Study On Preparation,structure And Properties Of Rare-earth Ions Eu²⁺/Ce³⁺ Doped Mgalon Transparent Ceramics

Spinel-type transparent ceramics are transparent-structure materials with excellent physicochemical properties,while a key research orientation about them is functionalized by doping functional ions.The rare earth Eu²⁺/Ce³⁺ions possess good luminous efficiency and spectral diversity,which makes them be frequently-used activators in optical functionalized organic and inorganic materials.Compared to other inorganic host materials,such as powder and single crystal,transparent ceramics possess remarkable structure features,that is,the basic microstructure is consist of crystal grains with periodic structure characteristic,and disordered grain boundaries.For the spinel type MgAlON,the narrow interspace of polyhedron indicates that the large-size rare earth ions could not dissolve in the crystal grain in quantity,hence,it could be an effective means that optically functionalizes it through grain boundaries?GB?doping.Through addition of Eu₂O₃/CeO₂ in raw powder of MgAlON transparent ceramic and pressureless sintering,the Eu²⁺/Ce³⁺doped MgAlON transparent fluorescent ceramics with excellent optical transparency and photoluminescence properties were obtained.The phase composition,microstructure and the closely related optical transparency,mechanical properties,thermal conductivities were investigated.The optical/X-ray energy absorption and transition properties introduced by the addition of rare earth cautions were characterized.The GB segregation behavior of rare earth ions was studied,combining with spectroscopy assisted by first principles calculation,the local environment of large-size rare earth ions in disordered grain boundaries was illuminated.The relationship of composition,local environment of rare earth ions and the optical transparency,photoluminescence properties were finally established.Moreover,by combining with the phase,element analysis and the SEM-CL method,the influence of MgAlON structure evolution in sintering process by addition of rare earth oxides was discussed.The conclusions are as follows:?1?The Eu²⁺doped MgAlON transparent fluorescent ceramics with doping content from 0.1 to 0.35 at%possess single phase and dense structure,exhibit good mechanical properties,thermal conductivity and excellent optical transparency from visible to intermediate IR.The strong photo-absorption in whole UV region caused by multiple absorption from host,impurities and 4f-5d energy transfer of Eu²⁺,made MgAlON transparent fluorescent ceramics be potential UV erasable window materials.Under the excitation of 340 nm UV light,the samples emit blue light centered at 420 nm,as well as show good fluorescence thermo-stability and optical converting efficiency,which made them applying as efficient UV excited and blue light emission material with high output power.?2?Lower content?0.005 and 0.01 at%?samples in 0.005⁰.02 at%Ce³⁺doped MgAlON transparent fluorescent ceramics possess single phase and dense microstructure,so they present good mechanical properties,thermal conductivity and optical transparency.Under the excitation of 320 nm UV light,the samples emit blue light centered at 410 nm.When the doping content get higher?0.02 at%?,the longer emission light from second phase impurities appear.The second phase seriously decreased the mechanical properties,thermal conductivity,optical transparency and fluorescence thermo-stability.The good mechanical properties,thermal conductivity and quantum efficiency of lower doping content sample made them potentially using in high power LED.Short fluorescent lifetime and good responsiveness to X-ray indicated they could be potentially applied as efficient scintillator.?3?Luminescence centers mapping and the spectral analysis show that the rare earth luminescence centers were concentrated on the GB region.The HRTEM-EDS analysis also proved that the large-size rare earth ions and impurities were located in narrow GB region.The first principles calculation result of polyhedron in spinel structure indicated that the coordination number of Al ions seriously affect the electron structure,and a greater coordination number means a strong ionicity.By combining the experimental TEM-EELS characteristic peak,it shows that the large size rare ions could replace the Al with large coordination number in incompact GB,and decreased the GB energy.So that the large size rare ions could stay stable in GB,but not formed a second phase.The Gaussian fitting results of fluorescent lifetime from single phase Eu²⁺/Ce³⁺doped MgAlON transparent fluorescent ceramics indicated that rare earth ions located in GB had two disparate lifetime,which corresponding to the GB transition and relax region environment.The MgAlON:Ce³⁺transparent fluorescent ceramic with high doping content had the third kind of luminescence center,which imported by the second phase.?4?The study based on the luminescence center tracing by cathode luminescence?CL?revealed that,compared to the CeO₂,the relatively more Eu₂O₃ could form mass liquid phase in initial stage of sintering process.There are three points of effect on microstructures of MgAlON ceramic in sintering process,that is,firstly in early stage of the sintering,pinning the crystal grain by forming second phase with the Al₂O₃ and preventing grain growth.Secondly,in the middle periods of sintering,chemical reaction with crystal grain and wetting the GB by forming liquid phase.Thirdly,dragging the GB by solid solution and controlling the grain size.