Design, Fabrication And Luminescence Properties Of Cerium-doped Lutetium Oxyorthosilicate Scintillation Ceramics

Cerium-doped lutetium oxyorthosilicate（Lu2SiO5:Ce, LSO:Ce） is a scintillation materials with excellent comprehensive properties. Due to high density（7.4g/cm3）, high light yield（27,300 photons/MeV） and short decay time（40 ns）, LSO:Ce has a good application prospect in nuclear medicine imaging. However, low segregation coefficient（⁰.22） of Ce ions in LSO lattice results in increasing of the total Ce concentration from top to bottom in the boule which brought about light yield fluctuation in obtained LSO:Ce crystal. In addition, extremely high melting point（²100 ℃） of LSO phase is very close to the breakdown temperature of iridium crucible and heat insulation material, which result in cost increased. Hence, LSO:Ce was studied in this dissertation. Processing conditions of polycrystalline LSO:Ce ceramics was systematically studied. Relationships between key physical and chemical conditions in processing and properties, microstructure and composition of LSO:Ce ceramics were illuminated.Through a sol-gel route, LSO:Ce precursors was fabricated by using LuCl3?6H2O, CeCl3?7H2O and tetraethyl orthosilioate（TEOS） as raw materials, propylene epoxide（PPO） as reaction promoter, isopropyl alcohol as liquid medium. The precursor through vacuum drying under the temperature of 130 ℃ and followed calcing at 1000 ℃ for 2 h in air, aggregated LSO:Ce powders with grain size of 70 nm was obtained. Undergoing planetary ball milling, the particle size of the aggregated LSO:Ce powders decreased to 200 nm from 1 μm. Onset of sintering temperature of green body pressed from ball milling treated powders compared to that of as-calcined LSO:Ce powders decreased to 1200 ℃ from 1500 ℃, liner shrinkage at 1650 ℃ increased from 4.75% to 6.92%. Spherical LSO:Ce precursor powders was obtained by spray drying. Two type of LSO:Ce powders with different structure were respectively obtained by calcing the spherical precursor powders at 1000 ℃ and 1100 ℃ for 2 h in air atmosphere. The LSO:Ce powders with particle size of 2 μm were spherical granule assembled from nano-sized grains.LSO:Ce ceramics with relative density of 99.8% and mean grain size of 1.3 μm was obtained by spark plasma sintering（SPS） under the sintering condition of 1200 ℃/80 MPa for 5 min starting from the micro-sized spherical granules. Under the same sintering condition, LSO:Ce ceramics with mean grain size of 0.6 μm was sintered starting from the submicro-sized LSO:Ce powders, which relative density was 99.9%. Tanslucent LSO:Ce ceramics with relative density of 99.7% and average grain size of 5 μm was sintered at 1700 ℃ for 2 h in a flowing H2 atmosphere starting from compacts pressed from the submicro-sized LSO:Ce powders. Relative light yield of the tanslucent LSO:Ce ceramics attains to 91% of that of LSO:Ce single crystal, its decay time is only 20 ns. The above mentioned compacts could be consolidated into LSO:Ce ceramics with porosity of 0.28% and grain size of 1.44 μm by pressureless sintering at 1650 ℃ for 4 h in air. Then as-consolidated LSO:Ce ceramics was densified into optical ceramics by post HIPing in Ar atmosphere at 1600 ℃ and 150 MPa for 1 h. Average grain size of the optical LSO:Ce ceramics was 1.7 μm. No second phase and pores were observed in grains and between grainboundaries. In-line transmisttance（1.4%） of the optical LSO:Ce ceramics in visible light range ascribes to high birefringence of LSO crystal structure up to 0.028. The in-line transmittance of the LSO:Ce ceramics with thickness of 1 mm reaches to 50% at 4.4 μm, which accord with value calculated from Rayleigh–Gans–Debye model.X-ray absorption near edge structure（XANES） spectra manifest that Ce ions in LSO:Ce ceramics annealed at 1300 ℃ for 4 h in air atmosphere remain Ce（Ⅲ）. After annealing, defect（oxygen vacancy） depth in the HIPed LSO:Ce ceramics decreased to 0.66 eV from 0.95 eV, defect concentration in the annealed LSO:Ce ceramics decreased to 6.7% of that of as-HIPed LSO:Ce ceramics. Light yield of the annealed LSO:Ce ceramics attain to 1.81 times of that of as-HIPed ceramics. Emission intensity of Ce1（CeLu1） is 9 times higher than that of Ce2（Ce Lu2） in LSO:Ce ceramics. Decay times of Ce1 and Ce2 are 22 and 40 ns, respectively, which were shorter than that of LSO:Ce single crystal. Thermal quenching of Ce2 emission results in decreasing of total emission in the temperature range of 50-300 K. Transmission rate of charge carriers increases with increasing temperature in the temperature of 150-300 K, which results in increasing of Ce1 emission and total emission.Effects of Y³⁺ doping on luminescent properties of LYSO:Ce scintillation ceramics was studied. Extend X-ray absorption fine structure（EXAFS） spectra manifest that Y³⁺ substituted Lu³⁺. PL emission intensity of LYSO:Ce ceramics decreases with increasing concentration of Y³⁺, which ascribe to increasing Ce2 emission intensity. Decay times of Ce1 and Ce2 both increase with increasing concentration of Y³⁺ at the range of concentration of Y³⁺ 40%. In the range of concentration of Y³⁺ >40%, decay time of Ce1 is 30 ns, that of Ce2 is 47 ns. X-ray excitated luminescence（XEL） and 137 Cs spectra of LYSO:Ce ceramics suggest that light yield of LYSO:Ce ceramics increase with increasing concentration of Y³⁺ in the range of concentration of Y³⁺ 40%, which ascribe to oxygen vacancy around Ce³⁺ tend to redistribute around Y³⁺, while light yield of the ceramics decrease with increasing concentration of Y³⁺ in the range of concentration of Y³⁺ >40% due to concentration of oxygen vacancy increasing with increasing concentration of Y³⁺.