Characterization of Yb2O3 based optical temperature sensor for high temperature applications

Characterization of Yb2O3 based optical temperature sensor for high temperature applications

Sensors and Actuators A 134 (2007) 348–356 Characterization of Yb2O3 based optical temperature sensor for high temperature applications Ayan Bhattach...

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Sensors and Actuators A 134 (2007) 348–356

Characterization of Yb2O3 based optical temperature sensor for high temperature applications Ayan Bhattacharya, R. Srinivasa Rao, M. Ghanashyam Krishna ∗ School of Physics, University of Hyderabad, Hyderabad 500 046, India Received 21 January 2006; received in revised form 7 May 2006; accepted 29 May 2006 Available online 10 July 2006

Abstract A rare earth oxide based high temperature sensor is presented in this work. The rare earth is used as a selective emitter, which emits narrow band radiation at a peak wavelength of 980 nm that falls within the band edge of the Si-photodetector. The radiation results from the transition of 4f electrons in the Yb3+ ions due to thermal excitation. The radiation from the emitter, and the output (short-circuit current) of the photocell, is a function of the temperature. By inversion of Planck’s law, the short-circuit current can directly be used as a measurement of temperature. The rare earth shows better performance in the range >1500 ◦ C. The sensor geometry has been tested under a variety of conditions such as varying distance (10–50 cm), area of photodetector (5 and 10 mm2 ) and materials such as pellets, coatings and powder. The temperature was calibrated by an optical pyrometer. It is found that the linearity of the sensor is within 5% over the temperature range 1200–1700 ◦ C, accuracy is 2% and repeatability is 6% within the mentioned temperature range. The diverse material forms that the sensor lends itself to make it an ideal candidate for variety of applications. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth; Optical temperature sensor; Yb2 O3

1. Introduction There are various temperature measurement techniques available that work over a range of temperatures [1]. However, the number of sensors available for application at temperatures greater than 1500 ◦ C is limited due to deterioration in material properties and practical limitations related to intrusive sensors. Some of the commonly used temperature sensors in this range are Pt–Rh thermocouples, optical pyrometers and optical fiber based temperature sensors, etc. [2–5]. Rare earth materials when heated to very high temperatures emit radiation selectively in the infrared region on relaxation to the ground state of 4f levels. This process is called selective emission [6]. The energy of radiation in most cases matches the band edge of known semiconductor materials. For example, the transition in Yb3+ ions matches the band edge of Si. This phenomenon has been previously used to convert incident radiation into electricity by a process called thermo photovoltaic ∗

Corresponding author. Tel.: +91 40 23134255; fax: +91 40 23010227. E-mail address: [email protected] (M. Ghanashyam Krishna).

0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.05.036

generation of electricity [7,8]. However, in principle, the same phenomenon can be employed to sense temperature by inversion of Planck’s law as shown recently by Chubb and Wolford [9]. They have demonstrated the concept using a fiber located at a fixed distance from the radiating surface. In this paper, we report on a cost-effective solution to demonstrate the sensor concept. It has been tested at different distances from rare earth to PV cell and with different areas of photocell and with different forms of the material such as bulk pellets and with coatings on substrate. 2. Sensor operation The working of the sensor is based on the principle that emissive power of a radiating surface can be converted into a corresponding temperature, based on Planck’s law. The sensor consists of a rare earth containing element, and a photo detector. The rare earth containing element is in contact with the sample being measured. The detector, which is a PV cell, kept some distance away from rare earth, collects the radiation from rare earth piece. A schematic view of the sensor is shown in Fig. 1.

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3. Experimental 3.1. Preparation of samples

Fig. 1. Schematic diagram of the sensor arrangement.

