Preparation And Luminescent Properties Of Phosphors Based On Tungstate For LED

White light-emitting diode(W-LED)has various advantages such as higher efficiency,energy conservation,environment protection,excellent reliability,long lifetime,which will illuminate the 21st century.LED chip emits monochromatic light,in order to obtain white light,the method that LED chip excites coated phosphors is adopted.One is that yellow phosphors(YAG:Ce3+)excited by a blue chip to obtain white light.Nevertheless,this method yields poor color rendering index and high correlated temperature owing to the lack of red light emission.The other method for producing white light is to excite tri-color phosphors with UV light.However,the efficiency of red phosphors(Y2O2S:Eu3+)is much lower than those of the green and blue phosphors,and the red phosphors is chemically unstable.It can be seen that both methods need to research and design efficient and stable red phosphors suitable for W-LED.In addition,there are the problems of luminescence re-absorption between multicolor phosphors,correlated color temperature drift caused by different aging rates,proportioning and modulation of three primary colors.The method that UV chip excites single matrix white phosphors can avoid the above problems,so it has attracted more and more attention.A series of phosphors for W-LED were prepared with tungstate with excellent physical and chemical stability as matrix and rare earth ions as activator,then luminescent properties of phosphors were studied.The main content of this paper is as follows:1.CaWO4:Eu3+/Sm3+red phosphors were synthesized for W-LED by co-precipitation method.We investigated the effects of Sm3+ions on the crystal orientations and luminescent properties of CaWO4:Eu3+,Sm3+phosphors.It was found that the peak ratios of X-ray diffraction,excitation spectra and emission spectra of the samples changed significantly.The doped Sm3+ions changed the crystal field around Eu3+ions in the host,enhanced the red emission of Eu3+(5D0→7F2,electric dipole transition,613 nm),supplemented red emission of Sm3+(4G5/2→6H9/2,electric dipole transition,642 nm)and improved the color purity of red emission.It shows that the samples are red-emitting phosphors with great development value.2.NaLa(WO4)2:Eu3+/Sm3+red phosphors with different concentrations were prepared by molten salt method.Sm3+ ions can strengthen the excitation peak at 403 nm of phosphors.Sm3+makes Eu3+occupy the non-inversion symmetry center,which enhances the emission of Eu3+.The effect of two-way energy transfer on luminescence was studied.In the synthesized samples,NaLa(WO4)2:15%Eu3+,1%Sm3+has the strongest red emission due to energy transfer from Sm3+to Eu3+.NaLa(WO4)2:15%Eu3+,5%Sm3+has the weakest emission due to energy transfer from Eu3+to Sm3+.The blue-green light emitted by the self-activation of WO42-was mixed with the yellow-orange-red light emitted by Eu3+ions to form white light.This study provides a useful idea and direction for further development of white phosphors based on NaLa(WO4)2:Eu3+.3.NaLa(WO4)2:Eu3+/Tb3+phosphors were synthesized in the hydrothermal processes.The emission spectra,excitation spectra and fluorescent decay curves of the samples were measured to investigate the luminescent properties.We have revealed the bi-directional energy transfer processes between Eu3+and Tb3+ions in NaLa(WO4)2,which effect the luminescence properties.By adjusting the concentrations of Tb3+ and Eu3+ ions,the obtained phosphors emitted white light.Under the excitation of 382 nm,the chromaticity coordinate of NaLa0.96(WO4)2:1%Eu3+,3%Tb3+was(0.31,0.33),the correlated color temperature was 6472 K and color rendering index was 81.07,which indicating that it may be used as a candidate for white phosphors under ultraviolet excitation.4.NaLa(WO4)2:Sm3+phosphors were prepared via a molten salt method.Under the excitation of UV,the blue-green light emitted by the self-activation of WO42-was mixed with the green-yellow-orange-red light emitted by Sm3+ ions to form white light.The phosphors emit cold white light under the excitation of 250 nm and warm white light under the excitation of 403 nm.With the increases of Sm3+ions doping concentration,the correlated color temperature of phosphors gradually decreases,and the color rendering index gradually increases.NaLa(WO4)2:Sm3+phosphors have thermal stability and can be effectively excited under near ultraviolet light,which can be used as a potential materials candidate for single matrix single-ion white phosphors.