Preparation And Properties Of Lanthanum Oxonitridosilicate Phosphors For White LED Application

Trichromatic phosphors excited by ultraviolet/near-ultraviolet LED chipsattract more and more attention for white light emitting diodes. In this thesis,lanthanide silicon oxynitride phosphors were studied, using La2O3, CeO2, SiO2andSi3N4as raw materials to prepare La5Si3O12N:Ce3+and LaSiO2N:Ce3+phosphors byhigh temperature solid-state reaction method. On this basis, adding the flux andadopting some post-treatments were proposed to improve the surface morphology,and to increase luminous efficiency of phosphors.The synthetic experimental results of La5Si3O12N:Ce3+and LaSiO2N:Ce3+phosphors indicated that, the reaction may not proceed sufficiently at lowtemperature and in short time. However, the phosphors may be partially decomposedat high temperature and in long time. Therefore, the optimum sintering temperatureand holding time for the two phosphors are1600℃and5h,1500℃and4h,respectively.The content of Ce3+doped was adjusted to investigate the impact on the crystalstructure, diffuse reflectance spectra, excitation and emission spectra, colorcoordinates of the phosphors, etc. The test indicated that, doping Ce3+has not asignificant effect on the crystal structure of the phosphors. However, with increasingof Ce3+doping concentration, the emission intensity is first increased and thendecreased. The optimum doping concentrations of the phosphors are5mol%and4mol%, respectively. A certain amount of NH4Cl was added as the flux, to explore theimpact of flux on the properties of phosphors. The characterization indicated that,adding the flux can improve the crystalline quality of the phosphors, and increasethe luminous intensity. But the excess flux would cause serious agglomeration,decreasing the luminous intensity. Therefore, the optimal dosages of the flux for thetwo phosphors were both3wt%.A series of post treatments were adopted for the as-synthetic phosphors to studythe impact on the performance of phosphors, such as annealing, washing and coating.The results showed that, the annealing can decrease the surface defects, and enhancethe luminous intensity. The washing can remove the impurity ions remaining in thephosphors, and improve the luminous performance. The coating can improve thedispersion of the phosphors and the surface morphology, in which the organiccoating may also improve the compatibility of the phosphor and organic solvents.The tests of luminous intensity and thermal stability for La5Si3O12N:Ce3+andLaSiO2N:Ce3+phosphors indicated that, LaSiO2N:Ce3+phosphors have largeremission intensity, and smaller temperature quenching effect compared with La5Si3O12N:Ce3+phosphors, which have a correlation with high N/O ratio, stablematrix structure. The LaSiO2N:Ce3+phosphors have good application prospect forthe blue phosphors in LED fields.