Browsing by Author "Lo, JR"

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Effect of LiCl on the crystallization behavior and luminescence of Y3Al5O12 : Tb
(1998-11-01)
Lo, JR
;
Tseng, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The single-phase cubic Y3Al5O12 : 1% Tb sol-gel-derived powders were prepared after firing at 650 degrees C for 2 h when a LiCl flux was used. The addition of the LiCl can increase the crystalline size of the powders at low temperatures. By using the broadening effect of X-ray patterns, the estimated crystalline size of the powder with 20 wt.% LiCl addition fired at 650 degrees C for 2 h was evaluated as 480 A. Both emission and excitation luminescent spectra of the samples were measured. Emission spectra of the synthesized powders mainly show D-5(4)-F-6 transition under 275 nm excitation. The excitation spectra of The Tb3+ ions are different between amorphous and crystalline phase because of the crystal field effect. The excitation spectra also help observing the degree of the crystallinity of the resulting YAG phase. (C) 1998 Elsevier Science S.A. All rights reserved.
Electron paramagnetic resonance and luminescence study of sol-gel derived YAG : Cr powder
(1999-03-01)
Lo, JR
;
Ezhilvalavan, S
;
Tseng, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Electron paramagnetic resonance (EPR) and luminescence studies were carried out for the sol-gel-derived YAG:Cr powders subjected to three kinds of heat treatment processes. The results show that powders processed using the two-step heat treatment process with the addition of 20 wt% LiCl and by direct heat treatment at 1500 degrees C exhibit good luminescence properties. The emission spectrum for the powders obtained by the two-step process shows the R-line at 688 nm, whereas the powders directly heat treated at 1500 degrees C display an additional R-line of Al2O3 : Cr at 694 nm: EPR measurements show that the presence or absence of absorption peaks depends on the Cr3+ environment in-the host lattice resulting from the different methods of heat treatment of the powder. The EPR spectra of YAG:Cr3+ phosphors were interpreted to indicate both Cr3+ single ion and Cr3+-Cr3+ pair (with spin three) transitions.
Enhanced light output in InGaN-based light-emitting diodes with onmidirectional one-dimensional photonic crystals
(2006-03-01)
Lin, CH
;
Tsai, JY
;
Kao, CC
;
Kuo, HC
;
Yu, CC
We have Successfully designed and fabricated GaN-based light-emitting diodes (LEDs) containing an omnidirectional one-dimensional photonic crystal (ID PC). The ID PC is composed of alternatively stacked TiO2 and SiO2 layers designed to possess a photonic band gap (PBG) within the blue regime of interest. With the same multiple quantum well (MQW) emission peak at approximately 450 nm and a driving Current of 20 mA, the light output powers of the LED with and without the ID PC are approximately 11.7 and 6.5 mW, respectively. The enhancement in light extraction of our LED with ID PC demonstrates that a property designed omnidirectional ID PC can have a higher reflectance and a wider reflection angle than a conventional distributed Bragg reflector (DBR). Our work shows promising potential for the enhancement of output powers of commercial light emitting devices.
Formation behavior of high-T-c Tl-based superconductors
(1999-02-01)
Koo, HS
;
Lo, JR
;
Tseng, TY
;
Lin, CS
;
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.
Luminescent properties of sol-gel prepared Y3Al5O12:Tb thin films
(1996)
Lo, JR
;
Tseng, TY
;
Tyan, JH
;
Huang, JCM
;
交大名義發表
A series of Tb-activated Y3Al5O12 films have been fabricated by spin coating on silicon substrate with sol-gel solutions. The formation of a single garnet phase is observed at 800 degrees C for 1 h in rapid thermal annealing furnace. The emission spectrum of the films induced by ultraviolet(UV) and electron excitations show the D-5 to F-6 transition and consisted of two groups, D-5(3) to F-6(before 485nm) and D-5(4) to 6F(after 485nm). The emission intensities of the films are dependent on the Tb concentrations.
Phase development and activation energy of the Y2O3-Al2O3 system by a modified sol-gel process
(1998-09-30)
Lo, JR
;
Tseng, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The phase development of Y3Al5O12 (YAG), YAlO3 (YAP), and Y4Al2O9 (YAM) synthesized by a modified sol-gel method has been studied. A chelate agent (ethyl acetoacetate) is added to control the hydrolysis and to modify the sol-gel process. The sol-gel-derived powder is then heat-treated to induce crystallization. The X-ray diffraction method is employed to determine the crystalline phases of the resulting powders. The formation of YAG, YAP, and YAM is described. For both YAP and YAM compositions, there is a phase transformation from YAP polymorphism to perovskite YAP. The activation energies of YAG are estimated at about 69 kcal/mol by the isothermal process as fitted with the Johnson-Mehl-Avrami equation and about 222 +/- 2 kcal/mol by the continuous heating method as fitted with Kissinger and Sotgiu plots. The difference of activation energies calculated from the two methods is mainly due to the complicated crystallization of YAG. (C) 1998 Elsevier Science S.A. All rights reserved.
Phase development and activation energy of the Y2O3-Al2O3 system by a modified sol-gel process
(1998-09-30)
Lo, JR
;
Tseng, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The phase development of Y3Al5O12 (YAG), YAlO3 (YAP), and Y4Al2O9 (YAM) synthesized by a modified sol-gel method has been studied. A chelate agent (ethyl acetoacetate) is added to control the hydrolysis and to modify the sol-gel process. The sol-gel-derived powder is then heat-treated to induce crystallization. The X-ray diffraction method is employed to determine the crystalline phases of the resulting powders. The formation of YAG, YAP, and YAM is described. For both YAP and YAM compositions, there is a phase transformation from YAP polymorphism to perovskite YAP. The activation energies of YAG are estimated at about 69 kcal/mol by the isothermal process as fitted with the Johnson-Mehl-Avrami equation and about 222 +/- 2 kcal/mol by the continuous heating method as fitted with Kissinger and Sotgiu plots. The difference of activation energies calculated from the two methods is mainly due to the complicated crystallization of YAG. (C) 1998 Elsevier Science S.A. All rights reserved.