Solid-State Fluorescence Above 1000°C:Application to High-Temperature Laser Therinoiretry

M. R. Cates, S. W. AllisonApplied Technology Division, Oak Ridge National Laboratory

Oak Ridge, Tennessee 37831-7280USA

G. J. PogatshnikSolid State Division, Oak Ridge National Laboratory

Oak Ridge, Tennessee 37831USA

A. R. BugosDepartment of Electric Engineering, The University of Tennessee

Knoxville, Tennessee 37996USA

Abstract

Rare-earth doped phosphors are discussed here which exhibitintense fluorescence well above 1000°C. This is a rarecharacteristic for solid—state materials. One immediateapplication for them is thermometry. For example, surfacetemperatures of rotating components, systems in hostile orrestricted environments, and systems in environments with very hightemperature backgrounds are measurable with phosphor thermographicmethods. The subject phosphors, Y203:Eu, LuPO4:Eu, YPO4:Eu, andLuPO4:Dy, provide the capability to extend these methods to veryhigh temperatures. The use of pulsed ultraviolet (UV) laseractivation of these phosphors leads to numerous practicalapplication possibilities. The phosphor characteristics plusvarious fluorescent decay times vs temperature are shown, alongwith discussion of their high-temperature applications.

The phosphor thermographic method

Several papers in previous ICALEO meetings have describedvarious aspects of the phosphor thermographic method.13 Since 1982,the method has been used to perform temperature measurements undera variety of conditions. The major elements of the technologyinclude the pulsed activation of a surface layer of phosphor,usually by a UV laser, and the recording of the characteristicdecay of selected fluorescent emission bands. For applicationsabove 1000°C the methodology remains basically the same. The addeddifficulties arise from the need to find and calibrate phosphorsthat are temperature active in this range and the need to rejectany effects from blackbody emissions or other background effectsspecific to high—temperature environments.

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This paper discusses actual characterization of candidate high-temperature phosphors from among the group of rare—earthorthophosphates and yttrium oxide and describes appropriatetechniques to reduce or eliminate background effects. The resultof this study is a clear demonstration of the viability of phosphorthermography for surface temperature measurement above 1000°C.

Properties of high—temperature phosphors

Oxides and orthophosphates are stable molecular structures thathave, high melting and dissociation points. In the work describedhere, temperatures >1500°C were used, in repeated cycles, and nodegradation of material properties was observed. When activatedwith a rare—earth dopant ion added in small percentage, compoundsof this type often produce efficient phosphors. The phosphorsproduced have emission lines characteristic of certain angularmomentum transitions in the dopant ion electronic levels. Therare—earth dopant ion, in effect, behaves as if it were an isolatedatom. This is because the outer shell electrons shield the opticaltransition electrons such that coupling to the lattice structureis very weak and only slightly perturbing. Hence, most of theabsorption transitions and all of the emission transitions beginand terminate on atomic states. The absorption and emission bandsare very narrow because these states are characteristically verynarrow with the exception of a wide absorption feature due to thelattice which appears at the blue end of the spectrum for thesematerials. As a rule, a large fraction of excited—state electronsreturn to the ground state by emitting fluorescence photons. Bothin the ground and excited states, the electron may reside in arange of vibrational levels governed by the Bolzmann distribution.At a sufficiently high temperature, some of the electrons willhave, within the excited electronic state, sufficient vibrationalenergy to be in resonance with a lattice transition known as thecharge transfer state. A fraction of the electrons will cross overto this state. Transitions from this state are exclusivelynonradiative. As the temperature increases more and more electronscross over to this state and de—excite, resulting in fewerelectrons being available for making fluorescence transitions.Hence, the fluorescence gets weaker at higher temperatures.

We use the term "onset quenching temperature" (OQT) for thetemperature at which this competition between photon-emitting andnonphoton—emitting processes becomes noticeable. The effectoccurs, of course, for certain groups of emissions that havesimilar quantum mechanical parameters and are normally clusteredin a particular wavelength region or band. Other quantumtransitions will have different OQTs. High-temperature phosphors,generally speaking, are those that have a band of photon emissionswhose OQT5 are very high. The four examples we discuss here haveOQT5 of 50O°C or higher.

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For temperature measurement purposes, the most obviouscharacteristics of the fluorescence to observe are the emissionintensity and the emission persistence. Both the intensity and theduration of the fluorescence decrease with increasing temperaturein the region above the onset quenching temperature because bothare electron—population dependent processes, and the population"available" to decay by photon emission is continuously decreasedby the lattice competition as the lattice heats up.5 In practice,the fluorescence intensity is best measured by ratioing a stronglytemperature—dependent emission band intensity with the intensityfrom another band without much temperature effect. Such anapproach eliminates the need for absolute calibration of thesystem. Similarly, if the fluorescence persistence, in the formof the decay time, for example, is measured, it, too, requires noabsolute calibration. In our experience, for most pastapplications, the decay—time measurement is the more practical.The mathematical form of the fluorescence intensity vs time formost emission lines of interest is given by

I = I exp(-t/T) I

where I is the initial intensity, t is the time, and T is thedecay time.

