• 한국신재생에너지학회:학술대회논문집
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• 2005.06a
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• pp.281-283
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• 2005

Water management in polymer electrolyte membrane fuel cells(PEMFCs) is one of the most challenging issues. Freeze start-up in the automotive applications is also important research topic in the PEMFC field. Transportation of proton and separation of reactant gases are main roles of polymer electrolyte membranes. It has been known that water in the membrane conducts as a vehicle for the proton transportation. At sub-zero temperature, the frozen water blocks the access of reactant gases to the active sites of electrode as well as occurs the physical destruction of fuel cell structures. In this study, property changes of electrolyte membranes in the freeze conditions $(at\;-25^{\circ}C)$ were investigated. For the various amount of water contained membranes, the property changes, especially for the proton conductivity, were observed after several times of freeze/thaw$(-25\~80^{\circ}C)$ cycle.

• Journal of Welding and Joining
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• v.16 no.6
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• pp.44-51
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• 1998

It is well known that the fine bainitic microstructure obtained by TMCP(thermo-mechanical control process) secures the high toughness of base metal. Besides, TMCP steel is very suitable for high heat input in welding as it has low carbon equivalent. In HAZ, however, the accelerated cooling effect imparted on the matrix by the weld thermal cycles is relieved and thus the weldment of TMCP steel has softening zone which shows low fracture toughness compared with base metal. Therefore, PHWT of weldment is carried out to improve the fracture toughness in weldment of TMCP steel which has softening zone. In this study, the effects of PWHT on the weldment of AH36-TMCP steel are investigated by the small punch (SP) test. From the several results such as SP energy and displacement at room temperature, the behavior of transition curves, the fracture strength at -196$^{\circ}C$, distribution of (DBTT)sp and (DBTT)sp, the PWHT condition of A.C. after 85$0^{\circ}C$-1 sec W.C. was suitable condition for recovering a softening zone of HAZ as welded.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.1057-1060
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• 2004

In this study, the wear characteristics of five different dental composite resins cured by conventional halogen light and LED light sources were investigated. Five different dental composite resins of Surefil, Z100, Dyract AP, Fuji II LC and Compoglass were worn against a zirconia ceramic ball using a pin-on-disk type wear tester with 15 N contact force in a reciprocal sliding motion with sliding distance of 10 mm/cycle at 1Hz under the room temperature dry condition. The wear variations of dental composite resins were linearly increased as the number of cycles increased. It was observed that the wear resistances of these specimens were in the order of Dyract AP ＆gt; Surefil ＆gt; Compoglass ＆gt; Z100 ＆gt; Fuji II LC. On the morphological observations by SEM, the large crack formation on the sliding track of Fuji II LC specimen was the greatest among all resin composites. Dyract AP showed the least wear with few surface damage. There is no significant difference in wear performance between conventional halogen light curing and light emitting diodes curing sources. It indicates that a light emitting diodes (LED) source can replace a halogen light source as a curing unit for composite resin restorations.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.12
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• pp.996-1001
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• 2010

Recently, the demand of ceramic metal halide lamp has been expanded. Therefore, the lamp with high efficiency and long lifetime are increasing and the evaluation of reliability is needed. In this paper, the degradation phenomena of ceramic metal halide lamp was studied. The lamp was tested for 3000 on/off cycles with each cycle having a duration of 20 minutes on and 20 minutes off based on the accelerated aging experiment based on "Reliability Standards RS C 0085". As result, the corrosion of arc tube and leak was appeared from reaction between inner wall of PCA and chemical elements, and distortion of electrode was resulted from difference of thermal expansion between arc tube of PCA and electrode. Also, the efficiency of lamp was decreased by the change of inner pressure, operation temperature, and driving voltage from wall blackening.

• Fashion & Textile Research Journal
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• v.20 no.1
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• pp.93-100
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• 2018

This study comparatively analyzed the changes in the dimensional stability and the mechanical properties of stretch knit fabrics associated with washings conditions in order to determine the most appropriate washing method. Polyester and spandex knit fabrics were washed 25 cycles with nine washing condition profiles that controled detergent, hardness. temperature and RPM. The knit fabrics with repeated washings were evaluated by dimensional stability and measured by the KES-FB system. Polyester and spandex fibers have good chemical resistance. However, the changes in the dimensional stability and the mechanical properties were observed in washed knit fabrics. After repeated washings, the knit fabrics washed in an alkali or neutral detergent had good dimensional stability compared with the knit fabric washed in functional detergent. The washed knit fabrics were extended, stiff, rough, and had a smaller volume. The changes were the result of the rearrange of polyester fibers which twisted around spandex core and chemical interactions involving the detergent and the physical and mechanical forces of washing. Especially, using an alkali detergent resulted in increased stiffness and roughness of the knit fabric. In conclusion, the washing in water with low hardness and a neutral detergent can minimize the changes in dimensional stability and mechanical properties of polyester and spandex knit fabric.

