Solution Combustion Synthesis And Optical Properties Of The Silicates Phoshors For White LED

White light emitting diode (WLED) is believed to be the fourth generation of lamp-house after incandescent lamp, fluorescent lamp and discharge light and there are the futures of illumination. WLED represents the future direction of development of the lighting industry due to its attravive advantages of energy consumption saving, high-efficiency, environmental and lifetime long-lasting aspects, which has been focus of attention and speed up research and development in around the world especially developed countries. From the realization point of view with WLED technology, the single-chip LED with inorganic luminescent materials (phosphors) of the combination relative of the multi-chip direct combination with good color, simple design and low cost advantages, which is the mainstream development of solid-state lighting. Therefore, it is a great significance to make novel, stable and efficient development of LED phosphors for WLED technology.Silicate phosphor is a class developed in recent years to meet the requirements of the new WLED inorganic luminescent materials. However, there is no systematic study the current matrix of silicate phosphor composition, crystal structure, electronic band structure, activating agent and synthesis method matching between luminescent properties. The solid-state reaction process has been used intensively for silicates phosphors, but this approach is the need for high-temperature and long-time calcinations. The unreacted phases appeared easily which reduced greatly its luminescent brightness and intensities characteristic. In the present works, the silicates phosphors synthesized by the solution combustion synthesis (SCS) method and researched the crystal structure, electronic structures and luminescence properties of synthetic materials. The main results obtained are as follows.The Ba2MgSi2O7:Eu2+ blue-green phosphors were synthesized by combustion synthesis method and its luminescent properties were also investigated. The results of XRD and SEM analysis show the sample is nonophasic and well-crystallized. The excitation wavelength of these phosphors can be effectively excited by UV-LED chip. The emission wavelength lies at 498 nm upon excited by 353 nm UV light. The Commission International de l'Eclairage (CIE) of the optimized sample was calculated (x, y)=(0.14, 0.41).The luminescent properties of Ba1.95(Mg1-y, Zny)Si2O7:Eu0.052+ phosphors were researched. The results shown that the introduction of Zn2+ into Ba2MgSi2O7:Eu2+ effectively increased its emission with UV excitation. In addition, the emission spectra presented an emission position red shift of up to 5 nm with increasing y value, which due to the crystal field enhancing and electron cloud expansion effect.The hardystonite phosphor Ba2ZnSi2O7:Eu2+ researched by the orthogonal design method. The results shown that the postannealed temperature at 1000℃for 3 h, the quantity of flux H3BO3 was 0.06 mol and the activator ions of Eu2+ was 0.05 mol could get the optimized phosphors. The critical quenching concentration of Eu2+ in Ba2ZnSi2O7: Eu2+ phosphor is about 0.05. The corresponding concentration quenching mechanism is verified to be a dipole-dipole interaction. The excitation wavelength of these phosphors can be effectively excited by UV-LED chip. The CIE of the Ba1.95ZnSi2O7:Eu0.052+was calculated (x, y)=(0.16,0.45).The Ba2ZnSi2O7:Eu2+ phosphors have also been prepared by a modified Sol-gel method in the reducing atmosphere. A blue-green emission with a peak at 500 nm is observed, which show the Eu2+ typical emission. Compared with the products of combustion synthesis, its emission peak blue-shift 3 nm, mainly due to activators in nano-size effect makes the vibration frequency between the bonds internal the crystal increases. At the same time, NaOH used as mineralizer to prepare Ba2ZnSi2O7:Eu phosphors by hydrothermal method. The results indicate that, the sample emission prominent peak locates at 614 nm, being attribute to 5Do→7F2 electric-dipole transitions of Eu3+ ions lie in non-centrosysmetrical sites. After the sample is reduced in the reducing atmosphere, a blue-green emission with a peak at 501 nm is observed, which shows the Eu3+ ions has been completely reduced to Eu2+ Comparison analysis of the Ba2ZnSi2O7: Eu2+ phosphor fabricated by SCS method, Sol-gel method and hydrothermal method respectively indicates that the phosphors can be excited by UV irradiation. However, the combustion method to prepare Ba2ZnSi2O7:Eu2+ phosphor powders have more uniform leading to the better performance. The luminescent intensity is higher than the Sol-gel and hydrothermal method. The process of combustion synthesis method is simple and more energy-saving.Morever, the crystal structure and luminescent property of silicates phosphors are considerable influenced by Ba/Zn ratio. The green-emitting phosphors of the BaZn2Si2O7: Eu2+ and BaZnSiO4:Eu2+ was prepared by SCS method. The emission of BaZn2Si2O7: Eu2+ phosphor showed one Gaussian symmetric peak at 522 nm. But in the structure of BaZnSiO4:Eu2+ phosphor, Eu2+ ions occupy three different lattice sites by substitution for Ba2+. Eu2+ions on Ba(1) and Ba(2) sites gave emissions at about 512 nm and 492 nm, while Eu2+ ions on Ba(3) sites showed an emission band at 403 nm.To further understand the photoluminescence properties of the melilites phosphors, the electronic band structures calculation were performed on Ba2MgSi2O7, Ba2ZnSi2O7, Sr2ZnSi2O7 and Ca2ZnSi2O7 by the first principle of density functional theory. The calculated results shown that the valence band top of these melilites silicate crystals was mainly composed of the O2p orbitals, also the transition-metal-element orbitals such as Zn3d orbitals contributed significantly to the valence band top and contracted the band gap. The low conduction band contained the Si3s and Si3p orbitals, also included the alkaline-metallic element orbitals such as the Ca3d, Sr4d and Ba5d orbitals. So, the rare earth activator Eu2+ ions occupy the alkaline earth metal ions in the melilite crystal structure, the host crystal band structure caused the different luminescence characteristics.A blue emitting phosphor BaCa2Si3O9:Eu2+ was prepared by the combustion synthesis method. The excitation spectrum is a broad extending from 260 to 450 nm, which matches the emission of UV-LED. The emission spectrum shows a single intensive band centered at 445 nm, which corresponds to the 4f65d1→4f7 transition of Eu2+. The CIE of the sample was calculated (x, y)= (0.16,0.11).Luminescent properties of hardystonite M2ZnSi2O7: Eu3+ (M=Ca, Sr, Ba) red phosphors have been systematically studied. The results showed that the excitation and emission spectra of phosphors did not change the shape and positions. The excitation peaks of these phosphors about 320, 362,383,394, 416 and 465 nm are assigned to correspond to 7F0→3H4,7F0→5D4,7F1→5F4,7F0→5L6,7F1→5L6 and 7F0←5D2 transitions of Eu3+, respectively. The spectrum exhibits five emission peaks at 579,591,614,652 and 703nm. These five emission peaks can be attributed to the 5D0→7F0,5D0→7F1,5D0→7F2, 5D0→7F3,5D0→7F4 transitions of Eu3+. These results show that the f-f transitions of Eu3+ basically dose not change with the different hosts. When the activator of Eu3+ codoped Ba2+, the ratio of the 5D0→7F2 (electric dipole transition) and 5D0→7F1 (magnetic dipole transition) is maximum. The experiment results show that the monoclinic Ba2ZnSi2O7 have the lowest crystal symmetry in the melilite structure. These phosphors have main excitation peaks located at 394 and 465 nm, which match the emission of UV and blue-LED, respectively. Thus, these luminescent materials could be used as red phosphors for WLED.