Study On Silicate Phosphors Activated By Rare Earth Ions

As a new kind of solid-state illumination light sourece,white light emitting diodes?WLED?have been recognized as a new generation lighting sources in 21 st century because of the advantages of energy-saving,high luminescent efficiency,long lifetime,small sizes,vibration-resistance,durability,flexible application,instantaneous startup,convenient adjustment and environmental protection,etc.However,along with the rapid development of WLED technology,the needs from consumers about the WLED products have been focused on higher technical parameters in luminescent efficiency,color rendering index and fluorescence decay,and emphasize more about thehealthy and comfortable environment instead of energy-saving and environmental protection during the use of WLED products.In order to satisfy the demands of high-level illumination market,WLED products with high color rendering index or even full-spectrum WLED with excellent color rendering index has gradually become a hot spot in the fields ofsemiconductor light source.Phosphorsplay a key role in WLED technology and many novel phosphors have been developed in recent years to obtain an ideal white light with high color rendering index.Among these phosphors,silicate-based phosphors show great potential applications for ideal color rendering index due to theircharacteristics of tunable excitation and emission spectra.It is well known that the crystal field of activated ions is strongly influenced by the host lattice.The luminescent property of silicate phosphors can be adjusted by introducing of different anion group.In this dissertation,several kinds of multi-color emission silicate-based phosphors suitable for the excitation of near ultra-violet and blue light source were investigated by the technical routes such as introducing of different anion-group,changing of the cation types in crystal lattice and exploration of new crystal structure,etc.The main research work in this dissertation focuses on the followings: 1.A series of M₅?PO₄?₂SiO₄:Eu²⁺?M=Sr,Ca,Ba?silicophosphate phosphorsA series of M₅?PO₄?₂SiO₄:Eu²⁺?M=Sr,Ca,Ba?silicophosphate phosphors were successfully synthesized under a reducing atmosphere by high temperature solid-state method.The synthesized phosphors have apatite crystal structure in the hexagonal system.The Sr₅?PO₄?₂SiO₄:Eu²⁺phosphors had wide-band excitation spectra in 200-500 nm region.Eu²⁺ had two luminescent centers,which were formed by occupied 7 and 9 coordinationcrystal lattices,and led to the formation of double emission peaks at blue lightand yellow light region.The wide-band emission spectra attributed to the 4f65d1-4f7 transition of Eu²⁺ ions.By the adjustment of Eu²⁺-doping concentration,the emission intensity varied and the location of emission peaks were red-shifted.The emission intensity reached its maximum with the emission wavelength of 569 nm when the Eu²⁺-doping concentration was 0.075 mol.The emission intensity decreased later due to Eu²⁺ ion concentration quenching caused by interaction between electric dipole and electric quadrupole.Investigations in the Sr_4.95-xCa_x?PO₄?₂SiO₄:0.05Eu²⁺ and Sr_4.95-xBa_x?PO₄?₂SiO₄: 0.05Eu²⁺ phosphors revealed that substitution of Sr by Ca or Ba ions did not change the crystal structure,and the doping ions formed limited-solid-solution with Sr.With the increase of Ca or Ba concentration,the spectra blue-shifted due to decrease of the upper shift of gravity center and splitting of energy level of Eu²⁺ caused by the electron cloud expansion effect or the decrease of crystal field strength.It was noted that when the Ba²⁺ doping concentration was 3mol,the amount of emission center formed by 9 coordination Eu?II?crystal lattice increased obviously,the emission spectra were single peak with the great increase of emission intensity.The Sr1.95Ba3?PO4?2SiO4:0.05Eu²⁺ phosphors with the blue-shifted emission peak of 515 nm and high transfer efficiency is a kind of green emission phosphors suitable for ultraviolet chip excitation.Package application results showed that the synthesized double-emission peak phosphors,single-emission peak yellow and green phosphors can be used in thehigh color rendering index WLED packed with near-ultraviolet or blue light LED chips.2.A series of M₈Mg?SiO₄?₄Cl₂:Eu²⁺?M=Ca,Sr,Ba?chlorosilicate phosphorsCa_8-xMg?SiO₄?₄Cl₂:x Eu²⁺chlorosilicate phosphorshave broad excitation and emission bands and can effectively excited by the light at the region of 300-470 nm,especially by the blue light with wavelength of 450-470 nm.The wavelength of emission peak was located in the blue-green light region at 508-511 nm.It wasdeduced from the calculation of empirical formulas that Eu²⁺ occupied Ca²⁺ sites with the coordination of 8 in host lattice.The synthesized Ca_7.87Mg?SiO₄?