The detector is operating in short-circuit mode and the output of detector is then converted to a temperature readout using a sensor electronic circuit (not described in this work). The upper temperature limit of sensor depends on the melting point of rare earth oxides. In the current work, Ytterbia (Yb2 O3 ), which has a melting point of approximately 3000 K, is used as the rare earth piece. The experimental arrangement of Chubb and Wolford [9] comprised of a rare earth containing end piece, an optical fiber, a narrow band optical filter, an optical detector and electronic circuit to convert detector output into temperature. In the current work, however the measurements have been carried out using a rare earth (of different geometries) directly in contact with the radiating surface, a Si-photodetector and electronics to convert the short-circuit current into temperature (this work is not discussed in this paper). The wavelength window of the rare earth oxide in the current study was assumed to be between 900 and 1000 nm. From Siphotodetector’s datasheet [10], it has been found that the spectral response of Si-photodetector falls to a minimum level of 80% from its peak within this region. It is further assumed that the emissivity of the rare earth does not vary much in the temperature range of interest. Based on these assumptions and following the treatment by Chubb and Wolford [9], the relation between temperature and detector output is   C2 Isc = C0 exp − (1) λTs

Highly pure Yb2 O3 (99.9%) powder was obtained from Sigma–Aldrich. The experiments were carried out in different forms such as pellets, coatings and using the powder itself. Pellets of thickness 1.2, 1.6, 2.4 and 3.3 mm were prepared and then sintered in a box furnace for 6 h at 1400 ◦ C. Apart from this, coatings of different thickness of Yb2 O3 on substrates were also put under observation as an emitter. Coatings were prepared from powder by diluting, using water, heated at 90 ◦ C and then mixed with polyvinyl alcohol, which acts as a binder. Slip coatings of Yb2 O3 were applied on molybdenum and tantalum boat surfaces. The coating thickness was typically 10 ␮m or higher as estimated from the weight change between coated and uncoated substance. 3.2. Measurement of radiation A schematic representation of the set-up used for measuring the radiance (in terms of short-circuit current Isc ) of the samples is shown in Fig. 1. The samples were placed in the direct contact with the hot surface (Ta, Mo boats). The boats were heated by passing current using a (15 V, 150 A) low-tension power supply in a standard thermal evaporation geometry for fabrication of thin films. The maximum temperature achieved in this arrangement is ∼1800 ◦ C. The sample temperature was gradually increased in an environment of 2 × 10−5 mbar pressure. The pressure was maintained using a mechanical and a diffusion pump in combination. Si-photodetector of different surface areas was kept at different distance (10–50 cm) from the rare earth to collect the radiation and convert to equivalent short-circuit current Isc . The schematic arrangement for measurement of the temperature in the vacuum environment is shown in Fig. 2. The short-circuit current was later converted to temperature and was also verified

C0 is determined by calibration procedure, it has a dimension of current. At some known calibration temperature, the shortcircuit current Isc is measured. Therefore,   C2 ln C0 = + ln(Isc ) (2) λTs where C2 is the hc/k, h the Planck’s constant, c the velocity of light in vacuum, k the Boltzman’s constant and Ts is the temperature of rare earth sample. Peak wavelength λ corresponds to the radiation emitted from rare earth oxide. The above equation gives the relation between the shortcircuit current and the temperature of the object and forms the basis for the sensor operation in the current work. Therefore, with appropriate analog electronics, the measured short-circuited current Isc , of the PV detector can be converted into a sample temperature, Ts .

Fig. 2. Schematic diagram of the arrangement to measure the radiance in vacuum.

