臺灣博碩士論文加值系統

目前市面上的白光LED主要是利用InGaN藍色發光二極體搭配YAG :Ce3+黃色螢光粉，但由於缺乏紅光區的光譜波段，無法將其應用在需高演色性(CRI > 90)的室內照明中。為獲得高演色性光源，室內照明用的LED是以三色(紅,藍,綠)螢光粉搭配紫外光發光二極體為主流。此類型白光LED光色的關鍵在於各色的發光效率以及其發光光譜之間的搭配，因此發展高發光效率具有可調變發光光譜的螢光粉系統係為目前研發的重要課題。
Sr4Al14O25 :Eu2+為一具有高發光效率和高化學穩定性的螢光材料，其發光波長約在490 nm，適合應用在白光LED的綠色螢光粉。另外，文獻指出，當主體晶格中的陽離子被改變，其發光特性也會跟著改變。因此本研究利用固相反應法(Solid reaction method)，燒結出此材料，並使用鋇和鈣離子進行摻雜，取代原先鍶離子的位置，觀察其對相變化、元素分佈、發光光譜、色度座標(chromaticity coordination)，量子產率(quantum yield)等影響。此外，量測在高溫下的發光光譜，探討摻雜對其活化能(activation energy)的影響。
鋇摻雜的螢光粉具有明顯的紅移(red shift)現象，在較低取代濃度時，此現象是由於電子雲膨脹效應大於晶場效應。且隨鋇濃度的增加，量子效率可以提升到96.5%。受到Stokes shift和晶格振動頻率降低的影響，活化能可以提升到0.450eV。在較高取代濃度時，紅移現象主要是SrAl2O4生成，但在過量的取代下，晶格扭曲和相生成造成發光強度、量子產率，活化能快速下降。
鈣摻雜的螢光粉同樣具有的紅移現象，因為晶場效應大於電子雲膨脹效應。並且由於鍶與鈣離子的差異較鍶與鋇離子小，晶格扭曲較小，在較高取代濃度下，可維持單一相和一定的發光強度。此外，隨鈣的取代量增加，量子產率穩定且活化能下降速率緩慢。
對於鋇和鈣同時添加的螢光粉，紅移現象介於鋇和鈣添加之間。由於晶格扭曲的效果被相互中和，在較高取代濃度下可維持單一相，和一定的發光強度。另外，隨著取代濃度上升，量子效率下降速率較鋇摻雜低，且活化能獲得顯著提升。
此外，從製程改良出發，經由醋酸清洗和二次熱處理後，螢光粉的發光強度和活化能皆可有效被提升。因為酸洗後，螢光粉中的非晶相會被去除；而熱處理後，球磨後的螢光粉獲得修復。

Currently, white LED is mainly using blue chip (InGaN) equipped with yellow phosphor (YAG: Ce3+). However, this type is lack of spectrum wave band for the red region, and failed to be applied in the indoor lighting which needs high color rendering index. To obtain light source with high color rendering, white LED applied in indoor lighting generally employ UV chip with green, red, and blue phosphor. The critical points of light color of this type are luminous efficiency and mutual collocation of different colors. Hence, the development of high luminous efficiency phosphor with tunable emission spectrum system is the critical issue.
Sr4Al14O25 :Eu2+ is of high luminous efficiency and high chemical stability fluorescent material with emission intensity around 490 nm. It is suitable used as green phosphor of white LED. However, in literature, luminescence properties were altered as cation of host lattice was changed. Therefore, this material was fabricated through solid reaction method with barium and calcium doping to replace strontium in this study. Then, phase transformation, elemental distribution, emission spectrum, chromaticity coordination and quantum yield were investigated. Besides, emission spectrum in high temperature was measured to investigate the effect of cation substitution on activation energy.
Barium-doped sample exhibited significant red shift phenomenon. In lower substitution concentration, this phenomenon was due to the fact that Nephelauxetic effect was higher than crystal field effect. Furthermore, with the increasing barium content, quantum yield could be promoted to 96.5 %.Activation energy increased to 0.450 eV. In higher substitution concentration, red shift phenomenon was mainly due to SrAl2O4 formation. However, with the excess substitution, lattice distortion, impurity and other phase formation would lead to the rapid reduction of emission intensity, quantum yield, and activation energy.
Calcium-doped sample also exhibited red shift phenomenon, since crystal field effect was higher than Nephelauxetic effect. In addition, because the size difference of strontium and calcium ions is lower than that of strontium and barium ions, lattice distortion was low and impurity formation was less. The phosphor retained single phase and a certain degree of emission intensity in higher substitution concentration. Besides, with the increasing calcium substitution, quantum yield was stable and the reduction rate of activation energy was slow.
For barium and calcium co-substitution, the degree of red shift phenomenon was between barium and calcium substitution. Because the effects of lattice distortion were mutually neutralized, phosphor retained single phase and a certain degree of emission intensity in higher substitution concentration. Besides, with the increasing substitution, reduction rate of quantum yield was lower as compared with barium substitution, and the activation energy was significant promoted.
Besides, from process improvements, emission intensity and activation energy of phosphor were effectively promoted through acetic acid washing process and secondary heat treatment. Especially, amorphous phase was removed after acid washing.

