• Journal of Hydrogen and New Energy
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• v.20 no.4
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• pp.317-322
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• 2009

To investigate degradation mechanism of PEMFC operated with repetitive startup/shutdown cycling, i-V characteristics, impedance, cyclic voltamograms were measured. OCV decreased from 0.967 to 0.951 V while the cell voltage at 800mA/$cm^2$ from 0.657 to 0.563V, implying that the electrodes rather than membrane electrolyte was damaged during the cycling operation. Electrochemical analyses supported that the performance degradation could be mainly attributed to degradation of the electrodes such as a decrease in electrochemical active surface area rather than degradation of membrane.

• Journal of Hydrogen and New Energy
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• v.14 no.4
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• pp.291-297
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• 2003

As an alternative bipolar plate material for polymer electrolyte membrane fuel cell (PEMFC), TiN-coated 316 stainless was evaluated in terms of electrical contact resistance and water contact angle. Performance and lifetime of the TiN-coated 316 bipolar plates were measured in comparison with those of graphite and bare 316 bipolar plates. At a cell voltage of 0.6 V, current density of the single cells using graphite, AISI 316, and TiN/316 bipolar plates was 996, 796, and $896mA/cm^2$, respectively. By coating 316 stainless steel with TiN layer, performance degradation rate determined to be the voltage degradation rate at a cell voltage of 0.6 V was reduced from 2.3 to 0.43 mV/h.

• Proceedings of the Korean Magnestics Society Conference
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• 2012.11a
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• pp.24-25
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• 2012

• Journal of Hydrogen and New Energy
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• v.28 no.5
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• pp.465-470
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• 2017

Sulfonated fluorinated block copolymers having diphenyl units were mixed with the sulfonated cationic conductive polymers at an optimum mixing ratio to form hybrid membranes for fuel cells and their characteristics were studied. 2D and 3D AFM topology analysis confirmed that the number of hydrophilic units in the hybrid membrane was improved. Through the FE-SEM, the microstructure of the hybrid membrane implied hydrogen bonding and pi-pi interactions, and EDAX confirmed carbon, oxygen, sulfur, and fluorine. The thermogravimetric analysis showed that the hybrid membrane was thermally stable and the hydrophilicity of the hybrid membrane was increased by the contact angle of water droplets. As a result, it was confirmed that the cation conductivity increased by a factor of 1.8 times as the number of acidic domains in the hybrid film increased.

• Journal of Magnetics
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• v.19 no.2
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• pp.116-120
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• 2014

The spin-lattice relaxation time, $T_1$, for $^{69}Ga$, $^{71}Ga$, and $^{75}As$ nuclei in GaAs:$Mn^{2+}$ single crystals was measured as a function of temperature. The values of $T_1$ for $^{69}Ga$, $^{71}Ga$, and $^{75}As$ nuclei were found to decrease with increasing temperature. The $T_1$ values in GaAs:$Mn^{2+}$ crystal are similar to those in pure GaAs crystal. The calculated activation energies for the $^{69}Ga$, $^{71}Ga$, and $^{75}As$ nuclei are 4.34, 4.07, and 3.99 kJ/mol. It turns out that the paramagnetic impurity effect of $Mn^{2+}$ ion doped in GaAs single crystal was not strong on the spin-lattice relaxation time.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1997.11a
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• pp.32-32
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• 1997

복합재 연소관의 구조적 건전성 평가 방법을 개발하기 위하여 표준 평가용 시편의 수압 시험 중, 연소관으로부터 방출되는 음향방출(acoustic emission: AE) 신호를 측정 장비를 이용하여 신호 처리함으로써 시편이 파괴되기 전, 대략적인 시편의 파열 위치 및 일정 압력 유지상태에서 hit rate 양상에 의한 건전성 평가 방법을 제시하였다. 데이터의 분석 기법 향상을 위하여 음향방출 데이터를 시간 및 센서별로 분류하는 프로그램을 개발하였다. 수압 시험을 낮은 압력 단계부터 높은 압력 단계까지 일정 압력 유지 상태(load-hold)에서 발생한 hit 수와 1분간 유지한 후 발생되는 hit rate(hit/sec)의 값의 크고 작음이 복합재 압력 용기의 결함수의 증감으로 나타났으며, 이로부터 복합재 압력 용기의 파열 압력과의 상관관계 및 건전성을 예측할 수 있었다. 복합재 압력 용기의 파열 위치는 energy rate(energy/sec) 측정값을 분석하여 예측하였으며, 파열 압력의 30∼50%에서 가능하였다.

