Preparation,Modification And Luminescence Properties Of Mn⁴⁺:K₂XF₆（X=Ti，Ge，Si） Phosphors

Mn⁴⁺-doped fluoride red phosphors are extensively used in white LEDs for their excellent luminescence performance to overcome the problems of low color rendering index and high color temperature.However,the luminescent center[Mn F₆]^2-group has the challenge of easy hydrolysis and fluorescence burst,which seriously affects the lifetime of LEDs.In this thesis,the moisture resistances of fluoride red phosphors are significantly improved by means of surface modification.The main studies are as follows:（1）Oxalate-covered K₂XF₆:Mn⁴⁺（X=Ti,Ge,Si）@MC₂O₄（M=Ca,Sr,Ba,Mn）was prepared by chemical precipitation.X-ray diffraction（XRD）results showed that MC₂O₄and K₂XF₆:Mn⁴⁺crystalline phases were synthesized,and scanning electron microscopy（SEM）results showed that K₂XF₆:Mn⁴⁺showed hexagonal sheet particles and MC₂O₄particles were encapsulated on its surface.The fluorescence spectra showed two broad excitation peaks from Mn⁴⁺:⁴A_2g→⁴T_1g,⁴T_2gnear 370 nm and 460nm,and a red narrow band emission peak from Mn⁴⁺:²E_g→⁴A_2gnear 630 nm.After60 days of immersion,the luminescence intensity of K₂TiF₆:Mn⁴⁺was almost zero,while the luminescence intensity of K₂TiF₆:Mn⁴⁺@Ca C₂O₄remained at 71.2%of the initial value.（2）Fluoride-covered K₂TiF₆:Mn⁴⁺@ZF₂（Z=Ca,Ba,Sr）was prepared by chemical precipitation.XRD results showed two crystalline phases of K₂TiF₆:Mn⁴⁺and ZF₂.SEM results showed that ZF₂particles were successfully encapsulated on the surface of K₂TiF₆:Mn⁴⁺.The fluorescence spectra showed broadband excitation peaks centered at 370 nm,460 nm,and a sharp emission peak at 630 nm.After 7 days of immersion,the luminescence intensity of K₂TiF₆:Mn⁴⁺maintained 1.9%of the initial value,while the luminescence intensity of K₂TiF₆:Mn⁴⁺@ZF₂maintained 72.6%of the initial value.（3）K₂TiF₆:Mn⁴⁺covered with oxalic acid to obtain homogeneous shell layer modified K₂TiF₆:Mn⁴⁺@K₂TiF₆.XRD results showed diffraction peaks of K₂TiF₆:Mn⁴⁺@K₂TiF₆corresponding to K₂TiF₆physical phase.SEM results showed irregular lamellar structure of K₂TiF₆:Mn⁴⁺@K₂TiF₆.The fluorescence spectrum showed a broad excitation band near 460 nm and a sharp red emission peak around 630 nm.After 7 days of immersion,the luminescence intensity of K₂TiF₆:Mn⁴⁺maintained2.1%of the initial value,while the luminescence intensity of K₂TiF₆:Mn⁴⁺@K₂TiF₆maintained 70.8%of the initial value.（4）G-K₂TiF₆:Mn⁴⁺was synthesized by green method and coated with Ca C₂O₄,Ca F₂and K₂TiF₆on its surface,respectively.XRD results showed that the diffraction peaks of G-K₂TiF₆:Mn⁴⁺corresponded to the K₂TiF₆phase,and the corresponding Ca C₂O₄and Ca F₂diffraction peaks also appeared after modification.SEM results showed that G-K₂TiF₆:Mn⁴⁺was a hexagonal lamellar particle,and the modified G-K₂TiF₆:Mn⁴⁺surface showed the corresponding small particles.After 48 h of submersion,the luminescence intensity of G-K2TiF6:Mn4+maintained 3.6%of the initial value,while the luminescence intensity of modified G-K2TiF6:Mn4+maintained 90.2%,78.0%and 82.2%of the initial value,respectively.