Browsing by Author "Koo, HS"

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Catalyzed growth model of carbon nanotubes by microwave plasma chemical vapor deposition using CH4 and CO2 gas mixtures
(2003-10-01)
Chen, M
;
Chen, CM
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Koo, HS
;
Chen, CF
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材料科學與工程學系
Catalyst growth carbon nanotubes (CNTs) have been synthesized successfully by microwave plasma chemical vapor deposition using CH4-CO2 gas sources, and Fe, Ti, Fe/Ti as catalysts. Significant difference of morphology in the carbon deposition was observed between Fe and Ti catalyst. By adjusting growth parameters of CH4 to CO2, a high yield of vertically aligned CNTs can be found in an Fe-deposited substrate. Ti is shown to be not suitable as a catalyst in CNT production. In the present work, we investigated the effect of H-2 plasma pretreatment on the CNT growth from the viewpoint of catalyst morphology, using Fe as the catalyst. After the H-2 pretreatment, significant catalyst particle sintering was observed and resulted in a broad size distribution of catalyst particles. The diameter of CNTs was governed by the catalyst particle size. The diameter of CNTs thus increased as the H-2 plasma pretreated time increased. The CNT diameters were distributed in the range approximately 10-20 nm when Fe-deposited substrate was not pretreated. However, the diameter of CNTs increased from 30 to 300 nm when Fe-deposited substrate was pretreated from 1 to 15 min. The CNT growth model in catalysts, as a function of a gas environment of CH4-CO2 gas mixture, was investigated. (C) 2003 Elsevier Science B.V. All rights reserved.
Fabrication and microstructure of the dc-magnetron-sputtered YBa2Cu3O7-x superconducting thin films
(1996-04-01)
Koo, HS
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Tseng, TY
;
Chang, WR
;
Tu, GC
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材料科學與工程學系
Systematic investigations were conducted to determine the effect of deposition conditions on the microstructure and superconducting properties of YBa2Cu3O7-x thin films produced by d.c. magnetron sputtering on (001) MgO substrate. The films were c-axis preferentially oriented with respect to the (001) MgO surface at substrate temperatures of 680-700 degrees C. X-ray diffraction patterns clearly indicated the existence of the c-axis alignment normal to the substrate surface and some second phases. The second phases, including a Cu-rich phase and Y2O3, were identified by energy dispersive X-ray spectrometer and the microstructures were analysed by electron and;atomic force microscopes.
Formation behavior of high-T-c Tl-based superconductors
(1999-02-01)
Koo, HS
;
Lo, JR
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Tseng, TY
;
Lin, CS
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電子工程學系及電子研究所
The effects of Ca, Cu, and Ca-Cu richness on the formation behavior and physical characteristics of high-T-c Tl-based superconductors were investigated systematically. In addition to sintering temperature and time, intermediate phases which proved important to the formation and properties of nearly single-phase, triple-CuO-layers Tl2Ba2Ca2Cu3Oy superconductors and optimal processing conditions were also investigated. Ca and Ca-Cu richness introduced positive effects on the properties of nearly single-phase Tl2Ba2Ca2Cu3Oy superconductors and helps in the formation of the appropriate intermediate phases, whereas Cu richness produced a negative effect. The higher sintering temperatures of the specimens accounted for the tailing of electrical resistance before the critical zero resistance temperature was approached, and for the instability of the superconducting phase. Lower post-annealing temperatures of the bulk samples slightly improved the superconducting properties. The resultant bulk Tl-based superconductors can be used as a Tl vapor source for the formation of superconducting thin films within a closed system.
Preparation and characteristics of Bi-Pb-Sr-Ca-Cu-O superconducting films by spray pyrolysis and post-annealing
(1998-10-15)
Koo, HS
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Tseng, TY
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Two kinds of high-T-c superconducting films, Bi1.8Pb0.2Sr2Ca2Cu3Oy and Bi2PbSr2Ca2Cu3Oy, were prepared by spray pyrolysis and postannealing with the aid of Bi-based bulk. Highly c-oriented and nearly single-phase 2223 high-T-c Bi-based superconducting films can be achieved for the as-sprayed Bi1.8Pb0.2Sr2Ca2Cu3Oy and the Bi2PbSr2Ca2Cu3Oy films annealed at 845 degrees C for 20 h. X-ray diffraction (XRD) patterns exhibit amorphous-like phases with microcrystals for the as-sprayed films and the transformation of amorphous-like phase into crystalline high-T-c (2223), low-T-c (2212) and Ca2PbO4 phases for the resultant films. For as-sprayed films, post-annealing in the closed system with Bi-based bulk sources is favourable to the formation of 2223 phase; on the contrary, the 2212 phase is formed in the open system, as demonstrated by XRD and scanning electron microscopy (SEM). SEM and XRD analyses show a plate-like microstructure for the 2223 phase and a needle-like one for the 2212 phase. This phenomenon explains that compensation and diffusion of Bi-based bulk sources into the as-sprayed films help in the formation of 2223 phase and these processes are more efficient in the closed system. The resultant film shows a high critical transition temperature (T-c,T-zero) of 104 K and a critical current density (J(c)) of 1.8 x 10(4) A/cm(2). (C) 1998 Elsevier Science S.A. All rights reserved.
Promotion of phase transformation and single-phase formation in silver-doped Tl-Ba-Ca-Cu-O superconducting thin films
(1999-05-01)
Koo, HS
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Tseng, TY
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Phase transformation and physical characteristics of the spray-pyrolyzed Tl-Ba-Ca-Cu-O superconducting films with 3 mol% silver dopant have been studied using electrical resistivity, x-ray diffraction, and scanning electron microscopy. The major phase formed in the resultant film, annealed at a temperature of 885 degrees C for 3 min, was found to be the nearly single-phase, high-T-c, Tl2Ba2Ca2Cu3Oy (Tl-2223), The multiple phase of Tl2Ba2Ca2Cu3Oy and TlBa2Ca2Cu3Oy (Tl-1223) appeared at annealing temperatures lower and higher than 885 degrees C. It was also observed that Ag dopant effectively reduces normal resistivity at 300 K and enhances the phase transformation of single-rrl-layer Tl-1223 to double-Tl-layer Tl-2223 phases and further helps to form the nearly single-phase Tl-2223 within a short duration. Critical transition temperature (T-c,T-zero) and current density (J(c), 77 K, 0 Tesla) of the best resultant film were shown to be 123 K and 5.7 X 10(4) A/cm(2), respectively.