Morphology Control And Luminescence Performance Analysis Of BaWO₄:Sm³⁺ Synthesized By Molten Salt Method

The traditional white LED yellow phosphor presents many shortcomings due to the lack of red light components,such as high color temperature,low color index,and so on.With high stability,monochromaticity and energy efficiency,long life and high quantum yield,barium tungstate is considered as a kind of important luminescent material and exceptional application potential,widely used in the field of flash equipment,batteries,photoelectric and microwave ceramic solid-state laser.The special structure of tetragonal scheelite-type,providing a good condition for rare earth ion doping,is regarded as a potential substrate of luminescent materials.It can effectively compensate the defects of yellow phosphor in traditional white LED.In this thesis,BaWO₄:Sm³⁺phosphor was synthesized by molten salt method.It was characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,energy spectrum analysis system?EDS?,ultraviolet visible absorption spectra,Raman spectroscopy and photoluminescence?PL?techniques.The crystal morphology was optimized by adjusting the process parameters.The morphology and dispersion of obtained BaWO₄ powder in composite salt of LiNO₃-NaNO₃,LiNO₃-KNO₃ and NaNO₃-KNO₃conditions were all better than in single salt of LiNO₃,NaNO₃ and KNO₃conditions.The effect of reaction temperature on partical morphology in composite salts was studied further.In LiNO₃-KNO₃,well developed shape of octahedron with the size of 2.51⁹.67?m was synthetised at 310?.With the increase of temperature,the phenomenon of morphology evolution was appeared.When the holding time was 12h and the molten salt content of1:12,growth of grains was fine with uniform sizes.In LiNO₃-KNO₃,the different morphologies of BaWO₄:Sm³⁺powders indicated that the phosphor crystals could evolve into octahedron-type in 310?and polyhedron-type in 500?,which are both of high purity scheelite tetragonal structure.Luminescent intensity of the polyhedral shape is 230%higher than octahedron,the color purity of polyhedron was only 89.69%compared with octahedron of 92.99%,and color temperature was 2358K which was lower than octahedron's 3382K.It showed that the octahedron-type phosphor has good luminescent properties with higher color purity and the color temperature is in the optimum color temperature region of WLED.The optimum doping concentration of BaWO₄:xSm³⁺phosphor was3mol%.The critical distance when energy interaction of Sm³⁺occurred was 18.52?.Under the excitation of near-UV light,the color coordinates of BaWO₄:3%Sm³⁺phosphor was?0.4585,0.5133?,color temperature was3382K,the purity of color was 93%.The results showed that BaWO₄:x Sm³⁺has great potential application in yellow phosphors.Lattice parameters of the BaWO₄ and BaWO₄:3%Sm³⁺crystal were obtained by the structure refinement,which were a=b=5.621?,c=12.703?,v=401.372?³ and a=b=5.614?,c=12.719?,v=400.865?³,respectively.Both of crystal structures were tetragonal scheelite-type.The bond lengths and bond angles of tetrahedral[WO₄]group and polyhedral[BaO₈]group were obtained by VESTA.It was found that the small differences in bond length and angle of two crystal structures.The results showed that Sm³⁺doping could cause slight change of the BaWO₄ crystal structure.The asymmetric properties of the BaWO₄:3%Sm³⁺crystal environment was given by J-O theory.The transition strength parameter?₅),magnetic dipole transition probability A_R?md?,the electric dipole transition probability A_R?ed?,radiative lifetimeand branching ratio₅)were also obtained.It is concluded that the yellow-emitting of BaWO₄:3%Sm³⁺phosphors is probably due to the larger branching of ⁴G_5/2 to ⁶H_5/2transitions of Sm³⁺ions.