Preparation, Structure, and Luminescent Property in Powder Phosphors of Yttrium/Terbium Aluminum Garnets Doping with Cerium, Terbium, and Magnesium Ions

• Main Authors: Hsuan-min Lee, 李玄閔
• Other Authors: Chi-yuen Huang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/32949853089718894893

博士 === 國立成功大學 === 資源工程學系碩博士班 === 97 === Blue light emitting diode (LED) chip coated with a yellow yttrium aluminum garnet doped with cerium ions (YAG:Ce) phosphor has led to the commercial production of white LED. The advantages of good reliability, simple preparation, low cost and so on are the reasons why this YAG product has been popular on the market for a long time. This dissertation not only investigates the spherical YAG:Ce and YAG:Tb (terbium) phosphors granulated by a spray-drying method, but also studies the host transition from YAG to TbAG (terbium aluminum garnet) synthesized with the maximum replacement of yttrium ions in YAG. The effects of dopant concentration, temperature and atmosphere used in the synthesis and so forth on structure and property of the system are mainly studied in the dissertation as follows: 1. Pure YAG:Ce can be successfully synthesized without second phase through the pH control. The flux, sodium hydroxide (NaOH) solution, actually acts as an important role. In addition, the spherical particles granulated with the spray-drying method produce the homogeneous agglomerate of phosphors and consequently achieve more favorable photoluminescence (PL) properties in this study. 2. The factors, such as activator concentration, phase impurity, calcination term, and powder crystallinity dominate the PL properties of phosphors. Most of these products (YAG:Ce) are identified as YAG and CeO2 phases after calcination, showing higher emission intensity resulted from higher crystallinity and less defect of phosphor (calcined at 1500oC/8 h). On the contrary, the product with CeO2 is found to have lower emission intensity (Ce3+ doping content: 2.50 at.%), and therefore decrease the reliability and goodness of fit when refining its atomic parameters. 3. Tb3+ ions have a strong trend to transform into Tb4+ ions under an oxidizing environment, and Tb4+ is induced and obtained by incorporating magnesium (Mg2+) ions into YAG host in the study. MgO co-doping restrains the grain growth behavior than single-doped YAG:Tb phosphors. Furthermore, the decrease of PL intensity is mainly caused by Tb4+ ions and the defect of microstructure of phosphor, and the co-existence of Mg2+ and Tb4+ in the YAG matrix also results in some changes in crystal structure. 4. TbAG is the complete solid solution with the maximum replacement for Y ion in YAG. The Tb3+ concentration quenchings of PL and cathodoluminescence (CL) emission intensities with higher Tb doping levels are shown, and the suitable Tb3+ concentration is between 15 ~ 20 at.% in this work. The Tb3+ concentration is the most important factor in YAG/TbAG system in the study, influencing the microstructure of powder, reflective and fluorescent properties, energy transfer of excitation light, and crystal structure. 5. Theta-Al2O3 is replaced by alpha-Al2O3 as one of the starting materials in the system. In addition, NaOH solution is selected for the synthesis as comparing with hydrochloric acid (HCl). The crystal field splitting of Ce3+ in YAG/TbAG host and the lowering of Ce3+-O2- bond lengths are correlated with luminescent properties in this study. PL excitation (PLE) and PL spectra, and chromaticity diagram of Ce3+-garnet shift to a longer wavelength while the structures expand with the increasing content of Tb3+ ions. The Tb3+ ions (or TbAG) not only act as a non-radiative role, but bring the concentration quenching in the (Y/Tb)AG:Ce material. 6. The absorption and luminescence spectra of compounds in YAG-TbAG solid solution synthesized in a N2/H2 (nitrogen/hydrogen) are significantly different from those synthesized in air. The oxidation of Tb from 3+ to 4+ when synthesized in air is the key factor in the variation of spectra, and it also results in smaller lattice constants for the Tb-garnets. The inhomogeneous broadening effect obviously occurs in the phosphors synthesized in N2/H2, but it disappears in the air-prepared system. The concentration quenching is still found because of an excess of Tb3+ doping, at more than 20 at.%, in the system.