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35 results on '"Choi, Ji-Hwan"'

31. Influence of AC-DC-DC converter on radial/axial flux permanent magnet wind power generators with mechanical energy storage system.

Author

Park, Yu-Seop, Jang, Seok-Myeong, Choi, Jang-Young, Choi, Ji-Hwan, and You, Dae-Joon

Abstract

This paper deals with the influence of AC-DC-DC converter on permanent magnet (PM) wind power generators. Differently from AC load conditions, since the phase current in PM generators with the converter contains harmonic components, their influence on generators' performance is investigated. In particular, in this paper, two PM generator types, radial flux PM generator and axial flux PM generator, are considered for the comparative investigation. The generators are connected to a mechanical energy storage system consisted of a permanent magnet synchronous motor with a heavy wheel, and the generator performance characteristics are dealt with while the mechanical energy storage system is being operated. [ABSTRACT FROM PUBLISHER]

Published
2013
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34. Atomic Layer Deposition of Ta-doped TiO2Electrodes for Dye-Sensitized Solar Cells

Author

Choi, Ji-Hwan, Kwon, Se-Hun, Jeong, Young-Keun, Kim, Il, and Kim, Kwang-Ho

Abstract

Ta-doped TiO2inverse opals were obtained by selective etching of a silica template after atomic layer deposition (ALD) of Ta-doped TiO2films and were applied as an electrode for dye-sensitized solar cells (DSSCs). Ta content in the Ta-doped TiO2film was controlled by the Ta/(Ta+Ti) unitcycle ratios in the Ta–TiO2supercycle of ALD. Also, excellent step coverage of nearly 100% in the inverse opal structure was confirmed by field-emission scanning electron microscopy (FE-SEM). Maximum photoconversion efficiency of 1.56% was achieved with the Ta (3.4 atom %)-doped TiO2inverse opal electrode due to increased photocurrent density. However, further Ta doping (> 4.9 atom %) decreased the Jscand photoconversion efficiency.

Published
2011
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35. The Effects of Pretreatment, CH4Gas Ratio and Bias Potential on the Microstructure of Microwave Plasma Enhanced Chemical Vapor Deposited Diamond Thin Films

Author

Park, Jong-Wan, Lee, You-Kee, Lee, Sei-Hyun, Cho, Kwang-Joon, Lee, Jae-Suk, Lee, Seoghyeong, Hyeongtag Jeon, Hyeongtag Jeon, and Ji-Hwan Choi, Ji-Hwan Choi

Abstract

Diamond thin films were deposited on p-type Si(100) wafers by microwave plasma enhanced chemical vapor deposition (MPECVD) using mixtures of H2, CH4and O2gases. Deposition was carried out at a substrate temperature of 900°C and a pressure of 40 Torr. In order to examine the effect of pretreatment on the film formation, SiC and diamond powders were used as pretreatment powders. Also, the methane gas ratio in the gas mixture was varied to observe its effect on the crystal structure of diamond films. Furthermore, the substrate bias effect on the diamond film formation was examined. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy were utilized to characterize the microstructures of diamond thin films. The highest nucleus density was obtained by pretreatment with diamond powders of 40mbox-60 µm in size and a negative bias potential to the substrates. The deposited diamond films were found to contain many defects. The density of the defects increased with CH4concentration in the gas mixtures. It is believed that the gas mixture with a high CH4ratio must have promoted the formation of a graphite phase, probably due to the insufficient amount of atomic hydrogen acting as a selective etchant for graphite. Diamond films with good quality and a nucleus density of 108/cm2could be deposited by utilizing the combined effects of a negative bias potential and a magnetic field.

Published
1997
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