• Polymer(Korea)
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• v.26 no.3
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• pp.344-352
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• 2002

In this work, two thermal cationic latent catalysts, i.e., triphenyl benzyl phosphonium hexafluoroantimonate (TBPH) and benzyl 2-methylpyrazinium hexafluoroantimonate (BMPH) were newly synthesized. And the thermal and mechanical properties of difunctional epoxy (diglycidylether of bisphenol h, DGEBA) resins initiated by 1 phr of either TBPH or BMPH catalyst were investigated. As experimental results, the epoxy/TBPH system showed higher curing temperature and critical stress intensity factor ($K_{IC}$) than those of epoxy/BMPH. This could be interpreted in terms of slow thermal diffusion rate and bulk structure of four phenyl groups in TBPH. However, the decomposed activation energy determined from Coats-Redfern method was lower in the case of epoxy/TBPH. This result was probably due to the fact that broken short chain structure was developed by steric hindrance of TBPH.

• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.820_821
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• 2009

전력기기는 일반적으로 상용주파수인 60 Hz의 교류 전원하에서 운용된다. 따라서 고온초전도체를 전력기기에 응용하는데 있어 교류전류나 교류자장에 의한 교류손실 발생 문제는 전력기기의 경제적 운용과 관련하여 중요하게 부각되어 왔다. 또한 고온초전도체에서 발생하는 교류손실은 에너지의 열적 변환으로 인한 냉동부하 문제와도 관련되어 있기 때문에 전력기기의 안정적인 운용을 위해서도 반드시 연구되어야 할 부분이다. 특히 변압기와 같이 유도형 권선이 필요한 전력기기의 경우에는 무유도 권선 형태가 적용되는 다른 전력기기에 비해 교류 손실의 크기가 매우 크기 때문에 이를 줄이기 위한 방안에 대한 연구가 다각도로 이루어지고 있다. 본 연구에서는 임계전류와 인덕턴스의 크기는 동일하지만 권선 형태를 다르게 할 수 있는 유도형 솔레노이드 코일을 제작하였다. 그리고 제작된 솔레노이드 코일의 병렬연결 구조를 달리하여 각각의 구조에 대한 교류손실의 차이를 측정하고 분석하였다. 그리하여 측정된 결과를 토대로 고온초전도 선재를 이용한 솔레노이드 코일에서 교류손실을 줄이기 위한 구조적 방안을 제시하고자 한다.

• Journal of Korean Ophthalmic Optics Society
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• v.9 no.1
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• pp.161-166
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• 2004

In this research, TiN films has been grown to the stainless steel substrate by ion beam assisted deposition. TiN film was grown to the nitrogen atmosphere of around $10^{-15}$ Torr with Arion bombardment. The chemical composition, color and adhesion of TiN films were examined as a variation of En(ion energy per atom). The N/Ti ratio increased linearly as the increase of En and saturated around 1.2 at high En. As a results, the bright golden color was obtained when En reached a certain value of Ecn. As a results, the N/Ti ratio is about 0.9.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.16 no.2
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• pp.213-221
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• 2012

In this paper, a dynamic power management system is proposed to reduce energy consumption for multiple clients in cloud computing environments. The proposed system monitors both keyboard and mouse input from the user, available memory, and CPU usage in the virtual machine. If the system detects no keyboard and mouse input for a certain amount of time and both available memory and CPU usage reach predefined threshold value, the manager in the virtual machine orders the client to shutdown the client machine, which results in significant power save. The developed system is applied to the real university computer lab and the performance of the system is evaluated.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.9
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• pp.1032-1041
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• 2004

In this study we decompose the 3-dimensional velocity field of isotropic turbulent flow into the coherent and the incoherent structure using the discrete wavelet. It is shown that the coherent structure, 3% wavelet modes, has 98% energy and 88% enstrophy and its statistical characteristics are almost same as the original turbulence structure. And it is confirmed that the role of the coherent structure is that it produces the turbulent kinetic energy at the inertia range then transfers energy to the dissipation range. The incoherent structure, with residual wavelet modes, is uncorrelated and has the Gaussian probability density function but it dissipates the kinetic energy in dissipation range. On the procedure, we propose a new but easy way to get the threshold by applying the energy partition percentage concept about coherent structure. The vorticity field extracted from the wavelet-decomposed velocity field has the same structure as the result of the precedent studies which decomposed vorticity field directly using wavelet. Therefore it has been shown that velocity and vorticity field are on the interactive condition.

