Synthesis And Characteristic Of High Performance Yellow Phosphor By Coprecipitation Method For White LEDs

Because phosphor is the critical material of white LED producing, the progress of phosphor technology has very important effect on development of the white LED. Yellow phosphor still can't meet the requirement completely for white LED products at present, so it is imperative to raise the efficiency, reduce the granularity, and prepare the spherical phosphor. Chemical co-precipitation is a widely applicable method of wet chemical synthesis, which raw materials' composition can be mixed on the molecule or atom rank, which is helpful to control the doping amount accurately, raise the activity of reacting, reduces the firing temperature notably. Therefore, the co-precipitation method is very hopeful to substitute solid state reaction and apply to the industrialization production of the phosphors.In this paper, (Y_3-x-yRE_x)Al₅O₁₂:Cey yellow phosphors were synthesized by co-precipitation method with NH₃·H₂O-NH₄HCO₃ system as the precipitating agent. The styles of red shift the emission light of the phosphors were studied. Luminescent properties of white LED based on (Y_3-x-yRE_x)Al₅O₁₂:Cey phosphors prepared and blue InGaN chip were also investigated. The main points of the thesis are as following:①The luminescence mechanism, realization methods of white LED were overviewed. Characteristics of various phosphors synthetic technologies for white LED were discussed briefly.②The formation process and structure of the phosphor powders were investigated by means of DTA-TGA and XRD. And the effect of fluoride (NH₄HF₂, NaF, BaF₂, and AlF₃) as additive on the morphology, crystallite and photo-luminescent properties of Y₃Al₅O₁₂:Ce was studied by photo-luminescent excitation and emission spectra and SEM. The result shows that completely crystallized phosphors are obtained at 900℃, which is more than 300℃lower than the temperature of solid-state method. The phosphor particles have spheroidal shapes with uniform distribution, and their sizes are 0.3～1μm. The emission spectrum excited by 468nm shows a characteristic wide band with the peak at about 530nm. A small quantity of NaF as additive is useful for enhancing photo-luminescent intensity of Y₃Al₅O₁₂:Ce phosphors, and improving the morphology.③(Y_3-x-yRE_x)Al₅O₁₂:Cey(RE=Lu, Yb, Tb, Gd, La) phosphors were synthesized by co-precipitation method. The crystal structures and luminescence properties of (Y_3-x-yRE_x)Al₅O₁₂:Cey phosphors have been investigated by means of XRD and photo-luminescent excitation and emission spectra. (Y_2.95-xTb_x)Al₅O₁₂:Ce and (Y_2.95-xGd_x)Al₅O₁₂:Ce have the same crystal structure which is garnet with a different lattice parameter. Substitution by Tb³⁺ or Gd³⁺ ions in the Y³⁺ sites will red shift the emission light of the phosphor. The mechanism of modulation of the emission wavelength in the phosphor could be explained by variation in crystal field.④The Y₃Al₅O₁₂:Ce phosphors prepared can be used together with a InGaN blue LED to achieve white light when excited by blue light of the LED. The achieved color temperature is 5571K, with a device luminous efficacy of 45lm·W^-1, a color rendering index of 79.9 and a color point near the coordinate x=0.3308,y=0.3476. This could basically meet requirement of white emitting light source for illumination.