Research On Optical Temperature Sensing Properties Of Rare-Earth Doped Tungstate Up-Conversion Luminescent Materials

Accurate measurement and real-time monitoring of temperature occupies a crucial position in product quality assurance,industrial and agricultural automation,environmental protection,biomedicine and other industries.Recently,the research on thermometry technology and the pursuit of supernal sensitivity values have become a hot spot in the world.Optical temperature sensing technology and optical temperature sensing technology based on rare earth luminescent materials have become an important research direction and drawn wide attention.At present,rare earth up-conversion temperature sensing materials based on fluorescence intensity ratio technology have potential application prospects in optical temperature detection due to their unique luminescence properties,low toxicity,deep tissue penetration,high precision and other advantages.This paper focuses on the up-conversion luminescent materials with tungstate as the matrix,Er³⁺or Ho³⁺as the activated ion and Yb³⁺as the sensitized ion.By designing the matrix and changing the type and concentration of doping ions,the effect of different doping ions on temperature sensing characteristics is studied based on fluorescence intensity ratio technology.The aim is to optimize the performance of up-conversion luminescence and temperature sensing,and further research direction is discussed on this basis.It is hoped that the research of this paper can provide new enlightenment for the new fluorescent temperature sensor in the future.The main research contents and results of this paper are as follows:（1）A series of scheelite tungstate up-conversion luminescent materials were synthesized by traditional high temperature solid state reaction method.Er³⁺was used as the activation ion and Yb³⁺as the sensitized ion,and the crystal structure,luminescence characteristics and temperature sensing performance of scheelite tungstate materials were controlled by different types of ion doping.We analyzed the efficient and stable green light emission mechanism of Er³⁺doped up-conversion luminescent materials,evaluated the fluorescence intensity ratio thermometry performance of classical thermal coupling energy level(²H_11/2/⁴S_3/2)and Stark sublevel based on Er³⁺,and studied and obtained the influence law of different types of ion doping on up-conversion luminescence and temperature sensing characteristics of tungstate materials:The green luminescence intensity of Li⁺/Nb⁵⁺co-doped CaWO₄:Er³⁺/Yb³⁺phosphors was significantly improved,and the thermometry characteristics were further optimized based on ²H_11/2/⁴S_3/2 fluorescence intensity ratio technique,and the measurement errorsδT were all less than 0.5 K at low temperature.In addition,based on Stark sublevel fluorescence intensity ratio technique,the temperature sensing performance of tungstate materials was also improved,the absolute sensitivity of ²H_11/2/⁴S_3/2（1）and ²H_11/2/⁴S_3/2（2）were 0.0134 K^-1 and 0.0104 K^-1,respectively,and the relative sensitivities were 0.0434 K^-1 and 0.0548 K^-1,respectively.（2）From the design of matrix materials,Yb-based tungstate up-conversion luminescence materials activated by Er³⁺were constructed,and the effects of co-doped Tm³⁺on luminescence and temperature sensing performance were discussed based on thermal coupling energy level and non-thermal coupling energy level respectively.After co-doping Tm³⁺,Er³⁺activated Yb-based tungstate phosphors show blue,green and red emission characteristics under 980 nm excitation.In terms of temperature sensing performance,the maximum absolute and maximum relative sensitivity of 0.0184 K^-1 and 0.0467 K^-1 were obtained by using the fluorescence intensity ratio technology of thermal coupling level ²H_11/2/⁴S_3/2 for single-doped Er³⁺ion,while the optimized maximum sensitivity of 0.0208 K^-1 and 0.0531 K^-1 were obtained by using the same thermal coupling level of Er³⁺ion after co-doping Tm³⁺.Moreover,a maximum sensitivity of 0.0563 K^-1 was obtained using the two-channel strategy with un-thermally coupled energy levels ⁴S_3/2(Er³⁺)/³F_2,3(Tm³⁺),and the sensitivity was higher than 0.0110 K^-1 over a broad range from 163 to 663 K.In addition,we use the negative thermal expansion effect of Er_2-xYb_xW₃O₁₂ to improve the heat quenching problem of rare earth up-conversion luminescent materials.Samples with higher Yb³⁺content can produce strong up-conversion luminescence intensity at low power excitation,and show excellent fluorescence thermal enhancement effect with the increase of temperature.Moreover,the increasing proportion of Yb³⁺can also improve the hygroscopicity of Er_2-xYb_xW₃O₁₂ system,which opens a new way for the synthesis of new luminescent thermal enhanced temperature-dependent materials and their applications in sensors and optoelectronic devices.（3）A series of multifunctional tungstate up-conversion luminescent materials with optical temperature and photothermal conversion were designed and synthesized by high temperature solid-phase synthesis method and hydrothermal method respectively.The effects of p H value of precursor solution,Yb/Nd ratio and complexing agent Na₃Cit on crystal structure and optical properties of the samples were systematically studied.Furthermore,the temperature sensing characteristics under various conditions based on Er³⁺thermal coupling energy level fluorescence intensity ratio technique are analyzed.The absolute sensitivity of the tungstate up-conversion luminescent material synthesized at high temperature is about 0.0100K^-1 in the wide temperature range of 410～753 K,and the minimum absolute sensitivity is 0.0066 K^-1 at room temperature,while the relative sensitivity is up to0.0123 K^-1 at RT.At the same time,Nd³⁺,as a matrix component,has abundant and compact energy levels,and these small energy gaps promote efficient heating of multi-photon decay process,achieving exceptionally excellent heating effect.By monitoring the photothermal conversion characteristics of these materials under various conditions of 808 nm and 980 nm excitation,a large photothermal temperature rise was obtained,which provides a new idea for the design and selection of near-infrared response self-monitoring photothermal materials.（4）Tungstate based up-conversion luminescence materials of co-doped transition metal ion TM³⁺were prepared by traditional high temperature solid-state reaction method.CaWO₄ with scheelite structure and Na La Mg WO₆ with double perovskite structure were selected as matrix materials respectively.The effects of TM³⁺doping on up-conversion luminescence and temperature sensing characteristics were studied based on the fluorescence intensity ratio technique of thermal coupling energy level of Er³⁺and red/green ratio technique of Ho³⁺.The co-doping of Cr³⁺improves the high temperature sensing characteristics of scheelite tungstate,and its linear sensitivity increases from 0.0163 K^-1 of CaWO₄:Ho/0Cr to 0.0233 K^-1 of CaWO₄:Ho/2Cr.At the same time,the best sample CaWO₄:Ho/2Cr also shows good linear temperature sensing performance,with an absolute sensitivity of 0.0217 K^-1 in the range of 173～663 K.In addition,the up-conversion luminescence and temperature sensing properties of Cr³⁺or Sc³⁺co-doped double perovskite tungstate materials were also studied,and the up-conversion luminescence characteristics have been significantly improved.These results indicate the potential application of tungstate up-conversion materials with different structures in optical temperature sensing.