• Journal of the Korean Society for Precision Engineering
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• v.12 no.6
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• pp.87-95
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• 1995

In order to predict the case depth and case width in laser transformation hardening, a finite element method was used to analyze the temperature distribution on the material. Laser hardening of the specimens of SM45C and STE11steels was experimented by using the continuous wave CO$_{2}$ laser with the various travel speeds and the defocused Gaussian beam mode. Phosphate coating was adopted on the surface of SM45C to increase the absorption of 10.6 .mu. m laser energy. Experimental data show good agreement with the theoretical predictions. The maximum possible case depth can be predicted for the given laser hardening conditions, such as laser power, and travel speed.

• Korean Chemical Engineering Research
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• v.52 no.5
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• pp.558-563
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• 2014

To study the effects of fabrication methods on the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs), membrane-electrode assemblies (MEAs) were fabricated using a Dr blade method, a spray method, screen print method and screen print + spray method. The performance of single cells assembled with the prepared MEAs were initially measured and compared. Electrode accelerated stress testing (AST) involving a potentiostatic step-wave with 10 s at 0.6 V followed by 30 s at 0.9 V was applied to test durability of MEAs. Before and after 6,000cycles of the AST, I-V curves, impedance spectra, cyclic voltammograms, linear sweep voltammetry (LSV) and transmission electron microscope (TEM) were measured. Under the operating conditions, the Dr Blde MEA exhibited the highest initial performance. After electrode accelerated stress testing, screen print + spray MEA showed lowest degradation rate.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.29 no.6
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• pp.70-77
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• 2015

This paper describes the experimental results obtained from various installation conditions of cascaded metal oxide varistor(MOV)-based SPDs with the objectives to analyze the coordination of the cascaded surge protective devices(SPDs) and to propose the proper selection and installation methods of the cascaded SPDs. The residual voltage, discharge current and energy sharing between the upstream and downstream SPDs in the $10/350{\mu}s$ direct lightning current wave were measured and discussed. The coordination of cascaded MOV-based SPDs is closely related to the varistor voltage and installation methods of SPDs. In cascaded SPDs without dedicated decoupling elements, the natural impedance of leads connecting two SPDs can act as a decoupler for the coordination of MOV-based SPDs. Even if the varistor voltage of the upstream SPD is higher than that of the downstream SPD at long distances between two SPDs, the energy coordination of cascaded SPDs could effectively be fulfilled in the conditions of large surge currents and the optimum voltage protection level can be achieved. Consequently, if the distance between voltage limiting type SPDs is long, the coordination of the cascaded SPDs should be determined by taking into account the decoupling effects due to the intrinsic inductance of leads connecting the upstream and downstream SPDs.

• Geophysics and Geophysical Exploration
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• v.19 no.1
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• pp.20-28
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• 2016

Recently many sinkholes have appeared in urban areas of Korea, threatening public safety. To predict the occurrence of sinkholes, it is necessary to investigate the existence of cavity under urban roads. Ground-penetrating radar (GPR) has been recognized as an effective means for detecting underground cavity in urban areas. In order to improve the understanding of the governing physical processes associated with GPR wave propagation, and interpret underground cavity effectively, a theoretical approach using numerical modeling is required. We have developed an algorithm employing a three-dimensional (3D) staggered-grid finite-difference time-domain (FDTD) method. This approach allows us to model the full electromagnetic wavefield associated with GPR surveys. We examined the GPR response for a simple cavity model, and the modeling results showed that our 3D FDTD modeling algorithm is useful to assess the underground cavity under urban roads.

• 한국지구물리탐사학회:학술대회논문집
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• 2009.10a
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• pp.28-33
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• 2009

Methane hydrates are considered important in terms of their effect on global warming and as potential energy resources. Now, mainly the presence of a BSR and seismic velocity are used for estimation of methane hydrate concentration in the seismic reflection survey. But recent studies on seismic attenuation show that it can be used also to estimate methane hydrates concentration. In this study, we tried to estimate attenuation from 2D seismic reflection data acquired at Nankai Trough in Japan and analyzed attenuation properties in methane hydrate bearing sediments. Seismic attenuation estimated by QVO method in an offset range $125{\sim}1,575m$. We observed high attenuation in methane hydrate bearing sediments over BSR in a frequency range of 30-70Hz. Thus, this result demonstrates that seismic reflection wave within this frequency range are affected significantly by the existence of methane hydrate concentration zone.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.2
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• pp.140-148
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• 2002

Anisotropic elastic constants of Zr-2.5Nb pressure tube materials were determined by a high temperature resonant ultrasound spectroscopy (RUS). The resonant frequencies were measured using alumina wave-guides and wide band ultrasonic transducers in a small furnace. The rectangular parallelepiped specimens were fabricated along with the axial, radial and circumferential direction of the pressure tube. A nine elastic stiffness tensor for orthotropic symmetry was determined in the range of room temperature ${\sim}500^{\circ}C$. As the temperature increases, the elastic constant tensor, cij gradually decreases. Higher elastic constants along the transverse direction compared to those along the axial or radial direction are similar to the case of Young's modulus or shear modulus. A crossing of shear elastic constants along axial direction and radial direction was observed near $150^{\circ}C$. This fact corresponds to the crossing of c44 and c66 of single crystal zirconium.

