Luminescent and Structural Analysis of Nanosized Yttrium Aluminum Garnet and Barium Magnesium Aluminate Phosphors Using Synchrotron Radiation Techniques
|關鍵字:||鎂鋁酸鋇;釔鋁柘榴石;同步輻射;螢光材料;奈米;Nano;Synchrotron radiation;Barium magnesium aluminate;Yttrium aluminum garnet;Phosphor||公開日期:||2004||摘要:||隨著科技的進展，各式新穎的顯示技術成為重要的研究焦點，電漿顯示器由於具有重量輕、體積小、廣視角、高對比等優點，因而成為極具發展潛力的顯示技術。螢光材料由於具有可吸收特定能量並進而放出可見光的特性，因此成為新式平面顯示器全彩化的關鍵材料。
本論文首先以固相法與乳膠法製備銪添加之釔鋁柘榴石紅光螢光粉，所合成之螢光粉體於空氣氣氛下煆燒，可發現兩法所得之粉體其反應機制並不相同；於乳膠法系統中，可於1400˚C下獲得單相粉體且其外型近似球狀之微米級YAG: Eu3+ 粉體，且其外型與粒徑分佈均較優於固相法所得粉體，此外由紫外光及真空紫外光激發下之放射光譜可知乳膠法所得之銪摻雜釔鋁柘榴石螢光粉具有較佳之螢光特性。
為了提昇釔鋁柘榴石之螢光特性，本論文第二部份利用微乳膠製程合成奈米級銪添加之釔鋁柘榴石螢光粉體，所合成之螢光粉體則以真空紫外光激發驗證其於電漿顯示器之應用。本研究結果發現在1100˚C煆燒下即可獲得單相、球狀且分散均勻之奈米級YAG: Eu3+ 螢光粉(~30 nm)。隨著熱處理溫度之上升，其螢光強度亦隨之增強；在Eu3+ 添加量超過5%時，則可觀察到濃度消光之現象。此外，增加水相對油相的體積比率將使所得粉體之粒徑增大及球狀粉體型態消失，同時其螢光特性亦隨之降低。藉由同步輻射之X光吸收光譜，探討釔鋁柘榴石中銪離子之局部結構、電子結構、原子排列雜亂程度與長距離有序結構等特性，以瞭解活化中心周圍結構對發光特性之影響。由前述結果發現微乳膠製程為合成奈米螢光粉體之有效方式，故進一步利用微乳膠法製備螢光特性較佳之釓添加YAG: Eu3+ 螢光粉體，隨著釓添加濃度之增加，螢光粉體之型態與粒徑並無明顯之變化，然而於紫外光及真空紫外光激發下，皆可發現其螢光強度、量子效率及色度卻有顯著之增強與改善。
論文的第三部分則利用溶膠-凝膠製程合成銪添加之鎂鋁酸鋇(BAM: Eu2+)藍光螢光粉體，研究中使用金屬硝酸鹽、檸檬酸及乙二醇進行溶膠-凝膠反應，探討製程與煆燒氣氛對螢光特性之影響。BAM: Eu2+ 單相粉體可在約1400˚C下得到。研究中亦發現本製程所得之螢光粉，其螢光性質隨著煆燒氣氛之改變而有明顯的差異，未經還原氣氛處理之螢光粉體均呈現微弱的Eu3+ 紅光放射，而經還原氣氛熱處理後之螢光粉體可明顯呈現強烈之Eu2+ 藍光放射。BAM: Eu2+ 螢光粉體經由電子順磁共振光譜之分析，可發現於還原氣氛熱處理後之試樣，顯現出Eu2+ 之訊號與螢光圖譜所發現吻合；而EPR結果顯示溶膠-凝膠法合成之試樣具有較不對稱之結構。進一步於500˚C熱處理後，可發現溶膠-凝膠法合成之試樣具有較佳之穩定性，且因部分Eu2+ 氧化形成Eu3+ 而導致其螢光強度減弱，且由EPR訊號可發現，Eu2+ 於兩法合成之BAM粉體中佔據不盡相似的位置，此亦為影響螢光粉體穩定性之因素。
Human culture has developed in accordance with communication skills by voice, letter, telephone, radio, and television. TV now is not only a necessary commodity in modern life for communication and entertainment but also one of the most important human-machine interfaces in our daily life. With the progress of technology, various new displays have been developed. The plasma display panel is one of the most promising candidates for its low weight, thinner volume, large view angle, high contrast. Phosphors, which absorb certain energy and emit the energy as visible radiation, play an important role in these newly developed displays. In this thesis, the solid-state-reaction method and the emulsion process were firstly adopted to prepare the europium-doped yttrium aluminum garnet, which is an important red PDP phosphor. It is observed that the reaction routes are different for the phosphors synthesized via these two processes. Through emulsion method, phase-pure and closely spherical YAG: Eu3+ phosphors with submicron particle size were obtained after calcination at 1400˚C. The morphology and particle size distribution of emulsion-derived phosphors are superior to those obtained via solid-state reaction. From the emission spectra upon the ultra-violet and vacuum ultra-violet radiation, it is found that emulsion-derived YAG: Eu3+ exhibits better luminescent properties. The reverse-microemulsion technique was employed to prepare nanosized YAG: Eu3+. The application of the obtained YAG: Eu3+ phosphors on plasma display panels as red component was examined through the measurement of emission spectra under the excitation of VUV radiation. The synthesis temperature of YAG: Eu3+ phosphor was reduced to 1100˚C, and well-dispersed, spherical particles (~30 nm) were obtained. The luminescent intensity of YAG: Eu3+ increased with the rise of calcined temperature due to the increase of the crystallinity of the obtained powders. The concentration quenching phenomenon was observed for the samples with Eu3+ concentration higher than 5%. Moreover, increasing the water to oil ratio (W/O) volume ratio resulted in an increase in the particle size and loss in spherical morphology. The luminescent intensity also reduced with increase in the W/O volume ratio. To investigate the effect of local environment on Eu3+ activators in YAG, X-ray absorption techniques were applied to investigate the local structure, electronic structure, atomic structure disorder, and long range ordering. According to the above results, this technique was considerable to be valuable in synthesizing nanosized phosphors; thereby, gadolinium-ion co-doped YAG: Eu3+ phosphors were prepared via this process. With increase in the Gd3+ doping concentration, the particle size and the morphology of YAG: Eu3+ remained unchanged. However, there are obvious sensitized-enhancements in the luminescence intensity, quantum efficiency, and chromacity. In the third part of this thesis, europium-doped barium magnesium aluminate (BAM: Eu2+) blue phosphors were prepared via a sol-gel polymer process employing citric acid and ethylene glycol as polymerizing agents. Phase-pure BAM: Eu2+ could be obtained after calcination at 1400˚C. It is observed that after calcined in reducing atmosphere, BAM: Eu2+ exhibits intensive and blue Eu2+ emission while samples before reducing treatment exhibit weak and red Eu3+ emission. The electron paramagnetic resonance (EPR) spectra confirmed the presence of Eu2+ ions in samples subjected to reducing atmosphere. The Eu2+ signals in sol-gel derived samples suggested the lower symmetry of Eu2+ ions in these samples. In samples subjected to additional 500˚C heat-treatment. The EPR signals for Eu2+ ions were reduced indicating that oxidation of Eu2+ took place to form Eu3+. Furthermore, Eu2+ ions substituted into different sites also had effects on the thermal stability.
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