Preparation And Optical Properties Of GAGG-based Transparent Ceramics

Transparent ceramics have attracted increasing interest from both the industry and scientific community,due to their unique optical,mechanical,physical and chemical properties.Transparent ceramics have current and potential application in aerospace,defense and security,lighting,energy,healthcare,optics and optoelectronics,and sensors and instrumentation,etc.Gadolinium-aluminum-gallium-based garnet?GAGG-based?transparent ceramic is prospective material that can be applied as the scintillators,laser host materials,ceramic phosphors for white LED and long persistent ceramic phosphors,due to its excellent physical and chemical properties including its cubic phase structure and uniform rare-earth?RE?activators doping.However,the studies on GAGG-based transparent ceramics are inadequate.In this thesis,GAGG-based transparent ceramics were prepared via optimized sintering method.Sintering behavior,spectroscopic properties and the optimization of GAGG ceramic composition were intensive studied and discussed.The main contents are as following:1.Highly transparent Ce: GAGG ceramics were prepared using commercially available oxide powders as starting materials,without sintering additives.After being sintered at 1650? in flowing dry O₂ atmosphere,the ceramics exhibits uniform microstructure.The highest transmittance of as prepared samples with thickness of 1 mm around wavelength of 558 nm reaches 62%,which is close to that value of its comparative single crystal.This is the first report about highly transparent GAGG-based ceramics obtained by pressureless sintering.Effects of holding time and sintering atmosphere on the microstructures of ceramics have been intensively studied.The spectroscopic properties of Ce: GAGG transparent ceramics have also been investigated.The crystal field splitting between the two lowest absorption bands of Ce³⁺ has been determined at about 0.93 eV.The room-temperature Stokes shift of Ce³⁺ is about 0.43 eV.2.To improve the transmittance of Ce: GAGG transparent ceramics,ZrO₂ was selected as the sintering additives.It was found that the addition of ZrO₂ can effectively prohibit grain growth while promoting the removal of residual pores.Effects of ZrO₂ concentration on the phase composition,microstructure and optical properties of GAGG-based transparent ceramics were intensively studied.With 0.045 at.% ZrO₂,the ceramic showed the maximum optical transmission of 73% at 558 nm,which is very close to the value of 76% for its equivalent single crystal.The emission peaks of the ZrO₂ doped Ce: GAGG transparent ceramics remain unchanged with different ZrO₂ content,while the ceramic doped with 0.36 at.% ZrO₂ showed the highest fluorescence intensity.3.Ce: GAGG nanopowders were synthesized via co-precipitation method using ammonium hydroxide as precipitant.The highest specific surface area of as prepared powder reached 29.6 m²/g after a calcination process at 850 ? for 1.5h.Two-step sintering strategy was implemented to make Ce: GAGG transparent ceramics.Samples were firstly pressureless sintered at 1650 ? in flowing dry O₂ atmosphere for 2h,when the relative density reached 95% or more,before the samples were HIPed at 1550 ? in 200 MPa Ar atmosphere for 2h.Finally,the Ce:GAGG transparent ceramics was annealed in air for 20h.The transmittance reached 77.6% at 558 nm,which is exceeding the value of 76% for its equivalent single crystal.The decay time for the sample is about 56.7 ns and the light output reaches 48000 photons/Me V,which satisfy the property requirements of solid state scintillators for modern medical X-CT.4.Transparent Ce³⁺-doped Gd₃(Al_1-xGa_x)₅O₁₂ ceramics with varying x value from 0.1 to 0.9 were prepared via an easy accessible solid state reaction-oxygen sintering method.Effects of Ga content on the structure,morphology and optical properties of these Ce: GAGG ceramics were investigated.The shift of energy level of Ce³⁺ with varying Ga content in the GAGG host was also analyzed.With increasing Ga content in the host,the blue shift is found for excitation 5d₁ band and emission 5d bands of Ce³⁺,whereas the red shift is observed for the excitation 5d₂ band.The highest quantum yield and relative scintillation light yield were obtained in the Ce: Gd₃Al₃Ga₂O₁₂ composition.5.Nd: GAGG transparent ceramics could be a promising laser medium.Ndx Gd_3-xAl₃Ga₂O₁₂ transparent ceramics were successfully prepared via a solid-state reactive sintering approach from a mixture of commercial powders for the first time.Optimum transmittance as high as 77% in near-infrared bands,which is close to its theoretical value of 81%,was achieved in the 0.8 at.% Nd: GAGG ceramics.The spectroscopic properties of the Nd: GAGG transparent ceramics were also investigated.Absorption cross-section of the 0.8 at.% Nd: GAGG ceramic at 808 nm is about 4.1×10^-20 cm².By means of Judd-Ofelt method,the emission cross-section at 1063 nm is about 1.8×10^-19 cm².