Strong visible up-conversion emissions and thermometric applications of Er³⁺–Yb³⁺ codoped Al₂O₃ prepared by the sol–gel method

The Er³⁺–Yb³⁺ codoped Al₂O₃ has been prepared by the sol–gel method using the aluminium isopropoxide [Al(OC₃H₇)₃]-derived Al₂O₃ sols with addition of the erbium nitrate [Er(NO₃)₃ · 5H₂O] and ytterbium nitrate [Yb(NO₃)₃ · 5H₂O]. The phase structure, including only two crystalline types of doped Al₂O₃ phases, θ and γ, was obtained for the 1 mol% Er³⁺ and 5 mol% Yb³⁺ codoped Al₂O₃ at the sintering temperature of 1,273 K. By a 978 nm semiconductor laser diodes excitation, the visible up-conversion emissions centered at about 523, 545, and 660 nm were obtained. The temperature dependence of the green up-conversion emissions was studied over a wide temperature range of 300–825 K, and the reasonable agreement between the calculated temperature by the fluorescence intensity ratio (FIR) theory and the measured temperature proved that Er³⁺–Yb³⁺ codoped Al₂O₃ plays an important role in the application of high temperature sensor.