Synthesis And Spectroscopic Studies Of Transparent Ceramic LuAG:Pr³⁺ And Upconversion Oxides Y₂O₃:Tm³⁺/Yb³⁺

This thesis consists of two parts: (1) preparation and luminescent properties of Lutetium aluminium garnet nanopowders and transparent ceramics doped with Pr³⁺ ions, and (2) enhancement of upconversion luminescence of nanosized Y₂O₃:Yb³⁺/Tm³⁺ codeped with Li⁺.1. Synthesis and Spectroscopic Studies of Transparent Ceramic LuAG:Pr³⁺Lutetium aluminum garnet (LuAG) precursors doped with different Pr³⁺ concentration (0.25, 0.5, 1.0, 3.0, 5.0at%) were synthesized via a co-precipitation method, using ammonium hydrogen carbonate as precipitant. The phase evolution and morphology of the precursor were characterized by XRD and TEM. XRD patterns show that the precursor remains amorphous below 900℃, and crystallization peaks begin to appear at 900℃. Calcining precursor at 1000℃results in complete transformation to LuAG and all diffraction peaks are well consistent with JCPDS No.73-1368. TEM image shows that the LuAG:Pr³⁺ nano-powder calcined at 1000℃presents near-spherical shape and is loosely agglomerated . The average particle size was in the range of 20³0nm in agreement with that estimated by Scherrer's equation. Such powder is beneficial to fabricate transparent polycrystalline ceramic.The resultant LuAG:Pr³⁺ powder was sintered into translucent ceramic without any additives in vacuum at 1150℃and then in nitrogen atmosphere at 1700℃. According to the SEM photograph of unpolished LuAG:Pr³⁺ ceramic, we can clearly see that the ceramic consists of tightly packed grains with irregular polyhedral morphologies, and the grain distribution of specimen varies typically between 1μm and 5μm. It is also obvious that the grain-boundary structure is well-defined and ceramic contains some residual submicron grains on the uneven surface. There is no pore to be observed in SEM photograph.Photoluminescence spectra of LuAG:Pr³⁺ powder and ceramic were measured at room temperature in vacuum ultraviolet (VUV) and ultraviolet (VU) region. For the 5d-4f transition of Pr³⁺ ions, dominant emission of ceramic samples peaking round 311nm has higher luminescence intensity. The suppression of host absorption and emergence of antisite defect absorption are observed in excitation spectra of ceramic sample. And the host absorption in ceramic samples is not as intensive as that in powder samples. In comparing with the ceramic sample, the host absorption band of powders becomes stronger, which reflects that there is a large energy loss in ceramic sample when host is to be excited. This phenomenon may relate the fact that the contamination and a high population of defects are brought onto the surface of unpolished ceramic in the ceramic molding and sintering process, and these defects serving as traps possibly lead to the inefficient energy transfer from host lattice to Pr³⁺ ions. The luminescent intensity of LuAG:Pr³⁺ vary with the Pr³⁺ contents and the quenching concentration is about 1.0at% for ceramic and 3.0at% for powder, which are much higher than 0.24 at% for LuAG:Pr³⁺ single crystals. This phenomenon shows that the ceramic has some superiority over single crystals.2. Enhancement of Y₂O₃:Yb³⁺/Tm³⁺ Upconversion Luminescence via Li⁺-dopingNanocrystals Y₂O₃:5%Yb³⁺/0.5%Tm³⁺ with different doping concentrations of Li⁺(0, 1, 3, 5, 7%) were prepared via a simple sol-gel method. The XRD results reveal that all the prepared Y₂O₃ powders belong to cubic structure, and the incorporation of Li⁺ ions can cause lattice distortion and then increase lattice parameter deduced from the slightly shift to lower angle of the (222) peak position.We measured the upconversion spectra of various concentrations of Li⁺ in Yb³⁺/Tm³⁺-codoped Y₂O₃ nanoparticles under 980 nm LD excitation at room temperature. And we mainly focus on the blue emission(488 nm). The upconversion luminescence intensity of Y₂O₃:Yb³⁺/Tm³⁺ nanoparticles at first increases significantly with increase of Li⁺ ions concentration and then rises to maximum on edge of 3 mol% Li⁺ ions concentration. When simply considering the peak strength, the maximum enhancement in quantity has been up to around 15 times.The underlying reason of UC luminescence enhancement by Li⁺ ions doping is revealed and the corresponding mechanism is proposed based on temperature enhancement effect, infrared (IR) transmittance spectra, Raman spectroscopy and phonon-assisted energy transfer rate equations. It can be reasonable concluded that the highest phonon energy available in the host lattice plays a prominent role in determining the efficiency of multiphonon-assisted upconversion process, like the Yb³⁺-Tm³⁺ pair.