The New Mechanisms Based On Luminescent Materials For Temperature Sensing

Temperature is one of the most basic physical parameters in various fields, and the accurate and reliable measurement of temperature is essential, such as in science, technology, industry and so on. Respective temperature sensors are widely used in daily life, in metrology, aerodynamics, climate and marine research, as well as in chemistry, medicine, biology, military technology. It is estimated that the share of sensors for temperature amounts to as much as 75-80% of the world’s sensor market. However, the traditional thermometers based on the principle of thermal expansion have limitations if applied in harsh or corrosive environments, and they are strongly affected by the detection speed and resolution, and the temperature detection could be performed only in the scale of micrometers or above. In this condition, novel temperature measurement concepts have emerged, especially for non-contact measurement based on the optical properties of the luminescent materials. They can satisfy the noncontact, high spatial resolution and fast response real time detection measurement requirements. Therefore, the noncontact optical thermometry has garnered the most attention recently.In a certain temperature range, the optical properties of luminescent materials or ions, including peak position, fluorescence intensity ratio, spectral line width and fluorescence intensity, polarization anisotropy and fluorescence decay lifetime, vary with the change of temperature. Therefore, we can use the variation of these optical properties to calibrate temperature. Among various luminescence thermometry methods, the methods based on the fluorescence intensity ratio and fluorescence decay curves are considered to be potential for applications in noncontact optical thermometry. That is because that they could not rely on the measurements, including the fluorescence loss, the fluctuation on the power of the excitation source, as well as the number of luminescence centers, and the measurements are believable with a small measuring error. As a consequence, this paper is purposefully focused on the temperature sensitive materials, whose main purpose is looking for new materials and new physical mechanisms available for noncontact optical thermometry, and the research results are generally presented as follows:The first chapter is the introduction part of this paper, which mainly introduces the significance and research background of the noncontact optical thermometry, as well as several methods of noncontact temperature sensing based on the fluorescence intensity ratio and luminescence decay curves, respectively. Furthermore, the basic knowledge and principles of rare earth doped luminescent materials are also illustrated in the introduction part, which includes the elements of rare earth, the spectrum theory, the luminescent materials, the upconversion luminescence, the common methods for spectral characterization, and so on.In chapter two, the optical thermometry based on the the thermally coupled energy levels of the Ho3+ ions is presented by means of the fluorescence intensity ratio. The Ho3+ doped oxyfuloride glass ceramic containing β-NaYF4 nanocrystals is successfully prepared via Melt-quenching method in air atmosphere, which is characterized by the X-ray diffraction and the transmission electron microscopy. Under the excitation of 980 nm, the Ho3+ ions in β-NaYF4 nanocrystals exhibits the favorable upconversion luminescence, suggesting that most Ho3+ ions are incorporated into the β-NaYF4 nanocrystals after crystallization. As a result, the multiphonon nonradiative relaxation of Ho3+ is reduced and the energy transfer from Yb3+ to Ho3+ is enhanced. Furthermore, the variation of the fluorescence ration intensity with temperature is studied in detail, taking the thermally coupled energy levels of 5F1/5G6 and 5F2,3/3K8 of Ho3+ ions for an example. In the temperature range from 390 K to 750 K, the fluorescence intensity ratio increases with the rise of temperature, which also meets the Boltzmann distribution. We could achieve the energy gap between the two thermally coupled energy levels as the value of 1438 cm-1, and the relative sensitivity curves could also be deduced with a maximum relative sensitivity of 1.37%·K-1 at 390 K. What’s more, the feasibility and the limitation of this method based on the fluorescence intensity ratio are discussed for potential applications.In chapter three, the optical thermometry based on the luminescence decay lifetime is a promising method with a broad prospect. In general, the crystal field splitting of Cr3+ ions varies with the change of the crystal field. In the LiAl5O8 lattice, the first excited energy level is 2E, which is characterized by a narrow band emission and relatively long lifetime, while the 4T2 state with a higher energy than the 2E level is characterized by broadband emission with a short lifetime. A series of LiAl5O8:Cr3+ have been successfully synthesized via the combustion method, and then structural and photoluminescent characterizations have been carried out. Strong deepred emission, being assigned to R1 line of Cr3+. has been observed under a broad wavelength range of visible-light excitation. The intensity, peak wavelength and decay lifetime vary strongly with temperature for all the samples. In particular, the lifetime of the Ri line for the sample LiAl5O8:0.2mol% Cr3- have been measured in detail for temperature sensing. A high average temperature sensitivity of 0.01 ms K-1 is obtained in a broad temperature range of 200 K-600 K, with a maximum of 0.015 ms·K-1 at 400 K and a maximal relative sensitivity of 0.83%K-1 at 447 K. Our results show that LiAl5O8:Cr3- is very promising for high-sensitive optical thermometry.Furthermore, the temperature sensitive rise time is also investigated for noncontact optical thermometry. The BaY2ZnOs:Eu3+ phosphors are synthesized via high temperature solid-state reaction and extensively characterized using various methods such as X-ray diffraction, photoluminescence excitation and emission spectra as well as fluorescence decay lifetime. Furthermore, the emission rise time of the 5D0 level of BaY2ZnO5:Eu3+ at various temperatures are investigated upon excitation at 469.1 nm into the 5D2 level. In the temperature range of 330-510 K, the rise time shows a strong temperature dependence, with the maximum relative sensitivity of 2.2%·K-1 at 490 K. Experimental results has demonstrated that luminescence thermometry based on emission rise time is reliable, and the BaY2ZnO5:Eu3+ phosphor is a promising candicate for applications in luminescence thermometry. Therefore, new avenues will be opened up for the exploration of novel noncontact luminescence thermometry.Finally, the main content of this paper are summarized, and the noncontact optical thermometry is looking forwarding to further applications.