A WIDE-RANGE PHOSPHOR THERMOMETRY TECHNIQUE

Transcription

1 A WDERANGE PHOSPHOR THERMOMETRY TECHNQUE S.W.Aison D. L. Beshears M. R. Cates 1 _. i. $ '3 ; \ G. T. Giies Department of Mechanica, Aerospace, and Nucear Engineering The University of Virginia Charottesvie, VA 2291 March 1998 Prepared by the Oak Ridge Nationa Laboratory Oak Ridge, Tennessee managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract DEAC596R22464 ",t

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their empoyees, make any warranty, expm or impied, or assumes any ega iabiity or responsibiity for the accuracy, competeness, or usefuness of any information, apparatus, product, or process discosed, or represents that its use woud not infringe privatey owned rights. Reference herein to any specific commercia product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessariy constitute or impy its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessariy state or refect those of the United States Government or any agency thereof.

3 A WDERANGE PHOSPHOR THERMOMETRY TECHNQUE S. W. Aison, D. L. Beshears, M. R. Cates Oak Ridge Nationa Laboratory Oak Ridge, TN G. T. Giies Department of Mechanica, Aerospace, and Nucear Engineering The University of Virginia Charottesvie, VA Key Words: fuorescence, thermometry. ntroduction Fuorescing materias exhibit a temperature dependence, which may be expoited for thermometry purposes, Soid state materias such as phosphors, gasses and crystas are exampes of such and have been used in commercia instruments and various oneofakind research and deveopment appication^',^^^'^. This area has been the subject of previous SA paper~~9~. t is generay the case that fuorescence decay time or ifetime is the parameter that is measured in order to determine temperature for appications that do not require imaging. There are severa good reasons for this. The decay time is a very sensitive function of temperature. Time and ratedependent methods are independent of ampitude fuctuations and are therefore not as susceptibe to optica noise. n some appications, however, other aspects of the temperaturedependent fuorescence can aso be usefu. What foows is a description that concerns intensitybased methods and the types of appications to which they appy. The emphasis of the present work is the advantage for situations demanding a wide range and rapidy varying temperatures. 11. Range and Sensitivity Athough intensity may not be as sensitive a function of temperature as decay time, it can provide for a wider range temperature measurement. Sometimes ratioing two different emission bands (or regions) that are spectray separated accompishes this. Figure 1 iustrates this contrast in sensitivity and range. Emission characteristics are depicted of an yttrium vanadate phosphor doped with dysprosium. The ratio data taken from previous work6 of two of the prominent emission ines is potted as a function of temperature. The ratio changes by a factor of about 4 over the range of ambient to 4 C. This contrasts with the pot of decay time versus temperature for the other prominent visibe ine of this phosphor. The decay time is essentiay constant unti a threshod temperature at about 3 C is attained. The decay time changes by a factor of 4 in a itte over 1 degrees. This particuar materia was used in an unpubished test where

4 temperatures were expected to vary over a wide range. When the temperature was beow the quenching threshod, the ratio method provided the temperature, above that, the decay time measurement determined the temperature. The term intensity here refers to either the average detected signa for continuous excitation or the integrated signa resuting from a pused excitation of fuorescence. Different mechanisms can be invoved in intensity and decay time. Generay, decay time, T:,is modeed and yieds a simpe reation2: simiar to the equation beow. ORNL EFG 1 4. x YVO,:Dy 5 i= Temperature ("C) Figure 1. YV4:Dy temperature dependence of emission ratio and decay time. Eq. 1 For this particuar phosphor sampe, a (65 s") is the reciproca of the unquenched decay time and b (5.5 X 115 s1 ) is a rate constant obtained from the fit to the data. The activation energy, E, which contros the temperature of quenching is determined from the fit to be 11,8 cm', T is absoute temperature and k is the Botmann constant. t may be noted that the decay times of these three emission ines for this phosphor in this range, appear to be identica within the experimenta capabiity of the setup, a few percent. f more than one energy eve is invoved, the equation must be modified, as for the case of ruby. This is discussed in reference 2.

