89-95
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• 2009

A comb-like copolymer consisting of a poly(vinyl chloride) backbone and poly(hydroxy ethyl acrylate) side chains, i.e. PVC-g-PHEA, was synthesized through atom transfer radical polymerization (ATRP). This comb-like copolymer was crosslinked with 4,5-imidazole dicarboxylic acid (IDA) via the esterification of the -OH groups of PHEA in the graft copolymer and the -COOH groups of IDA. Upon doping with phosphoric acid (PA, $H_3PO_4$) to form imidazole-PA complexes, the proton conductivity of the membranes continuously increased with increasing PA content. A maximum proton conductivity of 0.011 S/cm was achieved at $100^{\circ}C$ under anhydrous conditions. The PVC-g-PHEA/IDA/PA complex membranes exhibited good mechanical properties, i.e. 575 MPa of Young's modulus, as determined by a universal testing machine (UTM). Thermal gravimetric analysis (TGA) shows that the membranes were thermally stable up to $200^{\circ}C$.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.327-327
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• 2012

사파이어는 우수한 광학적, 물리적, 화학적 특성을 가지고 있는 물질 중의 하나이며, 청색 발광특성을 나타내는 GaN와 격자상수, 열팽창 계수가 가장 유사할 뿐만 아니라 가격도 상대적으로 저렴하기 때문에 GaN 성장을 위한 기판으로 사용될 수 있다. 실제로 사파이어는 프로젝터와 전자파 장치, 군사용 장비 등 다양한 분야에 응용되고 있으며, 발광 다이오드(LED)를 위한 기판으로 활용됨으로써 그 수요가 급격히 증가하고 있다. 그러나 사파이어 결정의 성장 중에 생길 수 있는 전위(dislocation)와 적층결함(stacking fault) 등의 결정 결함들은 결정 내에 존재하여 역학적, 전기적 성질에 큰 영향을 미칠 수 있다. 특히 사파이어가 청색 발광소자의 기판으로 사용되는 경우, 사파이어 기판 내부의 결정 결함은 증착되는 박막 특성에 영향을 미치게 된다. 따라서 사파이어의 보다 나은 응용을 위해서는 결정 결함에 대한 평가기술과 결함의 형성 메커니즘 등에 대한 이해가 필요하다. 특히, 결함의 정량적 평가 기술의 개발은 사파이어의 상용화에 중요한 핵심요소 중 하나이다. 결정을 산이나 염기 등을 이용하여 화학적 식각을 하게 되면 분자나 원자 간의 결합이 약한 부분이나 높은 에너지 상태에 있는 부분부터 반응을 하게 되는데, 이러한 반응을 통해 결정의 표면에 형성되는 것을 에치 피트(etch pit)라고 한다. 일반적으로 결정 내에 존재하는 전위는 높은 에너지 상태이므로, 이러한 에치 피트는 전위와 관련되어 있다. 따라서 사파이어 결정과 같은 결정질 물질은 표면의 식각을 통하여 관찰되는 에치 피트 등의 형상이나 반응성 등을 평가하여 결정 특성을 연구할 수 있다. 본 연구는 화학적 식각법으로 사파이어 결정의 특성을 평가하기 위하여 진행하였다. 사파이어 결정의 식각을 위하여 다양한 산-염기 용액들이 사용되었다. 식각 용액의 종류에 따른 사파이어 결정의 식각거동을 연구하고, 표면에 나타나는 형상을 연구하여 사파이어 결정의 구조적 특성을 파악하였다. 특히, 에치 피트 형성거동의 시간 및 온도 의존성에 관한 연구를 진행하였다.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.80.2-80.2
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• 2012

그라핀(graphene)은 탄소 원자의 2차원 육각형 $sp^2$ 결합체로서 탄소 나노구조체가 가지는 여러 가지 우수한 특성을 보유하면서 대면적 기판 위에서 소자구현 및 투명전극 등으로의 우수한 응용성 때문에 고품질 그라핀 제조와 물리적 특성, 소자응용에 관한 연구가 활발히 진행되고 있다. 최근 그라핀 제조를 위한 여러 가지 방법이 개발되고 있으나 화학적 박리법이 저비용으로 대량생산을 위해 가장 유리한 방법으로 주목을 받고 있다. 화학적 박리법은 벌크 그라파이트를 강한 산을 이용하여 산화시켜 형성된 산화 그라파이트(graphite oxide)을 열적으로 팽창시켜 박리하고 환원하여 그라핀으로 제조하는 것이다. 보통 열적팽창을 위해서 열처리 로를 사용하게 되는데, 본 연구에서는 박리를 보다 효율적으로 진행시키고 고품질의 그라핀을 얻기 위해 마이크로웨이브를 이용한 박리법을 적용하였다. 마이크로웨이브는 설비가 간단하고 매우 균일하게 열팽창을 시킬 수 있을 뿐만 아니라 대량생산에서도 유리할 것으로 기대하였다. 천연 그라파이트(99.9%, 평균입도 $200{\mu}m$)를 Hummer 방법에 따라 $H_2SO_4$와 $KMnO_4$를 사용하여 산화시키고 필터링 후 마이크로웨이브를 조사하였다. 이후 환원 처리를 거쳐 그라핀을 제조하였다. 라만스펙트럼 및 투과전자현미경으로 분석한 결과 우수한 품질의 그라핀이 형성되었음을 알 수 있었다. 그라핀의 두께 및 품질은 마이크로웨이브의 인가시간 및 반복 횟수가 증가함에 따라 크게 영향받는 것을 확인하였다. 본 발표에서는 마이크로웨이브를 사용한 산화 그라파이트 박리 및 그라핀 제조라는 새로운 시도와 주요변수에 따른 그라핀 특성에 관한 결과를 논의할 것이다.

