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 0.94Na(0.5)Bi(0.5)TiO(3)-0.06BaTiO(3) lead-free piezoelectric ceramics (NBT-0.06BT: xHo(3+), 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 F-5(4)/S-5(2)-> I-5(8) (~547 nm), F-5(5)-> I-5(8) (~656 nm), F-5(4)/S-5(2)-> I-5(7) (~756 nm) transitions of Ho3+ ions, respectively. The NBT-0.06BT: 0.015Ho(3+) 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.