• Journal of Advanced Marine Engineering and Technology
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• v.23 no.6
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• pp.770-777
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• 1999

In this study a phase diagram has been used to investigate the unsteadiness of two-dimensional lid-driven closed flows within a square cavity for twelve Reynolds numbers; $7.5{\times}10^3,\; 8{\times}10^3,\; 8.5{\times}10^3,\; 9{\times}10^3,\; 9.5{\times}10^3,\; 10^4,\;1.5{\times}10^4,\;2{\times}10^4,\; 3{\times}10^4,\; 7.5{\times}10^4$ and $10^5$. The results indicate that the first critical Reynolds number at which the flow unsteadiness of sinusoidal fluctuation appears from the temporal variation of total kinetic energy curves is assumed of sinusoidal fluctuation appears form the temporal variation of total kinetic energy curves is assumed to be in the neigh-bourhood of $Re=8.5{\times}10^3$ The second critical Reynolds number where the periodic amplitude and frequency collapse to random disturbance being existed around $Re=1.5{\times}10^4$ The exponentially decreasing vortices formed at the lower two corners are found commonly at the time-mean flow pattern of $Re=3{\times}10^4$.

• Laser Solutions
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• v.16 no.2
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• pp.1-6
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• 2013

Picosecond lasers are a very effective tool for micromachining metals, especially when high accuracy, high surface roughness and no heat affected zone are required. However, low productivity has been a limit to broadening the spectrum of their industrial applications. Recently it was reported that in the micromachining of copper with a 1064nm picosecond laser, there exist the optimal pulse energy and repetition rate to achieve the maximum volume ablation rate. In this paper, we used a 515nm picosecond laser, which is more efficient for micromachining copper in terms of laser energy absorption, to obtain its optimal pulse energy and repetition rate. Theoretical analysis based on the experimental data on copper ablation showed that using a 515nm picosecond laser instead of a 1064nm picosecond laser is more favorable in that the calculated threshold fluence is 75% lower and optical penetration depth is 50% deeper.

• Journal of the Korean Vacuum Society
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• v.6 no.3
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• pp.263-266
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• 1997

We studied an interfacial characteristics of $In_{0.1}Ga_{0.9}As$/ GaAs by photoreflectance (PR) measurement at room temperature. With increasing thickness of epitaxial layer, Franz-Keldysh oscillation (FKO) periods of PR signals were decreased, and interfacial electric field was decreased. This can be explained by the increases of defects due to lattice mismatch near the heterointerface between InGaAs and GaAs. For the thickness of epitaxial layer thinner than the 300$\AA$, InGaAs epitazial layer closed to critical thickness and increased strain, and then the bandgap energy shifted high greatly.

• Composites Research
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• v.13 no.6
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• pp.9-22
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• 2000

The Rayleigh-Ritz procedure based on energy method is used to present analytically the natural frequencies and the critical buckling loads for four types of loading conditions: (1) uniaxial, (2) biaxial, (3) positive shear and (4) negative shear, of the rectangular, composite plates unidirectionally stiffened with box beam type stiffeners. In analysis the discrete stiffener theory is adopted to present the effect of stiffeners in the plate structure. The convergence study is presented to demonstrate the accuracy of the results. Contour plots of the vibrated and buckled mode shapes are shown for some examples. The effect of various parameters such as numbers, position, aspect ratio of stiffener and layer angle, aspect ratio of plate are focused.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.10a
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• pp.274-279
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• 2004

Currently, carbon-fiber reinforced plastics(CFRP) are widely used in both space and civil aircraft due to their superior stiffness and strength to weight ratios compared to conventional metallic materials. This paper is to study the effects of curvature and stacking sequence on the penetration characteristics of composite laminated shell. And were performed to investigate the penetration characteristics of composite laminated shells by the oblique impact. They are stacked to [0$_3$/90$_3$]s, [90$_3$/0$_3$]s and [0$_2$/90$_3$/0]s, [90$_2$/0$_3$/90]s their interlaminar number two and fore. They are manufactured to varied curvature radius (R=100, 150, 200mm and $\infty$). When the specimen is subjected to transverse impact by a steel ball, the velocity of the steel ball was measured both before and after impact by determining the time for it to pass two ballistics-screen sensor located a known distance apart. In general, the critical penetration energy interface decrease and slope angle on the impact surface increased. [0$_3$/90$_3$]s and [0$_2$/90$_3$]s specimens higher than [90$_3$/0$_3$]s and [90$_2$/0$_3$/90]s specimens.