Summary of temperature—dependent fluorescence measurements

The emission spectra for the four phosphors, taken at roomtemperature, and for an optimum choice of UV excitation wavelengthare shown in Figs. 1-4. We show these to illustrate the types ofemissions seen from phosphors of this type, illustrating the linesand bands that are characteristic.

Fig. 1. Emission spectrum(expanded form) of dysprosium—doped lutetium phosphate atroom temperature (353 nm EX).

Fig. 2. Emission spectrumof europium-doped lutetiumphosphate at room temperature(395 nm EX).

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Using a laboratory arrangement similar to that diagrammed inFig. 5, we measured the characteristic decay times for emissionwavelengths with high—temperature OQTs, again using optimizedexcitation wavelengths. The activation was achieved using aNd:YAG—dye laser system which can produce nanosecond—durationpulses in a variety of UV wavelengths. Data, as shown in thefigure, were collected by selecting the emission wavelength bandof interest and recording it with a photomultiplier. Figure 6shows the decay times vs temperature for the four phosphors.

Fig. 5.calibration

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Fig. 3. Emission of Fig. 4. Emission spectrum ofeuropium-doped yttrium oxide europium-doped yttrium phosphateat room temperature (270 nm EX). at room temperature (396 nm EX).

Lens

Pulsed UVLaser

Diagram for phosphorsmeasurement system.

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Lifetime vs Temperature for Several Thermographic Phosphors

Fig. 6. Combined lifetime calibrationcurves for several high temperaturethermographic phosphors.

Clearly, each has measurable fluorescence well beyond 1000°C. Ourcontrolled temperature furnace as limited to 1200°C. Preliminarydata above l200C have already been taken in a 1700°C furnace andgive results consistent with the trends shown in Fig. 6. Thesehigher temperature results will be reported in later publications.

The OQT5 for the phosphors are 670°C for YP04:Eu, 755°C forLuPO4:Eu, 510°C for Y203:Eu, and 905°C for LuPO4:Dy. The definitionwe use for the onset quenching temperature is the intersection ofstraight-line fits of the logarithm of the data at temperaturesbelow and above where it changes slope (see Fig. 6).

Surface bonding for measurement applications

A thin layer of any phosphor used in a measurement applicationmust be bonded to the surface of interest. This bonding technologyhas been pursued since 1982, along with the phosphorcharacterizations. Considerable discussion of bonding and resultsin actual measurement situations is given in Reference 6. For thepurpose of this discussion, it is sufficient to note that pastexperience indicates that phosphor layers 50 microns thick, theapproximate thickness required for most applications, can beapplied and will survive in severe environments up to 1200°C. Weexpect to be able to go beyond 1200°C as well, but our research isonly in the preliminary stages.

Surfaces of particular interest at high temperatures are thoseof passivated nickel and niobium alloys, ceramics, ceramiccomposites, and various formulations of carbon. Our bondingexpertise to date has concentrated on the passivated metalsurfaces, but we have begun to concentrate on the other types ofsurfaces.

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C)G)1oU)

C)E io'C)

-J

200 400 600 800 1000 1200

Temperature (°G)

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High—tenperature applications

As technology advances, it is inevitable that numerous energyprocessing systems will be required to operate at highertemperatures. In jet turbine engines for aircraft, for example,the fuel burns at 2OOO°C; consequently, the stoichiometric limitof performance would require structures to survive at nearly thattemperature. Near—term goals aim at producing surfaces thatsurvive at 1400°C or greater. Similarly, for automobile engines,future designs call for ceramic and ceramic composite assembliesrequiring no water cooling and designed to work at far highertemperatures than present designs. Fusion energy power systems ofthe future will have confining structures that must survive smallexcursions of the operating plasma itself, heated to millions ofdegrees. Space vehicles and rockets will both operate at extremelyhigh—temperatures and be exposed to direct solar heating in thevacuum of orbit. In many instances, the problem of temperaturemeasurement will be of great significance because materials willbe used closer and closer to the limits of their survivability.Even now the problem is serious in turbine engines where a fewpercent increase in operating surface temperatures of nickel alloyscan lead to significant shortening of operating lifetime. High-temperature surface thermography can provide measurement solutionsfor many of these systems.