₄Cl₂:0.13Eu²⁺ phosphor achieved the highest luminescent intensity when Eu²⁺ doping concentrationwas x=0.13.The results of different fluxes experiments revealed that the luminescent intensity of the phosphor prepared by the addition of 2wt%BaCl2increased by 12%.The different substitutions of Ca²⁺ by Sr²⁺ and Ba²⁺ will result in the blue shift of emission peak along with the increase of Sr²⁺ and Ba²⁺ doping amount.Therefore,the emission bands can be easily adjusted at the blue-green light region of 450-510 nm.A blue phosphor can be obtained for x=4.8;when x=6.4,a light blue-green phosphor is obtained;while light-green phosphor was achieved for x=8.Combined the new designed blue,blue-green and green chlorosilicatephosphors in this research and the commercialred phosphor,the WLED with an ideal color rendering index?Ra>95?can be obtained.3.Lu₂CaMg₂Si₃O₁₂: Ce³⁺ silicate phosphors with garnet structureLu₂CaMg₂Si₃O₁₂: Ce³⁺silicatephosphorwith garnet structurewas successfully developed.The phosphors showed the broad excitation and emission bandsand the emission peak was located in orange light region of 580-597 nm under excitation of 450-480 nm blue light.The increasing Ce³⁺doping concentration resulted in the partial distortion of dodecahedron latticeof Ce³⁺ ions and the crystal field strength of Ce³⁺ is further enhanced,which finally promotes the energy splitting and formed spectroscopic red-shift.The effect of Ce³⁺ contents on the photoluminescent properties of Lu₂CaMg₂Si₃O₁₂: Ce³⁺ phosphor has also been included in this research and the phosphor showed the highest luminescent intensity whenCe³⁺ contents is 0.04 mol.It can be analyzed from Dexter energy resonance theorythat concentration quenching of luminescent center is attributed from the interaction of dipole-quadrupole.The effects of different fluxes on luminescent intensity were also thoroughly studied and the results demonstrated that luminescent intensity of Lu₂CaMg₂Si₃O₁₂: Ce³⁺ phosphor can be evidently improved using H₃BO₃ and NH₄Cl as flux,respectively.In Lu_0.95-xCe0.05)₂Ca_1+2xMg₂Si₃O₁₂ phosphors,the introduction of the more Ca²⁺ will results in a slight red-shift of emission spectrum.Thermal stability and water resistance test indicated that the silicate phosphors with garnet structure owned stable physical and chemical properties.Lu₂CaMg₂Si₃O₁₂: Ce³⁺ has been demonstrated an effective orange phosphor to improve color rendering index by LED package experiment.4.Full-spectrum WLED with excellent color rendering indexThe application of the multi-colorphosphors with various substrates developed in this thesis has been also investigated.It can be concluded from LED packaging experiment that a blue chip combined with light blue-green phosphor Ca_7.92Sr_0.08Mg?SiO₄?₄Cl₂:0.13Eu²⁺with the emission wavelength of 480 nm,yellow-green phosphorSr_2.95Ba₂?PO₄?₂SiO₄:0.05Eu²⁺with the emission wavelength of 522 nm,yellow-green phosphor Lu_1.8Ca_1.1Mg₂Si₃O₁₂:0.1Ce³⁺with the emission wavelengthof 595 nm,andcommercialred phosphor?Sr,Ca?_0.95AlSiN₃:0.05Eu²⁺with the emission wavelengthof 650 nmcan successfullyprepare full spectrum WLED withcolor correlated temperature of 4911 Kand excellent color rendering index?Ra=97.4?.5.Lu₂SrMg₂Si₃O₁₂:Eu²⁺silicate long afterglow phosphor with garnet structureThe novel long afterglow blue phosphors(Lu₂Sr_1-xMg₂Si₃O₁₂:xEu²⁺)was prepared by high temperature solid state method.The effects of different process factors on luminescent property were also thoroughly studied.Lu₂Sr_1-xMg₂Si₃O₁₂: xEu²⁺ long afterglow phosphors show a broad emission band of Eu²⁺ which is corresponding to 4f65d1-4f7 transition?8S7/2?of Eu²⁺ ions.The luminescent intensity enhanced along with the increase of Eu²⁺ doping concentration within the scope of quenching concentration and the best Eu²⁺ doping concentration was 0.05mol%.The application of different fluxis in favor of the high temperature synthesis reaction and the phosphor synthesized by H₃BO₃ flux exhibits optimum luminescent intensity.The trapping leveldepth of Eu²⁺ in Lu₂Sr_0.95Mg₂Si₃O₁₂ host lattice calculated by heat release spectrum testis 0.9025 eV,which is a medium level and in favor of the formation of excellent long lasting phosphorescence.Eu²⁺ and Dy³⁺co-doping in the host lattice can dramatically promote the long lasting phosphorescence,which is essentially attributed to the formation of a newdeeper trapping levelwhen Ce³⁺ is introduced.And the energy storage process was achieved through electron trapping with the result that afterglow luminescencewas enhanced.