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Fig. 3. The calibration of temperature, as estimated from Isc and the corresponding temperature readings from the pyrometer.

by an optical pyrometer, used for calibration, as shown in Fig. 3. At each temperature the pyrometer was also used to estimate the temperature of the rare earth and compare it with that of the radiating surface. 4. Results and discussion 4.1. Variation of short-circuit current as a function of tempearture The short-circuit current variation of Yb2 O3 pellet has been studied and presented in Fig. 4(a) and (b). The short-circuit current follows an exponentially increasing trend with respect to the rise in temperature of the hot surface as predicted by Eq. (1). The effect of the distance between detector and emitter on the response of the photodetector is also shown. A pellet of weight 2 g and 12 mm diameter was employed. Keeping all other physical parameters unchanged, the distance had been doubled

to 40 cm and its effect is shown in Fig. 4(b). As a result the short-circuit current decreased by one order of magnitude, at any particular temperature. From the figures it is evident that beyond a certain distance the amount of radiation may not be appreciable enough to convert into a suitable current. This would result in reduced sensitivity for the sensor operation. While the distance being 20 cm the Isc is in the 0.5–6 mA range, whereas at 40 cm it decreases to 0.05–0.3 mA. Clearly the sensitivity is a strong function of the distance. The short-circuit current variation as a function of temperature for a fixed weight of powder (=0.0125 gm) and area of detector (=5 mm2 ) for different distances is shown in Fig. 5(a)–(c). This is the minimum weight of the rare earth in the form of emitter that had been studied through out the experiment. The maximum temperature that could be sensed is near 1700 ◦ C. It is observed that the order of the short-circuit current falls with increase in distance between the photodetector and the rare earth. The effect of background sources also reduces in some order with the increase in distance. The background radiation is defined as the short-circuit current detected without the rare earth oxide in place. The range of operation of the rare earth in form of powder is much larger than in the case of its pellet form. 4.2. Linearity The main operation of the sensor is guided by Eq. (1) that relates the short-circuit current, to the temperature of rare earth. By taking logarithm on both sides, we get:   C2 ln(Isc ) = (ln C0 ) − (3) λTs The above equation shows that the ln(Isc ) maintains a linear relationship with temperature inverse; with a slope related to C2 (=hc/k) and peak wavelength λ, which are constants. The other constant C0 (with dimension of current) is a function of calibration procedure, i.e. it is dependent on the spectral emittance, transmittance of the medium and detector’s response. This is demonstrated in Figs. 6 and 7.

Fig. 4. Variation in short-circuit current at distances: (a) 20 cm and (b) 40 cm between the pellet of weight 2 g and photodetector with area 5 mm2 .

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Fig. 5. The variation of Isc with temperature for powder at distances of: (a) 24 cm; (b) 32 cm; (c) 42 cm between the detector and the emitter. Powder wt. = 0.0125 g, detector area = 5 mm2 .

Fig. 6. Linearity of T−1 and ln(Isc ) studied on different Yb2 O3 coatings at distance of: (a) 28 cm and (b) 42 cm from the detector of area 10 mm2 .

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Fig. 7. Linearity of rare earth in powder form at a distance of: (a) 33 cm and (b) 42 cm from photodetector of area 5 mm2 .

The graphs shown in Fig. 6(a) and (b) depict the variation of ln(Isc ) as a function of temperature inverse (T−1 ). It is evident from Figs. 6 and 7 that ln(Isc ) follows a linear relationship with T−1 (within 5% error) as predicted by the theory. Furthermore at all temperatures, the background contribution is between 20 and 50% lower than the contribution of rare earth towards Isc . As stated before, at each temperature the pyrometer is used to estimate and compare the temperatures of the rare earth and emitting surface. It was observed that the difference in the estimated temperatures is within a 5% error limit. Therefore, it can be concluded that the source of the difference is mainly the background contribution. Complete elimination of the background contribution is possible only by spectroscopic measurement of the emissive power. 4.3. Repeatability Repeatability is another major factor, which also stands as a figure of merit for a good sensor. There are several parameters, which affect the repeatability. A major concern among them is the upper limit of the temperature range of operation. If it is close to the melting point of the rare earth, then in pellet form, the sensor may result in to deformation. In case of powder this problem could be avoided up to a certain extent. Figs. 8 and 9 show the repeatability behaviour for the pellets at two thicknesses 2.4 and 3.3 mm. Repeatability in the present case is defined as the cycle-to-cycle variation in Isc at a given temperature (1400 ◦ C in the case). The same idea is extended to powder sample also, as shown in Fig. 10. It can be seen that the variation in Isc is within ±5% for the powder sample and ±7% for pellet. Several factors can be attributed to the cycle-to-cycle variation such as aging of the material, reduction of oxide and structural changes. It is further evident that the thinner pellet shows smaller cycle-to-cycle variation than the thicker pellet due to its lower thermal mass. Especially, in case of powder, the aging of material makes it difficult for further usage.

