The Characteristics Of Up-Conversion Emissions Of Rare Earth Doped Al₂O₃ Powders

The Er³⁺ doped and Er³⁺-Yb³⁺ codoped Al₂O₃ powders have been prepared by the nonaqueous sol-gel method, and the effects of Er³⁺, Yb³⁺ doping concentrations and sintering temperatures on phase structures and up-conversion emission properties were systematically investigated by using differential thermal analysis-thermogravimetry (DTA-TG), Fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), up-conversion emission measurement, to explore the up-conversion emission mechanism of Er³⁺ doped and Er³⁺-yb³⁺ codoped Al₂O₃ powders, confirming that two-photon absorption up-conversion process was involved for the green and red up-conversion emissions. The fluorescence intensity ratio (FIR) technique based on Er³⁺ doped and Er³⁺-Yb³⁺ codoped Al₂O₃ has been studied, and its potential application has been discussed.For the Er³⁺ doped Al₂O₃ powders, the phase contents diagram for the Er-Al-O system with the Er³⁺ doping concentration range of 0-5 mol% was described at the sintering temperature range of 550-1250℃, there were three crystalline types of Er³⁺ doped Al₂O₃ phases,γ,θandα-(Al, Er)₂O₃, and two stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases. The Er³⁺ doping suppressed crystallization ofγandθphases and delayed phase transition ofγ→θandθ→α. Compared with the undoped Al₂O₃ powders, the transition temperature ofγ→θandθ→αwas increased about 60 and 110℃for 5 mol% Er³⁺ doped Al₂O₃ powders, respectively. The green and red up-conversion emissions centered at about 523, 545 and 660 nm, corresponding to the ²H_11/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected for the Er³⁺ doped Al₂O₃ powders, and the up-conversion emission intensities increased with phase transition ofγ→θ→α, indicating that theα-(Al, Er)₂O₃ phase with lower symmetry was more appropriate for up-conversion emission. The two-photon absorption up-conversion process was involved for the green and red upconversion emissions of Er³⁺ doped Al₂O₃ powders. For 0.1 mol% Er³⁺ doping concentration, excited state absorption of ⁴I_11/2+a photon→⁴F_7/2 and ⁴I_13/2+a photon→⁴F_9/2 was major upconversion emission process; with increasing the Er³⁺ doping concentration to 1 mol%, cross relaxation of ⁴I_11/2+⁴I_11/2→⁴I_15/2+⁴F_7/2 and ⁴I_11/2+⁴I_13/2→⁴F_9/2+⁴I_15/2 was also responsible to upconversion emission. Yb³⁺ codoping enhanced greatly the up-conversion emission intensities of Er³⁺ doped Al₂O₃ powders. For the Er³⁺ doping concentration of 0.1-5 mol%, the up-conversion emission intensities of the Er³⁺-Yb³⁺ codoped Al₂O₃ first increased and then gradually decreased with increasing the Yb³⁺ codoping concentration. Compared with compound (Yb, Er)₃Al₅O_<sub>12, a higher solubility of Yb³⁺ inγ-(Al, Er, Yb)₂O₃ andθ-(Al, Er, Yb)₂O₃ phases benefited upconversion emissions because it can enhance energy transfer (ET) effect and absorption efficiency of pump energy. With Er³⁺ doping concentration of 0.1-1 mol%, a proper molar ratio of 1: 10 for Er³⁺ and Yb³⁺ resulted in the maximum intensities of both the green (I_green) and red up-conversions (I_red), which achived the level of application due to the red up-conversion emission being observed easily by naked eyes. The intensity ratio of the red to green upconversion emission (I_red/I_green) Was analysed with rate equations of Er³⁺-Yb³⁺ codoped Al₂O₃ system. A higher efficiency of ⁴I_13/2+a photon→⁴F_9/2 and ²F_5/2(Yb³⁺)+⁴I_13/2(Er³⁺)→²F_7/2(Yb³⁺)+⁴F_9/2(Er³⁺) led to apparent increase in I_red/I_green with an increase of Yb³⁺ codoping concentration. The high Yb³⁺ codoping concentration and the high pump electric current can enhance the ratio of 523 and 545 nm (I₅₂₃/I₅₄₅), which is a function of temperature and is independent of phase structure.The FIR technique was applied to the Er³⁺ doped and Er³⁺-Yb³⁺ codoped Al₂O₃. For the optical high temperature sensor based on the Er³⁺ doped and Er³⁺-Yb³⁺ codoped Al₂O₃, with operating temperature range of 295-773 K and 295-873 K, pump power with 800 mW and 350 roW, the maximum sensitivity is 0.0052 K^-1 and 0.0051 K^-1, respectively, and their resolution are about 0.3 K, suggesting that the Er³⁺ doped and Er³⁺-Yb³⁺ codoped Al₂O₃ have great potential of application in the optical high temperature sensor.