溶膠-凝膠法合成釔-鋁氧化物之相變化及其發光與電子自旋共振光譜之研究

Abstract

本論文是以溶膠-凝膠法分別製成釔鋁化合物為；Y3Al5O12，YAlO3和Y4Al2O9 之粉末並 探討其相形成的順序與添加活性劑後其發光性質與電子自旋共振光譜的研究。其中釔鋁比 為3:5的化合物為石榴石相簡稱YAG，1:1的相為鈣鈦礦結構簡稱為YAP，而4:2的相簡稱YAM 是因為它的結構為單斜晶體。並以旋轉塗佈的方式將YAG:Tb薄膜形成在矽基板上。 前驅溶液的製備中，添加螯合劑來控制水解及調整溶膠-凝膠的反應程序。由溶膠-凝膠 法製得的粉末經熱處理後形成結晶。其結晶相經由X光粉末繞射來鑑定。文中描述YAG，YAP ，YAM等相形成的順序，觀察到在形成YAP與YAM時有一個YAlO3的同分異構物至鈣鈦礦結 構YAlO3的相變化發生。YAG相形成的活化能由恆溫法以Johanson-Mehl-Avrami公式得到69 kcal/mol，以持續升溫法配合Kissinger圖與Sotgiu圖求得活化能222(2kcal/mol。這兩種 方法所得活化能的差異是由於YAG複雜的結晶過程所致。 本文亦使用熔鹽(LiCl)加入由溶膠-凝膠法製得的粉末來降低YAG相形成的溫度。添加20% LiCl ，YAG:1%Tb粉末可以在650oC, 2h下獲得單相。在低溫合成下，熔鹽的添加也能增加 結晶的大小。利用X光繞射圖譜估計添加20%LiCl的樣品，於650oC下2小時，形成的粉末結 晶大小為480禳C同時量測樣本的發光與激發光譜。在275nm波長激發下，所合成粉末的發光 光譜主要呈現能階5D4至6F群的能量釋放。在非晶質與結晶相下，Tb3+離子的激發光譜會在 晶格場的效應下會有所不同，藉此激發光譜也可幫助觀察YAG相的結晶化程度。 另方面，我們利用溶膠-凝膠溶液以旋轉塗佈法製成一系列含Tb為活性劑的Y3Al5O12的薄 膜。石榴石結構可以在800oC一小時的RTA爐下獲得。以紫外線與電子束激發的發光光譜呈 現了能階5D3到6F及5D3到 6F兩個群。這兩個群的比例與樣品中所含Tb離子濃度有關。 本文電子自旋共振(EPR)光譜的樣本是以溶膠-凝膠法製備的粉末經三種不同的熱處理方 式而得。樣本成份是YAG:Cr與YAM:Cr。YAG:Cr樣品除了進行EPR光譜的觀察外其發光性質亦 同時研究。兩階段熱處理及直接升溫至1500oC熱處理，添加20%LiCl YAG:Cr的樣本都呈現 良好的發光性質。經兩階段熱處理的樣品之發光光譜出現688nm的紅光而直接升溫至1500oC 的樣本則額外呈現Al2O3:Cr的紅光694nm。電子自旋共振光譜中吸收峰的出現與否依Cr3+離 子在主體晶格中的環境而定，而不同的熱處理方式則會影響Cr3+離子的環境。在YAG:Cr的 EPR光譜，它可以由Cr3+單離子吸收與Cr3+-Cr3+離* l間的吸收來解釋；同時YAM:Cr的EPR光譜包含Cr3+單離子吸收與YAM晶格本質的吸收組成 。藉由Cr3+單離子的吸收可以區別Cr3+的環境在經兩階段處理與直接熱處理所得粉末的不 同。經由EPR光譜的判斷可以更進一步確定兩階段熱處理所得的粉末有較佳的結晶結

The powders of phases in the Y2O3-Al2O3 system with Y/Al ratios of 3:5, 1:1 and 4:2 are synthesized by the sol-gel process. The phase development, their luminescence p roperties, and electron paramagnetic resonance spectrum are studied. We also investiga ted the sol-gel films of YAG:Tb on Si by using spin coating. The phase development of Y3Al5O12 (YAG), YAlO3 (YAP) and Y4Al2O9 (YAM) powders synthesized by a modified sol-gel method has been studied. For preparing solution, the chelate (ethylaceto acetate) agent 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 YAlO3 polymorphism to perovskite YAlO3. The activation ener gies of YAG are estimated at about 69 kcal/mol by isothermal process as fitted with ohnson-Mehl-Avrami equation and about 222(+-2 kcal/mol by continuous heating method as fitted with Kissinger plot and Sotgiu plot. The difference of activation energies calculated from two methods is mainly due to the complicated crystallization of Y3Al5O12 . We also added the melting salt (LiCl) into the sol-gel derived powders which helps to attain crystallization at lower temperatures. The single-phase cubic Y3Al5O12: 1% Tb sol-gel derived powders were prepared after firing at 650oC for 2h with the addition of LiCl flux. 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 cr ystalline size of the powder with 20 wt% LiCl fired at 650oC for 2h was evaluated as 480*. Both emission and excitation luminescent spectra of the samples were measured . Emission spectra of the synthesized powders mainly show 5D4-6F transition under 275nm 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 helps to determine the degree of crystallinity of the resulting YAG phase. A series of Tb -activated Y3Al5O12 films has been fabricated by spin coating on silicon substrate with sol-gel solution. The garnet phase is achieved by heat-treating the films at 800oC for 1 h in RTA furnace. The emission spectrum of the films induced by ultraviolet( UV) and cathode-ray excitations show the 5D to 6F transition and consisted of two gro ups, 5D3 to 6F(shorter than 485nm) and 5D4 to 6F(longer than 485nm).The ratio of intensity of two groups is dependent on the Tb concentration of the films. Electron pa ramagnetic resonance (EPR) studies are carried out for the sol-gel derived YAG:Cr and YAM: Cr powders subjected to three kinds of heat treatment processes. The luminescence p roperties of YAG:Cr are also studied. The results show that powders processed using two s tep heat treatment process with the addition of 20wt% LiCl and by direct heat treatme nt at 1500oC exhibit good luminescence properties. The emission spectrum for the powders obtained by two step process shows the R-line at 688nm whereas the directly heat treated powders at 1500oC display an additional R-line of Al3O3:Cr at 694nm. EPR measurements sho w that the presence or absence of absorption peaks depend on the Cr3+ environment in the h ost lattice resulted from the different methods of heat treatment of the powder. The EPR spectra of YAG:Cr3+ phosphors were interpreted as both Cr3+ single ion and Cr3+-Cr3+ pair (with spin three) transitions. The EPR spectra of the YAM:Cr includes the Cr3+ single ion and YAM structure nature transitions. We could identify the different environm ents of Cr3+ in the powders obtained from the two steps and directly heat- treated pr ocess using EPR analysis. The better crystallinity of the powders obtained by two-step process are identified by using EPR spectra.