Studies On Thermal Degradation Mechanism And Surface Coating Of Y₃Al₅O₁₂:Ce³⁺ Phosphor For White LED Application

Y₃Al₅O₁₂:Ce³⁺phosphor as an important aluminate luminescent material with excellent blue light absorption and yellow light emission characteristics can combine with GaN-based blue LED chip to produce white light,which is also currently the most mature phosphor for the commercialization of white LEDs.However,Y₃Al₅O₁₂:Ce³⁺phosphor has a serious thermal decay phenomenon.After working for a period of time at high junction temperature,the luminescent efficiency of Y₃Al₅O₁₂:Ce³⁺would reduce or even cause permanent damage to form thermal decay,which seriously affect the chromaticity and brightness of white LEDs.Many studies have shown that a layer of inorganic oxide film coated on the phosphor surface can improve the thermal stability and enhance resistance to thermal decay.But the studies on the thermal decay phenomenon of Y₃Al₅O₁₂:Ce³⁺phosphors are rarely reported at presents,and its relative mechanism are not clear.That also leads to the micro reasons for surface coating to improve the thermal decay of Y₃Al₅O₁₂:Ce³⁺phosphors are still not clear.Therefore,it is very necessary for us to have an in-depth study of the thermal degradation mechanism of Y₃Al₅O₁₂:Ce³⁺phosphor.On the basis of the above,the micro reasons for surface coating to improve the properties of phosphor are fully illuminated,which is an effective complement or verification for thermal degradation mechanism to form a complete theoretical system.To fully reflect the micro-structure changes before and after heat-treatment and thus to get accurate thermal degradation mechanism,a heat-treatment process was carried out to accelerate the luminous decay of Y₃Al₅O₁₂:Ce³⁺phosphor.The crystal phases,defect levels,micro-structure changes between surface and bulk phases and luminescent properties of Y₃Al₅O₁₂:Ce³⁺phosphors were characterized by XRD,PL,TL,XPS and FT-IR.The results were as follows:（1）The high temperature heat-treatment did not change the phase composition of phosphors,but phosphors produced obvious lattice distortion.（2）The emission intensity of Y₃Al₅O₁₂:Ce³⁺phosphor decreased with the increasing temperature.（3）The surface defects on the phosphor particles were gradually eliminated by the micro-structural adjustment and the adsorption of oxygen atoms,and formed a obvious micro-micro-structural distortion.Meanwhile,the degree of structural distortion increased gradually with the increasing temperature,the decrease of the concentrations of Ce³⁺and the distortion of its lying micro-environment were further aggravated.（4）Based on the above analysis results,the thermal degradation was jointly caused by the oxidation of Ce³⁺and the distortion of micro-structure.By establishing an intrinsic relationship between luminescent properties degradation and micro-structure changes,a thermal degradation based on micro-structural distortion was proposed.To precisely clarify the micro-reasons for surface coating to improve the properties of Y₃Al₅O₁₂:Ce³⁺phosphor,a heterogeneous precipitation method utilizing urea hydrolysis was adopted to coat a oxide film（SiO2,Al2O3,MgO）with different theoretical coating amounts on the phosphor surface.The effects of coating layer on the luminescent intensity and thermal stability were characterized by XRD,SEM,TEM,FTIR and PL.The variation tendencies of emission intensity and thermal stability with the increasing of theoretical coating amount were analyzed.The results were as follows:（1）A compact homogeneous oxide film was successfully coated on the phosphor surface,a coating combination based on chemical bond force was achieved.（2）The thermoluminescence peaks intensity decreased with the increasing theoretical amounts,and the surface lattice defects were gradually repaired and the micro-structure distortion was suppressed.（3）The luminescent intensity of coated phosphors had no obvious changes,but the thermal stability of coated phosphors was effectively improved by oxide layer.When the theoretical amount of SiO₂,Al₂O₃ and MgO was 4%,6%,6%respectively,the resistance to thermal decay was best.（4）Based on the above analysis results,we discussed the reasons for surface coating to improve the properties of phosphor:On the one hand,the coating layer isolated oxygen in the air and avoided the oxidation of activator ions(Ce³⁺).On the other hand,the surface defects on the phosphor particles were repaired and the formation and extension of micro-structural distortion were prevented by coating layer,the probability of non-radiative transition was reduced and the relative luminous efficiency increased.On this basis,a coating modification mechanism about the repair of surface defects or distortion was proposed.