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Thermometry Using Thermographic Phosphors Thermometry using thermographic phosphors 1. Introduction In recent years a new technique has been developed for remote measurements of surface temperature, but also with a potential for spray and even gasflow diagnostics. It has mainly been used in scientific and industrial applications of surface thermometry to complicated geometries, e.g., rotor engines, turbine engines, and also in medicine. Other quantities such as heat flux through a surface have been investigated, because of its high importance to science and engineering community. During the last years, as the applications of thermographic phosphors have expanded, some attempts have also been made in combustion environment. A useful review article [1] could be a good assistance as an introduction to the subject of phosphor thermometry. Thermometry based on the use of thermographic phosphors utilises the physical properties of the phosphor particles for assessing temperature. The phosphor particles used in thermometry are usually inorganic materials having the form of a white-brown powder some 1 - 10 µm in diameter. Such a phosphor consists of a host material and a doping agent from which the light is emitted. A large number of different phosphors are produced today. These cover a wide range of temperatures, from cryogenic temperatures up to 1700 C or higher, making them suitable for many different applications. Each phosphor that is selected is highly sensitive within a specific range of temperatures, exhibiting an accuracy in the order of 1-5 C. Once deposited on the surface of interest and excited by a suitable wavelength, mainly UV light, the phosphor particles emit an intense luminescent light. This emission is called phosphorescence or fluorescence, terms often used interchangeably, although fluorescence usually refers to emissions having a duration of 10-10 -10-7 s and phosphorescence to their having a duration in the order of 10-7-1 s. After excitation of the thermographic phosphor, the subsequent emission is imaged onto a detector. The temperature can then be deduced from the spectral or temporal properties of the recorded signal. This technique provides a high quantum yield, two- dimensional measurements and remote thermometry, as well as a high degree of accuracy. In contrast to pyrometry, it is not influenced by the emissivity of the material, background reflection from the surroundings or light absorption by optical windows or by surrounding gases. These advantages have allowed thermographic phosphors to be used in a wide variety of applications and in harsh environments. Thermographic phosphors (TP) have normally been used for hard non-combustible materials such as steel or concrete. For those surfaces the phosphor can be applied to the surface and only a very thin layer is required ensuring that the phosphor layer does not influence the flow and heat transfer from and to the surface. In this Chapter its application for combustion diagnostics are described. 2. Background Thermographic Phosphors which have temperature dependant emission are mostly inorganic and made of some ceramic material. The thermographic phosphor is composed by a carrier material which is doped with some activator material. The activator is often a rare earth metal. The doping concentration is typically about one percent, which is a small enough concentration for the activator atoms to be isolated from each other by the host matrix. The host material is mostly transparent to radiation, i.e. it is mostly the activators that absorb and emit radiation. Most phosphors used for thermometry are exited by laser radiation. The energy is absorbed by the rare earth metal in an electron excitation. The electron is then non- radiatively relaxed to a meta-stable energy level, i.e. a level from where no transitions are allowed. 1 The emission spectra of any material will change with temperature since the Boltzmann distribution affects vibrational levels in ground- and excited states. Thermographic phosphors have a strong temperature dependency due to several reasons. When reaching a certain temperature, the excited electrons in the activator atoms will populate high enough energy levels to transfer its energy to the surrounding host material. This energy level is called the Charge Transfer State (CTS) of the host material and is described in the energy level diagram in Figure 1. The Charge Transfer State is located at very low energies (<40*103 cm-1) for e.g. Eu3+ and Yb3+. Energy transferred to the CTS will relax non-radiatively to its lowest energy state. In the case of Eu3+ the CTS will then feed the D5 levels from where relaxation will occur in form of radiation. At high temperatures fast de-excitation via the CTS to the lowest CTS energy state will be more probable and thus shorten the lifetime of the phosphorescence. Measuring lifetime is consequently one way to determine the temperature of the phosphor. A high temperature will also increase vibrations in the host material