Contents
List of Tables IV
Figures Caption V
Abstract XIII
Chapter Ⅰ Introduction 1
1.1 Background 1
1.2 Motivation and goals in this study 2
Chapter Π Literature Review 5
2.1 Phosphor 5
2.2 Light emitting mechanism of phosphor 5
2.3 White light emitting diode phosphor 6
2.3.1 Light emitting diode 6
2.3.2 White light emitting diode 7
2.3.3 The condition of white LED phosphor 8
2.4 Strontium aluminates green phosphor 9
2.4.1 Microstructure of Sr4Al14O25: Eu2+ 9
2.4.2 Luminescence mechanism of Sr4Al14O25 :Eu2+ 9
2.4.3 Preparation methods of Sr4Al14O25 :Eu2+ 10
2.5 Cation substitution on SrAl2O4 :Eu2+ and Sr4Al14O25 :Eu2+ 14
2.5.1 Barium substitution 14
2.5.2 Calcium substitution 15
2.5.3 Barium and calcium co-substitution 15
2.5.4 Other cations doping 16
2.6 Terms relating to the phosphor 16
2.6.1 The Luminosity function 16
2.6.2 Nephelauxetic effect 17
2.6.3 Crystal field theory 17
2.6.4 Thermal quenching 18
2.6.5 Quantum yield and adsorption 19
2.6.6 The CIE chromaticity coordination 19
Chapter Ш Experimental Procedures 29
3.1 Powders fabrication of strontium aluminate green phosphor 29
3.2 Acetic acid washing process 30
3.3 Secondary heat treatment 31
3.4 Analysis of phosphor powders 32
3.4.1 Microstructure and phase identification 32
3.4.2 Mean particle size (D50) 32
3.4.3 Morphology evolution 33
3.4.4 Composition analysis 33
3.4.5 Fourier transforms Infrared spectroscopy 33
3.5 Analysis of luminescence properties 34
3.5.2Photoluminescence spectrum measurement 34
3.5.2 Temperature-dependent photoluminescence 34
3.5.3 Quantum yield and adsorption measurement 35
3.5.4 CIE chromaticity coordinates measurement 36
Chapter Ⅳ Results and Discussion 47
4.1 Effect of starting materials 47
4.1.1 Flux concentration 47
4.1.2 Al2O3 powder size and type 49
4.1.3 Activator concentration 51
4.2 Effect of cation substitution on Sr4Al14O25 :Eu2+ 58
4.2.1 Barium substitution 58
4.2.2 Calcium substitution 75
4.2.3 Barium and calcium co-substitution 87
4.3 Process improvement 96
4.3.1 Acid washing process 96
4.3.2 Secondary heat treatment 99
Chapter Ⅴ Conclusions 112
References 114

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