Fig. 8. Repeatability of a pellet of thickness 2.4 mm at distance of 13 cm and with photodetector of area 10 mm2 .

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4.4. Response time

Fig. 9. Repeatability at a distance of 33 cm between a pellet of thickness 3.3 mm and photodetector of area 5 mm2 .

The response time of the sensor to reach a steady state temperature has also been determined. The response time of the photodetector (τ d ) has to be much smaller than that of the rare earth emitter (τ e ) to reflect any change in the radiation intensity to an appreciable change in the short-circuit current. In general the response time could be studied in terms of rise time and relaxation time. Rise time is the time taken for a sensor to reach 90% of the maximum change in the temperature. Relaxation time is the time it requires to reach 10% of the maximum change. The results shown in Fig. 11 depict the response curve of powder sample under vacuum. Fig. 11(a) shows the time taken by the sample when the temperature of the hot surface was changed in steps of 150 ◦ C, approximately. Fig. 11(b) shows the response of the sample subjected to thermal shock, i.e. when the hot surface temperature was raised to 1700 ◦ C, suddenly from room temperature. In case of a powder sample it takes about 5 s to reach that high temperature, and the sample relaxes to <600 ◦ C in 2 s. The same observation had been continued for a thicker sample to estimate the effect of thickness on the response. The results are shown in Fig. 12. The maximum temperature reached is ∼1750 ◦ C in all the cycles. The overall rise and relaxation time in stepwise change in temperature narrowly differs from the previous sample. But the response due to thermal shock, in Fig. 12(b), makes it clear that the thick pellet retains the heat for a longer time, which makes the relaxation process quite long as 30 s. The rising profile has also increased in this case. A thinner dimension would be a better replacement for this unless thermal expansion induces any constraints in the structure. 4.5. X-ray diffraction The stability and repeatability of the sensor were analysed by means of X-ray diffraction. One of the major problems associ-

Fig. 10. Repeatability of powder samples at a distance of 42 cm and PD area 10 mm2 .

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Fig. 11. Response time studied for powder sample of Yb2 O3 by: (a) stepwise heating and (b) directly heating to maximum temperature.

Fig. 12. Response time studied for a pellet of thickness 1.6 mm by: (a) stepwise heating and (b) directly heating to highest temperature.