• Journal of the Korean earth science society
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• v.30 no.1
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• pp.40-48
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• 2009

The attenuation mechanism of seismic waves in the crust is controlled both by intrinsic absorption and scattering of energy. The amount of scattering and intrinsic energy losses from the total attenuation is separately estimated in this study for the southern Korean Peninsula. The formula to be deduced from the theoretical relationship between single back-scattered coda Q and multiple scattering theory was used to separate the total attenuation into the intrinsic Q and the scattering Q. It was found that the intrinsic Q was considerably lower than that of the scattering Q in the frequency range of 1.5 to 20 Hz. This fact implies that the energy loss caused by the intrinsic absorption is relatively larger than one by the scattering effect within the crust of the southern Korean Peninsula. Both intrinsic and scattering Q values appeared to be comparatively larger than those measured in other seismically active regions except for intrinsic Q in the frequency range of 1.5 to 3 Hz.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.26 no.1
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• pp.51-56
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• 1990

The authors carried out an experiment to investigate the echo fluctuations from ocean bottom due to ship's motion. The bottom echoes was continuously measured, by using a 50 kHz Echo sounder on board of the ship being at anchor under the sea condition of 15 knots in wind velocity and approximately 2 meters in wave height, to extract the information about the pulse stretching and the ship's motion from the first return and the second return. A data acquisition system was used to record digitally the envelope of the echoes, and the analysis was applied to the echo data collected from the continental shelf in the South China Sea. The results obtained can be summarized as follows: 1. The equivalent pulse width of the second return echoes from ocean bottom was 2.4 times longer than that of the first return echoes. 2. The echo peak values of the first return fluctuated markedly than that of the second return and was shown to be extremely sensitive to small change in ship's motion. 3. Energy target strength and peak target strength of the sandy-mud bottom were -13.4 dB and -14.6 dB, respectively.

• Journal of Powder Materials
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• v.23 no.2
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• pp.91-94
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• 2016

High-quality colloidal CdSe/ZnS (core/shell) is synthesized using a continuous microreactor. The particle size of the synthesized quantum dots (QDs) is a function of the precursor flow rate; as the precursor flow rate increases, the size of the QDs decreases and the band gap energy increases. The photoluminescence properties are found to depend strongly on the flow rate of the CdSe precursor owing to the change in the core size. In addition, a gradual shift in the maximum luminescent wave (${\lambda}_{max}$) to shorter wavelengths (blue shift) is found owing to the decrease in the QD size in accordance with the quantum confinement effect. The ZnS shell decreases the surface defect concentration of CdSe. It also lowers the thermal energy dissipation by increasing the concentration of recombination. Thus, a relatively high emission and quantum yield occur because of an increase in the optical energy emitted at equal concentration. In addition, the maximum quantum yield is derived for process conditions of 0.35 ml/min and is related to the optimum thickness of the shell material.

• Atmosphere
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• v.21 no.2
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• pp.163-172
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• 2011

Numerical forecasting depends on the initial condition error strongly because numerical model is a chaotic system. To calculate the sensitivity of some forecast aspects to the initial condition in the Korea Meteorological Administration (KMA) Unified Model (UM) which is originated from United Kingdom (UK) Meteorological Office (MO), an algorithm to calculate adjoint sensitivities is developed by modifying the adjoint perturbation forecast model in the KMA UM. Then the new algorithm is used to calculate adjoint sensitivity distributions for typhoon DIANMU (201004). Major initial adjoint sensitivities calculated for the 48 h forecast error are located horizontally in the rear right quadrant relative to the typhoon motion, which is related with the inflow regions of the environmental flow into the typhoon, similar to the sensitive structures in the previous studies. Because of the upward wave energy propagation, the major sensitivities at the initial time located in the low to mid- troposphere propagate upward to the upper troposphere where the maximum of the forecast error is located. The kinetic energy is dominant for both the initial adjoint sensitivity and forecast error of the typhoon DIANMU. The horizontal and vertical energy distributions of the adjoint sensitivity for the typhoon DIANMU are consistent with those for other typhoons using other models, indicating that the tools for calculating the adjoint sensitivity in the KMA UM is credible.