Preparation Of Sm³⁺ And Tb³⁺ Co-activated NaGd（moO₄）₂ Phosphors And Study Of Optical Temperature Sensing Properties

The optical temperature sensor based on FIR（Fluorescence Intensity Ratio）has attracted much attention because it has the advantages of non-contact and high sensitivity.In order to obtain better temperature sensing performance,it is very important to select appropriate rare earth ions as activation materials.First of all,the advantages of Sm³⁺,Tb³⁺co-doped materials lie in the use of fluorescence excitation in the luminous center,which is not affected by the photothermal effect of excitation.Secondly,the thermal quenching of Sm³⁺belongs to the"weak coupling"type,while Tb³⁺belongs to the"strong coupling"type,the quenching trend of Sm³⁺and Tb³⁺are different,the FIR of the two activated ions has a relatively significant change with the temperature,and has a high relative sensitivity.In addition,Sm³⁺and Tb³⁺have strong red emission and green emission,respectively,and the luminescence color of phosphors will change significantly with temperature.The alkali-metal-rare-earth double molybdate with scheelite structure has excellent chemical stability and thermal stability and can be used as substrate material.Therefore,in this paper,the optical temperature sensing properties of Sm³⁺,Tb³⁺co-doped NaGd（MoO₄）₂phosphors will be explored in detail.In this paper,sodium citrate（Na₃C₆H₅O₇-Na₃Cit）as chelating agent,using hydrothermal method,by changing the amount of sodium citrate and doping the proportion of Sm³⁺,NaGd（MoO₄）₂:Sm³⁺,Tb³⁺specimens were prepared.The crystal structure and morphology of the specimens were characterized by XRD（X-ray diffractometry）and FE-SEM（Field Emission-Scanning Electron Microscopy）.According to the XRD diffraction pattern,the specimens are all scheelite structure with tetragonal body center phase.By FE-SEM,it was found that the morphology of the specimen changed with the change of Cit^3-/Re³⁺value,while the doping concentration of Sm³⁺had little effect on it.The optical temperature sensing properties of NaGd（MoO₄）₂:Sm³⁺,Tb³⁺(Cit^3-/Re³⁺=0,0.5,1,2)phosphors were investigated in the temperature range of 298 K to 503 K.The experimental results showed that the sensitivity of the specimen increased with the increase of Cit^3-/Re³⁺ratio,when the temperature was 503 K and Cit^3-/Re³⁺=2,the sensitivity of the specimen reached the peak value of 0.346 K^-1.When the temperature range was from 298 K to 369 K,the relative sensitivity increased with the increase of Cit^3-/Re³⁺;from 374 K to 503 K,the relative sensitivity decreased with the increase of Cit^3-/Re³⁺,and reached the maximum value of 2.7%K^-1when Cit^3-/Re³⁺=0at 404 K.In addition,in the temperature range of 298 K to 503 K,the luminescence color of the specimen changed continuously from green to red.We also investigated the optical temperature sensing properties of NaGd（MoO₄）₂:x%Sm³⁺,5%Tb³⁺（x=0.1,0.3,0.5,1）phosphors when Cit^3-/Re³⁺=1 in the temperature range of 298 K to 503 K.The experimental results showed that the sensitivity of the specimen increased with the increase of the doping concentration of Sm³⁺.When the doping concentration of Sm³⁺was 1%and the temperature was 503 K,the maximum value of sensitivity was 0.374 K^-1.The relative sensitivity of the specimen increased with the increase of the doping concentration of Sm³⁺in the temperature range of 298 K-339 K,and decreased with the increase of the concentration of Sm³⁺in the temperature range of 382 K-503 K.When the doping concentration of Sm³⁺was the lowest(0.1%Sm³⁺),the relative sensitivity of the specimen reached the peak value of 2.72%K^-1at 413 K.The luminescence color of the specimen changed with the increase of temperature,in the temperature range of 298 K to 503 K,the luminescence color can change continuously from green to red.Sm³⁺,Tb³⁺co-doped NaGd（MoO₄）₂material has high relative sensitivity and good thermal stability,and its luminescence color changes obviously with the change of temperature,which can directly reflect the temperature through the luminescence color.The results of this paper provide a new direction for the material selection,sample preparation and the improvement of temperature sensing properties of FIR optical.