ated with repeated high temperature cycling of an oxide material is that it tends to dissociate in the reducing atmosphere. As a consequence the emissive power of the surface varies over a period of time. This variation is a strong function of the initial state of the materials in terms of its crystal structure, density, morphology and microstructure. A study of the crystal structure variations as a function of the thermal cycling was carried out. The results are presented in Figs. 13 and 14. Visually, it was observed that the pellets turned from a white colour to a very black colour under the influence of repeated temperature cycling. This clearly indicated the reduction of the Yb2 O3 to an unidentifiable non-stoichiometric phase. The structure and phase purity of all the samples were characterized by INEL CPS-120 powder diffractometer with Cobalt ´˚ This (Co) as the target and radiation wavelength λ = 1.7889 A. is a wide-angle position sensitive proportional counter diffractometer. Here, the Ge1 1 1 plane monochromator is used monochormatise the X-ray beam. The X-ray after coming out of the monochromator, gets scattered from the sample, the ray is been sensed by the curve position sensitive (0–120◦ ) detector, which consist of 8096 channels and a minimum resolution of 2θ as 0.015◦ . Each sample has been scanned for 20 min. In Fig. 13 planes 2 2 2, 4 0 0, 4 4 0 and 6 2 2 show peaks of Yb2 O3 . The results clearly indicate the appearance of a number of unidentified peaks caused by the dissociation of the starting material. The peak positions have a variation within 0.4%, with respect to standard JCPDS (1999) powder diffraction data. With the increase of number of cycles the peak broadening of the plane 2 2 2 is found to be within 15% for the powder sample. But overall it indicates that the rare earth in its different dimension is stable at high temperature range. Three different geometries of rare earth were tried in the current study; powder, pellet and coating. Of the three versions, it was found that powder had the fastest response time. However, under certain practical conditions the use of powders may not be possible. Thin pellets (thickness <1 mm) can be used as an alternate form of the rare earth since it has a lower thermal mass and hence a faster response. A further improvement on this situation is to apply coatings (thickness >10 ␮m) in proximity of the radiation surface. This is probably the most ideal form of the rare earth for application in temperature sensor. But under every new environment, the distance between radiating surface and detector and the area of photodetector have to be optimized. In the current study, it is evident that the powder sample, with a photodetector of area 5 mm2 , kept at a distance of 32 cm performs well as sensor (Fig. 5(b)). The background contribution is also less compared to a pellet, as an emitter (Fig. 9). It is clear from the results presented that the sensor concept can be usefully employed in a variety of harsh conditions. The ease with which the rare earth can be prepared in the form of coatings, powders or pellets and use of “off-the-shelf” photodetectors make the entire concept extremely cost-effective. The possibility of forming fibers using rare earth oxides can be exploited to extend this concept to achieve a truly non-contact optical temperature sensor.

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Fig. 13. The X-ray diffractograms of: (a) powder and (b) 1.2 mm pellet after each cycle of heating.

5. Conclusion An Yb2 O3 based optical temperature sensor is demonstrated. The linearity, repeatability, resolution and cost-effective nature of the sensor make it attractive for a number of situations that involves harsh environments. Acknowledgements The authors acknowledge, facilities provided under UGCSAP and UPE programs as well as the DST-FIST program. Fellowship for RSR and grants for part of the work under the ISRO-RESPOND program are also gratefully acknowledged. References

Fig. 14. X-ray diffratogram of Yb2 O3 coating after two cycles of heating.

[1] P.R.N. Childs, J.R. Greenwood, C.A. Long, Review of temperature measurement, Rev. Sci. Instrum. 71 (8) (2000) 2959–2977. [2] Limin Tong, J. Cryst. Growth 217 (2000) 281–286. [3] J. Canning, K. Sommer, M. Englund, Measure. Sci. Technol. 12 (2001) 824–828. [4] T. Sun, Z.Y. Zhang, K.T.V. Grattan, A.W. Palmer, Rev. Sci. Instrum. 69 (1998) 12. [5] H.C. Seat, J.H. Sharp, Measure. Sci. Technol. 14 (2003) 279–285.

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Biographies Ayan Bhattacharya obtained his MSc (Tech) degree in electronics from University of Hyderabad, India in 2004. He worked temporarily in the project titled “Development of ultra-high temperature sensors”. Presently he is working as

a Project Assistant in Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad. R. Srinivasa Rao completed MSc (Tech) degree in Electronics from University of Hyderabad, India in 2003. He worked as a Junior Research Fellow in the project titled “Development of ultra-high temperature sensors”, funded by ISRO from 2003 to 2005 and is currently registered as a research student in Department of Materials Science, Tohuku University, Japan. M. Ghanashyam Krishna obtained his PhD from the Department of Instrumentation, Indian Institute of Science, Bangalore, India in 1992. He worked as Senior Research Fellow in University of Warwick, UK from 1996 to 2001. In 2001 he joined the Faculty of School of Physics, University of Hyderabad, where he is currently a Reader. He has about 50 papers in international journals. His current research interests are nanostructured materials, thin films and sensors.