Optical Temperature Sensing Based On 4f-4f And 4f-5d Transitions Of Rare Earth Ions

The measurement of temperature is of foundmental importance in every field of human life.To meet different requirements on sensors and detection methods in different scientific fields,the exploration of new high performance temperature sensors has been proceeding continuously.In some particular fields,non-contact temperature measurement method is much more favorable than the conventional contact ones.Compared to the traditional contact thermometer,the advantage of non-contact temperature detection is that it does not need to be in contact with the object to be measured,so the temperature measurement is fast and does not cause intrusion damage to the object.Among those non-contact temperature sensing techniques,optical temperature sensing based on luminescence materials are of great intrests due to its unique advantages:A)temperature detection using nanometer-sized luminescent materials can have higher(nanoscale)spatial resolution which is suitable for temperature detection in cells;B)there exist many luminescent properties sensitive to temperature,which means luminescence-matrial-based optical temperature sensing can easily meet requirments in different stitutions.The most important part of the luminescence-material-based optical temperature sensor is the luminescent material used to detect temperature.The mechanism of temperature sensing using luminescent materials is that some optical properties of luminescent materials change with temperature,such as luminescence peak position,fluorescence intensity ratio(FIR)of two luminescence peaks,spectral linewidth and absolute fluorescence intensity.Therefore,it is possible to establish the relationship between these luminescence characteristics and temperature which can be used for detecting temperature by measuring the luminescence of materials.Among them,the FIR temperature sensing technique is considered to be the most potential one by the virtue of its weak dependence on measuring conditions and it is immune to the fluctuation of the intensity of the excitation light source.This will help to cut the costs of the measuring instrument thus conducive to saving production costs.The main content of this thesis is to explore new optical temperature sensing methods based on rare earth-doped luminescent materials.The main optical temperature measurement scheme is the FIR scheme.The research aims to improve the temperature sensitivity of existing temperature measurement schemes and find new temperature measurement approach for optical temperature sensing,the following is the content arrangement of this thesis:The first chapter is the introduction part of this thesis.It mainly introduces the research significance,background and development status of temperature sensing based on luminescent materials,as well as the advantages and limitations of existing research results.Then,by introducing rare earth luminescent materials,the 4f-4f and 4f-5d transition energy levels and the temperature measurement technology based on rare earth luminescent materials related to the content of this paper is introduced.In the second chapter,we try to realize FIR-type temperature sensing approach by using thermally coupled levels 5D0 and 5D1 of Eu3+ ions.In this work,the powder material of Eu3+ ion doping into YBO3 host was successfully synthesized through high temperature solid reaction method.The structure and morphology of the powder samples were characterized by X-ray diffraction and scanning electron microscopy.We measured the excitation and emission spectra of the sample at room temperature and identified the sources of the luminescence peaks.We chose 5D1 and 5D0 of Eu3+ions as the target thermally coupled energy levels,and measured and analyzed their temperature-dependent FIR.In the temperature range of 333 K to 773 K?the FIR of the emission peaks from 5D1 and 5D0 to 7FJ has a monotonous dependence and dramatical change with the change of temperature.The FIR shows an exponential growth with increasing temperature which can be fitted by Boltzmann formula.The fitting results show that the effective energy difference of the thermally coupled energy levels is 1400 cm-1,and its relative sensitivity has a maximal relative sensitivity of 1.8%K-1 at 333 K.We also performed thermogravimetric analysis on the synthesized samples.The results show the as-synthesized samples have a high temperature stability up to 1000 degrees which is fully qualified for temperature sensing within 500 degrees.Finally,we discussed the feasibility and limitations of temperature sensing technique based on thermally coupled energy levels of rare earth ions.For 4f-4f energy levels,in addition to the temperature sensing technique employing thermally coupled energy levels of excited states(5D1 and 5D0 of Eu3+ in the second chapter),temperature sensing based on thermally coupled levels of ground state energy level and low-lying energy levels is also a hot research topic.The third chapter is based on the thermally coupled levels of ground state energy level and low-lying energy levels of Eu3+ions.In this chapter,we chose the 7F0 and 7F2 of Eu3+as thermally coupled energy levels for temperature sensing.In the introduction part of this chapter,we first analyzed the relationship