• Journal of Hydrogen and New Energy
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• v.21 no.5
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• pp.398-404
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• 2010

Process conditions for MEA fabrications have significant effects on properties and performance of the MEAs for PEMFCs. In this study, effects of additives on the surface properties of the MEA was investigated to improve homogeneity of the coated catalyst layer. Another parameter that affects on characteristics of the MEAs is hot-pressing condition. Hot pressing condition was optimized by using DOE (design of experiment) method.

• Annual Conference of KIPS
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• 2001.10a
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• pp.117-120
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• 2001

화석 연료의 환경오염, 지구온난화현상 그리고 70년대 발생했던 두 차례의 오일 쇼크 등으로 대체 에너지에 대한 중요성은 날로 커지고 있다. 이에 따라 최근에는 무한정 에너지를 얻을 수 있는 태양에너지에 대한 연구가 활발하게 이루어지고 있다. 그러나 현재 사용되는 대체 에너지 관련 데이터베이스 시스템은 데이터의 이력, 공간 데이터 그리고 데이터가 갖는 속성 데이터 등을 동시에 처리한 수 있는 능력이 부족하다. 따라서 이 논문에서는 전국 20개 도시에 대한 엑셀 파일 포맷의 대체 에너지 관련 데이터를 효과적으로 분석하여 데이터베이스를 설계한다. 또한 GIS 도구를 활용하여 인터넷으로 보여 줌으로써 사용자는 쉽게 웹 브라우저를 통하여 지도 서비스와 함께 대체 에너지 데이터를 분석 할 수 있다.

• ETRI Journal
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• v.45 no.4
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• pp.678-689
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• 2023

This paper describes a novel end-to-end deep generative model-based speaker recognition system using prosodic features. The usefulness of variational autoencoders (VAE) in learning the speaker-specific prosody representations for the speaker recognition task is examined herein for the first time. The speech signal is first automatically segmented into syllable-like units using vowel onset points (VOP) and energy valleys. Prosodic features, such as the dynamics of duration, energy, and fundamental frequency (F₀), are then extracted at the syllable level and used to train/adapt a speaker-dependent VAE from a universal VAE. The initial comparative studies on VAEs and traditional autoencoders (AE) suggest that the former can efficiently learn speaker representations. Investigations on the impact of gender information in speaker recognition also point out that gender-dependent impostor banks lead to higher accuracies. Finally, the evaluation on the NIST SRE 2010 dataset demonstrates the usefulness of the proposed approach for speaker recognition.

• Journal of Nutrition and Health
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• v.38 no.10
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• pp.847-855
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• 2005

This study investigated and compared the daily activities and energy expenditure of normally-weighted and obese elementary school children. The participants were 115 boys and 115 girls (6th grade) at ten elementary schools in Busan. Time spent on each activity was evaluated using the twenty-four hour recall method. 1) The general characteristics of the normally-weighted and obese children did not differ, although the normally-weighted children exercised and used stairs more than the obese children.2) Among their classified activities, the children spent most of their time resting, study, leisure, physiology and hygiene, commuting, and housework in that decreasing order. Normally-weighted children spent less time tying down and in conversation with family and friends than obese ones. 3) The children's average activity factors were 1.47 - 1.50. The normally-weighted children's rest energy expenditure (REE) (1391.4 kcal,1264.7 kcal) was less than that of the obese children (1711.4 kcal. 1461.0 kcal) . The normally-weighted children's daily energy expenditure was less than that of the obese, but energy expenditure per body weight was less in obese children than in the normally-weighted. In conclusion, sedentary activities and energy expenditure per body weight was less in the obese compared to the normally-weighted children. Programs to help children perceive and practice desirable physical activities should be suggested for prevention of obesity in children. (Korean J Nutrition 38(10): 847$\sim$855,2005)