Combustion Synthesis And Luminescence Properties Of Li₂CaSiO₄:Eu³⁺ Phosphors

Rare earth phosphors are are an important class of functional materials, which have been widely used in the area of artificial lighting, displaying and detecting, etc. Silicates are considered as ideal host materials for rare earth phosphors owing to their excellent properties, such as abundant raw materials, easy and adaptable synthesis methods, stable crystal structure, easy to adjust compositions, stable thermal and chemical properties.In this study, Luminescent powders of Li2CaSiO4:M3+(M=Eu3+,Sm3+,Bi3+,Dy3+) and Li2CaSiO4:Eu3+,M3+(M=Sm3+,Bi3+,Dy3+) were synthesized by a combustion method with raw materials of LiNO3, Ca(NO3)2·4H2O, Eu2O3 and carbamide and so on. The phase, micro-structure, particle-size distribution and luminescence propeities of the samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), laser granularity analyzer and spectroscopic analysis. By means of experiments, the quantities of solvent of nitric acid and the optimal concentration of rare ions and combustion-supporting materials were ascertained. Discussions were made on the fluorescence spectra variation when samples were ion doping by charge compensation agent, fluxes and alkaline earth metal ions.(1) The results of XRD and laser size analysis showed that high purity and crystallinity of Li2CaSiO4:Eu3+could be obtained at 700 ℃ for 1 h. The optimal mole ratio of CON2H4 and Li2CaSiO4 was X=3.5. The prepared samples had a uniform distribution of grain-size, which the median diameter (D50) lied between 12-15 um.(2) In the fluorescence spectrum of Li2CaSiO4:Eu3+, the sample had a broad excitation band from 220 nm to 350 nm and a number of sharp small peaks extended from 350 nm to 500 nm, corresponding to the Eu3+â†'O2- and f-f inner-shell transitions of the Eu3+ ions. The emission spectra was described by the well-known 5D0â†'7FJ(J=1, 2,3,4) emission lines of the Eu3+ ions with the peak at 620 nm. And with the increasing amount of Eu3+ ions, the luminous intensity of samples improved, the optimal condition concentration of Eu3+ ions was 9 mol%. The chromaticity coordinates of sample was (0.641,0.362) which the color coordinates were located in the red region.(3) In the emission spectrum of Li2CaSiO4:Sm3+, Li2CaSiO4:Bi3+ and Li2CaSiO4:Dy3+ phosphors, the main peaks located in 605 nm,392 nm and 486 nm, respectively. The optimized doping concentrations were 3.5 mol%,0.1 mol% and 4 mol%, the corresponding color coordinates were (0.598,0.598), (0.151,0.151) and (0.289,0.289), respectively.(4) A small amounts of Mg2+, Sr2+ and Ba2+ ions could also improve the luminous intensity of the samples. The optimization concentration of Mg2+, Sr2+ and Ba2+ions were 5 mol%,10 mol% and 3 mol%, respectively. And the main emission peak position shifted from 620 nm to 617 nm when the concentration of Sr2+ exceed 40mol%. Except for K+, the addition of a small amount of Li+ and Na+ ions as charge compensation agent is helpful to enhance the luminance of the samples. When adding a proper amount of H3BO3 and NH4NO3 as flux respectively, both of them could improve the brightness of the samples, and the effect of H3BO3 was superior to NH4NO3.(5) When co-dopped Eu3+ and Bi3+ into Li2CaSiO4 host, the results showed the Eu3+ ions could be well sensitized by Bi3+ ions, and the pattern of the energy transferred between Bi3+ ions and Eu3+ ions was resonance energy transfer.(6) In the fluorescence spectrum of Li2CaSiO4:Eu3+,Sm3+ phosphors, after co-doping Sm3+, the excitation of Eu3+ was greatly enhanced, and its corresponding emission intensity was increased by the reabsorption of the excitation energy from Sm3+; it provided indirect evidence of the Sm3+â†'Eu3+ energy transfer. With increasing of the Sm3+ content, the Eu3+ emission intensity increased firstly, and reached a maximum at nSm3+/nEu3+=0.5.(7) A single-phased Li2CaSiO4:Eu3+, Dy3+ phosphors were synthesized by combustion method. While the Dy3+content increasd, the luminescent intensities of Eu3+ in Li2CaSiO4:Eu3+, Dy3+ phosphors enhanced, synchronously the intensities of Dy3+ were zero. It provided indirect evidence of the Dy3+â†'Eu3+ energy transfer. And when nDy3+/nEu33+=0.05, the luminous brightness was increased significantly.