• Journal of the Korean Electrochemical Society
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• v.13 no.3
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• pp.186-192
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• 2010

In situ electrochemical atomic force microscopy (ECAFM) observations of the surface of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in lithium bis(fluorosulfonyl)imide (LiTFSI) dissolved in 1-buthyl-2,3-dimethylimidazolium (BDMI)-TFSI to understand the interfacial reactions between graphite and BDMI-based ionic liquids. The formation of blisters and the exfoliation of graphene layers by the intercalation of $BDMI^+$ cations within HOPG were observed instead of reversible lithium intercalation and de-intercalation. On the other hand, lithium ions are reversibly intercalated into the HOPG and de-intercalatied from the HOPG without intercalation of the $BDMI^+$ cations in the presence of 15 wt% of 4.90 mol/$kg^{-1}$ LiTFSI dissolved in propylene carbonate (PC). ECAFM results revealed that the concentrated PC-based solution is a very effective additive for preventing $BDMI^+$ intercalation through the formation of solid electrolyte interface (SEI).

• Nuclear Engineering and Technology
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• v.27 no.1
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• pp.45-52
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• 1995

In influence of the vapor phase turbulent stress (i.e., the too-phase Reynolds stress) to the characteristics of two-phase system in a horizontal pipe has been theoretically investigated. The average two-fluid model has been constituted with closure relations for stratified flow in a horizontal pipe. A vapor phase turbulent stress model for the regular interface geometry has been included. It is found that the second order waves propagate in opposite direction with almost the same speed in the moving frame of reference of the liquid phase velocity. Using the well-posedness limit of the two-phase system, the dispersed-stratified How regime boundary has been modeled. Two-phase Froude number has been found to be a convenient parameter in quantifying the onset of slugging as a function of the global void fraction. The influence of the taper phase turbulent stress was found to stabilize the flow stratification.

• Journal of the Korean Chemical Society
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• v.46 no.2
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• pp.169-176
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• 2002

In this study, the chemistry achievement of the Korean 8th grade students in the Third Inter-national Mathematics and Science Study-Repeat(TIMSS-R), which was administered internationally in 1999,was analyzed. By the 6th national curriculum, 31 chemistry problems were selected from the categories of 'classification of matter', 'structure of matter', 'chemical reactivity and transformations', 'physical properties and physical changes of matter', 'heat and temperature', 'subatomic particles', 'pollution', 'scientific method',and 'scientific measurements' among the total 143 problems. Based on the analysis of the percentage of cor-rectness on each problem, the weak and strong points of the 6th curriculum were discussed. The implications for the development of national curriculum and textbook are suggested.

• Polymer(Korea)
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• v.38 no.6
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• pp.791-800
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• 2014

4-Chlorobenzoyl (CB) group-attached multi-walled carbon nanotube (c-MWNT) was prepared via a direct Friedel-Crafts acylation of MWNT with 4-chlorobenzoic acid (CBA) in a $P_2O_5$/poly(phosphoric acid) medium. c-MWNT with a maximum chlorine content of 5.3 wt% (CB group content of 20.9 wt%) was obtained by controlling the amount of CBA during the reaction. Using a self-condensation polymerization of 4-chlorobenzenethiol (CBT) to poly(phenylene sulfide) (PPS), MWNT-g-PPS was prepared by adding c-MWNT of chlorine content of 5.3 wt% during the self-polymerization of CBT and removing homo PPS after polymerization in order to increase the interfacial interaction between PPS and MWNT. Thermal and surface properties of the MWNT-g-PPS were characterized. The results showed that PPS was formed on the surface of c-MWNT by the condensation of c-MWNT and CBT.

• Korean Journal of Materials Research
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• v.27 no.6
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• pp.295-300
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• 2017

Aluminum-oxide($Al_2O_3$) thin films were deposited by electron cyclotron resonance plasma-enhanced atomic layer deposition at room temperature using trimethylaluminum(TMA) as the Al source and $O_2$ plasma as the oxidant. In order to compare our results with those obtained using the conventional thermal ALD method, $Al_2O_3$ films were also deposited with TMA and $H_2O$ as reactants at $280^{\circ}C$. The chemical composition and microstructure of the as-deposited $Al_2O_3$ films were characterized by X-ray diffraction(XRD), X-ray photo-electric spectroscopy(XPS), atomic force microscopy(AFM) and transmission electron microscopy(TEM). Optical properties of the $Al_2O_3$ films were characterized using UV-vis and ellipsometry measurements. Electrical properties were characterized by capacitance-frequency and current-voltage measurements. Using the ECR method, a growth rate of 0.18 nm/cycle was achieved, which is much higher than the growth rate of 0.14 nm/cycle obtained using thermal ALD. Excellent dielectric and insulating properties were demonstrated for both $Al_2O_3$ films.