Preparation And Photoluminescence Of New Type Rare Earth Polysilicate By Sol-gel Process

Fluorescent lamp is a kind of high light efficiency and long life light sources. Its luminous efficiency is 3 to 4 times than that of the electric-incandescent lamp, and its life is 8 to 10 times longer. The successful development of rare earth trichromatic phosphors has provided a technical support for the advent of compact fluorescent lamps. Generally, these phosphors have good resistance for high load and high temperature, and can save energy used for lighting significantly. Therefore, the improvement of phosphors and the explor of novel phosphors have become the focus of researchers.Rare-earth luminescent materials with the silicate as matrix have many advatages, such as cheap raw materials, good chemical stability and thermal stability,and high luminescence efficiency. Therefore,this kind of luminescent materials has atracted much attention for investigation and development of phosphors used in lamps. In present paper, several series of novel rare earth phosphors were synthesized by gel - combustion method. The formation process, phase structure, composition and luminescent properties of the as-synthesized phosphors were investigated by TG-DTA, infrared spectroscopy, X-ray powder diffraction, Energy Dispersive Spectrometer and Fluorescence spectrophotometer. According to the results, some valuable conclusions are obtained.(1) Novel phosphors Sr₃MgSi₃O₁0:Tb³⁺ were synthesized by gel-combustion method in weak reductive environment. The results show that Sr₃MgSi₃O₁0: Tb³⁺ phosphors possess the similar tetragonal crystal structure as that of Sr₂MgSi₂O₇. The excitation spectrum of Sr₃MgSi₃O₁0:Tb³⁺ is a broad band from 200nm to 300 nm, and the main peak is at 249 nm. The emission spectrum is composed of a series of peaks, located respectively at 491 nm, 544 nm, 586 nm, 624 nm. These emission peaks are ascribed to Tb³⁺ ions transition from ⁵D₄ to ⁷F_J (J =6,5,4,3). The main emission peak is at 544 nm due to the transition of 5D4→7F5, which results in a yellow-green emitting. In addition, it is found that the reductive temperature and the concentration of doped Tb³⁺ have great effects on the luminescent intensity of the sample, and the mechanism of concentration quenching is discussed. (2) The novel blue emitting phosphor Sr₃MgSi₃O₁0:Eu²⁺ with broad excitation band was synthesized by the gel-combustionethod in a weak reductive environment. The results show that Sr₃MgSi₃O₁0:Eu²⁺phosphors possess the tetragonal crystal structure similar to that of Sr₂MgSi₂O₇. The excitation spectrum of Sr₃MgSi₃O₁0:Eu²⁺ is abroad band in the range of 250–450 nm, and the main peak is at 366 nm and the secondary peak is at 394 nm. Therefore, this phosphorcan be excited efficiently by InGaN chip in the range of 350–410 nm. The emission spectrum excited by 366 nm shows a wide bandwith the main peak at about 466 nm, which is ascribed to the typical transition from 4f5d to 4f of Eu²⁺. It is found that the luminescentintensity of Sr₃MgSi₃O₁0:Eu²⁺ phosphors increases with the increase of reductive temperature. Moreover, the luminescent intensityincreases with the increase of Eu²⁺ content (in mole, the same below), and reaches the maximum when the content of Eu²⁺ is 10%, andthen decreases with the further increase of Eu²⁺ content, namely the concentration quenching occurs. The concentration quenching iscaused by the interaction of electric dipole-electric quadrupole of Eu²⁺ according to the Dexter theory.(3) High-brightness blue emitting phosphors Sr₃MgSi₃O₁0:Eu²⁺,Er³⁺ were synthesized successfully by gel-combustion method in weak reductive environment. The results show that Sr₃MgSi₃O₁0:Eu²⁺,Er³⁺ phosphors possess the tetragonal crystal structure similar to that of Sr₂MgSi₂O₇.The excitation spectrum shows a broad band from 250 nm to 450 nm, and the strongest excitation peak is at 357 nm. The emission spectrum excited by 357 nm also shows a wide band with the main peak at about 466 nm, which is ascribed to the typical transition from 4f65d1 to 4f7 of Eu²⁺. According to the emission spectrum of as-synthesized Sr₃MgSi₃O₁0:Eu²⁺,Er³⁺ and empirical formula of Van Uitert, it can be deduced that Eu²⁺ ions occupy Sr2+ sites with coordination number 8 in Sr3MgSi3O10 host. It is found that co-doped Er³⁺ can sensitize effectively the luninescence of Eu²⁺ ions in Sr3MgSi3O10 host. The luminescence intensity reaches the strongest when the mole fraction of co-doped Er³⁺ is 0.04, and it is about 3.3 times of single-doped Eu²⁺ sample.(4) A new red-emitting phosphor La₂Si₂O₇:Eu³⁺ was synthesized by gel-combustion method. The results show that the goal product La₂Si₂O₇:Eu³⁺ can be obtained by the precursor calcined at 1000℃for 2h. The as-synthesized sample has hexagonal crystal structure and unit cell parameter is a=b=0.6846 nm, c=2.4855 nm. The excitation spectrum of La₂Si₂O₇:Eu³⁺ is composed by two parts,the broad band from 200 nm to 350 nm is due to the charge transfer(CT) band of the Eu³⁺- O²- and the strongest excitation peak is at 263 nm; A seriers of excitation peaks from 350nm to 450nm are ascribed to f-f transition of Eu³⁺,and the strongest excitation peak is at 395 nm. The emission spectrum excited by 395 nm is similar to that excited by 263 nm, indicating that the two excitation peaks belong to the same luminescence center. The main emission peak is at 618 nm with strong red-emitting, due to the ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺. Co-doping Li⁺ ions have little effect on the shape of the excitation and emission spectra, but the intensity of excitation and emission peaks increases significantly, which indicats that co-doping Li⁺ ions can effectively enhance the luminescence performance of La₂Si₂O₇:Eu³⁺.