crystal lattice. This will broaden the linewidths of the emitting transitions in the phosphorescent process. Thermal expansion in the host lattice will also induce a frequency shift in the phosphorescence spectra, which accordingly can be used for temperature determination. Energy / 103 cm-1 40 Host CTS 30 3 5D 2 1 0 20 Eu3+-levels 6 10 5 4 7F 3 2 1 0 0 Figure 1. The charge transfer state (CTS) as it affects the de-excitation in the La2O2S:Eu 5 3+ phosphor. Excited electrons at high D-levels in the Eu -ion can relax via the La2O2S CTS to the Eu3+ 7F ground state level. There are, as mentioned above, a number of physical phenomena that influence the spectral shape of phosphorescence from thermographic phosphors depending on temperature. The 2 changes in the phosphorescence spectra will take form as changes in phosphorescence intensity, changes in phosphorescence lifetime and/or line shifts in the phosphorescence spectra. The two most common changes used for thermometry are lifetime changes and changes in intensity at specific wavelengths. 3. Temperature measurement approaches 3.1 Lifetime method As described above most thermographic phosphors have a lifetime which is temperature dependent. According to Eq. 1 the intensity will decay exponentially according to t − τ I =I oe (1) where I0 is the initial emission intensity, t is time and τ is the lifetime of the phosphorescence. τ is the amount of time to which the intensity has decreased to 1/e of the initial emission I0. In order to cover a large temperature interval several different phosphors may be used. Some of these and their corresponding lifetimes are shown in Fig. 2a. An example is shown in Fig 2b where the temporal decays are measured for three different temperatures. The phosphorescence lifetime will decrease with temperature depending on the phosphor material. Temperature can thus be calibrated and determined by calculating the phosphorescence lifetime from the measured intensity decay. This is normally done by fitting the intensity decay to the theoretical model (eq. 1), using a non-linear fitting procedure. The error in temperature from such a measurement can ideally be less than 1%.The lifetime method is usually used for point temperature measurements using a Photo-Multiplier Tube (PMT) for detection. Figure 2a. Temperature sensitivity of different phosphor materials. 3 Figure 2b.Phosphorescence decays measured at different temperatures. The lifetime becomes shorter when the temperature increases. 3.2 Intensity ratio method The intensity of certain peaks in the phosphorescence spectrum is often temperature dependant and can thus be calibrated to temperature. Simultaneous measurements of two different wavelengths allow an instantaneous temperature measurement. The phosphorescence spectra of the thermographic phosphor YAG:Dy at different temperatures is shown in Figure 3. Figure 3 - Phosphorescence spectra as a function of temperature. Comparison of the peak at 455 nm with the one at 493 nm is used for determination of temperature. 4 The non-radiative deexcitation rate is strongly dependent on temperature resulting in a decrease of emission efficiency and shortening of the emission decay time at high temperature, although not at the same rate for the different emitted wavelengths. The advantage of this technique is that the ratio between the two apparent peak intensities of the phosphor is only dependent on temperature. Following a UV excitation the emission signal, Sj, can be represented by: Sj = CjTeNj(T)τj I (2) Where Cj is the detection efficiency for the jth transition Te is the camera exposure time, Nj(T) is the temperature dependent quantum efficiency for the jth transition, τj is the optical filter transmission and I is the intensity of the laser beam. By rationing the signal for the two different transitions, the resulting value is only dependent on temperature as: S N (T) j+1 = K j+1 (3) S j N j (T) Where K is a constant 4. Calibration procedures In order to correlate the phosphorescence to temperature, a series of calibration measurements must be performed under well controlled conditions. Figure 4 shows the set-up of a calibration measurement using a laser for excitation, fibres for guiding light, filters for increasing the signal to background ratio, and detectors for acquiring the phosphorescence emission. Figure 4 Calibration set-up using a spectrometer, ICCD, laser, PMT, thermocouple and the test cell. During the calibration procedure several phosphors have been calibrated and both temperature measuring approaches were used and sometimes compared. In the experiments low laser intensity, 50 µJ, was enough to obtain phosphorescence. A power dependence test on the intensity of the emission did not show any noticeable variation of the phosphor lifetime to the laser power between 50 µJ and l mJ. 5 The phosphor was excited by the third or fourth harmonic of a Nd: YAG laser at 355 and 266 nm, respectively, with a pulse duration of about 7 ns and a repetition rate of 10 Hz.
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