• Journal of Welding and Joining
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• v.27 no.1
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• pp.95-101
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• 2009

In case of manufacturing the high quality welds or pipeline, the full penetration weld has to be made along the weld joint. Thus the root pass welding is very important and has to be selected carefully. In this study, an experimental method for the selection of optimal welding condition was proposed in the root pass welding which was done along the V-grooved butt weld joint. This method uses the response surface analysis in which the width and height of back bead were chosen as the quality variables of the weld. The overall desirability function, which is the combined desirability function for the two quality variables, was used as the objective function for getting the optimal welding condition. In the experiments, the target values of the back bead width and the height are 6mm and zero respectively for the V-grooved butt weld joint of 8mm thickness mild steel. The optimal welding conditions could predict the back bead profile(bead width and height) as 6.003mm and -0.003mm. From a series of welding test, it was revealed that a uniform and full penetration weld bead can be obtained by adopting the optimal welding condition which was determined according to the method proposed.

• Journal of Korean Society on Water Environment
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• v.21 no.6
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• pp.549-563
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• 2005

The study was carried out to develop and standardize the analytical method of 1,4-dioxane in water. The National Institute of Environmental Research in Korea has monitored for 1,4-dioxane in water since 1998 and it has been detected in environmental water occasionally. But the environmental pollution and analytical method in water is not well known over the world. The trace analytical method at low-${\mu}/L$ level is prerequisite in order to evaluate the residue of 1,4-dioxane in water. Evaluation of the method was demonstrated by comparing and analyzing with liquid-liquid extraction, solid-phase extraction and purge & trap technique. As a result of experiment by standard solutions and raw samples, the precision and accuracy for three methods were obtained within error rate of about 10%. Therefore, three methods were standardized as official monitoring method in Korea. Also, a convenient and simple liquid-liquid extraction method for the analysis of 1,4-dioxane in water was developed by combined with gas chromatography/mass spectrometry and applied to the water samples in Korea

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.38.1-38.1
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• 2017

We report the results of three-dimensional radiation hydrodynamic simulations of star cluster formation in turbulent molecular clouds, with primary attention to how stellar radiation feedback controls the lifetime and net star formation efficiency (SFE) of their natal clouds. We examine the combined effects of photoionization and radiation pressure for a wide range of cloud masses (10^4 - 10^6 Msun) and radii (2 - 80 pc). In all simulations, stars form in densest regions of filaments until feedback becomes strong enough to clear the remaining gas out of the system. We find that the SFE is primarily a function of the initial cloud surface density, Sigma, (SFE increasing from ~7% to ~50% as Sigma increases from ~30 Msun/pc^2 to ~10^3 Msun/pc^2), with weak dependence on the initial cloud mass. Control runs with the same initial conditions but without either radiation pressure or photoionization show that photoionization is the dominant feedback mechanism for clouds typical in normal disk galaxies, while they are equally important for more dense, compact clouds. For low-Sigma clouds, more than 80% of the initial cloud mass is lost by photoevaporation flows off the surface of dense clumps. The cloud becomes unbound within ~0.5-2.5 initial free-fall times after the first star-formation event, implying that cloud dispersal is rapid once massive star formation takes place. We briefly discuss implications and limitations of our work in relation to observations.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.1
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• pp.126-131
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• 2009

This study was carried out for the reliability of PD(Partial Discharge) pattern recognition. For the pattern recognition, the database for PD was established by use of self-designed insulation defects which occur and were mostly critical in GIS(Gas Insulated Switchgear). The acquired database was analyzed to distinguish patterns by means of PRPD(Phase Resolved Partial Discharge) method and stored to the form with to unite the average amplitude of PD pulse and the number of PD pulse as the input data of neural network. In order to prove the performance of genetic algorithm combined with neural network, the neural networks with trial-and-error method and the neural network with genetic algorithm were trained by same training data and compared to the results of their pattern recognition rate. As a result, the recognition success rate of defects was 93.2% and the neural network train process by use of trial-and-error method was very time consuming. The recognition success rate of defects, on the other hand, was 100% by applying the genetic algorithm at neural network and it took a relatively short time to find the best solution of parameters for optimization. Especially, it could be possible that the scrupulous parameters were obtained by genetic algorithm.

• Journal of Surface Science and Engineering
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• v.41 no.4
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• pp.163-168
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• 2008

The dental orthodontic bracket requires good mechanical properties, such as elastic strength and frictional resistance, combined with a high resistance to corrosion. The objective of this study was to investigate the effects of TiN and ZrN coating on corrosion resistance of orthodontic brackets using various electrochemical methods. Brackets manufactured by Ormco Co. were used, respectively, for experiment. Ion plating was carried out for coatings of bracket using Ti and Zr coating materials with nitrogen gas. Ion plated surface of each specimen was observed with field emission scanning electron microscopy(FE-SEM), energy dispersive Xray spectroscopy(EDS) and electrochemical tester. The corrosion potential of the TiN and ZrN coated bracket was comparatively high. The current density of TiN and ZrN coated bracket was smaller than that of non-coated bracket in 0.9% NaCl solution. Pit nucleated at angle of bracket slot.

