Structure And Luminescent Properties Of Tungstate And Dual-substituted Silicate Fluorescent Materials

White light-emitting diodes?w-LEDs?have been extensively considered as a new generation of green lighting sources due to their tremendous advantages of energy saving,environmental protection and green lighting,and thus exhibit broad application prospect in the field of lighting.Most of governments have issued corresponding development plans to promote the research and application of w-LEDs.Although the w-LEDs based on the combination of blue LED chip and YAG:Ce have been commercialized in recent years,there are still many problems to be solved,such as high production cost,poor color uniformity,insufficient color rendering,poor thermodynamic stability and short lifetime.For w-LEDs,the matrix material is one of the most important part of determining its performance.There are many strategies to improve the matrix materials,the regulation of local chemical environment and microscopic characteristics is the most promising method to enhance the luminescent properties of the materials.In this thesis,novel tungstate and dual-substituted silicate fluorescent materials have been developed by introducing various cations with different ion radii into tungstate and substituing(Ca_0.8Ba_1.2)²⁺ions or Ca²⁺/Si⁴⁺ions by Mg²⁺/Eu²⁺or Sc³⁺/Ga³⁺ions,and the relationship of phase structure,microscopic morphology and fluorescent properties of the materials have been detailedly investigated and corresponding mechanisms have been proposed.The main research contents and results are listed as follows:?1?Among inorganic phosphors,divalent tungstates have attracted considerable attention in recent years because of their excellent optical properties,stable chemical properties,wide intrinsic spectra,and high average refractive indices.Cations with various cationic radii have been introduced into MWO₄:0.04Eu³⁺?M=Zn,Cd,Ca,Sr,or Ba?to regulate the morphology and luminescence.A series of materials have been synthesized via a hydrothermal method and corresponding structure-property relationship has been studied.The results indicate that cationic species have an important influence on the crystal structure,micro-morphology and corresponding fluorescence properties of the material.For M=Zn or Cd,the sample exhibits a wolframite structure,while M=Ca,Sr or Ba,the sample exhibits a scheelite structure.Eu³⁺doped ZnWO₄,CdWO₄,CaWO₄,SrWO₄ and Ba WO₄ samples exhibit nanorods,rodlike,torispherical,dumbbell-like,and doubletapter-like grains,respectively.With increasing cationic radius,the MWO₄:0.04Eu³⁺grain size increases from 32 nm to 30?m,the MWO₄:0.04Eu³⁺emission band shifts to shorter wavelength,the emission intensity obviously increases with cationic radius increasing for the samples with a monoclinic phase;however,it is the opposite for the samples with a tetragonal phase,and CaWO₄:0.04Eu³⁺exhibits an optimal emission intensity.?2?The silicate materials have been widely used as hosts owing to the virtues of the availability of the raw materials,moderate cost,the excellent chemical and thermal stability,structure complexity,and efficient absorption in the near-UV light region.The dual-substitution for cations in silicate materials is infavor of regulating the local chemical environment to enhance the luminescent properties.This study designed the(Ca_0.8Ba_1.2)_1-xMg_xSiO₄:yEu²⁺materials via the dual-substitution of Mg²⁺-Eu²⁺for(Ca_0.8Ba_1.2)²⁺.The(Ca_0.8Ba_1.2)_1-xMg_xSiO₄:yEu²⁺rare earth luminescent materials have been successfully prepared by a high temperature solid-state method.The effects of Mg²⁺/Eu²⁺substitution on the structure,morphology and luminescent properties of the materials have been studied.For(Ca_0.8Ba_1.2)_1-xMg_xSiO₄:0.04Eu²⁺,the luminescence color changes from light blue to blue when x increases from 0 to 0.11;for(Ca_0.8Ba_1.2)_0.93Mg_0.07SiO₄:yEu²⁺,the emission band shows the opposite shift when y increases from1%to 8%.The Mg²⁺/Eu²⁺substitutions lead to blue-shift/red-shift 14/20 nm.This is ascribed to the comprehensive result of the crystal field splitting and nephelauxetic effect.Adjusting Mg²⁺content,FWHM decreases from 100 nm to 70 nm and emission intensity increases to 2 times.The energy transfer mechanism between Eu²⁺is dipole-dipole interaction.?3?The design scheme of chemical cosubstitution of[Sc³⁺-Ga³⁺]for[Ca²⁺-Si⁴⁺]pairs in the Eu²⁺doped Ca_1.65Sr_0.35SiO₄?CSSO?phosphors has been put into practice to form Ca_1.65-xSc_xSr_0.35Si_1-xGa_xO₄:Eu²⁺materials.Ca_1.65-xSc_xSr_0.35Si_1-xGa_xO₄:Eu²⁺rare earth luminescent materials have been successfully prepared by a high temperature solid-state method.The effects of Sc-Ga dual-substitution on the crystal microstructure and fluorescent properties has been investigated.Sc-Ga dual-substitution has no obvious effect on the crystal structure of Ca_1.65Sr_0.35SiO₄.All samples show a pure orthorhombic phase structure.With the increase of Sc-Ga concentration,the diffraction peak shifts slightly to a low angle and the main peak is cleaved.The fluorescence intensity decreases monotonically with the increase of Sc-Ga concentration,and the quantum yield decreases from 52%to 19%.The red shift of the emission spectra is attributed to the comprehensive result of the crystal field splitting and nephelauxetic effect caused by substitution of Sc-Ga for Ca-Sr in Ca_1.65Sr_0.35SiO₄:Eu²⁺material.As the Sc-Ga concentration increases to 0.50,the decay lifetime decreased from 5.34 ms to 7.97 ms and finally to 1.32 ms.Under the excitation of 365 nm,when the temperature increases to 100 ^oC?150 ^oC?,the emission intensities of the x=0 and 0.20 samples become 66%?53%?and 75%?69%?of those at 25 ^oC,respectively.These suggest that the cosubstitution of Sc³⁺/Ga³⁺for Ca²⁺/Si⁴⁺can enhance the thermal ability of the Ca_1.65Sr_0.35SiO₄:Eu²⁺materials.