Studies On Oxide-coating Of YAG Phosphor And Its Influence Mechanism Of Luminescent Properties

YAG(Y₃Al₅O₁₂:Ce³⁺)phosphor is one of the important components of a new generation of solid light source,white LED.It has excellent yellow emission characteristics and can be packaged with blue?In Ga N?LED chips to form white LED.Although YAG phosphor is the most mature commercial phosphor in white LED industry at present,its thermal degradation seriously affects and restricts the performance of white LED.A large number of studies have shown that the thermal stability of phosphors can be effectively improved by surface coating.However,the influence mechanism of coating on the luminescence properties and thermal degradation is still insufficient.In this paper,by coating different oxides on the surface of YAG phosphor and optimizing the process parameters,the effects of surface coating on the luminescent properties and thermal degradation of YAG phosphor were compared and analyzed,and then combined with the thermal degradation mechanism of YAG phosphor,the influence mechanism of surface coating on the luminescent properties and thermal degradation was discussed.A sol-gel method was adopted to coat Ti O₂ with different theoretical coating amounts on the phosphor surface,and a heterogeneous precipitation method was adopted to coat a oxide layer?Si O₂,Al₂O₃,Mg O?with different theoretical coating amounts on the phosphor surface.The effects of different coatings on the morphology,structure and luminescent properties of phosphors were studied by means of SEM-EDS,TEM,FTIR,XRD and PL.All kinds of coatings form compact and uniform coatings on the surface of phosphors,and do not change the phase structure of phosphors.Although the luminescent intensity of YAG phosphor before heat-treatment was weakened by surface coating,the luminescent intensity of YAG phosphor after heat-treatment was significantly enhanced compared with that of uncoated phosphor,that is,the thermal stability of phosphors was improved.The influence of theoretical coating amount on the luminescent properties and structural defects of YAG phosphor was studied.With the increase of theoretical coating amount,the luminescence intensity of phosphors decreased as a whole,while the thermal stability increased first and then decreased.When the theoretical coatings of Ti O₂,Si O₂,Al₂O₃ and Mg O were 4%,4%,6%and 6%,respectively,the phosphors had good thermal stability.With the increase of theoretical coating amount,the defect concentration of phosphors decreased continuously,which indicated that some lattice defects of phosphors were gradually repaired by coating layer.The effects of different coatings on the luminescent properties of YAG phosphors were studied.The weaken effect of Ti O₂coating on the luminescence intensity of phosphors was the most significant,but the enhancement effect on the thermal stability is the worst.The weaken effect of Mg O coating on the luminescent intensity of phosphors was the least,but on the thermal stability of phosphors was the best.This was related to the different refractive index and transmittance of different coatings.The influence mechanism of oxide coating on the luminescent properties of YAG phosphor was analyzed.On the one hand,the light transmittance of the coating decreased due to the refraction of light,which was shown as the obstruction of light emission and the reduction of light intensity;on the other hand,the coating could repair the surface defects of phosphors and inhibit the thermal expansion of micro-defects and structural distortion,which showed the improvement of thermal stability.On this basis,the influence of refractive index of the coating on the luminescent properties of phosphors was summarized,and a structural modification mechanism on improved thermal stability of coated phosphors was proposed.