5 Severa processes govern intensity. For simpe exponentia dependence it is as foows: Jo =A(h,T).a exp(:) dt or Eq.3 = A ( h, T).T Where A(X,T) is the initia ampitude of the fuorescence and is dependent on the amount of excitation ight absorbed which in turn is a function of excitation waveength, A, as we as temperature. Note that intensity is directy proportiona to the decay time (or integra of the time dependence). Figure 2 iustrates the temperature dependence of the utravioet eectronic absorption band of Y23:Eu. This is due to possiby two things. The first observation is that, as a resut of increased temperature, higher vibration eves in both the ground and the excited eectronic state are popuated. t may be that FranckCondon overap may be occurring and thus the absorption transition probabiity is increased. For atomic transitions, there is aso some broadening and shifting of the emission due to temperature, but the effect is smaer than that iustrated in Figure 2. Therefore, as the amount of absorption changes with temperature, the intensity wi be affected. ORNL EFG Figure 2. Excitation spectrum for 611 nm of Y23:Eu. nrn Quantum efficiency of a given fuorescence transition reative to another may change with temperature. An exampe of this is an effect known as thermaization. This appears to be the case for dysprosium uminescent materias. The emission from the buegreen emission band at 483 nm and the yeow emission at 575 nm both originate from the 4F9,2

6 excited eectronic state. This is ocated about 1 cm' beow the 4115,2 state from which the 453 nm emission band originates.. The popuation distribution in the 4F9/2 state wi be governed by a Botzmann distribution.. As temperature is increased, the popuation at 1 cm' increases and the energy crosses over to the 4115/2 state from which eve emission occurs. This process occurs even before the onset of quenching. The intensity of the 453 nm emission grows with temperature. n another materia, YAG:Dy, Goss et a1 demonstrated that this increases up to at east 13K'. Another exampe materia' is a crystaine doubedoped materia, LuP4:Dy( 1%), Eu(2%). ntensity versus temperature is shown in figure 3. The responses of severa of the characteristic emission bands are shown. They are activated by utravioet ight at either 353 nm or 39 nm. The trend with temperature for the Dy emission is simiar to that described for YVO4:Dy. The 595 nm ine (from Eu) shows a gradua decrease typica of emission beow the quenching temperature ~ 1 ~ 1 ~ c 'E c 3 ~ \A f(353 ex) 484 f(39 ex) ' 575 f(39 ex) 595 f(39 ex) Eu 453 f(353 ex) 1/2 \A 2.5 ( ~ A 484 f(353 ex) A 3. 1 \ 1.5 v. v) C 2 r Temperature ("C) Figure 3. Emission intensity vs. temperature for severa LuP4: Dy,Eu bands Transient Thermometry ntensity ratios are attractive for measurement of systems where surface temperature is changing rapidy. ntensities can be foowed continuousy and ratioed to give timedependent temperatures. Phosphor coatings can be very thin. n which case, the therma mass is ow. Because of this and the reativey short fuorescence decay times, rapid changes in temperature can be determined. This was demonstrated in a set up invoving

7 resistive heating of a phosphorcoated nichrome wire". The wire served as a resistor in a simpe RCcircuit. Discharging the capacitor produced a transient heat puse. A mechanica switch required about about 1 ms to cose. The ight source was an utravioet amp whose output foowed and rectified the 6 Hz ine votage. The output of the amp served as the cock for the test. A photodiode monitored the amp output. This signa was buffered and spit into two puses which were separatey deayed to trigger first the mechanica switch, and then, with sufficient deay, a digita oscioscope. Two 1 mm diameter optica fibers coected the emission. Each fiber couped to a photomutipier with intervening 1 nm wide bandpass fiters. A fiter centered at 62 nm coected emission from two ines at 62 and 617 nm. A fiter centered at 54 coected ight at 538 nm. Severa phosphors were evauated, however, the YV4:Eu was best suited with regard to the temperature range and signa eve. Figure 4 shows a portion of the spectrum. t may be noted that there is some ine broadening and shifting. These we known therma effects can be used for thermometry as noted by Kusama et a' but are usuay not as sensitive as the methods discussed here. ORNL9&1553 EFG Ambient nrn Figure 4. Spectrum of YV4:Eu at ambient and 3 C.

