With the increasing demands of industry and the requirement for precise temperature monitoring in harsh environments, the development of advanced temperature sensing technologies becomes extremely crucial. Traditional contact-based and slow-response thermometric methods cannot meet the real-time and non-invasive monitoring needs of modern applications. This growing need promotes the exploration of innovative materials for accurate, real-time, and non-invasive temperature sensing. Our research focuses on the synthesis and characterization of YSZ∶Yb/Tm nanophosphors, a new type of material with unique up-conversion luminescence properties. These nanophosphors, synthesized by a molten salt method, aim to provide a new solution for temperature sensing with high sensitivity and a wide dynamic range. The study explores the structural and luminescent properties of these nanophosphors to establish them as a reliable and effective tool for temperature measurement. The potential of these materials to revolutionize temperature sensing in various sectors is remarkable, providing a promising direction for further research and practical application development.

The synthesis of YSZ∶Yb/Tm nanophosphors starts with dissolving accurate amounts of zirconium, yttrium, ytterbium, and thulium salts in deionized water, followed by adding sodium chloride (NaCl) as a molten salt medium. The mixture is dried at 80 ℃ to remove water and form a solid precursor and then calcined at 900 ℃ for 3 h within a furnace. After calcination, the product is thoroughly washed to remove residual NaCl and then dried to obtain YSZ∶Yb/Tm nanophosphides. After synthesis, the nanophosphors are characterized to evaluate their structural and luminescent properties. X-ray diffraction (XRD) is used to determine the crystalline phase and structure while scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy are employed to examine the morphology and elemental distribution of the particles. The up-conversion luminescence properties are investigated using a fluorescence spectrometer under 980 nm laser excitation, and the temperature-dependent luminescence is measured in the temperature range of 303?573 K to assess the potential of these nanophosphors for temperature sensing applications.

The synthesized YSZ∶Yb/Tm nanophosphors show a uniform cubic phase structure with good crystallinity, which is verified by XRD analysis. The diffraction patterns match well with the standard cubic phase YSZ (JCPDS No. 30-1468), indicating the successful doping of Yb³⁺ and Tm³⁺ ions into the YSZ lattice without additional phases [Figs. 1(a), (b)]. The SEM image shows the microscopic morphology of the phosphor, and the particles tend to aggregate into larger clusters due to the large surface energy of the nanoparticles [Fig. 1(d)]. EDX analysis confirms the uniform distribution of doped elements in the matrix, ensuring efficient energy transfer processes within the nanophosphors [Fig. 1(e)]. Under 980 nm laser excitation, the up-conversion luminescence spectra show emissions at 489 nm (blue) and 690 nm (red), corresponding to the ¹G₄→³H₆ and ³F_2,3→³H₆ transitions of Tm³⁺, respectively [Fig. 3(a)]. These emissions result from the energy transfer from Yb³⁺ ions (sensitizers) to Tm³⁺ ions. The up-conversion mechanism involves a multiphonon-assisted energy transfer process, where Yb³⁺ ions absorb photons from the 980 nm laser and transfer energy to Tm³⁺ ions through a series of multiphonon processes [Fig. 3(b)]. The temperature-dependent luminescence tests display a fluorescence quenching effect for the blue emission band, while a thermally enhanced effect is observed for the red emission band [Fig. 5(a)]. This phenomenon is due to the temperature-dependent energy transfer efficiency between Yb³⁺ and Tm³⁺ ions, which is affected by the thermal population of energy levels and multiphonon relaxation processes. The integral intensity ratio of the red to blue emission peaks shows significant temperature sensitivity, with the maximum relative sensitivity of 1.10 %/K and absolute sensitivity of 31.55 %/K achieved at 423 K and 573 K, respectively (Fig. 6). These results demonstrate the potential of YSZ∶Yb/Tm nanophosphors for high-sensitivity temperature sensing. The temperature sensing performance is further analyzed by plotting the fluorescence intensity ratio (FIR) of the red and blue emission peaks against temperature. A nonlinear relationship is observed and fitted to a polynomial curve, revealing the material’s ability to sense temperature changes over a wide range [Fig. 6(a)]. The calculated absolute and relative sensitivities quantitatively measure the material’s performance as a temperature sensor, with the highest sensitivities obtained at the extreme temperatures tested [Fig. 6(b)]. These findings are important for the development of temperature sensors that can work in harsh environments where traditional contact-based sensors may fail or give inaccurate readings.

Our study has successfully synthesized YSZ∶Yb/Tm nanophosphors with excellent up-conversion luminescence properties, indicating their potential as highly sensitive temperature sensors for a wide range of applications, especially in extreme environments. The nanophosphors, synthesized by an efficient molten salt method, possess a uniform cubic phase structure with good crystallinity, which is essential for their superior luminescent properties. The up-conversion luminescence characteristics of these nanophosphors have been carefully studied, showing a unique temperature-dependent behavior where the blue emission undergoes fluorescence quenching and the red emission exhibits an unusual thermal enhancement. This phenomenon is caused by the complex interaction between Yb³⁺ and Tm³⁺ ions, affected by multiphonon relaxation processes and thermal population dynamics. The temperature sensing capabilities of the YSZ∶Yb/Tm nanophosphors are outstanding, with the maximum relative and absolute sensitivities of 1.10 %/K and 31.55 %/K respectively. These results emphasize the potential of these materials for high-precision, non-contact temperature monitoring in various industries such as aerospace, medical diagnostics, and industrial process control, where accurate temperature measurement is crucial. The results of this study contribute to the progress of luminescence-based temperature sensing technologies and open new paths for future exploration and innovation in this field.