• Transactions of Materials Processing
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• v.25 no.1
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• pp.12-20
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• 2016

Press machines and dies are commonly used for 3D curved sheet forming. Using conventional die forming can cause economic problems since various modifications of the die shape are required depending on the product shape. Various types of flexible forming such as multi-point dieless forming (MDF), flexible incremental roll forming have been developed to improve the needed process flexibility. Although MDF can reduce the production cost using reconfigurable dies, it still has significant material loss. Drawbacks such as wrinkling, dimpling, and forming errors can also occur despite continuous investigations to mitigate these defects. A novel sheet forming process for 3D curved surfaces, a flexibly-reconfigurable roll forming (FRRF), has been recently proposed to overcome the economic and technical limitations of current practice. FRRF has no limitation on blank size in the longitudinal direction, and also minimizes or eliminates forming defects such as wrinkling and dimpling. Feasibility studies of FRRF have been conducted using FE simulations for multi-curved shapes and various sheet thicknesses. Therefore, the fabrication of a FRRF apparatus is required for any follow-up studies. In the current study, experiments with reconfigurable rollers were conducted using a simple design pre-FRRF apparatus prior to fabricating the full size FRRF apparatus. There are three candidates for the reconfigurable roller: a bar-type shaft, a flexible shaft, a ground flexible shaft. Among these candidates, the suitable reconfigurable roller for FRRF is determined through various forming tests.

• Journal of the Korean Applied Science and Technology
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• v.32 no.1
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• pp.1-6
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• 2015

We investigated green phosphorescent organic light-emitting diodes with stepwise doping to improve efficiency roll-off and operational lifetime by efficient distribution of triplet excitons. The host material which was 4,4,N,N'-dicarbazolebiphenyl (CBP) of bipolar characteristic that can control the carrier in emitting layer (EML). When the EML devided into four parts with different doping concentration, each devices shows various efficiency roll-off and lifetime enhancement. The distribution of the carrier and excitons in the EML can be confirmed by using stepwise doping structure. The properties of device C exhibited luminous efficiency of 51.10 cd/A, external quantum efficiency of 14.88%, respectively. Lifetime has increased 73.70% compared to the reference device.

• 한국신재생에너지학회:학술대회논문집
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• 2010.06a
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• pp.69.2-69.2
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• 2010

Recently, bulk hetero-junction cells have been extensively studied by many researchers. Most of these cells were fabricated by spin coater. However, the spin coating process is not favorable to the large-scaled industry because it is not compatible with roll-to-roll process. One of the alternative methods is Doctor blading. In this study, we fabricated large OPV cells having total area of $100cm^2$. The buffer layer was Poly-(3,4-ethylenedioxythiophene) : poly-(styrenesulfonate) aqueous dispersion (PEDOT:PSS) and the active material is poly (3-hexythiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend in the solvent of Chlorobenzene. All of the organic layers were coated by dragging the blade with a speed of 5~20 mm/s on the stage with a temperature of $50^{\circ}C$. As-bladed PEDOT:PSS layer was baked at $120^{\circ}C$ for 10 minutes to eliminate the water. The cell structure is patterned ITO substrate/PEDOT:PSS/P3HT:PCBM/LiF/Al. The topmost electrode, LiF/Al, was deposited by thermal evaporation. After depositing electrode, and the cell was annealed at $150^{\circ}C$ for 30 minutes. The measured ISC, VOC, fill factor, and PCE were 2.95 A, 5.86 V, 0.32, and 0.78%, respectively. PCE was quite low but the large active area could be obtained successfully.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.5
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• pp.1193-1201
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• 1993

A new deformation resistance model for hot steel strip rolling process was formulated to improve the accuracy of roll force estimation. To improve the existing deformation resistance model more precisely, a modification function was introduced in this study. For the modification function, several factors considering material and operational conditions have been investigated and the optimal modification function was determined under the principle of minimum variability. The newly formulated modification function was applied to the deformation resistance model for ultra-low carbon steel and showed improved accuracy with about 30% decrease in terms of standard deviation of predicted roll force values against measured ones.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.3
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• pp.182-188
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• 2001

An experiment is carried out to investigate the characteristics of blanking for copper alloy C194 (t=0.254mm), a kind of IC lead frame material. By varying clearance between die and punch, the shapes of shear profile are examined. Finite element analysis with element deletion algorithm for ductile fracture mode is also carried out to study the effect of clearance theoretically and to compare with experimental results. The rectangular shape specimen with four different corner radius is used to study the characteristics of blanking for straight side and corner region simultaneously. As the result, the ratios measured from the experiment of roll over, burnish, and fracture zone based on intial blank thickness are compared with those of FE analysis. Both experiment and FE analysis show that the amount of roll over and fracture is increased as the clearance increases. It has been found that larger clearance is required than that of straight region when the radius of corner is less than thickness of blank, in order to maintain same quality of shear profile at the corner region.

