Solvothermal Synthesis Of YAG: Ce Phosphors

Ce³⁺-doped yttrium aluminum garnet (YAG: Ce) belongs to the isometric crystal system, and has the merit of high chemical stability, high hardness, high melt point and long life et al. It has good weatherabiltiy and will be very stability even if in the circumstance of humid and ultraviolet without coated. So, YAG:Ce is the most important fluorescent materials, and has been used widely as ultra-short afterglow phosphors for cathode ray tubes and high-resolution displays. At the same time, the phosphors and blue LED chip can make up of optic white light, namely white LED. This white LED has wide application future and will become the forth new illuminant. Firstly, YAG nanoparticles were prepared by solvothermal method low costly. The effects of the mass of dispersion, precipitants, concentration of mother solutions, holding time on the formation of YAG and powder properties were investigated. The results show that the addition of appropriate amount of ammonium sulfate can obviously improve the dispersion. The precursors composes of metal hydroxides of carbonate using CO(NH2)2 as the precipitant. However, the precursors prepared are mainly compound of carbonate using NH4HCO3 as the precipitant. If the NH4HCO3 as the precipitant and proper pH value are adopted, pure-phase YAG with nano-sized, well-dispersed and spherical morphology can be obtained by heated at 200℃for 5h without any other intermediate phase with low metal ions concentration ([Al3+]<0.2mol/l); but pure-phase YAG can not be synthesized with high metal ions concentration ([Al3+]>0.5mol/l), which need longer holding time. This is mainly caused by the segregation of the precipitation Al3+ and Y3+ in the precursors and aggleration of particles. The synthesizing reaction for long time promotes the growth of grains and results in the agglomeration of grains.The simplified methods of synthesizing YAG:Ce phosphors are performed based on the study on synthesizs conditions of YAG powders. The results show that the pure-phase YAG:Ce with spherical morphology can be synthesized under 200℃for 5h as the precipitants without the processes of washing and drying. The excitation spectra of YAG:Ce phosphors have two peaks. The major one is around 460nm attributed to the 4f→5d transitions. Its emission band is wide with the peak at 530nm and it belongs to the 5d→4f transitions. Ce³⁺ ions act as the activation in the phosphors, they effect the luminescent properties of YAG:Ce phosphors. In this thesis, the best concentration of Ce³⁺ is made certain by comparing the emission spectra intensity of YAG:Ce phosphors with different Ce³⁺concentration.The formation mechanism of synthesized YAG:Ce phase was discussed on the basis of the research above. The findings indicated that YAG:Ce phase is directly formed by the dissolution/crystallization mechanism of precursor. This is due to the solution to reach the critical nucleation value for the YAG:Ce crystallites and promotes the formation of YAG:Ce grains at a much lower temperature than that of other methods. Some of the results are creative and useful, which is the important significance to prepare compounds under solvothermal conditions.