| Temperature is one of the most fundamental physical quantities in thermodynamics. The measurement of temperature is crucial in industry, biomedicine, scientific research as well as our everyday life. Traditional thermometers based on the thermal contact approach, such as the liquid-in-glass thermometers, thermocouples, resistance thermometers, have played a key role in many fields during the past several decades. However, more complex demands are proposed for the control and measurement of temperature with the development of technology and the progress of society. Traditional temperature sensors can not be used in many cases due to the restriction of their intrinsic properties, such as the measurement in corrosive or electromagnetic interference environment, temperature sensing for tiny electronic device, cell or fast moving objects. Therefore, it is of great significance to develop the non-contact temperature sensors with high spatial and temperature resolution as well as quick response.Fluorescence intensity ratio (FIR) technique with rare earth ions has been regarded as a very promising approach for non-contact optical temperature sensing. Compared to the measurement of absolute fluorescence intensity, FIR method can effectively avoid the error caused by the fluorescence loss and the number of the luminescence center in the detction. The main content of this dissertation includes temperature sensing based on FIR technique with thermally coupled energy levels (TCELs) of rare earth ions and two novel strategies for FIR technique proposed to overcome the limitation on relative sensitivity resulted from the thermally coupled condition.In chapter one, we first introduced the necessary of the research for non-contact optical temperature sensing. Then, a brief introduction about the fundamental of luminescence, luminescent properties of rare earth ions, rare earth doped luminescent materials as well as the upconversion process was given. The principle of temperature sensing based on the FIR technique with TCELs, the research and difficulties of this approach were discussed in detail.In chapter two, temperature sensing based on FIR technique with TCELs in Er3+, Pr3+and Ho3+was presented. Yb3+/Er3+, Pr3+, Yb3+/Ho3+doped β-NaY(Lu)F4micron-sized powder materials were synthesized by hydrothermal method. Characterization was performed using X-ray diffractometer (XRD) and scanning electron microscope (SEM) for structure and morphology.Under980nm excitation, upconversion spectra of β-NaYF4:20%Yb3+,2%Er3+was obtained in the temperature region of160-300K, and the relationship between FIR of2H11/2and4S3/2in Er3+and temperature as well as the relative sensitivity was analyzed. The heating effect of980nm laser was pointed out by measuring the FIR under different power of980nm laser excitation.The laser heating effect can be avoided effectively by direct excitation. In addition, as for the TCELs with relative small gap, considerable population in up level can be achieved even in low temperature. The temperature dependence of FIR between3P1/1I6and3P0of Pr3+in β-NaYF4:0.8%Pr3+was studied in the region of120-300K. Due to the small energy gap, considerable emission intensity of up level was obtained in low temperature, showing large advantage. And the resulting absolute sensitivity was five times as high as that of β-NaYF4:Yb3+/Er3+at room temperature.Considering the principle of TCELs-based FIR technique, the relative sensitivity is proportional to the energy gap of the corresponding TCELs. Upconversion emission spectra of β-NaLuF4:10%Yb3+,0.5%Ho3+were studied at various temperatures from390to790K, indicating that5F1/5G6and5F2,3/3K8energy levels in Ho3+are thermally coupled. It is demonstrated that the resulting relative sensitivity is superior to most temperature sensors based on the same FIR technique due to the larger energy gap compared with other rare earth ions. In addition, the origins of443.6nm and482nm emissions of Ho3+were identified using the upconversion spectra in low temperature and pump power dependence of upconversion luminescence.In chapter three and chapter four, two novel FIR strategies were proposed in nano luminescent materials in order to overcome the limitation of TCELs-based FIR technique and satisfy the demand of temperature sensing in submicron or nano scale, which provide new ideas for FIR-based temperature sensing.In chapter three, monodispersed β-NaYF4:20%Yb3+,0.5%Tm3+/NaYF4:1%Pr+core shell nanoparticles were successfully synthesized by solvothermal method and characterization was performed for its structure and morphology. Upconversion emission spectra were measured at various temperatures in the region from302to510K. It is demonstrated that the FIR between two emissions from F2,3and1G4varied dramatically with temperature, and the reason was analyzed. A higher relative sensitivity was achieved compared with most temperature sensors using TCELs-based FIR technique, and the maximum reaches1.53%K-1at417K.In chapter four, we attempt to improve the relative sensing sensitivity using the temperature dependent luminescence of double rare earth ions doped materials. Eu3+and Nd3+doped Y2O3nano materials were synthesized by combustion method. Characterization was performed using XRD and SEM for their structures and morphologies. The thermally coupled property of7Fo and7F2in Eu3+was studied with excitation spectra measured at various temperatures, and the resulting relative sensitivity is superior to that of Er3+using its TCELs2H11/2and4S3/2. Under580.5nm excitation for Eu3+and Nd3+codoped Y2O3, the emission intensity of Eu3+decreases with the rise of temperature due to the nephelauxetic effect and the thermally coupled property of7Fo and7F2as well as the energy transfer from Eu3+to Nd3+. At the same time, the emission of Nd3+increases because of the thermally populated of4F5/2. Thus, dramatic temperature dependence was achieved. The obtained relative sensitivity was superior to many other temperature sensors based on FIR technique. |
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