Synthesis And Thermal Enhanced Luminescence Mechanism Of Yb³⁺/Er³⁺ Codoped Negative Thermal Expansion Materials For Optical Temperature Sensing

As the most fundamental physical quantity in the natural sciences,temperature plays a vital role in scientific research,industrial production and productive activities.The traditional contact temperature measurement technique is severely limited in its application due to its long response time and low sensitivity.In contrast,a new non-contact temperature measurement technique,namely optical temperature measurement,has received widespread attention in recent years due to its high resolution and sensitivity,which can be used for temperature detection in special environments such as cell temperatures and electronic components.Most of these optical temperature measurements in the fluorescence intensity ratio（FIR）mode are based on thermal quenching luminescent materials,which reduce the intensity of luminescence at high temperatures,leading to inaccurate temperature measurements.In this paper,the thermally enhanced luminescence is achieved by introducing Yb³⁺and Er³⁺ions into the negative thermal expansion（NTE）material as a host,and the temperature sensing performance of the FIR mode based on thermally enhanced luminescence is investigated,which has important research significance for accurate temperature measurement at high temperatures.A series of Yb³⁺and Er³⁺ions double-doped NTE materials Pb Ti O₃ phosphors were successfully synthesized by the sol-gel method.A series of typical emission peaks of Er³⁺were observed under 980 nm laser excitation,including two green emissions at 521 and 549 nm(²H_11/2/⁴S_3/2→⁴I_15/2)and a red emission at 661 nm(⁴F_9/2→⁴I_15/2),in addition,near-infrared（NIR）emission of Er³⁺ions was observed at 1546 nm(⁴I_13/2→⁴I_15/2).The luminescence mechanism and energy transfer mechanism of Er³⁺ions were investigated in detail,while the temperature sensing performance of the thermally coupled energy levels ²H_11/2 and ⁴S_3/2 of Er³⁺ions was studied using FIR,and a high sensitivity of 1.23%/K at 298 K was obtained.Unfortunately,thermally enhanced luminescence in a narrow temperature region was only achieved in the NIR emission and the desired thermally enhanced effect was not obtained.Therefore,new NTE materials were subsequently chosen as hosts and Yb³⁺and Er³⁺ions were used as dopants to investigate their thermally enhanced luminescence properties.A series of Yb³⁺and Er³⁺co-doped NTE materials,Zr Sc（WO₄）₂PO₄ phosphors,were synthesised by the sol-gel method and bright green light was observed under the excitation of a 980 nm laser.Firstly,the effect of Yb³⁺ion doping concentration on the luminescence intensity was investigated to explain the mechanism of the phosphor’s luminescence.Next,the hygroscopic properties of the sample were improved by the introduction of Zr and P elements,and the luminescence intensity of the sample was improved.Compared with the NTE phosphor Sc₂（Mo O₄）₃:Yb³⁺/Er³⁺,the luminescence intensity of Zr Sc（WO₄）₂PO₄:Yb³⁺/Er³⁺phosphor was2-fold as high in the range of visible light at room temperature.In the NIR optical region,it is38-fold,which well proves that the hygroscopic property has been significantly improved.Subsequently,thermal enhancement of phosphor luminescence was obtained by temperature-dependent emission spectroscopy,with I_{573 K}/I_{298 K} achieving an 8-fold thermal enhancement in the visible range and a 7-fold thermal enhancement in the NIR emission.The mechanism of the thermally enhanced luminescence is explained by temperature-dependent fluorescence lifetime tests.Finally,the temperature sensing performance of the Er³⁺ion thermally coupled energy levels ²H_11/2 and ⁴S_3/2 was investigated using the FIR technique,and the sensitivity was improved compared to that of the Sc₂（WO₄）₃:Yb³⁺/Er³⁺phosphor,obtaining a high sensitivity of 1.10%/K at 298 K,in addition to a low temperature uncertainty of less than 0.4 K.The above results indicate that the prepared Zr Sc（WO₄）₂PO₄:Yb³⁺/Er³⁺phosphors have great potential for temperature sensor applications.