and difference between the low-lying and high-excited thermally coupled energy levels temperature sensing techniques which are introduced in this chapter and in the previous chapter,respectively.In the body part,we synthesized a series of samples of Eu3+ doped into Ca3Sc2Si3O12 host by three-time calcination high temperature solid reaction method.The as-synthesized samples were characterized by XRD.The relationship between the luminescence properties of Eu3+ ions and the crystal field environment was analyzed.We used the 610.6 nm light as the excitation source,and some of the trivalent Eu ions in the low-lying states 7F2 are excited to the excited state 5D0.The temperature dependence of the luminescence from the downward transitions of 5D0 were studied and analyzed.The results showed that the 5D0 downward transition luminescence increases with increasing temperature when the excitation light intensity remains unchanged.We analyzed the experimental results and concluded that as the temperature increases,more Eu3+ ions are thermally excited from the ground state level 7F0 to the 7F2 level.This means that the amont of Eu3+ ions in 7F2 state will increase with the increasing temperature in the case of constant excitation intensity.The trivalent Eu ions in 7F2 level absorb the excitation light and jump to the 5D0 excited state to produce luminescence.This temperature sensing method based on low-lying states has higher temperature sensitivity and can reduce the heating effect of the excitation light on the sample.Another advantage of this technique for temperature sensing is that it is suitable for temperature ranges at relatively low temperatures such as below room temperature.In the fourth chapter,we try to jump out the stereotyped thinking pattern of temperature sensing based on thermally coupled energy levels,and turn to exploring temperature sensing method free of thermally coupled energy levels.Traditional temperature sensing based on thermally coupled energy levels generally uses the luminescence of trivalent rare earth ions,while the luminescences of trivalent rare earth ions are usually sharp peaks.Accurate measurement of the sharp peaks often requires more expensive instruments thus it is uneasy to reduce the production cost.The luminescence of 5d-4f transitions from divalent rare earth ions are usually broad bands,so we used a mixture of two divalent-rare-earth-doped luminescent materials for temperature sensing in this chapter.We prepared two powder samples doped with divalent Eu and divalent Sm synthesized by high temperature solid reaction method,and mixed the two phospors in a certain proportion.Under the excitation of an excitation light shared by both the two materials,as the temperature increases,the luminescent band of divalent Eu decreases while the luminescent band of divalent Sm increases.The FIR of the two bands has a dramatical change with increasing temperature.In the temperature range of 380 K to 700 K,the relative sensitivity of temperature sensing is on the average of 3%K-1,which is relatively high for the sensitivity of temperature sensing above room temperature.Therefore,the temperature sensing using divalent rare earth free of thermally coupled energy levels provides a new choice for non-contact temperature detection.The temperature sensing methods mentioned above are all fluorescence temperature measurement,which means,the excitation light must be employed during the temperature sensing process.However,the excitation light itself may have a heating effect,which affects the temperature of the sample thus unfavorable for the accuracy of optical temperature sensing.In the fifth chapter,we further explored a new type of temperature sensing,that is,temperature sensing based on long persistent luminescence(LPL)which is free from real-time excitation.In this chapter,we firstly introduced a yellow LPL material Ca2Al2SiO7:Eu2+,Tm3+ developed by our laboratory as an example to introduce the mechanism and research status of LPL materials.Then a temperature sensing method based on LPL of the famous LPL material SrAl2O4:Eu2+,Dy3+,Tb3+ was introduced.The experimental results showed that the persistent luminescence of this material was blue at 60 K.As the temperature increased,the blue light gradually became weaker and the green light gradually increased.The persistent luminescence of the sample at a relative high temperature of 240 K was shown as green light.Based on this phenomenon,we developed a new method of temperature sensing using FIR of the persistent luminescence of SrAl2O4:Eu2+,Dy3+,Tb3+.According to the fitting curve and analysis of the experimental data,the relative sensitivity of our new temperature sensing method reached 3%K-1.Moreover,the advantage of this LPL temperature sensing method is obvious,it greatly reduces the heating effect of the excitation light.It is also a broad-band FIR method so it retains the advantages of the fourth chapter which is relatively low requirements on the measuring instrument.We believe that the LPL temperature sensing method should be one of the development directions of future biological temperature sensing.At the end of this thesis,we summarized the main results of these works and looked forward to the future development direction of temperature sensing based on luminescent materials.