• Journal of Powder Materials
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• v.13 no.6 s.59
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• pp.408-414
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• 2006

[ $MoSi_2$ ] alloys with Al, B or Nb were prepared by an advanced consolidation process that combined mechanical alloying with pulse discharge sintering (complex forming) to improve the mechanical properties. Their microstructure and mechanical properties were investigated. The $MoSi_2$ alloys fabricated by complex forming method showed very fine microstructure when compared with the sample sintered from commercial $MoSi_2$ powders. Alloys made from powders milled in Ar gas had fewer silica or alumina phases as compared to their counterparts sintered from powders milled in air. In densification of the sintered body, addition of B was more effective than Al or Nb. Both Victors hardness and tensile test indicated that the alloy fabricated by the complex forming method showed better properties than the sample sintered from commercial $MoSi_2$ powders. The Al added alloy sintered from the powders milled in air had the superior mechanical properties due to the suppression of $SiO_2$ and formation of fine $Al_2O_3$ particles.

• Transactions of Materials Processing
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• v.19 no.5
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• pp.277-282
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• 2010

Magnesium alloy has been known as lightweight material in automobile and electronic industry with aluminum alloy, titanium alloy and plastic material. Friction welding is useful to join various metals and nonferrous metals that are difficult to join by such as gas welding, resistance welding and electronic beam welding. In this study, friction joining was performed to investigate mechanical properties of Mg alloy with 20mm diameter solid bar. Also the optimal joining conditions for its application were determined on the basis of tensile test, and hardness survey. The joining parameters were chosen as heating pressure, heating time, upsetting pressure, and upsetting time. Heating and upsetting pressure were executed under the range of 10~40MPa and 20~80MPa, respectively. From the experimental results, optimal joining conditions were determined as follows; rotating speed=2000rpm, heating pressure=35MPa, upsetting pressure=70MPa, heating time=1sec, upsetting time=5sec. Also the hardness of jointed boundary showed as HV50 which was similar to that of base metal at the optimal condition, and it was supposed that zone of HAZ was 8mm. Finally two materials were strongly mixed at interface part to show a well-combined microstructure without particle growth or any defect.

• Proceedings of the KAIS Fall Conference
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• 2010.05b
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• pp.769-772
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• 2010

The objective of the current research work is to investigate the usage of biodiesel combined with the use of EGR in order to reduce the emission of all regulated pollutants from diesel engines. A single cylinder, air cooled, constant speed direct injection diesel engine was used for the experimental work and a cold EGR system was developed and fitted to the engine. Concentrations of HCs, NOx, and CO from the exhaust gas along with the smoke opacity were measured. Engine performance parameters such as the brake thermal efficiency (BTE) and the brake specific energy consumption (BSEC) were also calculated from the measured data. The results from the present investigation suggest that 25-30% EGR rate could give excellent NOx reduction without any significance penalty on smoke opacity or BSEC under the engine load of up to 40%. Under the full load condition, 15% EGR rate was found to be an option while higher EGR rate resulted in inferior performance and heavy smoke.

• Journal of Hydrogen and New Energy
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• v.13 no.3
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• pp.214-223
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• 2002

Kinetic and effectiveness factors for methanol steam reforming using commercial copper-containing catalysts in a plug flow reactor were investigated over the temperature ranges of $180-250^{\circ}C$ at atmospheric pressure. The selectivity of $CO_2$/$H_2$ was almost 100%, and CO products were not observed under reaction conditions employed in this work. It was indicated that $CO_2$ was directly produced and CO was formed via the reverse water gas shift reaction after methanol steam reforming. The intrinsic kinetics for such reactions were well described by the Langmuir-Hinshelwood model based on the dual-site mechanism. The six parameters in this model, including the activation energy of 103kJ/mol, were estimated from diffusion-free data. The significant effect of internal diffusion was observed for temperature higher than $230^{\circ}C$ or particle sizes larger than 0.36mm. In the diflusion-limited case, this model combined with internal effectiveness factors was also found to be good agreement with experimental data.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.285-287
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• 2001

We have studied the nonlinear evolution of a magnetized disk of isothermal gas, which is sustained by its self-gravity. Our objective is to investigate how the Jeans, Parker, and convective instabilities compete with each other in structuring/de-structuring large scale condensations in such disk. The Poisson equation for the self-gravity has been solved with a fourth-order accurate Fourier method along with the Green function, and the MHD part has been handled by an isothermal TVD code. When large wavelength perturbations are applied, the combined action of the Jeans and Parker instabilities suppresses the development of the convection and forms a dense core of prolate shape in the mid-plane. Peripheral structures around it are filamentary. The low density filaments connect the dense core to the diffuse upper region. On the other hand, when small wavelength perturbations are applied, the disk develops into an equilibrium state which is reminiscent of the Mouschovias's 2-D non-linear equilibrium of the classical Parker instability under an externally given gravity.