Preparation And Luminescence Research Of Two Kinds Of Silicate Rare Earth Phosphors

Due to the outstanding physical, chemical and thermal stability, including theobvious low cost of raw materials, silicate phosphors have been widespread attractingresearchers, which becomes one of the most important photoluminescence materials.In recent years, some of silicate phosphors have been able to be appropriate for whiteLED via researchers constantly optimize and improve this kind of material. Duringthe silicate phosphors, alkaline earth metal silicate phosphors own rich emitting colorsand high brightness, some ternary silicate phosphors which composed of alkali metalpresente strong luminance as well.Because of the invisible emission of the LED chips, the impact on the LEDluminous of LEDs which fabricated by UV/NUV LED chips coated withwhite-light-emitting phosphors or RGB tri-color phosphors is much less than theconventional LEDs which made by means of blue LED chips combined with YAGpowder. So that this way is helpful to improve the performance of LEDs. In this paper,we started from looking for single host white-light-emitting phosphors and RGBtri-color phosphors, which have wide emission spectra and can be excited by UV orNUV light efficiently. Then we synthesized two kinds of phosphors, i.e., the yellowphosphor Sr3SiO5-6xN4x: Ce3+, Li+and the white phosphor Na2CaSiO4: Dy3+,respectively. Both the luminescence properties of two phosphors and the feasibility oftheir applications on the LED had been studied.(1) A yellow-green-emitting oxynitride phosphor Sr3-2xSiO5-6xN4x: yCe3+, yLi+(0≤x≤0.33,0.01≤y≤0.04) was synthesized within the reducing atmosphere. Thesamples were tested and characterized by X-ray diffractometer (XRD) analysis andfluorescence spectrometer. The results show that the crystallinity of samples are highand the structure of samples are isostructural with Sr3SiO5. The excitation spectrum(PLE) of phosphor covers from UV to blue region, whose main peaks are composed of two hump shape broadband excitation peaks. The peaks locate at325nm and414nm, and the second peak is higher. Corresponding to the5dâ†'4f transition of Ce3+, theemission peak is a broadband peak with475~575nm. Excited by blue light (460nm)and NUV light (414nm), the main emission peaks are557nm and538nm,respectively. During the synthesis process, we replaced part or all of SiO2with Si3N4as the silicon source, that is, the partial substitution of (SiN)+for the (SiO)2+in host.With the change of the crystal field, the emission peak shifts to red area and thebrightness descends. The optimized doping concentration of Ce3+is0.68mol%(y=0.02). The addition of a small amount of flux NH4F is conducive to the obviousincrease of luminous intensity, and its optimized content is0.5wt%.(2) A kind of white light Na2CaSiO4:Dy3+phosphors was synthesized by sol-gelmethod at750°C, holding3h, in air condition. The XRD patterns show that theNa2CaSiO4: Dy3+samples can be obtained at700~900°C, and the purity andcrystallinity of the sample which calcinated at750°C is higher than the other ones.The structure of samples are cubic with space group P213, all of which are inconformity with the crystal stucture of Na2CaSiO4. The PL spectrum of Na2CaSiO4:0.04Dy3+shows that the phosphor can be effectively excited by UV light. There aretwo character emission peaks in the spectrum, one lies in blue region (about486nm),the other one locates at yellow region (about578nm), corresponding to the4F9/2â†'6H15/2and the4F9/2â†'6H13/2transition of Dy3+ion, respectively. The yellowemission is higher than the blue one, so we deduce that Dy3+ions prefer occupy theasymmetric sites in the Na2CaSiO4host. The luminous intensity of samples increaseswith the adding of the dysprosium ions concentration, maximizing at about4mol%.The Commission International de I’Eclairage (CIE) chromaticity coordinates presentthat all the color coordinates of Na2CaSiO4: xDy3+(0.02≤x≤0.05) are in thewhite-light region, and the coordinates shift towards to yellow region with theincrease of Dy3+concentration. Except for Ba2+, the addition of a small amountalkaline earth metal ions M2+is helpful to enhance the luminance of samples. Theoptimized concentration of Mg2+, Sr2+is0.5mol%and1.0mol%, respectively.