A Study On The Co-precipitation Preparation And Luminescence Behavior Of Ce-doped Yttrium Aluminum Garnet Phosphor

White light emitting diodes (WLEDs) have the advantage of less energy consumption, no pollution, etc. WLEDs are called the forth generation lighting and have wide application prospect. Trivalent cerium activated yttrium aluminum garnet (YAG:Ce) is the primary phosphor for WLEDs. Recently, wet chemical method attracts more attention for preparation of phosphors, which has the advantage of reducing calcination temperature and shortening calcination time. Compared with other wet chemical methods, co-precipitation method has advantage of low cost, simple equipment and large scale production. Therefore, co-precipitation is a widely used method for preparation of phosphors. Previous research mostly focuses on luminescent properties of phosphor. The effect of Co-precipitation process on the physical, chemical and luminescent properties of phosphor is hardly systematically explored. For different lighting conditions, the phosphors owning different color coordinate and color temperature must be prepared. So, emission wavelength of YAG:Ce should be changed. Rare earth doping is an effective way to change emission wavelength. Coating process is an effective subsequent process. Film isolates phosphor from outside, eliminates the surface defects and improves the physical, chemical and luminescent properties of phosphor.This paper aims to develop YAG:Ce phosphor by co-precipitation and systematically researches effect of process conditions on the physical and chemical properties. The doping effect of rare earths on the physical, chemical and luminescent properties is explored. Meanwhile, coating treatment is applied to improve phosphor surface. Specific contents and main conclusions are shown as follows:(1) The influence of precipitant on the properties of YAG:Ce phosphor was investigated. The precursor has good dispersity, when selecting ammonia hydrocarbonate (AHC) as precipitant. The precursor directly transfers to YAG phase without appearance of other phases. Effect of concentration of precipitant, precipitation and aging temperature, aging time on the stoichiometry is discussed. The optimal process parameters are suggested. A molar concentration ratio between AHC and total cation is more than 4. Precipitation and aging temperature is room temperature (RT). Aging time is beyond 8h. The generation and control of agglomeration is analyzed. Ethanol–aqueous mixed solvents, adding polymeric surfactant and adopting heterogeneous azeortrope distillation (HAD) for drying precursor are constructive methods to reduce agglomeration.(2) Keeping Ce trivalence is a key circs during calcination. First, calcine the precursor in air. Then reduce the phosphor in reducing atmosphere. This procedure can reduce quadrivalent Ce sufficiently. The size of precursor comes into micron-size, when calcined above 1400℃. The luminescent decay contains two components, because of size effect. The long term reflects body Ce³⁺ luminescence, while the short term indicates surface Ce³⁺ luminescence. The most appropriate flux is BaF₂. The optimal added amount of BaF2 is 1/10 (mass ratios between fluxes and precursor). Systematic kinetic study is conducted on de-hydration, de-carbon, crystallization and crystal growth. The de-hydration fits the Jader function. The kinetics activity energy is 119.3 kJ/mol. Natural logarithm of Arrhenius frequency factor is lnA=24.6 s^-1. The de-carbon accords with Avrami-Erofeev function (n=3). The kinetics activity energy is 222.59 kJ/mol. Natural logarithm of Arrhenius frequency factor is lnA=24.7 s^-1. The apparent activity energy of crystallization is 1115.18 kJ/mol. Crystal growth coincides with the ninth order kinetic equation D 9 = kt = k 0 t exp( ? E / RT). The activity energy for crystal growth is 226.06 kJ/mol.(3) The effect of environmental temperature and lanthanide elements doping on the properties of YAG:Ce is investigated. The luminescence intensity decreases with the increase of environmental temperature, which is caused by the increase of the non-radiative relaxation rate. When environmental temperature increases above 150K, thermal quench effect is observed. The long decay component decreases with the increase of environmental temperature. While the short decay component shows intricate behavior, which results from effect of surface. The maximum emission intensity is obtained for 1mol% Ce additions. Concentration quench effect is observed when Ce amount increases beyond this limit. While Gd³⁺ doping concentration beyond 50 mol%, GdAlO₃ phase appears. When more than 20 mol% Y³⁺ is replaced by La³⁺, YAG phase completely disappears and LaAlO₃ phase emerges. As the doping concentration of Gd³⁺and La³⁺ increases, the maximum emission band red shifts, while the emission intensity decreases. As increment of Gd and La doping, the red content in the emission spectra increases, while blue content decreases.(4) Coating phosphor with Al by the sol-gel route and with Si by the solid-state reaction at room temperature was studied. The optimal parameters for sol-gel route are temperature above 90℃, HNO₃ as peptizer, concentration of HNO₃ 0.025mol/L. Al and Si coating treatment not change emission wavelength. Al coating slightly decreases the luminescent intensity. While, Si coating little increases the intensity.In this paper, the optimal parameters of co-precipitation are developed aimed at al cations coprecipitation. The effect of environmental temperature on the luminescent properties of YAG:Ce is investigated. The decay time of surface Ce³⁺ shows complex behavior, because of surface effect. The wavelength shows red shift, when YAG:Ce doped with Gd and La. The red content increases in the emission spectra of Gd and La doped YAG:Ce. Si coating using the solid-state reaction at room temperature makes the luminescent intensity slightly increases. These results provide instructive attempts of synthesizing YAG:Ce phosphor, and enrich the theoretical knowledge for further research.