Fig. 1. Morphology and structure characterization of samples. (a) XRD curves of samples with different Tm³⁺ doping mole fractions; (b) XRD curves of samples synthesized at different temperatures; (c) schematic of typical cubic phase ZrO₂ crystal structure; (d) scanning electron microscope image of YSZ∶10%Yb/1%Tm sample; (e) elemental distribution image of YSZ∶10%Yb/1%Tm sample

Fig. 2. Up-conversion fluorescence spectra of samples with different Tm³⁺ doping mole fractions under 980 nm laser excitation

Fig. 3. Up-conversion luminescence spectra and luminescence mechanism at room temperature. (a) Up-conversion luminescence spectra of the sample under excitation of 980 nm laser with 1 W power; (b) energy level diagram of Yb³⁺/Tm³⁺ and possible up-conversion mechanism

Fig. 4. Dependence of luminescence intensity on pump power. (a) Up-conversion luminescence spectra under the laser excitation with different pump power; (b) relationship between the emission peak intensity of red (690 nm) and blue (489 nm) light and the excitation power density

Fig. 5. Up-conversion luminescence changes of YSZ∶10%Yb/1%Tm at different temperatures (303‒573 K). (a) Luminescence spectra at different temperatures; (b) chromaticity diagram of the light emitted by the sample

Fig. 6. Temperature sensing sensitivity of the sample changed with fluorescence intensity ratio. (a) Relationship between the fluorescence intensity ratio (F) of red peak (690 nm) and blue peak (489 nm) and temperature; (b) absolute sensitivity (S_a) and relative sensitivity (S_r) of fluorescence thermometry

Fig. 7. Thermometry reliability. (a) Temperature resolution curve; (b) cyclic measurement of YSZ∶10%Yb/1%Tm in FIR mode