• Journal of the Korean Vacuum Society
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• v.16 no.3
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• pp.210-214
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• 2007

We have studied optical transitions of Gd-implanted GaN epilayers. Photoluminescence transition intensity at 590 nm at T=5 K diminishes and its center position moves to short avelength (blue shift) with increasing temperature up to 200 K. Above T=200 K, the transition intensity increases with increasing temperature while the center position remains the same. We believe that such anomalous optical transition behavior is due to the effect of rare-element in the semiconductor host material and lattice imperfection which was occurred during the implantation process well as.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.11
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• pp.999-1005
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• 2004

Thermal convection between two horizontal walls kept at small amplitude nonuniform temperatures of the form, $T_L=T_1+a{\Delta}T$ sin kx and $T_U=T_2+b{\Delta}T\;sin(kx-{\beta})$ with a, $b{\ll}1$, is numerically investigated. When the Rayleigh number is small, an upright cell is formed between two walls at ${\beta}=0$; the cell is tilted at ${\beta}={\pi}/2$, and a flow with two-tier-structure cells occurs at ${\beta}={\pi}$. As the Rayleigh number is increased, Nusselt number increases smoothly for ${\beta}=0\;and\;{\pi}/2$, but increases rather steeply for ${\beta}={\pi}$ near the critical Rayleigh number ($Ra_c=1708$). When the wave number is small (k=0.5), multicellular convection occurs over one wave length, for all phase differences, and multiple solutions are found.

• Journal of the Korean Vacuum Society
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• v.3 no.1
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• pp.51-58
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• 1994

확산 현상은 박막성장 과정 및 박막의 기계, 전기, 자기적 성질 이해에 중요한 역할을 한다. 열 처리에 의한 상호확산 때문에 생긴 Pd/Cu 다층박막의 조성변화를 AES depth-profiling 방법을 이용해 서 조사하였다. 열처리전 시료에서의 각형의 초기 조성분포가 여러온도에서의 열처리에 의해 정현파 모 양의조성분포로 변화되었다. 조성의존성을 고려하지 않은 상호확산 계수를 정현파 분포의 진폭으로부터 구하였으며, 1.66 eV의 값을 갖는 활성화에너지는 Arrhenius plot으로부터 산출하였다. 또한 Boltzmann-Matano 방법을 사용해서 15$0^{\circ}C$에서의 조성의존 상호확산계수도 구하였다. 열처리에 의해 조 성균일화가 되는 것으로만 알려졌던 본물질계에서도 상분리가 생성됨을 관찰하였고, 그 열처리 조건은 $180^{\circ}C$에서 150분 보다 짧아야함을 밝혔다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.6
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• pp.1319-1327
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• 1994

Since fatigue cracks in rail can be the source of fractures and subsequent derailments, quantitative evaluation of the fatigue behavior and fracture properities due to the analysis results of laboratory test are drawn on the basis for predicting fatigue life and making a decision of safe inspection interval. Charpy V-notch and fracture toughness behavior were evaluated from the results of Charpy impact test. Fatigue test was performed by using CT type specimen under constant amplitude loading, and finally the effects of the following parameters; crack orientation, temperature, and stress ratio, on the fatigue crack growth behavior were studied.

• Proceedings of the Korean Society for Emotion and Sensibility Conference
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• 1999.11a
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• pp.433-437
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• 1999

본 연구는 도로표면의 기복 또는 굴곡의 변화가 자동차 탑승자에게 미치는 자율신경계의 반응을 관찰하고자 하였다. 아스팔트, 시멘트, 비포장도로를 각각 30 km/h의 속력으로 정속 주행하면서 ECG, GSR, Skin Temperature 의 생리변화를 3분간 측정하였고, 주행 후에는 각각의 도로 주행시 느꼈던 감성의 변화를 주관적 평가지로 검정하였다. 건강한 5명의 지원자가 이 실험에 참여하였고 도로표면의 기복의 변화로부터 유발되는 감성에만 집중하도록 요구하여 다른 간섭효과로부터 유발되는 감성의 변화를 최소화하도록 하였다. 정차에 비해 각 도로 주행시 피험자는 아스팔트, 시멘트, 비포장도로 순서로 불쾌도와 긴장도가 증가하였다고 주관적 평가를 하였다. 또한 아스팔트, 시멘트, 비포장도로 순서로 평균 R-R 간격이 점차 감소하였고, GSR의 진폭은 증가하였으며, 피부온도는 감소하였다. 본 연구로부터 도로표면의 기복의 정도가 증가할수록 교감신경계가 활성화된다는 사실을 관찰할 수 있었고 이러한 결과는 주관적 평가결과와도 일치하였다.