8 To caibrate, a DC current heated the phosphor coated wire and the temperature was measured with a thermocoupe. Figure 5 shows the caibration. The photomutipier gain was adjusted so that the room temperature ratio was 1. Since the ight source osciates, the emission intensity foows it. Figure 6 shows the signa of the two channes as a function of time. A itte more than one cyce of the emission is depicted. Transient heating begins after approximatey 2 ms. The ratio begins to decrease around 3 ms after switch cosure, indicating a temperature rise. The ratio is converted to temperature in Figure 7 for various capacitor charging votages. Temperature increase rates are on the order of 1 C / ms. ORNL EFG F f.! YV4:Eu Ratio Caibration 62 nm / 54 nm Poynomia Curve Fit Coefficients: ao= a, =a Correation Coefficient = Ratio.8 1.o Figure 5. Caibration pot for transient thermometry. 1.2

9 ORNL EFG Transient Fuorescent Response.4 >.3 C E a Y 3 c. E.2 U Time (ms) Figure 6. Timedependence of fuorescence. ORNL EFG Transient Heating Rates YV4:Eu 62 nm/54 nm O C / r n s 3 F 25 Y g Ea, 2 e 15 z 1 e, Time (ms) 5 6 Figure 7. Heating rates vs. stored votage. A particuar advantage of using the ratio method for transient thermometry is the fact that data acquisition and computationa requirements are simpified. An anaog signa is produced that reates simpy to temperature.

10 V. Concusions There are severa issues to be considered regarding caibration. One is that a caibration may change if the test arrangement is atered. For instance, detector photocathodes have different spectra responses and switching detectors wi thereby change a ratio caibration. Specia attention must be given to optica components due to chromatic. effects. The power of a common ens wi vary somewhat with waveength. Changing the position of a simpe ens may ater the ratio of two separated emission waveengths even if the temperature has not changed. Optica noise and oss sources shoud be scrutinized for possibe spectra bias. Aso, significant heating of the optics can ater their optica and hence spectra characteristics. This paper presented some data regarding extended range and transient thermometry using the intensity or ratio technique. Twodimensiona thermometry aso utiizes the intensity or ratio method. A survey of such imaging appications is contained in reference 4. The Oak Ridge Nationa Laboratory is operated for the U. S. Department of Energy by Lockheed Martin Energy Research under contract DEAC596R22464.

11 e References 1. K. Wickersheim and M. Sun, "Phosphors and Fiber Optics Remove Doubt from Difficut Temperature Measurements," Res. Dev. 27, 114 (1985). 2. K. T. V. Grattan and Z. Y. Zhang, Fiber Optic Fuorescence Thermometry (Chapman & Ha, London, 1995). 3. B. W. Noe, W. D. Turey, and S. W. Aison, "ThermographicPhosphor Temperature Measurements: Commercia and DefenseReated Appications," in Proceedings of the 4th nternationa nstrumentation Symposium (nstrument Society of America, Research Triange Park, North Caroina, 1994), pp S. W. Aison and G. T. Giies, "Remote thermometry with thermographic phosphors: nstrumentation and appications," Rev Sci nstrum Vo. 68(7), p , D. L. Beshears, R. A. Abston, S. W. Aison, et. a., "Appication of phosphor thermometry to gavannea processing," Pubished in the Proceedings of the nstrument Society of America's 43rd nternationa Symposium on nstrumentation, pp. 7958, SBN , May 59, 1997, Orando, FL. 6. M. R. Cates, K. W. Tobin, and D. B. Smith, "Evauation of thermographic phosphor technoogy for aerodynamic testing," Martin Marietta Energy Systems, nc. Report No. ORNLATD4, August W. H. Fonger and C. W. Struck, "Eu3+5D Resonance Quenching to the ChargeTransfer States in YzO~S,La22S, and LaOC," J. Chem. Phys. 52,6364 (197). 8. L. P. Goss, A. A. Smith, and M. E. Post, "Surface Thermometry by Lasernduced Fuorescence," Rev. Sci. nstr. 6(12), pp , S. W. Aison, L. A. Boatner, and G. T. Giies, "Characterizationof HighTemperature Thermographic Phosphors: Spectra Properties of LuP4:Dy( 1 %),Eu(2%)," App. Opt. 34,5624 (1995). 1. K. W. Tobin, G. J. Capps, J. D. Muhs, D. B. Smith, and M. R. Cates, "Dynamic HighTemperature Phosphor Thermometry," Martin Marietta Energy Systems, nc. Report No. ORNLATD43, August 199,15 pp 1 1. Kusama,. J. Sovers, and T. Yoshioka, "Line Shift Method for Phosphor Temperature Measurements," Jap. J. App. Phys. 15,2349 (1976).

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