Comparison Of YAG:Ce³⁺ Phosphors For White-LEDs Application Prepared With Different Methods And Their Influence On Phosphor's Structure And Property

In order to deal with the global energy crisisi and environment pollution,research and development for environmental-friendly and energy efficient materialshas been the main goal for materials science scientists. Because of its energyefficiency, environmental friendness, high luminance efficiency and long life, theLED lamp has been considered the new generation light to replace the classicalincandescent lamp and fluorescent lamp.In this thesis, based on development of white-LEDs and YAG:Ce³⁺ phosphor'sphysical and chemical property, luminescent property and preparation methods, weproposed that comparison and relation among preparation route, structure andluminescence properties of YAG:Ce³⁺ phosphors prepared with different synthesismethods, were studied in order to develop new applicable synthesis method forYAG:Ce³⁺ phosphors.XRD,FT-IR,SEM and PL spectra were used to characterize the as-preparedphosphors which were prepared by different methods of solid-state reaction,co-precipitation method and sol-gel methods. Their aspects of synthesis temperature,crystal growth and morphology and luminescent property were compared andstudied. Considering the relative low white-LED's color rendering index asYAG:Ce³⁺ phosphor's lacks the red emission, the Pr³⁺ co-doped YAG:Ce³⁺,Pr³⁺phosphors were prepared by co-precipitation method in order to enhance the redemission and improve the color rendering index for white-LED's.The effects of flux on synthetic temperatures, luminescent properties andmorphologies of YAG:Ce³⁺ phosphors prepared by solid state reaction were studied.Results showed that flux could lower the formation temperature of YAG phase,improve luminescent intensity and accelerate the crystal growth. The formationtemperature of YAG phase, crystal growth status, microstructure and luminescentintensity of YAG:Ce³⁺ phosphors depend on preparation methods. It was found thatthe synthetic temperature of pure YAG phase was as high as 1300℃even if the fluxwas added in solid-state reaction. It was verified that the final YAG phase must betransformed from intermediate phase of YAM and YAP.For the wet-chemistry method, the synthetic temperature for pure phaseYAG:Ce³⁺ phosphors needs only 800℃-900℃in the co-precipitation and sol-gelmethods. In addition, the precursors prepared by co-precipitation method transformdirectly to crystalline YAG phase from its amorphous structure without anyintermediate phases which is obviously different from the case in solid-statereaction.. The nano-sized YAG:Ce³⁺ phosphors were obtained with an average particle size at about 35 nm through both sol-gel method and co-precipitationmethod.It was found that YAG:Ce³⁺ phosphors prepared by three different methodshave similar excitation and emission spectra. The excitation spectrum was composedof two peaks at 344 and 470 ran, respectively, which are due to the ²F_5/2â†'5D and²F_7/2â†'5D transitions of Ce³⁺ ion. The main peak of excitation was 470 nm which isin the range of blue light. Under blue light excitation, the YAG:Ce³⁺ phosphors emityellow light centered at 535-545 nm with broad band range from 500 to 700 nm.It was revealed that the emission intensity of phosphors increases withincreasing reaction temperature. Obvious difference of luminescence intensity wasfound among phosphors prepared at same reaction temperature by different method .The products prepared by sol-gel method have the poorest luminescent properties,and that prepared by co-precipitation and flux-aid solid-state reaction had goodluminescent intensity. The content of host phase of YAG and its growth status areconsidered two key factors determining the luminescent intensity. The Ce³⁺ ions donot emit in YAM and YAP phases while it has excellent luminescent property inYAG phase.It was found that co-precipitation method and flux-aid solid-state reaction weretwo routes to synthesize phosphors with practical value, because their products havegood crystal growth of crystalline and morphology and excellent luminescentproperty. Moreover, the co-precipitation method maybe the best way to prepareYAG:Ce³⁺ phosphors when synthetic temperature and phase control were taken intoaccount.YAG:Ce³⁺,Pr³⁺ phosphors with low synthsis temperature and high luminescenceintensity were successfully synthesized by co-precipitation method. It was found that,besides the emission band of Ce³⁺, the emission spectrum of YAG:Ce³⁺,Pr³⁺ has twoadditional sharp peaks at 610 and 640 nm which correspond to ¹D₂â†'³H₄ transitionand ¹D₂â†'³H₄ transition of Pr³⁺ ion, respectively. The red emission from Pr³⁺ ioncould improve the CIE of white-LEDs using YAG:Ce³⁺,Pr³⁺ phosphors. Also, it wasfound that Pr³⁺ ions had the strongest emission intensity with doping concentrationof 1.5 mol%, which is considered to be the best doping level because theluminescent intensity decreased at high Pr³⁺ doping concentration above 1.5 mol%.