Surface Uniform Coating And Its Luminescence Properties Research Of SrAl₂O₄:Eu²⁺,Dy³⁺ Phosphor

As a new type of long persistence luminescent material, SrAl₂O₄:Eu²⁺,Dy³⁺ luminescence material is widely used in lighting, display, decoration and other fields because of its high luminescent efficiency, high brightness, long afterglow time chemical stability and no radioactive. However, SrAl₂O₄:Eu²⁺,Dy³⁺ luminescence material also has some shortcomings such as hydrolysis, poor thermal stability and bad dispersion, which restrict its application in various fields and limit its use scope. Some researches indicate that, surface coating is an effective way to improve the existing problems of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor. But the present coat technology also exist some shortcomings. Especially the liquid phase method could result in the hydrolysis of luminescence materials in the coating process.In this paper, tetraethyl orthosilicate as the starting material was used to inhibit the hydrolysis of phosphor under certain conditions of hydrolysis and polymerization. Urea was used as precipitant. The heterogeneous precipitation method was adopted to coat a SiO2 layer on the surface of SrAl₂O₄:Eu²⁺,Dy³⁺ long persistence phosphor in aqueous medium under certain conditions of homogeneous reaction and coating principle of heterogeneous nucleation of urea. The phase 、 surface morphology 、 luminescent properties、water resistance and thermal resistance performance are analyzed by XRD、SEM-EDS and other test methods. The reaction time and the theory coat amount which affected the luminescent phosphor were researched. The quantitative relationship between the reaction conditions and the luminescent properties was established. On the condition of a continuous and uniform density film, the best process parameters were confirmed. The theory coat amount was 5% and the reaction time was 2 hours. Scanning electron microscopy test results showed that a continuous and uniform density SiO2 film layer was coated on the surface of phosphor successfully. XRD analysis results showed that the crystal structure of coated phosphor did not change. The impurity phase Sr3Al2（OH）12 generated in the deliquesce disappeared after coating. It could be dissolved in the coat system. Luminescent and afterglow performances test showed that the coat process did not change the crystal structure. With the increasing of the coat amount or the prolonging of reaction time, the intensity and brightness of coated sample presented a down trend. Water resistance test results showed that the water resistance of coated phosphor improved significantly. The pH value of system did not change after 24 hours dipping in the water. Compared with the uncoated sample, the intensity and brightness of coated sample changed unconspicuously under the same condition of heat treatment. It revealed that the coat did not improve the thermal stability property of phosphor.The key to improve the thermal stability of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor is to clarify the mechanism of thermal degradation. However, the study about thermal degradation mechanism of phosphor is less and there is no perfect explanation about it.In order to explore the thermal degradation mechanism of the long persistent luminescent material, the experiment took measures to the material with different heat treatment temperature. The phase, luminescent properties, afterglow properties and the surface composition were analyzed. The relationship between the peak intensity and heat treatment temperature was established and the rule between heat treatment temperature and the surface composition was also studied. The XRD test results showed that the crystal structure did not change with the increasing of heat treatment temperature. Through the luminescent properties test,the curve of peak intensity presented a regular change and two maximum intensity peaks appeared at 500℃ and 900℃ respectively. It corresponded to the two levels of trap energy level by heat damage successively. The XPS test results showed that with the increasing of heat treatment temperature, Sr and O composition on the surface of phosphor increased. On the condition of high temperature and oxygen, DySr�'or oxygen vacancy reacted with oxygen generated SrSr and OO and formed a lightless SrO layer on the surface which extended to the inside gradually. Therefore, the thermal degradation mechanism of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor was combined by the trap level of thermal damage and corrosion caused by surface structure.