Upconversion luminescence and non-contact optical temperature sensing in Ho3+-doped 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 lead-free ceramics

A series of Ho3+-doped 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 lead-free piezoelectric ceramics (NBT-0.06BT: xHo3+, 0.0025≤x ≤ 0.03) are prepared by a conventional solid-state reaction method. Under the excitation of 980 nm, all the samples exhibit strong green, red and near infrared upconversion emissions, which are ascribed to the 5F4/5S2→5I8 (∼547 nm), 5F5→5I8 (∼656 nm), 5F4/5S2→5I7 (∼756 nm) transitions of Ho3+ ions, respectively. The NBT-0.06BT: 0.015Ho3+ ceramic displays the optimal emission among these Ho3+-doped NBT-0.06BT solid solutions. Optical thermometry behavior is evaluated via fluorescent intensity ratio (FIR) of green to red emissions of Ho3+ ions. The temperature sensing performances is investigated in the temperature range of 293–473 K based on non-thermally coupled levels (NTCL). The maximum relative sensitivity is found to be 0.35% K−1 at 293 K. The present investigation results imply the promising application of NBT-0.06BT: Ho3+ in non-contact optical thermometers.