• Journal of Microbiology and Biotechnology
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• v.26 no.5
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• pp.829-836
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• 2016

Penicillin G acylase (PGA) was immobilized on magnetic Fe₃O₄@chitosan nanoparticles through the Schiff base reaction. The immobilization conditions were optimized as follows: enzyme/support 8.8 mg/g, pH 6.0, time 40 min, and temperature 25 ℃. Under these conditions, a high immobilization efficiency of 75% and a protein loading of 6.2 mg/g-support were obtained. Broader working pH and higher thermostability were achieved by the immobilization. In addition, the immobilized PGA retained 75% initial activity after ten cycles. Kinetic parameters V_max and K_m of the free and immobilized PGAs were determined as 0.113 mmol/min/mg-protein and 0.059 mmol/min/mg-protein, and 0.68 mM and 1.19 mM, respectively. Synthesis of amoxicillin with the immobilized PGA was carried out in 40% ethylene glycol at 25 ℃ and a conversion of 72% was obtained. These results showed that the immobilization of PGA onto magnetic chitosan nanoparticles is an efficient and simple way for preparation of stable PGA.

• The Journal of Engineering Geology
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• v.25 no.2
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• pp.165-178
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• 2015

Physical and chemical weathering degrades rock, affecting its structural properties and thus the stability of stone buildings or other structures. Confocal laser scan microscopy (CLSM) is used here to observe temporal changes in the surface roughness of rock samples under simulated accelerated weathering. Samples were pressurized to 50, 55, or 70 MPa using a pressure frame, and subjected to freeze/thaw cycling controlled by a thermostat. The temperature was cycled from -20℃ to 40℃ and back. After each 20 cycles, CLSM was used to assess the change in surface roughness, and roughness factors were calculated to quantify the progression of the surface condition over time. Variations in cross-section line-roughness parameters and surface-roughness parameters were analyzed for specific parts of the sample surfaces at 5× and 50× magnification. The result reveals that the highest and lowest values of the roughness factors are changed according to elapsed time. Freezing/thawing at high pressure caused larger changes in the roughness factor than at low pressure.

• Advances in concrete construction
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• v.5 no.3
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• pp.241-255
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• 2017

Concrete is known to be the most used construction material worldwide. The environmental and economic aspects of Ordinary Portland Cement (OPC) containing concrete have led research studies to investigate the possibility of incorporating supplementary cementitious materials (SCMs) in concrete. Metakaolin (MK) is one SCM with high pozzolanic reactivity generated throughout the thermal activation of high purity kaolinite clay at a temperature ranging from $500^{\circ}C$ to $800^{\circ}C$. Although many studies have evaluated the effect of MK on mechanical properties of concrete and have reported positive effects, limited articles are considering the effect of MK on durability properties of concrete. Considering the lifetime assessment of concrete structures, the durability of concrete has become of particular interest recently. In the present work, the influences of MK on mechanical and durability properties of concrete mixtures are evaluated. Various experiments such as slump flow test, compressive strength, water permeability, freeze and thaw cycles, rapid chloride penetration and surface resistivity tests were carried out to determine mechanical and durability properties of concretes. Concretes made with the incorporation of MK revealed better mechanical and durability properties compared to control concretes due to combined pozzolanic reactivity and the filler effect of MK.

• Steel and Composite Structures
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• v.28 no.5
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• pp.517-525
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• 2018

The specimens made by Z2CND18.12N austenitic stainless steel were conducted on a 100 kN closed loop servo hydraulic tension-compression testing machine with a digital controller. Uniaxial tension and uniaxial ratcheting effect tests were carried out at $25^{\circ}C$. Moreover, Uniaxial tension tests were conducted at $150^{\circ}C$, $250^{\circ}C$ and $350^{\circ}C$. Based on these experimental data, the prediction models of stress-strain curve and the relationship of ratcheting strain and number of cycles were established by the algorithm principle of BP neural network. The results indicated that the predicted results of neural network model were in well agreement with experimental data. It was found that the BP neural network model had high validity and accuracy.

• Transactions of the Korean Society of Automotive Engineers
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• v.11 no.1
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• pp.1-6
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• 2003

Engine emission regulations are becoming more stringent nowadays. In cold transient regime, about 80% THC is exhausted to the atmosphere in the first 200s (US FTP cycles). Accordingly, reducing emission levels in the cold period immediately after the engine start before the catalysts reach their working temperature will be an especially critical factor in meeting more stringent regulations in the future. In this study, the total hydrocarbon quantities are measured using a Fast FID with gasoline fuel for a 4-cylinde. Sl engine, including Secondary Air Injection (SAI) system. Commercial SAI device's direction is reverse to the exhaust flow. In this study, a swirl flow type SAI system which is positioned between the exhaust manifold and exhaust port, was developed. We compared the swirl type secondary air injection with a commercial secondary air injection of .everse flow. The swirl type SAI showed better results in reducing HC by 26% than the commercial flow type SAI of reverse flow which was caused by the better mixing between the exhaust gas and the secondary air.