Synthesis And Luminescent Properties Of Borate Phophors For White LEDs

As a new kind of lighting source in the field of lamps and display devices,white light emitting diodes have advantages including environmentally friendly,high efficiency and long persistence.One of the main approaches to obtain white light is to combine near-ultraviolet LED chips with the mixture of tricolor?blue,green and red?phosphors.With the development of research and application,people expect more for the luminescent efficiency of white LED.While,the high color temperature and low color-rendering index due to a deficiency of red color,as well as the lack of red and green phosphors that match well with near-UV LED chips are the main challenges in white LED field.It is important to develop new tri-color phosphors that can be excited by n-UV light and emit with high efficiency and good stability.Borates have low sinter temperature and stable structure,and are good phosphor hosts.In this paper,two kinds of borate phosphor Na₃Gd₂?BO₃?₃ and Y₂Ba₃B₄O₁₂ were prepared by high temperature solid-state method.The crystal structures,microstructures,luminescent properties,and the lumicescent mechamism were investigated comprehensively.Na₃Gd_2-x?BO₃?₃:xEu³⁺ phosphors of different calcination temperatures and different heat-retaining time were prepared.The results show that when the materials were calcined at 800 oC for 5h,the sample has best crystalinity and strongest emission intensity.The Na₃Gd_2-x?BO₃?₃:xEu³⁺ phosphors with different doping amount were researched.The phosphors have strong excitation peaks in the range of 350⁴00nm,especially at 394 nm.The dominant emission peaks are at 592 nm and 614 nm,and the peak at 614 nm is dominant.The optimal doping amount of Eu³⁺ is x=0.3 for the maximum emission.The CIE color coordinates of the Na₃Gd_2-x?BO₃?₃:xEu³⁺ phosphors are centred around?0.650,0.350?,which are close to the NTSC standard values of red.Multi-color emission phosphors can be obtained by doping more than one kind of rare earth ions into one host material,and this is an attractive topic now.In this paper,by the codoping of Tb³⁺ and Eu³⁺,Na₃Gd_2-x-y?BO₃?₃: xTb³⁺,yEu³⁺ phosphors that can be stimulated under UV and near-UV light were prepared.Under the excitation of 368 nm,the spectra of Na₃Gd_2-x-y?BO₃?₃: xTb³⁺,yEu³⁺ show character peaks of Eu³⁺ and Tb³⁺ simultaneously.With the increase of Eu³⁺ amount,Tb³⁺ enhances the emission intensity of Eu³⁺ by transferring energy through dipole-dipole interaction mechanism.The emission intensity of Eu³⁺ increases and the emission intensity of Tb³⁺ decreases,leading to the multi-color emission tunability of Na₃Gd_2-x-y?BO₃?₃:xTb³⁺,yEu₃ phosphor.The emission color varies as green?yellow?orange.Besides,take Dy³⁺ as the doping ions,single-phased white phosphor Na₃Gd_2-x?BO₃?₃:xDy³⁺ was synthesized.Under the 353 nm excitation,the samples' emission spectra peak at 479 nm and 574 nm,and the intensity of 574 nm yellow light is stronger than that of the 479 nm blue light,leading to the chromaticity coordinates of the phosphor locate in the white region where lean to the yellow scope.Y₂Ba₃B₄O₁₂ phosphors were prepared by the high-temperature solid state method.The optimized process parameters are emerged as following: the best temperature for presintering is 400 oC,the optimized calcination temperature is 1150 oC and the optimaized heat-retaining time is 3h.Under this process system,the as-prepared samples have pure Y₂Ba₃B₄O₁₂ phase,high crystalinity,smooth surface of the grains and best luminouscent properties.Under this optimum process parameters,Y_2-xBa₃B₄O₁₂:xDy³⁺ phosphor was prepared.The samples exhibit strong excitation spectra at 350⁴00nm,and two strong emission peaks are observed at 480 and 575 nm,leading to white light emission when emitted by near-UV light.The optimized doping amount of Dy³⁺ is x=0.20,and the color coordinates is?0.386,0.412?.Compare to ideal white light,Y_2-xBa₃B₄O₁₂:xDy³⁺ phosphor has lower colour temperature.Bisedes,the orange-red phosphors Y_2-xBa₃B₄O₁₂: xSm³⁺ were prepared and researched.The phosphors can be emitted efficiengtly by UV and near-UV light.The main excitation peak stands at 403 nm,and the main emission peaks are at 562 nm,602nm and 647 nm,respectively.The optimal doping amount of Sm³⁺ is x=0.15 for the maximum emission.The dipole-dipole interaction between Sm³⁺ ions is emerged to be the main mechanism in the concentration quenching process.The CIE coordinates of Y_2-xBa₃B₄O₁₂: xSm³⁺ phosphors are all at orange-red region,and this kind of phosphor has good prospect of research.