• Applied Chemistry for Engineering
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• v.8 no.1
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• pp.1-7
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• 1997

We show that the usage of the cyclic pressure function in the freeze drying is more effective than the drying rate of the constant pressure process. The ultimate object of this process is to reduce energy costs by shortening the drying time. In this work, the concept of cyclic pressure function to decrease the drying time and the effects of the following factors on drying rate are considered ; pulse type, size in amplitude of pressure, size of intermediate time. Temperature for drying has always been maintained constantly at $20^{\circ}C$ and the pulse type could have found to give useful results than the constant pressure by altering chamber pressure from low pressure to high pressure Also, when the amplitude of the pulse is 0.4 torr and the intermediate time is six hour the drying rate was increased considerably.

• Composites Research
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• v.17 no.6
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• pp.28-36
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• 2004

Micro-tow deformation during forming of PVC foam-fabric composite sandwich structure is investigated to find out the correlation between forming condition and material deformation. The foams used in this research are PVC foams which have 4 different densities and the fabric composite is Carbon/epoxy prepreg which is plain weave (3k) as a skin material. Tow parameters such as crimp angle and tow amplitude are measured using microscope and a proper image tool and are compared with each other. In order to find out the effect of foam deformation during forming on tow deformation the compressive tests of foams are performed in three different environmental temperatures ($25^{\circ}C$, $80{\circ}C$, $125^{\circ}C$). The microscopic observation results show that the micro tow deformations are quite different from each other with respect to the foam density and forming pressure.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.7
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• pp.911-917
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• 2010

In this study, we performed structural and fatigue analyses of the engine exhaust manifold that was subjected to thermo-mechanical cyclic loading. The methodologies used in this study are based on an approach in which the techniques for modeling the exhaust system, the temperature-dependent properties of the material, and thermal cyclic loading are taken into consideration and a reliable strategy is adopted for failure prediction. An application example shows that at an elevated temperature, considerable compressive plastic deformation is observed and that at a low temperature, tensile stresses remain in those parts of the test exhaust manifold where failure is observed. In order to predict fatigue life, mechanical damage is determined on the basis of the stress.strain hysteresis loops by using the classical Coffin.Manson equation and by adopting a method in which the dissipated plastic energy is taken into consideration.

• Composites Research
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• v.32 no.1
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• pp.78-84
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• 2019

This study focused on the prediction of the long-term performance of carbon fiber/epoxy composites using Dynamic Mechanical Analysis (DMA) and Time-Temperature Superposition (TTS). Single-frequency test, multi-frequency test, and creep TTS test were performed. A sinusoidal load of $20{\mu}m$ amplitude was applied while increasing the temperature from $-30^{\circ}C$ to $240^{\circ}C$ at $2^{\circ}C/min$ for the single-frequency test and the multi-frequency test. The frequencies applied to the multi-frequency test were 0.316, 1, 3.16, 10 and 31.6 Hz. In the creep TTS test, a stress of 15 MPa was applied for 10 minutes at every $10^{\circ}C$ from $-30^{\circ}C$ to $230^{\circ}C$. The glass transition temperature was determined by single-frequency test. The activation energy and the storage modulus curve for each temperature were obtained from glass transition temperature for each frequency by the multi-frequency test. The master curve for the reference temperature was obtained by applying the shift factor using the Arrhenius equation. Also, TTS test was used to obtain the creep compliance curves for each temperature and the master curve for the reference temperature by applying the shift factors using the manual shift technique. The master curve obtained through this process can be applied to predict the long-term performance of carbon fiber/epoxy composites for a given environmental condition.

• Journal of the Korean earth science society
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• v.37 no.7
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• pp.389-397
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• 2016

In order to investigate the light curve difference in visual and infrared wavelength of ${\delta}$ Scuti variable Bo Lyn, observations were performed using BOAO 1.8m reflecting telescope and an infrared detector, KASINICS, with J, H, and Ks filters. Infrared light curves of total 7 nights were obtained between March and April in 2011, and those were compared to the V-filter light curve to examine the differences in period, time of maximum light, amplitude, and shape. From the periodic analysis of infrared light curve, a single frequency of $f_1=10.712cycle/day$, $P=0.09335{\pm}0.00002days$ was obtained, and there was no difference in the period along different wavelengths. In the infrared light curve, a frequency of $2f_1$ was detected. This frequency well explains the asymmetric shape of light curve, one of the characteristics of high-amplitude ${\delta}$ Scuti variables. We compared the locations of the measured infrared maxima and the predicted maxima of V-filter, finding that the times of maxima were delayed about 0.3 phase at infrared wavelengths. Amplitude ratios were adopted to be ${\Delta}J/{\Delta}V=0.328$, ${\Delta}H/{\Delta}V=0.216$, and ${\Delta}Ks/{\Delta}V=0.211$, with the range of variation being smaller at longer wavelengths. It seems that the differences in the times of maxima and amplitude occurred because the changes in brightness of a pulsating variable star are mainly caused by the change in temperature.