Surface Coated Modification Of Strontium Magnesium Silicate Phosphors

Strontium magnesium silicate phosphors(Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺)are thought to be the most promising long-afterglow phosphors because they possess many excellent properties,such as good chemical stability,water resistance,and ultravioletirradiation resistance.It has extensive application prospect in emergency illumination installation,traffic signal,architectural decoration and other fields.However,when Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ phosphor is used at medium to high temperatures,its thermal stability is poor due to Eu²⁺ ions being easily oxygenated into Eu³⁺ ions.Therefore,in this thesis,surface coated modification of the Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺phosphors was carried out by chemical method to improve its thermal stability.An amorphous SiO₂ coating on the surface of Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ phosphor was successfully prepared by the hydrolysis,polymerization,and condensation of the tetraethyl orthosilicate?TEOS?precursor.The effects of the coating ratio,water bath temperature,pH value,heating temperature and heating time on coating,luminescent intensity and thermal stability were investigated in detail.A mechanism for the surface modification with SiO₂ was proposed.The experimental results showed that the thickness of SiO₂ coating was about 8.4nm,the luminescent intensity of the SiO₂-coated phosphor is 23% higher than that of uncoated phosphor,as the coating ratio was 3%,water bath temperature was 40? and pH value was 2.In addition,when the heating temperature was below 1100?,the Si O₂-coated phosphors show better thermal stability than that of as-received phosphors.However,when the heating temperature was beyond 1100?,the thermal stability of the SiO₂-coated phosphors were worse than that of as-received phosphors.In addition,a boehmite ?-AlOOH coating on the surface of Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ phosphor was successfully prepared by the hydrolysis of Al?NO3?3.The effects of the Al³⁺ ions concentration,coating ratio,pH value and heating temperature on coating,luminescent intensity and thermal stability were examined in detail.The experimental results demonstrated that the thickness of SiO₂ coating was about 40 nm,as the Al³⁺ ions concentration was 0.02mol/L,the coating ratio was 2%,and pH valuewas 8.Although the ?-AlOOH coating was five times of the amorphous SiO₂ coating in thickness,its densification and uniformity were inferior to the latter.The emission spectra of the coated phosphor heated at above 1000? for 10 min showed the two broad emission bands centered at 384 nm and 470 nm,respectively.When the coated phosphor was heated at 900,1000?,1150? and 1200?,the luminescent intensity of the broad emission band centered at 470 nm was higher than that of the uncoated phosphor.However,while the coated phosphor was heated at 1000 and 1050?,the result was opposite.On the other hand,the luminescent intensity of the broad emission band centered at 384 nm was higher than that of uncoated phosphor,when the coated phosphor was heated at above 1000?.