| Scintillators are functional materials that can convert high-energy particles into visible light.Scintillators coupled with light sensors, such as photomultiplier tubes, have been widely used inhigh-energy physics, nuclear medical imaging, geological exploration, security inspection andother fields. Single crystal and ceramics are two important scintillators. Nowadays, more and morepeople focus on the ceramic scintillators because they have many dramatic advantages over singlecrystals, such as easy accessible preparation process, low production cost, better machinability,high activator concentration and uniformity. Until now, the commercial available scintillators usedfor medical Computed Tomography are all polycrystalline materials, such as YGO and Gemstoneproduced by GE, and GOS utilized by Hitachi, Siemens and Toshiba. Gemstone has the bestperformance among all the existing CT scintillators, while the price of this material is extremelyexpensive due to its special composition including rare earth elements Lu and Tb. We hope to findother ingredient to replace those elements and finally prepare the ceramic scintillator with lowercost and better performance.Gadolinium aluminum gallium garnet (Gd3(Ga,Al)5O12:Ce，GAGG) is one of the mostpromising scintillators due to its cubic structure, high light yield, high density and short decay time.However, controllable synthesis of its ceramic material remains a huge challenge. This thesis aimsto explore the controllable preparation process of the GAGG:Ce scintillator material.GAGG:Ce powders were synthesized via co-precipitation method. The influence of rawmaterial ingredient and preparation process on the phase structure, morphology and luminescentproperties of the powders were studied and discussed. Finally, the GAGG:Ce ceramic scintillatorwith high crystallinity was successfully prepared. Main conclusions of this thesis are shown asfollows:1．Influence of raw material ingredient on the property of GAGG powdersNano-sized GAGG powders were successfully synthesized via co-precipitation method. Theinfluence of three different precipitators including NH4OH, NH4HCO3and mixed solution ofNH4OH and NH4HCO3on property of the powders were investigated. It is found that powders obtained from the mixed precipitation method exhibit the best micro structural morphology,highest luminescent intensity and good sintering ability. Pure crystalline phase of GAGG can beobtained after a calcination process at950oC for1.5h, with its average particle size around50nm.The density of sintered ceramic is99%of its theoretical density. In conclusion, the mixedprecipitant is of a good choice for preparing ceramic scintillators.Exploration of the influence of the Al(NO3)3, Al2(SO4)3and NH4Al(SO4)2on the properties ofpowders and ceramics were studied. It is found that powders prepared from NH4Al(SO4)2havegood uniformity, good sintering activity and specific surface area more than18m2/g. The assintered ceramic has good transparency.The macromolecule surfactant of polyethylene glycol (PEG) has high space steric effectwhich can effectively improve the powder dispersity. At the same time, PEG can be totallyremoved without introducing impurities into the powders because there are only C and H elementsin its molecule. The addition of PEG has little effect on the phase transition of the precursors.2. Parameters optimization of GAGG co-precipitation processThe influence of deionized water dosage during the filtration washing process on the phasestructure, morphology and dispersity of the powders were studied and discussed. Different amountof leaching water had evident influence on the morphology and phase structure of the powders.With less amount of water leaching, GAGG phase can be obtained at lower temperature, while asecond phase of Gd2O2SO4coexisted. On the contrary, when the washing amount is too much,some intermediate phases appeared, and the powders were seriously aggregated, which lead topoor ceramic sintering performance. Pure GAGG phase with good sinter ability can be obtained at950oC for1.5h when the leaching water is1500ml/50g. The powders are uniform with theaverage size about50nm.Three different drying methods including oven drying, freeze drying and microwave dryingwere studied. It is found that the powders obtained with the microwave drying method haveuniform particle size, good sintering activity. What’s more, this method is easy accessible andefficient. Drying time can be reduced to40min, which makes it suitable for big-batch powderpreparation.Particle size and luminescent intensity of powders were significantly influenced by thecalcination temperature. The powders with average particle sizes between35to80nm wereprepared at different calcination temperatures ranging from850~1100oC. And the powderscalcined at950°C for1.5h obtains pure GAGG phase and high luminescent intensity. Thosepowders would be suitable for the preparation of GAGG: Ce transparent ceramics due to theirspecific character including high purity, nano-sized particle and high sintering ability. 3. Exploration of the praperation process of GAGG ceramic scintillator with high structuralqualityCeramic green body with the density around43~45%can be obtained using cold isostaticpressing at300Mpa. With oxygen sintering, single-phase GAGG ceramic scintinllator can beobtained. Oxygen atmosphere can avoid the evaporation of Ga2O3at high temperature andguarantee the stability of its chemical composition. The density of as sintered ceramic is more than99%of its therical density, with small amount of porosity at grain boundary. Ceramictransmittance is about25%with its grain size about4~5m.HIP sintering can increase the ceramic density to more than99.5%and the porosity at thegrain boundary was effectively removed. Ceramic transmittance can be improved to more than60%. Fluorescence emission spectrum peak of as prepared ceramics is observed at536nm, whichcan match the Si photodiode well. The decay time after X-ray irridation is about64.5ns. It isexpected to be apllied to CT detector. |
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