Upconversion luminescence and temperature sensing characteristics of MgAl2O4 spinel doped with Er3+ and Yb3+ ions

Nanorod-shaped MgAl₂O₄: xEr³⁺, 0.05Yb³⁺ (where x ranges from 0.003 to 0.015) were synthesized using a low-temperature hydrothermal method. This study investigates the microstructure, upconversion luminescence performance, and temperature sensing characteristics of these phosphors. Results indicate that with increasing Er³⁺ doping, lattice distortions due to vacancy defects and lattice expansion first increase and then decrease, peaking at an Er³⁺ doping concentration of 0.015. A calcination temperature of 1100 °C was determined to be economically optimal, with a total weight loss of 38.5% during the precursor calcination process. Under 980 nm excitation, the intensity and ratio of red to green emissions peaked at an Er³⁺ doping concentration of 0.01. By controlling the doping level of Er³⁺ and the pump current of the excitation light, the chromatic transition from yellow to green and color purity can be finely tuned. The maximum temperature sensitivity (Sa_max) based on thermally coupled levels for Er³⁺ in MgAl₂O₄ is 40.28 × 10^{- 4} K^{- 1}, and based on non-thermally coupled levels, Sa_max is 143.65 × 10^{- 4} K^{- 1}, showing better performance than some common upconversion materials. Moreover, the calculated thermal activation energies for green and red transitions are ΔE₅₄₅ = 0.4863 eV and ΔE₆₅₈ = 0.4203 eV, respectively. The high temperature sensitivity and thermal activation energies suggest that these phosphors are promising candidates for non-contact temperature sensing applications.