Preparation Of Rare Earth Ion Doped Ba₃Y₄O₉ Upconversion Phosphors And Research On Its Temperature Sensing Performance

Optical temperature sensor using the fluorescence intensity ratio（FIR）technique is a type of promising non-contact temperature measuring tool.Among them,rare earth doped upconversion（UC）luminescent materials possess incomparable advantages over other luminescent materials in the field of temperature sensing.Unfortunately,temperature measurement property of the currently reported UC luminescent materials is generally poor,and it is difficult to be effectively improved,which restricts the practical application of the materials.Compared with fluoride,which is usually selected as the matrix material for the UC optical temperature sensor,oxides are more suitable for the field of temperature sensing due to the superior physical and chemical stability.Therefore,in this paper,Ba₃Y₄O₉,a composite oxide exhibits good physical and chemical stability,was selected as matrix material to develop novelty rare earth ions(Er³⁺,Ho³⁺,Tm³⁺,Yb³⁺)co-doped and tri-doped phosphors with excellent UC luminescence properties.On this basis,the temperature sensing performances based on the thermally coupled energy levels（TCLs）and non-thermally coupled energy levels（NTCLs）of rare earth ions were investigated.The main contents and results of this paper are described as follows:（1）A series of Er³⁺/Yb³⁺,Ho³⁺/Yb³⁺,Tm³⁺/Yb³⁺co-doped Ba₃Y₄O₉ phosphors were synthesized by solid-state reaction method.Excited by 980 nm near-infrared laser,the UC luminescence properties of the phosphors were investigated through regulating the Yb³⁺concentrations.With the increase of Yb³⁺content,the electrons located in the intermediate excited level tend to jump to the upper level,which is beneficial to promote multi-photon UC luminescence.On the other hand,the increase of Yb³⁺concentration shortens the distance between Er³⁺,Ho³⁺,Tm³⁺and Yb³⁺,thus the back-energy-transfer processes are accelerated.Under the combined action of the above two processes,the intensity ratio between the emission peaks changed obviously.For Ba₃Y₄O₉:Er³⁺/Yb³⁺phosphors,the variation of the intensity ratio leads to the output colors from green to red,which means that the UC luminescence of Ba₃Y₄O₉:Er³⁺/Yb³⁺can be tuned by controlling the Yb³⁺content.With the variation of the intensity ratio between the emission peaks,the Ba₃Y₄O₉:Ho³⁺/Yb³⁺and Ba₃Y₄O₉:Tm³⁺/Yb³⁺phosphors yield high-purity green and blue UC emission all along,respectively.（2）Using the FIR technique,the temperature sensing performances based on the TCLs from Stark sublevels of Er³⁺,TCLs from multiple energy levels of Ho³⁺,TCLs-like of Tm³⁺were studied,respectively.For Ba₃Y₄O₉:Er³⁺/Yb³⁺phosphors,under the influence of crystal field,the ²H_11/2,⁴S_3/2 and ⁴F_9/2 levels of Er³⁺are split into ²H_11/2（1）,²H_11/2（2）,⁴S_3/2（1）,⁴S_3/2（2）,⁴F_9/2（1）and ⁴F_9/2（2）levels,respectively.The maximum sensitivities of the TCLs by Stark sublevels reach 88.3×10^-4 K^-11 and 83.9×10^-4 K^-1 at elevated and subzero temperature,which are much higher than those of traditional TCLs of ²H_11/2/⁴S_3/2.For Ba₃Y₄O₉:Ho³⁺/Yb³⁺phosphors,the highest sensitivity based on the ⁵F₁/⁵G₆ and ⁵F_2,3/³K₈ levels reaches 79.62×10^-4 K^-1 at 573 K,which is higher than those of other Ho³⁺/Yb³⁺co-doped UC materials.For Ba₃Y₄O₉:Tm³⁺/Yb³⁺phosphors,the ¹G₄ and ³F_2,3 levels of Tm³⁺are essentially NTCLs.In consideration of the ¹G₄ levels is populated from ³H₄ level,while ³F_2,3,3 and ³H₄ levels are thermally coupled.Therefore,the ¹G₄ and ³F_2,3 levels are thermally-like coupled.Based on¹G₄/³F_2,3 levels,Ba₃Y₄O₉:Tm³⁺/Yb³⁺phosphors possess higher sensitivity than the previously reported other rare earth ions doped systems,with an absolute sensitivity of 193.51×10^-4 K^-1.（3）The Ho³⁺/Tm³⁺/Yb³⁺and Er³⁺/Tm³⁺/Yb³⁺tri-doped Ba₃Y₄O₉ phosphors were prepared by solid-state reaction method,and the temperature sensing performances based on NTCLs derived from two luminescent centers were investigated.The approximate equation of FIR based on the NTCLs versus temperature is obtained by reasonable derivation.Using the derived approximate equation as a fitting function to fit the FIR data of NTCLs,and the correlation coefficient above 99.6%,which demonstrates the rationality and universality of the derived equation.Under the combined action of non-radiation relaxation,thermally activated electron population and inter-energy transfer,the maximum sensitivity of Ho³⁺/Tm³⁺/Yb³⁺and Er³⁺/Tm³⁺/Yb³⁺reach as high as 0.0552 K^-1 and 0.0165 K^-1,respectively,which are much higher than those of the previously reported bulk UC temperature sensing materials.In addition,compared with TCLs,the NTCLs exhibits better signal discriminability,which is due to the monitored emission peaks are well separated.The results imply that the FIR technique based on the NTCLs may be a promising route to enhance the temperature measuring performance of the FIR-based temperature sensing material,and The Ho³⁺/Tm³⁺/Yb³⁺and Er³⁺/Tm³⁺/Yb³⁺tri-doped Ba₃Y₄O₉ phosphors are potential candidates for optical thermometer materials.