• Journal of Biosystems Engineering
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• v.14 no.4
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• pp.242-250
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• 1989

The performance of a vertical type centrifugal distributor of granular materials was studied by means of mathematical models and experimental investigations. To develop the mathematical description of particle motion, some assumptions were made. The distribution process consisted of three stages: the entrance of a particle to the blade, the motion of the particle on the blade, and the motion of the particle in the air. The physical properties of fertilizer, which affected the particle motion, were investigated: bluk density, coefficient of friction, coefficient of restitution, and particle size distribution. The particle motion were simulated by using a computer. A prototype distributor was designed and constructed for experimental tests. The following conclusions were drawn from the computer simulation and experiment results. 1. The fertilizer may slide or roll at the point of contact when they impact on the blade and move along the blade. 2. The interaction among fertilizers may prevent them from bouncing. 3. When fertilizers roll on the blade, rolling resistance is one of the factors affecting the particle's motion. 4. The trajectory angle and position of fertilizers from a disc depend on the blade position and particle shape, but the rotating speed of the disc affected them only slightly.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.319-319
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• 2010

Dye-sensitized solar cells (DSSCs) based on plastic substrates have attracted much attention mainly due to extensive applications such as ubiquitous powers, as well as the practical reasons such as light weight, flexibility and roll-to-roll process. However, conventional high temperature fabrication technology for glass based DSSCs, cannot be applied to flexible devices because polymer substrates cannot withstand the heat more than $150^{\circ}C$. Therefore, low temperature fabrication process, without using a polymer binder or thermal sintering, was required to fabricate necked $TiO_2$. In this presentation, we proposed polymer-inorganic composite photoelectrode, which can be fabricated at low temperature. The concept of composite electrode takes an advantage of utilizing elastic properties of polymers, such as good impact strength. As an elastic material, poly(methyl methacrylate) (PMMA) is selected because of its optical transparency and good adhesive properties. In this work, a polymer-inorganic composite electrode was constructed on FTO/glass substrate under low temperature sintering condition, from the mixture of PMMA and $TiO_2$ colloidal solution. The effect of PMMA composition on the photovoltaic property was investigated. Then, the enhanced mechanical stability of this composite electrode on ITO/PEN substrate was also demonstrated from bending test.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2011.06a
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• pp.261-262
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• 2011

Conventional automatic transmission system includes a hydrodynamic torque converter to transfer engine torque from an engine crank shaft to a rotatable input members, which are of complex design permitting them to serve several functions. These are clutches or brakes which couple the rotatable input member to member of a planetary gear set. The annulus gear for an automatic transmission is a monolithic gear having a set of gear teeth formed on an inner surface which is coupling with a set of planetary gear. In this study, the flow forming method is applied to the manufacturing of the annulus gear. This cold forming is proper method in order to manufacture dimensionally precise and round hollow components such as annulus gear. By pre-calculated amount of wall thickness reduction, the seamless tube of SAE1026 is compressed above its yield strength, plastically deformed and made to flow in several roll passes. According to this study, the desired geometry of the annulus gear can be achieved when the outer diameter and the thickness of the tube are properly decreased by compressed roll passes and the available material volume is easily forced to flow longitudinally over the shape of mandrel.

• Transactions of Materials Processing
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• v.22 no.5
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• pp.264-268
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• 2013

Aluminum clad sheets for brazing materials in the automotive heat exchangers are required to exhibit both high strength and excellent erosion resistance. In this study, the effects of microstructural changes on the property of clad sheets due to thermomechanical treatment were investigated. The clad sheets were fabricated by roll bonding of twin-roll-cast AA3527 and AA4343 alloys followed by cold rolling down to a thickness of 0.22mm. Partial or full annealing was conducted at the final thickness in order to improved the erosion resistance while keeping the proper strength. Since full annealing was achieved for a temperature of $400^{\circ}C$, annealing treatments were performed at 360, 380, and $400^{\circ}C$, respectively. The tensile strength of 3527/4343 clad material was found to be inversely proportional to the annealing temperature before the brazing heat treatment. After this latter treatment, however, the tensile strength of the clad material was about 195~200MPa regardless of the annealing temperature. The erosion depth ratio of the clad annealed at $400^{\circ}C$ was 8.8% (the lowest), while that of the clad annealed at $380^{\circ}C$ was 17% (the highest). The effect of annealing temperature on the tensile and erosion properties of 3527/4343 aluminum clad sheets was elucidated by means of microstructural analyses.

• Transactions of Materials Processing
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• v.20 no.3
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• pp.257-264
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• 2011

Multilayer(clad) sheets, composed of two or more materials with different properties, are fabricated using the roll-bonding process. A good formability is an essential property for a multilayered sheet in order to manufacture parts by plastic deformation. In this study, the influences of temperature and strain rate on the plastic properties of stainless steel-aluminum-magnesium multilayered(STS-Al-Mg) sheets were investigated. Tensile tests were performed at various temperatures and strain rates on the multilayered sheet and on each separate layer. Fracture of the multilayered sheet was observed to be temperature-dependent. At the base temperature of $200^{\circ}C$, all materials fractured simultaneously. At lower temperatures, the Mg alloy sheet fractured earlier than the other materials. Conversely, the other materials fractured earlier than the Mg alloy sheet at higher temperatures. The uniform and total elongations of the multilayered sheet were observed to be higher than that of each material at a temperature of $250^{\circ}C$. Larger uniform elongations were obtained for higher strain rates at constant temperature. The same trend was observed for the Mg alloy sheet, which exhibited the lowest elongation among the three materials. The tensile strengths and elongations of the single layer sheets were compared to those of the multilayer material. The strength of the multilayered sheet was successfully calculated by the rule of mixture from the values of each single layer. However, no simple correlation between the elongation of each layer and that of the multilayer was obtained.