發光二極體激發之磷酸鹽類螢光粉合成及其特性研究

Abstract

世界能源危機意識逐漸提升，各國均投入大量人力研發新穎之照明設備，而發展具省電、低污染、發熱量低、反應速度快、壽命長等優點之發光二極體(light emitting diode; LED)作為光源，已是現代照明之發展趨勢。自1996年，日本日亞化學公司(Nichia)以藍光晶片(InGaN)激發鈰掺雜之釔鋁石榴石(cerium doped yttrium aluminum garnet; YAG:Ce)之黃色螢光粉而研發白光發光二極體，造就人類照明之一大躍進，亦觸發LED邁入照明應用之門檻。 經過持續不斷地研究與發展，傳統藍光LED晶片搭配黃色螢光粉(YAG:Ce)所形成之白光，因藍光晶片所激發之螢光粉其能量轉換效率較紫外光激發低，且YAG型螢光粉之光譜所涵蓋範圍較狹隘，造成其生成轉換效率低、色溫高與演色性差之白光光源。鑒於目前研發多種之螢光粉，均須考慮和評估粉體於使用上所產生之發光效率、持效性、色溫、演色性指數、熱穩定性等性質。故本研究重點乃研發具高色純度且熱穩定性佳之紅綠藍(RGB)三原色磷酸鹽類化合物螢光粉，其可適用於紫外光與藍光所激發。 本研究主要乃利用固態反應法製備磷酸鹽類化合物之螢光粉，其通式為ABPO4 (A = Li, K；B = Sr, Ba)，採用LiSrPO4、KSrPO4、KBaPO4三系列為主體晶格，並添加銪(Eu2+)、鋱(Tb3+)、釤(Sm3+)稀土元素為發光中心。主體晶格係由陽離子與陰離子配位所構成，探討陽離子分別由鹼金屬(Li、K)與鹼土金屬(Sr、Ba)組成其晶格場改變之影響，並研究添加不同濃度之發光中心離子，其發光機制與濃度淬滅(concentration quenching)效應，最後利用不同之燒結環境，進而探討其放光特性與化學性質。 於本研究中所使用之分析方法主要有:以X-光粉末繞射儀(X-ray diffraction; XRD)鑑定樣品之純度與其長程有序晶體結構；利用光激發光光譜儀(photoluminescence; PL)分析螢光粉之激發光譜與放射光譜特性，並將放射光譜以程式轉換為其色度座標；以紫外可見光擴散式反射光譜(UV-vis. diffuse reflectance spectra; UV-vis. DRS)分析固態螢光粉末於紫外光與可見光其吸收特性；以掃描式電子顯微鏡(scanning electron microscope; SEM)進行樣品表面型態分析與觀察其粒徑大小之差異性；以熱重分析與差式掃描熱分析【(thermo gravimetric analysis(TGA) and differential scanning calorimetric(DSC)】分析樣品熱分解過程之重量變化與化學反應過程；以熱螢光光譜儀(thermoluminescence; TL)測試粉體實際應用之熱穩定性。本研究部分成果已發表於Appl. Phys. Lett. 90, 151108 (2007)期刊與申請一件專利。

The energy crisis awareness is improving gradually and many countries diverted their research propaganda towards the development of the novel lighting equipments. In this regard, the new trend has been developed to use light-emitting diodes (LEDs) as a source of light, which is considered to be advantageous in many respects such as energy saving, lower contamination, lower heat generation, higher response speed and longer lifetime. In 1996, the first white LED was developed by Nichia Chemical Company, Japan. They utilized a blue LED chip (InGaN) in combination with a yellow phosphor (cerium doped yttrium aluminum garnet; YAG:Ce) to generate white light. This opens up the new directions for the development of LEDs with great advantages that will leap forward and march towards its optimum use in illumination. However, the above white LED presented the major drawbacks of lower color rendering index (CRI), cool light and lower thermal stability. To overcome this problem, there is a kind of phosphor technology being developed at leading companies, which focuses on improving their efficiency, power stability for prolonged period, color temperature and thermal stability. The aim of this work is to develop the novel phosphors based on phosphate host matrix, which can produce superior blue, green and red lights under excitation by UV-LED or blue range and exhibit thermal stability at higher temperature. In this study, we prepared the phosphate compounds by using solid state reaction. The general chemical formula of these materials is ABPO4 (A = Li, K；B = Sr, Ba). The host matrixes were doped with different rare earth elements (Eu, Tb and Sm). We investigated the influence of change in crystal field which composed of different alkaline metal (Li, K) or alkaline-earth metal (Sr, Ba) cations. The effect of sintering of these materials at different temperatures to explore their PL and chemical properties has also been investigated. In addition to this we also studied their luminescent mechanism and concentration quenching effect. The characterization of the phosphors was carried out by X-ray diffraction (XRD) to study the crystal structure and photoluminescence (PL) measurements were carried out to show excitation and emission properties. The emission spectra were transformed to the Commission International de I ‘Eclairage (CIE) by the Colortt program to realize their color coordinates. The reflective ultraviolet/visible spectrometer (UV-Vis) revealed the absorption mechanism of phosphor in consistent with the PL measurements. We also used scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX) to obtain particle size, morphology, and chemical composition respectively. The thermo gravimetric analysis (TGA) and differential scanning calorimetric (DSC) analyses were performed to examine the weight loss and chemical reactions occur during the heat treatment. Finally, the thermoluminescence (TL) measurements suggested luminescence stability of the powders at higher temperature. The part of our work has been published in Appl. Phys. Lett. [90, 151108 (2007)] and also we have filed a patent.