Research On The Synthesis And Spectral Characterization Of The Rare Earth Doped Silicate Phosphors

Rare earth phosphors are rather important as a kind of functional materials, which have found wide applications in artificial lighting and displaying. Silicates are considered as ideal hosts for rare earth phosphors owing to their excellent properties, such as abundant raw materials, easy and adaptable synthesis methods, stable crystal structure, ease of the adjustment in their structures and compositions, stable thermal and chemical properties. Versatile luminescent properties can be obtained by doping rare earth ions in silicate hosts. In addition, the emission position and intensity of the luminescent center in these hosts can be easily tuned. White white light emitting diodes (LEDs) with high performance can be obtained by coving the blue or ultraviolet emitting chip with silicate phosphors. The development of the white LED technology has also rendered new requirements on the phosphors.Therefore, deep investigations of the processing, perperties, spectra adjusting and controlling of novel silicate luminescent materials are rather important scientifically and technologically, in order to promote the application ability of these materials in white LEDs, to improve the luminescent efficiency and color quanlity of the white LEDs, and to accelerate the population of the white LEDs.In present work, a systematic research was carried out on the processing methods, luminescent properties, spectral tuning and the application in white LEDs of the rare earth silicate phosphors. A series of silicate luminescence materials were produced by the tranditional solid state reaction synthesis and Pechini sol-gel methods. Thorough investigations were preceded on the luminescent characterization of Eu²⁺ in different silicate hosts, including the absorption spectra, photoluminescent excitation and emission, concentration quenching, energy transfer and luminescent decay behavior et al. The luminescent behavior of the Eu²⁺ et al. was deeply investigated in the silicate materisl. Through substituting the alkaline-earth ions in the host lattice, the crystal structure and chemical composition of the silicate hosts were controlled, therefore, the emission of the Eu²⁺ in the hosts were succefully tuned. The main work and achievements can be summarized as the following:1, A series of silicate based phosphors, such as M₂SiO₄ (M=Ba, Sr, Ca) rothosilicates, MSiO₃ (M=Ba, Ca) metasilicates, Li₂MSiO₄ (M=Ba, Sr, Ca) and M₂MgSi₂O₇(M=Ba, Sr, Ca), were prepared through the solid state reaction method and Pechini sol-gel method respectively. The crystallizing behavior, powder morphology and photoluminescent properties of the produced phosphors were investigated.2, The intersolubility in the Ca₂SiO₄-Sr₂SiO₄-Ba₂SiO₄ and Li₂CaSiO₄-Li₂SrSiO₄-Li₂BaSiO₄ systems and the tunability of the emission of Eu²⁺ in these hosts were systematically investigated. Well solid solution can be formed throughout the whole composition range between the orthosilicates Ca₂SiO₄, Sr₂SiO₄ and Ba₂SiO₄, which have the same crystal structure, and therefore the emission of Eu²⁺ in these hosts can be continuously tuned. In the Li₂CaSiO₄, Li₂SrSiO₄ and Li₂BaSiO₄ system, solid solutions can only be formed in some particular composition ranges while two phases co-exists or intermediates show up in the products in other compositions, due to the fact that these maters have different crystal structures. Therefore, the emission of Eu²⁺ in the Li₂Ca_1-x-ySr_xBa_ySiO₄ can only be tuned in the composition ranges under which solid solutions are formed.3,The crystal field environment in the silicate hosts was effectively tuned through substituting the alkaline earth ions in the host lattice, therefore the emission spectra of the Eu²⁺ were tuned by changing the crystal field strength and the covalence of the coordination environment. The crystal field strength in the host is stronger when the host lattice has a higher symmetry, therefore the crystal field splitting effect on the 5d energy level is stronger, which leads to a longer wavelength and broader full width at half maximum (FWHM) of the emission of Eu²⁺. Higher covalence of the coordinated anions makes the nephelauxetic effect stronger, which results in the lower energy transfer between 4f⁶5d¹â†’4f⁷ and longer wavelength emission of Eu²⁺.4, The energy transfer phenomenon and mechanisms in the Eu²⁺ and Ce³⁺ co-doped silicate phosphors were investigated. Through theoretical analysis the Ce³⁺→Eu²⁺ energy transfer process and mechanism were confirmed to be the dipole-dipole interaction mechanism during their electron transition process.5, The long afterglow properties of the Eu²⁺ and Dy³⁺ co-doped M₂MgSi₂O₇ melilites were studied and the corresponding mechanisms were discussed. The Eu²⁺ doped into the host lattice acts as the luminescent center, while the Dy³⁺, when it is co-doped with Eu²⁺, introduces defects in the host which act as the trap centers. The excitation energy is stored through electron or hole trapping process by the trap center and then de-trapped with the assistance of the lattice vibration. Therefore the afterglow emission of the Eu²⁺ and the afterglow time is prolonged.6, By a thorough investigation of the excitation and emission spectra, a series of silicate based rare earth phosphors which are suitable for white LEDs were obtained, such as: Li₂BaSiO₄:Eu²⁺, Li₂CaSiO₄:Eu²⁺, Li₂Ca_0.7Sr_0.3SiO₄:Eu²⁺ blue phosphors and Li₂SrSiO₄:Eu²⁺, Ca₂MgSi₂O₇:Eu²⁺ yellow phosphors. Warm white LED devices with brigheness of 50 lm, color rendering index of 75 and color temperature of about 5000K was obtained.