The study described here demonstrates the potential formeasurement of surface temperatures up to 1500°C or higher.Furthermore, the techniques involved are remote, requiring nocontact except the activating photon source, and will not impederotation or other necessary motions. In addition, and perhaps mostimportantly, this fluorescence method is independent of surfaceemissivity, which plagues pyrometric methods. Because of thenarrow wavelength band of the desired emissions and the pulsednature of the fluorescence, the method also can reject the intenseblackbody backgrounds of surfaces at elevated temperatures. Therejection ratio in this situation can be pushed a million to oneor greater.

Based on past experience and on the degree of temperaturedependence shown in the curves of Fig. 6, we expect that surfacetemperatures can be measured beyond 1000°C with <1% uncertainty.Since the fluorescence behavior is a function of absolutetemperature,7 each improvement in the calibratiori and measurementtechnique will lead to increased precision and accuracy.

Acknowledgment

This report is based on work performed at the Oak RidgeNational Laboratory, operated by Martin Marietta Energy Systems,Inc., for the U.S. Department of Energy under contract DE-ACO5-84OR2 1400.

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References

1 . , M. R. , et al . (1983) . "Remote Thermometry of MovingSurfaces by Laser Induced Fluorescence of Surface bonded Phosphor,"Proc. Laser Inst. of America 39: 50.

2. Cates, M. R., et al. (1984). "Applications of Pulsed—LaserTechniques and Thermographic Phosphors to Dynamic Theriroiitetry ofRotating Surfaces," Proc. Laser Inst. of America 45: 4.

3 . Gillies , et al . (1988) . "Noncontact Thermometry Via LaserPumped, Thermographic Phosphors: Characterization of SystematicErrors and Industrial Applications," Proc. Laser Inst. of America62: 15-21.

4 . Bugos , A. R. (May, 1989) . "Characterization of the EmissionProperties of Thermographic Phosphors for Use in High TemperatureSensing Applications." A thesis presented for the Master ofScience degree, The University of Tennessee, Knoxville, Tennessee.

5. Fonger, W. H. and Struck, C. W. (1971) . "Energy Loss and EnergyStorage from the Eu3+ Charger—Transfer States in Y and LaOxysulfides," J. Electrochem. Soc. 118: 272-280.

6 . Beshears, D. L. , et al . (April , 1986) . "Evaluation ofCommercially Available Coating Techniques for Application ofThermographic Phosphor to Nickel-Based Alloys," K/TS-ll,80l MartinMarietta Energy Systems, Inc., Oak Ridge Gaseous Diffusion Plant.

7 . 1 L. J . (May, 1989) . "Investigation and Development ofPhosphor Thermometry," A dissertation presented to the faculty ofthe School of Engineering and Applied Science, The University ofVirginia.

Meet the authors

Michael R. Cates is a native of Texas. He received his B.S. andM.S. in physics from Baylor University and his Ph.D. in physicsfrom Texas A&M University in 1969. From then until 1980 he was onthe staff at Los Alamos National Laboratory. Since late 1980 hehas been with Oak Ridge National Laboratory, where he is currentlyleader of the Photonics and Laser Applications group of the AppliedTechnology Division.

Stephen W. Allison, an Arkansas native, has a B.S. in physics andmathematics from Harding University, an M.S. in physics fromMemphis State University, and a Ph.D. in engineering physics fromthe University of Virginia (1979) . He has been with Oak RidgeNational Laboratory since 1978, working in fiber—optic and laserapplications in the Photonics and Laser Applications group of the

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Applied Technology Division. Currently, he is enjoying asabbatical year at the University of Virginia, studying imageapplications for treatment of brain tumors.

Gerald J. Pogatshnik received the B.S. degree in physics in 1980from the State University of New York, College at Cortland, and theM.S. and Ph. D. (1986) at the University of Connecticut. Dr.Pogatshnik held a joint Visiting Research Professorship from 1986to 1988, between the Solid State Division, Oak Ridge NationalLaboratory, and the Department of Materials Research andEngineering, at North Carolina State University. He is currentlyan Assistant Professor of Physics at Southern Illinois University—Edwardsville.

Alan R. Bugos is a native of Pittsburgh, Pennsylvania. Hereceived the B.S. degree in engineering physics and the M.S. inelectrical engineering (1989) at The University of Tennessee. Hehas been an instructor and graduate research assistant in theDepartment of Electrical and Computer Engineering at UT. Hisgraduate research was performed in the laboratory complex of theApplied Technology Division at Oak Ridge National Laboratory. Hehas expertise in fluorescence spectroscopy, fiber—optic deliverysystems, and high—power laser applications.

The submitted manuscript has beenauthored by a contractor of the U.S.Government under Contract No. DE-ACO5-840R2 1400. Accordingly, theU.S. Government retains a nonexclusive,royalty-free license to publish or repro-duce the published form of this contri-bution, or allow others to do so, forU.S. Government purposes.

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